Protein kinase C (PK-C) seems to be involved in the regulation of growth and differentiation of normal epithelial cells. Colonic adenomas and carcinomas show increased proliferation and decreased differentiation. We investigated the activity and subcellular distribution of PK-C in biopsies of normal, neoplastic, and malignant colonic epithelium to evaluate alterations in enzyme activity. In the control group (n = 7), the activity of PK-C was highest in the distal ileum (597 pmol/min/mg protein) and declined to the lowest amounts in rectal mucosa (225 pmol/min/mg protein).

In patients with colonic adenomas (n = 16), total PK-C activity was significantly reduced as compared to adjacent mucosa (146 versus 336 pmol/min/mg protein, P < 0.05) and to values determined in the control group (372 pmol/min/mg protein, P < 0.01). The reduction of total PK-C activity in the adenoma group was even more evident in intraindividual comparison to paired adjacent mucosa (41.8% of adjacent mucosa, P < 0.001). Specific activity of membrane-associated PK-C was equally decreased in colonic adenomas (36.3 pmol/min/mg protein) when compared to adjacent mucosa (102 pmol/min/mg protein, P < 0.05) or to the control group (107 pmol/min/mg protein).

In patients with colonic carcinomas (n = 10), the amount of total PK-C activity was also decreased (198 pmol/min/mg protein) when compared to adjacent mucosa or to the control group (P < 0.05). In addition, the amount of membrane-associated PK-C activity (89.1 pmol/min/mg protein) was significantly reduced in carcinomas when compared to adjacent mucosa (P < 0.05).

The ratio of membrane-associated/total PK-C was not altered in adenomas, while in patients bearing carcinomas the relative fraction of membrane-associated PK-C activity was increased in samples from carcinomas and equally from adjacent colonic mucosa (45.0 and 44.6 versus 28.9%, P < 0.05) when compared to controls.

These results indicate that alterations within the protein kinase C pathway occur as early events in the adenoma-carcinoma sequence of intestinal mucosa, suggesting an important role of PK-C in epithelial differentiation and growth.

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This study was supported by the Wilhelm Sander-Stiftung (Grant 86.018.2). Part of the data were presented at the Annual Meeting of the American Gastroenterological Association, San Antonio, Texas, May 1990, and published in abstract form in Gastroenterology (Suppl.), A-290.

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