THE LATE RETINOIC ACID INDUCTION OF LAMININ-B1 GENE-TRANSCRIPTION INVOLVES RAR BINDING TO THE RESPONSIVE ELEMENT

Transcription of the murine laminin B1 (LB1) gene is induced by retinoic acid (RA), but responds only 24-28 h after RA treatment in F9 EC cells. Here we have shown by gel retardation assay that all three retinoic acid receptors (RARs) alpha, beta and gamma expressed in Cos cells can bind directly to the previously characterized retinoic acid response element (RARE) of the LB1 promoter, albeit with a weaker affinity than to the RAR-beta gene RARE. Three stereo-aligned TGACC-like motifs are crucial for this binding. Interestingly, the capacity of RAR-alpha, -beta, and -gamma to bind the LB1 RARE appears to be differentially modulated by factor(s) present in HeLa cells infected with RAR-expressing vaccinia virus vectors. Analyses of LB1 RARE mutants provide a strong correlation between RA-inducibility in vivo and efficiency of RAR binding in vitro. Thus, RARs can participate directly in transcriptional induction of the LB1 gene, even though this induction is cycloheximide sensitive and RARs are present in F9 cells prior to RA addition.