Absence of opioid stress-induced analgesia in mice lacking beta-endorphin by site-directed mutagenesis

A physiological role for beta-endorphin in endogenous pain inhibition was investigated by targeted mutagenesis of the proopiomelanocortin gene in mouse embryonic stem cells. The tyrosine codon at position 179 of the proopiomelanocortin gene was converted to a premature translational stop codon. The resulting transgenic mice display no overt developmental or behavioral alterations and have a normally functioning hypothalamic-pituitary-adrenal axis. Homozygous transgenic mice with a selective deficiency of beta-endorphin exhibit normal analgesia in response to morphine, indicating the presence of functional mu-opiate receptors. However, these mice lack the opioid (naloxone reversible) analgesia induced by mild swim stress. Mutant mice also display significantly greater nonopioid analgesia in response to cold water swim stress compared with controls and display paradoxical naloxone-induced analgesia. These changes may reflect compensatory upregulation of alternative pain inhibitory mechanisms.

DNA elements with AT-rich core sequences direct pituitary cell-specific expression of the pro-opiomelanocortin gene in transgenic mice

Corticotrophs are the first fully differentiated cells to appear in the anterior pituitary during organogenesis and are distinguished by pro-opiomelanocortin (POMC) gene expression. Earlier studies in our laboratory defined three DNA regions (sites 1, 2 and 3) within promoter sequences at the 5'-end of the rat POMC gene (-323/-34) that cooperatively targeted cell-specific gene expression to corticotrophs and melanotrophs in transgenic mice. In this study we analysed the DNA-nuclear protein interactions underlying this functional activity. We demonstrated that the transcriptional activator SP1 interacts with GC-rich regions in sites 1 (-146/-136) and 2 (-201/-192) and an unidentified protein, which we call PP1 (putative pituitary POMC1), interacts with AT-rich regions in sites 2 (-202/-193) and 3 (-262/-253). The PP1-binding activity appears to be specific to cells that express the POMC gene because it was detected in nuclear extracts prepared from AtT20 corticotroph cells and mouse melanotroph tumours but not from GH4 pituitary tumour cells, HeLa cells or liver. Site-directed mutagenesis of core binding sequences demonstrated that PP1 is required for the correct cell-specific expression of the POMC gene in the pituitary gland of transgenic mice and SP1 appears to support such an expression. The best core binding sequence for PP1 is TAAT, a possible transcription factor homeodomain contact site. However, PP1 is distinct from Brn 3.0, a POU protein that also binds to site 3. We conclude that PP1 is a transcriptional activator for pituitary-specific POMC gene expression.

Cloning of the mouse gonadotropin beta-subunit-encoding genes, I. Structure of the follicle-stimulating hormone beta-subunit-encoding gene

More than 16kb of genomic sequence encompassing the mouse follicle-stimulating hormone beta subunit (FSH beta)-encoding gene was isolated from a 129SvEv mouse genomic library. Comparisons of the nucleotide and deduced amino-acid sequences to the FSH beta sequences of other mammalian species confirm an evolutionarily conserved role of this important protein in reproduction.

Targeted ablation of pituitary pre-proopiomelanocortin cells by herpes simplex virus-1 thymidine kinase differentially regulates mRNAs encoding the adrenocorticotropin receptor and aldosterone synthase in the mouse adrenal gland

We have produced and characterized lines of transgenic mice expressing a fusion gene composed of the pituitary expression-specific promoter region of the POMC gene, driving the herpes simplex viral-1 thymidine kinase. Adult mice were treated with the antiherpes agent ganciclovir at 70 mg/kg body weight (ip, twice daily for 10-12 days). Approximately 98% of the pituitary intermediate lobe melanotropes and anterior lobe corticotropes were ablated as determined by immunocytochemistry and RIA specific for the POMC-derived peptides, ACTH, beta-endorophin, and alpha-MSH. The number of lactotropes, somatotropes, thyrotropes, and gonadotropes was not altered compared with controls, indicating that in the adult pituitary, POMC products are not required to maintain the distribution of cell types. As expected, plasma corticosterone levels were substantially decreased after POMC cell ablation. In situ hybridization studies showed that the mouse ACTH receptor was expressed uniformly throughout the adrenal cortex, and RNase protection assays revealed that the ACTH receptor mRNA decreased after pituitary POMC cell ablation. Additionally, RNase protection assays showed that pituitary POMC cell ablation resulted in the decrease of adrenal p450c11 beta transcripts while p450c11AS (aldosterone synthase) mRNA levels remained constant. These data demonstrate differential regulation of steroid pathway-specific enzymes by POMC products. Our results also suggest that the thymidine kinase cell obliteration technique may not be dependent on cell division as a prerequisite for cytotoxicity, thus supporting the idea that targeted molecular ablation using cell- and tissue-specific promoter sequences to drive viral thymidine kinase expression can be refined further to study other nonmitotic cells.

Identification of two classes of prolactin-releasing factors in intermediate lobe tumors from transgenic mice

Targeted tumorigenesis, using the POMC gene promoter ligated to the simian virus 40 large T antigen, generated transgenic mice with massive tumors of the intermediate lobe (IL) of the pituitary. Inoculation of nude mice with the IL tumor cells resulted in very large secondary tumors. As the IL from several species produces a potent PRL-releasing factor (PRF), it was of interest to determine whether IL tumors from these mice also contain PRF. The objectives were to 1) measure serum PRL levels in mice with IL tumors, 2) determine whether these tumors contain PRF and examine its chromatographic properties, and 3) analyze whether this PRF is related to POMC, its derivatives, or other PRL secretagogues. Serum PRL levels were 5- to 6-fold higher in transgenic than in control mice. Primary and secondary IL tumors were acid extracted and successively fractionated using Sephadex G-100 gel filtration and reverse phase and gel permeation HPLC. PRF activity was determined using short term incubation of tissue extracts or column fractions with GH3 cells. Crude tumor extracts exhibited a strong and dose-dependent PRF activity. Upon chromatography, the PRF activity from either primary or secondary tumors resolved into two classes of compounds: a big PRF with an estimated mol wt of 70-80 kilodaltons and two small, very hydrophobic peptides. The elution profiles of the three PRFs differed from those of beta-endorphin, alpha MSH, beta MSH, ACTH, TRH, oxytocin, angiotensin II, vasoactive intestinal polypeptide, or corticotropin-like intermediate peptide. In summary, we have identified an animal model with IL tumors that has hyperprolactinemia and overproduces PRF. Two classes of PRFs, big and small, were resolved which differ from POMC derivatives and known regulators of PRL release. These data suggest that PRF is produced by melanotrophs, but is not a product of the POMC gene. The IL tumors should provide an excellent source for the purification and structural elucidation of PRFs.

Hormonal regulation of human follicle-stimulating hormone-beta subunit gene expression: GnRH stimulation and GnRH-independent androgen inhibition

Previous studies from our laboratory demonstrated that chronic testosterone administration to castrated transgenic mice suppressed human follicle-stimulating hormone-beta (FSH beta) mRNA levels transcribed from a human transgene to approximately 20% of control values. In the present study we used primary pituitary cultures prepared from the transgenic mice and in vivo experiments in hypogonadal (hpg) mice carrying the human transgene to assess the role of hypothalamic gonadotropin-releasing hormone (GnRH) in this inhibitory action. The levels of human FSH beta mRNA in monolayer cultures of pituitary cells were decreased by 24-hour treatments with 10 nM testosterone propionate or 5 alpha-dihydrotestosterone to 13 and 26% of control values, respectively, in the absence of GnRH. For the in vivo experiments we introduced the 10-kb human FSH beta transgene into the hpg genetic background by selective crossbreeding. Daily injections of 1 microgram GnRH for 14 days induced expression of the human FSH beta gene in male and female mice. Maximal effects were obtained by GnRH treatment of gonadectomized, hpg transgenic mice. Human FSH beta mRNA levels rose to approximately 4- or 10-fold that of control males and females, respectively. The stimulation was blocked completely by simultaneous administration of testosterone propionate in males and partially by estradiol in females. Pituitary content of immunoreactive FSH paralleled the mRNA changes. These data suggest that testosterone feedback inhibits the human FSH beta subunit gene directly at the pituitary gland in addition to the indirect mechanism of GnRH suppression. Furthermore, the in vitro data indicate that the suppression of human FSH beta gene expression is at least partly a direct androgen effect that does not require aromatization of testosterone to estradiol.

Localization of the melanocortin-4 receptor (MC4-R) in neuroendocrine and autonomic control circuits in the brain

POMC, the precursor of ACTH, MSH, and beta-endorphin peptides, is expressed in the pituitary and in two sites in the brain, in the arcuate nucleus of the hypothalamus and the commissural nucleus of the solitary tract of the brain stem. Little is known regarding the functions of melanocortin (ACTH and MSH) peptides in the brain. We report here the detailed neuroanatomical distribution of the MC4-R mRNA in the adult rat brain. The melanocortin 3 receptor (MC3-R), characterized previously, was found to be expressed in arcuate nucleus neurons and in a subset of their presumptive terminal fields but in few regions of the brainstem. The highly conserved MC4-R is much more widely expressed than MC3-R and is pharmacologically distinct. MC4-R mRNA was found in multiple sites in virtually every brain region, including the cortex, thalamus, hypothalamus, brainstem, and spinal cord. Unlike the MC3-R, MC4-R mRNA is found in both parvicellular and magnocellular neurons of the paraventricular nucleus of the hypothalamus, suggesting a role in the central control of pituitary function. MC4-R is also unique in its expression in numerous cortical and brainstem nuclei. Together, MC3-R and/or MC-4R mRNA are found in every nucleus reported to bind MSH in the adult rat brain and define neuronal circuitry known to be involved in the control of diverse neuroendocrine and autonomic functions. The high degree of conservation, distinct pharmacology, and unique neuronal distribution of the MC4 receptor suggest specific and complex roles for the melanocortin peptides in neuroendocrine and autonomic control.

In situ hybridization analysis of anterior pituitary hormone gene expression during fetal mouse development

We used 35S-labeled oligonucleotides and cRNAs (riboprobes) to detect the temporal order and spatial pattern of anterior pituitary hormone gene expression in (B6CBF1 x B6CBF1)F2 fetal mice from embryonic Day 9.5 (E9.5) to postnatal Day 1 (P1). Pro-opiomelanocortin (POMC) mRNA was expressed in the basal diencephalon on Day E10.5, in the ventromedial zone of the pars distalis on Day E12.5, and in the pars intermedia on Day E14.5. The common alpha-glycoprotein subunit (alpha-GSU) mRNA first appeared in the anterior wall of Rathke's pouch on Day E11.5 and extended to the pars tuberalis and ventromedial zone of the pars distalis on Day E12.5. Thyroid-stimulating hormone-beta (TSH beta) subunit mRNA was expressed initially in both the pas tuberalis and ventromedial pars distalis on Day E14.5, with an identical spatial distribution to alpha-GSU at the time. In contrast, luteinizing hormone-beta (LH beta) subunit and follicle-stimulating hormone beta (FSH beta) subunit mRNAs were detected initially only in the ventromedial pars distalis on Days E16.5 and E17.5, respectively, in an identical distribution to each other. POMC-, alpha-GSU-, TSH beta, LH beta-, and FSH beta-positive cells within the pars distalis all increased in number and autoradiographic signal with differing degrees of spatial expansion posteriorly, laterally, and dorsally up to Day P1. POMC expression was typically the most intense and extended circumferentially to include the entire lateral and dorsal surfaces of the pars distalis. The expression of both growth hormone (GH) and prolactin (PRL) started coincidentally on Day E15.5. However PRL cells localized in the ventromedial area similarly to POMC and the glycoprotein hormone subunits, whereas GH cells were found initially in a more lateral and central distribution within the lobes of the pars distalis. Somatotrophs increased dramatically in number and autoradiographic signal, extending throughout the pars distalis except for the most peripheral layer of cells on Day E17.5. Mammotrophs also increased in number but less abundantly than somatotrophs, and PRL expression remained more confined to central-medial and ventrolateral areas of the pars distalis up to Day P1. These data demonstrate distinctive patterns of expression for each of the major anterior pituitary hormone genes during development of the mouse pituitary gland and suggest that different groups of committed cells are the immediate precursors to the terminally differentiated hormone-secreting cell types.

Post-translational processing of proopiomelanocortin (POMC) in mouse pituitary melanotroph tumors induced by a POMC-simian virus 40 large T antigen transgene

Mice harboring a transgene composed of proopiomelanocortin (POMC) gene promoter sequences (nucleotides -706 to +64) ligated to the simian virus (SV) 40 early gene encoding large T antigen developed large POMC-expressing pituitary tumors. Histologically the tumors arose from the intermediate lobe, contained nuclear SV40 T antigen and POMC peptides, but did not express other pituitary hormones. POMC processing in the pituitary tumors was indistinguishable from normal mouse intermediate lobe melanotrophs and was characterized by high proportions of acetylated and carboxyl-terminal shortened beta-endorphins, and amino-terminal acetylated alpha-melanocyte-stimulating hormone, and virtually no adrenocorticotropic hormone (ACTH)(1-39), beta-lipotropin, or POMC. The tumors contained abundant levels of mRNA for the prohormone convertase PC2 and undetectable levels of PC1. Normal mouse neurointermediate lobe also has a high ratio of PC2/PC1 expression that is distinct from the relative abundance of PC1 in anterior lobe and AtT-20 corticotroph cells. In contrast, extracts from tumors transplanted subcutaneously in nude mice contained predominantly nonacetylated forms of beta-endorphin(1-31) and -(1-27), very little ACTH(1-39), almost no corticotropin-like intermediate peptide or alpha-melanocyte-stimulating hormone, and higher proportions of intact POMC. Surprisingly, despite the less efficient proteolytic cleavage, a transplanted tumor expressed both PC1 and PC2. These studies are the first biochemical documentation of a melanotroph pituitary tumor in a rodent species and provide a new model for the investigation of pituitary oncogenesis and the molecular basis of tissue-specific prohormone post-translational processing.

Identification of a receptor for gamma melanotropin and other proopiomelanocortin peptides in the hypothalamus and limbic system

Corticotropin (ACTH) and melanotropin (MSH) peptides (melanocortins) are produced not only in the pituitary but also in the brain, with highest concentrations in the arcuate nucleus of the hypothalamus and the commisural nucleus of the solitary tract. We have identified a receptor for MSH and ACTH peptides that is specifically expressed in regions of the hypothalamus and limbic system. This melanocortin receptor (MC3-R) is found in neurons of the arcuate nucleus known to express proopiomelanocortin (POMC) and in a subset of the nuclei to which these neurons send projections. The MC3-R is 43% identical to the MSH receptor present in melanocytes and is strongly coupled to adenylyl cyclase. Unlike the MSH or ACTH receptors, MC3-R is potently activated by gamma-MSH peptides, POMC products that were named for their amino acid homology with alpha- and beta-MSH, but lack melanotropic activity. The primary biological role of the gamma-MSH peptides is not yet understood. The location and properties of this receptor provide a pharmacological basis for the action of POMC peptides produced in the brain and possibly a specific physiological role for gamma-MSH.

Rat and mouse proopiomelanocortin gene sequences target tissue-specific expression to the pituitary gland but not to the hypothalamus of transgenic mice

The proopiomelanocortin (POMC) gene is expressed predominantly in corticotrophs of the pituitary anterior lobe, melanotrophs of the intermediate lobe and neurons of the arcuate nucleus of the hypothalamus. The different ontogeny of POMC mRNA as well as the complicated hormonal regulation of POMC gene expression in the three different cell types suggests a concerted interaction between several cis-acting elements in the POMC gene and transcription factors located in each of the three cell types. To investigate cell-specific elements in the POMC gene we tested two different constructs in transgenic mice. The construct -4000rPOMCLacZ, carrying 4 kb of the rat POMC promoter fused to the Escherichia coli beta-galactosidase gene, showed appropriate expression in melanotrophs in 50% of the mice analyzed. beta-Galactosidase activity was less evident in corticotrophs under basal environmental conditions. In brain, 7 out of 15 independently derived transgenic founders had ectopic expression of the transgene in different areas; however, none of the animals analyzed expressed beta-galactosidase in neurons of the arcuate nucleus. The construct HAL*, a 'tagged' 10.2-kb mouse genomic fragment, was more efficiently targeted to the pituitary. Using in situ hybridization, we detected uniform expression of HAL* in melanotrophs in 100% of the 6 pedigrees analyzed and transgenic mRNA levels paralleled those of the endogenous POMC mRNA. In corticotrophs, basal expression was low but after adrenalectomy HAL* mRNA levels were comparable to those of POMC. None of the 6 pedigrees had appropriate expression of HAL* in the brain; however, 2 lines had ectopic expression in the dentate gyrus of the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

Gonadal steroid hormone regulation of human and mouse follicle stimulating hormone beta-subunit gene expression in vivo

Transgenic mice that express a 10-kilobase human FSH beta (hFSH beta) gene exclusively in pituitary gonadotropes were used to study the regulation of hFSH beta gene expression by gonadal steroids. For comparison, the mouse FSH beta (mFSH beta) gene was studied in parallel in nontransgenic sibling (normal) mice. The hFSH beta gene showed a sexually dimorphic expression pattern, identical to mFSH beta, in the mouse environment. Intact normal and transgenic male mice had elevated (P < 0.05) levels of serum [16 +/- 2 ng/ml (normal); 38 +/- 6 (transgenic)] and pituitary FSH content [2 +/- 0.3 micrograms/mg protein (normal); 36 +/- 6 (transgenic)] and FSH beta mRNA [1.47 +/- 0.10 arbitrary density units (normal); 1.00 +/- 0.23 (transgenic)] compared to the corresponding female mice ([< 2.0 ng/ml (normal and transgenic)] [0.1 +/- 0.01 microgram/mg protein (normal); 0.2 +/- 0.03 (transgenic)] [< 0.03 arbitrary density units (normal and transgenic)]). Serum FSH levels were increased (P < 0.05) 2 weeks after castration of normal (22 +/- 2 ng/ml) and transgenic males (135 +/- 19 ng/ml) and were suppressed (P < 0.05) by testosterone [7 +/- 0.8 ng/ml (normal); 12 +/- 2 (transgenic)] or estradiol [14 +/- 1 ng/ml (normal); 16 +/- 1 (transgenic)] replacement. The increased serum FSH levels were associated with an inverse drop (P < 0.05) in pituitary FSH content to 1 +/- 0.1 microgram/mg protein in normal and 16 +/- 2 in transgenic males. Testosterone replacement further suppressed (P < 0.05) pituitary FSH content in transgenic (3 +/- 0.5 micrograms/mg protein) but not normal (1 +/- 0.1) males.(ABSTRACT TRUNCATED AT 250 WORDS)

Introduction of a point mutation into the mouse genome by homologous recombination in embryonic stem cells using a replacement type vector with a selectable marker

The introduction of small mutations instead of null alleles into the mouse genome has broad applications to the study of protein structure-function relationships and the creation of animal models of human genetic diseases. To test a simple mutational strategy we designed a targeting vector for the mouse proopiomelanocortin (POMC) gene containing a single nucleotide insertion that converts the initial tyrosine codon of beta-endorphin 1-31 to a premature translational termination codon and introduces a unique Hpal endonuclease restriction site. The targeting vector also contains a neo cassette immediately 3' to the last POMC exon and a herpes simplex virus thymidine kinase cassette to allow positive and negative selection. Homologous recombination occurred at a frequency of 1/30 clones of electroporated embryonic stem cells selected in G418 and gancyclovir. 10/11 clones identified initially by a polymerase chain reaction (PCR) strategy had the predicted structure without evidence of concatemer formation by Southern blot analysis. We used a combination of Hpa I digestion of PCR amplified fragments and direct nucleotide sequencing to further confirm that the point mutation was retained in 9/10 clones. The POMC gene was transcriptionally silent in embryonic stem cells and the targeted allele was not activated by the downstream phosphoglycerate kinase-1 promoter that transcribed the neo gene. Under the electroporation conditions used, we have demonstrated that a point mutation can be introduced with high efficiency and precision into the POMC gene using a replacement type vector containing a retained selectable marker without affecting expression of the allele in the embryonic stem cells. A similar strategy may be useful for a wide range of genes.

Fine structure of adrenal cortex in rats harbouring a medullary thyroid carcinoma transfected with a corticotrophin-releasing hormone cDNA expression vector

We report the light microscopic, transmission and scanning electron microscopic features of the adrenal cortices in rats bearing a medullary thyroid carcinoma cell line transfected with a corticotrophin-releasing hormone (CRH) cDNA expression vector. The animals had elevated CRH, ACTH and corticosterone blood levels, involuted thymuses and markedly enlarged adrenal glands with prominent lipid-depleted cortices and dilated congested capillaries, similar to those of animals treated with ACTH. Using electron microscopy it was found that the enlarged fasciculata and reticularis zones were composed of large, compact cells with abundant smooth endoplasmic reticulum, prominent Golgi complexes, increased number of large mitochondria with focal loss of cristae and cavitation of the internal compartments, numerous lysosomes and prominent elongated microvilli. In addition, small cytoplasmic fragments were seen within the capillary lumina; these structures resembled microvilli that were apparently detached from adrenocortical cells and entered the blood stream via discontinuous endothelium of dilated capillaries. By scanning electron microscopy it was found that the cells had bulging surfaces with scattered pits and numerous long microvilli pointing in different directions. This animal model allows analysis of the effects of protracted CRH excess resembling tumoural CRH-dependent Cushing's syndrome in human patients. Our findings call attention to the role of microvilli in adrenocortical secretion. The increased number and size of microvilli has been thought to lead to an increase in the surface area of adrenocortical cells, thereby facilitating hormone discharge. The detachment of microvilli from adrenocortical cells may represent a form of apocrine secretion and may contribute to hypercorticosteronaemia in CRH excess.

Identification of DNA elements cooperatively activating proopiomelanocortin gene expression in the pituitary glands of transgenic mice

The proopiomelanocortin (POMC) gene is highly expressed in adult mouse pituitary anterior lobe corticotrophs and intermediate lobe melanotrophs. To identify the DNA elements important for this tissue-specific expression, we analyzed a series of POMC reporter genes in transgenic mice. A DNA fragment containing rat POMC 5'-flanking sequences from -323 to -34 recapitulated both basal pituitary cell-specific and hormonally stimulated expression in adult mice when fused to a heterologous thymidine kinase promoter. Developmental onset of the reporter gene expression lagged by 1 day but otherwise closely paralleled the normal ontogeny of murine POMC gene expression, including corticotroph activation at embryonic day 14.5 (E14.5) followed by melanotroph activation at E15.5 to E16.5. AtT20 corticotroph nuclear protein extracts interacted with three specific regions of the functional POMC promoter in DNase I protection assays. The positions of these protected sites were -107 to -160 (site 1), -182 to -218 (site 2), and -249 to -281 (site 3). Individual deletions of these footprinted sites did not alter transgene expression; however, the simultaneous deletion of sites 2 and 3 prevented transgene expression in both corticotrophs and melanotrophs. Electrophoretic mobility shift and Southwestern (DNA-protein) assays demonstrated that multiple AtT20 nuclear proteins bound to these footprinted sites. We conclude that the sequences between -323 and -34 of the rat POMC gene promoter are both necessary and sufficient for correct spatial, temporal, and hormonally regulated expression in the pituitary gland. Our data suggest that the three footprinted sites within the promoter are functionally interchangeable and act in combination with promoter elements between -114 and -34. The inability of any reporter gene construction to dissociate basal and hormonally stimulated expression suggests that these DNA elements are involved in both of these two characteristics of POMC gene expression in vivo.

Pituitary corticotroph hyperplasia in rats implanted with a medullary thyroid carcinoma cell line transfected with a corticotropin-releasing hormone complementary deoxyribonucleic acid expression vector

CRH stimulates both the synthesis and release of ACTH and other derivatives of POMC by the adenohypophysis. It is uncertain, however, whether it also causes proliferation of corticotrophs. Patients with CRH-producing tumors develop Cushing's syndrome, and some have been reported to have pituitary corticotroph hyperplasia. We now report an animal model that accurately reproduces the human disorder of ectopic production of CRH by a neoplasm. Prolonged CRH secretion by a transplanted medullary thyroid carcinoma cell line stably transfected with a CRH cDNA under transcriptional control of a cytomegalovirus promoter resulted in corticotroph hyperplasia and hypertrophy; the percentage of ACTH-containing cells in animals bearing W2CRH tumors was increased at 9.8 +/- 0.5% (controls, 6.2 +/- 0.3%; W2 implanted tumors, 7.7 +/- 0.4%). Occasional mitotic figures were identified, and the cells were larger, with abundant cytoplasm but generally less intense immunohistochemical staining for ACTH due to relative degranulation compared to controls. Melanotrophs of the intermediate lobe were also increased in number and were larger, with abundant cytoplasm. No corticotroph adenomas were found. Our experiment accurately reproduces the gradually increasing CRH levels in the general circulation produced by a growing tumor, as found in the human ectopic CRH syndrome, and confirms that long term exposure to CRH excess, as produced by a tumor, results in an increased number of corticotrophs in the adenohypophysis.

Ectopic corticotropin-releasing hormone produced by a transfected cell line chronically activates the pituitary-adrenal axis in transkaryotic rats

Hypothalamic CRH is the primary positive regulatory factor of the pituitary-adrenal axis. The purpose of our study was to analyze the chronic effects of CRH on the production and secretion of POMC peptides from both the anterior lobe (AL) and neurointermediate lobe (NIL) of the pituitary by mimicking the syndrome of ectopic CRH secretion from neuroendocrine tumors. We first generated stably transfected W2 medullary thyroid carcinoma cell lines with a rat CRH expression vector under the transcriptional control of a cytomegalovirus gene promoter. These cell lines constitutively expressed the foreign gene, accurately processed the encoded prepro-CRH, and secreted biologically active CRH with an estimated potency equivalent to that of synthetic CRH-(1-41)NH2. The cell line designated W2CRH-7 was implanted sc in the syngeneic rat strain WAG/Rij and produced tumors that abundantly secreted CRH into the peripheral circulation. Four weeks postimplantation, W2CRH-7, but not wild-type W2, cells caused significant increases in the AL content of beta-endorphin-like immunoreactivity comparable to that caused by adrenalectomy (ADX). Plasma ACTH and serum beta-endorphin-like immunoreactivity were increased to a greater extent by ADX than by W2CRH-7 cell implantation. The NIL of both male and female rats showed either no change or a tendency to decreased beta-endorphin concentrations with no change in the acetylation or carboxy-shortening profiles judged by cation exchange chromatography in response to the ectopic CRH treatment. Rats of both sexes maintained a profound activation of the pituitary adrenal axis up to 16 weeks postimplantation, with normalized adrenal gland weights 5 times that of controls. The chronic secretion of CRH by W2CRH-7 cells resulted in a complete cessation of body growth in all rats up to the maximum time tested of 16 weeks. The lack of growth was partly ameliorated by concomitant ADX, suggesting an important role for adrenal glucocorticoids in these effects. We conclude that 1) the transplantable W2CRH-7 cell line provides a highly effective and reproducible means of sustained CRH treatment that mimics the syndrome of ectopic CRH expression by neuroendocrine tumors; 2) AL corticotrophs respond to chronic CRH by a sustained production and secretion of POMC peptides, leading to a marked adrenal cortical hyperplasia, with no evidence of biologically significant desensitization; 3) chronic CRH tends to decrease the NIL content of beta-endorphin,with remarkably little effect on posttranslational processing; and 4) the syndrome of chronic ectopic CRH in WAG/Rij rats includes a cessation of body growth at least partly due to products of the adrenal glands.

Histological, immunohistochemical, and ultrastructural features of a rat medullary thyroid carcinoma transfected with a corticotropin-releasing hormone cDNA expression vector

Clonal cell lines producing corticotropin-releasing hormone (CRH) have been generated by transfection of the W2 rat medullary thyroid carcinoma (MTC) cell with a CRH-encoding CMV/ SV40 expression vector. Here, we report the morphological, immunohistochemical, and ultrastructural features of rat tumors derived by implantation of CRH-producing W2CRH-7 cells and compare them with non-CRH-producing W2 MTCs. Both types of tumors grew rapidly and consisted of sheets and nests of pleomorphic cells infiltrating adjacent adipose tissue. Immunohistochemistry revealed CRH in only W2CRH-7 tumors; scattered cells in these tumors also were immunoreactive for chromogranin and for vasoactive intestinal peptide. Otherwise, the two tumor types exhibited similar profiles of various neuroendocrine markers, including neuron-specific enolase, synaptophysin, calcitonin, and somatostatin. Ultrastructurally, the tumors contained abundant dilated rough endoplasmic reticulum, a prominent Golgi apparatus, and numerous lysosomes. Very few secretory granules were noted in the W2 tumors; by contrast, secretory granules, although still not numerous in the majority of W2CRH-7 cells, were more abundant in scattered cells of those tumors. The positive immunostaining for CRH is consistent with the observations of increased plasma CRH and pituitary-adrenal activation induced by these transplanted tumors. This system provides a valuable model for CRH excess mimicking tumoral CRH-dependent Cushing's syndrome in human patients.

Gonadotrope-specific expression of the human follicle-stimulating hormone beta-subunit gene in pituitaries of transgenic mice

The molecular basis for the cell-specific and hormonal regulation of the pituitary gonadotropin beta-subunit genes is unknown. To identify the essential DNA regulatory elements of the human FSH beta (hFSH beta) gene, we introduced a 10-kilobase cloned fragment encompassing the gene into the germline of transgenic mice. We showed by a combination of Northern blot hybridization, RIA, and immunofluorescence histochemistry that the hFSH beta gene was expressed exclusively in mouse pituitary gonadotropes. Human FSH beta was actively secreted, presumably as an interspecies heterodimer, with mouse alpha-subunit, and no free hFSH beta was detected in serum. Male transgenic mice consistently had higher basal pituitary expression and secretion of hFSH beta than the female transgenic mice. Castration combined with testosterone replacement caused a marked reduction in hFSH beta gene expression in transgenic males, but had no effect on the pituitary levels of FSH in normal male mice. These experiments demonstrate that the essential DNA regulatory elements and gonadotrope transcription factors required for cell-specific expression of the FSH beta gene must be conserved between human and mouse. There appear to be important differences in the hormonal regulation of the gene between species, however.

Pituitary-specific and hormonally regulated gene expression directed by the rat proopiomelanocortin promoter in transgenic mice

All aspects of POMC biosynthesis exhibit tissue-specific regulation. The single copy gene is highly expressed in anterior lobe (AL) corticotrophs and intermediate lobe (IL) melanotrophs of the pituitary gland and in the arcuate nucleus of the hypothalamus. POMC gene transcription in corticotrophs is induced by hypothalamic CRH and vasopressin and inhibited by adrenal glucocorticoids, while in melanotrophs it is predominantly regulated by beta-adrenergic neural input and dopamine. To identify the rat POMC (rPOMC) gene sequences necessary and sufficient to target expression and hormonal regulation in corticotrophs and melanotrophs, we generated 13 transgenic mice carrying rPOMC fusion genes. The genes consisted of 706 or 480 basepairs of rPOMC 5' flanking sequences ligated to either the E. coli LacZ gene encoding beta-galactosidase or the K1 mutant of the SV40 large T-antigen gene. Overall, half of the transgenic lines had reporter gene expression in their AL and IL in a pattern indistinguishable from ACTH immunohistochemistry. In three of these lines, beta-galactosidase or K1 T-antigen was localized by double immunofluorescence exclusively to ACTH-positive corticotrophs and melanotrophs. Transcriptional regulation of the rPOMC-LacZ fusion gene in response to hormonal manipulation was quantified by a fluorescence assay for beta-galactosidase enzyme activity in pituitary extracts. There was a 15-fold increase in AL enzyme activity after adrenalectomy and a 3-fold increase in IL activity after haloperidol treatment. X-gal histochemistry of pituitaries from hormonally treated mice confirmed the cellular specificity of these effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin gene regulation: an overview

The somatostatinergic system has proven to be one of the best models of neuropeptide biology. Originally characterized as a hypothalamic regulator of growth hormone secretion, somatostatin also regulates the secretion of several other pituitary, pancreatic, and gastrointestinal (GI) hormones including thyrotropin-stimulating hormone, insulin, glucagon, and gastrin. Disorders in somatostatin metabolism have been proposed to contribute to the pathogenesis of Alzheimer's disease, epilepsy, GI motility disorders, and diabetes. On a more basic level, studies of somatostatin action have integrated divergent concepts of intracellular signal transduction. Advances in the understanding of somatostatin biosynthesis have had an impact on areas outside the field of endocrinology by providing new concepts of eukaryotic gene regulation. This report focuses on the transcriptional regulation of somatostatin gene expression. Two aspects of somatostatin gene transcription will be considered--regulated expression by second messengers and tissue-specific basal expression.

Cryptic human growth hormone gene sequences direct gonadotroph-specific expression in transgenic mice

Our laboratory reported previously that chimeric genes encoding either rat somatostatin (SS) or human GH (hGH), but containing the identical mouse metallothionein-I (MT) promoter/enhancer sequences and hGH 3'-flanking sequences, were selectively expressed in the gonadotrophs of transgenic mice. The experiments reported here were designed to identify the DNA sequences responsible for this unexpected cell-specific expression within the anterior pituitary. We produced new transgenic mice expressing fusion genes that tested separately the requirement of the MT or 3'-hGH sequences for gonadotroph expression. A fusion gene that retained the original MT and SS sequences, with a simian virus 40 polyadenylation signal exchanged for the 3'-hGH sequences, no longer directed strong pituitary expression, but was active in the liver. In contrast, a cytomegalovirus promoter/enhancer-SS-hGH fusion gene was expressed at the same high level in the anterior pituitaries of transgenic mice as the originally studied MT-SS-hGH gene. Immunohistochemical analysis indicated that pituitary expression of the cytomegalovirus promoter/enhancer-SS-hGH fusion gene was also restricted to gonadotroph cells in adult mice. These studies indicate that sequences within the 3'-flanking region of the hGH gene can direct expression of chimeric genes to pituitary cells that do not normally produce growth hormone.