Phorbol ester effect in platelets, lymphocytes, and leukemic cells (HL-60) is associated with enhanced phosphorylation of class I HLA antigens. Coprecipitation of myosin light chain.

Phorbol-12-myristate-13-acetate (PMA) is a potent tumor promoter. However, the mechanism of its effect is still unknown. In the present study we use two dimensional gel electrophoresis to show that the PMA effect on platelets is associated with enhanced phosphorylation of a series of polypeptides at 44K which migrate close to but distinct from a previously reported 47K protein. We identified these proteins as the class I molecules of the human histocompatibility antigens (HLA A,B). We further demonstrate that the PMA effect is also associated with a dramatic phosphorylation of HLA antigens in HL-60 leukemic cells and in human lymphocytes, showing that an increase in phosphorylation of HLA antigens is intimately related to the signal of PMA in various cellular systems. Immunoprecipitation of HLA proteins resulted in coprecipitation of phosphorylated myosin light chain (20K). HLA antigens are transmembrane proteins which interact with cytoskeletal elements, probably via their intracellular region, which has been previously shown to be phosphorylated. It is suggested that phosphorylation of HLA membrane proteins may represent an important mechanism in the effects induced by PMA.