Differential Signaling by Regulatory Subunits of Phosphoinositide-3-kinase Influences Cell Survival in INS-1E Insulinoma Cells

Class 1A phosphoinositide 3- kinase ( PI3K) is essential for beta- cell growth and survival. Although PI3K has been studied extensively in diabetes the effect of alternatively spliced isoforms of the catalytic subunit p85a on beta cell proliferation and survival remains to be defined. We examined expression and signaling of alternatively spliced PI3K regulatory subunits p85a, p55a and p50a in insulinoma cells ( INS- 1E), an insulin- producing beta cell line. PI3K regulatory isoforms were knocked down by siRNA transfection or overexpressed by adenoviral gene delivery. Knockdown of p85a elevated PI3K activation determined by Akt phosphorylation at baseline and after stimulation with growth factors. In contrast, Akt phosphorylation was inhibited by overexpression of all isoforms of p85a. Correspondingly, p55a and p85a overexpression decreased downstream kinase GSK- 3 phosphorylation as well, whereas p50a overexpression resulted in an activation of GSK- 3. Moreover, overexpression of p50a and p85a lead to retinoblastoma protein hyperphosphorylation and S- phase entry. Upon challenge of INS- 1E cells with a cytotoxic cytokine cocktail, levels of p85a were reduced and p50a was upregulated. Selective overexpression of p50a prevented cytokine induced apoptosis in INS- 1E cells. In conclusion, signalling of p50a, p55a and p85a is similar at the level of Akt, but differentially influence downstream GSK- 3 activation and cell cycle entry. PI3K isoform p50a induction by cytokines provides a link between regeneration and cell survival under cytotoxic stress in insulin- producing pancreatic beta- cells.