Homologous induction of growth hormone receptors in cultured rat hepatocytes

The administration of growth hormone (GH) to animals has been reported to induce GH receptors in liver and adipose tissue. However, GH addition to cultured fibroblasts and lymphoblasts downregulated GH receptors, suggesting an indirect mechanism for GH upregulation of its receptors in vivo. We evaluated the direct role of GH by adding it to rat hepatocytes cultured in serum-free medium supplemented as previously described (Barash et al. (1988) Endocrinology 122, 1151-1158). After 3 days in culture the initial 125I-bGH specifically bound (0.18 ng per mg protein) had declined 5-fold. Binding continued to decrease thereafter to 0.008 ng by day 9 of culture. When added after 3 days in culture both hGH and bGH induced GH receptors. The maximum level (0.1 ng/mg protein) was attained 2 days later (day 5 of culture) and remained at this plateau through day 9 of culture. Induction occurred with 10 ng/ml hGH and was maximal (4- to 12-fold control) at 250 ng/ml. At a supramaximal dose of 1000 ng/ml hGH downregulated GH receptor. GH receptor induction was equally seen with hGH, bGH and rGH and did not occur on incubation with oPRL or ACTH. Thyroxine (1 X 10(-5) M) augmented 125I-bGH binding to levels 3-fold those of control but did not further augment the inductive effect of GH alone. We conclude that hepatic GH receptors are upregulated by GH acting through its own receptor. The induction occurs rapidly without a lag phase. The failure to restore fully receptor levels to those seen in freshly prepared hepatocytes implies a role for other modulating factor(s).     

Hormonal regulation of growth hormone receptors in primary cultured rat hepatocytes

The hormonal regulation of GH receptors was studied by measuring specific binding of [125I]human GH to primary cultured rat hepatocytes. The binding of labeled GH to primary cultured hepatocytes decreased during culture, but addition of dexamethasone (100 nM) compensated for this decrease and even increased GH binding. After addition of dexamethasone, the binding increased to a maximum after 10 h, and after 24 h was about 6 times that of control cells. Glucagon (100 nM) did not have any significant effect on GH binding by itself, but enhanced the increased binding caused by dexamethasone about 1.5-fold. For this effect, glucagon could be replaced by (Bu)2cAMP. Insulin (10 nM) and epidermal growth factor (20 ng/ml) reduced the increase by dexamethasone plus glucagon by about half. Scatchard plot analysis showed that the changes of GH binding induced by various hormones were due to changes in the number of binding sites without significant changes in their affinity. The GH bound to dexamethasone or dexamethasone plus glucagon-treated cells was not replaced by unlabeled ovine PRL. This strongly suggests that the number of somatogenic (GH) receptors may be subject to hormonal regulation: dexamethasone alone or with glucagon may induce GH receptors, whereas insulin and EGF may suppress the induction of GH receptors. These patterns of hormonal regulations were almost the same as those of proteins whose expressions were known to be differentiated functions of liver. On the other hand, the increase of GH binding by dexamethasone was inhibited by cycloheximide and actinomycin D, though the GH binding was inhibited by cycloheximide, but not by actinomycin D in the cells cultured without dexamethasone. This result suggests that the increased binding induced by dexamethasone is dependent on the synthesis of new protein and is probably regulated at a pretranslational level.     

Prolactin (PRL)-releasing peptide stimulates PRL secretion from human fetal pituitary cultures and growth hormone release from cultured pituitary adenomas

The hypothalamic peptide PRL-releasing peptide (PrRP) has recently been cloned and identified as a ligand of an orphan pituitary receptor that stimulates in vitro PRL secretion. PrRP also induces PRL release in rats in vivo, especially in normal cycling females. However, no information on the effects of PrRP in the human is available. To elucidate the role of PrRP in regulating human anterior pituitary hormones, we used human PrRP-31 in primary cultures of human pituitary tissues, including fetal (20--27 weeks gestation) and normal adult pituitaries, as well as PRL- and GH-secreting adenomas. PrRP increased PRL secretion from human fetal pituitary cultures in a dose-dependent manner by up to 35% (maximal effect achieved with 10 nM), whereas TRH was slightly more potent for PRL release. Coincubation with estradiol resulted in enhanced fetal PRL response to PrRP, and GH release was only increased in the presence of estradiol. Although PRL secretion from PRL-cell adenomas was not affected by PrRP, PrRP induced PRL release from cultures of a GH-cell adenoma that cosecreted PRL. PrRP enhanced GH release in several GH-secreting adenomas studied by 25--27%, including GH stimulation in a mixed PRL-GH-cell tumor. These results show for the first time direct in vitro effects of PrRP-31 on human pituitary cells. PrRP is less potent than TRH in releasing PRL from human fetal lactotrophs and is unable to release PRL from PRL-cell adenomas in culture, but stimulated GH from several somatotroph adenomas. Thus, PrRP may participate in regulating GH, in addition to PRL, in the human pituitary.     

Luteal cell receptor content for prolactin (PRL) and luteinizing hormone (LH): regulation by LH and PRL.

The effect of LH and PRL during the differentiation of granulosa cells to luteal cells was examined by determining the ability of LH and PRL to regulate luteal cell receptor content for these hormones and to increase production of progesterone. Preovulatory follicles and corpora lutea were hormonally induced in immature hypophysectomized female rats by sequential treatment with estradiol, hFSH and oLH. The content of receptor for LH was high in granulosa cells of large antral follicles. Administration of LH caused receptor for LH to decrease markedly within 24 h and to remain low for 96 h. In contrast, granulosa cell content of receptor for PRL increased progressively for 48 h following LH stimulation and remained elevated in fully luteinized cells at 96 h. This increase in PRL receptor appears to be functionally related to the ability of luteal cells to respond to PRL. When PRL was given for 4 days after LH, both luteal cell progesterone production and LH receptor content increased progressively after, but not before, 48 h. Since these changes occurred in the absence of LH, the increase in LH receptor appears to be a consequence of, but not a requirement for, the PRL-induced increase in progesterone production. If daily injections of PRL were delayed for 72 or 96 h following LH induction of lutenization, luteolytic rather than luteotropic effects of PRL were observed. Since receptor for PRL remained elevated at 72 and 96 h, intracellular mechanisms and not receptor content, appear to be effecting the response of luteal cell to PRL.     

Rat growth hormone (GH) but not prolactin (PRL) induces both GH and PRL receptors in female rat liver

This study was designed to elucidate which hormone is responsible for the induction of GH and PRL receptors in rat liver. Intact female rats were implanted with osmotic minipumps delivering rat GH (rGH) or ovine GH (oGH) or PRL at various rates from 75 to 800 micrograms/day for 7 days, and binding of radioiodinated bovine GH or ovine PRL (oPRL) tracer was measured on liver microsomal membranes. MgCl2 treatment was used to remove bound hormones from receptors before tracer binding. Infusion of rGH resulted in a significant increase (P less than 0.001) in both GH and PRL binding, the effect being maximal (2.5- to 3-fold for both ligands) at rGH infusion rates from 150 to 400 micrograms/day. Serum rGH levels were elevated 3- to 5-fold in these animals, but somatomedin-C concentrations were not higher than in controls. MgCl2 treatment showed that GH, but not PRL, binding sites in rGH-treated animals were significantly occupied by administered hormone. Analysis of competitive binding curves indicated that receptors for both GH and PRL increased in concentration without changes in binding affinity. In contrast to the rGH effect, oGH infusion from 75 to 400 micrograms/day failed in two experiments to consistently alter either bovine GH or oPRL binding sites. This was not explained by the potency of the preparation at the somatogenic receptor; oGH was in fact more potent than rGH. The effects of rat PRL and oPRL infusion on receptor levels were also assessed. In contrast to previous reports, neither preparation caused induction of either PRL or GH binding sites. oPRL decreased PRL binding by 30-40% when infused between 200 and 400 micrograms/day, whereas rat PRL had a less consistent effect. MgCl2 stripping of membranes suggested that administered PRL preparations did not significantly occupy PRL receptors. GH receptors were unaffected in any PRL-treated group. It is concluded that in intact female rats, rGH regulates the concentration of both GH and PRL receptors. The slight down-regulation of PRL receptors resulting from PRL infusion casts further doubt on the concept that PRL induces its own hepatic receptors.     

Prolactin (PRL), follicle-stimulating hormone, and luteinizing hormone are regulators of testicular PRL receptors in golden hamsters

The ability of PRL, FSH, and LH to regulate testicular PRL receptors in golden hamsters was evaluated using a variety of experimental protocols. Exposure to a photoperiod of 5 h of light and 19 h of darkness (5L:19D) for 11 weeks precipitated a 94% reduction in content (femtomoles per testes) of testicular PRL receptors and, concomitantly, a decrease (P less than 0.05) in plasma PRL, but not LH or FSH. One pituitary gland under the kidney capsule in 5L:19D-housed hamsters increased (P less than 0.05) both the concentration (femtomoles per mg protein) and content of PRL receptors, as well as those of plasma PRL and FSH. Similar treatment in 14L:10D-housed hamsters produced comparable changes in plasma PRL and FSH without affecting PRL receptors. Injections of L-dopa for 7 days into hamsters housed in 5L:19D for 11 weeks significantly elevated serum FSH concentrations, had no measurable effect on serum PRL and LH, and induced a greater than 2-fold increase in PRL receptor levels. In a separate study, hamsters housed in 5L:19D for 12 weeks were injected for 3 days with 250 microgram ovine (o) PRL, 25 microgram oLH, or 5 microgram oFSH, and results were compared with vehicle-injected, 5L:19D- and 14L:10D-housed controls. Injections of oPRL and oLH increased (P less than 0.05) PRL receptor concentration and content, with PRL being more efficacious. No anti-oPRL antibodies were produced by oPRL injections. In this study, injections of oFSH were without effect on PRL receptors. To ascertain the effects of each hormone in the absence of other trophic influences, experiments were conducted in hypophysectomized hamsters injected daily for 3 days (2-4 days posthypophysectomy) with one of the following: 5 or 25 microgram oLH; 10, 50, or 250 microgram oPRL; or 1 or 5 microgram oFSH. Hypophysectomy reduced the concentration and content of PRL receptors by 85%, and treatment with 50 or 250 microgram oPRL increased (P less than 0.05) these low levels almost 3-fold. Again, no anti-oPRL antibodies were induced. Injection of 5 microgram oLH or 25 microgram oFSH also induced increases (P less than 0.05) in PRL receptors. Hypophysectomy reduced basal and hCG-stimulated in vitro testicular testosterone production (nanograms per testes/4 h) to levels less than 20% of control values. None of the hormonal treatments affected (P less than 0.05) basal testosterone production in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dexamethasone inhibits both growth hormone (GH)-induction of insulin-like growth factor-I (IGF-I) mRNA and GH receptor (GHR) mRNA levels in rat primary cultured hepatocytes.

Glucocorticoids are potent inhibitors of growth. In this work, we investigated whether glucocorticoids inhibit the stimulatory action of GH on IGF-I gene expression in rat hepatocytes. GH increased IGF-I mRNA levels 11-fold after 24 h, whereas high doses of DXM (10(-6)M) caused a slight (2.6-fold) increase of IGF-I mRNA levels. However, high doses of DXM (10(-6)M) inhibited the induction of IGF-I mRNA by GH. To assess the role of GHR in this inhibition, we investigated the regulation of GHR expression. High doses of DXM decreased GHR mRNA levels. This effect was already detectable 6 h after addition of 10(-6)M DXM and was dose-dependent, with a maximal inhibition observed at a concentration of 10(-6)M. In conclusion, our results show that high doses of DXM inhibits the GH-induced IGF-I gene expression and the GHR gene expression. The parallel decrease of GHR and GH-induced IGF-I mRNA suggests that the GH resistance caused by DXM is mediated by diminished GH receptor synthesis.     

Insulin-like growth factor binding protein-3 secretion from cultured rat sertoli cells: dual regulation by follicle stimulating hormone and insulin-like growth factor-I.

The insulin-like growth factors (IGFs) are found in extracellular fluids bound to carrier proteins which influence the biological activity of the IGFs. Three structurally different binding proteins (BPs) have been isolated and cloned; each has distinct tissue specific expression and unique properties. We report here that testicular cells synthesize a specific subset of these binding proteins. Ligand blot analysis and RNA blot hybridization indicates that cultured peritubular cells synthesize primarily IGFBP-2. In contrast, as determined by ligand blot, RNA blot hybridization and N-linked deglycosylated studies, IGFBP-3 is predominantly synthesized by the Sertoli cell. In a dose dependent fashion, FSH markedly reduces the levels of IGFBP-3 in Sertoli cell conditioned medium. Similarly, isoproterenol, (Bu)2cAMP and cholera toxin also markedly reduce the abundance of IGFBP-3 in conditioned media. In contrast, IGF-I increases the concentrations of IGFBP-3 with the concentration required for half-maximal stimulation, approximately 20 ng/ml. Consistent with a peritubular cell origin, IGFBP-2 may be the predominant species found in interstitial fluid. In summary, our data reveal that the IGFBPs are expressed in a cell type specific manner in the testis. The opposing effects of FSH and IGF-I on Sertoli cell IGFBP-3 expression suggests a mechanism by which the IGF-I biological activity on Sertoli cell might be influenced.     

Prolactin (PRL) regulation of maternal behavior in rats: bromocriptine treatment delays and PRL promotes the rapid onset of behavior

Recent findings indicate that PRL helps stimulate the onset of maternal behavior in inexperienced hypophysectomized steroid-treated female rats. In a series of five experiments we have further examined the involvement of PRL in maternal behavior using nonhypophysectomized ovariectomized rats treated concurrently (type I) or sequentially (type II) with progesterone (P) and estradiol (E2) and administered either bromocriptine (to suppress endogenous PRL secretion) or bromocriptine plus ovine PRL. In Exp 1 plasma PRL concentrations were measured in ovariectomized rats treated for 2 weeks with a combination of E2 and P Silastic implants. Type I steroid-treated (2mm E2, days 1-24; three 30 mm P, days 3-13) rats exhibited elevated plasma PRL levels throughout the sampling period compared with nonsteroid-treated controls. In contrast, PRL concentrations in type II steroid-treated (P, days 3-13; E2, days 13-24) females were low (similar to controls) from days 3-13 when the type II steroid-treated females were exposed to P only. Like type I treated rats, PRL levels in type II steroid-treated rats were elevated from day 13 onward after E2 capsule insertion. In Exp 2, treatment of both type I and type II steroid-treated rats with bromocriptine (2 mg/kg, sc) twice daily beginning on treatment day 13 suppressed basal PRL concentrations and prevented the estrogen-induced diurnal PRL surge. Whereas PRL was effectively suppressed by bromocriptine in both steroid-treated groups, the absolute levels of PRL were lower in rats treated with the type II steroid regimen. Behavioral analyses in Exp 3, 4, and 5 revealed that bromocriptine administration, while failing to interfere with the onset of maternal behavior in rats treated with the type I concurrent steroid regimen, disrupted the onset of maternal care in rats treated with the type II sequential steroid regimen. When a separate set of type II steroid-treated rats was given both bromocriptine (2 mg/kg) plus ovine PRL (0.5 mg, sc) twice daily, maternal behavior rapidly appeared. Thus, suppression of endogenous PRL secretion delays the onset of maternal behavior in nonhypophysectomized steroid-primed rats, an effect prevented by concurrent administration of ovine PRL. In addition to providing further experimental support for PRL's role in maternal behavior, the development of this endocrine regimen provides researchers with a potentially fruitful model to examine neural sites and mechanisms of PRL regulation of maternal behavior in mammals.     

Regulation of growth hormone (GH) receptor (GHR1 and GHR2) mRNA level by GH and metabolic hormones in primary cultured tilapia hepatocytes

Growth hormone (GH) regulates essential physiological functions in teleost fishes, including growth, metabolism, and osmoregulation. Recent studies have identified two clades of putative receptors for GH (GHR1 clade and GHR2 clade) in fishes, both of which are highly expressed in the liver. Moreover, the liver is an important target for the anabolic effects of GH via endocrine IGFs, and liver sensitivity to GH is modulated by metabolic hormones. We investigated the effects of GH, insulin, glucagon, cortisol and triiodothyronine on GHR1 and GHR2 mRNA levels in primary cultured tilapia hepatocytes. Physiological concentrations of GH strongly stimulated GHR2 mRNA level (0.5-50×10(-9)M), but did not affect GHR1 mRNA level. Insulin suppressed stimulation of GHR2 mRNA level by GH (10(-8)-10(-6)M). Insulin increased basal GHR1 mRNA level (10(-8)-10(-6)M). Cortisol increased basal GHR2 mRNA level (10(-7)-10(-6)M), but did not consistently affect GH-stimulated GHR2 mRNA level. Cortisol increased basal GHR1 mRNA level (10(-9)-10(-6)M). Glucagon suppressed GH-stimulated GHR2 mRNA level and increased basal GHR1 mRNA level at a supraphysiological concentration (10(-6)M). A single injection of GH (5μg/g) increased liver GHR2 mRNA level, and insulin injection (5μg/g) decreased both basal and GH-stimulated GHR2 mRNA levels after 6h. In contrast, insulin and GH injection had little effect on liver GHR1 mRNA level. This study shows that GHR1 and GHR2 gene expression are differentially regulated by physiological levels of GH and insulin in tilapia primary hepatocytes.     

Prolactin (PRL)-like protein J, a novel member of the PRL/growth hormone family, is exclusively expressed in maternal decidua.

A search of a nonmouse, nonhuman, expressed sequence tag database for messenger RNAs in the PRL/GH family has identified a novel rat complementary DNA clone. The encoded protein, designated PRL-like protein J (PLP-J), is predicted to be synthesized as a precursor of 211 amino acids, modified by N-linked glycosylation, and secreted as a mature glycoprotein of 182 residues. PLP-J messenger RNA synthesis is limited to early pregnancy with abundant expression on day 7, slightly declining expression on day 9, and no detectable expression by day 11. Unlike most other PRL family members, PLP-J does not appear to be synthesized by placental trophoblasts but, rather, by decidual cells surrounding the implantation site. By sequence similarity to rat PLP-J, a murine clone was identified in a mouse expressed sequence tag database. Mouse PLP-J was used to map the gene to a 700-kb region of mouse chromosome 13 that includes other members of the PRL/GH family.     

Glucocorticoid regulation of growth hormone (GH) secretagogue-induced growth responses and GH secretagogue receptor expression in the rat.

Synthetic GH-releasing peptides such as GHRP-6 are potent GH secretagogues (GHSs) in several species, but attempts to stimulate growth by continuous GHS exposure have had limited success. GHSs also release ACTH and adrenal steroids. Since glucocorticoid excess is associated with poor linear growth, stimulation of the hypothalamo-pituitary-adrenal (HPA) axis by continuous GHS administration may compromise their growth-promoting effects. We have now examined the effects of continuous GHRP-6 infusion (100 mg/day, s.c. for 14 days) in normal 150-day-old female rats, and in adrenalectomized (Adx) rats with or without dexamethasone (Dex) replacement. Infusion of GHRP-6 did not significantly affect body weight gain compared with excipient-treated controls in either intact rats (controls, 9.0 +/- 1.6 vs GHRP-6, 11.8 +/- 0.9 g) or Adx rats (4.4 +/- 1.5 vs 7.9 +/- 2.7 g). However, GHRP-6 significantly increased weight gain in Adx rats treated with Dex (controls, 3.5 +/- 1.4 vs GHRP-6, 15.4 +/- 1.6 g;P<0.01). Adrenalectomy decreased plasma triglycerides (P<0.01), and Dex treatment increased plasma cholesterol (P<0.001), GHRP-6 treatment did not affect these plasma lipids. Dex treatment also reduced plasma GH-binding protein levels and hepatic GH binding (P<0.05). Pituitary GH content was decreased in Adx rats (P<0.05) but not in Dex-treated Adx rats. Adrenalectomy markedly decreased GHS-receptor mRNA expression in the arcuate (P<0. 001) and ventromedial nuclei (P<0.01), whilst Dex treatment normalized GHS-receptor expression. These results suggest that adrenal steroids are necessary for normal GHS-receptor expression and GHRP-6-induced weight gain, but long-term stimulation of the HPA axis by continuous GHS exposure may be detrimental to the growth response.     

Prolactin (PRL) receptor gene expression in mouse adipose tissue: increases during lactation and in PRL-transgenic mice

There are indications that PRL may exert important metabolic actions on adipose tissue in different species. However, with the exception of birds, the receptor has not been identified in white adipose tissue. The present study was designed to examine the possible expression and regulation of the PRL receptor (PRLR) in mouse adipose tissue. The long PRLR messenger RNA (mRNA) splice form (L-PRLR) and two short splice forms (S2- and S3-PRLR) were detected in mouse adipose tissue by RT-PCR. Furthermore, L-PRLR mRNA was detected by ribonuclease protection assay. Immunoreactive PRLR with a relative molecular mass of 95,000 was revealed by immunoblotting. Furthermore, L-PRLR mRNA expression was demonstrated in primary isolated adipocytes. In mouse adipose tissue, the level of L-PRLR mRNA expression increased 2.3-fold during lactation compared with those in virgin and pregnant mice. In contrast, in the liver the expression of L-PRLR increased 3.4-fold during pregnancy compared with those in virgin and lactating mice. When comparing the levels of L-PRLR expression in virgin female and male mice, no difference was detected in adipose tissue. However, in virgin female liver the expression was 4.5-fold higher than that in male liver. As PRL up-regulates its own receptor in some tissues, we analyzed L-PRLR expression in PRL-transgenic female and male mice. In PRL-transgenic mice L-PRLR expression was significantly increased in both adipose tissue (1.4-fold in females and 2.4-fold in males) and liver (1.9-fold in females and 2.7-fold in males) compared with that in control mice. Furthermore, in female PRL-transgenic mice retroperitoneal adipose tissue was decreased in weight compared with that in control mice. However, no difference was detected when comparing the masses of parametrial adipose tissue. Our results suggest a direct role for PRL, mediated by PRLR, in modulating physiological events in adipose tissue.     

Growth hormone (GH) and 17beta-estradiol regulation of the expression of mouse GH receptor and GH-binding protein in cultured mouse hepatocytes.

In the present study, primary mouse hepatocytes from 8- to 10-week-old virgin female Swiss-Webster mice were perfused with collagenase (100 U/ml) using the two-step method. Isolated hepatocytes were plated in a rat tail type I collagen sandwich configuration to examine the regulation of GH receptor (GHR) and GH-binding protein (GHBP) expression by GH and 17beta-estradiol (E2). After 48 h of initial plating, hepatocytes were divided into groups of five replicates and treated for 24 h with medium containing no hormones (controls), GH (100 ng/ml), E2 (10(-9) M), E2 (10(-9) M) plus GH (100 ng/ml), or E2 plus GH and ICI 182-780 at different concentrations. Treatment of hepatocytes with GH or E2 alone did not have any effect on the cellular concentrations of GHBP and GHR. However, the combination of E2 and GH up-regulated the cellular concentrations of GHBP and GHR 2- to 3-fold. GHBP and GHR messenger RNA concentrations were also up-regulated 2- to 3-fold. ICI 182-780, a competitive inhibitor of E2 for the estrogen receptor (ER), at different concentrations inhibited the E2 and GH-induced stimulation of GHBP and GHR. Furthermore, ER concentrations increased 5- to 7-fold in hepatocytes treated with E2 and GH compared with those in untreated cells or cells treated with either E2 or GH alone. In the present study we have shown that in cultured hepatocytes from virgin female mice, GH or E2 alone did not affect the concentrations of GHBP and GHR. However, E2 and GH together significantly up-regulated GHR and GHBP expression.     

Hormonal regulation of pseudocholinesterase activity in cultured rat hepatocytes

Plasma pseudocholinesterase (PsChe) activity was examined in adult female rat hepatocytes isolated by collagenase perfusion and maintained in a chemically defined medium supplemented with dimethyl sulfoxide. Time course studies on PsChe activity in cultured hepatocytes indicate that cells maintained in a chemically defined medium lacking human GH and 17 beta-estradiol (E2) exhibit a decrease in activity after the first 3 days in culture followed by a stabilization of PsChe activity for up to 15 days. GH (0.02, 0.2, and 2 micrograms/ml) increased PsChe activity in a dose-dependent manner. Addition of E2 (10(-5)-10(-7) M) alone to hepatocyte cultures did not cause an increase in PsChe activity. The increases produced by both the 2 micrograms/ml and 0.2 micrograms/ml GH doses plus E2 (10(-7) M) were significantly greater than controls and similar to the increase produced by GH alone. The ability of the hepatocytes to express PsChe activity was not dependent upon the continuous exposure of the cells to GH, since control cultures, maintained for 12 days in medium lacking GH, were able to express a high level of PsChe activity after the addition of GH (2 micrograms/ml) on day 12. This increase was observed in hepatocytes in culture for 30 days. These results indicate that GH plays a pivotal role in the regulation of PsChe activity in vitro, and that under the conditions used in this study, E2 does not influence the ability of hepatocytes in culture to express this enzyme.     

Prolactin (PRL) induction of cyclooxygenase 2 (COX2) expression and prostaglandin (PG) production in hamster Leydig cells

Serum prolactin (PRL) variations play a crucial role in the photoperiodic-induced testicular regression-recrudescence transition in hamsters. We have previously shown that cyclooxygenase 2 (COX2), a key enzyme in the biosynthesis of prostaglandins (PGs), is expressed mostly in Leydig cells of reproductively active hamsters with considerable circulating and pituitary levels of PRL. In this study, we describe a stimulatory effect of PRL on COX2/PGs in hamster Leydig cells, which is mediated by IL-1β and prevented by P38-MAPK and JAK2 inhibitors. Furthermore, by preparative isoelectric focusing (IEF), we isolated PRL charge analogues from pituitaries of active [isoelectric points (pI): 5.16, 4.61, and 4.34] and regressed (pI: 5.44) hamsters. More acidic PRL charge analogues strongly induced COX2 expression, while less acidic ones had no effect.Our studies suggest that PRL induces COX2/PGs in hamster Leydig cells through IL-1β and activation of P38-MAPK and JAK2. PRL microheterogeneity detected in active/inactive hamsters may be responsible for the photoperiodic variations of COX2 expression in Leydig cells.