Epigenetic patterns of the retinoic acid receptor beta2 promoter in retinoic acid-resistant thyroid cancer cells.

Treatment with retinoic acid (RA) is effective to restore radioactive iodine uptake in metastases of a small fraction of thyroid cancer patients. In order to find predictive markers of response, we took advantage of two thyroid cancer cell lines, FTC133 and FTC238, with low RA-receptor (RAR)beta expression but differing in their response to RA. We report that in both cell lines, RA signalling pathways are functional, as transactivation of an exogenous RARbeta2 promoter is effective in the presence of pharmacological concentrations of all-trans RA, and enhanced in RA-resistant FTC238 cells after ectopical expression of RARbeta, suggesting a defective endogenous RARbeta2 promoter in these cells. Further analyses show that whereas the RARbeta2 promoter is in an unmethylated permissive status in both FTC133 and FTC238 cells, it failed to be associated with acetylated forms of histones H3 or H4 in FTC238 cells upon RA treatment. Incubation with a histone deacetylase inhibitor, alone or in combination with RA, restored histone acetylation levels and reactivated RARbeta and differentiation marker Na+/I- symporter gene expression. Thus, histone modification patterns may explain RA-refractoriness in differentiated thyroid cancer patients and suggest a potential benefit of combined transcriptional and differentiation therapies.