CONTROL OF RETINOIC ACID RECEPTOR EXPRESSION IN MOUSE MELANOMA-CELLS BY CYCLIC-AMP

Retinoic acid receptor (RAR) alpha and gamma mRNAs were constitutively expressed in B16 melanoma cells with or without retinoic acid (RA) tr eatment. RAR beta mRNA, however, was significantly expressed only afte r exposure to RA. Induction of RAR beta by RA occurred within 1 h and was not inhibited by cycloheximide (i.e., did not require new protein synthesis). All three RAR mRNA levels were dramatically decreased with 8-bromo-cyclic AMP treatment and could not be rescued by addition of RA. Analysis of RAR gamma revealed that this decrease occurred within 1 h of exposure to 8-bromo-cyclic AMP and was not blocked by simultane ous treatment with cycloheximide. The stability of RAR?I mRNA was not altered by cyclic AMP treatment. Nuclear extracts from 8-bromo-cyclic AMP-treated cells showed a large decrease in protein binding to a reti noic acid response element (RARE) oligonucleotide compared to control cells. This correlated with a marked reduction of RA-stimulated RARE-r eporter gene activity in transfected cells which were treated with cyc lic AMP. Pretreatment of B16 cells with cyclic AMP prior to RA additio n dramatically reduced induction of PKC alpha, an early marker of RA-i nduced cell differentiation. Thus, cyclic AMP can antagonize the actio n of RA most likely via its ability to inhibit RAR expression. (C) 199 6 Wiley-Liss, Inc.