5 OF 6 PROTEIN-KINASE-C ISOENZYMES PRESENT IN NORMAL MUCOSA SHOW REDUCED PROTEIN-LEVELS DURING TUMOR-DEVELOPMENT IN THE HUMAN COLON

Protein kinase C (PKC) isoenzyme patterns were analyzed from human col onic epithelial cells of normal, premalignant and malignant origin. PK Cs alpha, beta and zeta were found predominantly in the cytosol and th e subtypes delta, epsilon and eta almost exclusively in the particulat e fraction. Of the isoenzymes found beta, epsilon and eta were low in abundance and could only be detected after partial purification of cel lular fractions on DE52-cellulose. Only PKC beta was similar in abunda nce in normal mucosa, premalignant and malignant colonic epithelial ce lls, while all other isoenzymes were decreased in abundance in tumor c ells. The loss of PKC protein in tumor cells correlated with a loss in enzyme activity, as has been described before by other groups, especi ally affecting the Ca2+-dependent isoenzymes. On the other hand, activ ation of PKC by phorbol ester treatment in vivo was only possible in c arcinoma cells (4/4) and a subset of adenomas (3/7). Normal human colo nic epithelial cells did not respond to TPA treatment with either stim ulation of PKC activity or translocation of cytosolic enzymes to the p articulate fraction. Instead, TPA treatment resulted in a rapid loss o f protein for the isoenzymes alpha, delta and to a lesser degree also beta. We assume that this reflects qualitative differences in response between normal and tumor cells, that may be due to the differences in isoenzyme distribution.