IB1 reduces cytokine-induced apoptosis of insulin-secreting cells

IB1/JIP-1 is a scaffold protein that interacts with upstream components of the c-Jun N-terminal kinase (JNK) signaling pathway. IB1 is expressed at hi gh levels in pancreatic beta cells and may therefore exert a tight control on signaling events mediated by JNK in these cells. Activation of JNK by in terleukin 1 (IL-1 beta) or by the upstream JNK constitutive activator Delta MEKK1 promoted apoptosis in two pancreatic beta cell Lines and decreased I B1. content by 50-60%. To study the functional consequences of the reduced IB1 content in beta cell lines, we used an insulin-secreting cell line expr essing an inducible IB1 antisense RNA that lead to a 38% IB1 decrease. Redu cing IB1 levels in these cells increased phosphorylation of c-Jun and incre ased the apoptotic rate in presence of IL-1 beta. Nitric oxide production w as not stimulated by expression of the IB1 antisense RNA. Complementary exp eriments indicated that overexpression of IB1 in insulin-producing cells pr evented JNK-mediated activation of the transcription factors c-Jun, ATF2, a nd Elk1 and decreased IL-1 beta- and Delta MEKK1-induced apoptosis. These d ata indicate that IB1 plays an anti-apoptotic function in insulin-producing cells probably by controlling the activity of the JNK signaling pathway.