Insulin-like growth factor-1 increases intracellular calcium concentration in human primary neuroendocrine pancreatic tumor cells and a pancreatic neuroendocrine tumor cell line (BON-1) via R-type Ca2+ channels and regulates chromogranin a secretion in BON-1 cells

Abstract

Insulin-like growth factor 1 (IGF-1) is a potent mitogenic and secretory factor that acts on voltage operated Ca(2+) channels (VOCCs). VOCCs are categorized into L-type channels (Ca(V)1.1-1.4), P/Q-type channels (Ca(V)2.1), N-type channels (Ca(V)2.2), R-type channels (Ca(V)2.3), and T-type channels (Ca(V)3.1-3.3). Aside from regulating membrane excitability, VOCCs influence chromogranin A (CgA) secretion in neuroendocrine tumor (NET) cells. It is not known, whether VOCCs play a role in the IGF-1-dependent regulation of CgA secretion in NET cells. We therefore studied the effects of IGF-1 on individual VOCC subtypes and characterized their role in mediating IGF-1-dependent regulation of CgA secretion in NET cells. Using specific modulators of VOCC subtypes, we identified the functional expression of L-, N-, P/Q- and R-type channels in primary as well as permanent models of NET. The IGF-1-induced intracellular Ca(2+) increase in NET cells was mainly due to the activation of R-type channel activity. The effects on intracellular calcium, observed in whole-cell patch-clamp recordings and fluorescence imaging, were partially blocked by the specific R-type channel blocker SNX-482 and antisense oligonucleotides against the alpha(1) subunit of this channel. IGF-1 potently induced CgA secretion. The effect of IGF-1 was reduced by both, inhibition of R-type channel activity and a reduction of R-type channel expression using antisense oligonucleotides. Since R-type channels exist in NET cells and couple to both, IGF-1 receptor signaling as well as CgA secretion, pharmacological interference with R-type channels may represent a new therapeutic option by blocking Ca(2+) signaling thereby abrogating IGF-1-dependent hypersecretion in NET disease.