Ectopic p21^sdi1 gene transfer induces retinoic acid receptor β expression and sensitizes human cancer cells to retinoid treatment

The biological effects of retinoic acid (RA) are mediated by nuclear retinoic acid receptors (RARs) that function as ligand-activated transcriptional factors. The response of human cancer cells to RA is known to be associated with the expression of RARβ. Recent studies have demonstrated that the loss of RARβ expression is involved in the development of a variety of human malignancies. We show that recombinant adenovirus-mediated p21^sdiI gene transfer enhances RARβ mRNA expression as well as protein expression and induces the sensitivity to all-trans RA (ATRA) in human cancer cells. Semi-quantitative reverse transcription-polymerase chain reaction analysis demonstrated that infection with adenovirus carrying human p21^sdiI gene (Ad5CMV-p21), which encodes a cyclin-dependent kinase inhibitor, induced RARβ mRNA and protein expression in H1299 human non-small cell lung cancer cells and DLD-I human colorectal cancer cells. We also found that exogenous introduction of the p21^sdiI gene transcriptionally activated the upstream promoter function of the RARβ gene. Treatment with I μM of ATRA showed no significant inhibitory effects on the growth of H1299 and DLD-I cells; after Ad5CMV-p21 infection, however, cells underwent apoptosis with ATRA treatment at the same concentration, suggesting that p21^sdiI gene transfer sensitized H1299 and DLD-I cells, presumably, through RARβ upregulation. We also demonstrated the efficacy of intratumoral injection of Ad5CMV-p21 in combination with systemic administration of ATRA in a nude mice xenograft model. Our results indicate that recombinant adenovirus-mediated p21^sdiI gene transfer could be potentially useful for the local induction of RA sensitivity in human premalignant and malignant lesions lacking appropriate RARβ expression.