The export of major histocompatibility complex class I molecules from the endoplasmic reticulum of rat brown adipose cells is acutely stimulated by insulin

Major histocompatibility complex class I (MHC-I) molecules have been implic ated in several nonimmunological functions including the regulation and int racellular trafficking of the insulin-responsive glucose transporter GLUT4. We have used confocal microscopy to compare the effects of insulin on the intracellular trafficking of MHC-I and GLUT4 in freshly isolated rat brown adipose cells. We also used a recombinant vaccinia virus (rVV) to express i nfluenza virus hemagglutinin (HA) as a generic integral membrane glycoprote in to distinguish global versus specific enhancement of protein export from the endoplasmic reticulum (ER) in response to insulin. In the absence of i nsulin, MHC-I molecules largely colocalize with the ER-resident protein cal nexin and remain distinct from intracellular pools of GLUT4. Surprisingly, insulin induces the rapid export of MHC-I molecules from the ER with a conc omitant approximately three-fold increase in their level on the cell surfac e. This ER export is blocked by brefeldin A and wortmannin but is unaffecte d by cytochalasin D, indicating that insulin stimulates the rapid transport of MHC-I molecules from the ER to the plasma membrane via the Golgi comple x in a phosphatidylinositol 3-kinase-dependent and actin-independent manner . We further show that the effect of insulin on MHC-I molecules is selectiv e, because insulin does not affect the intracellular distribution or cell-s urface localization of rVV-expressed HA. These results demonstrate that in rat brown adipose cells MHC-I molecule export from the ER is stimulated by insulin and provide the first evidence that the trafficking of MHC-I molecu les is acutely regulated by a hormone.