Retinoic acid down-regulation of fibronectin and retinoic acid receptor α proteins in NIH-3T3 cells: Block of this response by ras transformation

All-trans-retinoic acid (RA) markedly reduced the level of intracellular 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 biosynthesis rather than its turnover rate. Steady state levels of FN transcripts did not change after treatment of the cells with RA for various times or concentrations, suggesting that HA acts at the translational level. Similar effects were observed in other fibroblasts. In NIH-3T3 cells, RA had distinct effects on different receptors; it down-modulated retinoic acid receptor (RAR) α protein and transcript levels, it up-regulated RARβ transcripts, and it had no effect on RARγ. Transformation of NIH-3T3 cells with an activated Ha-ras oncogene 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 RARα proteins by the use of the retinoid X receptor- selective compound, SR11237, by stable overexpression of a truncated form of the RARα gene, RARα403, with strong RAR dominant negative activity, and by overexpression of RARα. We conclude that: 1) RA-dependent FN down- modulation is mediated by RARs, 2) retinoid X receptors mediate the observed reduction of RARα by RA, and 3) the block of RA responsiveness in Ha-ras cells cannot be overcome by overexpression of RARα. These studies have defined fibronectin and RARα as targets of RA in fibroblast cells and have shown that oncogenic transformation renders the cells resistant to RA action.