ANTI-AP-1 ACTIVITY OF ALL-TRANS-RETINOIC ACID IN GLOMERULAR MESANGIALCELLS

Functional antagonism between retinoic acid (RA) receptors and activat or protein-1 (AP-1) transcription factors might regulate expression of genes involved in the response to injury in the kidney. We designed e xperiments to analyze the mechanisms by which RA inhibits AP-1-directe d transcriptional responses in glomerular mesangial cells. RA inhibite d serum-stimulated mesangial cell proliferation as assessed by measure ments of [H-3]thymidine uptake and cell number. In transient transfect ion assays with a chloramphenicol acetyltransferase reporter, RA compl etely blocked transcription directed by an AP-1 cis-element in cells s timulated by serum. AP-1 DNA binding was analyzed in electrophoretic g el mobility shift assays using nuclear extracts from control or RA-pre treated cells stimulated with serum. RA did not abolish AP-1 DNA bindi ng activity under the conditions of this assay. The apparent equilibri um dissociation constant, maximal density of binding, and association rate for the AP-1-DNA interaction were similar in serum-stimulated cel ls or RA-pretreated cells stimulated with serum. RA repressed serum-st imulated induction of the immediate early genes c-fos and c-jun, whose protein products dimerize to form AP-1. Repression was relatively sel ective for c-fos/c-jun; induction of other immediate early transcripti on factors (junB, c-myc, and egr-1) was not downregulated by RA. That repression of c-fos by RA might contribute to anti-AP-1 activity was s uggested by experiments with an antisense c-fos expression vector, whi ch demonstrated that c-fos induction was required for serum-stimulated AP-1 activity. Together, these data demonstrate that RA antagonizes A P-1-directed transcription without inhibiting AP-1 DNA-binding in mesa ngial cells. Selective repression of c-fos and c-jun might contribute to the anti-AP-1 activity of RA.