Efficacy of chimeric molecules directed towards multiple somatostatin and dopamine receptors on inhibition of GH and prolactin secretion from GH-secreting pituitary adenomas classified as partially responsive to somatostatin analog therapy.

OBJECTIVE: This study compared the potency of a somatostatin receptor (sstr)2-sstr5 analog, BIM-23244, of an sstr2-dopamine D2 receptor (sstr2-DAD2) molecule, BIM-23A387 and of new somatostatin-dopamine chimeric molecules with differing, enhanced affinities for sstr2, sstr5 and DAD2, BIM-23A758, BIM-23A760 and BIM-23A761, to suppress GH and prolactin (PRL) from 18 human GH adenomas that are partially responsive to octreotide or lanreotide. MATERIALS AND METHODS: The sstr2, sstr5 and DAD2 mRNA levels were determined by RT-PCR. The effect of drugs was tested in cell cultures at various concentrations. RESULTS: In all tumors, the sstr2, sstr5 and DAD2 mRNA levels were coexpressed (mean levels+/-s.e.m. 0.4+/-0.1, 5.3+/-1.9 and 2.0+/-0.4 copy/copy beta-glucuronidase). In 13 tumors, the maximal suppression of GH secretion produced by BIM-23A387 (30+/-3%) and BIM-23244 (28+/-3%) was greater than that produced by octreotide (23+/-3%). In six out of 13 tumors, BIM-23A758, BIM-23A760 and BIM- 23A761 produced greater maximal suppression of GH secretion than octreotide (33+/-5, 38+/-2 and 41+/-2 vs 24+/-2%). Their EC(50) values were 10, 2 and 4 pmol/l. BIM-23A761 was more effective than BIM-23A387 in GH suppression (41+/-2 vs 32+/-4%). The new chimeric molecules produced maximal PRL suppression greater than octreotide (62+/-8 to 74+/-5 vs 46+/-11%). CONCLUSIONS: Novel dopamine-somatostatin chimeric molecules with differing, enhanced activity at sstr2, sstr5 and DAD2, consistently produced significatly greater suppression of GH and PRL than either octreotide or single-receptor-interacting ligands in tumors from patients classified as only partially responsive to octreotide therapy. The higher efficacy of the chimeric compounds was, at least partially, linked to their high affinity for sstr2 (IC50 1-10 pmol/l). The other mechanisms by which such molecules produce an enhanced inhibition of GH remain to be elucidated.

Combined pituitary hormone deficiency due to the F135C human Pit-1 (pituitary-specific factor 1) gene mutation: functional and structural correlates.

The pituitary-specific transcription factor Pit-1 (pituitary-specific factor 1) is known to play a key role in the differentiation of PRL-, GH-, and TSH-secreting cells, and in the regulation of expression of the corresponding genes. In recent years, 12 distinct mutations of the Pit-1 gene have been shown to be responsible for a phenotype of multiple congenital pituitary hormone deficiency involving PRL, GH, and TSH. We had previously identified, in four siblings with GH, PRL, and TSH deficiencies, a mutation (F135C) resulting in a single amino acid change within the POU-specific binding domain of the Pit-1 molecule. In the present report, we have explored the functional effect of the F135C mutation. In vitro activity tests performed by transfection in human HeLa cells showed decreased transactivation capacity on the PRL, GH, and Pit-1 genes. The DNA binding experiments performed by gel shift showed that the F135C mutation generated a protein capable of binding to DNA response elements. To analyze how the F135C mutation might affect functionality of the transcription factor despite a normal DNA binding, we used a structure modelization approach and also analyzed two other Pit-1 mutant proteins (F135A and F135Y). The loss of functionality in these two mutants was similar to that of F135C. This finding was in keeping with our molecular modeling studies. According to structural data derived from the crystallographic analysis of the DNA/Pit-1 POU domain complex, the conformation of the first helix of the F135C-mutated POU-specific domain could be perturbed to such an extent that any interaction with other transcription cofactors might be definitively prevented.

Different pathways of secretion for glycosylated and nonglycosylated human prolactin.

Human prolactinoma cells in culture secrete the monomeric nonglycosylated form of human PRL (NG-hPRL) and its glycosylated variant (G-hPRL). We have performed pulse-chase experiments to investigate the individual patterns of release of these two molecular variants. The cells were pulse labeled for 10 min with [35S]methionine and then chased for increasing periods of time up to 24 h. The secretion of newly synthesized G- and NG-hPRL was followed by immunoprecipitation of the chase medium and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. Both forms were rapidly released (10 min of chase), but presented with different rates of secretion. Half-maximal release of G-hPRL occurred with 60-min chase, while 110 min were necessary for NG-hPRL. More than 50% of initially labeled G-hPRL was released in the medium vs. only 20% for NG-hPRL. Incubation of the cells with 8-chloroadénosine-cAMP during a 2-h chase period resulted in a 3.6-fold increase in the release of newly synthesized NG-hPRL and had only a slight effect on newly synthesized G-hPRL release (1.7-fold increase). The intracellular transit of labeled G- and NG-hPRL was investigated in cells treated by the ionophore monensin. The secretions of both newly synthesized forms were inhibited to the same extent, probably via an arrest of the transit at the level of the median Golgi, as judged by the delay of acquisition to endoglycosidase-H resistance for G-hPRL in monensin-treated cells. In contrast, Western blot analysis of the same medium-showed that monensin abolished the secretion of G-hPRL and had little effect on NG-hPRL. Our results on the different rates of secretion of G- and NG-hPRL indicate a sorting of the two forms into different compartments in the secretory pathway, with G-hPRL being secreted at a higher rate than NG-hPRL, possibly via a different intracellular route. The differential effects of 8Cl-cAMP and monensin further suggest that G-hPRL may be constitutively secreted after synthesis, while NG-hPRL secretion may involve a storage step.

Polymorphism of prolactin secreted by human prolactinoma cells: immunological, receptor binding, and biological properties of the glycosylated and nonglycosylated forms.

Multiple forms of PRL differing in their physicochemical and biological characteristics have been described. We have analyzed the molecular forms of human (h) PRL released in culture by pure hPRL-secreting tumors with a particular attention to glycosylated hPRL. The prolactinoma cells from six different tumors released, in serum-free conditions, 10-28 mg hPRL. The combination of polyacrylamide gel electrophoresis and immunoblotting techniques using a [125I]anti-hPRL monoclonal antibody allowed qualitative and quantitative analysis of the hPRL variants. The ratio of the glycosylated 25,000-mol wt form (G-hPRL) to the 23,000-mol wt nonglycosylated monomeric hPRL (NG-hPRL) varied from 0.13 to 0.25. Under the conditions of our studies, cleaved forms of the hormone (19,000 and 15,000 mol wt) accounted for less than 5% of the total immunoreactivity. G- and NG-hPRL were subsequently purified by gel filtration and lectin affinity chromatography. G-hPRL appeared fully sensitive to endoglycosidase F digestion, further supporting the presence of a freely accessible N-linked carbohydrate chain. When assayed for their ability to react with polyclonal antibodies directed against hPRL in a competitive RIA, G-hPRL was 3 times less immunoreactive than NG-hPRL. However, both types of hPRL exhibited superimposable displacement curves when tested in an immunoassay using an anti-hPRL monoclonal antibody. In binding studies using crude rabbit mammary gland membranes G-hPRL was half as potent as NG-hPRL. In stimulating the growth of the Nb2 lymphoma cell line, G-hPRL was 50% less active than NG-hPRL. Thus 1) under basal conditions, hPRL undergoes partial and variable glycosylation; 2) glycosylation of the hormone may modulate its immunoreactivity; 3) glycosylation of hPRL not only lowers its mammary gland receptor binding capacity but also its growth-promoting activity.

Direct action of opiates on bromocriptine-inhibited prolactin release by human prolactinoma cells in primary culture.

The present study was undertaken in order to examine the existence of opioid binding sites on cell membranes of human PRL-secreting tumors. Determination of opioid binding sites using different opiate ligands revealed one class of high affinity (KD, 1.3 nM) binding sites. Pharmacological characterization revealed kappa-1 selectivity (high affinity ethylketocyclazocine (EKC) binding, insensitive to 5 microM (D-Ala2, D-Leu5]enkephalin). Subsequently EKC was added to hPRL-secreting tumor cells in primary culture, alone or in combination with the dopaminergic agonist bromocriptine, and PRL release was measured. Opiates had no direct effect on PRL release by prolactinoma cells. When cells were preincubated with bromocriptine [6.6 +/- 4.8 (SD) X 10(-11) M], EKC (10(-11) to 10(-9) M) antagonized, in a dose-dependent manner, the dopaminergic inhibition of PRL release. The opiate effect was reversed by the opiate antagonist diprenorphine (10(-7) M). Cross-competition studies indicated that this effect was not due to the interaction of opiates with the dopaminergic receptor. In conclusion, opioid binding sites are found on prolactinoma cells. The binding of kappa-1 type opioid ligands modulates the inhibitory effect of dopamine upon PRL release.

Bromocriptine effects on cultured human prolactin-producing pituitary adenomas: in vitro ultrastructural, morphometric, and immunoelectron microscopic studies.

In an effort to better characterize the ultrastructural, morphometric, and immunocytochemical changes induced by 10(-8) M bromocriptine (BR), tumor cells from three surgically removed PRL-producing pituitary adenomas were cultured on an extracellular matrix in serum-free medium. In each instance, the treated cultures were compared to control cells at the end of 24 h and 16 days. PRL RIAs were performed on culture medium. A decrease in cell and nucleus surface area was found on day 16 in two cultures. This supports the well known shrinkage of BR-treated PRL-producing adenomas. BR induced no change in these parameters in the tumor from a third patient who was partly resistant to the drug. Changes in the secretory process were discernible as of day 1 in all three tumors, with a dramatic reduction of exocytosis and intracellular accumulation of PRL-immunoreactive granules. This induced delayed inhibition of protein synthesis, demonstrated by preembedding immunocytochemistry on day 16. These results, obtained for the first time in human PRL-producing adenomas, are informative as to the subcellular events subsequent to short term BR treatment and illustrate that secretory inhibition and tumor shrinkage are not necessarily linked.

Pit-1 gene expression in the human pituitary and pituitary adenomas.

The cellular basis for pituitary neoplasia is poorly understood. The POU domain protein Pit-1 is a pituitary-specific transcription factor involved in the generation, differentiation, and proliferation of three pituitary cell types: lactotrophs, somatotrophs, and thyrotrophs. In this study, we analyzed the expression of Pit-1 gene in a series of 15 different human pituitary tumors and compared it with that observed in normal tissue. Pit-1 transcripts, identical in size (2.4 and 4.5 kilobases) and sequence to those observed in normal tissue were evidenced in PRL-, GH-, and TSH-secreting tumors. Pit-1 is overexpressed (2.5- to 5-fold) in the PRL- and GH-secreting tumors, but to an extent consistent with the predominant cellular type of these adenomas. An isoform of Pit-1, with an insertion of 26 amino acids in the trans-activation domain as a result of alternative splicing, is also present in both normal and tumoral tissues. It is concluded that human pituitary tumorigenesis does not seem to be associated with a gross alteration of Pit-1 gene expression.

In vitro corticotropin-releasing hormone (CRH) stimulation of adrenocorticotropin release from corticotroph adenoma cells: effect of prolonged exposure to CRH and its interaction with cortisol.

To examine if down-regulation of CRH-induced ACTH release occurs in corticotroph adenoma cells as well as CRH-glucocorticoid interactions in these cells, we established primary cultures of pituitary adenoma cells obtained by transphenoidal surgery from five patients with Cushing's disease. To prevent binding of glucocorticoids by serum proteins, we used a serum-free medium containing insulin, transferrin, selenium, and epidermal growth factor. The latter was found to be essential for both basal and CRH-stimulated ACTH secretion. CRH acutely stimulated, in a dose-dependent manner, ACTH release by all adenomas studied, with an IC50 of 0.5 X 10(-9) mol/L. Prolonged exposure (10 days) to a half-maximal stimulatory concentration of CRH led to continuous stimulation of ACTH secretion. A 4-day incubation with cortisol induced a dose-dependent decrease in both basal and long term CRH-stimulated ACTH release, with no difference in the IC50 (1 X 10(-8) mol/L). These data suggest that long term exposure to CRH does not desensitize corticotroph adenoma cells. Thus, it is unlikely that long-acting analogs of CRH will be useful in the treatment of Cushing's disease. ACTH secretion from corticotroph adenomas is restrained by glucocorticoids; the sensitivity of these cells to the negative effect of glucocorticoids is not modified by long term stimulation with CRH.

Immunoradiometric analysis of circulating human glycosylated and nonglycosylated prolactin forms: spontaneous and stimulated secretions.

The monoclonal antibodies (MAbs) obtained in mice immunized with human PRL coupled to an anti-PRL MAb were screened for their ability to distinguish the glycosylated (G-) and nonglycosylated (NG-) forms of PRL. The 431-29 MAb exhibited high affinity binding for NG-PRL but little or no cross-reactivity to G-PRL. Using this antibody in conjunction with other MAbs which equally recognized both forms, we developed 2 immunoradiometric assays which were used to determine the amount of G- and NG-PRL in plasma. In 85 normal subjects, NG-PRL baseline levels averaged 6.6 +/- 3 micrograms/L, and represented 76 +/- 8% of the total PRL immunoreactivity. In 74 pregnant women, this proportion was significantly higher during the last 2 trimesters (84 +/- 4% and 85 +/- 6%), as compared to the first trimester (76 +/- 7%). In 6 healthy volunteers studied over 24 h, 79% of the NG-PRL peaks detected using the cluster algorithm occurred concomitantly to a G-PRL peak. The mean NG-PRL/PRL ratio was significantly higher during NG-PRL pulses (81 +/- 9%) than during valleys (71 +/- 12%). Similarly, this ratio was significantly increased during TRH or metoclopramide stimulated PRL secretion (to 88 +/- 7% and 86 +/- 6%, respectively). We conclude that 1) NG-PRL is the predominant immunoassayable form of PRL in plasma; 2) both G- and NG-PRL are cosecreted but NG-PRL is the main PRL form released during spontaneous or pharmacologically induced PRL secretion.

Somatostatin receptors on thyrotropin-secreting pituitary adenomas: comparison with the inhibitory effects of octreotide upon in vivo and in vitro hormonal secretions.

The in vivo and in vitro inhibitory effects of a somatostatin (SRIH) analog, octreotide, upon TSH, alpha-subunit, GH, and PRL have been studied, as well as SRIH receptors and their coupling to adenylate cyclase, in nine TSH-secreting pituitary adenomas. From in vivo and cell culture studies, the TSH- and alpha-subunit-secreting adenomas appeared heterogeneous, with four out of the nine tumors cosecreting GH and/or PRL. A single sc injection of octreotide (100 micrograms) lowered plasma concentration of TSH by 40 +/- 5% (mean +/- SE of 5), of alpha-subunit by 27 +/- 9% (n = 5), of GH by 60 +/- 5% (n = 4), and of PRL by 27 +/- 9% (n = 4). In cells cultures, octreotide (10(-8) mol/L) inhibited equally TSH, alpha-subunit, and GH release. 125I-Tyr0-DTrp8-SRIH binding sites were measurable in the nine TSH-secreting adenomas either on membrane preparations (n = 6; Bmax: 152 +/- 73 fmol/mg protein) or on frozen sections by radioautography (n = 3). Their density was variable among TSH adenomas and was lower than that measured in GH-secreting adenomas but higher than in nonfunctioning tumors. Two out of three TSH-secreting adenoma displayed an heterogeneous distribution of 125I-Tyr0-DTrp8-SRIH binding sites. 125I-Tyr0-DTrp8-SRIH specific binding was inhibited by guanosine triphosphate (GTP: 10(-4) mol/L). SRIH inhibited adenylate cyclase in 5/5 TSH-secreting adenomas and a good correlation (r = 0.92, P less than 0.02) was found between 125I-Tyr0-DTrp8-SRIH binding capacity (Bmax) and maximal adenylate cyclase inhibition by SRIH. These results demonstrate in vivo and in vitro inhibition of TSH, alpha-subunit, PRL, and GH secretion by octreotide in TSH-secreting pituitary adenomas. Functional SRIH receptors are present on these tumors and the effect of SRIH on hormonal secretion could be mediated, at least in part, by inhibition of adenylate cyclase. These findings support the medical treatment of this rare type of tumors by SRIH analogs.

Effects of the dopamine agonist CV 205-502 in human prolactinomas resistant to bromocriptine.

In 21 patients with prolactinomas resistant to bromocriptine, we studied the effects of CV 205-502 on PRL hypersecretion and tumor mass, as assessed by consecutive computed tomography examinations. Cell culture studies were performed in 9 of such tumors. In 11 patients (group I; 52%) with a mean baseline plasma PRL level of 468 +/- 160 micrograms/L (+/- SE), normal PRL values were achieved after 1-6 months of treatment with 0.1-0.5 mg/day CV 205-502. Tumor size was reduced by 25% or more in 6 of 11 patients. In group II (n = 10), PRL levels (948 +/- 538 micrograms/L at baseline) were reduced by 48% after treatment with 0.1 mg/day CV 205-502. A progressive increase in the daily dose up to 0.5 mg did not further improve the partial reduction of PRL. No reduction in tumor size was observed in this group. The cell culture studies showed that 1) a brief exposure to both drugs provoked PRL suppression lasting 3 days; 2) in group I, CV 205-502 suppressed PRL release more efficiently than bromocriptine, with a maximal inhibition of 72% at 10(-9) mol/L; and 3) in group II, CV 205-502 only achieved a 26% inhibition of PRL release at 10(-8) mol/L, superimposable to that of bromocriptine. These data indicate that in at least half of such adenomas resistant to bromocriptine, CV 205-502, probably due to its higher affinity toward the D2 dopamine receptor, can overcome such resistance.

A human pituitary adenoma secreting thyrotropin and prolactin: immunohistochemical, biochemical, and cell culture studies.

A 43-yr-old woman, who had previously had a subtotal thyroidectomy, presented with hyperthyroidism and amenorrhea-galactorrhea due to a pituitary adenoma secreting TSH, TSH-alpha, and PRL. Her serum T4 concentration was 14 micrograms/dl; T3, 5.7 ng/ml, and TSH, 19-33 microU/ml. Serum TSH was not altered by TRH stimulation or T3 suppression. Basal plasma PRL levels were 19-27 ng/ml and plasma PRL doubled after TRH stimulation. A 900-mg pituitary tumor, removed by transphenoidal surgery, was studied in cell culture. After dispersion, tumor cells were maintained on an extracellular matrix produced by bovine corneal endothelial cells in a defined serum-free medium. The hormones released in the culture medium were analyzed by high pressure gel chromatography. Three fractions of tumor TSH were found, with respective apparent mol wts of 45,000 (11%), 28,000 (70%), and 20,000 (19%). Tumoral PRL eluted as a single peak of apparent mol wt of 24,000. Pharmacological studies of TSH, TSH-alpha, and PRL release using thyroid hormones (T3), dopamine agonist (bromocriptine), TRH, and cholera toxin yielded the following results: 1) T3 after 3 days of incubation produced a dose-dependent inhibition of TSH, TSH-alpha, and PRL release. Maximal inhibition (81%) was obtained at 10(-9) M and half-maximal inhibition at 4-6 X 10(-11) M. 2) Bromocriptine produced rapid and partial inhibition of hormone release. Maximal inhibition (51%) was obtained at 10(-8) M and half-maximal inhibition at 5 X 10(-10) M. 3) TRH at 10(-8) M concentration significantly stimulated PRL release but it had no effect on TSH release. 4) Adenylate cyclase activation by 10(-11) M cholera toxin increased TSH (152%), TSH-alpha (150%), and PRL (220%). Immunohistochemical analysis of serial 2 micron sections of the tumor showed that: 1) TSH-alpha immunoreactive cells were the most numerous, 2) TSH-beta positive cells were always positive for TSH-alpha, 3) PRL immunoreactivity was found either uniquely in some cells and colocalized with TSH-alpha immunoreactivity in other cells. However, by electron microscopy, the tumor cells were thyrotrophs. These data indicate that in this patient's tumor: 1) cells secreting TSH were responsive in vitro to near physiological concentrations of thyroid hormones. 2) The colocalization of PRL and TSH-alpha immunoreactivities in some cells raises the possibility either of fusion of differentiated pituitary cells synthesizing distinct hormones or of transformation of less differentiated multipotential pituitary cells.

Impact of gsp oncogene on the expression of genes coding for Gsalpha, Pit-1, Gi2alpha, and somatostatin receptor 2 in human somatotroph adenomas: involvement in octreotide sensitivity.

The impact of the gsp oncogene on the expression of genes engaged in the somatotroph cell phenotype remains poorly understood in human somatotroph adenomas. As the gsp oncogene is associated with an increased octreotide (somatostatin agonist) sensitivity, a group of 8 somatotroph adenomas bearing the gsp mutation (gsp+) and another group of 16 adenomas without the mutation (gsp-) were analyzed, all of them presenting variable octreotide sensitivities. The expressions of genes encoding for G(s)alpha, Pit-1, G(i2)alpha, and SSTR2, involved in the regulation of secretory activity in somatotroph cells, were assessed by Northern blot. A decreased expression of the G(s)alpha gene was found in gsp + tumors, suggesting the existence of a negative feedback of the oncogenic protein upon its own messenger ribonucleic acid (mRNA). In contrast, G(i2)alpha, Pit-1, and GH messengers were not significantly different in the groups. A positive correlation between the in vitro and in vivo GH octreotide-induced secretory inhibition and the expression of SSTR2 mRNA was found. However, the expression of the gene for SSTR2 appeared not to be different between gsp + and gsp-, even when the octreotide sensitivity was significantly higher in the adenomas carrying the mutation. Interestingly, the SSTR2 gene expression was significantly correlated to those of G(i2)alpha and Pit-1. In the same way, the G(s)alpha mRNA expression was positively correlated with those of Gi2alpha and Pit-1. Such correlations strongly suggest a concerted dysregulation of the expression of these genes in both categories of adenomas. The loss of the octreotide sensitivity represents one aspect of the dysregulation process that partially results from the decreased SSTR2 expression. However, the improvement of the sensitivity associated with the presence of the gsp oncogene seems to proceed in a way different from SSTR2 expression.

Resistance to bromocriptine in prolactinomas.

Bromocriptine therapy normalizes PRL secretion in most, but not all, patients with prolactinomas. This study was undertaken to determine the mechanism(s) responsible for bromocriptine resistance in patients with a PRL-secreting macroadenomas (n = 5) or microadenomas (n = 3). Their mean basal plasma PRL value was 807 +/- 220 (+/- SE) micrograms/L before treatment, and their nadir mean value was 354 +/- 129 micrograms/L during chronic therapy with 15-30 mg bromocriptine daily; four of the eight patients had an increase in tumor size during therapy. In cultures of prolactinoma cells from patients normally responsive to bromocriptine therapy (n = 10), considered as controls, 10(-9) mol/L bromocriptine inhibited PRL release by 71 +/- 6% (+/- SE), and the half-inhibitory dose was 7 x 10(-11) mol/L. In contrast, in cultures of prolactinoma cells from five patients resistant to bromocriptine, PRL release was inhibited by only 3-42% at 10(-9) mol/L bromocriptine. This partial inhibition was reversed by a 100-fold excess of haloperidol. In contrast, the effects of other inhibitors of PRL release (10(-8) mol/L T3 and 10(-8) mol/L somatostatin) or of a stimulator (10(-8) mol/L angiotensin-II) on cells from resistant and normally responsive patients were similar. In cell membranes from five bromocriptine-responsive adenomas the density of dopaminergic binding sites, labeled by [3H] spiroperidol was 243 +/- 65 (+/- SE) fmol/mg protein. In adenomas from the eight patients resistant to bromocriptine therapy the density of [3H]spiroperidol-binding sites lower (145 +/- 31 fmol/mg protein). In adenomas from five resistant patients whose tumor had grown during therapy the density of binding sites was 25 +/- 3 fmol/mg protein, 10% of that in normally responsive patients. The effects of dopamine on adenylate cyclase activity also were different in the three groups of adenomas. Dopamine inhibited adenylate cyclase activity by 28.8 +/- 5.6% in five bromocriptine-responsive tumors and by 16.5 +/- 4.3% in adenomas from eight resistant patients. In contrast, in the five patients whose tumors grew during therapy dopamine paradoxically stimulated adenylate cyclase activity (+26.4 +/- 9.8%). There was a very good correlation between the density of dopaminergic binding sites and maximal inhibition of adenylate cyclase activity in bromocriptine-responsive prolactinoma patients (r = 0.90) and resistant patients who had no tumor growth during therapy (r = 0.94).(ABSTRACT TRUNCATED AT 400 WORDS)

Bim-23244, a somatostatin receptor subtype 2- and 5-selective analog with enhanced efficacy in suppressing growth hormone (GH) from octreotide-resistant human GH-secreting adenomas.

Although both somatostatin receptor subtype 2 (SSTR2) and SSTR5 messenger ribonucleic acid (mRNA) are consistently expressed in GH-secreting adenomas, SSTR2 has been believed to be the key modulator of somatostatin-mediated inhibition of GH release. The somatostatin agonists currently in clinical use, octreotide and lanreotide, are directed mainly to SSTR2 (IC(50) 12- to 18-fold higher than for SSTR5). Recently, however, it was demonstrated that an SSTR5 preferential agonist, BIM-23268, not only suppressed PRL release from prolactinomas and mixed GH-PRL adenomas, but also inhibited GH release in about half of GH adenomas. In addition, the SSTR5-preferring analog showed a slight additive effect when used in combination with SSTR2 preferential drugs at submaximal concentrations in octreotide partially sensitive adenomas. In the present study we quantified SSTR2 and SSTR5 mRNA expression and the GH-suppressive effects of somatostatin-14; octreotide; a SSTR2-preferential compound, BIM-23197; a SSTR5-preferential compound, BIM-23268; and a new SSTR2- and SSTR5-bispecific compound, BIM-23244, in GH-secreting tumors classified as either full responders to octreotide (n = 5) or partially sensitive to octreotide (n = 5). The octreotide-sensitive GH secretory adenomas presented with a high level of both SSTR2 and SSTR5 mRNA expression [222 +/- 61 and 327 +/- 136 pg/pg glyceraldehyde-3-phosphate dehydrogenase (GAPDH), respectively]. In these tumors the suppression of GH release was similarly achieved at picomolar ranges by octreotide, BIM-23197, and BIM-23244 (EC(50) = 25 +/- 15, 3 +/- 2, and 3 +/- 3 pmol/L, respectively). The compounds preferential for only SSTR5 were unable to inhibit GH release in such tumors. Among the octreotide partially responsive tumors, SSTR2 mRNA expression was 9-fold lower than in the octreotide-sensitive tumors (25 +/- 12 vs. 222 +/- 61 pg/pg GAPDH; P < 0.015), whereas SSTR5 mRNA expression was approximately 7-fold higher than in the octreotide-sensitive tumors (2271 +/- 1197 pg/pg GAPDH). In these octreotide partially responsive tumors, the SSTR5-preferential compound, BIM-23268, and the SSTR2- and SSTR5-bispecific compound, BIM-23244, were quite effective in suppressing GH secretion (EC(50) = 25 +/- 13 and 50 +/- 31 pmol/L, respectively). Similarly, BIM-23244, was able to suppress by 51 +/- 5% PRL release from five mixed GH- and PRL-secreting adenomas. These data indicate that due to heterogeneous expression of SSTR2 and SSTR5 receptor subtypes, in GH-secreting tumors, a bispecific analog, such as BIM-23244, that can activate both receptors could achieve better control of GH hypersecretion in a larger number of acromegalic patients.

Demonstration of enhanced potency of a chimeric somatostatin-dopamine molecule, BIM-23A387, in suppressing growth hormone and prolactin secretion from human pituitary somatotroph adenoma cells.

In acromegaly, the combination of somatostatin (SS) and dopamine (DA) agonists has been shown to enhance suppression of GH secretion. In the present study, a new chimeric molecule, BIM-23A387, which selectively binds to the SS subtype 2 receptor (sst(2); K(i) = 0.10 nM) and to the DA D2 receptor (D2DR; K(i) = 22.1 nM) was tested in cultures prepared from 11 human GH-secreting tumors for its ability to suppress GH and prolactin (PRL) secretion. The chimeric compound was compared with individual sst(2) and D2DR agonists of comparable activity at the individual receptors. All tumors expressed both sst(2) and D2DR mRNAs (0.8 +/- 0.2 and 4.7 +/- 0.7 copy/copy beta-glucuronidase mRNA, respectively). In cell cultures from seven octreotide-sensitive tumors, the maximal inhibition of GH release induced by the individual sst(2) and D2DR analogs and by BIM-23A387 was similar. However, the mean EC(50) for GH suppression by BIM-23A387 (0.2 pM) was 50 times lower than that of the individual sst(2) and D2DR analogs, either used individually or combined. Similar data were obtained in four tumors that were only partially responsive to octreotide. The inhibition of GH release by BIM-23A387 was only partially reversed by the D2R2 antagonist, sulpiride, or by the sst(2) antagonist, BIM-23454. Only when both antagonists were combined was the GH suppressive effect of BIM-23A387 totally reversed. Finally, BIM-23A387 produced a mean 73 +/- 6% inhibition of PRL in six mixed GH plus PRL tumors. These data demonstrate an enhanced potency of the chimeric molecule, BIM-23A387, in suppressing GH and PRL secretion from acromegalic tumors, which cannot be explained merely on the basis of binding affinity for SS and/or DA receptors.

Human somatostatin receptor subtypes in acromegaly: distinct patterns of messenger ribonucleic acid expression and hormone suppression identify different tumoral phenotypes.

Recently, studies using somatostatin (SRIF) analogs preferential for either the SRIF receptor 2 (SSTR2) or the SSTR5 subtype demonstrated a variable suppression of GH and PRL release from GH-secreting human adenomas. These data suggested the concept of SSTR subtype specificity in such tumors. In the present study the quantitative expression of messenger ribonucleic acid (mRNA) for the 5 SSTR subtypes and the inhibitory effects of SRIF14; SRIF28; octreotide; the SSTR2-preferential analog, BIM-23197; and the SSTR5-preferential analog, BIM-23268, on GH and PRL secretion were analyzed in cells cultured from 15 acromegalic tumors. RT-PCR analysis revealed a consistent pattern of SSTR2 and SSTR5 mRNA expression. SSTR5 mRNA was expressed at a higher level (1052 +/- 405 pg/pg glyceraldehyde-3-phosphate dehydrogenase) than SSTR2 mRNA (100 +/- 30 pg/pg glyceraldehyde-3-phosphate dehydrogenase). However, only SSTR2 mRNA expression correlated with the degree of GH inhibition induced by SRIF14, SRIF28, and BIM-23197. The SSTR5-preferential compound inhibited GH release in only 7 of 15 cases. In cells cultured from the 10 mixed adenomas that secreted both GH and PRL, RT-PCR analysis revealed a consistent coexpression of SSTR5, SSTR2, and SSTR1 mRNA. In all cases SRIF14, SRIF28, and the SSTR5-preferential analog, BIM-23268, significantly suppressed PRL secretion, with a mean maximal inhibition of 48 +/- 4%. In contrast, the SSTR2-preferential analogs, BIM-23197 and octreotide, were effective in suppressing PRL in only 6 of 10 cases. In cells cultured from adenomas taken from patients partially responsive to the SRIF analog, octreotide, partial additivity in suppressing both GH and PRL secretion was observed when the SSTR2- and SSTR5-preferring analogs, BIM-23197 and BIM-23268, were tested in combination. Our data show a highly variable ratio of the SSTR2 and SSTR5 transcripts, according to tumors. The SSTR2-preferring compound consistently inhibits GH release, whereas the SSTR5-preferring compound is the main inhibitor of PRL secretion. When both drugs are combined, the partial additivity observed in mixed GH- plus PRL-secreting adenomas may be of interest in the therapeutic approach of such tumors.

Pit-1 gene expression in human lactotroph and somatotroph pituitary adenomas is correlated to D2 receptor gene expression.

The expression of the pituitary-specific transcription factor Pit-1 gene was analyzed in a series of 30 human lactotroph and somatotroph pituitary tumors. Northern blot analysis failed to reveal any quantitative differences in Pit-1 gene expression between somatotroph and lactotroph tumors, and reverse transcription-PCR analysis showed similar patterns of Pit-1 isoforms expression in both populations of tumors. The expression of the D2 receptor gene was subsequently analyzed in the same adenomas. In the prolactinomas, which presented with a variable sensitivity to dopamine agonist treatment, the intensity of the D2 receptor transcripts (2.8 kilobases) was variable and was related to the sensitivity to the dopamine agonist treatment. Notably, the individual D2 receptor messenger ribonucleic acid (mRNA) levels were highly correlated to the Pit-1 mRNA levels measured in the same tumors (r = 0.90; P < 0.0001). In the GH-secreting tumors, a significant expression of the D2 receptor gene was evidenced by Northern blot in all mixed somato-lactotroph adenomas and in some of the pure somatotroph adenomas; again, a positive correlation was found between D2 mRNA and Pit-1 mRNA levels (r = 0.68; P < 0.01). These results suggest the existence of mechanisms responsible for a coordinate control of Pit-1 and D2 receptor genes that remain to be determined.

A new mutation of the gene encoding the transcription factor Pit-1 is responsible for combined pituitary hormone deficiency.

The pituitary-specific transcription factor Pit-1/GHF1 regulates the expression of PRL, GH, and TSH beta genes through binding to specific regions of the promoters of these genes. Mutations of the Pit-1 gene have been shown to be responsible for a syndrome of combined pituitary hormone deficiency (CPHD), including complete GH and PRL deficiencies and central hypothyroidism. We studied four siblings presenting with CPHD born to healthy consanguinous parents. All four affected children had complete GH deficiency diagnosed in early childhood. They later developed hypothyroidism and were found to have undetectable PRL levels. The pituitary gland was hypoplastic at magnetic resonance examination in one of the patients. Amplification of genomic DNA and subsequent sequencing of the six exons of the Pit-1 gene allowed identification in the four patients with CPHD of an as yet undescribed mutation in exon 3. A substitution of T go G induced a change from a Phe to a Cys residue at position 135 within the hydrophobic core of the POU-specific DNA-binding domain of the Pit-1 protein. All affected children were homozygous for the mutation, whereas the mother was heterozygous, suggesting a recessive mode of inheritance. Molecular studies in other affected families will allow instructive genotype-phenotype correlations concerning the Pit-1 gene.

Quantitative and functional expression of somatostatin receptor subtypes in human prolactinomas.

Recently, it was demonstrated that somatostatin analogs preferential for the SSTR5 subtype suppress PRL release from prolactinoma cell cultures by 30-40%. These data supported the idea of somatostatin receptor subtype-specific control of PRL secretion in such tumors. The present study examines the quantitative profile of SSTRs messenger ribonucleic acid (mRNA) in 10 PRL-secreting tumors and correlates the expression with the ability of native somatostatins (SS14 and SS28), SSTR2 preferential analogs (octreotide and BIM-23197), and the SSTR5 preferential analog BIM-23268 to suppress PRL secretion. RT-PCR quantitative analysis showed a large predominance of SSTR5 mRNA [5648 +/- 1918 pg/pg glyceraldehyde-3-phosphate dehydrogenase (GAPDH)] vs. SSTR2 mRNA (148 +/- 83 pg/pg GAPDH). The SSTR1 transcript was also highly expressed in prolactinomas (1296 +/- 669 pg/pg GAPDH). SSTR5 mRNA expression correlated with PRL inhibition induced by both SRIF14 and SRIF28. Among the different analogs tested, only BIM-23268 produced inhibition of PRL release similar to that achieved with the native peptides. Its EC50 for PRL suppression was 0.28 +/- 0.10 nmol/L. No additive effects on PRL suppression were achieved by cotreatment of the tumor cells with SSTR2 and SSTR5 preferential analogs. In the same tumor cell cultures, quinagolide, a potent dopamine agonist, produced a dose-dependent inhibition of PRL with an EC50 at least 10 times lower than that of BIM-23268. Coincubation of quinagolide and BIM-23268, particularly in tumor cells resistant to dopamine agonist treatment, did not produce additive effects on PRL suppression. In conclusion, prolactinomas have a specific pattern of SSTR subtype mRNA expression (SSTR5 and SSTR1). SSTR5 expression is correlated to PRL regulation. These inhibitory effects are superimposable, at a higher concentration, to those of the dopamine agonists, but are not additive, particularly in the adenomas resistant to dopaminergic suppression of PRL release.