MARCKS protein is transcriptionally down-regulated in v-Src-transformed BALB/c 3T3 cells.

Activation of protein kinase C (PKC) by tumor-promoting phorbol esters leads to the phosphorylation of an 80-kilodalton PKC substrate (known as MARCKS) in murine fibroblasts. In BALB/c 3T3 cells stably transformed by v-Src, phorbol esters were unable to induce phosphorylation of MARCKS. Western blot analysis and in vitro kinase assays showed that both PKC protein levels and kinase activity were unchanged in v-Src-transformed relative to the parental nontransformed BALB/c 3T3 cells. However, MARCKS protein levels were reduced in v-Src-transformed cells relative to nontransformed cells. MARCKS RNA levels were also correspondingly reduced in v-Src-transformed cells. Nuclear "run-on" assays showed decreased transcription of MARCKS in v-Src-transformed cells. Thus, the absence of MARCKS in v-Src-transformed cells could be explained by a down-regulation of MARCKS transcription. Inhibiting the protein tyrosine kinase activity of v-Src with herbimycin A restored MARCKS RNA levels, MARCKS transcription, and MARCKS protein, suggesting that down-regulation of MARCKS in v-Src-transformed BALB/c 3T3 cells is a direct effect of v-Src.