Phorbol-12-myristate-13-acetate (PMA) inhibited growth of
human mammary carcinoma cell lines and increased mainly the phosphorylation of two cytosolic
phosphoproteins (pp) of 27 kD with isoelectric points of 5.5 (pp27a) and 5.0 (pp27b). The time course of pp27 phosphorylation closely paralleled the rapid PMA-induced subcellular redistribution of
protein kinase C (PKC) activity and its subsequent down regulation. Addition of
phospholipase C and
fetal calf serum to intact cells or purified PKC to a cell free system enhanced the phosphorylation of both pp27 suggesting that the two
polypeptides are specific substrates for PKC. Exposure of
human mammary carcinoma cells to stress inducers such as
arsenite or
cadmium increased the 32P incorporation of both pp27 to an extent comparable to PMA. The increased
phosphorus content following stress was rather due to a higher rate of synthesis of both pp27 than to a higher phosphorylation state of these
polypeptides as determined by [3H]-
leucine labeling. These results indicate that the major substrates of PKC, phosphorylated during the PMA-induced growth inhibition of
human mammary carcinoma cells, are members of the
stress protein family, suggesting a new possible function for these
proteins.