Role of coactivators and corepressors in the induction of the RARbeta gene in human colon cancer cells.

We previously reported that the retinoic acid (RA) insensitivity of RARbeta induction is a general feature of human colon cancer cells (Biochem. Pharmacol., 59: 485-496, 2000). In the present investigation, we analyzed potential transcriptional defects associated with the expression of the RARbeta gene in colon cancer cells. Transfection of reporter constructs containing the RARbeta gene promoter as well as truncated fragments of the promoter showed a significant induction of reporter activity by RA treatment in RA-sensitive HCT-15 cells, but not in RA-resistant DLD-1 cells. The results suggest that the transcriptional defect of RARbeta expression may not be due to the presence of a specific cis-element in the RARbeta promoter. Next we examined whether coactivators and core-pressors of nuclear receptors were involved in the RA sensitivity of colon cancer cells. Transfection of coactivators such as CREB binding protein (CBP) and p300 up-regulated the RA-responsive element present in the RARbeta promoter (betaRARE) in DLD-1 cells up to the level in HCT-15, while coexpression of the nuclear receptor corepressor (NCoR) suppressed the betaRARE activity in HCT-15 cells. The expression level of CBP protein was consistently higher in HCT-15, while that of NCoR and Sin3A was higher in DLD-1 cells. Treatment with the histone deacetvlase inhibitor trichostatin A (TSA) increased both basal and RA-induced betaRARE activity in DLD-1, indicating that histone deacetylase is involved in the regulation of RARbeta gene expression. Taken together, our results show that differential function of coactivators and corepressors may determine the level of RARbeta induction that may mediate retinoid action in colon cancer cells.