Two compartments for insulin-stimulated exocytosis in 3T3-L1 adipocytes defined by endogenous ACRP30 and GLUT4

Insulin stimulates adipose cells both to secrete proteins and to translocat e the GLUT4 glucose transporter from an intracellular compartment to the pl asma membrane. We demonstrate that whereas insulin stimulation of 3T3-L1 ad ipocytes has no effect on secretion of the alpha 3 chain of type VI collage n, secretion of the protein hormone adipocyte complement related protein of 30 kD (ACRP30) is markedly enhanced, Like GLUT4, regulated exocytosis of A CRP30 appears to require phosphatidylinositol-3-kinase activity, since insu lin-stimulated ACRP30 secretion is blocked by pharmacologic inhibitors of t his enzyme. Thus, 3T3-L1 adipocytes possess a regulated secretory compartme nt containing ACRP30. Whether GLUT4 recycles to such a compartment has been controversial. We present deconvolution immunofluorescence microscopy data demonstrating that the subcellular distributions of ACRP30 and GLUT4 are d istinct and nonoverlapping; in contrast, those of GLUT4 and the transferrin receptor overlap. Together with supporting evidence that GLUT4 does not re cycle to a secretory compartment via the trans-Golgi network, we conclude t hat there are at least two compartments that undergo insulin-stimulated exo cytosis in 3T3-L1 adipocytes: one for ACRP30 secretion and one for GLUT4 tr anslocation.