Biologically active lipids promote trafficking and membrane association of Rac1 in insulin-secreting INS 832/13 cells

Abstract

Despite emerging evidence to suggest that glucose-stimulated insulin secretion (GSIS) requires membrane targeting of specific small G proteins (e.g., Rac1), very little is known with regard to the precise mechanisms underlying subcellular trafficking of these proteins in the glucose-stimulated islet β-cell. We previously reported activation of small G proteins by biologically active lipids via potentiation of relevant GDP/GTP exchange activities within the β-cell. Herein, we studied putative regulatory roles for these lipids in the trafficking and membrane association of Rac1 in cell-free preparations derived from INS 832/13 β-cells. Incubation of INS 832/13 cell lysates with polyphosphoinositides (e.g., PIP2), phosphatidic acid, phosphatidylcholine, and phosphatidylserine significantly promoted trafficking of cytosolic Rac1 to the membrane fraction. Lysophosphatidic acid, but not lysophosphatidylcholine or lysophosphatidylserine, also promoted translocation and membrane association of Rac1. Arachidonic acid, diacylglycerol, calcium, and cAMP failed to exert any clear effects on Rac1 translocation to the membrane. Together, our findings provide the first direct evidence in support of our recent hypothesis (Kowluru A, Veluthakal R. Diabetes 54: 3523–3529, 2005), which states that generation of biologically active lipids, known to occur in the glucose-stimulated β-cell, may mediate targeting of Rac1 to the membrane for optimal interaction with its putative effector proteins leading to GSIS.