Differential expression of an 80-kDa protein kinase C substrate in preneoplastic and neoplastic mouse JB6 cells.

An 80-kDa protein (p80), previously reported to be a major protein kinase C substrate in preneoplastic JB6 mouse epidermal cells, has been shown to be transiently phosphorylated by phorbol 12-O-tetradecanoate 13-acetate. Phosphorylation was maximal at 2 hr of phorbol 12-O-tetradecanoate 13-acetate treatment and returned to basal levels by 24 hr. In contrast, using a p80-specific antibody, we found that phorbol 12-O-tetradecanoate 13-acetate treatment produced no increase in p80 concentration. p80 showed a progressive decrease in JB6 cells during progression from a preneoplastic to neoplastic phenotype. The lack of p80 expression in neoplastic cells was not attributable to lack of protein kinase C; the protein kinase activity and protein concentration were similar in cells of all three phenotypes. When p80 mRNA was analyzed by hybridization to a putative p80 cDNA clone, its relative concentration paralleled that of p80 protein, with high levels present in preneoplastic JB6 cells, and little or no evidence for p80-hybridizing RNA in transformed cells. Thus, p80 appears to be regulated pretranslationally at the level of mRNA concentration during preneoplastic progression in mouse epidermal JB6 cells.