In vitro testing of new somatostatin analogs on pituitary tumor cells

Abstract

It has been indeed demonstrated that native SRIF inhibits secretion and proliferation of both normal and neoplasic pituitary cells by inducing several intracellular pathways, depending on receptor subtype and target tissue (Ferjoux et al. 2000). Therefore, many studies subsequently explored SSTR subtype expression in pituitary adenomas, demonstrating that SSTR2 is the most frequently expressed subtype (Panetta and Patel, 1995 A c c e p t e d M a n u s c r i p t 3 SSTR 2 and 5 mRNA were always expressed, while SSTR1 and 3 mRNA were expressed in 11 and 12 tumors, respectively. Primary cultures deriving from these tumors were tested with 1 M SRIF or lanreotide (Lan), a SRIF analog preferentially binding with high affinity SSTR2 and less affinity SSTR5, or with SSTR2-and SSTR5-selective analogs. After 24 hours no effect was observed in some cultures, but in others (as well as in the GH3 rat cell line) a parallel reduction in cell viability and DNA synthesis was observed. This study also highlighted a dissociation between the in vitro effects of SRIF analogs on tumor cell proliferation and on GH secretion. In nearly 60% of the tumors, treatment with SRIF, Lan, SSTR2-or SSTR5-selective agonists significantly inhibited GH secretion.by a median of 33%. However, not all these primary cultures responded to SRIF and its analogs in terms of cell viability reduction, suggesting that inhibition of cell proliferation occurs independently of the effects on GH secretion (Danila et al. 2001). Similarly, an important tumor shrinkage has been described in an acromegalic patient during primary therapy with SRIF analogs, in the absence of hormonal normalization . In this case, the lack of antisecretory action of octreotide, a SRIF analog preferentially binding with high affinity SSTR2 and less affinity SSTR5, was in keeping with the absence of SSTR2 and SSTR5 expression by the tumor, which expressed almost exclusively SSTR3. The latter SSTR subtype has been indeed demonstrated to transduce important antiproliferative signals, since its activation causes apoptosis in heterlogous expression systems (Sharma et al. 1996). Further support to the hypothesis that antiproliferative effects are independent of antisecretory actions of SRIF analogs is provided by a recent report, describing the case of an acromegalic patient displaying dramatic tumor shrinkage during primary therapy with octreotide, without hormonal normalization. Again, in vitro SRIF and its analogs at 10 nM induced a scant antisecretory effect, but potently inhibited thymidine incorporation. This effect was evident for compounds interacting with SSTR5, in keeping with the finding that SSTR5 expression levels were greater than SSTR2 in the tissue (Resmini et al. 2007). In light of the recent discoveries concerning SSTR biology, we attempted to clarify the possible functional interaction between SSTR and dopamine (DA) receptor subtype 2 (DR2). To this aim, we investigated DR2 and SSTR2 and 5 expression in 25 GH-secreting pituitary adenomas, which were tested in primary culture...

Somatostatin Dramatically Stimulates Growth Hormone Release from Primate Somatotrophs Acting at Low Doses Via Somatostatin Receptor 5 and Cyclic AMP

“…The fact that sst2 agonists, and not high concentration SST, could block basal GH release may be related to the fact that SST binds to all sst subtypes, where each is known to differentially couple to G‐proteins (5) and upon ligand binding can form homo‐ or heterodimers that can in turn modify their association with intracellular signals (28,29). In support of the results obtained in the present study, studies using SST receptor specific agonists/antagonists in GH‐producing cell lines, primary pituitary cultures (pigs, chickens, rats) and/or human pituitary GH‐producing adenomas cultures have revealed that only sst2 consistently confers the inhibitory actions of SST on GH release, whereas activation of the other SST receptor subtypes is variable depending on the concentration of agonist supplied and the model tested (1,10,12,14,30–32). It has also been shown, using nonprimate models, that sst2‐mediated inhibition of GH release requires intact AC activity and is associated with inhibition of cAMP accumulation and voltage‐gated Ca 2+ channels, and/or stimulation of membrane K + channels (1,7,10,12,14).…”

Molecular Characterization of Growth Hormone-producing Tumors in the GC Rat Model of Acromegaly

“…Octreotide is a clinically used somatostatin analogue that binds primarily to SSTR2 and to a lesser extent to SSTR5, BIM-23120 exhibits a very specific affinity to SSTR2, whereas SOM-230 is a multi-somatostatin receptor ligand with high affinity for SSTR1, SSTR2, SSTR3 and SSTR5 42 53 54 . These somatostatin analogues have been previously shown to inhibit GH secretion in primary cultures of human GH-secreting adenomas (for reviews, see 55 56 ). In keeping with these observations, all somatostatin analogues cause a marked decrease in GH secretion in GC tumor cells.…”

Hypersecretion of the α-subunit in clinically non-functioning pituitary adenomas: Diagnostic accuracy is improved by adding α-subunit/gonadotropin ratio to levels of α-subunit

“…27,28 The α-subunit measurements have been used to study the effects of experimental therapy; i.e., somatostatin analogues, dopamine-D2-agonists, the combination of somatostatin analogues and dopamine-D2-agonists. 26 Future studies will elucidate the effects of selective somatostatin analogues 29,30 and BIM 31,32 on α-subunit, gonadotropins, and tumour size. Temozolomide therapy for resistant pituitary adenomas including NFPAs 12 is a new experimental therapy, where monitoring of αsubunit and gonadotropins may be of value.…”