LOSS OF RETINOIC ACID RECEPTOR-GAMMA FUNCTION IN F9 CELLS BY GENE DISRUPTION RESULTS IN ABERRANT HOXA-1 EXPRESSION AND DIFFERENTIATION UPONRETINOIC ACID TREATMENT

Retinoic acid (RA) signal transduction is believed to be mediated thro ugh several high-affinity nuclear receptors [RA receptors (RARs) and r etinoid X receptors], which are members of the steroid/thyroid/vitamin D superfamily and function as transcription factors. Why multiple RAR s exist and what gene targets are regulated by each of the three recep tors remain compelling questions in developmental biology. Through tar geted disruption of both RARgamma alleles, we have identified several differentiation-specific genes that are regulated either directly or i ndirectly by RARgamma in F9 embryonal carcinoma cells. These include g enes encoding Hoxa-1 (Hox-1.6) and the extracellular matrix proteins l aminin B1 and collagen type IV (alpha1), all of which are RA inducible in wild-type F9 embryonal carcinoma cells but are not significantly i nduced in the RARgamma-/- lines. In contrast, transcripts encoding Hox b-1 (Hox-2.9) and cellular RA binding protein II (CRABPII) are activat ed by RA for a longer period of time in the RARgamma-/- lines compared to the wild-type F9 line. Not all RA-responsive genes are aberrantly expressed; Rex-1, RARbeta, and SPARC transcripts are regulated in the RARgamma-/- lines as they are in F9 wild-type cells. Our results suppo rt the idea that each RAR may regulate different subsets of RA-respons ive genes, which may explain, in part, the complex regulation of devel opmental processes by retinoids.