Sequence and organization of the human mitochondrial genome

The complete sequence of the 16,569-base pair human mitochondrial genome is presented. The genes for the 12S and 16S rRNAs, 22 tRNAs, cytochrome c oxidase subunits I, II and III, ATPase subunit 6, cytochrome b and eight other predicted protein coding genes have been located. The sequence shows extreme economy in that the genes have none or only a few noncoding bases between them, and in many cases the termination codons are not coded in the DNA but are created post-transcriptionally by polyadenylation of the mRNAs.

Transcriptional interference between c-Jun and the glucocorticoid receptor: mutual inhibition of DNA binding due to direct protein-protein interaction

Glucocorticoids are potent inhibitors of collagenase induction by phorbol esters and inflammatory mediators. The target for this negative effect is the AP-1 site within the collagenase promoter, which also mediates its induction. Negative regulation is due to repression of AP-1 activity by the glucocorticoid receptor (GCR). While the GCR is a potent inhibitor of AP-1 activity (Jun/Fos), both c-Jun and c-Fos are potent repressors of GCR activity. In vitro experiments using purified GCR and c-Jun proteins suggest that mutual repression is due to direct interaction between the two. Direct interaction between GCR and either c-Jun or c-Fos is demonstrated by cross-linking and coimmunoprecipitation. These findings reveal a cross talk between two major signal transduction systems used to control gene transcription in response to extracellular stimuli, and a novel mechanism for transcriptional repression.

A different genetic code in human mitochondria

Comparison of the human mitochrondial DNA sequence of the cytochrome oxidase subunit II gene and the sequence of the corresponding beef heart protein shows that UGA is used as a tryptophan codon and not as a termination codon and suggests that AUA may be a methionine and not an isoleucine codon. The cytochrome oxidase II gene is contiguous at its 5' end with a tRNAAsp gene and there are only 25 bases at its 3' end before a tRNALys gene. These tRNA'S are different from all other known tRNA sequences.

Ptx1, a bicoid-related homeo box transcription factor involved in transcription of the pro-opiomelanocortin gene

The pituitary gland contains six distinct hormone-producing cell types that arise sequentially during organogenesis. The first cells to differentiate are those that express the pro-opiomelanocortin (POMC) gene in the anterior pituitary lobe. The other lineages, which appear later, include cells that are dependent on the POU factor Pit-1 and another POMC-expressing lineage in the intermediate pituitary lobe. Using AtT-20 cells as a model for early expression of POMC in the anterior pituitary, we have defined a regulatory element conferring cell specificity of transcription and cloned a cognate transcription factor. This factor, Ptx1 (pituitary homeo box 1), contains a homeo box related to those of the anterior-specific genes bicoid and orthodenticle in Drosophila, and Otx-1 and Otx-2 in mammals. Ptx1 activates transcription upon binding a sequence related to the Drosophila bicoid target sites. Ptx1 is the only nuclear factor of this DNA-binding specificity that is detected in AtT-20 cells, and it is expressed at high levels in a subset of adult anterior pituitary cells that express POMC. However, Ptx1 is expressed in most cells of Rathke's pouch at an early time during pituitary development and before final differentiation of hormone-producing cells. Thus, Ptx1 may have a role in differentiation of pituitary cells, and its early expression pattern suggests that it may have a role in pituitary formation. In the adult pituitary gland, Ptx1 appears to be recruited for cell-specific transcription of the POMC gene.

A pituitary cell-restricted T box factor, Tpit, activates POMC transcription in cooperation with Pitx homeoproteins

The pituitary gland has provided unique insight into molecular mechanisms and regulatory factors controlling both differentiation and gene transcription. We identified Tpit, a novel T box factor only present in the two pituitary POMC-expressing lineages, the corticotrophs and melanotrophs, and apparently in no other tissue, including hypothalamic POMC neurons. In pituitary cells, Tpit activation of POMC gene transcription requires cooperation with Pitx1, the two factors binding to contiguous sites within the same regulatory element. In gain-of-function experiments, Tpit induces POMC expression in undifferentiated pituitary cells, indicating that it can initiate differentiation into POMC-expressing lineages. TPIT gene mutations were found in patients with isolated deficiency of pituitary POMC-derived ACTH, in support of an essential role of Tpit for differentiation of the pituitary POMC lineage.

A homeodomain gene Ptx3 has highly restricted brain expression in mesencephalic dopaminergic neurons

The mesencephalic dopaminergic (mesDA) system regulates behavior and movement control and has been implicated in psychiatric and affective disorders. We have identified a bicoid-related homeobox gene, Ptx3, a member of the Ptx-subfamily, that is uniquely expressed in these neurons. Its expression starting at E11.5 in the developing mouse midbrain correlates with the appearance of mesDA neurons. The number of Ptx3-expressing neurons is reduced in Parkinson patients, and these neurons are absent from 6-hydroxydopamine-lesioned rats, an animal model for this disease. Thus, Ptx3 is a unique transcription factor marking the mesDA neurons at the exclusion of other dopaminergic neurons, and it may be involved in developmental determination of this neuronal lineage.

Novel dimeric Nur77 signaling mechanism in endocrine and lymphoid cells

Within the nuclear receptor family, Nur77 (also known as NGFI-B) distinguishes itself by its ability to bind a target sequence (the NBRE) as a monomer and by its role in T-cell receptor (TCR)-induced apoptosis in T cells. We now report on a novel mechanism of Nur77 action that is mediated by homodimers. These dimers bind a Nur77 response element (NurRE), which has been identified as a target of CRH-induced Nur77 in the pro-opiomelanocortin (POMC) gene promoter. Both halves of the palindromic NurRE are required for responsiveness to physiological signals, like CRH in pituitary-derived AtT-20 cells. Similarly, in T-cell hybridomas, TCR activation induced NurRE but not NBRE reporters. The in vivo signaling function of Nur77 thus appears to be mediated by dimers acting on a palindromic response element of unusual spacing between its half-sites. This mechanism may represent the biologically relevant paradigm of action for this subfamily of orphan nuclear receptors.

Most of the coding region of rat ACTH beta--LPH precursor gene lacks intervening sequences

The peptide hormones ACTH, beta-endorphin, alpha- and beta-melanotropin(MSH) and possibly gamma-MSH are synthesized in the pituitary gland by the processing of a 32,000-molecular weight (MW) polypeptide called proopiomelanocortin (POMC). The existence of a further precursor (pre form) to POMC containing an additional N-terminal 'leader' peptide has been suggested by analysis of the in vitro translation products of poly(A)-containing RNA from AtT-20 cells, a mouse ACTH-producing cell line of pituitary origin. Nakanishi et al. cloned and sequenced a cDNA copy of the bovine prePOMC mRNA. This sequence confirmed the known structure of the carboxyl half of POMC and revealed the presence of a new MSH-like moiety, gamma-MSH, within the 16,000-MW amino half of the precursor (16K fragment). Recent experiments have suggested that this peptide may act in synergy with ACTH to increase corticosterone and aldosterone production in vivo and in vitro. We have now isolated from a rat genomic DNA library a segment of a DNA encoding most of POMC, using as probe a mouse 144-base pair cloned cDNA fragment encoding beta-MSH and beta-endorphin. The cloned rat gene is one of two (or more) closely related POMC genes. The DNA sequence obtained shows that the cloned POMC gene is not interrupted by any intervening sequence (IVS) between the codon for amino acid 19 and the presumptive poly(A) addition site. This region of POMC encodes all the biologically active peptides mentioned above. The DNA sequence encoding the putative gamma-MSH and the coding sequence that precedes it are highly conserved between rat and cow. This may indicate an as yet unrecognized biological function(s) for the NH2-terminal portion of the 16K fragment.

Different pattern of codon recognition by mammalian mitochondrial tRNAs

Analysis of an almost complete mammalian mitochondrial DNA sequence has identified 23 possible tRNA genes and we speculate here that these are sufficient to translate all the codons of the mitochondrial genetic code. This number is much smaller than the minimum of 31 required by the wobble hypothesis. For each of the eight genetic code boxes with four codons for one amino acid we find a single specific tRNA gene with T in the first (wobble) position of the anticodon. We suggest that these tRNAs with U in the wobble position can recognize all four codons in these genetic code boxes either by a "two out of three" base interaction or by U.N wobble.

Hindlimb patterning and mandible development require the Ptx1 gene

The restricted expression of the Ptx1 (Pitx1) gene in the posterior half of the lateral plate mesoderm has suggested that it may play a role in specification of posterior structures, in particular, specification of hindlimb identity. Ptx1 is also expressed in the most anterior ectoderm, the stomodeum, and in the first branchial arch. Ptx1 expression overlaps with that of Ptx2 in stomodeum and in posterior left lateral plate mesoderm. We now show that targeted inactivation of the mouse Ptx1 gene severely impairs hindlimb development: the ilium and knee cartilage are absent and the long bones are underdeveloped. Greater reduction of the right femur size in Ptx1 null mice suggests partial compensation by Ptx2 on the left side. The similarly sized tibia and fibula of mutant hindlimbs may be taken to resemble forelimb bones: however, the mutant limb buds appear to have retained their molecular identity as assessed by forelimb expression of Tbx5 and by hindlimb expression of Tbx4, even though Tbx4 expression is decreased in Ptx1 null mice. The hindlimb defects appear to be, at least partly, due to abnormal chondrogenesis. Since the most affected structures derive from the dorsal side of hindlimb buds, the data suggest that Ptx1 is responsible for patterning of these dorsal structures and that as such it may control development of hindlimb-specific features. Ptx1 inactivation also leads to loss of bones derived from the proximal part of the mandibular mesenchyme. The dual role of Ptx1 revealed by the gene knockout may reflect features of the mammalian jaw and hindlimbs that were acquired at a similar time during tetrapod evolution.

Novel glucocorticoid receptor complex with DNA element of the hormone-repressed POMC gene

Previous studies defined a DNA element necessary for glucocorticoid repression of the pro-opiomelanocortin (POMC) gene. The glucocorticoid receptor (GR) binds this negative glucocorticoid response element (nGRE) with an in vitro affinity similar to that of GR for positive GREs. However, whereas GR binds GREs as homodimers, a novel GR complex which forms with nGRE appears to contain three GR molecules. Biochemical characterization of this complex as well as equilibrium binding studies suggest that it is formed by sequential binding of a GR homodimer followed by binding of a GR monomer on the opposite side of the double helix. The DNA-binding domain (DBD) of GR is sufficient for differential binding of GRE and nGRE, as bacterially-expressed DBD formed unique nGRE complexes that contain three GR polypeptides. Thus, the POMC nGRE provides the first example of an interaction between GR and DNA in which GR binds otherwise than as a homodimer. Despite its high affinity for GR, the nGRE differs significantly from GREs in that it does not activate transcription in any context. As the nGRE appears insufficient on its own to confer hormone responsiveness, other POMC promoter elements are likely to be required to mediate glucocorticoid repression.

Egr-1 is a downstream effector of GnRH and synergizes by direct interaction with Ptx1 and SF-1 to enhance luteinizing hormone beta gene transcription

Pituitary gonadotropins are critical regulators of gonadal development and function. Expression and secretion of the mature hormones are regulated by gonadotropin-releasing hormone (GnRH), which is itself secreted from the hypothalamus. GnRH stimulation of gonadotropin expression and secretion occurs through the G-protein-linked phospholipase C/inositol triphosphate intracellular signaling pathway, which ultimately leads to protein kinase C (PKC) activation and increased intracellular calcium levels. Transcription factors mediating the effects of GnRH-induced signals on transcription of gonadotropin genes have not yet been identified. Recent studies have identified key factors involved in luteinizing hormone beta (LHbeta) gonadotropin gene transcription: the nuclear receptor SF-1, the bicoid-related homeoprotein Ptx1 (Pitx1), and the immediate-early Egr-1 gene. We now show that GnRH is a potent stimulator of Egr-1, but not Ptx1 or SF-1, expression. Further, Egr-1 activation of the LHbeta promoter is specifically enhanced by PKC, in agreement with a role for Egr-1 in mediating a GnRH effect on transcription. Egr-1 interacts directly with Ptx1 and with SF-1, leading to an enhancement of Ptx1- and SF-1-induced LHbeta transcription. Thus, Egr-1 is a likely transcriptional mediator of GnRH-induced signals for activation of the LHbeta gene.

Heterodimerization between members of the Nur subfamily of orphan nuclear receptors as a novel mechanism for gene activation

We have recently shown that the orphan nuclear receptor Nur77 (NGFI-B) is most active in transcription when it is interacting with a cognate DNA sequence as a homodimer. Further, we have shown that the target for Nur77 dimers, the Nur response element (NurRE), is responsive to physiological stimuli in both endocrine and lymphoid cells, whereas other DNA targets of Nur77 action are not. The Nur77 subfamily also includes two related receptors, Nur-related factor 1 (Nurr1) and neuron-derived orphan receptor 1 (NOR-1). Often, more than one member of this subfamily is induced in response to extracellular signals. We now show that Nur77 and Nurr1 form heterodimers in vitro in the presence or absence of NurRE, and we have documented interactions between these proteins in vivo by using a two-hybrid system in mammalian cells. These heterodimers synergistically enhance transcription from NurRE reporters in comparison to that seen with homodimers. The naturally occurring NurRE from the pro-opiomelanocortin gene preferentially binds and activates transcription in the presence of Nur77 homo- or heterodimers, while a consensus NurRE sequence does not show this preference. Taken together, the data indicate that members of the Nur77 subfamily are most potent as heterodimers and that different dimers exhibit target sequence preference. Thus, we propose that a combinatorial code relying on specific NurRE sequences might be responsible for the activation of subsets of target genes by one of the members of the Nur77 subfamily of transcription factors.

The pan-pituitary activator of transcription, Ptx1 (pituitary homeobox 1), acts in synergy with SF-1 and Pit1 and is an upstream regulator of the Lim-homeodomain gene Lim3/Lhx3

The Ptx1 (pituitary homeobox 1) homeobox transcription factor was isolated as a transcription factor of the pituitary POMC gene. In corticotrope cells that express POMC, cell-specific transcription is conferred in part by the synergistic action of Ptx1 with the basic helix-loop-helix factor NeuroD1. Since Ptx1 expression precedes pituitary development and differentiation, we investigated its expression and function in other pituitary lineages. Ptx1 is expressed in most pituitary-derived cell lines and as is the related Ptx2 (Rieger) gene. However, Ptx1 appears to be the only Ptx protein in corticotropes and the predominant one in gonadotrope cells. Most pituitary hormone-coding gene promoters are activated by Ptx1. Thus, Ptx1 appears to be a general regulator of pituitary-specific transcription. In addition, Ptx1 action is synergized by cell-restricted transcription factors to confer promoter-specific expression. Indeed, in the somatolactotrope lineage, synergism between Ptx1 and Pit1 is observed on the PRL promoter, and strong synergism between Ptx1 and SF-1 is observed in gonadotrope cells on the betaLH promoter but not on the alphaGSU (glycoprotein hormone alpha-subunit gene) and betaFSH promoters. Synergism between these two classes of factors is reminiscent of the interaction between the products of the Drosophila genes Ftz (fushi tarazu) and Ftz-F1. Antisense RNA experiments performed in alphaT3-1 cells that express the alphaGSU gene showed that expression of endogenous alphaGSU is highly dependent on Ptx1 whereas many other genes are not affected. Interestingly, the only other gene found to be highly dependent on Ptx1 for expression was the gene for the Lim3/Lhx3 transcription factor. Thus, these experiments place Ptx1 upstream of Lim3/Lhx3 in a cascade of regulators that appear to work in a combinatorial code to direct pituitary-, lineage-, and promoter-specific transcription.

NeuroD1/beta2 contributes to cell-specific transcription of the proopiomelanocortin gene

NeuroD1/beta2 is a basic helix-loop-helix (bHLH) factor expressed in the endocrine cells of the pancreas and in a subset of neurons as they undergo terminal differentiation. We now show that NeuroD1 is expressed in corticotroph cells of the pituitary gland and that it is involved in cell-specific transcription of the proopiomelanocortin (POMC) gene. It was previously shown that corticotroph-specific POMC transcription depends in part on the action of cell-restricted bHLH factors that were characterized as the CUTE (corticotroph upstream transcription element) (M. Therrien and J. Drouin, Mol. Cell. Biol. 13:2342-2353, 1993) complexes. We now demonstrate that these complexes contain NeuroD1 in association with various ubiquitous bHLH dimerization partners. The NeuroD1-containing heterodimers specifically recognize and activate transcription from the POMC promoter E box that confers transcriptional specificity. Interestingly, the NeuroD1 heterodimers activate transcription in synergy with Ptx1, a Bicoid-related homeodomain protein, which also contributes to corticotroph specificity of POMC transcription. In the adult pituitary gland, NeuroD1 transcripts are detected in POMC-expressing corticotroph cells. Taken together with the restricted pattern of Ptx1 expression, these results suggest that these two factors establish the basis of a combinatorial code for the program of corticotroph-specific gene expression.

The bicoid-related homeoprotein Ptx1 defines the most anterior domain of the embryo and differentiates posterior from anterior lateral mesoderm

Ptx1 is a member of the small bicoid family of homeobox-containing genes; it was isolated as a tissue-restricted transcription factor of the pro-opiomelanocortin gene. Its expression during mouse and chick embryogenesis was determined by in situ hybridization in order to delineate its putative role in development. In the head, Ptx1 expression is first detected in the ectoderm-derived stomodeal epithelium at E8.0. Initially, expression is only present in the stomodeum and in a few cells of the rostroventral foregut endoderm. A day later, Ptx1 mRNA is detected in the epithelium and in a streak of mesenchyme of the first branchial arch, but not in other arches. Ptx1 expression is maintained in all derivatives of these structures, including the epithelia of the tongue, palate, teeth and olfactory system, and in Rathke's pouch. Expression of Ptx1 in craniofacial structures is strikingly complementary to the pattern of goosecoid expression. In addition, Ptx1 is expressed early (E6.8) in posterior and extraembryonic mesoderm, and in structures that derive from these. The restriction of expression to the posterior lateral plate is later evidenced by exclusive labelling of the hindlimb but not forelimb mesenchyme. In the anterior domain of expression, the stomodeum was shown by fate mapping to derive from the anterior neural ridge (ANR) which represents the most anterior domain of the embryo. The concordance between these fate maps and the stomodeal pattern of Ptx1 expression supports the hypothesis that Ptx1 defines a stomodeal ectomere, which lies anteriorly to the neuromeres that have been suggested to constitute units of a segmented plan directing head formation.

Antagonism between Nur77 and glucocorticoid receptor for control of transcription

Two important functions of glucocorticoids (Gc), namely, suppression of immune system function and feedback repression of the hypothalamo-pituitary-adrenal (HPA) axis, are mediated through repression of gene transcription. Previous studies have indicated that this repression is exerted in part through antagonism between the glucocorticoid receptors (GR) and the AP-1 family of transcription factors. However, this mechanism could not account for repression of the pro-opiomelanocortin (POMC) gene, an important regulator of the HPA axis. Our recent identification of the orphan nuclear receptor Nur77 as a mediator of CRH induction of POMC transcription led us, in the present work, to show that Gc antagonize this positive signal at two levels. First, Gc partly blunt the CRH induction of Nur77 mRNA, and second, they antagonize Nur77-dependent transcription. GR repression is exerted by antagonism of Nur77 action on the NurRE element of the POMC gene. Gc antagonism of NurRE activity was observed in response to physiological stimuli in both endocrine (CRH induction of POMC) and lymphoid (T-cell receptor activation) cells. In transfection experiments, transcriptional activation by Nur77 and the repressor activity of liganded GR titrated each other on their cognate DNA target. In vitro binding experiments as well as mutation analysis of GR suggest that the mechanism of GR antagonism of Nur77 is very similar to that of the antagonism between GR and AP-1. The convergence of positive signals mediated by Nur77 (and also probably by related family members) and negative signals exerted by GR appears to be a general mechanism for control of transcription, since it is active in both endocrine and lymphoid cells.

Selective effect of androgens on LH and FSH release in anterior pituitary cells in culture

A possible direct effect of androgens at the pituitary level on gonadotropin release was studied using rat anterior pituitary cells in primary culture. The preincubation of cells with 3 X 10(-9)M testosterone (T) for 40 h increased the concentration of luteinizing hormone-releasing hormone (LHRH) required for half-maximal stimulation (ED50) of LH release from 3 X 10(-10)M to 1 X 10(-9)M. In the same experiment, the LHRH ED50 for FSH release (3 X 10(-10)M) was not affected by preincubation with T, while a slight stimulatory effect of the androgen was observed on balal FSH release and on the maximal FSH response to LHRH. Time-course experiments showed that the inhibitory effect of T on the LH response to LHRH was maximal after about 48 h of incubation and that 54 h after the removal of T, only 50% of the inhibition was reversed. 5 alpha-Dihydrotestosterone (DHT) and T led to the same maximal inhibition (15-20% of control) of the LH response to 10(-10)M LHRH. DHT was, however, about 3 times more potent than T, their ED50 values being 1.6 X 10(-10)M and 5 X 10(-10)M, respectively. In contrast to the effect on LH, the FSH response to 10(-10)M LHRH was only slightly, but not significantly, inhibited by increasing concentrations of DHT or T. The finding that total LH (medium + cell content) remained constant after incubation with T or DHT clearly indicates that the inhibition of the LH response to LHRH is really due to changes in the sensitivity of the releasing mechanisms in the LH-secreting cells. Androgens did, however, lead to increased total FSH. The present data indicate an independent control of LH and FSH secretion by a direct action of androgens at the pituitary level.

Structure of the rat pro-opiomelanocortin (POMC) gene

The gene encoding pro-opiomelanocortin (POMC) presents unique regulatory features. In particular, glucocorticoids inhibit transcription of the POMC gene in the anterior pituitary, but not in the intermediate pituitary. In order to study the mechanism leading to transcriptional inhibition of POMC by glucocorticoid and the interaction of the glucocorticoid receptor complex with specific DNA sequences along the POMC gene, we have cloned the rat POMC gene and determined its structure. The gene is composed of three exons and appears to be present at a single copy per haploid genome. Besides the usual regulatory signals like 'TATA' and 'CCAAT' boxes, the upstream region contains sequences homologous to known enhancer sequences and to the glucocorticoid receptor binding site observed in glucocorticoid-responsive genes.

Neuroendocrine dysplasia in mice lacking protein tyrosine phosphatase sigma

Protein tyrosine phosphatase sigma (PTP-sigma, encoded by the Ptprs gene) is a member of the LAR subfamily of receptor-like protein tyrosine phosphatases that is highly expressed during mammalian embryonic development in the germinal cell layer lining the lateral ventricles of the developing brain, dorsal root ganglia, Rathke's pouch, olfactory epithelium, retina and developing lung and heart. On the basis of its expression and homology with the Drosophila melanogasterorthologues DPTP99 and DPTP100A (refs 5,6), which have roles in the targeting of axonal growth cones, we hypothesized that PTP-sigma may also have a modulating function in cell-cell interactions, as well as in axon guidance during mammalian embryogenesis. To investigate its function in vivo, we generated Ptprs-deficient mice. The resulting Ptprs-/-animals display retarded growth, increased neonatal mortality, hyposmia and hypofecundity. Anatomical and histological analyses showed a decrease in overall brain size with a severe depletion of luteinizing hormone-releasing hormone (LHRH)-immunoreactive cells in Ptprs-/- hypothalamus. Ptprs-/- mice have an enlarged intermediate pituitary lobe, but smaller anterior and posterior lobes. These results suggest that tyrosine phosphorylation-dependent signalling pathways regulated by PTP-sigma influence the proliferation and/or adhesiveness of various cell types in the developing hypothalamo-pituitary axis.

Specific protein-protein interaction between basic helix-loop-helix transcription factors and homeoproteins of the Pitx family

Homeoproteins and basic helix-loop-helix (bHLH) transcription factors are known for their critical role in development and cellular differentiation. The pituitary pro-opiomelanocortin (POMC) gene is a target for factors of both families. Indeed, pituitary-specific transcription of POMC depends on the action of the homeodomain-containing transcription factor Pitx1 and of bHLH heterodimers containing NeuroD1. We now show lineage-restricted expression of NeuroD1 in pituitary corticotroph cells and a direct physical interaction between bHLH heterodimers and Pitx1 that results in transcriptional synergism. The interaction between the bHLH and homeodomains is restricted to ubiquitous (class A) bHLH and to the Pitx subfamily. Since bHLH heterodimers interact with Pitx factors through their ubiquitous moiety, this mechanism may be implicated in other developmental processes involving bHLH factors, such as neurogenesis and myogenesis.

Opposite regulation of pro-opiomelanocortin gene transcription by glucocorticoids and CRH

Pro-opiomelanocortin (POMC) is the pituitary precursor for adrenocorticotropin (ACTH), beta-endorphin, beta-lipotropin and the melanotropins. The level of ACTH secretion from the anterior pituitary is largely determined by the competing action of the stimulatory hypothalamic hormone, corticoliberin (corticotropin-releasing hormone, CRH), and the inhibitory effect of glucocorticoids. We now demonstrate that these two hormones, glucocorticoids and CRH, also inhibit and stimulate, respectively, the transcription rate of the POMC gene as measured by nuclear run-on transcription assays. Indeed, we show both by in vivo treatment and with rat anterior pituitary cells in primary culture that glucocorticoids inhibit within 30 min transcription of the POMC gene. Similarly, we find that CRH stimulates POMC gene transcription within 15 min. CRH and glucocorticoids can compete with each other to set the rate of POMC transcription. Our results indicate that CRH and glucocorticoids regulate anterior pituitary POMC gene transcription in addition to their well-documented role in the control of POMC peptide release.

Estradiol-induced increase of the LH responsive to LH releasing hormone (LHRH) in rat anterior pituitary cells in culture

The effect of 17 beta-estradiol (E2) on LH secretion was studied using rat adenohypophyseal cells in primary culture. Preincubation of cells with 1 X 10(-9) M E2 for 40 h decreased the concentration of LHRH required for half-maximal stimulation (ED50) of LH release from 3.0 +/- 0.3 to 1.6 +/- 0.2 X 10(-10)M (P less than 0.01). Basal LH release was increased from 84 +/- 4 to 182 +/- 8 ng LH-RP-1/ml/4h (P less than 0.01) by E2 pretreatment. Time-course experiments showed that the stimulatory effect of 10(-8)M E2 on the LH response to LHRH can be first measured after 10 h of incubation in the presence of E2 and that this effect is maximal after 24 h of incubation with the steroid. While E2 increases the LH responsiveness to LHRH, androgens decrease the sensitivity of LH-secreting cells to the neurohormone. The LHRH ED50 value of testosterone-treated cells is of 7.2 +/- 0.4 vs. 3.7 +/- 0.3 X 10(-10)M for control cells (P less than 0.01). E2 can only partially reverse this inhibitory effect of androgens on the LH response to LHRH. These data show clearly that E2 can have a direct stimulatory effect on LH-secreting cells to increase the sensitivity of their response to LHRH.