Neuropeptides of anterior pituitary origin

Several neuropeptides classically associated with the hypothalamus have been found in the anterior pituitary. The question arises whether they are locally synthesized and if they play a paracrine or autocrine role on pituitary hormone secretion. Using normal and tumoral human pituitaries we found neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries, while stimulatory hormones (TRH and GHRH) were predominantly found in tumoral tissue, implying an imbalance of pathophysiological importance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. The in situ synthesis was demonstrated for SRIH by the evidence of SRIH mRNA, the detection of SRIH immunoreactivity in peculiar cells and the presence of SRIH precursor. The possible role of these pituitary neuropeptides was suggested for instance by the negative correlation found in vitro between SRIH and GH secretions. Moreover neuropeptides could interact on each other. Indeed DA stimulated TRH release while PRL secretion decreased at the same time. Pulses of TRH had differential effects on SRIH release according to the nature of the tissue as TRH inhibited SRIH release from adenoma while it stimulated SRIH release from normal pituitary. Concerning the effects of SRIH and GHRH on GH secretion, there was an endogenous regulatory pattern comparable to that described in rat portal blood vessels. Pulses of GHRH induced GH secretion only when endogenous SRIH release was not stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of vasoactive intestinal polypeptide on growth hormone secretion in perifused acromegalic pituitary adenoma tissues

The effects of vasoactive intestinal polypeptide (VIP), dopamine, and somatostatin (SRIF) on GH secretion were examined in vitro in perifused pituitary adenoma tissues obtained at surgery from seven patients with acromegaly. The perifusion of VIP at 5 x 10(-8) M resulted in a significant increase in effluent GH levels in five of the seven adenomas. A dose-related GH response was observed from 5 x 10(-9) to 5 x 10(-7) M VIP in two adenomas examined. SRIF at 5 x 10(-8) to 10(-7) M suppressed not only baseline secretion of GH but also inhibited GH rises elicited by VIP in six of the seven adenomas. Dopamine at 5 x 10(-7) to 5 x 10(-6) M decreased the baseline secretion of GH in six of the seven adenomas. In four of the six adenomas responsive to dopamine, dopamine suppressed VIP-induced GH release when perifused simultaneously. In the remaining two dopamine-sensitive adenomas in which VIP alone failed to affect GH release, the inhibition by dopamine of GH release was blocked by VIP perifused concomitantly with dopamine. Synthetic TRH or theophylline perifused at the end of the experiment stimulated GH release in all of the adenomas, indicating the viability of tumor cells throughout the study. These results suggest that VIP stimulates GH release by its direct action on pituitary adenoma cells of acromegalic patients and that VIP, SRIF, and dopamine interact at the pituitary level in modulating GH secretion from these adenomas.     

Acromegaly due to ectopic growth hormone (GH)-releasing hormone (GHRH) production: dynamic studies of GH and ectopic GHRH secretion

Dynamic studies of GH and GH-releasing hormone (GHRH) secretion were performed in a man with a GHRH-producing carcinoid tumor and acromegaly. Insulin hypoglycemia stimulated and metoclopramide inhibited both GH and GHRH acutely. Bromocriptine suppressed GH both acutely and chronically without altering circulating GHRH levels and also blunted the GH response to exogenous GHRH. TRH acutely stimulated GH, but not GHRH, secretion, and iv bolus doses of synthetic GHRH-(1-40) stimulated GH release acutely. Somatostatin infusion decreased both GH and GHRH concentrations and blunted the GH responses to TRH and GHRH-(1-40). We conclude that prolonged exposure of the pituitary gland to high concentrations of GHRH is associated with chronic GH hypersecretion and may be accompanied by a preserved acute GH response to exogenous GHRH; a paradoxical response of GH to TRH may be mediated at the pituitary level, consequent to prolonged pituitary exposure to GHRH; bromocriptine suppression of GH in acromegaly is due to a direct pituitary effect of the drug; and somatostatin inhibits both ectopic GHRH secretion as well as GH responsiveness to GHRH in vivo. Since GH secretory responses in patients with somatotroph adenomas are similar to those in this patient, augmented GHRH secretion may play a role in development of the "classic" form of acromegaly.     

GLYCOPROTEIN HORMONE ALPHA-SUBUNIT AND PROLACTIN RELEASE BY CULTURED PITUITARY ADENOMA CELLS FROM ACROMEGALIC PATIENTS: CORRELATION WITH GH RELEASE

In-vitro data of pituitary adenoma cells from 28 acromegalic patients were evaluated. In addition to GH, PRL was produced by 16 adenomas (57%) and alpha-subunit by 15 adenomas (54%) while there was a significantly higher incidence of tumours producing PRL and alpha-subunit simultaneously. From 26 pituitary adenomas enough cells were obtained in order to perform secretion studies. Percentage basal hormone release (medium: (medium + intracellular hormone)) x 100% of GH and alpha-subunit by 11 adenomas showed a close correlation while such a correlation for GH and PRL was present only in a subgroup of 10 of 13 adenomas. The responses of GH and alpha-subunit release to 10nM SMS201-995, 10nM bromocriptine, 100 nM TRH and 10nM GHRH were closely related in that a response or an absent response of GH release to the four secretagogues was virtually always attended with a response or an absent response respectively of alpha-subunit release. Such a relationship was less evident with respect to the effects of SMS201-995, bromocriptine. TRH and GHRH on GH and PRL release. We conclude that basal and secretagogue-induced alpha-subunit release by cultured pituitary adenoma cells from acromegalic patients closely follows the pattern of GH release while such a relationship for GH and PRL is present only in a subgroup of the adenomas secreting GH and PRL simultaneously.     

Pituitary vasoactive intestinal peptide regulates prolactin secretion in the hypothyroid rat

In this study, we demonstrated that the cell content and basal secretion of vasoactive intestinal peptide (VIP) in primary rat pituitary cell cultures were increased in hypothyroidism. VIP release from hypothyroid pituitary cells in vitro was stimulated by thyrotropin releasing hormone (TRH 10(-8) to 10(-6) M) and growth hormone (GH)-releasing hormone (GHRH 10(-9) to 10(-8) M) but not by corticotropin-releasing hormone or luteinizing hormone-releasing hormone in concentrations up to 10(-6) M. In the presence of anti-VIP antisera, there was a significant decrease in basal prolactin secretion from cultured hypothyroid pituitary cells (p less than 0.005) indicating that VIP exerts a tonic stimulatory effect on prolactin (PRL) secretion. The increment in PRL secretion following TRH was not affected by exposure to anti-VIP indicating that PRL release after TRH is not mediated by VIP at the pituitary level. In contrast to changes in PRL, exposure to anti-VIP had no effect on basal GH secretion, indicating that the PRL changes are hormone specific. Similarly, GHRH-induced GH release was unaffected by VIP immunoneutralization.     

Heterogeneity of pituitary adenoma cell subpopulations from acromegalic patients obtained by Percoll density gradient centrifugation

Pituitary adenoma cells from 6 acromegalic patients were separated on continuous Percoll density gradients according to differences in their density. Two adenomas produced GH only in culture, the other 4 adenomas produced either GH and PRL (one adenoma) or GH and alpha-subunit (one adenoma) or GH, PRL and alpha-subunit (2 adenomas). The cell subpopulations obtained by this technique differed in the amount of hormone production per 10(5) cells: GH release decreased from the low density fractions to the higher density fractions in 5 of 6 adenomas. Intracellular GH levels completely followed this profile. In the mixed GH/alpha-subunit adenomas the alpha-subunit profile completely paralleled the GH profile, whereas in the mixed GH/PRL adenomas the PRL profile showed a pattern different from that of GH (and alpha-subunit). In neither of the adenomas did we find any differences between the subpopulations with respect to the responsiveness of GH, PRL or alpha-subunit release to GHRH, TRH and the somatostatin analogue SMS 201-995. Conclusions: 1. Within pituitary adenomas from acromegalic patients heterogeneity exists with respect to hormone production per cell. 2. The cell subpopulations obtained by density gradient centrifugation are not different in their responsiveness to SMS 201-995, GHRH or TRH. 3. Because GH and alpha-subunit release by the fractions from the mixed GH/alpha-subunit secreting adenomas were completely parallel, further evidence for co-release of GH and alpha-subunit by the same tumoural cells is provided.     

Normal and growth hormone (GH)-secreting adenomatous human pituitaries release somatostatin and GH-releasing hormone

Neuropeptides such as vasoactive intestinal peptide, LHRH, or TRH have been found in rat pituitary tissue and could act via paracrine or autocrine actions in this tissue. In this study we investigated whether normal human pituitary tissue and GH-secreting human pituitary adenomas could release somatostatin (SRIH) and GHRH. Fragments from three human pituitaries and dispersed cells from six GH-secreting adenomas (four adenomas were studied for GHRH release and five for SRIH release) were perifused using a Krebs-Ringer culture medium, and the perifusion medium was collected every 2 min (1 mL/fraction for 5 h). GH, GHRH, and SRIH were measured by RIA under basal conditions and in the presence of 10(-6) mol/L TRH or SRIH. Both normal pituitaries and GH-secreting pituitary adenomas released SRIH and GHRH. SRIH release commenced 90-180 min after initiation of the perifusion, at which time GH secretion had decreased significantly. TRH stimulated SRIH release from normal pituitary tissue and inhibited SRIH release from adenoma tissue. GHRH was present at the start of the perifusion, but rapidly disappeared. However, SRIH stimulated GHRH release from normal pituitary tissue, but not from adenoma tissue. Significant amounts of GHRH and SRIH were released during the experiments, suggesting their local synthesis. These results indicate that pituitary cells can release hypothalamic peptides. The liberation of these neuropeptides is regulated, and moreover, their regulation differs between normal and adenomatous pituitaries.     

Secretion of glycoprotein hormone alpha-subunit by pituitary tumors

Serum glycoprotein hormone alpha-subunit levels were determined in 165 patients with pituitary adenomas. Elevated serum alpha-subunit levels were found in 17 patients (acromegaly, 5 of 58; prolactinoma, 6 of 56; nonfunctioning adenoma, 5 of 32; and ACTH-producing adenoma, 1 of 19), most of whom had normal serum TSH and gonadotropin levels. When TRH (0.5 mg) was injected iv in the 6 prolactinoma patients with elevated serum alpha-subunit levels, serum PRL and alpha-subunit levels increased in only 1 patient. Four acromegalic patients with high serum alpha-subunit levels received TRH; serum GH and alpha-subunit increased in 1 patient and did not change in 2, and only serum GH increased in the remaining patient. Oral administration of bromocriptine (5 mg), on the other hand, consistently decreased serum alpha-subunit and PRL levels in 2 patients with prolactinoma and alpha-subunit and GH levels in 1 acromegalic patient. When serum from 3 patients was subjected to Sephadex G-100 gel filtration, immunoreactive alpha-subunit eluted in a single peak, which emerged in fractions corresponding to [125I]TSH alpha. Concanavalin A (Con A) affinity chromatography revealed that the major portion of immunoreactive alpha-subunit was retained to Con A. A pituitary adenoma removed at surgery from a patient with acromegaly was studied in monolayer cell culture. Secretion of both alpha-subunit and GH from cultured adenoma cells was stimulated by TRH and suppressed by dopamine in a dose-dependent manner. Immunohistochemistry of the pituitary adenomas removed from patients with prolactinoma and acromegaly who had high serum alpha-subunit levels demonstrated alpha-subunit-containing cells as well as PRL- or GH-containing cells. These results suggest that elaboration of glycoprotein hormone alpha-subunit occurs without concurrent production of glycoprotein hormones in a substantial number of patients with pituitary adenomas and that alpha-subunit responses to stimuli in such adenomas are generally parallel with those of the concomitantly produced hormones.     

Phorbol ester, not growth hormone releasing factor, consistently stimulates growth hormone release from somatotroph adenomas in culture

In order to study the mechanism of GH secretion from somatotroph adenoma cells, we have compared the effect of 12–O-tetradecanoyl phorboi-13-acetate (TPA) with that of growth hormone releasing factor (GRF) on GH secretion from human somatotroph adenoma cells cultured in monolayer. Pituitary adenoma cells were obtained from 13 patients with acromegaly undergoing surgery. On the 7th day of culture, the cells were exposed for 2 h to secretagogues. All 13 adenoma cell cultures (100%) responded to TPA (1·6–16·0 nmol/I) with a two- to six-fold increase in GH release (240·37% Increase of control: mean±SE). The response was detectable within 10 min, and was maximal at 2 h. Phosphollpase C (7·7 mmol/I) also stimulated a two-to ten-fold Increase In GH release in all four adenomas examined (100%). GH release was stimulated by GRF (2·0 nmol/I) in eight out of 12 adenoma cells (67%), but the magnitude of the responses to GRF (60·18% Increase of control: mean ± SE) were much smaller than that of TPA. Five out of 13 adenomas secreted detectable amount of PRL Into the medium and these five adenomas (100%) responded to TPA (16·0 nmol/I) with a two- to six-fold Increase. These observations indicate that the activation of protein kinase C is the consistent stimulator in GH and PRL secretion In human somatotroph adenoma cells. However, It is not determined whether the protein kinase C     

Long-term culture of rat mammotrope and somatotrope subpopulations separated on continuous Percoll density gradients: effects of dopamine, TRH, GHRH and somatostatin

Normal adult female rat mammotrope and somatotrope subpopulations were separated on continuous Percoll density gradients according to differences in their density. Viable cells were recovered in 16 fractions. The cells from each fraction were cultured during 7 days after which period 4-h incubations were performed. rPRL secretion per cell increased towards the higher density fractions. No major difference in TRH, dopamine and somatostatin responsiveness was observed between mammotropes that were recovered in the different gradient fractions. In addition, no differences in somatostatin responsiveness between the somatotrope cells in the different gradient fractions were observed. However, somatotropes that were recovered in the highest density region of the gradient appeared to be more responsive to GHRH than the lower density somatotropes. In the various gradient fractions there were no paradoxical effects of TRH and dopamine on rGH release and of GHRH on rPRL release. Conclusions: 1. In long-term cultures there is no evidence for functionally different subpopulations of mammotropes and somatotropes, separated according to differences in their density, with regard to dopamine and TRH responsiveness and with regard to somatostatin responsiveness, respectively. 2. There is no evidence for a (mammosomatotrope?) subpopulation of cells showing paradoxical responses of PRL or GH release to GHRH and dopamine or TRH, respectively.     

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.     

Effect of thyrotropin-releasing hormone and bromoergocriptine on growth hormone and prolactin secretion in perfused pituitary adenoma tissues of acromegaly.

To determine the site of action of TRH and 2-brom-alpha-ergocriptine (CB154) on pituitary hormone release in acromegalic patients, the effect of these substances on GH and PRL secretion was examined in perfused pituitary adenoma tissues obtained at surgery from subjects with acromegaly. Relatively stable baseline secretion levels of GH and PRL were followed by an abrupt and marked discharge of the hormones after TRH infusion in all of the experiments. The pattern of GH response was essentially the same as that of PRL. Moreover, a dose-response relationship was obtained between the TRH concentrations infused and the magnitude of GH and PRL responses. The infusion of CB154, on the other hand, inhibited both GH and PRL secretion in three experiments performed on different adenoma tissues. This effect of CB154 was prompt and lasted for a long period even after the infusion was discontinued. When TRH was perfused concomitantly with CB154, the stimulatory effect of TRH on GH release was maintained, while TRH-induced PRL secretion was completely blocked. The results suggest that both TRH and CB154 possess a direct action on pituitary adenoma cells of acromegaly and that aberrant GH responses to TRH and dopaminergic agonists in acromegalic patients may be explained by the altered cellular membrane receptors of the adenoma of these subjects.     

Functional in vitro studies on the role and regulation of interleukin-6 in human somatotroph pituitary adenomas

OBJECTIVE: Interleukin-6 (IL-6), a member of the gp130 cytokine family, is considered to be an important modulator of function and growth in endocrine anterior pituitary cells. In pituitary adenomas, where IL-6 is often produced by the tumour cells, it is thought to be involved in pituitary adenoma pathophysiology via autocrine/paracrine mechanisms. METHODS: We have studied in primary cell cultures of human somatotroph adenomas whether IL-6 stimulates growth hormone secretion and whether intratumoral IL-6 is affected by various IL-6-regulating factors. RESULTS: Interleukin-6 stimulated GH secretion in 10 out of 11 somatotroph adenoma cultures (1.4- to 6.5-fold above basal levels). In comparative studies the GH-stimulatory potency of IL-6 was identical, or even stronger, than that of GHRH. In eight out of 11 adenoma cell cultures, IL-6 production was observed. This suggests that GH production might be stimulated by IL-6 in an autocrine/paracrine manner in these tumours. Dexamethasone strongly inhibited basal IL-6 secretion in all IL-6-producing adenoma cell cultures, whereas the IL-6 inhibitory or stimulatory action of other factors (octreotide, transforming growth factor-beta1, insulin-like growth factor-I, pituitary adenylate cyclase-activating peptide and oestradiol) were heterogeneous in the different adenomas. Only transforming growth factor-alpha consistently stimulated IL-6 secretion in all of the adenomas studied. CONCLUSIONS: Intratumoral IL-6, which is differently regulated by various factors, might contribute to excessive GH production in the majority of somatotroph adenomas.     

Heterogeneity of growth hormone (GH) release by individual pituitary adenoma cells from acromegalic patients, as determined by the reverse hemolytic plaque assay: effects of SMS 201-995, GH-releasing hormone and thyrotropin-releasing hormone

We used the reverse hemolytic plaque assay to study the dynamics of GH secretion by individual pituitary adenoma cells from eight acromegalic patients. There was a considerable variation between the adenomas with respect to the percentages of GH-secreting cells (25-78.5%) and also with respect to the amount of GH released per individual pituitary adenoma cell (mean plaque areas varying from 901-3559 micron 2). The GH plaque area frequency distributions from the adenoma cells were not normally distributed, but revealed a preponderance of small plaques, defined as those with areas smaller than the mean plaque area. The large plaques, that is those with areas larger than the mean plaque area, constituted 24-38% of the total cell population from different tumors and accounted for a large fraction (63-80%) of the total plaque area (the total amount of GH released by the adenoma cells). The somatostatin analog SMS 201-995 caused a shift in the GH plaque area frequency distributions toward smaller plaques, but had no effect on the overall percentages of GH plaque-forming cells in three of the five adenomas in which it was studied. This finding suggests that the adenoma cells from these patients that formed large plaques were preferentially inhibited by SMS 201-995. GHRH (studied in two adenomas) and TRH (studied in one adenoma) had no preferential effect on any subpopulation of adenoma cells. We conclude that GH secretion by individual somatotroph adenoma cells is highly variable both within and between adenomas and that SMS 201-995 has a preferential inhibitory effect on a subpopulation of adenoma cells in some adenomas.     

Mechanism of the inhibitory action of dopamine and somatostatin on prolactin secretion from human lactotrophs in culture

In an attempt to delineate the mechanism(s) of PRL secretion from human lactotrophs, the effects of dopamine and somatostatin on PRL release from adenomatous and nonadenomatous human pituitary cells in culture was studied. High K+ and the divalent cation ionophore A23187 both elevated PRL secretion, which was blocked by dopamine and somatostatin. When the cells were incubated in low calcium medium, PRL secretion was significantly inhibited. The addition of dopamine or somatostatin to low calcium medium further decreased PRL release. The stimulatory action of ionophore A23187 on PRL release was found even in the absence of extracellular calcium. Theophylline and isobutylmethylxanthine, when added to the incubation medium, increased PRL secretion, and dopamine as well as somatostatin again inhibited PRL release induced by phosphodiesterase inhibitors. No qualitative difference in these PRL responses was found in adenomatous and nonadenomatous human lactotrophs. In prolactinoma cells obtained from three different patients, cAMP generation was correlated with hormone release. Exposure of the cells to dopamine or somatostatin resulted in a parallel decrease in intracellular cAMP content and PRL secretion. The inhibitory effect of dopamine on PRL secretion and cAMP accumulation was blocked by coincubation of the cells with haloperidol. These results suggest that an increase in cytosol calcium caused by either mobilization from intracellular calcium pools or influx from the extracellular compartment and intracellular cAMP accumulation may be involved in the mechanism of PRL secretion from human lactotrophs, and dopamine and somatostatin may influence these two messengers to suppress PRL secretion.     

Reciprocal interactions of somatostatin with thyrotropin-releasing hormone and vasoactive intestinal peptide on prolactin and growth hormone secretion in vitro

Reciprocal interactions of somatostatin (SRIF) and vasoactive intestinal peptide (VIP) or TRH on in vitro PRL and GH release from male rats hemipituitaries were investigated. SRIF did not modify basal PRL release, but TRH- or VIP-induced release was inhibited by SRIF in a dose-dependent manner [effective concentration-fifty (EC50) = 1.7 +/- 0.9 nM for SRIF inhibition of TRH stimulation and EC50 = 0.8 +/- 0.5 nM for SRIF inhibition of VIP stimulation]. VIP and TRH did not affect GH release by themselves, but reduced the inhibition of GH secretion elicited by SRIF (EC50 = 7.6 +/- 3.4 nM for TRH blockade of SRIF inhibition and EC50 = 4.6 +/- 3.1 nM for VIP blockade of SRIF inhibition). Secretin, a partial structural analog of VIP, also blocked SRIF-induced inhibition of GH and stimulated PRL release. Secretin stimulation of PRL release was also prevented by SRIF. [D-Trp8,D-Cys14]SRIF, a potent analog of SRIF, antagonized VIP stimulation of PRL secretion with the same apparent affinity as the native peptide. The maximal stimulation, but not the apparent affinity of VIP action on prolactin release was reduced by SRIF, suggesting that the interaction is of a noncompetitive nature. This conclusion as further substantiated by the observation that neither TRH nor VIP were able to displace specific 125I-labeled [Tyr1] SRIF high affinity binding to pituitary membranes. The three peptides tested thus appear to exhibit reciprocal interactions mediated by independent receptor sites on GH as well as on PRL-producing cells.     

Effect of GnRH-associated peptide on prolactin secretion from human lactotrope adenoma cells in culture

The effect of GnRH-associated peptide on PRL secretion by human pituitary lactotropes in culture was studied. Pituitary adenomas obtained at selective transsphenoidal adenomectomy from a patient with prolactinoma, and two patients with mixed GH- and PRL-secreting pituitary adenomas were cultured in monolayer. When cells were incubated with dopamine (10 nmol/l), a significant inhibition in PRL secretion was observed in all the experiments, which was blocked by co-incubation with haloperidol. In mixed GH- and PRL-secreting adenoma cells, dopamine likewise decreased GH secretion. Incubation of cells with synthetic GnRH-associated peptide at concentrations up to 100 nmol/l, on the other hand, failed to affect both PRL and GH secretion. These results suggest that synthetic GnRH-associated peptide has no inhibitory effect on PRL secretion in human pituitary lactotropes.     

Pituitary somatotroph adenoma producing growth hormone (GH)-releasing hormone (GHRH) with an elevated plasma GHRH concentration: a model case for autocrine and paracrine regulation of GH secretion by GHRH.

An acromegalic patient with a pituitary somatotroph adenoma associated with an extremely elevated plasma GHRH concentration is presented. The preoperatively high concentration of plasma GHRH returned to the normal level after successful removal of the adenoma. GHRH production and GHRH gene expression were confirmed in the adenoma by studies including immunohistochemistry and in situ hybridization. Expression of GHRH receptor messenger ribonucleic acid was verified by in situ hybridization. Immunohistochemical double staining for GH and GHRH revealed their colocalization in single adenoma cells. These findings confirmed the autocrine or paracrine regulation of GH production by endogenous GHRH from the adenoma cells. GHRH synthesis in the pituitary gland has recently been demonstrated, however, there have been no previous reports of a GHRH-producing pituitary somatotroph adenoma associated with an elevated plasma GHRH concentration. The existence of this GHRH-producing adenoma suggests a possible role of locally generated GHRH in the progression of somatotroph adenomas, i.e. the monoclonally established somatotroph adenomas develop further under the influence of locally produced GHRH. The demonstration of GHRH production by this somatotroph adenoma is of importance in clarifying the autocrine or paracrine regulation of GH production and the progression of human somatotroph adenomas.     

Hormone secretion in vitro by plurihormonal pituitary adenomas of the acidophil cell line.

Pituitary tumors producing GH and PRL are morphologically classified as monomorphous bihormonal acidophil stem cell adenomas (ASCAs) which cause hyperprolactinemia and two tumor types which are usually associated with acromegaly, the monomorphous plurihormonal mammosomatotroph adenomas (MSAs) and bimorphous mixed somatotroph-lactotroph adenomas. We studied 12 MSAs, 2 ASCAs, and 10 mixed adenomas in vitro to assess the secretory behavior of these tumors diagnosed by immunohistochemistry and electron microscopy. GH release by MSAs and all but one mixed tumor was greater than that of PRL; the opposite was true of the ASCAs. One mixed tumor which caused impotence and hyperprolactinemia contained predominantly lactotrophs and released greater amounts of PRL than of GH in vitro. All 12 MSAs and 6 of 10 mixed tumors released alpha-subunit of glycoprotein hormones. Incubation with GHRH increased release of GH and PRL by all tumors and of alpha-subunit when present; the responses of all hormones were parallel among MSAs whereas among mixed adenomas, GH and alpha-subunit had greater responses than PRL. TRH stimulated GH, PRL, and alpha-subunit release by MSAs in parallel; among mixed adenomas, PRL response was generally greater than that of GH or alpha-subunit. SRIH markedly reduced GH release by all MSAs; it inhibited GH and alpha-subunit release by mixed tumors more than it affected PRL. Bromocriptine inhibited GH, PRL, and alpha-subunit release by most MSAs and mixed tumors but did not inhibit GH or PRL release by ASCAs. This study demonstrates release of GH, PRL, and alpha-subunit by these morphologically classified plurihormonal tumors in vitro. Variable quantities of GH and PRL released by the different tumor types correlate with immunohistochemical and clinical data. The dynamic studies indicate that regulation of GH, PRL, and alpha-subunit release can be affected by GHRH, TRH, SRIH, and bromocriptine in these adenomas and suggest differences in receptor status. Our data strengthen the view that these three plurihormonal adenomas of the acidophil cell line are not only morphologically but also functionally different and warrant separation.     

Prolactin secretion by mixed ACTH-prolactin pituitary adenoma cells in culture

To characterize the functional aspect of prolactin (Prl) cells coexisting with corticotroph adenomas, pituitary adenoma cells obtained from a patient with Cushing's disease and a patient with Nelson's syndrome, who were associated with hyperprolactinaemia, were cultured in monolayer and their Prl responses to various secretagogues were compared with those of prolactinoma cells in culture. Immunohistochemistry performed in one of these two adenomas demonstrated the presence of Prl-containing cells in addition to ACTH cells. When ACTH-Prl adenoma cells were exposed to ovine corticotrophin-releasing factor (CRF), a dose-dependent increase in both ACTH and Prl secretion was observed, which was blocked by coincubation with hydrocortisone. In contrast, no stimulatory effect of CRF on Prl release was observed in all of the experiments using prolactinoma cells. Thyrotrophin-releasing hormone, which consistently stimulated Prl secretion in ACTH-Prl adenomas, was effective in triggering Prl release in only 25% of the prolactinomas. Exposure of the cultured cells to lysine vasopressin, growth hormone-releasing factor and vasoactive intestinal peptide resulted in an increase in ACTH and Prl secretion in one ACTH-Prl adenoma, however, none of the prolactinomas responded to these stimuli to secrete Prl. Dopamine and somatostatin, on the other hand, uniformly suppressed Prl secretion from ACTH-Prl adenomas as well as from prolactinoma cells. These results suggest that the mode of Prl secretion by mixed ACTH-Prl pituitary adenomas is not identical to that by pure prolactinomas and is, at least in part, common to that of ACTh secretion.