Ha-ras oncogene transformation abolishes retinoic acid-induced reduction of intracellular fibronectin
1Laboratory of Cellular Carcinogenesis and Tumor Promotion, National Cancer Institute, NIH, Bethesda, Maryland 20892, USA.
Abstract:
All-trans-retinoic acid (RA) is a master regulator of cell differentiation and in this process it greatly influences cell adhesion and the elaboration of the extracellular matrix. Therefore, we were interested in the effect of RA on the biosynthesis of fibronectin (FN). RA reduced the level of intracellular 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. Since the steady-state level of FN transcripts did not change after treatment of the cells with RA for various times or concentrations, RA probably acts at the translational level. In NIH-3T3 cells, RA had distinct effects on different receptors, from decreasing retinoic acid receptor (RAR)alpha to increasing RAR beta expression to no effect on RAR gamma. Transformation of NIH-3T3 cells with an activated Ha-ras oncogene downmodulated RAR expression and also abolished responsiveness to RA. A variety of approaches permitted the following conclusions: 1) RA-dependent FN downmodulation is mediated by RARs, 2) retinoid X receptors (RXRs) mediate the observed reduction of RAR alpha by RA, and 3) the blockade of RA responsiveness by Ha-ras-transfected cells cannot be overcome by overexpression of RAR alpha. These studies have identified fibronectin and RAR alpha as RA targets in fibroblast cells and have shown that oncogenic transformation renders the cells resistant to RA action.
Insights
All-trans-retinoic acid (RA) reduces fibronectin (FN) levels in fibroblasts by affecting translation, not transcription. Oncogenic Ras transformation blocks this RA response, highlighting RA
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- All-trans-retinoic acid (RA) is a crucial regulator of cell differentiation, impacting cell adhesion and extracellular matrix.
- Fibronectin (FN) is a key extracellular matrix protein involved in cell adhesion and differentiation processes.
Purpose of the Study:
- To investigate the effect of all-trans-retinoic acid (RA) on fibronectin (FN) biosynthesis in NIH-3T3 cells.
- To determine the mechanism by which RA influences FN levels and the role of retinoic acid receptors (RARs).
Main Methods:
- Treatment of NIH-3T3 cells and Ha-ras-transformed NIH-3T3 cells with varying concentrations and durations of RA.
- Analysis of intracellular FN levels and FN transcript levels.
- Assessment of retinoic acid receptor (RAR) and retinoid X receptor (RXR) expression and function.
Main Results:
- RA significantly reduced intracellular FN levels in NIH-3T3 cells in a time- and dose-dependent manner.
- RA's effect on FN occurred at the translational level, as FN transcript levels remained unchanged.
- RA-dependent FN reduction is mediated by RARs, with RXRs involved in reducing RAR alpha expression.
- Ha-ras oncogene transformation abolished RA responsiveness and downmodulated RAR expression.
Conclusions:
- Fibronectin and RAR alpha are direct targets of RA action in fibroblast cells.
- Oncogenic transformation by Ha-ras confers resistance to RA's effects on FN and RAR expression.
- RA's influence on cell differentiation and adhesion is complex and can be disrupted by oncogenic signaling.
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