Hypothalamic peptides modulate cytosolic free Ca2+ levels and adenylyl cyclase activity in human nonfunctioning pituitary adenomas

The effects of hypothalamic peptides (TRH, GnRH, arginine vasopressin, vasoactive intestinal peptide, GHRH, CRH, and SRIH) on cytosolic free calcium concentrations ([Ca2+]i) and adenylyl cyclase (AC) activity were evaluated in 12 nonfunctioning pituitary adenomas. TRH, GnRH, and arginine vasopressin induced a marked [Ca2+]i rise in 10/12, 4/12, and 2/5 tumors, respectively. The transients induced by these peptides were due to both Ca2+ mobilization from the intracellular stores and Ca2+ influx from the extracellular medium. AC activity was evaluated in 10 adenomas; 1 microM vasoactive intestinal peptide induced a 2- to 6-fold stimulation of the enzyme activity in all tumors, while neither GHRH nor CRH were effective. Moreover, in 5/10 tumors 1 microM SRIH reduced both AC activity and [Ca2+]i, while in 2/10 the peptide caused a significant rise in [Ca2+]i despite the AC inhibition and in 3/10 SRIH did not modify either AC activity or [Ca2+]i. This study indicates that in nonfunctioning pituitary adenomas a wide spectrum of hypothalamic peptides modulate [Ca2+]i and AC activity. Moreover, the presence of biologically active receptors may offer a possible target for therapeutic intervention.

Clinical, biochemical, and morphological correlates in patients bearing growth hormone-secreting pituitary tumors with or without constitutively active adenylyl cyclase

Somatic mutations in the alpha-chain (alpha s) of the stimulatory regulatory protein of adenylyl cyclase (Gs) causing constitutive activation of the enzyme have been identified in a subset of human GH-secreting pituitary adenomas. This study reports on the differences between acromegalic patients bearing tumors without (group 1; n = 51) or with (group 2; n = 29) this alteration. No difference in age, sex, clinical features, duration of the disease, or cure rate was observed between the two groups. By contrast, group 2 patients had higher basal GH levels than group 1. Moreover, a significant difference in sellar morphology was found; group 2 patients more frequently showed sellas of normal size (grade I) than group 1. Hypersecretory activity of group 2 tumors was also apparent at electron microscopy; contrary to those of group 1, cells of group 2 tumors were densely granulated and showed prominent rough endoplasmic reticulum and Golgi complex. With respect to group 1, group 2 patients were less responsive to GH-releasing hormone, while they were more sensitive to somatostatin- and dopamine-induced GH inhibition. These results suggest that patients with constitutively active adenylyl cyclase have hyperactive tumors; the sensitivity of these tumors to inhibitory agents (somatostatin and dopamine), possibly counteracting the expression of activating mutations, might explain the low rate of tumor growth.

Thyroxine 5'-deiodinase in human anterior pituitary tumors

The activity of T4 5'-monodeiodinase (5'D) in the pituitary was measured in 12 patients with pituitary adenoma (3 patients with acromegaly, 2 with prolactinoma, 1 with Cushing's disease, 1 with TSH-producing tumor, and 5 with nonfunctioning tumor) and, as a control, in a patient who died of parotid cancer. The pituitaries, obtained at operation or autopsy, were homogenized in 0.1 mol/L potassium phosphate buffer, pH 7.0, and centrifuged at 800 x g. Supernatants were incubated with [125I]T4 and 20 mmol/L dithiothreitol (DTT) at 37C for 90 min. T4 5'-D was measured by the release of 125I- with the ion exchange method. The activity of T4 5'-D in the pituitaries from patients with prolactinoma and parotid cancer was dependent on protein concentration, incubation time, incubation temperature, and T4 concentration, and was labile to prior heating at 70 C for 30 min. T4 5'-D was not inhibited by 1 mmol/L propylthiouracil, but was inhibited 95% by 0.1 mmol/L iopanoic acid. The apparent Km and maximum velocity for T4 5'-D in homogenates of prolactinoma at 20 mmol/L DTT were 11 nmol/L and 1.54 pmol/mg protein.h, respectively. This reaction followed sequential-type reaction kinetics when the DTT concentration was varied. All other homogenates of pituitary tumors, except two nonfunctioning tumors, also demonstrated T4 5'-D activity. These results indicate that 1) the human pituitary express a low Km and PTU-insensitive T4 5'-D activity which is very similar to the type II enzyme activity in the rat pituitary; and 2) various types of pituitary tumor cells contain T4 5'-D activity.

A new constitutively activating mutation of the Gs protein alpha subunit-gsp oncogene is found in human pituitary tumours

A new 227 Gln----Lys oncogenic mutation of the Gs alpha subunit (the GTP-binding protein that stimulates adenylyl cyclase) has been found in three out of eight human growth hormone-secreting pituitary tumours of the group 2, characterized by high cAMP and constitutively activated adenylyl cyclase. The additional five group 2 tumours expressed another, previously described mutation, 201 Arg----Cys, while the tumours of group 1, characterized by normal cAMP and adenylyl cyclase activity, expressed neither these nor the two other mutations of the same sites previously described in group 2. The Gln 227 site of alpha s corresponds to the Gln 61 site in the low Mw G protein, p21 ras, where the Leu substitution had already been shown to be oncogenic. The oncogenic potential (gsp oncogene) of the mutations so far observed depends on the reduced GTPase activity intrinsic to alpha s, with constitutive activation of the protein and marked elevation of cAMP. The latter is the intracellular messenger that stimulates growth of pituitary growth hormone-secreting cells and other cell types.

Inhibition of basal and corticotropin-releasing hormone-stimulated adenylate cyclase activity and cytosolic Ca2+ levels by somatostatin in human corticotropin-secreting pituitary adenomas

The effects of CRH and somatostatin (SRIH) on adenylate cyclase (AC) activity, intracellular free calcium concentrations [( Ca2+]i) and in vitro ACTH release were investigated in six human ACTH-secreting pituitary adenomas. In all tumors, CRH induced a marked stimulation (from 69-210% at 10 nM), whereas SRIH caused a definite inhibition (from 29-50% at 100 nM) of membrane AC. When added together, CRH and SRIH caused a purely additive effect on AC. In adenomatous corticotrophs CRH (10 nM) caused [Ca2+]i to rise from 160 +/- 30 nM (mean +/- SD) to 410 +/- 95 nM. CRH-induced transients were biphasic, with an initial peak predominantly due to redistribution from intracellular Ca2+ stores and a secondary phase due to Ca2+ influx. The effects of CRH on [Ca2+]i were totally independent of the stimulation of AC. In fact, cAMP-elevating agents other than CRH did not modify [Ca2+]i. SRIH (100 nM) decreased resting [Ca2+]i (approximately 20-40%) as well as [Ca2+]i rises induced by CRH, arginine vasopressin, or high K+. The effect of SRIH on [Ca2+]i was maintained in presence of high cAMP levels, while was totally abolished after pertussis toxin pretreatment. CRH (10 nM) stimulated ACTH release (from 22.5 +/- 3.5 to 45.0 +/- 8.5 pmol/L) by an extent similar to that elicited by calcium ionophore and forskolin. By contrast, SRIH (0.1 microM) inhibited both basal and CRH-stimulated ACTH release. In conclusion, in human adenomatous corticotrophs SRIH exerts an inhibitory action by reducing both AC activity and, independently, [Ca2+]i. In this way, SRIH can efficiently counteract the stimulatory action of CRH that in these cells involves activation of both cAMP and Ca2+ pathways.

The importance of dopamine in the pathogenesis of experimental prolactinomas

The factors responsible for the production of prolactin-secreting tumors are obscure. One hypothesis, that chronic loss of dopamine control from the hypothalamus may be associated with prolactinoma formation, was tested. Female adult Fischer 344 rats were subjected to ovariectomy and were then given subcutaneous implants of diethylstilbestrol (DES) Silastic capsules to produce lactotrophic hyperplasia. Sequential studies assessed the neuronal activity of the tuberoinfundibular dopaminergic neurons of the arcuate nucleus of the hypothalamus (A12) during and after this estrogen-induced pituitary growth. Immunocytochemical staining for tyrosine hydroxylase was used as a marker for dopamine synthesis, plasma radioimmunoassay provided plasma prolactin levels, and magnetic resonance imaging and histological studies were performed to examine the structural changes occurring in the pituitary gland. Animals were sacrificed from 3 to 67 days after DES implantation. To determine the reversibility of the estrogen-induced changes, rats were also sacrificed at different time intervals after the removal of 30-, 40-, or 60-day DES implants. After 30 days of DES treatment, plasma prolactin levels increased 40-fold and pituitary weight increased more than threefold. Tyrosine hydroxylase immunoreactivity diminished gradually and was almost completely depleted at 30 days. Pituitary histology revealed marked prolactin cell hyperplasia. These changes were completely reversible; removal of the capsule after 30 days resulted in eventual normalization of plasma prolactin levels and pituitary size and in restoration of tyrosine hydroxylase immunoreactivity in the A12 region. Sixty days of DES treatment produced large hemorrhagic tumors with sustained high plasma prolactin levels and an irreversibly distorted A12 area. These observations suggest that in these animals loss of dopamine regulation secondary to estrogen stimulation initially produces prolactin hyperplasia but that prolonged loss leads to adenoma formation.

Regulation of GH3 pituitary tumour-cell adenylate cyclase activity by activators of protein kinase C

The influence of protein kinase C (PKC) activation on cyclic AMP production in GH3 cells has been studied. The stimulation of cyclic AMP accumulation induced by forskolin and cholera toxin was potentiated by 4 beta-phorbol 12,13-dibutyrate (PDBu). Moreover, PDBu, which causes attenuation of the maximal response to vasoactive intestinal polypeptide (VIP), also induced a small right shift in the dose-response curve for VIP-induced cyclic AMP accumulation. PDBu-stimulated cyclic AMP accumulation was unaffected by pretreatment of cells with pertussis toxin or the inhibitory muscarinic agonist, oxotremorine. PDBu stimulation of adenylate cyclase activity required the presence of a cytosolic factor which appeared to translocate to the plasma membrane in response to the phorbol ester. The diacylglycerol-generating agents thyroliberin, bombesin and bacterial phospholipase C each stimulated cyclic AMP accumulation, but, unlike PDBu, did not attenuate the stimulation induced by VIP. These results suggest that PKC affects at least two components of the adenylate cyclase complex. Stimulation of cyclic AMP accumulation is probably due to modification of the catalytic subunit, whereas attenuation of VIP-stimulated cyclic AMP accumulation appears to be due to the phosphorylation of a different site, which may be the VIP receptor.

GTPase inhibiting mutations activate the alpha chain of Gs and stimulate adenylyl cyclase in human pituitary tumours

A subset of growth hormone-secreting human pituitary tumours carries somatic mutations that inhibit GTPase activity of a G protein alpha chain, alpha(s). The resulting activation of adenylyl cyclase bypasses the cells' normal requirement for trophic hormone. Amino acids substituted in the putative gsp oncogene identify a domain of G protein alpha-chains required for intrinsic ability to hydrolyse GTP. This domain may serve as a built-in counter-part of the separate GTPase-activating proteins required for GTP hydrolysis by small GTP-binding proteins such as p21ras.

Protein kinase C stimulates adenylate cyclase activity in prolactin-secreting rat adenoma (GH4C1) pituicytes by inactivating the inhibitory GTP-binding protein Gi

The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and thyroliberin exerted additive stimulatory effects on prolactin release and synthesis in rat adenoma GH4C1 pituicytes in culture. Both TPA and thyroliberin activated the adenylate cyclase in broken cell membranes. When combined, the secretagogues displayed additive effects. TPA did not alter the time course (time lag) of adenylate cyclase activation by hormones, guanosine 5'-[beta,gamma-imino]triphosphate or forskolin, nor did it affect the enzyme's apparent affinity (basal, 7.2 mM; thyroliberin-enhanced, 2.2 mM) for free Mg2+. The TPA-mediated adenylate cyclase activation was entirely dependent on exogenously added guanosine triphosphate. ED50 (dose yielding half-maximal activation) was 60 microM. Access to free Ca2+ was necessary to express TPA activation of the enzyme, however, the presence of calmodulin was not mandatory. TPA-stimulated adenylate cyclase activity was abolished by the biologically inactive phorbol ester, 4 alpha-phorbol didecanoate, by the protein kinase C inhibitor polymyxin B and by pertussis toxin, while thyroliberin-sensitive adenylate cyclase remained unaffected. Experimental conditions known to translocate protein kinase C to the plasma membrane and without inducing adenylate cyclase desensitization, increased both basal and thyroliberin-stimulated enzyme activities, while absolute TPA-enhanced adenylate cyclase was maintained. Association of extracted GTP-binding inhibitory protein, Gi, from S49 cyc- murine lymphoma cells with GH4C1 cell membranes yielded a reduction of basal and hormone-stimulated adenylate cyclase activities, while net inhibition of the cyclase of somatostatin was dramatically enhanced. However, TPA restored completely basal and hormone-elicited adenylate cyclase activities in the Gi-enriched membranes. Finally, TPA completely abolished the somatostatin-induced inhibition of adenylate cyclase in both hybrid and non-hybrid membranes. These data suggest that, in GH4C1 cells, protein kinase C stimulation by phorbol esters completely inactivates the n alpha i subunit of the inhibitory GTP-binding protein, leaving the n beta subunit functionally intact. It can also be inferred that thyroliberin conveys its main effect on the adenylate cyclase through activation of the stimulatory GTP-binding protein, Gs.

Synthesis and release of acetylcholine by normal and tumoral pituitary corticotrophs

Incubation of cultured rat pituitary cell aggregates with [3H]choline ([3H]Chol) yielded a derivative that was identified as [3H]acetylcholine ([3H]ACh) by several criteria: 1) the [3H]Chol derivative with the highest retention time coeluted with a [14C]ACh standard in cation exchange and reverse phase HPLC; 2) cholinesterase treatment converted this derivative to a substance with the retention time of [3H]Chol; 3) two blockers of ACh production, hemicholinium and 4-[(1-naphthylvinyl)pyridinium], eliminated 3H-labeled material in the HPLC fractions with ACh retention time. Spontaneous [3H]ACh release was increased by depolarizing potassium concentrations, and both synthesis and release of ACh were increased by the glucocorticoid hormone dexamethasone. Double immunostaining of choline acetyltransferase (CAT) and, respectively, of ACTH, GH, PRL, TSH, S100, LH, and FSH in rat pituitary cells revealed that most of the CAT-immunoreactive cells were also ACTH immunoreactive. A small proportion (less than 10%) of the PRL-immunoreactive cells also showed CAT immunoreactivity, but all other cell types were negative. The immunocytochemical evidence for colocalization of CAT within the ACTH cell was strengthened by the finding of a significantly higher rate of [3H]ACh synthesis in a corticotroph-enriched cell population obtained by separating pituitary cells on a velocity sedimentation gradient. In addition, the mouse pituitary corticotropic cell line AtT20 contained CAT immunoreactivity, converted [3H]Chol to [3H]ACh, and released bioactive ACh-like material. In conclusion, the present data provide strong evidence that pituitary corticotrophs synthesize and release ACh, and that the activity of this intrapituitary cholinergic transmission system is under regulatory control.

Sodium butyrate induces pyroglutamyl peptidase I and decreases thyrotropin-releasing hormone receptors in GH3 cells

The effect of sodium butyrate treatment on TRH-degrading enzymes and TRH receptors in GH3 cells was investigated. The specific activity of pyroglutamyl peptidase I (EC 3.4.19.3) was increased by exposure to sodium butyrate in a time- and concentration-dependent manner, whereas the specific activity of prolyl endopeptidase (EC 3.4.21.26) was unchanged. The maximal effect occurred at a concentration of 1 mM sodium butyrate and 16 h after exposure. The increase was reversible upon removal of sodium butyrate from the cell culture. Cycloheximide totally blocked the stimulation, indicating that the increase was due to new protein synthesis. Sodium butyrate had no effect on pyroglutamyl peptidase I activity in the AtT-20 cell line. [methyl-3H]TRH binding to intact GH3 cells was reduced to 70% of the control value when cells were exposed to 1 mM sodium butyrate for 8 h. A maximal decrease in binding to 40% of the control value occurred after 16 h of exposure. The Kd of [methyl-3H]TRH binding was not changed. Sodium butyrate altered GH3 cell morphology, but the morphological changes occurred after alterations of pyroglutamyl peptidase I activity and [methyl-3H]TRH-binding sites. Other agents known to alter GH3 cell morphology had no effect on pyroglutamyl peptidase I activity. These results indicate that sodium butyrate can in some respects mimic the action of T3 on GH3 cells. Moreover, they provide further evidence that the activity of pyroglutamyl peptidase I, but not prolyl endopeptidase, is subject to regulation in the GH3 cell.

Hormone-sensitive adenylate cyclase of prolactin-producing rat pituitary adenoma (GH4C1) cells: molecular organization

Hormonal activation and inhibition of the GH4Cl1 cell adenylate cyclase complex is delineated. In the presence of the guanyl nucleotide GTP, enzyme activity was enhanced twofold by thyroliberin, sixfold by vasoactive intestinal peptide (VIP), twofold by prostaglandin E2 and twofold by isoproterenol. The diterpene, forskolin, increased, the activity 14-fold. In the presence of high GTP (400 microM) and NaCl (150 mM) concentrations, somatostatin inhibited (ED50 = 0.5 microM) the cyclase activity by 40%. In the presence of 10 microM somatostatin, the ED50 values (5 nM) for thyroliberin- and VIP-stimulated adenylate cyclase activities were shifted to 20 nM. Forskolin-elicited activation was, however, not affected by somatostatin. Cholera-toxin and pertussis-toxin pretreatment of the enzyme brought about some 20-fold and twofold activation, respectively. Inhibition by somatostatin was abolished upon pre-exposure to pertussis toxin. Mild alkylation by N-ethylmaleimide increased basal and hormone-activated adenylate cyclase while somatostatin again failed to express its inhibitory potential. Further alkylation caused a gradual decline and convergence of hormone-modulated cyclase activities towards zero. The N-ethylmaleimide-induced attenuation of thyroliberin-elicited activity was paralleled by a decrease in [3H]thyroliberin binding. Trifluoperazine and an anti-calmodulin serum reduced basal and net thyroliberin-, VIP- and forskolin-enhanced cyclase activities by some 30%, 100%, 70% and 80%, respectively. The Vmax of basal and thyroliberin-stimulated adenylate cyclase was diminished by 65%, leaving the apparent Km values (7.2 mM and 2.6 mM, respectively) for Mg2+ unaltered. Finally, the phorbol ester 12-O-tetra-decanoyl-phorbol 13-acetate (TPA) doubled the activity. This effect was counteracted by the protein kinase C inhibitor, polymyxin B, while thyroliberin-enhanced adenylate cyclase remained unaffected. In summary, we have described an adenylate cyclase with stimulatory (Rs) and inhibitory (Ri) receptors coupled to a calmodulin-sensitive holoenzyme through the Gs and Gi type of GTP-binding proteins. The ratio of the Gs to Gi is high. It appears that the GH4C1 cell adenylate cyclase is also activated by protein kinase C by interference with Gi. Apparently, thyroliberin activates the cyclase both directly through Gs and indirectly via protein kinase C stimulation.

Malignant pituitary tumors

Pituitary malignancies are rare. The definition of pituitary malignancy is still subject to debate. Three types of malignancy with the description of clinical course and complete autopsy were presented; i) true carcinosarcoma, ii) ectopic sphenoid invasive tumor, and iii) post-irradiation sarcoma. The first true carcinosarcoma had a long natural tumor history of 46 years, ending in malignant transformation with extracranial metastasis from a pituitary adenoma. The second ectopic sphenoid tumor was prolactin-producing, locally invasive, with the intact pituitary gland. The third post-irradiation sarcoma was followed by chromophobe pituitary adenoma. The interval between irradiation to the adenoma and malignant transformation to fibrosarcoma was about 14 years.

Phorbol esters and thyroliberin have distinct actions regarding stimulation of prolactin secretion and activation of adenylate cyclase in rat pituitary tumour cells (GH4C1 cells)

The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) enhances the effects of TRH on phase II of prolactin secretion as well as on hormone synthesis at both low and high TPA receptor occupancy. Furthermore TPA, but not the biologically inactive substance 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), stimulates the particulate bound adenylate cyclase with a time course paralleling that of TRH activation. However, the combined additions of TRH and TPA activate this cyclase in an additive manner while the Gpp(NH)p- and the forskolin-sensitive enzyme are unaffected by TPA addition. Polymyxin B, which inhibits protein kinase C, abolishes activation of adenylate cyclase by TPA without interfering with the stimulatory action of TRH. Also, when phosphatase activity is preferentially inhibited by pretreatment of the cells with sodium vanadate, the TRH-sensitive cyclase is unaltered, while TPA activation is obliterated. Maximal stimulation of adenylate cyclase by cholera toxin pretreatment, obliterated the actions of TRH and TPA. Cells pretreated with pertussis toxin retained their TRH-sensitive cyclase, however, TPA-responsiveness was lost. We therefore suggest that the action of TPA as it relates to activation of adenylate cyclase, is probably mediated via the Gi component of the adenylate cyclase complex, while TRH stimulates the enzyme via the classical pathway involving the stimulatory GTP binding protein (Gs).

Glandular kallikrein in estrogen-induced pituitary tumors: time course of induction and correlation with prolactin

Glandular kallikrein (a trypsin-like serine protease) is an estrogen-induced and dopamine-repressed protein in the rat anterior pituitary which appears to be associated with lactotrophs. This study examined glandular kallikrein levels in diethylstilbestrol (DES)-induced pituitary tumors in F344 rats and compared it to plasma and pituitary prolactin, and pituitary wet weight. Ovariectomized F344 rats were implanted with Silastic tubes containing 0 or 5 mg DES for 1, 3, 5, 7, or 9 weeks. Glandular kallikrein was measured by microenzymatic assay using D-valylleucylarginyl-p-nitroanilide following trypsin treatment of extracts to activate latent forms of glandular kallikrein. Prolactin was measured by radioimmunoassay. DES induced steady time-dependent increases in pituitary wet weight with 7- and 16-fold increases observed by 5 and 9 weeks, respectively. Growth rates averaged 11.4 mg/week during the first 5 weeks of DES exposure, and then increased to 23.2 mg/week between weeks 5 and 9. Glandular kallikrein total activity (nmol/min/pituitary) increased 130- and 240-fold after 3 and 5 weeks of DES exposure, respectively, and then abruptly plateaued. The specific activity (nmol/min/mg protein) of glandular kallikrein peaked at 3-5 weeks (36-fold increase compared to controls) and then declined as pituitary protein but not glandular kallikrein continued to increase. Total pituitary prolactin constantly rose during DES exposure with 12- and 26-fold increases after 5 and 9 weeks, respectively. Plasma prolactin levels also continuously rose during exposure to DES with 130- and 290-fold increases after 5 and 9 weeks, respectively. No major strain differences were found with regard to sensitivity to the acute effects of estrogen or dopaminergic stimulation on glandular kallikrein induction. DES-induced pituitary tumors in F344 rats are well known to arise via lactotroph proliferation, and the striking elevation in glandular kallikrein and prolactin during the early phases of tumor growth provide further support for a localization of glandular kallikrein in lactotrophs. However, the abrupt stabilization in glandular kallikrein levels by week 5 was unexpected and may signal a biochemical transformation of the tissue during tumor progression.

The effects and interactions of substrates, inhibitors, and the cellular thiol-disulfide balance on the regulation of type II iodothyronine 5'-deiodinase

Thyroid hormones rapidly inhibit type II iodothyronine 5'-deiodinase (5'DII) activity in the rat central nervous system, anterior pituitary gland, and GH3 pituitary tumor cells. To gain insight into the cellular mechanisms responsible for this down-regulation, the effects of substrates, inhibitors, and alterations in the cellular thiol-disulfide balance on 5'DII regulation were investigated. The results demonstrate that in vitro competitive inhibitors, such as iopanoic acid, as well as iodothyronine substrates induce a rapid and irreversible loss of enzyme activity in rat cerebral cortex and anterior pituitary tissue. The potency of these agents in down-regulating this enzymatic process in intact GH3 cells is directly related to their competitive inhibitory effects on 5'DII activity in vitro. Additional studies demonstrated that treatment of intact GH3 cells with the sulfhydryl-oxidizing agent diamide mimicks the effect of substrate and results in the rapid inactivation of 5'DII. In contrast, preincubation of cells with the sulfhydryl-reducing agent dithiothreitol renders the enzyme less susceptible to the down-regulatory effects of substrate. Sulfhydryl-reducing agents thus appear to play a dual role in the 5'DII process by serving as cosubstrates and by modulating the enzymes susceptibility to substrate-induced down-regulation. These findings suggest that the in vivo inhibition of 5'DII by thyroid hormones involves a unique mechanism of enzyme regulation whereby the binding of ligand to the active site induces the rapid and irreversible inactivation and/or degradation of the enzyme. This inactivation of 5'DII initiated by substrate binding may be mediated by alterations in the sulfhydryl state of the enzyme as it progresses through the catalytic cycle.

Elevated adipose tissue lipoprotein lipase activity in craniopharyngioma patients

The activity of lipoprotein lipase (LPL) was measured in adipose tissue (AT-LPL) and postheparin plasma (PH-LPL) of 13 obese patients (aged 11 to 31 years) who had surgery for craniopharyngioma 1 to 13 years earlier. AT-LPL activity (mean +/- SEM) was higher in them than in subjects matched with respect to age, sex, and relative body weight (4.6 +/- 1.1 v 2.1 +/- 0.4 mumol free fatty acids (FFA).h-1.g-1, P less than .05). The activity was also higher when expressed per fat cell.

Phorbol esters induce two distinct changes in GH3 pituitary cell adenylate cyclase activity

Phorbol esters alter cyclic AMP levels in a number of tissues, including the anterior pituitary. We report that membrane preparations from GH3 cells exposed to phorbol esters exhibit decreased vasoactive intestinal peptide (VIP)-stimulated and enhanced forskolin-stimulated adenylate cyclase activity. The responsiveness of adenylate cyclase activity to NaF, guanylyl-imidodiphosphate, and Mn2+ was also reduced by phorbol ester treatment. The ability of somatostatin to inhibit forskolin-stimulated adenylate cyclase activity was reduced while phorbol ester exposure had no apparent effect on somatostatin inhibition of VIP-stimulated adenylate cyclase activity. We suggest that protein kinase C alters at least two distinct components of the adenylate cyclase system. One modification disrupts hormone receptor-Gs interaction (lowering VIP efficacy) and the second perturbation augments the activity of the adenylate cyclase catalytic subunit.

Estrogen induces accumulation of the mitochondrial ribonucleic acid for subunit II of cytochrome oxidase in pituitary tumor cells

The gene for subunit II of cytochrome oxidase is part of the mitochondrial genome. 17 beta-Estradiol, 1 nM, increased the levels of cytochrome oxidase II mRNA in the GH4C1 pituitary tumor cell line; the increases ranged from 3- to 16-fold over controls in different experiments. Insulin, 300 nM, estradiol, 1 nM, and epidermal growth factor, 10 nM, together caused a larger increase in cytochrome oxidase II mRNA accumulation than did estradiol alone. The dose-response relationship for the induction of cytochrome oxidase II mRNA by estradiol was similar to that for PRL mRNA; maximal induction occurred at about 10(-9) M. This concentration is 10-fold greater than that required for maximal stimulation of cell proliferation and of 1C28, another estrogen-inducible mRNA, indicating that the increase in cytochrome oxidase II mRNA is not a result of increasing the growth rate of the cells. The increase in cytochrome oxidase II mRNA was not caused by an increase in the number of copies of the cytochrome oxidase II gene. Estradiol therefore must induce in the mitochondria an increase in transcription or a decrease in degradation of cytochrome oxidase II mRNA.

Phorbol esters increase adenylate cyclase activity and stability in pituitary membranes

In this report, we demonstrate that calcium and phorbol esters enhance cAMP production in GH4C1 cell homogenates. The mechanism for this is a reduction in the rate of decay of adenylate cyclase activity over the course of the assay. Purified protein kinase C can reconstitute calcium- and phorbol ester-dependent adenylate cyclase. Phorbol ester-activated protein kinase C increases both the initial rate of cAMP synthesis and reduces the time-dependent decay of adenylate cyclase activity in membrane preparations. The rate of cAMP production is fit to an equation derived from a model which assumes that adenylate cyclase initially exists in a high activity state which decays exponentially into a low activity state. We suggest that protein kinase C can both prevent the decay of the high activity state and convert the low activity state into the high activity state.

Pituitary and hypothalamic glucose-6-phosphate dehydrogenase: effects of estradiol and age in C57BL/6J mice

Activities of glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6-PGDH) were measured in the mediobasal hypothalamus and pituitary gland of C57BL/6J mice throughout their lifespan. Activities of pituitary G6PDH, pituitary 6-PGDH, and hypothalamic G6PDH increase with age in female mice, assayed 7 days after ovariectomy, but not in intact females or males. Pituitary G6PDH specific activity is increased by middle-age (13-14 months) in females, before the onset of acyclicity, and remains elevated throughout the acyclic phases of persistent vaginal cornification and persistent diestrus. This increase in activity is ovary dependent, because it can be prevented by long term (12-month) ovariectomy. The increased activity is not linked to pituitary tumorigenesis and does not result from trapped blood cells, as evaluated by studies with 51Cr-labeled erythrocytes. Responses to physiological levels of estradiol (E2) were analyzed with a graded series of chronic polyethylene implants or after a single sc injection in ovariectomized mice aged 5-22 months. The responsiveness of pituitary G6PDH to E2 is not altered during aging. Young cycling (6 months old) and older acyclic mice (19 months old) displaying persistent vaginal cornification show equivalent increases of about 100% in G6PDH specific activity after chronic E2 treatment and similar time courses of induction after a single E2 injection. Pituitary G6PDH is maximally induced (30% increase) by 48 h after E2 injection in all age groups. In addition, the rates of decline in pituitary G6PDH specific activity after ovariectomy are similar in young and older mice (half-life, 4 days). The specific activity of G6PDH in the mediobasal hypothalamus and blood is unaffected by E2 administration. The relatively low doses of E2 used here fail to alter 6-PGDH specific activity in pituitary or brain. These findings indicate that female reproductive senescence in mice is not associated with generalized losses of sensitivity and responsivity to E2 throughout the neuroendocrine axis.

Presence of tyrosine-hydroxylase activity in anterior pituitary adenomas and ectopic anterior pituitaries in male rats

There is evidence for local regulatory effects of dopamine (DA) and norepinephrine (NE) on the release of prolactin (PRL) and other hormones from the anterior pituitaries transplanted to an ectopic site. In order to further explore these suspected regulatory mechanisms and to determine if they may exist also in pituitary tumors, we have examined the activity of tyrosine-hydroxylase (TH) in ectopic pituitaries and in hyperplastic pituitaries of estrogen-treated rats. Adult male rats with a pituitary graft in each kidney and sham-operated controls were examined 5 weeks after surgery. Rats implanted for 3 months with Silastic capsules containing diethylstilbestrol (DES) or with empty capsules were used 10 weeks after the capsules were removed. TH activity in ectopic anterior pituitaries of grafted rats was significantly higher than that measured in the eutopic anterior pituitaries of control animals. Similarly, TH activity was significantly higher in DES-induced pituitary tumors than in control pituitaries. These data support the possible existence of local catecholaminergic mechanism(s) that could be modulating PRL secretion from pituitary tumors and from ectopic pituitaries.

Regulation of a thyrotropin-releasing hormone-degrading enzyme in GH3 cells: induction of pyroglutamyl peptidase I by 3,5,3'-triiodothyronine

The effect of exposure of GH3 cells to T3 on the TRH-degrading enzymes pyroglutamyl peptidase I (EC 3.4.19.3) and prolyl endopeptidase (EC 3.4.21.26) was studied. T3 produced a dose-dependent increase in the specific activity of pyroglutamyl peptidase I after 3 days of exposure. The EC50 for T3 was 5 X 10(-10) M. The specific activity of prolyl endopeptidase was unaffected by exposure to T3. The increase in pyroglutamyl peptidase I activity was dependent upon the time of exposure of the cells to this hormone. A maximal effect occurred at 72 h. The stimulation of pyroglutamyl peptidase I by T3 was totally blocked by cycloheximide, indicating that this enzyme is induced in GH3 cells by T3. The effect of T3 on the two TRH-degrading enzymes was also studied in the ACTH-secreting cell line AtT20. T3 had no effect on these enzymes in the AtT20 cell, suggesting that the effect of T3 on pyroglutamyl peptidase I may be cell specific. These studies indicate that the induction of pyroglutamyl peptidase I by T3 may contribute to the negative feedback regulation of T3 levels.

Calmodulin-dependent phosphatases of PC12, GH3, and C6 cells: physical, kinetic, and immunochemical properties

Calmodulin-dependent phosphoprotein phosphatase (CaMDP) activity has been found in each of three cultured cell lines: rat pheochromocytoma (PC12), glioma (C6), and pituitary adenoma (GH3) cells. These CaMDP activities bind to immobilized calmodulin in the presence of Ca2+ and are eluted by EGTA. Sucrose density centrifugation revealed that the phosphatase activities exhibited sedimentation coefficients of 4.37, 4.23, and 4.59 for proteins derived from C6, GH3, and PC12 cells, respectively. The Stokes radii measured for the PC12 and C6 activities were 41.8 and 40.0 A, respectively. The estimated molecular weights calculated for the enzymes from these data are 79,100 and 72,200. The phosphatase activities required the presence of divalent cations such as Ca2+ or Mn2+ for expression of activity, which was optimal only in the presence of calmodulin. The apparent Km for phosphorylated myelin basic protein substrate was 8 microM. Affinity-purified antibodies to the B subunit of bovine brain CaMDP were found by immunoblot (Western blot) to cross-react with a single protein among proteins extracted from PC12, C6, and GH3 cells that had been resolved by two-dimensional electrophoresis. In each case, the cross-reacting protein exhibited an Mr of 16,000 and an isoelectric point of 4.7, values virtually identical to those reported previously for the B subunit of bovine brain CaMDP (sometimes called calcineurin). This cross-reacting protein was found among cellular proteins eluted from immobilized calmodulin by EGTA. Immunocytochemical localization of the cross-reacting protein in undifferentiated PC12 cells or in cells differentiated in response to nerve growth factor revealed its presence diffusely throughout the cytoplasm. These experiments support the contention that each of these cell lines contains a calmodulin-regulated phosphatase homologous physically and kinetically, and immunologically related to bovine brain CaMDP.

Effect of calcium on adenylate cyclase of rat anterior pituitary gland

The effect of free calcium (Ca2+) on adenylate cyclase (AC) activity of rat anterior pituitary gland have been investigated in order to shed some light on the interrelationships between the two second messengers (cAMP and calcium) which operate in pituitary cells. Anterior pituitary homogenates or crude membranes preparations (obtained using buffers free of divalent cation chelators) were assayed and the concentrations of Ca2+ in the assay mixture containing EGTA were calculated by a computer program for each addition of CaCl2. A wide range of Ca2+ concentrations (from 2 X 10(-9) to 6 X 10(-4)M) was spanned. Ca2+ was found to markedly inhibit pituitary AC and the mathematical analysis of data indicated the presence of two inhibition The two KiS were: 1.78 +/- 0.48 X 10(-7) M and 2.47 +/- 0.52 X 10(-4) M for the homogenates and 1.71 +/- 0.45 X 10(-7) M and 3.15 +/- 0.85 X 10(-4) M for the membrane preparations. No stimulation of the enzyme could be detected at any Ca2+ concentration tested. Furthermore, because of our experimental conditions it is unlikely that there was substantial loss of endogenous calmodulin, or other calcium binding protein(s) required to mediate AC stimulation by calcium. The lack of a calcium-calmodulin stimulation of pituitary AC was confirmed by experiments with anticalmodulin drugs (trifluoperazine and calmidazolium, R24571) and experiments with EGTA-washed membranes in the presence of exogenous calmodulin. At any Ca2+ concentration, the same AC activity was observed in the presence and in the absence of anticalmodulin drugs or added calmodulin. The mechanism of pituitary AC inhibition by Ca2+ was investigated focusing on a range of Ca2+ concentrations near the Ki for the high affinity calcium site and thus similar to the intracellular Ca2+ concentrations. Ca2+ was found to act as a competitive inhibitor of the Mg2+ activation of AC and as a noncompetitive inhibitor with respect to the MgATP2-, the substrate of the enzyme. The effects of Ca2+ on AC were also studied in cell populations and tissues extremely rich in PRL-secreting cells (cell fractions purified from rat anterior pituitaries and human prolactinomas). The pattern of Ca2+ action was found to be nearly superimposable on that observed in total pituitary.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased creatine kinase activity in pituitary tumours of rat and man

The demonstration that phosphocreatine is used as an energy source by rat PRL-secreting pituitary tumours prompted the study of the enzyme creatine kinase in both rat and human pituitary tumours. Rats treated with diethylstilbestrol developed greatly enlarged pituitaries and hyperprolactinaemia. Total creatine kinase was significantly increased and fractionation on diethylaminoethyl Sephadex showed that the 'brain' form was increased, whereas the 'muscle' and mitochondrial forms showed no change. Exposure to large concentrations of oestradiol caused similar changes in creatine kinase which increased over a period of 25 weeks. The total creatine kinase content of a series of human pituitary tumours was highly variable, but the mean value of 183 +/- 46 (SEM) units per gram protein was significantly higher than the mean for normal pituitary tissues (28.4 +/- 2.9). The brain:muscle isozyme ratio was measured in six human PRL-secreting tumours with a mean of 3.47 +/- 0.73, significantly higher than in 'non-functional' tumours (1.57 +/- 0.29) or normal tissue (1.77 +/- 0.28). Three of four GH-secreting tumours had a ratio below 0.6. The highest ratio found (8.66) was in an ACTH-secreting tumour. Previous reports have shown that oestradiol rapidly induces brain creatine kinase in oestrogen responsive tissues. This is not the case with the rat pituitary gland or oestrogen responsive human tumour cells in culture. Chronic oestrogen treatment, however, does increase creatine kinase in the proliferating gland and many human pituitary tumours have increased enzyme activity. These results suggest that the phosphocreatine/ATP system and in particular the brain isozyme of creatine kinase are of particular importance in lactotropes.

Translocation of protein kinase C in anterior pituitary tumor cells

Previous studies have shown that phorbol esters and lithium each stimulate the secretion of adrenocorticotropic hormone (ACTH) by the anterior pituitary tumor cell line AtT20/D16-16. Pretreatment with either lithium or phorbol ester desensitizes the cells to subsequent stimulation by phorbol ester. An early consequence of phorbol ester action in other systems is the translocation of protein kinase C from cytosol to membranes. We have assayed protein kinase C activity in cytosol and membranes of AtT20 cells after treatment with phorbol dibutyrate, lithium, or other agents that stimulate secretion of ACTH in these cells. Phorbol dibutyrate clearly induced translocation of protein kinase C, but lithium treatment did not cause translocation itself, nor did pretreatment with lithium affect the translocation induced by phorbol dibutyrate. These results are consistent with a role for translocation of protein kinase C in the stimulatory and desensitizing effects of phorbol esters but fail to implicate translocation in the actions of lithium on AtT20 cells.

Elevated glandular kallikrein in estrogen-induced pituitary tumors

Anterior pituitary tumors were induced in F344 rats by implants of diethylstilbesterol (DES) for 8 weeks. DES-induced tumors contained 250 times more glandular kallikrein than anterior pituitaries from rats not exposed to estrogen, and 6.6 times more glandular kallikrein than anterior pituitaries from rats exposed to large doses of estrogen for 10 days. DES-induced anterior pituitary tumors in F344 rats arise from lactotroph proliferation. Thus, the results suggest that anterior pituitary glandular kallikrein is associated with lactotrophs.

Biochemical evidence for existence of immunoreactive renin in human prolactinoma tissue

High activity of renin was demonstrated in human prolactinoma tissue. This activity was almost completely inhibited by specific antibody raised against human renal renin, indicating that it was not due to the nonspecific action of proteases. The specific activity of renin was 5.04 ng of angiotensin I generated/mg of protein per h, comparable to that of the pituitary tissue prepared from postmortem human subjects. The biochemical properties of the prolactinoma renin were generally similar to those of well-known kidney enzyme, such as molecular mass (Mr = 46,000), optimum pH (6.0), and glycoprotein nature. However, the isoelectric points (pI) of the prolactinoma renin (pI = 4.90, 5.04, 5.24 and 5.41) differed somewhat from those of plasma and kidney renins reported hitherto. These results indicate that true renin can be produced in human prolactinoma tissue.

Inhibition of peptide amidation by disulfiram and diethyldithiocarbamate

Peptidylglycine alpha-amidating monooxygenase is a copper- and ascorbate-dependent enzyme that converts peptides with COOH-terminal glycine residues into the corresponding alpha-amidated product peptides. The relatively selective copper chelator N,N-diethyldithiocarbamate (DDC) and its disulfide dimer, disulfiram (Antabuse), were used to determine whether the availability of copper affects the production of two alpha-amidated pro-ACTH/endorphin-derived peptides, alpha-melanotropin (alpha MSH) and joining peptide. When mouse pituitary corticotropic tumor cells (AtT-20) were grown in medium containing micromolar concentrations of disulfiram or DDC, alpha-amidation of newly synthesized joining peptide was specifically inhibited in a dose-dependent manner. In rats injected twice with disulfiram or DDC, the ability of the intermediate pituitary to alpha-amidate newly synthesized alpha MSH and joining peptide was inhibited in a dose-dependent manner; at disulfiram doses equivalent to those used in alcohol abuse therapy (4 mg/kg/day), only about 10% of the newly synthesized peptides were correctly alpha-amidated. Chronic treatment of rats with DDC or disulfiram produced a dose-dependent increase in the pituitary content of glycine-extended alpha MSH and joining peptide; the total amount of pro-ACTH/endorphin-related material was unaltered. After 11 days of treatment with 4 mg/kg/day disulfiram, about one-third of the pituitary alpha MSH and joining peptide were present in the glycine-extended rather than the alpha-amidated form; pituitary extracts normally contain almost entirely alpha-amidated peptides.

Presence of renin, angiotensinogen, and converting enzyme in human pituitary lactotroph cells and prolactin adenomas

Renin, angiotensinogen, and converting enzyme were detected in 10 normal human pituitary glands by immunohistochemical techniques. Renin was stained by polyclonal and monoclonal antibodies directed against human renin, and an antibody directed against the renin prosegment revealed the presence of prorenin. Immunoreactive angiotensinogen and angiotensin I-converting enzyme were found in the same cells as renin. Using serial sections and double immunohistochemical labeling with a PRL antiserum, all of the proteins of the renin-angiotensin system appeared to be localized within the lactotroph cells, and no component of the renin system was detected in any of the other pituitary cells. Renin, angiotensinogen, and angiotensin I-converting enzyme also were found in 6 PRL-secreting adenomas as well as in a mixed PRL/GH-secreting adenoma. The renin content of a PRL adenoma was about 1/100th that of a normal kidney. Renin activity could be blocked by an anticatalytic human renin antibody. No renin, angioten-sinogen, or angiotensin I-converting enzyme was found in 6 GH-secreting adenomas, 1 corticotroph adenoma, or 10 nonsecreting pituitary adenomas. The colocalization of proteins of the renin-angiotensin system suggests production of angiotensin II within the lactotroph cells and favors the hypothesis of a paracrine action of this peptide.

Regulation of thyroxine 5'-deiodinase by thyroid hormones and activators of protein kinase C in GH4C1 cells

The regulation of T4 5'-deiodinase activity was studied in cultured GH4C1 cells. Enzyme activity was measured in cell sonicates as the release of radioiodide from [125I]T4. Enzyme activity was stimulated 2- to 3-fold by hypothyroid serum and activators of protein kinase C, such as TRH and phorbol esters. The hypothyroid serum effect was maximal by 3 h, whereas TRH and phorbol esters required 6 h to achieve a maximal effect. The hypothyroid serum effect was gone within 4 h of returning the cells to control medium. In contrast, the TRH and phorbol ester effects persisted 24-48 h after removal of those agents. Both T4 and rT3 were at least as potent as T3 in blocking the effect of hypothyroid serum. The stimulation of 5'-deiodinase induced by hypothyroid serum was additive with that induced by kinase C activators. Trifluoperazine blocked the effect of TRH and phorbol esters, but not that of hypothyroid serum. It is concluded that stimulation of 5'-deiodinase activity can occur by at least two independent mechanisms: one involving hypothyroidism and another involving activation of protein kinase C. The relative potencies of various iodothyronines for abolishing the hypothyroid effect differ markedly from the relative binding affinities of these agents for the nuclear T3 receptor, suggesting that this thyroid hormone effect may not be mediated by the classical nuclear thyroid hormone receptor.

Thyrotropin-releasing hormone activates a Ca2+-dependent polyphosphoinositide phosphodiesterase in permeable GH3 cells. GTP gamma S potentiation by a cholera and pertussis toxin-insensitive mechanism

Numerous hormones are known to rapidly activate polyphosphoinositide turnover in target cells by promoting phosphodiesteratic cleavage of the phospholipids; however, little is known about the enzymology of receptor-mediated phosphoinositide breakdown. In the present study, thyrotropin-releasing hormone (TRH) stimulation of polyphosphoinositide turnover has been characterized in electrically permeabilized, [3H]myoinositol-labeled GH3 cells. The permeable cells allow the influence of small molecular weight (Mr less than or equal to 1000) cofactors to be determined. We present evidence for the following: 1) TRH stimulates inositol phosphate generation in permeable cells; 2) optimal hormone-stimulated inositol phosphate generation requires Mg2+, ATP, and Ca2+; 3) Mg2+ and ATP requirements reflect polyphosphoinositide kinase reactions; 4) in the absence of MgATP, TRH stimulates the phosphodiesteratic breakdown of pre-existing polyphosphoinositides in a reaction which requires only low Ca2+ (10(-7) M); 5) hormone activation is potentiated in the presence of the stable guanine nucleotide, GTP gamma S; neither TRH-stimulated nor GTP gamma S-potentiated hydrolysis is inhibited by cholera or pertussis toxin treatment. These results demonstrate that hormone-induced phospholipid hydrolysis involves activation of a phosphoinositide phosphodiesterase; activation results in lowering the Ca2+ requirement of the phosphodiesterase such that maximal activity is observed at Ca2+ levels characteristic of a resting cell (10(-7) M). Furthermore, TRH regulation of polyphosphoinositide hydrolysis is modulated by guanine nucleotides; however, nucleotide regulation appears to involve a GTP-binding factor (Np) other than Ns or Ni.

Evidence for regulation of a thyrotropin-releasing hormone degradation pathway in GH3 cells

GH3 cells, cloned from a rat anterior pituitary tumor, synthesize and secrete PRL in response to TRH. One of the pathways of TRH degradation is removal of the N-terminal pyroglutamyl residue catalyzed by pyroglutamyl peptide hydrolase (PPH; EC 3.4.11.8). We recently described the synthesis and properties of 5-oxoprolinal, a specific and potent (Ki = 26 nM) inhibitor of PPH. The effect of long term exposure of GH3 cells to 5-oxoprolinal on PPH activity was studied by incubating cells with inhibitor for 3 days, harvesting, washing to remove inhibitor, and assaying for PPH. Unexpectedly, we found a marked (300%) increase in PPH activity. This effect was dependent on the concentration of 5-oxoprolinal (EC50 = 10(-7) M) and was time dependent, with a rapid increase in enzyme activity occurring during the first 24 h. Cycloheximide did not block the increase. The results suggest that the activity of PPH in GH3 cells is subject to complex regulatory mechanisms.

Fate of immunoprecipitable protein kinase C in GH3 cells treated with phorbol 12-myristate 13-acetate

The effect of phorbol 12-myristate 13-acetate (PMA) on protein kinase C was studied by metabolically labeling GH3 cells with [35S]methionine and using a polyclonal antibody raised against rat brain protein kinase C to immunoprecipitate the enzyme. PMA accelerates the loss of immunologically reactive protein kinase C from the cells in a time- and dose-dependent manner. The half-life of the enzyme in cells treated with 400 nM PMA was 2 h whereas in control cells 60-70% of the enzyme was still detectable after 24 h. The concentration of PMA required to reduce cellular protein kinase C 50% after 24 h was 130 nM. PMA also induced the translocation of [35S]Met-labeled protein kinase C from the cytosol to the membranes in a concentration-dependent manner. Less protein kinase C was translocated to membranes when cells were treated with 20 nM PMA than when they were exposed to 400 nM PMA. In the latter case, most of the labeled protein kinase C became membrane-associated. Maximal translocation was evident after 15 min of incubation with either concentration of PMA and was followed by degradation of the membrane-associated enzyme. The rate of degradation of membrane-associated protein kinase C was the same with both concentrations of PMA. In cells treated with 20 nM PMA, disappearance of [35S]Met-labeled protein kinase C from the cytosolic fraction occurred in two phases, a rapid decrease characteristic of the membrane-associated enzyme, followed by a slower loss similar to that seen in control cells. The results indicate that turnover of protein kinase C is enhanced by membrane association.

Metabolic effect of 3,3',5'-triiodothyronine in cultured growth hormone-producing rat pituitary tumor cells. Evidence for a unique mechanism of thyroid hormone action

Physiologic levels of 3,3',5'-triiodothyronine (rT3) are generally believed to have minimal metabolic effects in the pituitary gland and other tissues. In the present studies, the regulatory role of rT3 and other thyroid hormones on iodothyronine 5'-deiodinase (I5'D) activity was studied in a growth hormone-producing rat pituitary tumor cell line (GH3 cells). I5'D activity was thiol-dependent and displayed nonlinear reaction kinetics suggesting the presence of two enzymatic processes, one having a low Michaelis constant (Km for thyroxine [T4] of 2 nM) and a second with a high Km value (0.9 microM). Growth of cells in hormone-depleted medium resulted in a two- to 3.5-fold increase in low Km I5'D activity (P less than 0.001). The addition of thyroid hormones to the culture medium resulted in a rapid, dose-dependent inhibition of low Km I5'D activity with the following order of analogue potency: rT3 greater than or equal to T4 greater than 3,5,3'-triiodothyronine (T3). Using serum-free culture conditions, rT3 was approximately 50 times more active than T3. These inhibitory effects were noted within 15 min of hormone addition and could not be attributed to substrate competition with T4. These findings suggest that the control of T4 to T3 conversion by thyroid hormones in the anterior pituitary gland is mediated by a unique cellular mechanism that is independent of the nuclear T3 receptor; and under some circumstances, rT3 may play a regulatory role in controlling this enzymatic process.

Modulation (feminization) of hepatic enzymes by an ectopic pituitary tumor

The ontogeny and endocrine regulation of sex-differentiated hepatic metabolism is mediated via the hypothalamic-pituitary axis. Using in vitro-in vivo systems, we demonstrate alterations in activity levels of six sex-differentiated enzyme systems in male rats bearing ectopic pituitary tumors after the injection of a pituitary cell line, C811RAP. Activity levels of hepatic glutathione S-transferase, UDP-glucuronyltransferase, and aryl hydrocarbon hydroxylase are reduced to activity levels of control females, while histidase, 5 alpha-reductase, and serum cholinesterase levels are increased to levels of control females, i.e. feminization of all of these enzymes. RIAs of testosterone, estrogen, FSH, and PRL are similar in tumor-bearing and control animals, but GH levels are significantly higher in tumor-bearing animals than in the controls. It is suggested that GH may be the pituitary factor responsible for the expression of sex-differentiated hepatic metabolism.

Thyrotropin-releasing hormone induces redistribution of protein kinase C in GH4C1 rat pituitary cells

Thyrotropin-releasing hormone (TRH) affects hormone secretion and synthesis in GH4C1 cells, a clonal strain of rat pituitary cells. Recent evidence suggests that the intracellular mediators, inositol 1,4,5-trisphosphate and 1,2-diacylglycerol, which are generated as a result of TRH-induced hydrolysis of the polyphosphatidylinositols, may be responsible for some of the physiological events regulated by TRH. Because diacylglycerol is an activator of protein kinase C, we have examined a role for this enzyme in TRH action. The subcellular distribution of protein kinase C in control and TRH-treated cells was determined by measuring both enzyme activity and 12,13-[3H]phorbol dibutyrate binding in the cytosol and by measuring enzyme activity in the particulate fraction. Acute exposure of GH4C1 cells to TRH resulted in a decrease of cytosolic protein kinase C, and an increase in the level of the enzyme associated with the particulate fraction. The redistribution of protein kinase C induced by TRH was dose- and time-dependent, with maximal effects occurring within the first minute of TRH treatment. Analogs of TRH which do not bind to the TRH receptor did not induce redistribution of protein kinase C, while the active analog, methyl-TRH, did promote redistribution. Treatment of GH4C1 cells with phorbol myristate acetate also resulted in a shift in protein kinase C distribution, although the response was slower than that produced by TRH. TRH-induced redistribution of protein kinase C implies translocation of the enzyme from a soluble to a membrane-associated form. Because protein kinase C requires a lipid environment for activity, association with the membrane fraction of the cell suggests activation of the enzyme; thus, protein kinase C may play a role in some of the actions of TRH on GH4C1 cells.

Subcellular distribution of protein kinase C of GH3 cells: quantitation and characterization by polyacrylamide gel electrophoresis

Quantitative estimation of cytosolic Ca2+- and phospholipid-dependent protein kinase (PKC) activity was performed by polyacrylamide gel electrophoresis under nondenaturating conditions (PAGE). With this method less than 50 micrograms of cytosol protein can be accurately quantitated for PKC activity. The amount of cytosolic PKC activity recovered after PAGE was comparable to the amount obtained by DEAE-cellulose chromatography. Homogenization of GH3 cells in the presence of 2 mM EGTA/EDTA revealed that 80% of the total cellular PKC activity resided in the cytosol. However, omission of the ion chelator during cell disruption followed by subcellular fractionation and extraction of subcellular fractions by EDTA/EGTA showed that 80% of the total PKC was found in the lysosomal-mitochondrial and microsomal extracts. Detailed analysis of PKC activities demonstrated that cytosolic PKC was identical with the PKC solubilized by EDTA/EGTA from subcellular fractions. In conclusion, GH3 cells appear to contain one species of PKC with an apparent molecular weight of 90,000 which seems to be associated with membranes via a calcium-dependent mechanism (or mechanisms).

Immunocytochemistry of folliculo-stellate cells of normal and neoplastic human pituitary gland

Five normal human pituitaries and 20 pituitary neoplasms were investigated by immunocytochemical methods. Glial fibrillary acidic protein and S100 have been shown in the anterior lobe of the pituitary. Both these markers were present in the folliculo-stellate cell. Evidence is presented for the presence of a transitional folliculo-stellate cell which is immunoreactive for S100. The role of the folliculo-stellate cell is discussed.

Protein kinase C translocates from cytosol to membrane upon hormone activation: effects of thyrotropin-releasing hormone in GH3 cells

The subcellular distribution of protein kinase C (PK C) was examined in thyrotropin-releasing hormone (TRH)--responsive GH3 pituitary cells. TRH treatment, which is known to stimulate polyphosphoinositide turnover and diacylglycerol generation, resulted in a rapid (less than or equal to 15 sec) and transient redistribution of the enzyme from cytosol to membrane fraction. Other agents which either stimulate PK C directly (1-oleoyl-2-acetyl-sn-glycerol and 12-O-tetradecanoyl phorbol-13-acetate) or elevate cellular diglyceride levels (phospholipase C) also promoted a redistribution of the enzyme from cytosol to membrane. These results provide evidence for the concept that cell-surface receptor-mediated phosphoinositide breakdown activates PK C. It appears that translocation of PK C to the membrane is an early step in the cellular activation of this enzyme.

Somatostatin inhibits growth hormone-releasing factor-stimulated adenylate cyclase activity in GH, cells

GH3 cells were used to study the effect of rat growth hormone-releasing factor on adenylate cyclase activity and its interaction with somatostatin. Rat GRF stimulates adenylate cyclase activity (ED5 0 = 6 X 10(-8) M) and somatostatin-14 inhibits this GRF-stimulated activity in a non-competitive manner without affecting the basal enzyme levels. Inhibition by somatostatin-14 is observed at concentrations as low as 10(-11) M and the half-maximal effect is obtained with 10(-8) M. Somatostatin-28 is more potent than SS-14 and has an ED5 0 of 3 X 10(-11) M. VIP is more active than rat GRF in stimulating adenylate cyclase activation. We conclude that in GH3 cells rat GRF behaves as a partial VIP agonist by interacting with VIP-preferring receptors and its effects are inhibited by somatostatin.

In vitro conversion of progesterone in the human testis at different ages, pathophysiological conditions, and during treatment with estrogens or gonadotrophic hormones

The enzyme 17 alpha-hydroxylase catalyzes the conversion of progesterone to 17 alpha-hydroxyprogesterone in the testis and may be studied with the use of incubation of testicular tissue with [3H] progesterone in vitro. The enzyme 20 alpha-hydroxysteroid dehydrogenase catalyzes the conversion of progesterone to 20 alpha-dihydroprogesterone. These enzymes were studied in testicular tissue from 105 human males regarding the effects of aging, different pathophysiological conditions, and gonadotrophic or estrogenic treatment. 17 alpha-Hydroxylase activity was low in vitro in testicular tissue from prepubertal boys, adult men with pituitary tumors, and estrogen-treated elderly men. In nontreated elderly men and certain infertile adult men, 17 alpha-hydroxylase was more active than in the above-mentioned patient groups, albeit lower than in adolescent and adult men. Gonadotropic treatment increased the conversion mediated by 17 alpha-hydroxylase in prepubertal as well as adult testicular tissue. In conclusion, the activity of 17 alpha-hydroxylase may be influenced by gonadotrophic hormones and may be an indicator of the testicular endocrine state at the moment of biopsy. Testicular 20 alpha-hydroxysteroid dehydrogenase seems to be active during low gonadotrophic influence and might even be inhibited by gonadotrophic stimulation.

Erythrocyte catechol-O-methyltransferase, platelet monoamine oxidase, and platelet phenol sulfotransferase activities in patients with prolactin-secreting pituitary adenomas

Dopamine is metabolized by oxidative deamination catalyzed by monoamine oxidase, O-methylation catalyzed by catechol-O-methyltransferase, and sulfate conjugation catalyzed by phenol sulfotransferase. This study was performed to determine whether platelet monoamine oxidase, red blood cell catechol-O-methyltransferase and platelet phenol sulfotransferase enzymatic activities in patients with prolactinomas were quantitatively different from the same enzyme activities in blood samples from normal subjects. The mean enzyme activities in blood samples from 22 women with histologically proven prolactinomas were compared to the mean enzyme activities in blood samples from 32 normal women. The blood levels of these 3 enzymatic activities were not significantly different between the two groups (P greater than 0.4). If the regulation of these catecholamine-metabolizing enzyme activities in blood elements reflects the regulation of the enzymes in the hypothalamic-hypophyseal region, these results suggest that a defect in the regulation of dopamine-metabolizing enzymes is not associated with the pathogenesis of prolactinomas.

Alterations in serum sialyltransferase activities in patients with brain tumors

The activities of serum sialyltransferase were determined in patients with brain tumors. Blood samples from normal volunteers were used as controls. Serum specimens were obtained from patients with brain tumors both before and after operations. The preoperative serum sialyltransferase activities of the brain metastasis group showed significant increase, but the enzyme levels decreased after removal of the tumor. The serum sialyltransferase activities in the glioma group and the neurilemoma group increased significantly after operation, but no significant difference was found between the preoperative means of these two groups and that of the control. Surgical treatment produced significant differences between the preoperative and the postoperative serum sialyltransferase activities in the brain metastasis and the glioma and neurilemoma groups. Alterations of this enzyme in the blood of patients with brain tumors and its possible clinical applications are discussed.

A mu-opiate receptor in 7315c tumor tissue mediates inhibition of immunoreactive prolactin release and adenylate cyclase activity

Cells of the 7315c tumor released immunoreactive PRL (IR-PRL). Cholera toxin enhanced this release. Morphine and other opiate agonists inhibited IR-PRL release from both untreated and cholera toxin-treated tumor cells. The opiate-induced inhibition of IR-PRL release was concentration dependent and naloxone sensitive. Cholera toxin also enhanced the adenylate cyclase activity of 7315c tumor tissue. Opiates inhibited enzyme activity in both untreated and cholera toxin-treated 7315c tissue in a concentration-dependent and naloxone-sensitive manner. FK 33824 was more potent than [D-Ala2,D-Leu5]enkephalin in inhibiting IR-PRL release and adenylate cyclase activity. In cholera toxin-treated 7315c tumor tissue, GTP was required for opiate-induced inhibition of adenylate cyclase activity. Nonhydrolyzable analogs of GTP inhibited toxin-stimulated cyclase activity in the absence of an opiate. These results suggest that the 7315c tumor possesses a mu-opiate receptor; stimulation of this receptor inhibits both IR-PRL release and adenylate cyclase activity. An inhibitory guanyl nucleotide component may link the mu-opiate receptor to adenylate cyclase.

Secretion and regulation of two biosynthetic enzyme activities, peptidyl-glycine alpha-amidating monooxygenase and a carboxypeptidase, by mouse pituitary corticotropic tumor cells

Secretion of peptidyl-glycine alpha-amidating monooxygenase (PAM) activity and enkephalin convertase (a carboxypeptidase B-like enzyme) activity from AtT-20 mouse corticotrope tumor cells was compared with secretion of pro-ACTH/endorphin-derived peptides. Upon stimulation of secretion by corticotropin-releasing factor, a beta-adrenergic agonist, or barium ions, secretion of PAM activity, enkephalin convertase activity, and immunoactive hormone rose in parallel. During inhibition of stimulated secretion with glucocorticoids or cobalt ions, secretion of PAM activity, enkephalin convertase activity, and immunoactive hormone fell in parallel. No measurable secretion of cytosolic, mitochondrial, or lysosomal marker enzymes was detected, indicating that the secretion of PAM activity and enkephalin convertase activity was specific and not a result of cell damage or lysis. The kinetic characteristics, apparent mol wt, and enzymatic properties determined for the secreted enzymes were similar to those for the enzymes from secretory granules. In AtT-20 cells treated with glucocorticoids for up to 8 days, cellular levels of immunoactive ACTH/endorphin-related peptides decreased to one third of control levels. Levels of PAM activity in cell extracts declined in parallel with levels of hormone. In contrast, levels of enkephalin convertase activity, cell protein, lactate dehydrogenase, fumarase, and lysosomal carboxypeptidase did not decline in response to chronic glucocorticoid exposure.

Adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase activity in rodent pituitary tissue: possible role in cAMP-dependent hormone secretion

A cAMP-dependent protein kinase occurs in the intermediate lobe of the rat pituitary gland and the ACTH-secreting tumor AtT-20/D16-16 derived from the mouse pituitary gland. Exposure of either tissue to drugs increasing cAMP production and hormone release (forskolin, cholera toxin, or isoproterenol in the case of the intermediate lobe; forskolin or isoproterenol in the case of the AtT-20 cells) increases the cAMP-dependent protein kinase activity of a tissue homogenate in the absence, but not in the presence, of added cAMP. The potencies of these drugs to induce changes in the protein kinase activity ratio (i.e. enzyme activity in the absence of cAMP to enzyme activity in the presence of 3 microM cAMP) are comparable with their potencies as stimulants of hormone secretion. In either tissue, A23187, a calcium ionophore that stimulates hormone release but not cAMP production, does not change the protein kinase activity ratio. In the case of the AtT-20 cells, dexamethasone blocks the release of ACTH simulated by either isoproterenol or forskolin, but does not alter the enhancement of protein kinase activity induced by these drugs. Conversely, dexamethasone does not block the A23187-stimulated release of ACTH. The data suggest that cAMP modulates (but does not trigger) hormone secretion from the rodent pituitary gland by a mechanism involving activation of the cAMP-dependent protein kinase. Several possible sites for this modulatory effect of cAMP are discussed.

Thyrotropin-releasing hormone stimulates GTP hydrolysis by membranes from GH4C1 rat pituitary tumor cells

Thyrotropin-releasing hormone (TRH) stimulates prolactin production by GH4C1 rat pituitary tumor cells, which possess high-affinity membrane receptors for the peptide. TRH caused up to a 50% increase in the activity of a low-Km GTPase in membranes from GH4C1 cells. The TRH stimulatory effect was maximal at GTP concentrations of 1 microM or lower. TRH caused an increase in GTPase activity of between 0.2 and 20 pmol of GTP hydrolyzed per mg of protein per min, depending on GTP concentration, while TRH binding was 0.3 pmol/mg of protein. TRH did not stimulate GTPase activity in membranes from GH12C1, or GH-Y cells, two pituitary lines lacking TRH receptors. Stimulation of GTPase depended on occupancy of the TRH receptor; half-maximal increases in GTPase activity required 46 nM TRH and 25 nM [N3-methyl-His]TRH, but the TRH free acid was inactive. The apparent Kds of these peptides for receptors were similar when measured under the same conditions. The fact that TRH binding to receptors is regulated by guanyl nucleotides, together with the demonstration of TRH stimulation of low-Km GTPase activity, suggests that the TRH receptor is associated with a guanyl nucleotide regulatory protein in the lactotroph membrane.