Characterization of the intracellular mechanisms mediating somatostatin and lanreotide inhibition of DNA synthesis and growth hormone release from dispersed human GH-secreting pituitary adenoma cells in vitro.
OBJECTIVE
Somatostatin is an endogenous inhibitor of hormone secretion and cell proliferation. Treatment with somatostatin analogues in humans causes a reduction in size and secretory activity of endocrine tumours, including GH-secreting pituitary adenomas. This study was aimed to characterize the intracellular mechanisms mediating the in vitro antiproliferative and antisecretory effects of somatostatin and its analogue lanreotide, on primary cultures of GH-secreting pituitary adenoma cells.


METHODS
Thirteen GH-secreting pituitary adenoma postsurgical specimens were analysed for somatostatin receptor (SSTR) mRNA expression and a subset of them was analysed in vitro for the effect of somatostatin on cell proliferation, assessed by means of [3H]-thymidine uptake, and GH release, using an immunoradiometric assay. Moreover, the intracellular signalling involved in such effects has been studied.


RESULTS
All the adenomas analysed expressed at least one somatostatin receptor subtype mRNA. SSTR2 mRNA was identified in 77% of the adenomas, SSTR1 and SSTR3 in 69% and SSTR5 in 60%. Somatostatin and lanreotide inhibited cell proliferation in phorbol ester (PMA)-stimulated conditions (10/13 adenomas), as well as after fetal calf serum (3/3 adenomas) or IGF-I stimulation (2/2 adenomas). Conversely, GHRH or forskolin treatments did not significantly affect DNA synthesis in adenoma cells in the presence or absence of somatostatin (2/2 and 4/4 adenomas, respectively). Vanadate pretreatment reversed somatostatin inhibition of PMA-induced DNA synthesis suggesting an involvement of tyrosine phosphatase in this effect (2/2 adenomas); this was confirmed by the direct induction of tyrosine phosphatase activity in two adenomas after somatostatin treatment. Somatostatin and also lanreotide caused significant inhibition of phorbol ester, forskolin, GHRH and KCl-dependent increase of GH secretion in the culture medium. Moreover, voltage-sensitive calcium channel activity induced by 40 mm KCl depolarization in microfluorimetric analysis, was significantly reduced (5/5 adenomas).


CONCLUSIONS
These data show that somatostatin and lanreotide inhibit human GH-secreting pituitary adenoma cell proliferation and hormone release in vitro, and suggest that the activation of tyrosine phosphatases may represent intracellular signals mediating the antiproliferative effects and that the inhibition of the voltage-dependent calcium channels and adenylyl cyclase activities may control GH secretion.
Florio
Tullio
T
Pharmacology and Neuroscience, National Institute for Cancer Research, c/o Advanced Biotechnology Center (CBA), Genova, Italy. florio@cba.unige.it
Thellung
Stefano
S
Corsaro
Alessandro
A
Bocca
Liliana
L
Arena
Sara
S
Pattarozzi
Alessandra
A
Villa
Valentina
V
Massa
Alessandro
A
Diana
Fabrizio
F
Schettini
Daria
D
Barbieri
Federica
F
Ravetti
Jean Louis
JL
Spaziante
Renato
R
Giusti
Massimo
M
Schettini
Gennaro
G
D000236:Adenoma / Q000378:metabolism*; D000262:Adenylyl Cyclases / Q000378:metabolism; D000328:Adult; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000494:pharmacology*; D015220:Calcium Channels / Q000378:metabolism; D002455:Cell Division; D005576:Colforsin / Q000494:pharmacology; D004247:DNA / Q000096:biosynthesis; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D013006:Growth Hormone / Q000378:metabolism* / Q000494:pharmacology; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology; D006801:Humans; D008297:Male; D008875:Middle Aged; D010456:Peptides, Cyclic / Q000494:pharmacology*; D010911:Pituitary Neoplasms / Q000378:metabolism*; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis; D017481:Receptors, Somatostatin / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D013004:Somatostatin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D000894:Anti-Inflammatory Agents, Non-Steroidal; D015220:Calcium Channels; D004791:Enzyme Inhibitors; D010456:Peptides, Cyclic; D012333:RNA, Messenger; D017481:Receptors, Somatostatin; C060347:lanreotide; D005576:Colforsin; D013004:Somatostatin; D013006:Growth Hormone; D004247:DNA; D013007:Growth Hormone-Releasing Hormone; D017027:Protein Tyrosine Phosphatases; D000262:Adenylyl Cyclases