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 beta2 (RARbeta2) expression due to epigenetic silencing impairs the activation of retinoid target genes including RARbeta2, and has been associated with the development of cancer. In this study we developed a strategy to monitor the re-activation of RARbeta2 by chromatin remodeling agents combined with retinoids in real time, and to correlate the RARbeta2 re-activation with anti-tumor activity.

Methods: We selected the RARbeta2-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 RARbeta2 promoter (pGL2-RARbeta2-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 RARbeta2 re-activation, we tested the anti-tumor activity of this drug combination.

Results: Following combination treatment with MS-275 and CRA, we observed endogenous RARbeta2 re-expression, acetylation at the RARbeta2 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 RARbeta2 activation may represent a useful tool to study the pharmacodynamics of combination therapy with HDAC inhibitors and retinoids.