Abstract

OBJECTIVE Phosphatidylinositol 3-OH kinase (PI3K) has a long-recognized role in β-cell mass regulation and gene transcription and is implicated in the modulation of insulin secretion. The role of nontyrosine kinase receptor–activated PI3K isoforms is largely unexplored. We therefore investigated the role of the G-protein–coupled PI3Kγ and its catalytic subunit p110γ in the regulation of insulin granule recruitment and exocytosis.

RESEARCH DESIGN AND METHODS The expression of p110γ was knocked down by small-interfering RNA, and p110γ activity was selectively inhibited with AS605240 (40 nmol/l). Exocytosis and granule recruitment was monitored by islet perifusion, whole-cell capacitance, total internal reflection fluorescence microscopy, and electron microscopy in INS-1 and human β-cells. Cortical F-actin was examined in INS-1 cells and human islets and in mouse β-cells lacking the phosphatase and tensin homolog (PTEN).

RESULTS Knockdown or inhibition of p110γ markedly blunted depolarization-induced insulin secretion and exocytosis and ablated the exocytotic response to direct Ca²⁺ infusion. This resulted from reduced granule localization to the plasma membrane and was associated with increased cortical F-actin. Inhibition of p110γ had no effect on F-actin in β-cells lacking PTEN. Finally, the effect of p110γ inhibition on granule localization and exocytosis could be rapidly reversed by agents that promote actin depolymerization.

CONCLUSIONS The G-protein–coupled PI3Kγ is an important determinant of secretory granule trafficking to the plasma membrane, at least in part through the negative regulation of cortical F-actin. Thus, p110γ activity plays an important role in maintaining a membrane-docked, readily releasable pool of secretory granules in insulinoma and human β-cells.