The 27 kDa protein, a major component of rat liver gap junctions, was shown to be phospho~iated in vitro by protein kinase C. The stoichiometry of the phospho~lation indicated that approx. 0.33 mol phosphate was incorporated per moi 27 kDa protein. Phosphorylation was entirely dependent on the presence of calcium and was virtually specific for serine residues. For comparison, the gap junction protein was also examined for its phosphorylation by CAMP-dependent protein kinase, the extent of phosphorylation being one-tenth that exerted by protein kinase C.

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The down‐regulation of protein kinase C (PKC) was examined by Western blot procedure on about 30 tissue samples derived from human colorectal cancer and the corresponding normal mucosa. PKC‐α down‐regulation was detected in 60% of the cancer tissues compared with the respective normal mucosa and was observed in a higher frequency with the tissues under more advanced cancer stages. However, the frequencies of the down‐regulation of PKC‐δ and PKC‐ζ were lower than that of PKC‐α. These results suggest that a decreased level of PKC‐α may affect the cell growth and tumor promotion in colorectal tissue.

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