Although some evidence supports the antitumoral effects of
somatostatin (SRIF) and related agonists, the available data in
prostate cancer (PCa) model systems and clinical studies are few, conflicting and not conclusive. This study investigated the effects of
lanreotide and new mono- and bi-specific SRIF agonists on proliferation,
ligand-driven SRIF receptor (sst) dimerization and secretory pattern of the IGF system in LNCaP cells, a model of
androgen-dependent PCa. LNCaP expressed all sst(s), but sst(4). Among them, sst(1) and sst(3) were inversely regulated by serum concentration. sst(1)/sst(2) and sst(2)/sst(5) dimers were constitutively present and further stabilized by treatment with BIM-23704 (sst(1)/sst(2)) and
BIM-23244 (sst(2)/sst(5)), respectively. Dose-response studies showed that
lanreotide and
BIM-23244 were significantly more potent in inhibiting LNCaP cell proliferation than
BIM-23120 (sst(2)) and
BIM-23206 (sst(5)) alone or in combination. Treatment with
BIM-23926 [corrected] (sst(1)) markedly reduced cell proliferation, whereas exposure to BIM-23704 resulted in a lower cell growth inhibition. The antiproliferative effects of
BIM-23244,
lanreotide and BIM-23704 were unchanged, reduced and abolished by the sst(2) antagonist
BIM-23627, respectively. All SRIF analogs caused a significant induction in p27(KipI) and p21 and down-regulation of
protein expression of
cyclin E, as well as reduced
IGF-I and
IGF-II secretion. In particular, the administration of exogenous
IGF-I, at variance to
IGF-II, counteracted the inhibitory effect on cell proliferation of these compounds. Moreover, SRIF agonists reduced endogenous
IGFBP-3 proteolysis. These results show that, in LNCaP cells, activation of sst(1) and sst(2)/sst(5) results in relevant antiproliferative/antisecretive actions.