The protein‐kinase‐C (PKC) family of iso‐enzymes regulates mitogenic signal transduction in colorectal‐cell lines. Its function in human colonic mucosal proliferation is controversial. Our study investigated the role of PKC with regard to proliferation and changes of PKC iso‐enzyme expression in colonic biopsies compared with small adenomas. In short‐term tissue‐culture experiments of colonic mucosal biopsies, we found reduced S‐phase labeling in the 2 apical compartments of longitudinally sectioned crypts when PKC was activated by 200 nM of the phorbol ester TPA (n = 8). Thus, PKC inhibited growth of differentiated colonocytes which may influence cell homeostasis in colonic crypts. Furthermore, we have determined the expression of PKCα, ‐β1, ‐β2, ‐δ and ‐ϵ in colonic adenomas smaller than 1 cm in diameter of 18 patients and found a significant increase of PKCα in the cytosolic fraction and decreased membrane levels of PKCβ2 in adenomas compared to normal, neighboring mucosa while protein levels of PKCβ1, ‐δ and ‐ϵ were not altered. Moreover PKCδ but not PKCα mRNA expression was significantly lowered in adenoma tissue in 7 patients, as determined by ribonuclease‐protection analysis. Changes in the regulation patterns of PKC isoforms suggest a decreased activation state of PKC even in small adenomas. This is compatible with an anti‐proliferative function of PKC serving to protect mucosa from expanding mutated cells. Int. J. Cancer 80:47–53, 1999. © 1999 Wiley‐Liss, Inc.

A point mutation of protein kinase Calpha (PKCalpha) has been described in pituitary adenomas and in follicular adenomas and thyroid carcinomas. The mutation results in an exchange of aspartic acid into a glycine of the amino acid 294 of PKCalpha, which is located adjacent to the Ca (2+)-binding hinge region and has been proposed as an activation inhibitor. To investigate its biochemical sequelae, we constructed the mutated enzyme and expressed it in human embryonic kidney cells (HEK). The K M of the purified enzyme for Ca (2+) and its K M for the substrate MBP 4 - 14 was not altered by the mutation. Translocation of PKCalpha to HEK cell membranes upon activation was not changed and the mutant potently inhibited cell proliferation upon 5-fold stable overexpression in HEK cells. Thus, loss of function in mutated PKCalpha was excluded. A screen for the mutation using a restriction assay with a sensitivity of at least 8 % for the mutated DNA did not show any mutation in 11 carcinoma and 13 adenomatous thyroid samples. We conclude that the A294G mutation of PKCalpha does not detectably affect its biochemical properties in vitro or in vivo, and is at least rare in thyroid neoplasias, in Germany.