Dysregulation of Aromatase in Breast, Endometrial, and Ovarian Cancers: An Overview of Therapeutic Strategies.

Aromatase is the rate-limiting enzyme in the biosynthesis of estrogens, which play crucial roles on a spectrum of developmental and physiological processes. The biological actions of estrogens are classically mediated by binding to two estrogen receptors (ERs), ERα and ERß. Encoded by the cytochrome P450, family 19, subfamily A, polypeptide 1 (CYP19A1) gene, aromatase is expressed in a wide variety of tissues, as well as benign and malignant tumors, and is regulated in a pathway- and tissue-specific manner. Overexpression of aromatase, leading to elevated systemic levels of estrogen, is unequivocally linked to the pathogenesis and growth of a number malignancies, including breast, endometrium, and ovarian cancers. Aromatase inhibitors (AIs) are routinely used to treat estrogen-dependent breast cancers in postmenopausal women; however, their roles in endometrial and ovarian cancers remain obscure. While AI therapy is effective in hormone sensitive cancers, they diminish estrogen production throughout the body and, thus, generate undesirable side effects. Despite the effectiveness of AI therapy, resistance to endocrine therapy remains a major concern and is the leading cause of cancer death. Considerable advances, toward mitigating these issues, have evolved in conjunction with a number of histone deacetylase (HDAC) inhibitors for countering an assortment of diseases and cancers, including the aforesaid malignancies. HDACs are a family of enzymes that are frequently dysregulated in human tumors. This chapter will discuss the current understanding of aberrant regulation and expression of aromatase in breast, endometrial, and ovarian cancers, and potential therapeutic strategies for prevention and treatment of these life-threatening diseases.

Regulation of the steroidogenic acute regulatory protein expression: functional and physiological consequences.

Steroid hormones are synthesized in steroidogenic cells of the adrenal, ovary, testis, placenta and brain and are essential for normal reproductive function and bodily homeostasis. The rate-limiting and regulated step in steroid biosynthesis is the intramitochondrial transport of cholesterol, a process that is mediated by the steroidogenic acute regulatory (StAR) protein. The importance of StAR has been illustrated by analyses of patients with lipoid congenital adrenal hyperplasia (lipoid CAH), an autosomal recessive disorder that markedly disrupts the synthesis of all gonadal and adrenal steroids. Molecular and physio-pathological analyses have demonstrated that alterations in the StAR gene are the only known cause of lipoid CAH. Furthermore, StAR knockout mice have been generated and display a phenotype that is essentially identical to the human condition. Recent advances in tissue-specific and hormone-induced expression of the StAR protein provide insights into a number of human endocrinological health issues including developmental and reproductive abnormalities. Several factors and processes have been demonstrated to influence StAR expression in steroidogenic cells and there is increasing evidence that a transcription factor-binding site-rich region present in the proximal region of the StAR promoter is highly instrumental in StAR gene expression. In this review we focus on the significant findings that have been made with regards to the regulation of StAR expression and also on the clinical and endocrinological consequences of a non-functioning StAR gene.

Biphasic action of prolactin in the regulation of murine Leydig tumor cell functions.

We investigated in this study the effects of ovine PRL on endocrine functions of cultured murine Leydig tumor cells (mLTC-1). The parameters studied were the activation of signal transduction systems involving cAMP and intracellular free Ca(2+), the expression of Janus kinase 2 (JAK2), expression and function of LH and PRL receptors (R), expression of the steroidogenic acute regulatory (StAR) protein, and stimulation of steroidogenesis. Very similar biphasic dose- and time-dependent responses of all the parameters studied were found upon PRL stimulation, comprising a fast inhibition within 24 h in response to high PRL doses (>/=30 microgram/liter), and a slow stimulation, between 48-72 h, in response to lower PRL doses (1-10 microgram/liter). In addition, extracellular Ca(2+) (1.5 mmol/liter) increased the effect of PRL on human CG (hCG)-stimulated StAR messenger RNA expression and progesterone (P) production. Importantly, the biphasic effects of PRL on LHR gene expression and hCG-mediated P production were abolished in the presence of anti-PRL antiserum, demonstrating specificity of PRL action. The PRL effects on StAR expression, and steroid and cAMP production, apparently reflect its effects on LHR function. The relevance of the PRL effects observed in mLTC-1 cells was supported by demonstration of similar PRL responses in hCG-stimulated testosterone production of isolated mouse Leydig cells. Collectively, these findings clearly demonstrate the biphasic regulatory actions of PRL, and clarify some facets of the controversial role of this hormone in Leydig cell function.

Functional assessment of the calcium messenger system in cultured mouse Leydig tumor cells: regulation of human chorionic gonadotropin-induced expression of the steroidogenic acute regulatory protein.

The steroidogenic acute regulatory (StAR) protein, a 30-kDa mitochondrial factor, is a key regulator of steroid hormone biosynthesis, facilitating the transfer of cholesterol from the outer to the inner mitochondrial membrane. StAR protein expression is restricted to steroidogenic tissues, and it responds to hormonal stimulation through different second messenger pathways. The present study was designed to explore the mechanisms of extracellular calcium (Ca2+) involved in the hCG-stimulated expression of StAR protein and steroidogenesis in a mouse Leydig tumor cell line (mLTC-1). Extracellular Ca2+ (1.5 mmol/liter) enhanced the hCG (50 microg/liter)-induced increases in StAR messenger RNA (mRNA) and protein levels (1.7 +/- 0.3-fold; 4 h), as monitored by quantitative RT-PCR and immunoblotting. The potentiating effect of Ca2+ on the hCG-stimulated StAR response correlated with the acute progesterone (P) response. In accordance, omission of Ca2+ from the extracellular medium by specific Ca2+ chelators, EDTA or EGTA (4 mmol/liter each), markedly diminished the hCG-stimulated P production. The Ca2+ effect on hCG-induced StAR mRNA expression was dramatically suppressed by 10 micromol/liter verapamil, a Ca2+ channel blocker. The Ca2+-mobilizing agonist, potassium (K+; 4 mmol/liter), greatly increased the hCG responses of StAR expression and P production, which conversely were attenuated by Ca2+ antagonists, further supporting the involvement of intracellular free Ca2+ ([Ca2+]i) in these responses. The interaction of Ca2+ or K+ with hCG accounted for a clear increase in the StAR protein level (1.4-1.8-fold; 4 h) compared with that after hCG stimulation. Inhibition of protein synthesis by cycloheximide (CHX) drastically diminished the hCG-induced StAR protein content, indicating the requirement for on-going protein synthesis for hCG action. The transmembrane uptake of 45Ca2+ was increased by 26% with hCG and was strongly inhibited by verapamil. [Ca2+]i moderately augmented the response to hCG in fura-2/AM-loaded mLTC-1 cells within 30-40 sec, reaching a plateau within 1-3 min. Interestingly, the calcium ionophore (A23187) clearly increased (P < 0.01) StAR mRNA expression, in additive fashion with hCG. Northern hybridization analysis revealed four StAR transcripts at 3.4, 2.7, 1.6, and 1.4 kb, with the 1.6-kb band corresponding to the functional StAR protein; all of them were up-regulated 3- to 5-fold upon hCG stimulation, with a further increase in the presence of Ca2+. The mechanism of the Ca2+ effect on hCG-stimulated StAR expression and P production was evaluated by assessing the involvement of the nuclear orphan receptor, steroidogenic factor 1 (SF-1). Stimulation of hCG significantly elevated (2.1 +/- 0.3-fold) the SF-1 mRNA level, which was further augmented in the presence of Ca2+, whereas EGTA and verapamil completely abolished the increase caused by Ca2+. Cells expressing SF-1 marginally increased StAR expression, but coordinately elevated StAR mRNA levels in response to hCG and hCG plus Ca2+ compared with those in mock-transfected cells. On the other hand, overexpression of the nuclear receptor DAX-1 remarkably diminished (P < 0.0001) the endogenous SF-1 mRNA level as well as hCG-induced StAR mRNA expression. In summary, our results provide evidence that extracellular Ca2+ rapidly increases [Ca2+]i after hCG stimulation, presumably through opening of the transmembrane Ca2+ channel. Neither extracellular Ca2+ nor K+ alone has a noticeable effect on StAR expression and steroidogenesis, whereas they clearly potentiate hCG induction. The Ca2+-mediated increase in hCG involved in StAR expression and P production is well correlated to the levels of SF-1 expression. The stimulatory effect of hCG that rapidly increases [Ca2+]i is responsible at least in part for the regulation of SF-1-mediated StAR expression that consequently regulates steroidogenesis in mouse Leydig tumor cells.

Progesterone action in a murine Leydig tumor cell line (mLTC-1), possibly through a nonclassical receptor type.

In a recent report we demonstrated that a high (micromolar) concentration of progesterone (P) specifically down-regulates LH receptor (R) expression and function in murine Leydig tumor cells. The aim of the present study was to characterize further the putative novel R, mediating these P effects in the murine Leydig tumor cell line, mLTC-1. The binding of [3H]P to these cells revealed a high (Kd, approximately 9.3 nmol/liter) and a low affinity (Kd, approximately 284 nmol/liter) component, and the binding displayed with specificity (P > dehydroepiandrosterone > 17-OHP). The binding was apparently different from that of the classical nuclear PR in the following ways. 1) The P/glucocorticoid antagonist RU 486 did not compete with [3H]P binding to the mLTC-1 cells. 2) No expression of the classical PR messenger RNA was detected, despite clear P binding to these cells, by Northern hybridization or RT-PCR. 3) An antibody against the C-terminal end of the classical PR (alpha c-262) revealed in mLTC-1 cells several molecular size protein bands between 45-57 kDa on Western hybridization, whereas these immunoreactive proteins were faintly recognized by another antibody (alpha-PR) directed toward the NH2-terminal region of the classical PR. The sizes of the immunoreactive molecules were relatively similar to those detected using the same antibodies in human sperm lysates, but were at variance with the classical PR (120, 94, and 60 kDa), detected with these antibodies in human uterus. The immunoreactive proteins bound peroxidase-labeled-P, which could be displaced in the presence of a 10-fold excess of free P. 4) An immediate increase in the intracellular free calcium level was observed after P treatment in cultured mLTC-1 cells, whereas it also increased the 45Ca2+ entry within 15 min in these cells. 5) Increasing doses of P (0.1-10 micromol/liter) demonstrated significant inhibition of LH receptor messenger RNA levels in a dose-dependent manner in mLTC-1 cells. In conclusion, a nonclassical PR is expressed and functional in these cells, and it is clearly distinct from the classical nuclear PR. It is apparent that recently reported inhibitory effects of P on LH receptor gene expression and function are mediated through this novel type PR in mouse Leydig cells.

Cloning and functional expression of the luteinizing hormone receptor complementary deoxyribonucleic acid from the marmoset monkey testis: absence of sequences encoding exon 10 in other species.

Based on sequence homologies among the human, porcine, rat, and mouse genes for the LH receptor (LHR), overlapping partial fragments of LHR complementary DNAs (cDNAs) were multiplied from marmoset monkey testicular RNA using reverse transcription-PCR. Ligations of the individual cDNA fragments generated a full-length monkey LHR cDNA (2031 bp) containing the complete amino acid-coding sequence (676 amino acids). Northern hybridization analysis of monkey testicular RNA, using a complementary RNA probe corresponding to the full-length cDNA, demonstrated major transcripts of 5.5 and 1.4 kilobases and minor ones of 4.0, 2.7, and 1.9 kilobases. Sequence analysis of the monkey LHR cDNA revealed a striking feature, i.e. the absence of an 81-bp nucleotide sequence corresponding to exon 10, present in the LHR cDNAs of all other species studied to date. The monkey LHR cDNA displayed 83-94% overall sequence homology with the other mammalian LHR cDNAs. Reverse transcription-PCR with human exon 10-specific primers demonstrated the total absence of this sequence from the monkey LHR messenger RNA. Southern hybridization of monkey genomic DNA using a human exon 10 probe demonstrated its presence in the monkey gene and that it is totally spliced out from the primary transcript. COS cells transfected with the monkey LHR cDNA showed similar high affinity (Kd = 0.25 nmol/liter) of [125I]iodo-hCG binding as those transfected with human LHR cDNA (Kd = 0.20 nmol/liter). The cells expressing the recombinant monkey and human LHR displayed similar responses of extracellular cAMP and inositol trisphosphate to hCG. In conclusion, marmoset monkey LHR seems to lack the sequence corresponding to exon 10 of the LHR gene in other mammalian species. The truncation does not alter LHR function, as the monkey receptor protein bound hCG and evoked cAMP and inositol trisphosphate responses comparable to those of the human LHR containing the exon 10-encoded structure. As the sequence homologous to exon 10 is missing in the other two glycoprotein receptors, i.e. those of FSH and TSH, this extra exon is apparently inserted into the LHR messenger RNA of some species during evolution from intronic sequences by a change in alternative splicing.

Assessment of mechanisms of thyroid hormone action in mouse Leydig cells: regulation of the steroidogenic acute regulatory protein, steroidogenesis, and luteinizing hormone receptor function.

Recently, we demonstrated that triiodothyronine (T(3)) stimulated steroid hormone biosynthesis and steroidogenic acute regulatory (StAR) protein expression in mLTC-1 mouse Leydig tumor cells through the mediation of steroidogenic factor 1 (SF-1). We now report a dual response mechanism of T(3) on steroidogenesis and StAR expression, and on LH receptor (LHR) expression and binding in mLTC-1 cells. T(3) acutely (8 h), induced a 260% increase in StAR messenger RNA (mRNA) expression over the basal level which was coincident with an increase in progesterone (P) production. In contrast, chronic stimulation with T(3) (beyond 8 h), resulted in an attenuation of StAR expression and P production. This attenuation was most likely caused by a decrease in cholesterol delivery to the inner mitochondrial membrane as demonstrated by incubations with the hydrophilic steroid precursors, 22R hydroxycholesterol and pregnenolone, which restored P synthesis. In similar studies, chronic treatment with T(3) increased the levels of cytochrome P450scc mRNA by 83%, whereas those of cytochrome P450 17alpha-hydroxylase and 3ss-hydroxysteroid dehydrogenase decreased. The diminished response in steroidogenesis following chronic T(3) exposure was not a result of alterations in StAR mRNA stability, but rather was due to inhibition of transcription of the StAR gene. Similar acute stimulatory and chronic inhibitory responses to T(3) were found when LHR mRNA expression and LHR ligand binding were examined. Transfections with an LHR or StAR promoter/luciferase reporter construct demonstrated that a 173-bp fragment of the LHR promoter containing an SF-1 binding motif was involved in T(3) response, as was the SF-1 recognition site at -135 bp in the StAR promoter. Furthermore, the importance of SF-1 in T(3) function was also verified employing mutation in the bases of SF-1 sequences using electrophoretic mobility shift assays. The potential physiological relevance of these findings was demonstrated when similar responses were obtained in mice rendered hypo and hyperthyroid. Collectively, these observations further characterize the thyroid-gonadal connection and provide insights into the mechanisms for a dual regulatory role of thyroid hormone in Leydig cell functions.

Transcriptional regulation of the mouse steroidogenic acute regulatory protein gene by the cAMP response-element binding protein and steroidogenic factor 1.

Transcriptional induction by cAMP is mediated through the interaction of the cAMP response-element binding protein (CREB) with a cAMP response element (CRE) in the promoter of target genes. The steroidogenic acute regulatory (StAR) protein gene is regulated by cAMP-mediated signaling in steroidogenic cells even though its promoter lacks a consensus CRE. Previously, we have identified three highly conserved 5'-CRE half-sites within the -96/-67 bp region of the mouse StAR gene, and a member of the CREB family (CREB/CRE modulator (CREM)) was shown to be involved in its expression and regulation. Here we show that CREB and CREMtau (but not CREMalpha and CREMbeta) have qualitatively similar effects on StAR promoter activity in response to (Bu)(2)cAMP. Studies on the effects of the functional integrity of the CRE half-sites on CREB-dependent (Bu)(2)cAMP-mediated StAR gene transcription demonstrated the greater importance of the CRE2 site in comparison with the CRE1 and CRE3 sites. The CRE2 sequence was also found to bind specifically to recombinant CREB protein and nuclear extract from MA-10 mouse Leydig tumor cells. The cAMP and CREB/CREM responsive region (-151/-1 bp) of the mouse StAR promoter also contains three recognition motifs for steroidogenic factor 1 (SF-1). Electrophoretic mobility shift assays and reporter gene analyses demonstrated the involvement of different SF-1 elements in StAR gene expression with the order of importance being SF-1/3>SF-1/1>SF-1/2. Specific mutations that eliminated the binding sites of CRE and SF-1 elements, either alone or in combination, resulted in an attenuation of StAR promoter activity, indicating that CREB and SF-1 can regulate StAR gene transcription in a cooperative fashion. In addition, mammalian two-hybrid assays revealed a high affinity protein-protein interaction between CREB/CREMtau and SF-1 which appeared to be dependent upon CREB protein phosphorylation. These findings further demonstrate CREB's role in StAR gene transcription and also provide evidence that the combined action of CREB/CREMtau and SF-1 results in enhanced activation of the StAR promoter.

Molecular mechanisms of leptin action in adult rat testis: potential targets for leptin-induced inhibition of steroidogenesis and pattern of leptin receptor messenger ribonucleic acid expression.

Leptin, the product of the ob gene, is a pivotal signal in the regulation of neuroendocrine function and fertility. Although much of the action of leptin in the control of the reproductive axis is exerted at the hypothalamic level, some direct effects of leptin on male and female gonads have also been reported. Indeed, recent evidence demonstrated that leptin is able to inhibit testosterone secretion at the testicular level. However, the molecular mechanisms behind this effect remain unclear. The focus of this study was twofold: (1) to identify potential targets for leptin-induced inhibition of steroidogenesis, and (2) to characterize in detail the pattern of expression and cellular distribution of leptin receptor (Ob-R) mRNA in adult rat testis. In pursuit of the first goal, slices of testicular tissue from adult rats were incubated with increasing concentrations of recombinant leptin (10(-9)--10(-7 )M) in the presence of human chorionic gonadotropin (hCG; 10 IU/ml). In this setting, testosterone secretion in vitro was monitored, and expression levels of mRNAs encoding steroidogenic factor 1 (SF-1), steroidogenic acute regulatory protein (StAR), cytochrome P450 cholesterol side-chain cleavage enzyme (P450 scc) and 17 beta-hydroxysteroid dehydrogenase type III (17 beta-HSD) were assessed by Northern hybridization. In pursuit of the second goal, the pattern of cellular expression of the Ob-R gene in adult rat testis was evaluated by in situ hybridization using a riboprobe complementary to all Ob-R isoforms. In addition, testicular expression levels of the different Ob-R isoforms, previously identified in the hypothalamus, were analyzed by means of semi-quantitative RT-PCR. In keeping with our previous data, recombinant leptin significantly inhibited hCG-stimulated testosterone secretion. In this context, leptin, in a dose-dependent manner, was able to co-ordinately decrease the hCG-stimulated expression levels of SF-1, StAR and P450 scc mRNAs, but it did not affect those of 17 beta-HSD type III. In situ hybridization analysis showed a scattered pattern of cellular expression of the Ob-R gene within the adult rat testis, including Leydig and Sertoli cells. In addition, assessment of the pattern of expression of Ob-R subtypes revealed that the long Ob-Rb isoform was abundantly expressed in adult rat testis. However, variable levels of expression of Ob-Ra, Ob-Re, and Ob-Rf mRNAs were also detected, whereas those of the Ob-Rc variant were nearly negligible. In conclusion, our results indicate that decreased expression of mRNAs encoding several up-stream elements in the steroidogenic pathway may contribute, at least partially, to leptin-induced inhibition of testicular steroidogenesis. In addition, our data on the pattern of testicular expression of Ob-R isoforms and cellular distribution of Ob-R mRNA may help to further elucidate the molecular mechanisms of leptin action in rat testis.

Mechanisms of desensitization of follicle-stimulating hormone (FSH) action in a murine granulosa cell line stably transfected with the human FSH receptor complementary deoxyribonucleic acid.

The desensitization of follicle-stimulating hormone (FSH)-evoked cAMP synthesis occurs upon continuous or repeated hormonal stimulation, and it involves the hormone-receptor interaction and post-receptor events. These mechanisms were studied in a murine granulosa cell line (KK-1) stably transfected with the human FSH receptor (hFSHR) complementary deoxyribonucleic acid (cDNA) under a powerful viral promoter. Hence, the FSHR transcriptional regulation was eliminated from the experimental model. Stimulation of the cells with recombinant human FSH (rhFSH) or a phorbol ester, 12-O-tetradecanoylphorbol-13 acetate (TPA), resulted in clear desensitization, i.e. subsequent rhFSH-stimulated cAMP formation was 73.4 +/-2.2%, (P < 0.001) and 66.3 +/-3.4%, (P < 0.0001), respectively, of that of cells preincubated in medium. TPA prestimulation evoked also clear inhibition (65-74% of control) of rhFSH or forskolin (a non-specific activator of adenylate cyclase) induced progesterone production. The suppression by TPA preincubation of the rhFSH-induced cAMP synthesis was completely abolished by the protein kinase C (PKC) inhibitor staurosporine (STR). Preincubation with STR exhibited a significant (P < 0.0001) increasing effect on the rhFSH-stimulated cAMP accumulation. The specific involvement of PKC was further evidenced by other inhibitors, all of them exerted significant elevation of cAMP synthesis following rhFSH restimulation. Furthermore, only the PKC beta isoform appeared to be constitutively expressed in these cells during desensitization. Prestimulation of the G-protein activity by sodium fluoride (NaF) or cholera toxin (CT), followed by rhFSH challenge, accounted for a decrease in the cAMP-mediated responsiveness, down to 69.4 +/- 2.8 or 74.2 +/- 1.9%, of control (P < 0.001), respectively, indicating that the post-receptor events are critical for desensitization. [125I]iodo-rhFSH binding to the cells did not change significantly during desensitization and the different stimulations. In contrast, approximately 50% increase (P < 0.001) occurred in the steady-state levels of FSHR mRNA in the cells stimulated with FSH. This was apparently due to prolonged half-time of mRNA, and not to altered transcription, since the FSHR cDNA was driven by a powerful viral promoter. In accordance, the cells transfected with Simian Virus (SV40) promoter-driven luciferase gene did not display alterations in luciferase activity following stimulatory treatments. The effects of the post-receptor stimulations (NaF or CT) on [125I]iodo-rhFSH binding were minor (8-12% reduction). Taken together, these data provide evidence that the agonist-responsive hFSHR desensitization appears through a PKC-beta isoform-mediated modulation of cAMP production. The desensitization of FSH action involves modifications of functional properties of the existing components of the FSH signal transduction complex, and does not require concomitant suppression of transcription or translation of the FSHR gene.

Rat gonadotropin-releasing hormone receptor expressed in insect cells induces activation of adenylyl cyclase.

Increasing evidence exist that multiple G proteins mediate the effects of gonadotropin-releasing hormone (GnRH) on the synthesis and release of pituitary gonadotropins. In the present study, we have expressed the rat GnRH receptor (GnRH-R) in insect cells, by infection with a recombinant baculovirus. Under the conditions used, insect cells expressed, 48 h post-infection, a maximum of 7800 +/- 650 receptors/cell which bound GnRH agonist [D-Trp6]GnRH with a Kd = 0.52 +/- 0.06 nM indicating characteristics similar to those of the natural receptor. No binding was observed in non-infected cells or cells infected with wild-type baculovirus. In presence of GnRH, GnRH-R expressing cells elicited a time- and dose-dependent production of inositol trisphosphate, with a maximum level reached within 30 min and an EC50 = 5 nM. These recombinant insect cells also produced cAMP in response to GnRH. However, in contrast to other heterologous systems, or rat pituitary gonadotropes wherein GnRH induced a weak and delayed elevation of cAMP, in insect cells the rise of cAMP was comparatively rapid, attaining a maximum level after 2 h, and the EC50 was 5 nM. Finally, a clear activation of adenylyl cyclase (AC) in response to GnRH was shown for the first time by measuring the conversion of [alpha-32P]ATP into labeled cAMP, using membrane preparations from GnRH-R expressing insect cells. These data demonstrate that rat GnRH-R has the potential for dual coupling to both phosphoinositidase C and AC and suggest a major influence of the host cell for this coupling and/or its expression, probably in relation with the G protein repertoire and preference. This notion could be extended to several target cells other than pituitary gonadotropes that normally express the GnRH-R in mammals, including hippocampal, Leydig, granulosa, placental and GnRH-secreting hypothalamic cells.

Elements involved in the regulation of the StAR gene.

The steroidogenic acute regulatory protein (StAR) mediates the transfer of cholesterol from the outer to the inner mitochondrial membrane, the regulated step in steroidogenesis. A most interesting facet of this protein is the manner in which its expression is acutely regulated. In this regard, a number of studies have concentrated on the search for consensus cis regulatory elements within its promoter, and, more importantly, on whether these elements are involved in its expression. This short review will summarize some of the findings that have been reported concerning the nature of how the expression of this gene is regulated.

Functional relevance of luteinizing hormone receptor in mouse uterus.

The presence of high-affinity luteinizing hormone (LH)/human chorionic gonadotropin (hCG) receptors has been reported in porcine, rabbit, rat and human uteri. We have demonstrated binding of [125I]LH to mouse uterus, which was saturable. Scatchard plot analysis indicated Kd to be 1.37 x 10(-10) mol/l and the maximum binding capacity to be 5.24 nmol/kg protein. Attempts have been made to observe the functional relevance of gonadotropin receptor in the mouse uterus. The size and weight of the uterus remarkably decreased as a result of ovariectomy; administration of LH to ovariectomized (OVX) mice significantly increased the uterine weight in comparison to the OVX control (p < 0.01), indicating a direct effect of LH on the uterus. There was a two-fold decrease of uterine ascorbic acid content in LH-treated OVX mice as compared to the intact control. The gain in uterine weight of OVX mice by LH was due to the increase in uterine protein synthesis. The stimulatory effect of LH on OVX mice uterus appears to be mediated via steroid hormones because it significantly augmented uterine mitochondrial steroidogenesis. Since 17 beta-estradiol (E2) is known to stimulate uterine protein synthesis, the circulatory level of E2 was determined in intact, OVX and OVX + LH-treated mice. A fall in the circulatory level of E2 occurred in OVX mice as compared to the control, while treatment of LH for 7 days (three injections) significantly elevated E2 levels in OVX mice (p < 0.001). This higher level of E2 in OVX mice remains unaltered on adrenalectomy, indicating that adrenals are not the source for increased E2 levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of thyroid hormone and steroidogenic acute regulatory (StAR) protein in the regulation of murine Leydig cell steroidogenesis.

The steroidogenic acute regulatory (StAR) protein, a novel phosphoprotein, is a crucial factor involved in intramitochondrial cholesterol transportation, the rate-limiting step in steroidogenesis. The present investigations were undertaken to elucidate involvement of thyroid hormone and StAR protein in the regulation of steroidogenesis in mouse Leydig cells. Treatment of cells with triiodothyronine (T(3)) coordinately augmented the levels of StAR protein, StAR mRNA, and steroid production, and these responses were progressively dependent on expression of steroidogenic factor 1 (SF-1). With regard to steroidogenesis and StAR expression, the T(3) response requires both on-going mRNA and protein synthesis. In addition, the effects of T(3) were acutely modulated at the steroidogenic machinery and luteinizing hormone receptor (LHR) function, while these levels were suppressed following longer periods of exposure to T(3). Furthermore, the inhibition of SF-1 expression by DAX-1 markedly abolished T(3)-mediated StAR expression in a time frame, which was consistent with decreased steroid biosynthesis. Specific involvement of SF-1 was further confirmed by assessing the 5'-flanking region of the mouse StAR gene, which identified a region between -254 and -110 bp that was essential for T(3) function. Importantly, it was found that the SF-1 binding site at position -135 bp of the 5'-flanking region was greatly involved in T(3)-mediated reporter activity. Electrophoretic mobility shift assays (EMSA) also demonstrated involvement of SF-1 in T(3) function. The relevance of T(3)-mediated LHR function was investigated in mice rendered hypo-and hyperthyroid, which accounted for up-regulation in the former and down-regulation in the latter group, respectively. These findings demonstrate a key role of thyroid hormone in maintaining mouse Leydig cell function, where thyroid hormone and StAR protein coordinately regulate steroid hormone biosynthesis.

Homologous radioimmunoassay and radioreceptorassay of gonadotropic hormone for an Indian carp Catla catla.

A homologous radioimmunoassay (RIA) for Indian carp (C. catla) gonadotropin (GtH) was developed by using purified Catla GtH and its specific antiserum. In Ouchterlony agar diffusion, antisera raised against purified Catla GtH (cGtH), showed clear crossreaction. Catla-anti GtH (anti-cGtH) did not cross-reacted with Catla TSH enriched fraction. Immunocrossreaction was further confirmed with competitive binding inhibition studies where displacement of radiolabelled cGtH was precisely linear against increasing concentrations of cGtH, hence this was later used as standard curve for RIA. Competitive binding inhibition was also observed with purified murrel GtH, silver carp GtH and Cyprinus GtH, using varied doses. Both murrel and silver carp GtH showed clear parallelism with cGtH, while Cyprinus GtH inhibition slope was less steeper. Mammalian GtHs (hCG, oLH, oFSH), bTSH and bPRL had no crossreaction with anti-cGtH. Radioreceptorassay (RRA) for cGtH was developed by preparing plasma membranes from Catla oocytes. Binding of 125I-cGtH to oocyte plasma membranes showed saturability with high affinity (Ka = 0.11 X 10(13)M-1) and low capacity (17 fmol/mg protein). Displacement of 125I-cGtH in receptorassay by cold cGtH was linear and therefore served as standard curve. The interassay and intrassay variability in RIA was 7.9% and 3% while that of RRA was 5% and 3% respectively. Sensitivity of RIA was in the picogram level whereas it was in nanogram level by RRA. Determination of carp pituitary and serum GtH content by RIA and RRA showed the consistency, precision and validity of these assays. Although RRA was comparatively less sensitive than RIA, it was convenient, quick and less expensive.

Human chorionic gonadotropin binding sites in the human endometrium.

The existence of high-affinity and low-capacity specific binding sites for luteinizing hormone (LH)/human chorionic gonadotropin (hCG) has been reported in porcine, rabbit and rat uteri. We have identified hCG binding sites in the human endometrium collected from 35-42-year-old ovulatory and anovulatory women. The binding characteristics of hCG to endometrial tissue preparations from ovulatory and anovulatory women showed saturability with high affinity and low capacity. Scatchard plot analysis showed the dissociation constant of specific binding sites in the ovulatory women to be 3.5 x 10(-10) mol/l and in anovulatory women to be 3.1 x 10(-10) mol/l. The maximum binding capacity varied considerably between ovulatory (3.85 nmol/kg protein) and anovulatory (6.12 nmol/kg protein) endometrium. Among the divalent metal ions tested (Zn2+, Mg2+, Mn2+, Ca(2+)--4 mol/l), Zn2+ effected a remarkable increase in [125I]hCG binding to the endometrium (p < 0.005) whereas Mn2+ showed a marginal increase and other metal ions did not have any effect. Data obtained with human endometrium indicate an influence of the functional state of the ovary on [125I]hCG binding to endometrium.

Molecular mechanisms of thyroid hormone-stimulated steroidogenesis in mouse leydig tumor cells. Involvement of the steroidogenic acute regulatory (StAR) protein.

Using a mouse Leydig tumor cell line, we explored the mechanisms involved in thyroid hormone-induced steroidogenic acute regulatory (StAR) protein gene expression, and steroidogenesis. Triiodothyronine (T3) induced a approximately 3.6-fold increase in the steady-state level of StAR mRNA which paralleled with those of the acute steroid response ( approximately 4.0-fold), as monitored by quantitative reverse transcriptase-polymerase chain reaction assay and progesterone production, respectively. The T3-stimulated progesterone production was effectively inhibited by actinomycin-D or cycloheximide, indicating the requirement of on-going mRNA and protein synthesis. T3 displayed the highest affinity of [125I]iodo-T3 binding and was most potent in stimulating StAR mRNA expression. In accordance, T3 significantly increased testosterone production in primary cultures of adult mouse Leydig cells. The T3 and human chorionic gonadotropin (hCG) effects on StAR expression were similar in magnitude and additive. Cells expressing steroidogenic factor 1 (SF-1) showed marginal elevation of StAR expression, but coordinately increased T3-induced StAR mRNA expression and progesterone levels. In contrast, overexpression of DAX-1 markedly diminished the SF-1 mRNA expression, and concomitantly abolished T3-mediated responses. Noteworthy, T3 augmented the SF-1 mRNA expression while inhibition of the latter by DAX-1 strongly impaired T3 action. Northern hybridization analysis revealed four StAR transcripts which increased 3-6-fold following T3 stimulation. These observations clearly identified a regulatory cascade of thyroid hormone-stimulated StAR expression and steroidogenesis that provides novel insight into the importance of a thyroid-gonadal connection in the hormonal control of Leydig cell steroidogenesis.

Molecular cloning of the mouse follicle-stimulating hormone receptor complementary deoxyribonucleic acid: functional expression of alternatively spliced variants and receptor inactivation by a C566T transition in exon 7 of the coding sequence.

The gonadotropin receptors, i.e., those of LH and FSH (FSHR), are pivotal elements in the regulation of gonadal function. Recently, extensive efforts have been made to elucidate the structure-function relationship of these receptors as well as the modulatory mechanism(s) of their function. In the present study, we report 1) characterization of the mouse (m) FSHR cDNA coding sequence and 2) the functional consequences of coexpression of several splice variants of the mFSHR. In addition, we evaluate 3) the impact on mFSHR function of a C566T transition in exon 7 of the coding sequence, a substitution analogous to the inactivating mutation in the human FSHR gene responsible for a hereditary form of hypergonadotropic ovarian failure. Molecular cloning of the mFSHR cDNA was carried out by reverse transcription-polymerase chain reaction (RT-PCR) using 129/Sv mouse testicular RNA and primers complementary to the rat or the partially characterized mouse FSHR sequence. Overlapping partial fragments of receptor cDNA were amplified, sequenced, and engineered to produce the entire cDNA coding sequence, subcloned into the pSG5 expression vector. Using a similar approach, 4 different receptor splice variants, selectively lacking exons 2, 2 and 5, 5 and 6, and 2, 5, and 6 of the coding region, were cloned. Finally, PCR-based site-directed mutagenesis was used to generate the C566T mutant of mFSHR. Sequence analysis showed an open reading frame of 2076 base pairs for the mFSHR cDNA, predicting a putative 17-amino acid signal peptide and a 675-amino acid mature receptor protein, and overall sequence homology of 94% with rat, 87% with human, and 85-84% with bovine, and ovine FSHRs. Functional expression in human embryonic kidney (HEK 293) and mouse granulosa (KK-1) cells demonstrated for the cloned receptor high-affinity binding to recombinant human (rh) FSH and ability to elicit cAMP, inositol trisphosphate (IP3), and progesterone responses. In contrast, transient transfection studies showed that despite successful transcription, the exon-lacking FSHR variants were unable to bind rhFSH either in intact or in solubilized HEK 293 cells, or to elicit cAMP or progesterone responses in KK-1 cells. Furthermore, cotransfections of the splice variants in the context of an ovarian cell line stably expressing the full-length mFSHR failed to demonstrate modulatory effects on the holoreceptor function. Finally, transient expression of the C566T mFSHR mutant in HEK 293 cells revealed that, in accordance with observations on human FSHR, this substitution profoundly impaired the ligand binding and cAMP and IP3 responses to rhFSH stimulation. In conclusion, the present data indicate that, despite extensive splicing of the mFSHR message, a potential role of the exon-lacking receptor transcripts in modulating FSH actions is unlikely. In addition, we provide evidence for mFSHR inactivation by a C566T transition in exon 7 of the coding sequence, thus paving the way for further development of animal models of hypergonadotropic ovarian failure.

Requirement of extracellular calcium in fish pituitary gonadotropin release by gonadotropin hormone-releasing hormone.

Influence of extracellular calcium on gonadotropin hormone-releasing hormone (GnRH)-stimulated gonadotropin hormone (GtH) release from a teleostean fish (Channa punctatus) pituitary was examined in vitro by preparing enzymatically dispersed pituitary cell incubation. Effect of Ca2+ on GnRH-augmented GtH release was evaluated with partially purified C. punctatus GnRH (cGnRH) and synthetic mammalian GnRH (mGnRH). Cells were dispersed by 0.3% collagenase plus 0.05% trypsin in culture medium and a high yield of viable cells were obtained. Addition of cGnRH (10 micrograms/ml) to pituitary cells in Ca2+-free medium resulted in a significant increase in GtH release, but the addition of Ca2+ (2 mM) enhanced it to about four- and threefold over cGnRH and mGnRH, respectively. Increasing concentrations of Ca2+ (0.1-2.0 mM/well) with fixed concentrations of GnRH (10 micrograms/ml) or increasing doses of GnRH (2.5 to 20 micrograms/ml) with fixed amount of Ca2+ (2 mM/well) resulted in a dose dependent increase in GtH release. EDTA or EGTA (2 mM/well) completely suppressed the Ca2+-augmenting effect of GnRH-stimulated GtH release. Addition of lanthanum (La3+, 4 mM/well), a competitive inhibitor of Ca2+, reduced 60% of the Ca2+ (2 mM/well) stimulation. Verapamil, a specific Ca2+ channel blocker, when added in increasing concentrations (1-100 microM/well) to pituitary cell incubations containing GnRH-stimulated GtH release in Ca2+-free medium could be waived by EGTA (2 mM/well), indicating availability of extracellular calcium from tissue sources. The uptake of radioactive Ca2+ by pituitary cells was greatly enhanced by GnRH while the addition of verapamil (10 microM/well) not only inhibited the GnRH-stimulated uptake, but also reduced it below the control level.(ABSTRACT TRUNCATED AT 250 WORDS)