Activation of thyrotropin-releasing hormone gene expression in cultured fetal diencephalic neurons by differentiating agents

The present studies were undertaken to investigate the effect of 5-bromo-2'-deoxyuridine (BrdU; 50 microM) or forskolin/3-isobutyl-1-methylxanthine (F/I; 10/500 microM) on TRH gene expression in cultured fetal diencephalic cells. BrdU as well as drugs such as F/I that raise intracellular cAMP levels had been previously termed differentiating agents because they cause morphological and functional differentiation of IMR-32 neuroblastoma cells. We postulated that neurons of fetal diencephalons may remain relatively undifferentiated in vitro and that this might be the reason for low or undetectable TRH production. We hypothesized that treatment with differentiating agents might increase neuropeptide expression. Both BrdU and F/I dramatically (P < 0.01) raised intracellular TRH and pro-TRH messenger RNA concentrations in cultured diencephalic neurons. Although a short BrdU exposure during the first 4 days of culture was sufficient to irreversibly change TRH neurons and to cause maintenance of high TRH levels after withdrawal of the drug, F/I had to be present continuously throughout the observation period of 16 days to significantly elevate TRH expression. This suggests that BrdU and F/I act at different intracellular sites to activate TRH expression in cultured diencephalic neurons. The reduction of glial cells that occurs concurrent with the BrdU treatment was not observed after F/I exposure, and therefore, this effect does not appear to be a key factor for the induction of TRH expression. As the intracellular accumulation of somatostatin and arginine vasopressin, which were determined in parallel, was similarly enhanced after treatment with BrdU or F/I, our culture system might provide a valuable tool for the study of these and possibly other neuropeptides in vitro.

Thyrotropin-releasing hormone gene expression in cultured anterior pituitary cells: role of gender

The present studies were undertaken to investigate the effect of gender on thyrotropin-releasing hormone (TRH) gene expression in cultured anterior pituitary (AP) cells. AP cells derived from 15-day-old male, female, or female pups that had been neonatally treated with testosterone propionate (TP), were cultured for up to 18 days in a modified DMEM/L-15 medium containing 10% fetal calf serum. TRH and AP hormones including GH, prolactin (PRL), luteinizing hormone (LH) and thyrotropin (TSH) were measured by RIA, proTRH mRNA was determined by in situ hybridization using a full-length riboprobe followed by quantification with a computer-assisted image analysis system. Cultures derived from female rats contained significantly (p < 0.01) higher amounts of TRH and secreted approximately twice (p < 0.01) as much TRH under basal conditions and in response to activators of the protein kinase A and C pathways, respectively. In situ hybridization studies revealed that 'female' cultures contained significantly higher amounts of proTRH mRNA compared to 'male' cultures. Computer-assisted image analysis demonstrated that proTRH mRNA levels were 3.5 times higher in 'female' compared to 'male' cultures (p < 0.01), an effect that was the result of a significantly higher number (3 times; p < 0.01) of cells expressing proTRH mRNA in 'female' cultures. Neonatal TP treatment did not affect either proTRH mRNA or TRH peptide levels. In vitro testosterone treatment resulted in a moderate rise (p < 0.05) of intracellular TRH accumulation in cultures from both sexes, however, proTRH mRNA levels remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Onset of pro-thyrotropin-releasing hormone gene expression in cultured rat anterior pituitary cells is expedited by dexamethasone

The early onset of proTRH gene expression in anterior pituitary (AP) cells in culture and its regulation by dexamethasone (DEX) were investigated. AP cells derived from 15-day-old rats were cultured for up to 4 days in the presence or absence of 10(-7) M DEX. TRH peptide levels, which could be detected only after 3 days of culture in control cells, were detectable after 1 day in DEX-treated cells. Levels rose from undetectable (< 35 fmol/well/0.2 x 10(6) cells) to 121 +/- 11 fmol/well in control cells and from 59 +/- 3 to 2978 +/- 88 fmol/well in DEX-treated cells (Day 1 to Day 4; means +/- SEM, n = 6). ProTRH mRNA levels as analyzed by in situ hybridization showed an excellent correlation with TRH peptide levels: mRNA was already detectable on Day 1 in DEX-treated cells and on Days 2-3 in control cells. DEX stimulated proTRH mRNA levels as determined by Northern blot analysis within 4 h. The half-life of proTRH mRNA was calculated based on a first-order decay model by measuring mRNA levels after addition of 5 micrograms/ml actinomycin D with or without DEX. The t1/2 of proTRH mRNA in control cells was 13.1 +/- 2.8 h and was not influenced by DEX treatment (12.5 +/- 2.8 h). Since DEX stimulated proTRH mRNA levels acutely without any increase in mRNA stability, we propose that DEX expedites proTRH gene expression in our AP cell culture system by acting at the transcriptional level.

Thyrotropin-releasing hormone gene expression in the anterior pituitary. III. Stimulation by thyroid hormone: potentiation by glucocorticoids

The present study was designed to investigate the effect of thyroid hormone on TRH gene expression in cultured anterior pituitary (AP) cells. AP cells derived from 15-day-old male rats were cultured for up to 14 days in Dulbecco's Modified Eagle's Medium-L-15 medium supplemented with either fetal calf serum (FCS) or FCS devoid of thyroid hormones. T4 or T3 were added at various concentrations to the medium for a duration of 2-14 days. TRH and GH were measured by RIA, and pro-TRH mRNA levels were determined by semiquantitative in situ hybridization. Addition of both T3 and T4, but not the biologically inactive diiodothyronine, significantly stimulated TRH accumulation in AP cells. T3 increased TRH content in a time- and dose-dependent fashion and was much more potent than T4. Dexamethasone (Dex) also raised the content of TRH significantly. The combination of 10(-9) M T3 and 10(-8) M Dex dramatically potentiated the effect of either treatment alone (T3, 8.9-fold rise; Dex, 37.2-fold rise) and increased TRH accumulation 251.2-fold (all P < 0.01). Levels of pro-TRH mRNA mirrored TRH content data. T3, Dex, or the combination of both raised pro-TRH mRNA levels 1.9-, 2.7 (both P < 0.05)-, and 11.1 (P < 0.01)-fold, respectively. The visualization of pro-TRH mRNA by in situ hybridization revealed that the combination of T3 and Dex treatment caused a substantial increase in the number of cells expressing pro-TRH. The results presented here demonstrate that T3 increases pro-TRH gene expression in cultured AP cells and that glucocorticoids markedly potentiate this effect. As pro-TRH is expressed in a subpopulation of somatotrophs, our data suggest that the TRH gene in this location may be coordinately regulated with the GH gene.

Thyrotropin-releasing hormone (TRH) gene expression in the anterior pituitary. I. Presence of pro-TRH messenger ribonucleic acid and pro-TRH-derived peptide in a subpopulation of somatotrophs

We have previously reported the presence of authentic pro-TRH-derived peptides in cultured anterior pituitary (AP) cells. The present studies were undertaken to determine whether pro-TRH mRNA could be demonstrated in the AP and to elucidate the cell type expressing pro-TRH. AP cells were cultured for up to 18 days, during which time the content of both TRH and prepro-TRH-(25-50) rose significantly (P < 0.01). In contrast, the cellular contents of LH, FSH, TSH, and ACTH fell significantly (P < 0.01), whereas that of GH increased by 45.9% (P < 0.05). Northern blot analysis revealed that the levels of pro-TRH mRNA extracted from AP cells (18 days in culture) were similar to those in hypothalamic tissue from adult male rats, indicating a high relative abundance of this mRNA in the AP. In situ hybridization experiments showed a dense accumulation of silver grains over a subpopulation of cultured AP cells. A combination of in situ hybridization for pro-TRH mRNA and immunocytochemistry for pituitary hormones revealed colocalization of pro-TRH mRNA and GH in a subpopulation of somatotrophs. No colocalization with LH-, TSH-, PRL-, or beta-endorphin-containing cells was observed. Immunocytochemistry at the electron microscopic level demonstrated that prepro-TRH-(25-50) was contained in a subpopulation of secretory granules in AP cells expressing this pro-TRH-derived sequence. These studies demonstrate that pro-TRH mRNA is present in cultured AP cells in high concentration and that the pro-TRH gene is expressed within a subpopulation of somatotrophs.

Thyrotropin-releasing hormone gene expression in the anterior pituitary. II. Stimulation by glucocorticoids

The present studies were undertaken to determine whether glucocorticoids (GC) regulate TRH gene expression in cultured anterior pituitary (AP) cells. AP cells derived from 15-day-old male rats were cultured for up to 18 days in Dulbecco's Modified Eagle's Medium-L-15 medium supplemented with 1) fetal calf serum (FCS), 2) charcoal-treated FCS, 3) normal rat serum, or 4) serum from rats that were adrenalectomized, rendered hypothyroid, and gonadectomized (ATG rat serum). Dexamethasone (Dex) or corticosterone (Cort) was added to the culture medium at various concentrations with exposure times ranging from 4-18 days. TRH and prepro-TRH-(25-50) in cellular extracts and release media were measured by RIA, and pro-TRH mRNA was determined by Northern blot analysis and in situ hybridization. Dex substantially stimulated cellular TRH and prepro-TRH-(25-50) accumulation under all culture conditions investigated, i.e. in medium supplemented with any of the four sera. TRH gene expression did not occur in medium supplemented with charcoal-treated FCS or ATG rat serum. Pretreatment with 10(-8) M Dex caused a significant increase in basal as well as cAMP- or phorbol ester-stimulated release of the peptide. Steady state pro-TRH mRNA levels rose 6.8- and 4.2-fold (both P < 0.01) after treatment with 10(-8) M Dex for 4 and 12 days, respectively. In situ hybridization experiments revealed that this rise in pro-TRH mRNA levels was probably the result of an increase in the number of AP cells expressing pro-TRH. Both Dex and Cort caused a dose-dependent increase in TRH accumulation, but Cort was approximately 40 times less potent than Dex. These results indicate that GC stimulate TRH gene expression in cultured AP cells. The presence of GC in culture medium is a prerequisite for the occurrence of TRH gene expression in the AP. As GC have been reported to reduce pro-TRH mRNA levels in the hypothalamus in vivo, our results may provide an example of the tissue-specific effects of GC on TRH gene expression.

Multiple molecular forms of gonadotropin-releasing hormone in the brain of an elasmobranch: evidence for IR-lamprey GnRH

These studies investigated brains of skate, Raja erinacea (order Rajiformes, class Chondrichthyes), for gonadotropin-releasing hormone (GnRH) peptides by chromatograph and immunoreactivity with region-specific antisera raised against mammalian GnRH and lamprey GnRH. The region-specific antibody to lamprey GnRH-I was produced following conjugation to bovine serum albumin using the bis-diazotized benzidine method. This antibody was characterized by assaying a range of increasing dilutions of the known vertebrate GnRHs, as well as analogs to lamprey GnRH-I. Two analogs, lamprey [Phe2]GnRH-I and lamprey [Leu7]GnRH-I, were synthesized by solid phase peptide synthesis using a benzhydrylamine resin as the supporting medium and purified by chromatography. This antibody demonstrated less than 0.01% cross-reactivity with all GnRH peptides tested, suggesting a highly specific antibody with a region of amino acids 2-8 that appears essential for binding. In the skate brain, five immunoreactive (IR) GnRH forms were detected, four of which eluted in the same positions as synthetic mammal and chicken GnRH-I (which coelute): lamprey GnRH-I, salmon and chicken GnRH-II, and one that was an unidentified form. A minor peak coeluted with lamprey GnRH-III. The major form in the skate brain is considered to have eluted with synthetic mammalian GnRH. These studies confirm an earlier report of an IR-mammalian GnRH peptide and provide new evidence for IR-lamprey GnRH in the brain of an elasmobranch.

Changes in brain gonadotropin-releasing hormone, plasma estradiol 17-beta, and progesterone during the final reproductive cycle of the female sea lamprey, Petromyzon marinus

Changes in ovarian morphology, brain gonadotropin-releasing hormone (GnRH), plasma estradiol, and progesterone were examined during the 1988 and 1989 spawning migrations of the adult female sea lamprey, Petromyzon marinus. There were significant increases through time in brain GnRH (1989) and plasma estradiol (1988 and 1989), with progesterone levels fluctuating (1988 and 1989) during the freshwater phase of the spawning migrations. In 1989, brain GnRH and plasma estradiol levels gradually increased through time until just prior to spawning when levels decreased. During 1988, there were no significant changes in GnRH, which may reflect lower temperatures in that year. These data provide new information on brain GnRH during the final maturational processes in the female sea lamprey.

Protrh peptides are synthesized and secreted by anterior pituitary cells in long-term culture

The expression of two ProTRH derived peptides, thyrotropin--releasing hormone (TRH) and PrePro-TRH25-50 (PYE27) was studied in anterior pituitary (AP) cells cultured in monolayer for up to 21 days. TRH levels in extracted cells rose from undetectable at 3 days to 267 +/- 22.5 fmol/well (p less than 0.01) at 21 days in culture. When AP tissue was extracted without dissociation or culture TRH was undetectable. The molar ratio of TRH/PYE27 was approximately 5:1 as predicted by the structure of PreProTRH. Extracts of cultured AP cells coeluted with TRH and PYE27 standards when subjected to HPLC analysis. Basal TRH secretion was 13.2 +/- 1.8 fmol/well/30 min at 18 days in culture; depolarizing concentrations of K+ (55 mM) caused a 2.2 fold (p less than 0.01) Ca++ dependent increase in TRH release. Immunostaining for PYE27 was found in approximately 10% of the cell population. Our results suggest that authentic ProTRH peptides are synthesized by AP cells in long term culture but not in situ. While the mechanism of activation of the PreProTRH gene needs to be elucidated we propose that TRH and/or other ProTRH derived peptides may exert paracrine effects on AP function.