Receptor regulation and target cell responses: studies in the ovarian luteal cell

Constitutive and Agonist-dependent Self-association of the Cell Surface Human Lutropin Receptor

The human lutropin receptor (hLHR) is a G protein-coupled receptor (GPCR) that plays an essential role in reproductive physiology. The present studies were undertaken to determine whether the hLHR self-associates. We show that high molecular weight complexes of the hLHR can be co-immunoprecipitated from 293 cells transfected with differentially tagged hLHRs. These complexes are detected only in extracts from cells that have been co-transfected and not in extracts combined from cells expressing only one form of tagged hLHR, confirming the in vivo self-association of the receptor. In transiently transfected cells, in which a small percentage of cells overexpress hLHR and most of the hLHR is located intracellularly in the ER, the self-associated hLHR is composed predominantly of immature hLHR. When cells were transiently co-transfected with wild-type hLHR and a misfolded mutant of the hLHR, a physical association of the ER-localized misfolded mutant with the immature hLHR was observed, resulting in a decreased cell surface expression of the wild-type receptor. In contrast, in stably transfected cells, where the majority of cells express receptor and there is much less intracellular accumulation of hLHR, the self-associated forms of the hLHR are composed predominantly of cell surface receptor. The abundance of cell surface hLHR dimers and oligomers, as detected on SDS gels, is increased further upon human choriogonadotropin treatment of the stably transfected cells. In addition to documenting the self-association of cell surface hLHR, our results underscore the importance of the cellular distribution of recombinant GPCR as it relates to the nature of the GPCR dimerization and oligomerization.

Early hCG-induced desensitization of rat ovarian LH/hCG receptors is associated with altered physical state of membranes

Injection of pseudopregnant rats with pharmacological doses of hCG produced desensitization of adenylylcyclase and steroidogenic systems in ovarian membranes and luteal cells. Membrane lipid rigidity, as determined by fluorescence polarization of DPH, decreased as early as 0.5 h after injection of hCG. Desensitization also modified the differential scanning calorimetric profile characteristic of control membranes. The accessibility of ovarian LH/hCG receptors was unchanged. The results indicate that the hCG-induced decrease of membrane lipid rigidity is preceded by the process of desensitization of the rat ovary.

Juvenile hormone regulates the steroidogenic competence of Manduca sexta prothoracic glands

The acquisition of steroidogenic competence by prothoracic glands of last instar Manduca sexta larvae is regulated by juvenile hormone (JH). Topical treatment of pre-commitment larvae with JH I or (7S)-hydroprene (a JH analog) delays development by increasing the time to pupal commitment and wandering. Prothoracic gland competence is suppressed in JH-treated larvae: Unstimulated and prothoracicotropic hormone (PTTH)-stimulated rates of in vitro ecdysone secretion are decreased relative to rates of secretion by competent glands. (7S)-Hydroprene also suppresses the competence of glands in head-ligated pre-commitment larvae, suggesting the hormone acts directly on the glands. Two results indicate PTTH plays a role in controlling competence, and that JH regulates competence indirectly by inhibiting PTTH release: (1) head-ligation prevents the acquisition of full competence, and (2) cAMP levels are elevated in glands from JH-treated larvae. Thus, the decrease in the JH titer that precedes pupal commitment in Manduca is permissive for the acquisition by prothoracic glands of steroidogenic competence.

Desensitisation of LH-stimulated cyclic AMP accumulation in isolated bovine luteal cells--effect of phorbol ester

In this study, we have characterized the LH-mediated desensitisation of receptor-linked cAMP generation in bovine luteal cells. Furthermore, the possibility that protein kinase C could play a role in this process has been investigated. The results obtained, show that the preincubation of Percoll-purified bovine luteal cells with LH diminished the cAMP response during reincubation with LH, depending upon the duration of prior exposure to LH and the concentration of LH used in the first incubation. This desensitisation was specifically dependent upon the prior exposure of the cells to the hormone only, as preincubation with either forskolin or cholera toxin did not result in a desensitised cAMP response to subsequent LH stimulation. On the other hand, LH-desensitised cells retained undiminished responsiveness to restimulation with cholera toxin. Neither the maximum binding capacity nor the affinity of the LH-receptor was affected by exposure of the cells to a desensitising dose of LH. The results demonstrate that in bovine luteal cells, LH produces a homologous desensitisation of the cAMP response which is not mediated by cAMP and that a hormone-receptor interaction appears to be a prerequisite for this process. Preincubation of the cells with varying concentrations of the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) did not result in any reduction of LH-induced cAMP response during reincubation. The affinity of LH-receptor was also not affected by PMA pretreatment. In contrast, PMA-pretreated cells consistently produced increased amounts of cAMP when challenged with any of the agonists, LH, cholera toxin or forskolin. The preincubation of the cells with LH in the presence of PMA appears to prevent, at least partially, the desensitising effect of LH. It is concluded that in bovine luteal cells there is no evidence for a role of protein kinase C in LH-induced desensitisation. On the contrary, PMA pretreatment increased the response of adenylate cyclase to a subsequent hormonal stimulation without changing the affinity of the receptors for the hormone. Either an attenuation of the inhibitory N protein or a direct activation of the catalytic unit of adenylate cyclase could be the explanation for the observed effects of PMA. However, available data at present do not offer a choice between the two possibilities.

Colonic cyclic AMP metabolism following chronic ethanol consumption in the rat: effect of hormonal secretagogues

The colonic cyclic AMP system is known to be involved in intestinal secretion and can be stimulated by a variety of gastrointestinal hormones including prostaglandins. We have investigated the effect of chronic ethanol ingestion on the activity of the key enzymes in cyclic AMP metabolism--adenylate cyclase and cyclic AMP phosphodiesterase--in the colonic mucosa of the rat. Chronic ethanol consumption by feeding a nutritionally adequate liquid diet enhanced basal colonic adenylate cyclase activity significantly by 168% (p less than 0.01), but had no effect on colonic low Km cyclic AMP phosphodiesterase activity. In addition, various hormonal secretagogues were used to stimulate colonic adenylate cyclase. Colonic adenylate cyclase exhibited a significantly greater sensitivity and efficacy to prostaglandins and vasoactive intestinal peptide after chronic ethanol ingestion. Since increased intestinal cyclic AMP production due to an increased activity of intestinal adenylate cyclase is known to promote intestinal secretion of water and electrolytes, the frequently observed diarrhea in alcoholics may be explained at least in part by an enhanced production of colonic cyclic AMP.

Follicle-stimulating hormone regulates in vivo testicular phosphodiesterase

The effect of FSH on testicular phosphodiesterase was studied in immature rats in order to verify that the regulation of response to hormone in the gonad involves an increased cyclic AMP catabolism. Hydrolysis of cyclic AMP and cyclic GMP was measured in the homogenates of seminiferous tubules and interstitium of control animals and animals injected i.p. with 50 micrograms ovine FSH twice, 24 h and 12 h before necroscopy. After hormonal treatment, cyclic AMP phosphodiesterase activity in the seminiferous tubules was markedly increased whether results were expressed per testis or per mg protein, while cyclic GMP phosphodiesterase present in the same compartment was apparently unaffected. In the interstitium, cyclic nucleotide hydrolysis was usually decreased after FSH injection. The stimulation of phosphodiesterase, a slow process reaching maximal stimulation after 12 h, was dependent on the dose of FSH injected. In addition, DEAE-cellulose chromatography of cytosol prepared from control and treated seminiferous tubules confirmed that stimulation was restricted to a cyclic AMP hydrolysing enzyme while the activity of the cyclic GMP hydrolysing form was not modified. Thus it is demonstrated that testicular phosphodiesterase is under FSH control. It is proposed that this in vivo regulation is a relevant phenomenon in the modulation of Sertoli cell function and contributes to the refractoriness that follows gonadotropin treatment.

Desensitization of steroidogenesis in cultured Leydig tumor cells: role of cholesterol

We have reported previously that the human choriogonadotropin (hCG) receptors of cultured Leydig tumor cells can be down-regulated with hCG or mouse epidermal growth factor (mEGF) and that such down-regulation results in a loss of the steroidogenic response of the cells to hCG. Exposure of the cells to hCG, but not to mEGF, also resulted in a decrease in steroidogenic responses to cholera toxin and cAMP. The results presented herein show that the hCG-induced loss of steroidogenic response to cAMP is due to the depletion of intracellular cholesterol and that this depletion can be prevented by the addition of low density lipoprotein. Our results also show that after exposure of the cells to mEGF or hCG their steroidogenic response to hCG is limited by the number of hCG receptors, regardless of the presence of low density lipoprotein.

Early decrease in secretin-, glucagon-, and isoproterenol-stimulated cardiac adenylate cyclase activity in rats treated with isoproterenol

Male Wistar albino rats received three times daily for one to five days 0.25 to 5.0 mg/kg D,L-isoproterenol intraperitoneally. D.L-Isoproterenol injections provoked a time dependent- and dose-related cardiac hypertrophy. With moderate hypertrophy, a selective decrease in secretin-stimulated adenylate cyclase activity occurred. When heart hypertrophy was more pronounced, greater losses in secretin-, as well as in D,L-isoproterenol-, glucagon-, guanine nucleotide-, and fluoride-stimulated enzyme activity developed. Hormone stimulations of adenylate cyclase were more severely, curtailed (60 to 65%) than guanine nucleotide or fluoride stimulations (40 to 45%). The accompanying loss in beta-receptors (35%) was proportionately lower than the loss of D,L-isoproterenol sensitivity of adenylate cyclase. This complex pattern of adenylate cyclase desensitization in heart membranes from animals chronically treated with D,L-isoproterenol is reminiscent of that observed in heart membranes from spontaneously hypertensive rats.

Effect of peripheral sex steroid metabolism on pituitary gonadotropin reserve of women with hypothalamic amenorrhea

Women with hypothalamic amenorrhea without evidence of pituitary tumor were studied to ascertain (1) whether affected patients have altered pituitary sensitivities and reserves to a constant gonadotropin-releasing hormone (GnRH) infusion and (2) whether altered sex steroid metabolism contributed to altered pituitary synthesis or secretion of gonadotropin. Women with hypothalamic amenorrhea displayed abnormal gonadotropin reserves without altered pituitary sensitivity to GnRH. Those altered reserves may have reflected, abnormally low estradiol levels, which directly or indirectly affected, pituitary gonadotropin synthesis. However, sex steroid metabolism or normally cycling and amenorrheic women were not significantly different. The ovaries of both groups of patients were equally reactive to stimulation with endogenous gonadotropin, as reflected by circulating estradiol levels attained at the end of the GnRH infusion.

Desensitization to gonadotropins in cultured Leydig tumor cells involves loss of gonadotropin receptors and decreased capacity for steroidogenesis

The ability of human choriogonadotropin (hCG) to regulate its receptors and target cell responses has been studied in a clonal strain of cultured Leydig tumor cells (MA-10). Exposure of the MA-10 cells to hCG results in decrease in hCG binding activity which is dependent on time and the concentration of hCG. This decrease is due to a change in the number of receptors rather than in the affinity of the receptors, and it is accompanied by a corresponding reduction in the ability of hCG to stimulate steroidogenesis. Exposure of the MA-10 cells to hCG also resulted in a reduction of the steroidogenic responses to cholera toxin and 8-Br-adenosine cyclic 3',5'-monophosphate. The hCG-induced loss of steroidogenic responses to these stimuli seems to be due to the stimulation of steroidogenesis rather than to the decrease in hCG receptors because it also can be induced when steroidogenesis is stimulated with cholera toxin or 8-Br-adenosine 3',5'-monophosphate under conditions such that the number of hCG receptors is not reduced.

Characterization of the desensitized state of Leydig tumor cells

A perifusion system has been used to study the in vitro desensitization of isolated Leydig tumor cells. It was observed that the cells become refractory, as measured by decreased rates of steroidogenesis, during continuous perifusions with saturating concentrations of either human choriogonadotropin (CG), cholera toxin, or 8-bromo-cyclic AMP. Furthermore, an initial perifusion of the cells with either human CG, cholera toxin, or 8-bromo-cyclic AMP causes subsequent desensitization towards all three stimuli. Thus, each of these stimuli is equally effective in inducing a state of desensitization in these cells that is manifested by a steroidogenic lesion(s) distal to cyclic AMP formation. It was found that the post-cyclic AMP lesion(s) in the desensitized state occurs prior to the formation of pregnenolone. However, the decreased rates of steroidogenesis do not seem to arise from a depletion of intracellular cholesterol.

Inactivation of porcine corpus luteum lutropin receptors by a microsomal enzyme. Activation in ageing and regressing corpora lutea

The binding of 125I-labelled human choriogonadotropin (hCG) to homogenates of porcine corpora lutea showed a marked departure from ideal behaviour, due to a time- and temperature-dependent inactivation of both free hormone and unoccupied receptors. Occupied receptors were not affected. Lutropin (LH)-receptor-inactivating activity was detected after preincubation of homogenates, particulate fractions and microsomes, but little activity could be demonstrated in cytosol fractions. Inactivation was dependent on the temperature and pH of preincubation, and on tissue concentration: LH-receptor inactivation was first-order with respect to preincubation time. Lutropin-receptor-inactivating activity was low in early-luteal and mid-luteal phase in pig corpora lutea, but was increased significantly in late-luteal and regressing corpora lutea.

Secretin and VIP-stimulated adenylate cyclase from rat heart. II. Impairment in spontaneous hypertension

Cardiac adenylate cyclase activity was normal in 3 weeks-old spontaneously hypertensive rats of the Wistar-Okamoto substrain. The hormone-sensitive adenylate cyclase activity was reduced in 10 weeks-old or older animals, and secretin- and VIP-activations were definitely more impaired (by 64% and 69%, respectively) than isoproterenol- and glucagon-activations (17% and 22%, respectively). By contrast, the fluoride- and p[NH]ppG-stimulations of the enzyme were unaffected. These alterations in the adenylate cyclase system coupled to secretin and VIP appeared specific to the heart as the isolated pancreatic acinar cells from spontaneously hypertensive animals responded normally to secretin, as a liver particulate fraction responded normally to secretin and VIP, and both brain synaptic membranes and a particulate fraction of anterior pituitary to VIP.

Aggregation of luteinizing hormone receptors in granulosa cells: a possible mechanism of desensitization to the hormone

The temporal relationship between redistribution of receptors to lutropin (luteinizing hormone)/human chorionic gonadotropin in cultured rat ovarian granulosa cells and the cellular response to hormonal challenge were studied. Visualization of receptor-bound human chorionic gonadotropin by indirect immunofluorescence, with hormone-specific antibodies after fixation with 2% formaldehyde, revealed the existence of small clusters around the entire cell circumference 5--20 min after exposure to the hormone at 37 degrees C. Such small receptor aggregates were also evident if hormone incubation was at 4 degrees C or if cells were fixed with 2% formaldehyde before incubation. Larger clusters were evident after prolonged incubation with the hormone (2--4 hr) at 37 degrees C. The later change coincided with diminished cyclic AMP accumulation in respose to challenge with fresh hormone. When the fixation step was omitted and antibodies to human chorionic gonadotropin were applied after hormonal binding, acceleration of both receptor clustering and the desensitization process was observed. This maneuver also induced capping of the hormone receptors. In contrast, monovalent Fab' fragments of the antibodies were without effect. Internalization of the bound hormone in lysosomes, and subsequent degradation, was evident 8 hr after hormonal application and was not accelerated by the antibodies. It is suggested that clustering of the luteinizing hormone receptors may play a role in cellular responsiveness to the hormone. Massive aggregation of the receptors may desensitize the cell by interferring with coupling to adenylate cyclase.

Comparative study of high-dose chorionic gonadotropin on the human and rat corpus luteum and effect of gonadotropin-releasing hormone on human luteal function

Human chorionic gonadotropin (hCG) was administered to pseudopregnant rats with 4-day-old corpora lutea and to normal women on days 16 to 18 following onset of menses. In the rat serum progesterone levels fell by 90% within 8 hours as did unoccupied luteal luteinizing hormone (LH) receptors following treatment with hCG (100 IU). Total receptor number for LH, however, remained unchanged until after 12 hours. In the woman 20,000 or 40,000 IU of hCG given on day 16 and day 18 of the cycle did not reduce serum progesterone or serum estradiol levels although the serum hCG level was similar to that achieved in the rat. In fact, serum progesterone levels rose and the cycle length was extended by hCG treatment in the human. Conversely, treatment of the woman with gonadotropin-releasing hormone (GnRH, 500 microgram on day 16 and on day 18) caused an initial rapid rise, then a fall in serum progesterone levels and the cycle length was shortened. It was concluded that the human corpus luteum may be resistant to densensitization by hCG but possibly not to LH. However, the possibility cannot be excluded that GnRH may compromise luteal function through mechanisms independent of effects on pituitary gonadotropin secretion or action.

Effects of lysomotropic agents, and of microfilament- and microtubule-disrupting drugs on the activation of casein-gene expression by prolactin in the mammary gland

The organ-culture technique was used to investigate the effects of lysomotropic agents (NH4Cl and chloroquine) and of modifiers of microfilaments (cytochalasin B) and microtubules (colchicine) on the induction of casein synthesis and the accumulation of casein mRNA by prolactin in the rabbit mammary gland. Neither chloroquine nor NH4Cl altered the lactogenic action of prolactin. Cytochalasin B attenuated the response to prolactin in terms of casein synthesis. However, this drug did not hamper the accumulation of casein mRNA. Colchicine exhibited a marked specific inhibitory effect on the induction of casein synthesis. It also prevented the accumulation of casein mRNA. These results suggest that a putative degradation of the internalized prolactin--receptor complex by lysosomes is not strictly involved in prolactin action. In addition, the integrity of the microfilaments seems unnecessary in the process of casein-gene activation by prolactin. By contrast, the integrity of the microtubule network seems absolutely necessary to ensure the transmission of prolactin information to the nucleus.

Localization of LH receptors on luteal cells with a ferritin--LH conjugate

In order to study the distribution of LH (HCG) receptors on luteal cells ferritin was coupled to ovine LH with glutaraldehyde and purified by gel chromatography. The conjugate (FELH) competed with 125I-hCG for binding to isolated luteal membranes and stimulated a dose-dependent release of progesterone (P) from isolated luteal cells which was inhibited by PGF2 alpha. FELH was distributed as single molecules or in small clusters at intervals on the surfaces of luteal cells labeled at 37 degrees C, 4 degrees C or with formaldehyde prefixation. Capping or preferential labeling at one site was not observed. The general distribution of LH (hCG) binding sites at 37 degrees C was confirmed by light-microscopic autoradiography. The distribution at 4 degrees C or with prefixation was more diffuse than at 37 degrees C suggesting that FELH binding induces small changes in receptor aggregation. Binding of FELH was specific since excess hCG reduced FELH binding to luteal cells. In cells labeled at 4 degrees C, rinsed and warmed to 37 degrees C FELH was observed along cell surfaces and within some coated vesicles and a few lysosomes within minutes suggesting that receptor internalization is a rapid and possibly continual process.

Hormonal regulation of peptide receptors and target cell responses

Regulation of plasma membrane receptors for peptide hormones by the prevailing ligand concentration often causes altered target cell function. Receptor number is determined by hormone-induced changes in membrane conformation, irreversible ligand binding, and processing of ligand-receptor complexes during hormone action.