Protein kinase C regulates MHC-class II expression on endothelial cells

Cultured endothelial cells (EC) were negative for class II antigen in native state, whereas 49% of the endothelial cells began to express this antigen after 24 h of interferon-gamma (IFN-gamma) stimulation. IFN-gamma induced relatively slowly the elevation of class II antigen on endothelial cells, since it took more than 10 h before the first signs of mRNA signal of class II were detected. Class II antigen instead began to appear during 16-20 h after the initiation of IFN-gamma treatment. Committed step analysis revealed that IFN-gamma could not be washed away at any time point without affecting the number of class II positive cells after a 24-h incubation period. Protein kinase C (PKC) activator phorbol-12-myristate-13-acetate (PMA) could partially mimic IFN-gamma effect in inducing class II expression on endothelial cells. PMA together with another PKC activator arachidonic acid (AA) induced class II expression on endothelial cells as well as IFN-gamma. The crucial role of activation of PKC in the IFN-gamma induced class II expression can also be demonstrated by using PKC inhibitors in combination with IFN-gamma. PKC inhibitor H7 was able to decrease almost totally IFN-gamma induced class II induction both on the mRNA as well as on the protein level. PKC activation has often been linked to its translocation from the cytosolic compartment to the inner surface of the plasma membrane. IFN-gamma induced a transient 2.4-fold increase in the membrane-associated PKC in endothelial cells within 10 min after the initiation of the stimulus. Taken together these data show that IFN-gamma requires a long time before class II induction. The regulation of class II expression occurs at transcriptional level and requires de novo protein synthesis as shown by cycloheximide inhibition.