RETINOIC ACID DOWN-REGULATION OF FIBRONECTIN AND RETINOIC ACID RECEPTOR-ALPHA PROTEINS IN NIH-3T3 CELLS - BLOCK OF THIS RESPONSE BY RAS TRANSFORMATION

All trans-retinoic acid (RA) markedly reduced the level of intracellul ar fibronectin (FN) in a time- and concentration-dependent fashion in NIH-3T3 cells, but not in NIH-3T3 cells transformed by an activated Ha -ras oncogene. Pulse/chase experiments indicated that RA affects FN bi osynthesis rather than its turnover rate. Steady state levels of FN tr anscripts did not change after treatment of the cells with RA for vari ous times or concentrations, suggesting that RA acts at the translatio nal level. Similar effects were observed in other fibroblasts. In NIH- 3T3 cells, RA had distinct effects on different receptors; it down-mod ulated retinoic acid receptor (RAR) alpha protein and transcript level s, it up-regulated RAR beta transcripts, and it had no effect on RAR g amma. Transformation of NIH-3T3 cells with an activated Ha-ras oncogen e down-modulated RAR expression and abolished responsiveness to RA. We identified the retinoid signal transduction pathways responsible for the effects of RA on FN and RAR alpha proteins by the use of the retin oid X receptor-selective compound, SR11237, by stable overexpression o f a truncated form of the RAR alpha gene, RAR alpha 403, with strong R AR dominant negative activity, and by overexpression of RAR alpha. We conclude that: 1) RA-dependent FN down-modulation is mediated by RARs, 2) retinoid X receptors mediate the observed reduction of RAR alpha b y RA, and 3) the block of RA responsiveness in Ha-ras cells cannot be overcome by overexpression of RAR alpha. These studies have defined fi bronectin and RAR alpha as targets of RA in fibroblast cells and have shown that oncogenic transformation renders the cells resistant to RA action.