Impaired retinoic acid (RA) signal leads to RARβ2 epigenetic silencing and RA resistance

Resistance to the growth-inhibitory action of retinoic acid (RA), the bioactive derivative of vitamin A, is common in human tumors. One form of RA resistance has been associated with silencing and hypermethylation of the retinoic acid receptor beta 2 gene (RAR beta 2), an RA-regulated tumor suppressor gene. The presence of an epigenetically silent RAR beta 2 correlates with lack of the RA receptor alpha (RAR alpha). Normally, RAR alpha regulates RAR beta 2 transcription by mediating dynamic changes of RAR beta 2 chromatin in the presence and absence of RA. Here we show that interfering with RA signal through RAR alpha (which was achieved by use of a dominant-negative RAR alpha, by downregulation of RAR alpha by RNA interference, and by use of RAR alpha antagonists) induces an exacerbation of the repressed chromatin status of RAR beta 2 and leads to RAR beta 2 transcriptional silencing. Further, we demonstrate that RAR beta 2 silencing causes resistance to the growth-inhibitory effect of RA. Apparently, RAR beta 2 silencing can also occur in the absence of DNA methylation. Conversely, we demonstrate that restoration of RA signal at a silent RAR beta 2 through RAR alpha leads to RAR beta 2 reactivation. This report provides proof of principle that RAR beta 2 silencing and RA resistance are consequent to an impaired integration of RA signal at RAR beta 2 chromatin.