Integrative Mitogenic Role of Protein Kinase B/Akt in β‐Cells

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Abstract
Protein kinase B/Akt (PKB/Akt) is activated by phosphatidylinositol 3‐kinase (PI 3‐K) and is a central mediator of cellular proliferation and protection against apoptosis. Insulin, insulin‐like growth factor (IGF‐1), and glucagon‐like peptide‐1 (GLP‐1) act as glucose‐dependent growth factors for pancreatic β‐cells. We assessed signaling pathways and stimulation patterns of PKB/Akt activation by these ligands in the β‐cell line INS‐1. Insulin, IGF‐1, and GLP‐1 induced distinctive time dependent, dose dependent, and glucose dependent phosphorylation of PKB/Akt. Insulin and IGF‐1 stimulated PI 3‐K activity was mainly associated with insulin receptor substrate (IRS) isoforms IRS‐1 and IRS‐2 and less so with the IRS‐isoform Grb‐2 associated binder‐1 (Gab‐1). In contrast, GLP‐1 induced PI 3‐K activity mainly in Gab‐1 and also in IRS‐2 immunoprecipitates, although in an attenuated kinetic. Thus, activation pathways of PKB/Akt by insulin, IGF‐1, and GLP‐1 converge at the level of IRS‐isoforms and PI 3‐K inducing differential activation of PKB/Akt. These data indicate an essential role of PKB/Akt in regulation of β‐cell proliferation.