Intermediary metabolism in the liver and adipose tissue of rats with hormone-secreting pituitary tumors

Effects of long-term experimental diabetes on adrenal gland growth and phosphoribosyl pyrophosphate formation in growth hormone-deficient dwarf rats

The availability of growth hormone (GH)-deficient dwarf rats with otherwise normal pituitary function provides a powerful tool to examine the relative role of hyperglycaemia and the reordering of hormonal factors in the hypertrophy-hyperfunction of the adrenal gland that is seen in experimental diabetes. Here, we examine the effects of long-term (6 months) experimental diabetes on the growth of the adrenal glands; their content of phosphoribosyl pyrophosphate (PRPP); and the activity of the PRPP synthetase, G6P dehydrogenase and 6PG dehydrogenase enzymes in GH-deficient dwarf rats compared to heterozygous controls. These parameters were selected in view of the known role of PRPP in both de novo and salvage pathways of purine and pyrimidine synthesis and in the formation of NAD, and in view of the role of the oxidative enzymes of the pentose phosphate pathway in both R5P formation and the generation of the NADPH that is required in reductive synthetic reactions. This study shows that GH deficiency prevents the increase in adrenal gland weight, PRPP synthetase, PRPP content and G6P dehydrogenase and 6PG dehydrogenase. This contrasts sharply with the heterozygous group that showed the expected increase in these parameters. The blood glucose levels of the groups of long-term diabetic rats, both GH-deficient and heterozygous, remained at an elevated level throughout the experiment. These results are fully in accord with earlier evidence from studies with somatostatin analogues which showed that the GH-insulin-like growth factor I (IGF-I)-axis plays a key role in the adrenal diabetic hypertrophy-hyperfunction syndrome.

MMQ cells: a model for evaluating the role of G proteins in the modulation of prolactin release

It is well known that dopamine (DA) inhibits while vasoactive intestinal peptide (VIP) and angiotensin II (ANG II) stimulate prolactin (PRL) release from normal anterior pituitary lactotrophs; however, elucidation of the intracellular mechanisms involved in these effects has been hindered by the cellular heterogeneity of the anterior pituitary. MMQ cells, isolated from the PRL-secreting rat pituitary tumor 7315a is an interesting model since they only secrete PRL. In order to determine whether and which GTP-binding (G) proteins are involved in the modulation of cyclic 3',5'-adenosine monophosphate (cAMP) accumulation and phospholipids turnover and eventually PRL release, we have performed studies with MMQ cells. For this purpose, the levels of various G proteins (alpha o, alpha s, alpha i, alpha q and beta) and their mRNAs, measured by Western and Northern blots respectively, were correlated with intracellular cAMP accumulation in response to DA, VIP or DA plus VIP, and with inositol phosphates (IPx) formation in response to ANG II, DA or DA plus ANG II. This study shows that, when compared to normal pituitary tissue, the levels of alpha o, alpha o2 and alpha i3 were significantly decreased in MMQ cells; those of alpha o1, alpha i (alpha i1 + alpha i2), alpha s42 and alpha q were very low or undetectable while those of alpha s47 and beta were normal. DA was unable to inhibit basal PRL release and cAMP accumulation. VIP increased both cAMP accumulation and PRL release, while cAMP accumulation elicited by VIP could be suppressed by DA. BAY K 8644-induced PRL release also could be suppressed by DA.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of prolactin release by gonadotropin-releasing hormone-associated Peptide in benign, dopamine-sensitive and in malignant, dopamine-resistant pituitary tumors

Since the gonadotropin-releasing hormone-associated peptide (GAP) has been reported to be capable of inhibiting prolactin release from normal lactotrophs, with the present study we have examined the in vitro effects of GAP on prolactin release in an estrone-induced, dopamine-sensitive rat pituitary adenoma and two malignant, transplantable and dopamine-resistant rat pituitary tumors, 7315a and MtTW15. Enzymatically dispersed cells obtained from the three types of tumor were cultured in multiwell dishes for 4 days. On the fifth day, the cells were exposed for 4 h to human GAP 1-56 or to the analog GAP 42-56 or to rat GAP 1-53, at various concentrations. In some experiments, the effect of a pretreatment of the cells for 16 h with pertussis toxin before exposure to human GAP was also evaluated. In the three tissues, rat GAP was able to inhibit prolactin release in a dose-dependent manner. Human GAP 1-56 and GAP 42-56 were able to inhibit prolactin release in a dose-dependent manner in all cells except those of the MtTW15 tumor. Furthermore, in adenomatous cells, the inhibitory effects of these peptides were suppressed by pretreatment of the cells with pertussis toxin. These findings indicate that GAP is capable of inhibiting prolactin release even in dopamine-resistant pituitary tumors. This inhibition is exerted through a pertussis toxin-sensitive G-protein-dependent signaling mechanism in adenomatous cells.

Comparison of efficacy of cysteamine in depleting prolactin immunoreactivity in different hyperprolactinemic animal models

We have examined the effects of cysteamine on its ability to deplete prolactin in various states of hyperprolactinemia. Administration of subtoxic doses of cysteamine (75 and 150 mg/kg,sc) dramatically reduces serum prolactin levels as well as pituitary prolactin content in a dose-dependent manner in estrogen-primed brown Irish ACI female rats. A similar dose-dependent decrease in anterior pituitary prolactin levels was observed in two ectopic prolactin secreting pituitary tumor models (MtTW15 and 7315a). However, a significant reduction in serum prolactin levels was seen in these same tumor bearing animals at only the 150 mg/kg dose of cysteamine. Interestingly, the prolactin content of each of the prolactin secreting tumors, although reduced by cysteamine administration, the effect was neither dose-dependent nor as dramatic as that observed in the anterior pituitary gland proper. These data demonstrate that cysteamine can significantly lower prolactin concentrations in hyperprolactinemia. Further, ectopic prolactin secreting pituitary tissue appears less sensitive to the prolactin-depleting effects of cysteamine. This latter finding may explain, in part, why serum prolactin levels were not as severely reduced in the ectopic tumor bearing female rats as in estrogen-induced hyperprolactinemic animals.

Control of phospholipid turnover and prolactin release in a dopamine-sensitive, prolactin-secreting rat pituitary adenoma and in two dopamine-resistant, prolactin-secreting rat pituitary tumors

Abstract The secretion of prolactin by the pituitary gland is under a tonic inhibitory control exerted by tubero-infundibular dopamine. Recently, it has been suggested that dopamine may exert its action by inhibiting production of inositol phosphates and mobilization of intracellular Ca(2+). To study the effects of dopamine on the production of inositol phosphates and prolactin release, we have utilized an estrone-induced, dopamine-sensitive rat pituitary adenoma and two transplantable and dopamine-resistant rat pituitary tumors, 7315a and MtTW15. Purified cells, obtained from the three tissues, were incubated for 30 min in media with drugs (thyrotropin-releasing hormone or angiotensin II) stimulating inositol phosphates and prolactin release, in the presence or the absence of dopamine. Basal production of inositol phosphates and prolactin release by adenomatous lactotrophs were inhibited by dopamine. Thyrotropin-releasing hormone and angiotensin II stimulated inositol phosphates by adenomatous and 7315a cells. This effect was antagonized by dopamine in adenomatous cells. Prolactin release by adenomatous cells only was stimulated by thyrotropin-releasing hormone and angiotensin II. This stimulation was inhibited by dopamine. The results show differences, in the mechanisms of regulation of prolactin release, between adenoma and transplantable pituitary tumors as well as between the two tumors themselves. These differences may be responsible, in part, for the resistance of the two transplantable pituitary tumors to the inhibitory effects of dopamine on prolactin release and tumor size. Our results obtained both with adenoma and tumoral cells also suggest that inositol phosphates probably intervene only in the late phases of dopamine inhibition of prolactin release and only in the presence of a normal Ca(2+) signaling system.

Estradiol attenuates prolactin secretion and phosphoinositide hydrolysis in MMQ cells

We previously isolated a clonal cell line, designated MMQ, which only secretes prolactin (PRL) and whose secretory process is nonresponsive to thyrotropin releasing hormone (TRH) and angiotensin II (AII). In the present study, we injected MMQ cells into rats to determine whether the tumor cells would become responsive to secretagogues when subsequently propagated in vitro. We also investigated what effects in vivo administration of 17 beta-estradiol would have on secretagogue-induced PRL release and on intracellular biochemical mechanisms in these cells. MMQ cells were implanted subcutaneously in the backs of female rats. One group was injected with 100 micrograms polyestradiol phosphate (PEP) every 5 days, a second with saline. The inoculants grew into solid tumors within 3 weeks. The day after the tumors were removed and enzymatically dispersed, the cells, now designated MMQt cells, were perifused in vitro. Basal PRL released by MMQt cells was approximately 1 ng/min/10(7) cells and perifusions with 100 nM TRH or AII for 5 min significantly increased PRL release above baseline (integrated areas: 1.8 +/- 0.4 and 5.2 +/- 1.3 ng/10(7) cell, respectively; P less than 0.01). Two ng/ml maitotoxin (MTX), a calcium channel activator, increased PRL release (38.2 +/- 6.7 ng/10(7) cells; P less than 0.01). In PEP-treated perifused MMQt cells, basal in vitro PRL release was not different from that observed in the control group, but the responses to TRH, AII and MTX were greatly attenuated (TRH: 0.6 +/- 0.1, AII: 1.3 +/- 0.2 and MTX: 9.2 +/- 2.5 ng/10(7) cells).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of Basal and stimulated prolactin release in prolactin-secreting rat pituitary tumors*

Abstract To better understand the mechanisms of the inhibitory effects of dopamine on pituitary prolactin release, we have utilized an estrone-induced, benign and dopamine-sensitive rat pituitary adenoma and two malignant, transplantable and dopamine-resistant rat pituitary tumors, 7315a and MITW15. Enzymatically dispersed and Percoll purified cells obtained from the three tissues were incubated for 30 min in media with or without Na(+) and in the presence or the absence of dopamine and/or various prolactin releasers for evaluating the secretion of prolactin under baseline and experimental conditions. In some experiments, the cells were pretreated for 16 h with pertussis toxin to evaluate the eventual presence and role of pertussis toxin-sensitive G proteins. Dopamine inhibited baseline prolactin release by adenomatous lactotrophs in a Na(+)-dependent manner, but was totally inactive with 7315a and MtTW15 cells. The Ca(2+) channel agonist BAY K 8644 stimulated prolactin release with all three preparations and its effects were enhanced by a Na(+)-free medium. Dopamine antagonized the stimulatory effects of BAY K 8644 with adenomatous and 7315a cells only, even in the absence of Na(+). Pertussis toxin pretreatment significantly increased baseline prolactin release by adenomatous and MtTW15 cells and abolished dopamine inhibition of adenomatous lactotrophs baseline hormone release. BAY K 8644, TRH and vasoactive intestinal peptide, stimulated prolactin release by adenomatous cells and this effect was antagonized by dopamine in a pertussis toxin-sensitive manner. All prolactin releasers, except TRH, were effective also with 7315a cells, and its actions were not blocked by pertussin toxin. The stimulatory effects of BAY K 8644 and vasoactive intestinal peptide on 7315a cells were enhanced by pertussis toxin pretreatment. The results obtained with an almost pure preparation of adenomatous lactotrophs confirm the existence of a dual mechanism of dopamine inhibitory action on prolactin release: 1) a Na(+)-dependent action exerted on baseline, and 2) a Na(+)-independent action exerted on stimulated prolactin release. They also indicate that both actions are exerted through pertussis toxin-sensitive G proteins. Furthermore, our results show the presence in transplantable pituitary tumors 7315a and MtTW15 of multiple and diverse anomalies in the regulation of prolactin release probably due, at least partly, to anomalies of one or more G proteins and/or neurotransmitter receptors.

Plasma lipids and prolactin in patients with breast cancer

In a comparative study of pre- and postmenopausal women with benign and malignant breast disease, a number of differences were observed in circulating plasma prolactin and lipid concentrations. Plasma lipids, phospholipids, triglycerides, cholesterol and free fatty acids were all higher in blood obtained from breast cancer patients prior to surgery. HDL-Cholesterol levels were significantly lower in these patients. These differences remained when the patient groups were sub-divided according to menopausal status. Plasma prolactin concentrations were also found to be higher in cancer compared with non-cancer patients, this effect being more marked in premenopausal than in postmenopausal patients. Premenopausal patients with invasive or poorly differentiated disease had significantly higher prolactin levels than those with non-invasive disease. No correlations were found between plasma prolactin and any of the lipid fractions.

Effects of growth hormone on lipogenesis and glucose oxidation in genetically GH-deficient mice

Late inhibitory effects of growth hormone were observed when the hormone (0.1-1.0 microgram/ml) was incubated with adipose tissue from GH-deficient and normal mice. Early insulin-like effects of GH occurred only when glucose was used as precursor. Adult GH-deficient mice have a much higher basal acetyl-CoA carboxylase activity than normal mice. Administration of GH caused a marked reduction of enzyme activity in both groups.

Growth hormone releasing factor increases growth hormone release from MtTW15 pituitary tumors

The MtTW15 pituitary tumor secretes growth hormone and prolactin. Perfusion of these dispersed MtTW15 tumor cells with 10 nM growth hormone releasing factor (GRF) increases growth hormone release without affecting prolactin release. This effect is dose-dependent between 0.001 and 0.1 nM and is blocked by 100 nM somatostatin. These findings suggest that this tumor and clones derived from it may be valuable tools in studying the cellular mechanisms of action of GRF and somatostatin.

Prolactin release from MtTW15 and 7315a pituitary tumors is refractory to TRH and VIP stimulation

We studied the in vitro responsiveness of prolactin-secreting MtTW15 and 7315a pituitary tumor cells to stimulation by selected secretagogues using a perifusion technique. Prolactin release by these cells was refractory to thyrotropin-releasing hormone (TRH) and vasoactive intestinal peptide (VIP). In contrast, 50 mM K+, dibutyryl cAMP, theophylline, phospholipase A2 and phorbol myristate acetate all increased prolactin release from both tumor cell types. Phospholipase C increased prolactin release from 7315a but not from MtTW15 cells. TRH increased 32P incorporation into phosphatidylinositol in the 7315a but not in the MtTW15 tumor cells. Therefore, the refractoriness of these tumors to TRH and VIP may be at least partially due to a defect in the receptor or in the process that couples receptor binding and intracellular biochemical processes. In the MtTW15 tumor at least part of the defect may be related to phospholipid hydrolysis.

The pentose phosphate pathway of glucose metabolism. Influence of a growth-hormone-secreting pituitary tumour on the oxidative and non-oxidative reactions of the cycle in liver

1. Measurements were made of the activities of enzymes of the pentose phosphate cycle, glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, ribose 5-phosphate isomerase, ribulose 5-phosphate epimerase, transketolase and transaldolase, as well as of the related or competing enzymes glucokinase, hexokinase, phosphoglucose isomerase and phosphoglucomutase, in control rats and in rats bearing the growth-hormone- and prolactin-secreting pituitary tumour MtTW5, to study the effect of high endogenous concentrations of growth hormone on this pathway in liver. 2. There was a twofold increase in liver weight. Glucokinase activity/g. of liver decreased to half the control value in the experimental group, although on a total liver basis it remained unchanged. Hexokinase activity increased in parallel with the liver weight, so that the total activity was doubled in rats with a high endogenous concentration of growth hormone. No differences in response were found between heat-stable and heat-labile forms of hexokinase. 3. The activity/g. of liver of the two oxidative enzymes of the pathway decreased slightly in the experimental group, but this was offset by the increase in liver weight, and the resultant effect was a 50% increase in the total activity. 4. Of the non-oxidative enzymes of the cycle the most marked increase on a total liver basis was in ribose 5-phosphate isomerase activity, to 2.5 times the control value. Ribulose 5-phosphate epimerase activity showed the smallest increase. Transketolase and transaldolase activities were also increased. The latter is the rate-limiting enzyme of the non-oxidative reactions of the cycle in these animals. 5. The results are discussed in relation to the glycolytic pathway and synthesis of glycogen, and more particularly to the increased requirement for ribose 5-phosphate for RNA synthesis.