A synthetic peptide corresponding to amino acid residues 90 to 95 of human follicle-stimulating hormone beta-subunit delays the onset of puberty in female Swiss Webster mice

We have recently reported that a synthetic peptide corresponding to amino acid residues 81-95, a receptor-binding region of the human FSH-beta-subunit, and a subdomain within this region, hFSH-beta-(90-95) (DSTDCT), prolonged vaginal estrus when administered intraperitoneally (ip) to normally cycling Swiss Webster mice. These results were similar to those we reported for a synthetic peptide corresponding to hFSH-beta-(34-37) [TRDL, a subdomain within receptor-binding region hFSH-beta-(33-53)] in the same model system. TRDL also accelerated the onset of puberty in immature mice. We now report the effects of hFSH-beta-(90-95) in prepubertal female mice. In two separate experiments, a single ip injection of 200 microg/g body weight (BW) hFSH-beta-(90-95) in phosphate buffered saline (PBS, vehicle) administered to mice on day 28 delayed first vaginal estrus by 3 days in 50% (4/8) and 62.5% (6/8) when compared to mice given vehicle alone on day 28. Vaginal opening was also delayed in mice receiving hFSH-beta-(90-95) when compared to mice injected with vehicle alone. Serum estradiol levels of vehicle-injected control mice in first vaginal estrus were three-fold higher than in mice treated with hFSH-beta-(90-95), whose vaginal smears showed no evidence of first estrus. No significant differences in ovarian or uterine weights, or serum progesterone levels, were observed between vehicle-injected control mice achieving first vaginal estrus and mice receiving hFSH-beta-(90-95) in whom first estrus was delayed. The contrasting effects on the onset of puberty of hFSH-beta-(90-95) (delay) and hFSH-beta-(34-37) (acceleration) may reflect synthetic peptide binding to different domains of the FSH receptor, resulting in variable post-binding effects. These results are consistent with our earlier study suggesting that FSH-beta-subunit-related synthetic peptides can induce significant in vivo effects on the onset of puberty in female mice.

Cysteine residues in a synthetic peptide corresponding to human follicle-stimulating hormone beta-subunit receptor-binding domain 81-95 [hFSH-beta-(81-95)] modulate the in vivo effects of hFSH-beta-(81-95) on the mouse estrous cycle

We have previously reported that synthetic peptide amides corresponding to subdomains of the human FSH 3-subunit, hFSH-beta-(33--53) and hFSH-beta-(81--95), interact with the external domain of the FSH receptor in two in vitro model systems. Consistent with these in vitro observations, we found that intraperitoneal (i.p.) administration of each of these peptides prolonged vaginal estrus in normally cycling mice in vivo. Both hFSH-beta-(33--53) and hFSH-beta-(81--95) contain cysteine (Cys) residues with free sulfhydryl groups of potential significance in receptor interactions. To assess the possible involvement of these groups in the in vivo effects of hFSH-beta-(33--53) and hFSH-beta-(81--95), synthetic peptide analogs were prepared in which all Cys residues were replaced with serine (Ser). In the present study, we demonstrate that the in vivo effect of hFSH-beta-(33--53) on the mouse estrous cycle, extension of vaginal estrus, was not changed by substitution of Cys-51 with Ser. In contrast, mice receiving the Ser-substituted analog of hFSH-beta-(81--95) had normal estrus stages, but were arrested in diestrus. hFSH-beta-(33--53)-(81--95), a linear peptide encompassing both domains, also prolonged vaginal estrus. The Ser-substituted analog of this peptide, however, prolonged vaginal estrus in some of the mice tested and induced cycle arrest at diestrus in others. hFSH-beta-(90--95), the active subdomain at the C-terminus of hFSH-beta-(81--95), extended vaginal estrus, but diestrus stages were of normal duration. Its Ser-substituted analog, however, prolonged the estrus stage of the majority of mice treated, but induced diestrus arrest in some. The differing responses to these peptides are presumably due to interactions of the synthetic peptides with different regions of the FSH receptor. This further suggests that one consequence of ligand interaction with multiple receptor binding domains may be variable effects on ovarian function, and that Cys to Ser analogs may have value in the design of a novel class of synthetic peptides capable of fertility regulation and control.

In vivo effects of human follicle-stimulating hormone-related synthetic peptide hFSH-beta-(81-95) and its subdomain hFSH-beta-(90-95) on the mouse estrous cycle

We have previously reported that a synthetic peptide corresponding to amino acid residues 81-95 of the human (h) FSH-beta subunit inhibited binding of [125I]hFSH to bovine calf testis membranes and stimulated estradiol biosynthesis in primary cultures of rat Sertoli cells. We have now obtained several lines of evidence demonstrating in vivo effects of hFSH-beta-(81-95) on the mouse estrous cycle. 1) A single i.p. injection of 200 micrograms/g BW hFSH-beta-(81-95) significantly (p < 0.001) prolonged vaginal estrus in comparison to that in vehicle-injected control mice. 2) Vaginal smears taken at estrus from mice given hFSH-beta-(81-95) were characterized by the complete absence of epithelial casts, a hallmark of spontaneous ovulation in mice. 3) Mice receiving hFSH-beta-(81-95) had significantly (p < 0.001) lower serum estradiol at proestrus and serum progesterone at diestrus than vehicle-injected control mice. 4) The proestrous effects of estrogen on uterine ballooning and weight gain, clearly evident in vehicle-injected control mice, were not observed in mice treated with hFSH-beta-(81-95). A synthetic peptide corresponding to the carboxy-terminal region of hFSH-beta-(81-95), hFSH-beta-(90-95), inhibited binding of [125I]hFSH to bovine calf testis membranes, antagonized FSH-stimulated estradiol biosynthesis by primary cultures of rat Sertoli cells, and prolonged vaginal estrus in normally cycling mice. A synthetic peptide corresponding to the amino-terminal domain, hFSH-beta-(81-86), was inactive in vitro and had no effect on the mouse estrous cycle. The results of the present study provide additional evidence for in vivo effects of FSH-related synthetic peptides.

Cultured human granulosa cells secrete a follicle stimulating hormone receptor-binding inhibitor

We previously reported that human follicular fluid contained a protein that inhibits binding of 125I-human FSH to its membrane receptor (FSH-BI) and demonstrates FSH-like agonist activity in vitro. The cellular origin of FSH-BI was unknown, although ovarian granulosa cells seemed a likely source. To address this question, human granulosa cells were collected from patients during routine in-vitro fertilization (IVF) or gamete intra-Fallopian transfer (GIFT) procedures. Cells from 98 patients were cultured and then examined for their ability to secrete FSH receptor-binding inhibitory activity into the culture medium. The function of the cultured cells was confirmed by their ability to respond to added FSH with conversion of exogenous androstenedione to oestradiol. Radioreceptor assays were performed individually on cell culture medium obtained from granulosa cell cultures from these 98 patients. Cultured granulosa cells, under basal conditions (in the absence of FSH stimulation), secreted significant FSH-BI activity into the culture medium. In order to accumulate enough material for further study, this culture medium was pooled and lyophilized. The lyophilized medium retained FSH-BI activity, and also demonstrated FSH agonist activity by stimulating oestradiol synthesis in cultured rat Sertoli cells. A fraction showing a single component after purification by polyacrylamide gel electrophoresis had an estimated molecular weight of 25000, and inhibited 125I-human FSH binding to receptor by 50% at 2.5 microg/ml. The results indicate that human granulosa cells secrete a protein with FSH-like activity having potential significance as a local regulator of FSH action in the ovary.

A synthetic peptide corresponding to amino acid residues 34 to 37 of human follicle-stimulating hormone beta-subunit accelerates the onset of puberty in male and female mice

We have previously reported that a synthetic peptide amide corresponding to residues 34-37 (TRDL, threonine, arginine-aspartic acid-leucine) of the beta-subunit of human FSH induced prolonged vaginal estrus in normally cycling female mice (see Ref. 15). These results represented the first demonstration of an in vivo effect of a gonadotropin-related synthetic peptide on reproductive processes. We have extended these studies to examine possible effects of TRDL on the onset of puberty in female mice. In two replicated experiments, vehicle-injected control mice attained first vaginal estrus by day 39. An ip injection of 200 ug TRDL/g BW to 28-day-old prepubertal female mice, however, accelerated the onset of first vaginal estrus by 7 days in 11 of 12 (11/12) (Exp 1) and 7/9 (Exp 2) mice. Serum estradiol levels were significantly (P = 0.017) elevated in TRDL-treated mice, whereas progesterone was unchanged. Uteri of TRDL-treated mice were significantly (P = 0.003) heavier than uteri of vehicle-injected control animals of the same age and body weight. Intraluminal fluid accumulation (ballooning) at proestrus was absent in 20/21 TRDL-treated females, as were oviductal ova and ovarian corpora lutea. These phenomena are characteristic of the first estrous cycles of female mice isolated from males. To obtain further evidence for in vivo effects of TRDL, we assessed the ability of TRDL to accelerate the onset of puberty in male mice. When given as five consecutive daily ip injections of 200 ug/g BW to 28-day-old prepubertal male mice, TRDL significantly increased testis weight, when compared with vehicle-injected control mice of the same age and BW (171.3 +/- 3.8 mg vs. 151.6 +/- 4.3 mg, P = 0.001) and induced a 6.5-fold increase in serum testosterone levels. These studies confirm the previously reported in vivo activity of a synthetic peptide corresponding to human FSH-beta subunit 34-37 (TRDL) in adult female mice and extend its effects to the acceleration of the onset of puberty in immature male and female mice.

In vivo effects of follicle-stimulating hormone-related synthetic peptides on the mouse estrous cycle

We have previously shown that a synthetic peptide amide corresponding to residues 34-37 (TRDL) of the human (h) FSH beta-subunit inhibited binding of [125I]hFSH to bovine calf testis membrane receptors and antagonized FSH-stimulated estradiol biosynthesis in primary cultures of rat Sertoli cells. These in vitro effects would have additional significance if they could be confirmed in an in vivo model system. We have obtained several lines of evidence supporting in vivo effects of TRDL on the mouse estrous cycle. 1) A single i.p. injection of 200 microg/g BW TRDL induced persistent vaginal estrus, characterized by the complete absence of epithelial casts in 87% of the mice treated, as determined by vaginal cytology. 2) A synthetic peptide representing a larger receptor-binding domain of the hFSH beta-subunit, hFSHbeta-(33-53), that contains the TRDL sequence had a similar effect, but hFSHbeta-(38-53) lacking the TRDL sequence, did not. 3) A series of unrelated synthetic peptides, tested at a comparable dose (200 microg/g BW), were also without effect, as was a D-amino acid analog of TRDL, TR(D)DL. 4) Serum estradiol levels at proestrus in TRDL-treated mice were significantly lower than those in vehicle-injected control mice. 5) The effect of estrogen on uterine ballooning and weight gain, seen in all vehicle-injected control mice at proestrus, did not occur in 97% of the mice treated with TRDL. 6) The ovaries of TRDL-treated mice taken during persistent vaginal estrus contained a greater number of large hemorrhagic preovulatory follicles and fewer corpora lutea than those in ovaries taken at estrus from vehicle-injected control mice. Taken together, these results indicate disruption of the normal mouse estrous cycle by the TRDL peptide and represent the first demonstration of in vivo effects of gonadotropin-related synthetic peptides on reproductive processes.

D-amino acid substitution of residues 32 to 46 of the glycoprotein hormone common alpha-subunit: development of a synthetic glycoprotein hormone antagonist

We have used single- or double-point D-amino acid substitutions to study the structure-function relationships involving residues 32 to 46 of the glycoprotein hormone common alpha-subunit (GPHa) and the testicular follicle-stimulating hormone (FSH) and luteinizing hormone (LH/hCG) receptors. D-Amino acid substitution analogs of GPHa(32-46) were synthesized and tested in both FSH and hCG radioligand receptor assays using bovine calf testis membranes as receptor source. Correct orientation of the amino acid side chains was generally of paramount importance for peptide interaction with receptor and bioactivity. Most substitutions with corresponding D-amino acids did not enhance the potency of native GPHa(32-46). A significant increment in peptide potency, however, was observed by inversion of configuration at positions Ser-34 and Thr-39 with D-amino acid isoforms. Based on the relative potency of each peptide analog. [D-Ser-34, D-Thr-39]GPHa(32-46) was approximately twofold more potent than native peptide GPHa(32-46) in both FSH and hCG radioligand receptor assays. [D-Ser-34, D-Thr-39]GPHa(32-46) also markedly inhibited FSH-stimulated estradiol biosynthesis in cultured rat Sertoli cells. The present study is unique in that it represents the first report of utilizing D-amino acid substitution to develop more potent peptide analogs related to the glycoprotein hormone common alpha-subunit region 32-46. Our results offer hope for the development of more potent and stabile peptide antagonists of possible usefulness in fertility regulation and control.

Synthetic peptides corresponding to residues 551 to 555 and 650 to 653 of the rat testicular follicle-stimulating hormone (FSH) receptor are sufficient for post-receptor modulation of Sertoli cell responsiveness to FSH stimulation

We have recently demonstrated that synthetic peptides corresponding to the third cytoplasmic (3i) loop (residues 533 to 555) and a region in the carboxy-terminal cytoplasmic tail (residues 645 to 653) of the rat testicular follicle-stimulating hormone receptor (FSHR) affected signal transduction in rat testis membranes and cultured rat Sertoli cells. In order to define more precisely the peptide domains involved, we synthesized truncated peptide amides corresponding to FSHR residues 551-555 (KIAKR) and 650-653 (RKSH), respectively. These two regions were chosen since they contained a minimal structural motif present in G protein activator regions of several other G protein-coupled receptors (i.e., B-X-X-B-B or B-B-X-B, B representing a basic amino acid). Neither peptide inhibited binding of FSH to testis membrane receptors. Each peptide significantly reduced FSH-stimulated estradiol biosynthesis by intact cultured rat Sertoli cells. The same results were obtained with streptolysin O-permeabilized Sertoli cells. No effect was noted on forskolin-induced steroidogenesis, indicating that the peptide effects were not due to interaction with adenylyl cyclase. Each peptide amide, however, induced concentration-dependent increases in guanine nucleotide exchange in rat testis membranes. Our results indicate that interaction of FSH receptor with its associated G protein may involve relatively restricted peptide sequences, and include residues 551-555 (KIAKR) in the third cytoplasmic loop, and residues 650-653 (RKSH) in the carboxy-terminal cytoplasmic tail of the FSH receptor.

A synthetic peptide corresponding to glycoprotein hormone alpha subunit residues 32-46 inhibits gonadotropin binding to receptor

A synthetic peptide strategy was used to study structure-function relationships between residues 32 to 46 of the glycoprotein hormone alpha subunit (GPH alpha) and the testicular follicle-stimulating hormone (FSH) and luteinizing hormone (LH/hCG) receptors. A peptide amide corresponding to this region [GPH-alpha-(32-46)] inhibited both 125I-hFSH and 125I-hCG binding to their respective calf testis membrane receptors. The concentration at which GPH-alpha-(32-46) peptide amide inhibited FSH binding by 50% (IC50) was 36 microM, and for hCG it was 54 microM. GPH-alpha-(32-46) peptide amide also inhibited FSH-stimulated estradiol biosynthesis in cultured rat Sertoli cells. In order to determine the involvement of individual residues within this region of the glycoprotein hormone alpha subunit in receptor binding inhibitory activity, truncated and alanine-substituted peptide analogs were synthesized and tested in both FSH and hCG radioligand receptor competition assays. Based on the relative potency of each peptide, we conclude that Phe-33, Arg-35, Arg-42, Ser-43 and Lys-44 may be important, and Cys-32 is required, for inhibition of FSH and hCG binding to their respective receptor. Our results demonstrate involvement of the glycoprotein hormone alpha-subunit in receptor binding, identify residues 32 to 46 as a receptor binding domain, and define the relative importance of specific residues within this region of the alpha subunit for hormone-receptor interaction.

Selective effects of charge on G protein activation by FSH-receptor residues 551-555 and 650-653

Two cytosolic regions of the rat testicular FSH receptor (FSHR), residues 533-555 and 645-653, have been identified as G protein-coupling domains. We localized the activity in these domains to their C-terminal sequences, residues 551-555 (KIAKR, net charge +3) and 650-653 (RKSH, net charge +3), and examined the effects of charge on G protein activation by the C-terminal peptides, using synthetic analogs containing additions, through alanine (A) linkages, of arginine (R, +), histidine (H, +) or both. RA-KIAKR (net charge +4) mimicked the effect of FSHR-(551-555) on guanine nucleotide exchange in rat testis membranes, but reduced its ability to inhibit FSH-stimulated estradiol biosynthesis in cultured rat Sertoli cells. Further increasing net charge by the addition of H (HARA-KIAKR, net charge +5) increased guanosine 5'-triphosphate (GTP) binding, but eliminated FSHR-(551-555) effects on FSH-stimulated steroidogenesis. HA-RKSH (net charge +4) significantly inhibited guanine nucleotide exchange in rat testis membranes, but stimulated basal and potentiated FSH-induced estradiol biosynthesis in cultured rat Sertoli cells. Addition of two H residues (HAHA-RKSH, net charge +5) restored GTP binding and further potentiated basal and FSH-stimulated steroidogenesis. These results suggest that positive charges in G protein-coupling domains of the FSHR play a role in modulating G protein activation and postbinding effects of FSH, such as steroidogenesis.

Identification of amino acid residues 300-315 of the rat FSH receptor as a hormone binding domain: evidence for its interaction with specific regions of FSH beta-subunit

We previously reported that residues 9-30 of the extracellular N-terminus domain of the rat FSH receptor, which has no homologous sequence in receptors for related pituitary glycoprotein hormones, represented a specific FSH binding domain. Further examination of its deduced primary structure identified another region, residues 300-315, which was also unique to the FSH receptor. To determine whether this region of the FSH receptor was involved in hormone binding, a synthetic peptide corresponding to residues 300-315 was studied with respect to its ability to bind FSH, as well as a series of nine overlapping synthetic peptides corresponding to the entire primary structure of the hormone specific FSH beta-subunit. 125I-FSH rec-(300-315) peptide bound to immobilized human, ovine and bovine FSH, but not to prolactin or ovalbumin. Of the nine synthetic peptides studied, binding was restricted to FSH beta residues 21-35, and to a much lesser extent (20%) to residues 11-25. All binding was abolished in the presence of excess solubilized FSH receptor. Earlier studies indicated that although FSH binds to FSH rec(9-30) peptide, residues 11-25 or 21-35 of the FSH beta-subunit were not involved. Our results suggest the FSH receptor N-terminus, extracellular residues 300-315, may define a FSH binding site, and that binding of FSH beta-subunit may occur via interactions with FSH beta 21-35 and 11-25.

A decapeptide corresponding to the partial amino acid sequence of a high molecular weight human FSH receptor-binding inhibitor is a specific inhibitor of FSH binding

We previously reported purification of a protein (approximately equal to 57 kDa) from human follicular fluid having FSH binding inhibitory (FSH-BI) activity. Purified hFSH-BI was cleaved with cyanogen bromide and trypsin. The resulting peptide fragments were separated by HPLC and sequence information for individual fragments was obtained. A ten amino acid sequence of hFSH-BI derived from this procedure was identified, and a corresponding peptide amide (BI-10) was synthesized and utilized for further study. A protein database search revealed no significant identity between this decapeptide and other known proteins. We examined the ability of BI-10 to inhibit binding of 125I-hFSH to FSH-receptor enriched bovine testes membranes utilizing a radioligand receptor assay (RRA). BI-10 inhibited binding of 125I-hFSH to its receptor in a concentration-related manner, with an ED50 of 300 microM. BI-10 had no effect on 125I-hCG binding to receptor even at concentrations up to 1000 microM, suggesting that the effect of BI-10 was specific for the interaction between FSH and its receptor. To assess bioactivity of BI-10, we investigated its effect on FSH-stimulated conversion of androstenedione to estradiol by rat Sertoli cells in primary culture in vitro. Inhibition of FSH-stimulated estradiol synthesis (FSH antagonist activity) was significant at a BI-10 concentration of 1000 microM. BI-10 also significantly inhibited FSH-stimulated cAMP accumulation in primary cultures of Sertoli cells when examined at the same concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

A synthetic peptide corresponding to the third cytoplasmic loop (residues 533 to 555) of the testicular follicle-stimulating hormone receptor affects signal transduction in rat testis membranes and in intact cultured rat Sertoli cells

Involvement of the third cytoplasmic (3i) loop (residues 533 to 555) of the rat testicular FSH receptor in the mechanism of FSH signal transduction was examined using light membranes prepared from immature rat testes, monolayer cultures of rat Sertoli cells, and a synthetic peptide strategy. This region of the FSH receptor is structurally related to G protein-activator regions identified in other G protein-coupled receptors. FSHR-(533-555) peptide amide stimulated guanine nucleotide exchange in rat testis light membranes, presumably via its interaction with membrane-associated G protein. The peptide failed to inhibit FSH binding to testis membrane receptors, indicating that the nucleotide exchange effect was not a result of peptide interaction with receptor. When incubated with cultured Sertoli cells from immature rat testes, FSHR-(533-555) peptide amide consistently and significantly inhibited FSH stimulation of cAMP and estradiol biosynthesis, but failed to inhibit forskolin stimulation of each. The peptide effect, therefore, was not due to a direct interaction with adenylyl cyclase. Since FSHR-(533-555) peptide amide did not inhibit FSH binding to membrane receptor, these results imply entry of the peptide into the Sertoli cell, possibly by vesicular internalization or diffusion. Indeed, the inhibitory effects of FSHR-(533-555) peptide amide on FSH-stimulated estradiol biosynthesis were prevented by pretreating Sertoli cells with phenylarsine oxide, an inhibitor of FSH receptor internalization. FSHR-(533-555) was without effect on basal levels of cAMP and estradiol biosynthesis, indicating absence of toxicity at the concentrations tested.(ABSTRACT TRUNCATED AT 250 WORDS)

A synthetic peptide corresponding to residues 645-653 in the carboxyl terminal cytoplasmic domain of the rat testicular follicle stimulating hormone receptor modulates G protein coupled-receptor signaling in rat testis membranes and in intact cultured rat Sertoli cells

We examined the involvement of the carboxyl terminal cytoplasmic domain (residues 645-653) of the rat testicular FSH receptor in FSH signal transduction utilizing light membranes prepared from immature rat testes, intact cultured rat Sertoli cells, and a synthetic peptide approach. This region of the FSH receptor was selected because of its structural similarity to receptor-G protein contact sites identified in other G protein-coupled receptors. FSHR- (645-653) peptide amide promoted guanine nucleotide exchange in rat testis membranes, presumably via its interaction with membrane-associated G protein, but did not inhibit FSH binding to testis membrane receptors. When incubated with intact cultured Sertoli cells from immature rat testes, FSHR- (645-653) peptide amide consistently and significantly stimulated basal cAMP and estradiol biosynthesis. The peptide had no effect on forskolin stimulation of cAMP and estradiol, but inhibited FSH stimulation of each. FSH binding to receptor was unaffected by the peptide, these results suggest peptide interaction with receptor-associated G protein. The effects of FSHR- (645-653) peptide amide on FSH-stimulated estradiol biosynthesis were prevented by pretreating Sertoli cells with phenylarsine oxide, an inhibitor of FSH receptor internalization. These results suggest that peptide effects in intact Sertoli cells were related to peptide entry into the cell, presumably during receptor-mediated endocytosis of FSH, or by diffusion. Synthetic peptide amides not satisfying structural criteria for G protein coupling had no effect on either guanine nucleotide exchange or estradiol biosynthesis, even at concentrations significantly higher than used for FSHR- (645-653) peptide amide.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification in human ovarian follicular fluid of proteins that share an epitope region unique to the extracellular domain of the follicle-stimulating hormone receptor

Recently we identified a unique region, residues 9-30 in the extracellular domain of the FSH receptor, capable of binding FSH but not LH or TSH. We have shown that polyclonal antibodies raised against this region specifically recognized intact FSH receptors present on plasma membranes of cultured rat Sertoli cells. In the present study, plasma membranes from human granulosa-lutein cells were solubilized and subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis, followed by Western blot analysis. Antireceptor peptide antibody, but not preimmune serum control, recognized intact human FSH receptors, suggesting that human and rat FSH receptors share this unique N-terminus epitope region. Recent cloning studies have identified, in addition to full length receptor, the presence of FSH receptor-spliced messenger RNA variants, which encode receptor proteins with variable lengths of hydrophilic extracellular domains, but lacking transmembrane domains. Such proteins could theoretically represent secreted forms of the receptor. In this study, we used a polyclonal anti-FSH receptor (residues 9-30) peptide antibody to investigate whether FSH receptor-related soluble proteins might also be present in human ovarian follicular fluid (FF). In an enzyme-linked immunosorbent assay specific for the FSH receptor, antireceptor peptide antibody, but not preimmune serum serving as control, identified significant immunoreactivity in several human ovarian FF samples, suggesting that protein(s) present in FF share a common epitope with the extracellular domain of the FSH receptor. The apparent levels of FSH receptor-related activity in FF samples, expressed relative to the FSH receptor (residues 9-30) peptide, ranged from 31-55 ng/mL. Passing of the samples through 0.22-micron filters or subjecting the samples to high speed centrifugation did not alter the activity profiles of the samples ruling out effects due to contamination with plasma membranes from granulosa cells. When human FF samples were subjected to gel permeation chromatography, at least four distinct protein peaks were resolved, in the molecular mass range between 70-460 kilodaltons, each of which was recognized by the FSH receptor 9-30 peptide antibodies. Our results provide initial evidence for the presence in human ovarian FF of proteins sharing epitope with the extracellular domain of the FSH receptor and presumably derived from the granulosa cell. Since we have previously shown that the epitope region, represented by residues 9-30 in the extracellular domain of the FSH receptor specifically binds FSH, the proteins in human FF sharing this epitope may have functional significance.

Evidence that a calmodulin-like calcium-binding domain of the FSH beta-subunit is involved in FSH-induced calcium uptake by Sertoli cells

We have previously shown that a synthetic peptide amide corresponding to residues 1-15 of the human FSH beta-subunit (hFSH-beta-(1-15)) possesses structural characteristics and calcium-binding properties similar to the calcium-binding loops of calmodulin (CaM). The calcium-binding property of hFSH-beta-(1-15) correlated well with its ability to stimulate uptake of calcium (as 45Ca2+) by cultured rat Sertoli cells and proteoliposomes enriched with bovine calf testis FSH receptors. A sequence found in the calcium-binding loops of CaM and a number of other calcium-binding proteins can be represented by the motif +-+-+-+-+--+, where + represents a calcium-binding residue and - represents a non-binding residue. A sequence containing a similar motif appears in hFSH-beta-(1-15) between residues 4 and 15: +-++-+---+-+. Using a synthetic peptide strategy, we undertook to determine whether the first three residues of hFSH-beta-(1-15) were required to induce uptake of calcium by cultured rat Sertoli cells and FSH receptor-enriched proteoliposomes, and to assess whether rearrangement of the putative calcium-binding ligands (+) of hFSH-beta-(1-15) to correspond to their linear sequence in CaM would enhance the ability of hFSH-beta-(1-15) to induce calcium uptake in these two model systems.(ABSTRACT TRUNCATED AT 250 WORDS)

A tetrapeptide within a receptor-binding region of human follicle-stimulating hormone beta-subunit, hFSH-beta-(34-37), regulates sodium-calcium exchange in Sertoli cells

In a previous study, we showed that binding of FSH by cultured rat Sertoli cells significantly inhibited basal levels of Na+/Ca2+ exchange. Similar inhibition was observed when proteoliposomes enriched with bovine calf testis follicle-stimulating hormone (FSH) receptors were stimulated with FSH. In the present study, we screened a series of overlapping synthetic peptide amides, representing the entire primary structure of the beta-subunit of hFSH, for their effects on sodium-dependent calcium uptake (as 45Ca2+) by monolayer cultures of Sertoli cells from immature rats. hFSH-beta-(33-53), previously identified as a receptor binding region of hFSH-beta-subunit, significantly (P < 0.05) inhibited Na+/Ca2+ exchange. A tetrapeptide [TRDL, hFSH-beta-(34-37)] contained within this sequence, was observed to be equally as active as hFSH-beta-(33-53) at 200 microM, suggesting that the regulatory effect of hFSH-beta-(33-53) on sodium-dependent 45Ca2+ influx was due to residues 34-37. hFSH-beta-(81-95) also inhibited Na(+)-dependent calcium influx, although to a lesser extent than hFSH-beta-(33-53) or hFSH-beta-(34-37). Sodium-dependent 45Ca2+ entry into Sertoli cells was enhanced in a concentration-related manner when extracellular sodium was replaced by equimolar concentrations (up to 135 mM) of choline chloride. hFSH-beta-(34-37) significantly reduced basal uptake of 45Ca2+ in choline-containing buffer, but was without effect in buffer containing 135 mM NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional properties of polyclonal antibodies raised against the N-terminus region (residues 9-30) of the follicle-stimulating hormone (FSH) receptor: significance of this receptor region in FSH recognition and signal transduction

In this study, we raised polyclonal antibodies in rabbits against a synthetic peptide corresponding to a unique region of the FSH receptor, residues 9-30, with no sequence homology to receptors for LH and TSH, and examined their characteristics relevant to receptor function. Binding of [125I]human (h) FSH to membrane-bound receptors was inhibited in a concentration-dependent manner by the anti-FSH receptor-(9-30) peptide antibody. Preimmune serum had no effect. Lineweaver-Burke plot analysis of [125I]hFSH binding to membrane receptors in the presence or absence of antireceptor peptide antibody indicated that the antibody effectively competed with FSH at a hormone-binding site on the receptor. Also, antireceptor peptide antibody, but not preimmune serum, inhibited the ability of FSH to stimulate the conversion of androstenedione to estradiol in cultured immature rat Sertoli cells. Stimulation of estradiol synthesis by Sertoli cells caused by cholera toxin or forskolin (which are known to act through the Gs-protein and catalytic unit of adenylate cyclase, respectively) was not inhibited by antireceptor peptide antibody. Indirect immunofluorescence staining of cultured rat Sertoli cells showed binding of antibody to plasma membrane receptor. No fluorescent staining of receptor was observed when cells were incubated with preimmune serum or antireceptor peptide antibody in the presence of excess receptor-(9-30) peptide or hFSH. These results were consistent with specific labeling of membrane-bound FSH receptors by anti-receptor-(9-30) peptide antibody. When detergent-solubilized membrane preparations from rat Sertoli cells were fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions and then subjected to Western blot analysis, antireceptor peptide antibody, but not preimmune rabbit serum, specifically recognized intact FSH receptor. Although the antireceptor peptide antibody occupied the N-terminus 9-30 epitope region in the FSH receptor, it did not induce postbinding events, such as receptor patching (aggregation), as shown by indirect immunofluorescence staining of rat Sertoli cells and the estradiol response. In contrast, a polyclonal antibody against the FSH holoreceptor capable of interacting with multiple epitopes on the receptor could induce FSH-like effects, such as receptor patching and estradiol response in Sertoli cells. In conclusion, antibody raised against the N-terminus region (9-30) of the FSH receptor recognized intact FSH receptor, inhibited FSH binding, and behaved as an antagonist, suggesting that this N-terminus epitope region of the receptor is involved in hormone binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification of a high molecular weight follicle-stimulating hormone receptor-binding inhibitor from human follicular fluid

In a previous study we reported the presence in human follicular fluid (hFF) of a FSH receptor-binding inhibitor (hFSH-BI) with FSH agonist activity, which was immunologically similar to FSH but could be distinguished from FSH on the basis of its greater stability in acid. We have now purified hFF-derived hFSH-BI after molecular sieving on Sephracyl S-100 ion exchange chromatography using Diethyl-aminoethyl-cellulose followed by polyacrylamide gel electrophoresis (PAGE). The purified hFSH-BI had a potency approximately 12,000-fold greater than that of dialyzed hFF, based on its ability to inhibit the binding of [125I]hFSH to its membrane receptor. The purified hFSH-BI also had FSH agonist activity, stimulating estradiol synthesis in cultured rat Sertoli cells. Upon sodium dodecyl sulfate (SDS)-PAGE, hFSH-BI migrated as two bands of almost identical mobility, with an estimated mol wt of 57,000, compared with 30,000 for pituitary FSH run simultaneously. A monoclonal antibody to hFSH that also recognizes hFSH-BI was used for Western blot analysis of the SDS-PAGE fraction. The Western blot confirmed the detection of two bands with very similar mobilities and estimated mol wt of 57,000, which were clearly distinguishable from that of immunologically reactive hFSH run in parallel. The hFSH-BI bands showed similar profiles upon cyanogen bromide cleavage and had indistinguishable amino acid compositions. The amino acid composition of hFSH-BI was clearly distinct from those of hFSH, hLH, hCG, and the alpha-subunit of human inhibin. Our studies confirm the presence in hFF of a unique agonist protein which is of potential importance in the regulation of gonadal function.

Identification of regions of the follitropin (FSH) beta-subunit that interact with the N-terminus region (residues 9-30) of the FSH receptor

We have recently identified a region, N-terminus residues 9-30, in the extracellular domain of the follicle-stimulating hormone (FSH) receptor capable of binding FSH, but not luteinizing hormone (LH) or thyroid-stimulating hormone (FSH) (Dattatreyamurty and Reichert (1992) Mol. Cell. Endocrinol. 87, 9-17). The objectives of the present study were to examine the interaction between a synthetic peptide corresponding to this receptor sequence and the beta-subunit of FSH, and to identify which FSH-beta regions are involved in the interaction. FSH-beta subunit and synthetic FSH-beta peptides 1-15, 71-85 and 101-111 effectively bound 125I-labeled FSH rec-(9-30) peptide, and binding was inhibited by excess unlabeled FSH receptors. Scatchard analysis indicated that the synthetic FSH-beta peptides had affinities for FSH rec-(9-30) peptide in the order of 10(6) M-1 (Ka), with the sum of individual peptide affinities (Ka = 1.21 x 10(7) M-1) closely approximating that of the intact beta-subunit (1.02 x 10(7) M-1). Polyclonal antibodies raised against FSH rec-(9-30) peptide completely inhibited the binding of 125I-labeled receptor peptide to hFSH, hFSH-beta, and hFSH-beta peptides 1-15, 71-85 and 101-111. Our results indicate that recognition of FSH-beta by N-terminus region (9-30) of the FSH receptor involves contact with residues in three discontinuous binding regions on FSH-beta.(ABSTRACT TRUNCATED AT 250 WORDS)

Transforming growth factor-beta 1 modulates calcium metabolism in Sertoli cells

The mechanism of signal transduction by transforming growth factor-beta 1 (TGF-beta 1) is largely unknown. In this study, TGF-beta 1 was able to stimulate 45Ca2+ uptake in cultured rat Sertoli cells. In contrast to the rapid stimulation of 45Ca2+ uptake previously observed in response to FSH (< 1 min), the response to TGF-beta 1 required several hours and was abolished by actinomycin-D. Although TGF-beta 1 had no effect on basal calcium uptake before 1 h, it completely inhibited the FSH-stimulated calcium uptake in short term incubations (2 and 20 min). These effects of TGF-beta 1 on calcium metabolism may be directly related to the ability of this cytokine to regulate cell growth and differentiation in many cell systems.

Effects of mono(2-ethylhexyl) phthalate, a testicular toxicant, on follicle-stimulating hormone binding to membranes from cultured rat Sertoli cells

The widely used plasticizer di(2-ethylhexyl) phthalate (DEHP) is a male reproductive toxicant. Its toxicity has been shown to be due primarily to the action of its metabolite mono(2-ethylhexyl) phthalate (MEHP) on Sertoli cells. We have previously shown that at least one of the sites of action of MEHP on the Sertoli cell is the cAMP second messenger system. MEHP inhibits the ability of FSH but not isoproterenol, forskolin, or cholera toxin to stimulate cAMP accumulation in cultured Sertoli cells in a dose- and time-dependent manner. To further characterize this effect of MHP, we prepared a light membrane fraction from control and MEHP-treated Sertoli cells cultured from 18-day-old Fischer 344 rats and measured FSH binding in a radioligand receptor assay using 125I-labeled human FSH (125I-hFSH). MEHP inhibited FSH binding when preincubated with Sertoli cells in culture but not when added simultaneously with 125I-hFSH to the purified membrane preparation. Attenuation of FSH binding was evident after a 3-h preincubation with 100 microM MEHP (18%) and was maximal after 15-24 h of preincubation (70-90%). Preincubation of Sertoli cells for 24 h with 100 microM DEHP had no effect on FSH binding. Half-maximal inhibition occurred at approximately 0.1 microM MEHP. Scatchard analysis indicated a four-fold decrease in FSH affinity with no change in receptor concentration. Exposure of Sertoli cells to MEHP amplified the attenuating effect of guanosine triphosphate (GTP) on FSH binding, suggesting that the action of MEHP may be at the level of the GTP-binding protein that couples the FSH receptor to the adenylate cyclase catalytic subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

An explanation for the disparate effects of synthetic peptides corresponding to human follicle-stimulating hormone beta-subunit receptor binding regions (33-53) and (81-95) and their serine analogs on steroidogenesis in cultured rat Sertoli cells

We have recently reported that synthetic peptide amides corresponding to regions of human FSH beta-subunit, hFSH-beta-(33-53) and hFSH-beta-(81-95), bind to receptor and stimulate estradiol biosynthesis by cultured rat Sertoli cells. Because of experimental difficulties caused by the presence of free sulfhydryl groups in these peptides, synthetic analogs were prepared in which all Cys residues were replaced with Ser. These analogs, [Ser-51]-hFSH-beta-(33-53) and [Ser-82, 84, 87, 94]-hFSH-beta- (81-95), also bound to receptor but did not stimulate estradiol biosynthesis by cultured rat Sertoli cells. In order to explain this observation, we compared the effects of hFSH-beta-(33-53) and hFSH-beta-(81-95) and their Ser analogs on another recently recognized effect of FSH in Sertoli cells, namely its ability to promote influx of extracellular calcium. We and others have shown that estradiol biosynthesis by these cells is markedly decreased in the presence of high intracellular calcium. Cys-containing hFSH-beta-(33-53) and hFSH-beta-(81-95) did not increase influx of extracellular calcium over basal levels, whereas [Ser-51]-hFSH-beta-(33-53) and [Ser-82, 84, 87, 94]-hFSH-beta-(81-95) induced 2.8- and 1.8-fold increases, respectively. Cellular cAMP and estradiol biosynthesis in response to each Ser-substituted peptide were not significantly different from basal levels. Thus, the explanation for the observed disparate effects of Cys and Ser analog peptides on estradiol biosynthesis may be related to the ability of the Ser peptides to stimulate calcium entry but not cAMP accumulation in cultured rat Sertoli cells.

Correlation of follicle-stimulating hormone (FSH)-receptor complex internalization with the sustained phase of FSH-induced calcium uptake by cultured rat Sertoli cells

We have previously reported that synthetic peptide amides corresponding to regions of the beta-subunit of human FSH [hFSH-beta-(1-15) and hFSH-beta-(51-65)] have the ability to bind calcium and to facilitate its entry into liposomes. In the present study, we have examined the ability of synthetic peptides corresponding to the entire primary structure of hFSH-beta-subunit, to induce calcium influx in cultured rat Sertoli cells. Calcium (as 45Ca2+) uptake in response to 50 microM hFSH-beta-(1-15), hFSH-beta-(21-35), or hFSH-beta-(51-65) peptide amides was 2.5-, 2.4-, and 2.0-fold higher, respectively, than basal uptake. Pretreatment of Sertoli cells for 5 min with phenylarsine oxide (PAO, 80 microM), an inhibitor of receptor-mediated endocytosis, significantly (P < 0.05) reduced 45Ca2+ influx in response to hFSH-beta-(1-15), hFSH-beta-(21-35), and hFSH-beta-(51-65). A delay of 20 min was required, however, before the inhibitory effect of PAO on 45Ca2+ uptake was observed. Specific binding of [125I] hFSH to receptor at 4 C was unaffected by PAO. After 2 h at 37 C, however, approximately 1.6-fold more [125I]hFSH specifically bound at 4 C could be dissociated from the cell surfaces of PAO-pretreated Sertoli cell monolayers, compared to untreated monolayers. This result is consistent with an inhibitory effect of PAO on FSH receptor internalization. Chloroquine (at 100 microM), a lysosomotropic agent known to block FSH degradation, also significantly (P < 0.05) inhibited FSH-induced 45Ca2+ uptake. Extending our earlier studies, these results suggest that the sustained (> 20 min) phase of FSH-induced calcium uptake, also seen in response to synthetic hFSH-beta-(1-15), hFSH-beta-(21-35), and hFSH-beta-(51-65) peptide amides, may occur as a consequence of FSH-receptor complex internalization and FSH degradation. Vesicular uptake of extracellular calcium, which accompanies internalization of FSH-receptor complexes, and release of channel-forming peptides by lysosomal hydrolysis of FSH suggests a novel mechanism whereby FSH increases intracellular calcium levels in Sertoli cells.

Structural response of the hamster Sertoli cell to hypophysectomy: a correlative morphometric and endocrine study

Reproductively active hamsters were hypophysectomized and examined 6 or 20 days later in a combined morphometric and endocrine study of the Sertoli cell to determine 1) the morphological and endocrine effects of hypophysectomy of both short- and long-term duration, 2) if regression of Sertoli cells after hypophysectomy in a seasonal breeder resembles regression due to seasonal changes, and 3) if effects of hypophysectomy in a seasonal breeder are equivalent to the effects of hypophysectomy in a nonseasonal breeder. Six days after hypophysectomy, at a period when germ cell degeneration is first noted, there was a significant decrease in testis weight, interstitial space, tubule diameter and length, volume of seminiferous tubule, and tubular lumen. There were no significant changes in Sertoli cell nuclear and cytoplasmic volume although cell surface area was decreased significantly. Most organelles exhibited no significant change in volume or surface area except for secondary lysosomes which expectedly increased in volume as the result of phagocytosis of germinal cells. Thus at an early time period when functional changes in germ cells and Leydig cells are clearly evident (Russell et al. [1992] Endocrinology), the Sertoli cell shows minimal changes. Twenty days after hypophysectomy, the cell, nuclear and cytoplasmic volumes and surface area of the Sertoli cells, and volumes and surface areas of nearly all organelles were significantly decreased from values measured in normal and in short-term hypophysectomized hamsters. The exceptions were the total volumes of lipid which increased significantly and lysosomes which were similar to normal but significantly lower than short-term hypophysectomized animals. The long-term hypophysectomized hamster Sertoli cell, like that of the short-day hamster (Sinha Hikim et al. [1989b] Endocrinology, 125:1829-1843) is structurally regressed as a whole rather than exhibiting selective decreases in cellular and subcellular components. The size of the Sertoli cell in pituitary-intact, long- and short-term hypophysectomized animals showed positive and significant correlations with the volumes and surface areas of all its cytoplasmic organelles except the volume of lipid which showed a negative, significant correlation. Comparisons of long-term hypophysectomized hamsters (in long-day light exposure) and short-day exposed animals (Sinha Hikim et al. [1989b] (Endocrinology, 125:1829-1843) suggested that hypophysectomy, in general, resulted in similar, but slightly more severe regressive changes in the testis and germ cell population than those seen during seasonal regression.(ABSTRACT TRUNCATED AT 400 WORDS)

Carbohydrate moiety of follitropin receptor is not required for high affinity hormone-binding or for functional coupling between receptor and guanine nucleotide-binding protein in bovine calf testis membranes

Previously, we have extensively studied FSH-receptor interactions using bovine calf testis membranes, and demonstrated that the high-affinity FSH binding to receptors and coupling of FSH receptors with guanine nucleotide-binding protein (Gs protein) in a GTP-sensitive state are important initial events in FSH action. In this study, using the same plasma membrane system, we examined the glycoprotein nature of the FSH receptor and determined the contribution of carbohydrate moieties to these functions of the FSH receptor. Our approach involved enzymic deglycosylation of FSH receptors present in calf testis plasma membranes and then removal of incompletely deglycosylated FSH receptors by lectin affinity chromatography. Following treatment of testis membranes with peptide N-glycosidase, the receptor, as identified by ligand-blot analysis, had a higher electrophoretic mobility indicating a decrease in M(r) from 240-200K. Treatment of testis membranes with neuraminidase caused a reduction (to approximately 225K) in the size of the receptor consistent with desialylation. However, digestion with O-glycosidase (endo-alpha-N-acetylgalactosaminidase) did not affect the mobility of the FSH receptor. These results suggest that bovine testis FSH receptor contains predominantly N-linked oligosaccharide chains, a finding which is consistent with recent predictions that N-linked glycosylation, but not O-linked glycosylation sites are present in cloned FSH receptor from rat testis. Moreover, calf testis membranes after treatment with peptide N-glycosidase F, were solubilized with Triton X-100 under optimum conditions that preserve the physical and functional coupling of FSH receptors with guanine nucleotide-binding protein, and then subjected to lectin affinity chromatography. Scatchard analysis indicated that intact and deglycosylated FSH receptors bound 125I-human FSH with similar affinities. In the presence of GTP, the binding of 125I-human FSH to intact and deglycosylated receptors decreased similarly and in a noncompetitive manner. Treatment of testis membranes with NAD plus cholera toxin, but not NAD plus pertussis toxin, eliminated the GTP effect on FSH binding to enzymic deglycosylated as well as intact receptors, suggesting that the guanine nucleotide binding protein mediating GTP regulation of FSH binding in these membranes is probably Gs protein. Our results suggest that the bovine testis FSH receptor contains predominantly N-linked oligosaccharide chains consistent with recently predicted N-linked glycosylation sites of cloned FSH receptor of rat testis. The bovine testis FSH receptor does not require N-linked carbohydrate for high-affinity hormone binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Solution structure of a synthetic peptide corresponding to a receptor binding region of FSH (hFSH-beta 33-53)

The receptor binding surface of human follicle-stimulating hormone (hFSH) is mimicked by synthetic peptides corresponding to the hFSH-beta chain amino acid sequences 33-53 [Santa-Coloma, T. A., Dattatreyamurty, D., and Reichert, L. E., Jr. (1990), Biochemistry 29, 1194-1200], 81-95 [Santa-Coloma, T. A., Reichert, L. E., Jr. (1990), J. Biol. Chem. 265, 5037-5042], and the combined sequence (33-53)-(81-95) [Santa-Coloma, T. A., Crabb, J. W., and Reichert, L. E., Jr. (1991), Mol. Cell. Endocrinol. 78, 197-204]. These peptides have been shown to inhibit binding of hFSH to its receptor. Circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy were used to determine the structure of the first peptide in this series, the 21 amino acid peptide hFSH-beta-(33-53), H2N-YTRDLVYKDPARPKIQKTCTF-COOH. Analysis of CD data indicated the presence of approximately equal amounts of antiparallel beta-pleated sheet, turns including a beta-turn, "other" structures, and a small amount of alpha-helix. The major characteristics of the structure were found to be relatively stable at acidic pH and the predominant effect of increased solvent polarity was a small increase in alpha-helical content. One- and two-dimensional NMR techniques were used to obtain full proton and carbon signal assignments in aqueous solution at pH 3.1. Analysis of NMR results confirmed the presence of the structural features revealed by CD analysis and provided a detailed picture of the secondary structural elements and global folding pattern in hFSH-beta-(33-53). These features included an antiparallel beta-sheet (residues 38-51 and 46-48), turns within residues 41-46, and 50-52 (a beta-turn) and a small N-terminal helical region comprised of amino acids 34-36. One of the turns is facilitated by prolines 42 and 45. Proline-45 was constrained to the trans conformation, whereas proline-42 favored the trans conformer (approximately 70%) over the cis (approximately 30%). Two resonances were observed for the single alanine residue (A-43) sequentially proximal to P-42, but the rest of the structure was minimally affected by the isomerization at proline-42. The major population of molecules, containing trans-42 and trans-45 prolines, presented 120 NOEs. Distance geometry calculations with 140 distance constraints and energy minimization refinements were used to derive a moderately well-defined model of the peptide's structure.(ABSTRACT TRUNCATED AT 400 WORDS)

The size of the mature membrane receptor for follicle-stimulating hormone is larger than that predicted from its cDNA

A 240 kDa protein isolated from bovine calf testis has been shown to have properties characteristic of an FSH receptor. However, rat testis FSH receptor has, on the basis of cloning experiments, been found to have a much lower molecular mass of 75 kDa (peptide only). To examine this point, the size of the FSH receptor in membranes obtained from cultured Sertoli cells of immature rats was determined after polyacrylamide gel electrophoresis under non-reducing conditions, followed by transfer to polyvinylidene difluoride membranes and direct identification of the FSH receptor by ligand blot analysis utilizing radioiodinated human FSH. In this system, the rat Sertoli cell membrane FSH receptor also showed a molecular mass of 240 kDa. Bovine testis contains LH and FSH receptors. We compared the sizes of FSH and LH receptors present in the same bovine testis membrane preparation by ligand blot analysis. The FSH receptor again showed a molecular mass of 240 kDa, whereas the LH receptor showed a molecular mass of 90 kDa. The latter value is similar to that deduced by cloning techniques (75 kDa, peptide only). The evidence seems to suggest that, whereas the molecular mass deduced for the LH receptor on the basis of its cDNA is similar to that of the mature membrane receptor, the size of the FSH membrane receptor is considerably different from that deduced on the basis of its cDNA, presumably as a result of post-translational processing. The marked difference in size between mature FSH (240 kDa) and LH (90 kDa) receptors may reflect significant structural differences of importance with regard to mechanisms of signal transduction.

Stabilization of follicle-stimulating hormone-receptor complexes may involve calcium-dependent transglutaminase activation

Calcium-dependent transglutaminase (TGase) activity, determined by incorporation of [1,4-14C]diaminobutane dihydrochloride (putrescine) into casein, was demonstrated in a light membrane fraction prepared from bovine calf testicular homogenates. Purification of these membranes by sucrose density gradient centrifugation produced a follicle-stimulating hormone (FSH) receptor-enriched fraction containing TGase activity which cosolubilized with the FSH receptor and could be incorporated with detergent-solubilized receptor into liposomes. In the present study, we show that calcium increases specific binding of FSH to receptor in a concentration-related manner, and is associated with an increase (13.2-fold at 20 mM) in the affinity (Ka) of the receptor with no significant (P greater than 0.05) change in receptor concentration. Treatment of the light membrane fraction with monodansylcadaverine (MDC, 1 mM), a specific inhibitor of TGase, did not affect specific binding of FSH, but resulted in only a 3.9-fold increase in Ka at 20 mM calcium with no change in receptor concentration. Specific binding of FSH to receptor at 4 degrees C was also enhanced by calcium. Scatchard analysis of competitive binding inhibition data showed a Ka at 20 mM calcium similar to that observed with MDC. Dissociation of [125I]hFSH-receptor complexes formed at 30 degrees C in the presence of calcium was significantly less than dissociation of complexes formed at 30 degrees C in the absence of calcium. When [125I]hFSH-receptor complexes were formed at 30 degrees C in the presence of calcium and dissociated in calcium-deficient buffer, dissociation increased 3-fold. Similar results were obtained in the presence of MDC.(ABSTRACT TRUNCATED AT 250 WORDS)

A synthetic peptide corresponding to amino acids 9-30 of the extracellular domain of the follitropin (FSH) receptor specifically binds FSH

As deduced on the basis of cloning experiments, the putative extracellular domain of pituitary glycoprotein hormone (lutropin (LH), thyrotropin (TSH), and FSH) receptors (rec) is sufficiently large to suggest involvement in hormone binding. Comparison of the amino acid sequences of the extracellular domains of the glycoprotein hormone receptors indicates that the FSH receptor has a peptide sequence in the external domain close to the amino terminus (residues 9-30) which has no sequence homology to receptors for LH or TSH. To examine whether this region is involved in FSH-receptor interaction, we studied the hormone-binding properties of a corresponding synthetic peptide in several systems. (1) Binding of 125I-hFSH to receptor-containing bovine testis membranes was inhibited by preincubation with FSH rec-(9-30) peptide amide in a concentration-dependent manner. (2) 125I-labeled rec-(9-30) peptide amide bound to ovine, bovine, or human FSH preparations, and the binding was inhibited by solubilized bovine FSH receptor. 125I-labeled rec-(9-30) peptide amide, however, did not bind to LH or TSH. (3) 125I-hFSH bound to unlabeled rec-(9-30) peptide amide, and the binding was inhibited by excess unlabeled FSH, but not by LH or TSH. (4) Scatchard analysis indicated that the FSH rec-(9-30) peptide amide contained a single class of FSH binding sites with a Ka = 1.1 x 10(6) M-1. (5) The binding of 125I-labeled rec-(9-30) peptide amide to hFSH, bFSH or oFSH was effectively inhibited by rabbit polyclonal antibodies raised against rec-(9-30) peptide amide but not by preimmune rabbit serum.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural manifestations of the rat Sertoli cell to hypophysectomy: a correlative morphometric and endocrine study

Although the Sertoli cell is a key cell mediating the actions of FSH- and LH-stimulated testosterone (T) in the testis, there is little information to indicate how this cell responds structurally to hormonal insufficiency. The present study used morphometric techniques to study the structural manifestations of the Sertoli cell in adult rats hypophysectomized for 6 and 28 days. Six days posthypophysectomy, a period when germ cell degeneration is first evident, tubular diameter and length, testis weights, volume of the interstitium, and volume of most parameters that comprise the interstitium (except blood vessels) showed significant regressive features. However, virtually no parameter relating to the volume and surface area of the Sertoli cell or its subcellular components was significantly reduced compared with that in the normal animal. Thus, at a time when germ cell degeneration is seen in the testis, the Sertoli cell showed no significant structural response to the changing endocrine status of the animal. In contrast, 28 days after hypophysectomy, virtually all parameters relating to the Sertoli cell and its organelles were significantly decreased compared with those in normal animals. Plasma and tissue testosterone, PRL, and FSH showed a very significant decrease 6 days after hypophysectomy compared with intact animals, but at 28 days there was no further significant decrease in the levels of these hormones. There was no correlation of most organelle volumes and surface areas with endocrine parameters. The size of the Sertoli cell showed positive and significant correlations with the volumes and surface areas of all of its cytoplasmic organelles, except the lipid volume. Short term hypophysectomy resulted in no significant change in either the concentration (femtomoles per mg protein) or the content (femtomoles per testis) of FSH receptors, nor was there a significant change in the number of FSH receptors per cell. However, 28 days after hypophysectomy only the content, not the concentration, of FSH receptors decreased significantly along with a decrease in the number of FSH receptors per cell. Although the marked degenerative changes that are seen in the testis 6 days after hypophysectomy parallel endocrine decline, the Sertoli cell responded slowly from a structural standpoint compared with the Leydig cell. After long term hypophysectomy, significant morphometric changes were observed in all of its structural parameters.

Serine analogues of hFSH-beta-(33-53) and hFSH-beta-(81-95) inhibit hFSH binding to receptor

Synthetic peptides corresponding to hFSH-beta-(33-53) and hFSH-beta-(81-95) each contain free sulfhydryl groups, inhibit binding of FSH to receptor and are partial agonists of estradiol synthesis in Sertoli cells. Recently, we have reported that sulfhydryl groups are important in FSH- receptor interaction and that peptides containing free sulfhydryl groups or disulfide bridges, such as glutathione, may nonspecifically inhibit FSH binding to receptor. In the present study, Cys residues of hFSH-beta-(33-53) and hFSH-beta-(81-95) were replaced by Ser residues and the peptides tested for their ability to inhibit binding of FSH to receptor. Results similar to those obtained previously with natural sequence peptides were obtained with the Ser analogs, indicating that Cys residues were not essential for binding inhibition. However, the partial agonist activity of the hFSH-beta-(33-53) and (81-95) in cultured Sertoli cells could not be detected when Cys residues were replaced by Ser. Thus, replacement of Cys residues with Ser does not effect receptor binding activity but is deleterious to the agonist activity of these peptides.

Hormonal regulation of the growth and steroidogenic function of human granulosa cells

The aim of this study was to assess development-related interactions between gonadotropins and insulin-like growth factor (IGF-I) on DNA synthesis and steroidogenesis in human granulosa cells. "Immature" granulosa cells were obtained from follicles during the late luteal phase or first half of the follicular phase; "mature" granulosa cells came from follicles during the second half of the follicular phase but before the midcycle LH surge; and granulosa-lutein cells were obtained as a by-product of in vitro fertilization. Granulosa cells were cultured for 96 h in serum-free medium 199 with and without LH or FSH, and in the presence and absence of IGF-I. The cell monolayers were then incubated with [3H]methyl thymidine to assess DNA synthesis. Spent culture medium was assayed for progesterone and estradiol content. Immature granulosa cells: Tritiated thymidine uptake in granulosa cell cultures from immature follicles were significantly increased by IGF-I. FSH was able to maintain or increase basal and IGF-I stimulated growth whereas LH had no effect. Basal progesterone production was low and not increased by either FSH or LH. However, treatment with FSH, but not LH, increased aromatase activity. Mature granulosa cells: IGF-I also stimulated thymidine uptake. However, whereas FSH either maintained or increased thymidine uptake by these cells, LH dose dependently suppressed thymidine uptake. This inhibitory action of LH was accentuated by the presence of IGF-I. Despite the inhibitory effect of LH on thymidine uptake, the gonadotropin markedly stimulated steroid production and the maximal steroidogenic response to LH was equivalent to 3-fold greater than that to FSH. Granulosa-lutein cells: Patterns of basal and IGF-I- and gonadotropin-stimulated steroid synthesis were similar to those observed for mature granulosa cells but steroid production rates were higher. Suppression of basal and IGF-I-stimulated thymidine uptake by LH was even more pronounced. These results suggest that the granulosa cell LH receptor, once expressed, negatively regulates cell growth and, simultaneously, positively regulates steroid synthesis. This development related event could be crucial to the mechanism whereby granulosa cells cease to divide and commence maximal rates of steroid synthesis in response to the LH surge.

Synthetic human follicle-stimulating hormone-beta-(1-15) peptide-amide binds Ca2+ and possesses sequence similarity to calcium binding sites of calmodulin

To determine the basis for the previously demonstrated calcium requirement for specific binding of FSH to receptor, 11 overlapping peptide amides representing the entire primary structure of human FSH (hFSH)-beta-subunit were tested for their ability to bind 45Ca2+ as an approach to identifying possible calcium binding regions of the hormone. hFSH-beta-(1-15)-peptide amide bound significant amounts of 45Ca2+. This peptide contains an amino acid sequence similar to that found in the loop structures of the calcium-binding domains of calmodulin. The affinity of hFSH-beta-(1-15)-peptide amide for calcium (Kd = 1.2 +/- 0.3 mM) was similar to that previously reported for a synthetic peptide corresponding to calmodulin binding site III. No such sequence is predicted in the recently deduced primary structure of the FSH receptor. FSH-beta-(1-15) may, therefore, be associated with the calcium requirement for specific binding of FSH to receptor. The calcium binding property of this calmodulin-like peptide also correlates well with its ability to induce uptake of calcium into liposomes via transmembrane channel formation, as previously reported by this laboratory.

Control of immunoactive inhibin production by human granulosa cells

OBJECTIVE

The aim was to determine the relation between stage of antral follicular development and granulosa cell production of immunoactive inhibin.

DESIGN

Primary granulosa cell cultures in serum-free Medium 199 were incubated at 37 degrees C for 96 hours with a change of medium at 48 hours. Inhibin and steroid levels in culture medium were determined by radioimmunoassay. The inhibin assay was based on the N-terminal 1-26 amino acid sequence of the alpha-chain of porcine 32 kDa inhibin using pl alpha 1-26-GLY27-TYR28 as the immunogen, tracer and standard.

PATIENTS

Granulosa cells were obtained from the ovaries of women with regular menstrual cycles undergoing hysterectomy with unilateral or bilateral oophorectomy to treat non-malignant gynaecological disease.

RESULTS

Basal production of immunoactive inhibin by granulosa cells from presumptive preovulatory follicles (greater than 15 mm diameter) was 5-13 times higher than that by granulosa cells from immature (less than 10 mm diameter) or intermediately mature (10-15 mm diameter) follicles. Basal production of progesterone and oestradiol followed a qualitatively similar pattern, establishing a positive relation between functional granulosa cell maturity and inhibin production. Treatment of granulosa cell cultures from immature follicles with follicle-stimulating hormone (FSH), but not luteinizing hormone (LH), increased inhibin production, time and dose dependently. FSH, but not LH, also brought about similar increases in steroid hormone synthesis by granulosa cells from immature follicles. The stimulatory effect of FSH on granulosa cell inhibin production was augmented at least twofold by the presence of testosterone or 5 alpha-dihydrotestosterone (1.0 mumol/l) but was unaffected by oestradiol. Granulosa cells from intermediately mature follicles undertook variable degrees of both FSH and LH-responsive inhibin production which generally corresponded with gonadotrophin-responsive steroid production. Granulosa cells from presumptive preovulatory follicles showed inconsistent inhibin responses to FSH. However, LH caused marked (at least twofold) increases in inhibin production, paralleling LH-responsive steroid production.

CONCLUSION

These results show that for human beings, granulosa cell capacity to produce immunoactive inhibin in vitro increases with follicular maturity. FSH, but not LH, stimulates inhibin production by immature granulosa cells and this response to FSH is subject to modulation by androgen. During preovulatory follicular development, production of inhibin, like steroids, becomes increasingly responsive to LH. Such a development-related pattern of granulosa cell inhibin production helps explain how, post-ovulation, the corpus luteum is able to secrete inhibin as well as steroids. It is also compatible with the concept that locally produced inhibin could participate in the paracrine control of follicular development during the human menstrual cycle.

A synthetic peptide corresponding to hFSH-beta-(81-95) has thioredoxin-like activity

The thioredoxin-like activity of human follicle stimulating hormone (hFSH), hFSH-beta-(83-88) peptide amide (hFSH-beta-(83-88) which has a sequence similar to the thioredoxin active center (-His-Cys-Gly-Lys-Cys-Asp-)) and thioredoxin-(31-36)-peptide amide (TD-(31-36) which contains the redox-active dithiol of thioredoxin (-Trp-Cys-Gly-Pro-Cys-Lys-)) was characterized by their ability to reactivate reduced and denatured bovine pancreatic ribonuclease (RNase). This assay reflects the recently recognized ability of thioredoxin to catalyze disulfide bond formation in proteins. Compared to uncatalyzed refolding of reduced, denatured substrate, hFSH was approximately 10-fold more active than thioredoxin on a molar basis. The catalytic activity of hFSH-beta-(83-88) and TD-(31-36) was equivalent to that of an equimolar concentration of thioredoxin. Screening of 11 overlapping peptide amides representing the entire primary structure of hFSH-beta-subunit indicated that hFSH-beta-(81-95), which contains the sequence similar to the thioredoxin active center within a receptor-binding region of the hFSH-beta-subunit, possesses strong thioredoxin-like activity and was more active than an equimolar concentration of thioredoxin. In contrast, hFSH-beta-(33-53), a thiol-containing peptide which corresponds to a second FSH receptor-binding domain but lacks the sequence similar to the thioredoxin active center, was inactive. Synthetic peptide amides corresponding to other regions of hFSH-beta-subunit were less effective than hFSH-beta-(81-95) in reactivating reduced and denatured RNase. Our data provide evidence that the recently reported thioredoxin-like catalytic activity of FSH may be due, at least in part, to the redox-active dithiol present within a receptor-binding domain of its beta-subunit, and thus may have a physiological role in receptor binding or signal transduction.

A synthetic peptide encompassing two discontinuous regions of hFSH-beta subunit mimics the receptor binding surface of the hormone

Synthetic peptides corresponding to discontinuous segments of the hFSH-beta subunit, amino acids 33-53 and 81-95, have been shown to interact with the follicle-stimulating hormone (FSH) receptor. In this study, we demonstrate that hFSH-beta-(33-53)-(81-95)-peptide amide, a synthetic peptide encompassing these binding regions, possesses higher affinity for the FSH receptor than either synthetic hFSH-beta-(33-53) or hFSH-beta-(81-95). This increased affinity suggests that each binding component is effectively interacting with the receptor, providing evidence that these two separate receptor binding regions of hFSH-beta form a continuous binding surface on the native molecule. These results also suggest that binding surfaces of very complex proteins, such as the heterodimeric glycoprotein hormone FSH, may be mimicked by a linear arrangement of its binding domains. A model based on energetics of the peptide-receptor interaction is also described. The results indicate that the affinity (Ka) of a peptide containing different binding domains can be approximated utilizing the product of the affinity constant of each binding domain (Ka = k1.k2...kn).

Synthetic peptides corresponding to human follicle-stimulating hormone (hFSH)-beta-(1-15) and hFSH-beta-(51-65) induce uptake of 45Ca++ by liposomes: evidence for calcium-conducting transmembrane channel formation

We have previously described FSH receptor-mediated influx of 45Ca++ in cultured Sertoli cells from immature rats and receptor-enriched proteoliposomes via activation of voltage-sensitive and voltage-independent calcium channels. We have further shown that this effect of FSH does not require cholera toxin- or pertussis toxin-sensitive guanine nucleotide binding protein or activation of adenylate cyclase. In the present study, we have identified regions of human FSH-beta-subunit which appear to be involved in mediating calcium influx. We screened 11 overlapping peptide amides representing the entire primary structure of hFSH-beta-subunit for their effects on 45Ca++ flux in FSH receptor-enriched proteoliposomes. hFSH-beta-(1-15) and hFSH-beta-(51-65) induced uptake of 45Ca++ in a concentration-related manner. This effect of hFSH-beta-(1-15) and hFSH-beta-(51-65) was also observed in liposomes lacking incorporated FSH receptor, suggesting that the peptide amides may act as ionophores or channel-formers. Reducing membrane fluidity by incubating liposomes (containing no receptor) with hFSH-beta-(1-15) or hFSH-beta-(51-65) at temperatures lower than the transition temperatures of their constituent phospholipids resulted in no significant (P greater than 0.05) difference in 45Ca++ uptake. The effectiveness of the calcium ionophore A23187, however, was abolished. Ruthenium red, a voltage-independent calcium channel antagonist, was able to completely block uptake of 45Ca++ induced by hFSH-beta-(1-15) and hFSH-beta-(51-65) whereas nifedipine, a calcium channel blocker specific for L-type voltage-sensitive calcium channels, was without effect. These results suggest that in addition to its effect on voltage-sensitive calcium channel activity, interaction of FSH with its receptor may induce formation of transmembrane aqueous channels which also facilitate influx of extracellular calcium.

Sulfhydryl groups are involved in the interaction of FSH with its receptor

FSH has recently been reported to possess thioredoxin-like activity, presumably explained by the homology between a region of FSH-beta subunit and the active site of thioredoxin. The homologous sequence lies within a receptor binding region, which suggests a possible role for sulfhydryl groups in the formation of an active hormone-receptor complex and subsequent signal transduction. In order to determine the relevance of sulfhydryl groups on FSH-receptor interaction, we studied the effect of N-ethylmaleimide (NEM) and glutathione on FSH binding. The results indicate that free sulfhydryl groups, probably derived from the FSH receptor, are involved in ligand-receptor interaction.

Structure--function relationships of the glycoprotein hormones and their receptors

The primary structures of the glycoprotein hormones follitropin (FSH), lutropin (LH), human choriogonadotropin (hCG) and thyrotropin (TSH) have been determined, hCG has been crystallized and initial diffraction data obtained. Studies with synthetic peptides have provided information on regions involved in receptor interaction and signal transduction. The receptors for the glycoprotein hormones have been prepared by gene cloning methods and their primary structures deduced. As Leo Reichert and colleagues discuss here, although cAMP is involved in glycoprotein hormone signal transduction, recent evidence also implicates other second messengers, especially Ca2+ and may include both the phosphatidylinositol pathway and activation of Ca2+ channels.

Human follicular fluid contains a follicle-stimulating hormone (FSH) receptor binding inhibitor which has FSH agonist activity, is immunologically similar to FSH, but can be distinguished from FSH

We have previously reported the presence in human follicular fluid (hFF) of a high (greater than 5000) mol wt FSH receptor binding inhibitor (FSH-BI). This hFF FSH-BI was further purified by removal of material insoluble in acidified acetone (pH 4.1) but soluble in diethyl ether (pH 10.5), followed by molecular sieving through Sephacryl S-100. FSH-BI activity eluted from S-100 with an elution volume similar to that of hFSH, but could be distinguished from hFSH on the basis of a differential sensitivity to acid inactivation. Human FSH was inactivated in acetone at pH 4.1 (1 h, 25 C), whereas hFF FSH-BI retained activity under these conditions. Human FF FSH-BI also demonstrated FSH-like agonist activity, defined as the ability to stimulate basal levels of estradiol synthesis in cultured rat Sertoli cells. Human FSH-BI strongly cross-reacted to a commercially available monoclonal antibody used to measure serum hFSH. Indeed, recovery of FSH immunologic activity was significantly greater (134-fold on a mass basis) after partial purification, indicating that antibody recognition sites were apparently masked in unfractionated hFF. In summary, large mol wt hFSH-BI has been partially purified from hFF and found to be similar in size to pituitary hFSH and to have FSH-like agonist activity in vitro. Although distinguishable from pituitary hFSH on the basis of stability to acid, hFSH-BI appears immunologically related to pituitary hFSH so that measurements of hFSH levels in hFF using immunologic techniques should be interpreted with caution.

Determination of alpha-subunit contact regions of human follicle-stimulating hormone beta-subunit using synthetic peptides

Follicle-stimulating hormone (FSH) is a glycoprotein hormone composed of two different subunits designated FSH-alpha and FSH-beta. Using synthetic peptides corresponding to the primary structure of human (h) FSH-beta subunit, we previously identified two regions of the beta-subunit, hFSH-(33-53) and hFSH-(81-95), as receptor binding regions. In this report, we tested the ability of synthetic peptides to interact with hFSH-alpha-subunit. Synthetic peptides corresponding to hFSH-beta-(11-25), (41-55), (51-65), and (101-111) were able to bind specifically radioiodinated hFSH-alpha-subunit, suggesting that they represent regions of interaction with the alpha-subunit. These experimental results were in agreement with the location of alpha-subunit contact regions predicted by sequence analysis. Peptides of hFSH-beta-subunit showing maximum specific binding to the alpha-subunit were those possessing minimum interaction with receptor whereas those not binding to alpha-subunit corresponded to regions shown to interact with receptor (hFSH-beta-(33-53) and hFSH-beta-(81-95]. The hFSH-beta-subunit, therefore, seems to have two discontinuous receptor binding regions flanked by three alpha-subunit contact regions.

Differential roles of high and low affinity guanosine 5'-triphosphate binding sites in the regulation of follicle-stimulating hormone binding to receptor and signal transduction in bovine calf testis membranes

We have previously shown that FSH receptors are physically and functionally associated with a guanine nucleotide regulatory protein (Gs) in membranes of calf testis. Using N-ethylmaleimide (NEM), forskolin, and cholera toxin as probes, we have investigated the role of low and high affinity GTP-binding sites of stimulatory guanine nucleotide-binding protein of adenylate cyclase (Gs) in the activation of adenylate cyclase. When calf testis membranes were exposed to NEM (1 mM), FSH binding to receptors was slightly (30%) decreased, but the receptors showed continued sensitivity to GTP, resulting in a further decrease in [125I]human FSH binding to receptors. Pretreatment of membranes with NEM (up to 20 microM) produced no effect on GTP-binding. A dose-dependent decrease in high affinity GTP-binding sites, however, was observed at higher (greater than 50 microM) NEM. Adenylate cyclase activity was reduced in response to GTP gamma S or NaF concomitant to a decrease in high affinity GTP-binding sites in membranes treated with 50-100 microM NEM, or completely abolished in membranes exposed to 300 microM NEM. Stimulation by forskolin indicated that the significant inhibition of adenylate cyclase activity occurring in membranes exposed to low NEM (50-100 microM) was not due to inactivation of catalytic unit of adenylate cyclase by NEM. Pretreatment of membranes with 100 micrograms/ml cholera toxin and NAD slightly (18%) reduced specific FSH binding but did not affect Gpp(NH)p-binding. However, adenylate cyclase stimulation by GTP plus FSH in these membranes was significantly enhanced. When membranes were treated with higher concentration of cholera toxin (250 micrograms/ml), the adenylate cyclase stimulation by GTP plus FSH was abolished due to uncoupling of FSH receptors from Gs and a significant decrease in high affinity GTP-binding sites. Our results suggest that high affinity GTP-binding sites of Gs coupled to FSH receptors are essential for FSH and guanine nucleotide activation of adenylate cyclase. The low affinity binding sites bind GTP and thereby regulate FSH binding but are not involved in the activation of adenylate cyclase.

A new role for follicle-stimulating hormone in the regulation of calcium flux in Sertoli cells: inhibition of Na+/Ca++ exchange

Elucidation of mechanisms regulating intracellular calcium levels in steroidogenic tissues is important for understanding control of cellular function. We have previously described FSH receptor-mediated flux of 45Ca++ into cultured rat Sertoli cells and receptor-enriched proteoliposomes via voltage-sensitive and voltage-independent calcium channels. In the present study, we report heretofore unrecognized inhibitory effects of FSH on Na+/Ca++ exchange in these two systems. An outwardly directed Na+ gradient, developed by preincubating Sertoli cell monolayers in buffer made hypertonic with NaCl, resulted in uptake of 45Ca++ that was unaffected by calcium channel blocking agents, ruthenium red or methoxyverapamil, but was enhanced by ouabain, a specific inhibitor of Na+/K(+)-ATPase. Sodium-dependent 45Ca++ flux into Sertoli cells was inhibited in a concentration-related manner by increased extracellular Na+ (up to 135 mM). FSH consistently and reproducibly (28.9 +/- 3.8%, 10 separate assays) reduced sodium-dependent 45Ca++ influx in the absence or presence of ouabain. A lesser effect on Na+/Ca++ exchange was seen when Li+ replaced Na+ in the preincubation buffer, and a marked reduction occurred when Sertoli cells were incubated in buffer containing KCl, presumably due to membrane depolarization. FSH-sensitive Na+/45Ca++ exchange was also observed when using FSH receptor-enriched proteoliposomes. Our earlier calcium channel studies indicated that FSH affects Ca++ entry into Sertoli cells via a receptor-mediated process. The results reported here demonstrate that the interaction of FSH with its receptor is associated with changes in Na+/Ca++ exchange as well, and suggest that this activity may also be involved in regulating intracellular free Ca++ levels in the Sertoli cell.

Follicle-stimulating hormone receptor-mediated uptake of 45Ca2+ by cultured rat Sertoli cells does not require activation of cholera toxin- or pertussis toxin-sensitive guanine nucleotide binding proteins or adenylate cyclase

We have previously reported that FSH stimulates flux of 45Ca2+ into cultured Sertoli cells from immature rats via voltage-sensitive and voltage-independent calcium channels. In the present study, we show that this effect of FSH does not require cholera toxin (CT)- or pertussis toxin (PT)-sensitive guanine nucleotide binding (G) protein or activation of adenylate cyclase (AC). Significant stimulation of 45Ca2+ influx was observed within 1 min, and maximal response (3.2-fold over basal levels) was achieved within 2 min after exposure to FSH. FSH-stimulated elevations in cellular cAMP paralleled increases in 45Ca2+ uptake, suggesting a possible coupling of AC activation to 45Ca2+ influx. (Bu)2cAMP, however, was not able to enhance 45Ca2+ uptake over basal levels at a final concentration of 1000 microM, although a concentration-related increase in androstenedione conversion to estradiol was evident. Exposure of Sertoli cells to CT (10 ng/ml) consistently stimulated basal levels of androstenedione conversion to estradiol but had no effect on basal levels of 45Ca2+ uptake. Similarly, CT had no effect on FSH-induced 45Ca2+ uptake, but potentiated FSH-stimulated estradiol synthesis. PT (10 ng/ml) augmented basal and FSH-stimulated estradiol secretion without affecting 45Ca2+ influx. The adenosine analog N6-phenylisopropyladenosine, which binds to Gi-coupled adenosine receptors on Sertoli cells, inhibited FSH-stimulated androgen conversion to estradiol in a dose-related (1-1000 nM) manner, but FSH-stimulated 45Ca2+ influx remained unchanged. Our results show that in contrast to FSH-stimulated estradiol synthesis, the flux of 45Ca2+ into Sertoli cells in response to FSH is not mediated either directly or indirectly by CT- or PT-sensitive G protein, nor does it require activation of AC. Our data further suggest that the FSH receptor itself may function as a calcium channel.

Identification of low and high molecular weight follicle-stimulating hormone receptor-binding inhibitors in human follicular fluid

Various fractions of human follicular fluid (FF) were tested for follicle-stimulating hormone (FSH) receptor-binding inhibitory activity, as well as for their ability to form a dansyl derivative previously reported to be associated with FSH receptor-binding inhibitory activity in bovine FF. Pooled human FF was found to inhibit binding of radioiodinated human FSH (125I-hFSH) to its receptor in a concentration-related manner. Fractionation of human FF by sequential ultrafiltration through membranes of calibrated pore size resulted in human FF components of low (500 to 5,000) and high (greater than 5,000) molecular weight, (Mr) respectively. Each of these inhibited 125I-hFSH binding to receptor in a concentration-related manner. Dansylation of the above components revealed the Rf = 0.15 dansyl derivative to be present in unfractionated human FF and in the low (500 to 5,000 Mr), but not the high (greater than 5,000 Mr), component of human FF. These findings are consistent with those previously reported for bovine FF and porcine FF and indicate that human FF contains FSH receptor-binding inhibitors with properties similar to those observed in animal species.

Purification of follitropin receptor from bovine calf testes

Follitropin (FSH) receptors were solubilized from pure light membranes of bovine calf testis, using an optimum detergent to protein ratio of 0.01. The soluble FSH receptor fraction was gel filtered through Sepharose 6B to isolate an active fraction (6B-Fr-1) which behaved as a complex of FSH receptor and Gs protein. The 6B-Fr-1 was concentrated by ultrafiltration and further purified by sequential Sepharose 4B gel filtration, DEAE-cellulose chromatography (to separate the receptor from Gs protein), and wheat germ lectin affinity chromatography. The purified receptor had an FSH-binding capacity of approximately 3.47 nmol/mg of protein with a Kd of 1.9 X 10(-10) M. Yield was 526 micrograms/11.5 kg tested. Radioiodinated, as well as unlabeled purified FSH receptor, migrated on sodium dodecyl sulfate-polyacrylamide gels as a single major band of Mr approximately 240,000. This band was not affected by 8 M urea treatment prior to analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but treatment with dithiothreitol induced the loss of the 240-kDa band, with appearance of an Mr approximately 60,000 band. The availability of highly purified, stable FSH receptor should allow direct studies on its structure-function relationships.