8th Annual International Conference on Obesity and Non-Insulin Dependent Diabetes Mellitus: novel drug developments

The 8th Obesity & Non-Insulin Dependent Diabetes Mellitus and Novel Drug Developments Conference organised by International Business Communication, Inc. was held on 19th - 20th, April 1999 in London, followed by an one-day symposium on Novel Drug Developments for NIDDM & Insulin Resistance. More than 100 delegates from both academic and industrial institutes attended the two meetings. The presentations provided insights into the understanding of mechanisms and developments of novel drugs for treatments of obesity and Type 2 diabetes. This review offers a general overview of the fields in appetite suppression, thermogenesis and insulin sensitisation. Discussions focused on several emerging therapeutic areas, including novel compound developments and target identification of receptors, proteins and viruses with the use of conventional methods and recently emerged technologies, such as bioinformatics and invertebrate modelling of human systems.

The coactivator LXXLL nuclear receptor recognition motif

Nuclear receptors require coactivator binding in order to activate transcription of their cognate target genes. Ligands regulate nuclear receptor (NR)-mediated recruitment of coactivators by binding to the ligand-binding domain of the receptor and inducing a conformational change allowing for recognition of a specific motif contained within the coactivator protein. This motif is known as the NR box or LXXLL (where L is leucine and X is any amino acid) domain. Here, we review the discovery of the domain as well as its characterization.

Thioether side chain cyclization for helical peptide formation: inhibitors of estrogen receptor-coactivator interactions

Cystine, lanthionine, and cystathionine containing cyclic peptides incorporating the signature nuclear receptor (NR) box (LXXLL) motif have been synthesized and the abilities of these peptides to inhibit estrogen receptor (ER)-coactivator interactions have been determined. We found that helicity of these peptides directly correlated with their bioactivity. Cystathionine proved to be a redox-stable, isosteric replacement for the cystine disulfide. Cystathionine containing peptide 3 showed higher helical character and a lower inhibition constant (Ki, 7 nm) when compared with its cystine counterpart.

Ligands Specify Coactivator Nuclear Receptor (NR) Box Affinity for Estrogen Receptor Subtypes

Nuclear receptors (NRs) require coactivators to efficiently activate transcription of their target genes. Many coactivators including the p160 proteins utilize a short NR box motif to recognize the ligand-binding domain of the NR when it is activated by ligand. To investigate the ability of various ligands to specify the affinity of NR boxes for a ligand-bound NR, we compared the capacity of p160 NR boxes to be recruited to estrogen receptor (ERα) and ERβ in the presence of 17β-estradiol, diethylstilbestrol, and genestein. A time-resolved fluorescence-based binding assay was used to determine the dissociation constants for the 10 NR boxes derived from the three p160 coactivators for both ER subtypes in the presence of the each of the agonists. While the affinity of some NR boxes for ER was independent of the agonist, we identified several NR boxes that had significantly different affinities for ER depending on which agonist was bound to the receptor. Therefore, an agonist may specify the affinity of an NR for various NR boxes and thus regulate the coactivator selectivity of the receptor.

Correlation of farnesoid X receptor coactivator recruitment and cholesterol 7alpha-hydroxylase gene repression by bile acids

Cholesterol conversion to bile acids in the liver is regulated by the rate-limiting enzyme cholesterol 7alpha-hydroxylase (CYP7A1). CYP7A1 activity is regulated by feedback repression by bile acids at the transcriptional level. The farnesoid X receptor (FXR), a member of the nuclear hormone receptor superfamily, was recently demonstrated to function as the bile acid receptor and its high level of expression in the liver implicates it in the transcriptional regulation of CYP7A1. This study compares the potencies of various bile acids in their ability to mediate recruitment of the transcriptional coactivator protein, steroid receptor coactivator-1 (SRC-1), to the FXR ligand binding domain with their ability to repress CYP7A1 expression in HepG2 cells. A mammalian two-hybrid assay was utilized to assess the ability of FXR to recruit SRC-1 in a ligand-dependent manner. Chenodeoxycholic acid (CDCA) was the most potent and efficacious compound in the SRC-1 recruitment assay (EC(50) = 11.7 microM) followed by deoxycholic acid (DCA; EC(50) = 19.0 microM). Ursodeoxycholic acid (UDCA) displayed minimal activity while cholic acid (CA) was inactive. In order to directly compare the potencies of the bile acids in the coactivator recruitment assay to their ability to repress CYP7A1 expression, a branched DNA assay was developed to rapidly measure CYP7A1 mRNA levels from HepG2 cells cultured in 96-well plates. The rank order and absolute potency was conserved (CDCA IC(50) = 8.7 microM, DCA IC(50) = 27.2 microM, UDCA and CA inactive) consistent with bile acid repression of CYP7A1 being mediated by FXR.

A dominant-negative peroxisome proliferator-activated receptor gamma (PPARgamma) mutant is a constitutive repressor and inhibits PPARgamma-mediated adipogenesis

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) promotes adipocyte differentiation, exerts atherogenic and anti-inflammatory effects in monocyte/macrophages, and is believed to mediate the insulin-sensitizing action of antidiabetic thiazolidinedione ligands. As no complete PPARgamma antagonists have been described hitherto, we have constructed a dominant-negative mutant receptor to inhibit wild-type PPARgamma action. Highly conserved hydrophobic and charged residues (Leu(468) and Glu(471)) in helix 12 of the ligand-binding domain were mutated to alanine. This compound PPARgamma mutant retains ligand and DNA binding, but exhibits markedly reduced transactivation due to impaired coactivator (cAMP-response element-binding protein-binding protein and steroid receptor coactivator-1) recruitment. Unexpectedly, the mutant receptor silences basal gene transcription, recruits corepressors (the silencing mediator of retinoid and thyroid receptors and the nuclear corepressor) more avidly than wild-type PPARgamma, and exhibits delayed ligand-dependent corepressor release. It is a powerful dominant-negative inhibitor of cotransfected wild-type receptor action. Furthermore, when expressed in primary human preadipocytes using a recombinant adenovirus, this PPARgamma mutant blocks thiazolidinedione-induced differentiation, providing direct evidence that PPARgamma mediates adipogenesis. Our observations suggest that, as in other mutant nuclear receptor contexts (acute promyelocytic leukemia, resistance to thyroid hormone), dominant-negative inhibition by PPARgamma is linked to aberrant corepressor interaction. Adenoviral expression of this mutant receptor is a valuable means to antagonize PPARgamma signaling.

HIV protease inhibitors block human preadipocyte differentiation, but not via the PPARgamma/RXR heterodimer

A recent prospective clinical study has shown that antiviral therapy with HIV protease inhibitors (PIs) is associated with a syndrome of peripheral fat wasting (lipodystrophy) and disordered glucose and lipid metabolism (Carr et al. 1999). We have studied the effects of indinavir and saquinavir, two HIV protease inhibitors, on cultured primary human preadipocytes and report that these compounds inhibit their differentiation. However, we find that these agents do not inhibit either transcriptional activation or adipocyte P2 gene induction by the PPARgamma/RXR nuclear receptor heterodimer. Together, our findings suggest that impaired adipogenesis is the basis of PI-associated lipodystrophy, but that this occurs via a PPARgamma/RXR-independent mechanism.

PPARgamma activation induces the expression of the adipocyte fatty acid binding protein gene in human monocytes

The peroxisome-proliferator activated receptor gamma (PPARgamma), a member of the nuclear receptor superfamily of ligand activated transcription factors, plays a key role in the anti-diabetic actions of the thiazolidinediones (TZDs). PPARgamma induces the expression of many genes involved in lipid anabolism, including the adipocyte fatty acid binding protein (aP2), and is a key regulator of adipocyte differentiation. PPARgamma is also expressed in hematopoietic cells and is up-regulated in activated monocytes/macrophages. Activation of PPARgamma may play a role in the induction of differentiation of macrophages to foam cells that are associated with atherosclerotic lesions. We report that both natural and synthetic PPARgamma agonists induce time- and dose-dependent increases in aP2 mRNA in both primary human monocytes and the monocytic cell line, THP-1. These data suggest that PPARgamma activation may play a role in monocyte differentiation and function analogous to its well-characterized role in adipocytes.

A novel method for analysis of nuclear receptor function at natural promoters: peroxisome proliferator-activated receptor gamma agonist actions on aP2 gene expression detected using branched DNA messenger RNA quantitation

Peroxisome proliferator-activated receptor-gamma (PPARgamma), a member of the nuclear hormone receptor superfamily, plays an essential role in the mediation of the actions of antidiabetic drugs known as thiazolidinediones (TZDs). PPARgamma activates many target genes involved in lipid anabolism including the adipocyte fatty acid binding protein (aP2). In this study, induction of aP2 gene expression by PPARgamma agonists was examined in both cultured cells and diabetic mice using branched DNA (bDNA)-mediated mRNA quantitation. bDNA technology allows for the direct measurement of a particular mRNA directly within cellular lysate using a 96-well plate format in a time frame comparable to a reporter gene assay. In cultured human subcutaneous preadipocytes, the TZDs, troglitazone and BRL-49653, both rapidly induced aP2 mRNA as detected with the bDNA method. In these cells, the effect of BRL-49653 on aP2 mRNA levels was detectable as early as 30 min after treatment (47% increase) and was maximal after 24 h of treatment (12-fold increase). The effects of troglitazone on aP2 mRNA induction were similar to those of BRL-49653 except that the maximal level of induction was consistently lower (e.g. 24 h treatment = 4-fold increase). Dose-response relationships for both of the TZDs were also determined using the 24-h treatment time point. EC50s for both BRL-49653 and troglitazone were estimated to be 80 nM and 690 nM, respectively. A natural PPARgamma ligand, 15-deoxy-delta12,14-PGJ2, was also active in this assay with a maximal induction of aP2 mRNA of approximately 5-fold when tested at 1 microM. Since the PPARgamma:retinoid X receptor (RXR) heterodimer has been characterized as a permissive heterodimer with respect to RXR ligands, the ability of 9-cis-retinoic acid (9-cis-RA) to induce aP2 mRNA was examined. Although 9-cis-RA had very low efficacy (2-fold induction), the maximal effect was reached at 100 nM. No synergism or additivity in aP2 mRNA induction was detected when 9-cis-RA was included with either of the TZDs used in this study. Significant induction of aP2 mRNA in bone marrow of db/db mice treated with either troglitazone or BRL-49653 was also detected, indicating that the bDNA assay may be a simple method to monitor nuclear receptor target gene induction in vivo.

DAX1 mutations map to putative structural domains in a deduced three-dimensional model

The DAX1 protein is an orphan nuclear hormone receptor based on sequence similarity in the putative ligand-binding domain (LBD). DAX1 mutations result in X-linked adrenal hypoplasia congenita (AHC). Our objective was to identify DAX1 mutations in a series of families, to determine the types of mutations resulting in AHC and to locate single-amino-acid changes in a DAX1 structural model. The 14 new mutations identified among our 17 families with AHC brought the total number of families with AHC to 48 and the number of reported mutations to 42; 1 family showed gonadal mosaicism. These mutations included 23 frameshift, 12 nonsense, and six missense mutations and one single-codon deletion. We mapped the seven single-amino-acid changes to a homology model constructed by use of the three-dimensional crystal structures of the thyroid-hormone receptor and retinoid X receptor alpha. All single-amino-acid changes mapped to the C-terminal half of the DAX1 protein, in the conserved hydrophobic core of the putative LBD, and none affected residues expected to interact directly with a ligand. We conclude that most genetic alterations in DAX1 are frameshift or nonsense mutations and speculate that the codon deletion and missense mutations give insight into the structure and function of DAX1.

Betidamino acid scan of the GnRH antagonist acyline

Strong clinical evidence suggests that GnRH antagonists will replace GnRH agonists in a number of indications because of their ability to inhibit gonadotropin secretion as long as an adequate concentration of the analogue is present in the circulation whereas superagonists will take approximately 2 weeks to desensitize the gonadotrophs. Until recently, antagonists were either too weak and/or would release histamine. Azaline B {[Ac-D2Nal1,D4Cpa2,D3Pal3, 4Aph5(atz),D4Aph6(atz),ILys8,DAla10] GnRH} and long-acting members of the azaline family {Ac-D2Nal-D4Cpa-D3Pal-Ser-4Aph(X)-D4Aph(Y) -Leu-ILys-Pro-DAla-NH2}, however, appear to be promising drug candidates. Because these antagonists tend to form gels (due to the formation of beta-sheet structures) and, as a result, are not readily amenable to formulation for long-term delivery, we have investigated ways of increasing hydrophilicity while retaining high potency and lack of histamine releasing activity. Betidamino acids (a contraction of "beta" position and "amide") are N'-monoacylated (optionally, N'-monoacylated and N-mono- or N,N'-dialkylated) aminoglycine derivatives in which each N'-acyl/alkyl group may mimic naturally occurring amino acid side chains or introduce novel functionalities. We have used unresolved N alpha-Boc,N'alpha-Fmoc-aminoglycine, and N alpha-Boc,N'alpha-(CH3)Fmoc-aminoglycine as templates for the introduction of betidamino acids in acyline (Ac-D2Nal-D4Cpa-D3Pal-Ser-4Aph(Ac)-D4Aph(A c)-Leu-Ilys-Pro-DAla-NH2), a long acting member of the azaline B family, to test biocompatibility of these betide derivatives. Diastereomeric peptides could be separated using RP-HPLC in most cases. Biological results obtained in vitro (binding affinity to rat pituitary gland membranes) and in vivo (rat antiovulatory assay, AOA) indicate in most cases small differences in relative potencies (< 5-fold) between the D- and L-nonalkylated betidamino acid-containing acylines. Importantly, most betide diastereomers have high affinity for the GnRH receptor and were equipotent with acyline in the AOA. Greater differences in affinity and potency between diastereomers were observed after introduction of a methyl group on the side chain nitrogen ("beta" position) of the same analogues, with one of the diastereomer having an affinity and a potency in the AOA equivalent to that of acyline. These results suggest that chirality at the alpha-carbon coupled to side chain orientation is important for receptor recognition. The duration of action of some of the most potent analogues was also determined in the castrated male rat in order to measure the extent (efficacy and duration of action) of inhibition of luteinizing hormone release. Data suggest that introduction of a betidamino acid results in reduction of duration of action. Also, introduction of betidamino acids results in peptides with increased hydrophilicity (as determined by elution times on C18 silicas at pH 7.3) compared to that of the parent compound. N'-Methyl substitution results in parallel increase in retention times on C18 silicas as expected.

Ahch, the mouse homologue of DAX1: cloning, characterization and synteny with GyK, the glycerol kinase locus

We cloned the murine full-length cDNA encoding Ahch, the mouse homologue of DAX1 (DSS-AHC Region on Human X Chromosome, Gene1) which is the gene responsible for human X-linked adrenal hypoplasia congenita (AHC) and hypogonadotropic hypogonadism (HH). Sequence analysis revealed that the murine and human cDNAs have 65% aa identity and 75% aa similarity overall. The cysteine residues in the putative DNA binding domain, which may interact with Zn2+ ions to form zinc fingers, are 100% conserved between the two species, indicating that the novel zinc-finger structures in DAX1 may be functional. In addition, mouse interspecific backcrosses show that the Ahch gene is closely linked to the glycerol kinase locus, GyK, on the mouse X chromosome, indicating that the order of the loci is conserved in this syntenic region between mouse and human.

Genomic sequence of the DAX1 gene: an orphan nuclear receptor responsible for X-linked adrenal hypoplasia congenita and hypogonadotropic hypogonadism

The gene responsible for X-linked adrenal hypoplasia congenita, DAX1, encodes a member of the nuclear hormone receptor superfamily. We sequenced 8851 bp that contained the DAX1 genomic region. The DAX gene was composed of two exons and one 3.4-kilobase intron. Putative TATA and GC boxes and a putative steroidogenic factor 1 response element were present in the 5'-flanking region. Two potentially polymorphic short tandem repeats were identified. The first exon encoded two putative novel zinc finger motifs within a putative DNA binding domain and part of the ligand binding domain, and the second exon encoded the remainder of the ligand binding domain. Although the putative DNA binding domain of DAX1 does not contain substantial sequence similarity to other nuclear hormone receptor superfamily members, the putative ligand binding domain had remarkable similarity to other family members. Single-strand conformational polymorphism analysis permitted identification of three new mutations in DAX1. In conclusion, single-strand conformational polymorphism analysis facilitates identification of mutations in the DAX1 gene, and the short tandem repeats may permit linkage analysis in families in which mutations are not yet identified. We speculate that DAX1 may be the most primitive member of the nuclear hormone receptor superfamily identified in mammals.

A nuclear hormone receptor-associated protein that inhibits transactivation by the thyroid hormone and retinoic acid receptors

Nuclear hormone receptors are transcription factors that require multiple protein-protein interactions to regulate the expression of their target genes. Using the yeast two-hybrid system, we identified a protein, thyroid hormone receptor uncoupling protein (TRUP), that specifically interacts with a region of the human thyroid hormone receptor (TR) consisting of the hinge region and the N-terminal portion of the ligand binding domain in a hormone-independent manner. Interestingly, TRUP inhibits transactivation by TR and the retinoic acid receptor but has no effect on the estrogen receptor or the retinoid X receptor in mammalian cells. We also demonstrate that TRUP exerts its action on TR and retinoic acid receptor by interfering with their abilities to interact with their DNA. TRUP represents a type of regulatory protein that modulates the transcriptional activity of a subclass of the nuclear hormone receptor superfamily by preventing interaction with their genomic response elements.

Expression of DAX-1, the gene responsible for X-linked adrenal hypoplasia congenita and hypogonadotropic hypogonadism, in the hypothalamic-pituitary-adrenal/gonadal axis

DAX-1, an orphan member of the nuclear hormone receptor superfamily, is responsible for X-linked adrenal hypoplasia congenita (AHC) and the frequently associated hypogonadotropic hypogonadism (HH). The entire DAX-1 genomic region has been sequenced and a putative steroidogenic factor-1 response element has been identified in the promoter region of the gene. The purpose of these investigations was to determine if DAX-1 was expressed in the central nervous system, particularly the hypothalamus and pituitary, in order to better understand the relationship of mutations in this gene to HH associated with AHC. We used Northern blot analysis and reverse transcription PCR to demonstrate that DAX-1 was expressed in the hypothalamus and the pituitary, and to confirm its expression in adrenal cortex and gonads. The expression of DAX-1 in these tissues indicates the involvement of DAX-1 in the development of the reproductive system at multiple levels within the hypothalamic-pituitary-adrenal/gonadal axis. We also observed the expression of DAX-1 in a human adrenocortical carcinoma cell line, NCI-H295, that has features characteristic of the fetal adrenal cortex. Therefore, NCI-H295 cells will be a useful cellular model for investigating the involvement of DAX-1 in the regulation of steroidogenesis.

Identification of a putative steroidogenic factor-1 response element in the DAX-1 promoter

The nuclear hormone receptor, DAX-1, is responsible for X-linked adrenal hypoplasia congenita and hypogonadotrophic hypogonadism. We recently cloned the 5' flanking region of the human DAX-1 gene and in this report we describe the identification of a putative steroidogenic factor 1 (SF-1) response element approximately 110 bases upstream of the TATA box. Both DAX-1 and SF-1 are expressed in similar tissues including the adrenal cortex, gonads, hypothalamus, and the pituitary gland. Like DAX-1, SF-1 expression has been shown to be essential for the development of the adrenal cortex. We demonstrate that SF-1 is able to efficiently bind to the putative SF-1 response element found in the DAX-1 promoter in vitro. This suggests that SF-1 may directly regulate the expression of DAX-1 and that these two transcription factors may be components of a cascade required for development of steroidogenic tissues.

The tau 4 activation domain of the thyroid hormone receptor is required for release of a putative corepressor(s) necessary for transcriptional silencing

The C terminus of nuclear hormone receptors is a complex structure that contains multiple functions. We are interested in the mechanism by which thyroid hormone converts its receptor from a transcriptional silencer to an activator of transcription. Both regulatory functions are localized in the ligand binding domain of this receptor superfamily member. In this study, we have identified and characterized several functional domains within the ligand binding domain of the human thyroid hormone receptor (TR beta) conferring transactivation. Interestingly, these domains are localized adjacent to hormone binding sites. One activation domain, designated tau 4, is only 17 amino acids in length and is localized at the extreme C terminus of TR. Deletion of six amino acids of tau 4 resulted in a receptor that could still bind hormone but acted as a constitutive silencer, indicating that tau 4 is required for both transactivation and relief of the silencing functions. In addition, we performed in vivo competition experiments, the results of which suggest that in the absence of tau 4 or hormone, TR is bound by a corepressor protein(s) and that one role of hormone is to release corepressor from the receptor. We propose a general model in which the role of hormone is to induce a conformational change in the receptor that subsequently affects the action of tau 4, leading to both relief of silencing and transcriptional activation.

The yeast SIN3 gene product negatively regulates the activity of the human progesterone receptor and positively regulates the activities of GAL4 and the HAP1 activator

The activation of gene transcription in eukaryotic organisms is regulated by sequence-specific DNA-binding proteins as well as by non-DNA-binding proteins. In this report we describe the modulatory functions of a non-DNA-binding protein, SIN3 (also known as SDI1, UME4, RPD1, and GAM2) on the transactivation properties of the human progesterone receptor (hPR), GAL4, and the HAP1 activator in yeast. Our data suggest that SIN3 is a dual function protein. It negatively regulates the transcriptional activities of hPR-A and hPR-B by affecting the N-terminal activation domain (AF1). SIN3 positively regulates the transcriptional activities of GAL4 and the HAP1 activator. However, it has no effect on the transcriptional activities of the human glucocorticoid receptor (hGR) and GCN4. The SIN3 protein contains four copies of a paired amphipathic helix (PAH) motif. Deletion analysis of the SIN3 PAH motifs shows that the PAH3 motif is essential for SIN3-mediated regulation of hPR, GAL4, and the HAP1 activator. In contrast, the PAH1, PAH2, and PAH4 motifs are not required for SIN3-mediated regulation of these activators. Additionally, we examined the mechanism(s) by which the SIN3 protein modulate the activities of various activators. We are unable to demonstrate the direct interaction of SIN3 protein with these activators using the yeast two-hybrid system or co-immunoprecipitation. These data suggest that SIN3 regulates the transactivation functions of hPR, GAL4, and the HAP1 activator by an indirect mechanism.

Comparison of the forms of the dopamine D2 receptor expressed in GH4C1 cells

The effects of dopamine (DA) on prolactin (PRL) secretion, phosphoinositide metabolism, cytosolic calcium concentrations ([Ca2+]i), and cAMP production in GH4C1 cells expressed either the short (GH4ZR7 cells) or long (GH4I12 cells) form of the rat DA D2 receptor were compared in this study. The GH4C1 cell line is derived from the rat anterior pituitary gland and lacks functional DA receptors. The GH4ZR7 and GH4I12 cell lines have been transfected with either the short or the long form of the D2 receptor, respectively. In this study, the functional coupling of these receptors to both basal and stimulated PRL secretion and to common second messenger systems was examined. Both cell types expressing DA receptors exhibited similar saturable binding to the D2 antagonist [3H]spiperone (Kd GH4ZR7 = 96 +/- 8 pM, Kd GH4I12 = 107 +/- 49 pM). In GH4ZR7 cells, 1 and 10 microM DA inhibited basal PRL secretion by 37% and 58%, respectively. In GH4I12 cells, 1 and 10 microM DA inhibited basal PRL secretion by 63% and 54%, respectively. In GH4ZR7 cells, 10 microM DA completely reversed the stimulatory effects of 1-100 nM thyrotropin-releasing hormone (TRH), and 1 microM DA attenuated the stimulatory effects of 10 and 100 nM TRH. Interestingly, GH4I12 cells were not responsive to TRH. In both cell lines, the inhibitory effects of DA were blocked by the specific D2 antagonist, eticlopride. The stimulatory effects of TRH on [Ca2+]i were dose dependent and could be blocked (at least in GH4ZR7 cells) by prior treatment of the cells with 1 microM DA. The ability of dopamine to block the TRH-mediated increase in [Ca2+]i was attenuated by eticlopride. DA (1 microM) had no effect on resting [Ca2+]i in either cell line expressing DA receptor. TRH (100 nM) maximally stimulated total inositol phosphate (IP) accumulation to values approximately three times greater than controls in GH4C1 and GH4ZR7 cells only. DA had no effect on basal or TRH-stimulated IP accumulation in any of the cell lines. DA (1 and 10 microM) inhibited cAMP production by 40% and 39%, respectively, in GH4ZR7 cells. Similarly, in GH4I12 cells, DA (1 and 10 microM) inhibited cAMP production by 48% and 60%, respectively. These data indicate that both the long and short forms of the D2 receptors play similar roles when expressed in GH4C1 cells. However, maximal inhibition of PRL in these cell lines is approximately 20% less than that of normal lactotrophs.

Low concentrations of dopamine increase cytosolic calcium in lactotrophs

Dopamine (DA) plays a dual role in the neuroendocrine regulation of PRL secretion from the anterior pituitary gland. DA normally exerts a tonic inhibitory effect on PRL secretion. However, DA at concentrations much lower than those required for maximal inhibition of PRL secretion actually stimulates PRL secretion. In this study, the effects of stimulatory concentrations of DA on common second messenger systems were examined in pituitary cells enriched for lactotrophs in an attempt to identify the signal transduction mechanisms used in DAergic stimulation of PRL secretion. Rat pituitary cells were enriched for lactotrophs (88% lactotrophs) by differential sedimentation on a discontinuous Percoll gradient. Enriched cells in monolayer culture were responsive to both inhibitory and stimulatory doses of DA; 10 microM DA inhibited PRL secretion by 80%, and 0.1 nM DA stimulated PRL secretion by 40% after a 4-h incubation. No dose of DA (10 pM to 10 microM) had an effect on phosphatidylinositol turnover. However, 1 and 10 microM DA inhibited cAMP formation by 58% and 72%, respectively. DA at doses in the stimulatory range had no effect on cAMP formation. Single cell cytosolic calcium concentrations ([Ca2+]i) were examined by quantitative fluorescence microscopy using fura-2 as a probe. Inhibitory doses of DA (1 microM) decreased [Ca2+]i in 83% of the cells examined. Two subpopulations of cells were noted that varied in the degree of response to 1 microM DA. One subpopulation responded to DA by decreasing [Ca2+]i 50-100 nM. The other subpopulation responded to DA by decreasing [Ca2+]i 300-400 nM. A stimulatory dose of DA (0.1 nM) increased [Ca2+]i in 44% of the cells examined. This dose of DA increased [Ca2+]i by 100-400 nM. These data indicate that the second messenger mediating DAergic stimulation of PRL secretion is most likely Ca2+.

Infusion of dopamine at low concentrations stimulates the release of prolactin from alpha-methyl-p-tyrosine-treated rats

Prolactin (PRL) secretion from the anterior pituitary gland is inhibited by dopamine (DA) released into the hypophysial portal vasculature from neurons in the hypothalamus. We have shown previously that DA also stimulates PRL secretion in vitro. Here we report that DA has a dual effect on PRL release in vivo. Injection of rats with alpha-methyl-p-tyrosine (200 mg/kg, ip) induced an immediate 35-fold enhancement of PRL secretion which reached a plateau by 90 min after injection on diestrus 1. When DA was infused intravenously at varying doses beginning at 90 min after alpha-methyl-p-tyrosine, differing effects on PRL secretion were observed. These effects were dose dependent: higher doses of DA (1000 ng/kg/min) inhibited and lower doses (10 ng/kg/min) stimulated PRL secretion. These data suggest that DA may be an important stimulator of PRL secretion in vivo.

The stimulatory and inhibitory effects of dopamine on prolactin secretion involve different G-proteins

The reverse hemolytic plaque assay (RHPA) was used in this study to further characterize the mechanism whereby low concentrations of dopamine (DA) stimulate PRL secretion in vitro. Female Sprague-Dawley rats were used as a source of anterior pituitary cells for the RHPA. Pituitary cells were infused into Cunningham chambers along with a suspension of protein-A-coated ovine red blood cells. Excess cells were rinsed from the chambers leaving a monolayer of cells attached to the glass. The cells were then incubated with solutions containing PRL antiserum (1:40) and various concentrations of DA. After 4 h, a solution containing guinea pig complement (1:60) was infused into the chambers. Thirty minutes later, the cells were fixed and plaques (zones of hemolysis) surrounding PRL-producing cells (lactotrophs) were measured and used as an index of the amount of PRL secreted. Control cells that received no DA had a mean plaque area of 8,000 microns 2 and two distinct subpopulations of plaque sizes. This biphasic population of cells consisted of a small and a large plaque producing population. The mean plaque area surrounding lactotrophs was significantly (P less than 0.05) decreased if 1 microM or 10 microM DA was present (4,500 microns 2 and 3,500 microns 2, respectively). These cells which received inhibitory concentrations of DA demonstrated a monophasic distribution of plaque-forming cells. On the other hand, mean plaque area was significantly (P less than 0.05) increased if 0.1 nM or 1 nM DA was presented to the cells (15,000 microns 2 and 14,500 microns 2, respectively). These cells receiving stimulatory doses of DA exhibited a multiphasic distribution of plaque-forming cells. The possibility that the two physiological opposing actions of DA on PRL secretion might be mediated by different GTP binding proteins was also examined using cholera toxin (CTX) and pertussis toxin (PTX). Anterior pituitary cells were pretreated with either CTX (50 micrograms/ml) or PTX (5 micrograms/ml) for 1 h before initiation of the RHPA. In the RHPA, cells received no DA, a stimulatory dose of DA (0.1 nM), or a inhibitory dose of DA (10 microM). The effects of toxin pretreatment on mean plaque area of DA-treated cells was determined. PTX pretreatment significantly attenuated the inhibitory effects of DA while having no effect on the stimulatory effects of DA on PRL secretion. CTX significantly (P less than 0.05) potentiated the stimulatory effects of DA on PRL secretion and had no effect on inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of endothelins on the secretion of prolactin, luteinizing hormone, and follicle-stimulating hormone are mediated by different guanine nucleotide-binding proteins

Different bacterial toxins capable of modifying specific alpha-subunits of G-proteins were used to characterize the guanine nucleotide-binding protein (G-protein) dependency of the effects of endothelins (ETs) on PRL, LH, and FSH secretion. Primary cultures of anterior pituitary cells obtained from female rats were preincubated for 24 h with 20 ng/ml pertussis toxin (PTX) or 2 micrograms/ml cholera toxin (CTX) before challenge with ETs. Both ET-1 and ET-3 elicited a concentration-dependent inhibition of PRL secretion and stimulated the release of LH and FSH secretion on pituitary cells not treated with toxins. Based on the calculated ration of the half-maximal effective concentrations (EC50) of ET-1 and ET-3, ET-1 showed 7800, 20, and 14 times greater potency than ET-3 on PRL, LH, and FSH secretion, respectively. PTX, a selective inhibitor of Gi and several other G proteins, increased the basal secretion of PRL and completely eliminated the responsiveness of lactotroph cells to ET-1 and ET-3. Pretreatment with PTX caused a markedly different effect on LH and FSH secretion: while basal LH release was slightly increased, FSH secretion was markedly depressed by PTX. Moreover, while ET-induced LH secretion was enhanced by PTX, the effectiveness of ETs on FSH release was completely abolished. CTX, known as an activator of Gs proteins, decreased the basal secretory activity of lactotrophs but did not influence the ET-induced decrease of PRL release. CTX pretreatment (like PTX before) elicited a strikingly different effect on LH and FSH: while basal LH secretion was enhanced, basal FSH secretion was markedly inhibited by CTX. Moreover, while the effectiveness of ETs on LH secretion was not changed significantly, the stimulatory effect of ETs on FSH secretion was diminished after CTX pretreatment. Thus, the inhibition of PRL secretion by ETs requires a PTX-sensitive G protein while the ET-induced stimulation of FSH secretion involves both PTX- and CTX-sensitive elements. The fact that pretreatments with PTX or CTX influenced basal secretion of PRL, LH, and FSH suggests that PTX- and/or CTX-sensitive G proteins are directly involved in the process of exocytosis. Additionally, these findings might indicate an active paracrine/autocrine regulation of pituitary cells in culture that are impaired or enhanced by the bacterial toxins employed. Though the broad substrate specificity of PTX and CTX and the multiplicity of G protein families did not allow us to identify the specific G protein(s) involved, these data reveal the diversity of ET-induced intracellular signaling mechanisms in lactotrophs and gonadotrophs.

Pharmacologic evidence that a D2 receptor subtype mediates dopaminergic stimulation of prolactin secretion from the anterior pituitary gland

The ability of low concentrations of dopamine (DA) to stimulate the secretion of prolactin (PRL) was examined in perifused or monolayer cultures of anterior pituitary cells. In cultures perifused with media containing 100 nM DA, changing the DA concentration to either 1 or 100 pM caused a significant dose-dependent stimulatory PRL secretory response within 6 min when compared to the PRL secretory response to removal of DA altogether. Picomolar concentrations of DA caused a biphasic PRL secretory response. This response is characterized by an immediate increase in the rate of PRL secretion similar to that seen when the cells were treated with 100 nM thyrotropin-releasing hormone followed by a decrease in the rate of PRL secretion to levels comparable to cells receiving media alone. In a monolayer culture system DA, at concentrations between 10 nM and 1.0 pM, caused significant stimulation of PRL secretion relative to media alone. Maximal stimulation occurred at nanomolar concentrations of DA (approximately 60% greater than control). Although the D2 agonists, bromocriptine and 2-(N-phenethyl-N-propyl)-amino-5-hydroxytetralin hydrochloride (PPHT) caused significant (p less than 0.05) inhibition of PRL secretion at nanomolar concentrations and above, neither had stimulatory activity. The D1 agonists, SKF 38393 and SKF 82958, had no effect on PRL secretion when tested at 0.1 pM to 1 microM. These data suggest that DA not only inhibits PRL secretion in vitro, but also stimulates PRL secretion at relatively low concentrations. Stimulation is mediated by a DA receptor which is neither recognized by D2 nor D1 agonists, suggesting a possible third DA receptor subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of prolactin secretion by endothelin-3 is pertussis toxin-sensitive

The effect of pertussis toxin (PTX) pretreatment on endothelin-3 (ET-3)-mediated inhibition of prolactin secretion from primary cultures of rat anterior pituitary cells was examined. Monolayer cultures of anterior pituitary cells were treated with either 20 ng/ml PTX dissolved in media or with media alone (control) on the third day of culture. Exactly 24 h after PTX pretreatment, cells were challenged with either 100 nM ET-3 dissolved in media or media alone (control) for 4 or 48 h. ET-3 significantly (P less than 0.01) inhibited prolactin secretion in both the 4 and 48 h incubations. However, if the cells had been previously treated with PTX, ET-3 did not significantly affect prolactin secretion. These data suggest that a PTX-sensitive G protein mediates ET-3-induced inhibition of prolactin secretion and that ET-3 may invoke a signal transduction mechanism in the lactotroph which is distinct from those described in other cell types.

Endothelin-3 inhibits prolactin and stimulates LH, FSH and TSH secretion from pituitary cell culture

The influence of endothelin-3 (ET-3) on anterior pituitary hormone secretion was investigated over a wide range of concentrations (from 10(-14) to 10(-6) M) and incubation times (from 4 to 48 hours). ET-3 elicited a concentration-dependent inhibition of prolactin (PRL) secretion and stimulated the release of luteinizing hormone (LH), follicle stimulating hormone (FSH) and thyroid stimulating hormone (TSH) from primary monolayer cultures of anterior pituitary cells derived from female rats. The responsiveness of different pituitary cells to ET-3 differs markedly in terms of onset and duration: the maximal inhibition of PRL secretion occurred after 12 hours and the stimulation of LH, FSH and TSH reached the maximum after 4, 48 and 48 hours of incubation, respectively. These data corroborate the concept that ET-3 has an important role as a neuroendocrine modulator. Moreover, the data presented suggest different intracellular mechanisms underlying ET-3 actions.