Regulation of protein kinase C gene expression by retinoic acid in B16 mouse melanoma cells

Abstract

We have previously shown that the retinoic acid (RA)-induced growth arrest and differentiation of B16 mouse melanoma cells is accompanied by a large increase in the amount and activity of protein kinase C (PKC). Since PKC is a multigene family, we investigated which isoforms were expressed in control and RA-treated B16 melanoma cells, and characterized the manner by which RA regulates PKC gene expression. We found that RA treatment of B16 cells resulted in an increase in PKCα mRNA beginning at 4–8 h and reached a maximum of 10- to 12-fold over control levels by 48 h. There was also a small amount of PKCγ mRNA, present only in 48-h RA-treated cells, but no PKCβ mRNA was detected. The effect of RA on PKCα mRNA induction was not direct since the induction was abolished when cycloheximide was included in the incubation medium. Nuclear run-on experiments showed that the RA-induced increase in PKCα steady-state mRNA was not entirely due to an increase in transcriptional activity, as the increase in PKCα transcription was only 2- to 3-fold over control, which is not enough to account for the 10- to 15-fold increase in steady state levels. There was also no change in PKCα mRNA stability in RA-treated B16 cells compared to untreated cells. The 10.9-kb PKCα message in both control and RA-treated cells was less stable than the 3.8-kb PKCα message. Therefore, we propose that the major level of control of PKCα mRNA levels by RA is post-transcriptional, either RNA processing or transport out of the nucleus.