In vivo imaging of retinoic acid receptor beta2 transcriptional activation by the histone deacetylase inhibitor MS-275 in retinoid-resistant prostate cancer cells

BACKGROUND. In retinoid resistant epithelial tumors, the lack of retinoic acid receptor beta 2 (RAR beta 2) expression due to epigenetic silencing impairs the activation of retinoid target genes including RAR beta 2, and has been associated with the development of cancer. In this study we developed a strategy to monitor the re-activation of RAR beta 2 by chromatin remodeling agents combined with retinoids in real time, and to correlate the RAR beta 2 re-activation with anti-tumor activity. METHODS. We selected the RAR beta 2-negative retinoid resistant human prostate carcinoma cell line PC3 and stably transfected it with a luciferase expression vector under the control of a functional segment of RAR beta 2 promoter (pGL2-RAR beta 2-PC3). Then, we used the bioluminescence technology to monitor the reporter gene expression in real time both in vitro and in vivo following combination treatment with the histone deacetylase inhibitor MS-275 and 13-cis retinoic acid (CRA). Based on the effective dose for the RAR beta 2 re-activation, we tested the antitumor activity of this drug combination. RESULTS. Following combination treatment with MS-275 and CRA, we observed endogenous RAR beta 2 re-expression, acetylation at the RAR beta 2 promoter level, and synergistic activation of the luciferase reporter gene by real time imaging both in vitro and in vivo. Combination treatment with MS-275 and CRA restored retinoid sensitivity in human prostate carcinoma cell lines, and had a greater inhibitory effect on tumor cell growth than single agents in vitro and in vivo. CONCLUSIONS. This study provides evidence that HDAC inhibitors restore retinoid sensitivity in prostate cancer cells, and in vivo real time imaging of RAR beta 2 activation may represent a useful tool to study the pharmacodynamics of combination therapy with HDAC inhibitors and retinoids.