Histone Deacetylase Inhibitors Restore Cell Surface Expression of the Coxsackie Adenovirus Receptor and Enhance CMV Promoter Activity in Castration-Resistant Prostate Cancer Cells

Adenoviral gene therapy using the death receptor ligand TRAIL as the therapeutic transgene can be safely administered via intraprostatic injection but has not been evaluated for efficacy in patients. Here we investigated the efficacy of adenoviral TRAIL gene therapy in a model of castration resistant prostate cancer and found that intratumoral injections can significantly delay tumor growth but cannot eliminate established lesions. We hypothesized that an underlying cause is inefficient adenoviral delivery. Using the LNCaP progression model of prostate cancer we show that surface CAR expression decreases with increasing tumorigenicity and that castration resistant C4-2b cells were more difficult to transduce with adenovirus than castration sensitive LNCaP cells. Many genes, including CAR, are epigenetically silenced during transformation but a new class of chemotherapeutic agents, known as histone deacetylase inhibitors (HDACi), can reverse this process. We demonstrate that HDACi restore CAR expression and infectivity in C4-2b cells and enhance caspase activation in response to infection with a TRAIL adenovirus. We also show that in cells with high surface CAR expression, HDACi further enhance transgene expression from the CMV promoter. Thus HDACi have multiple beneficial effects, which may enhance not only viral but also non-viral gene therapy of castration resistant prostate cancer. 1. Introduction Epigenetic alterations, such as aberrant activity of histone deacetylases, are frequently observed in malignancies. Acetylation of histones is associated with less condensed chromatin and a transcriptionally active gene status, while deacetylation is associated with transcriptional silencing. Histone deacetylase inhibitors (HDACiS) were originally found to reverse the malignant phenotype of transformed cells and have subsequently been developed as a new group of chemotherapeutic agents. HDACi can affect numerous signaling pathways to inhibit growth or angiogenesis and induce apoptosis or senescence [1, 2]. Using two HDACi under evaluation for the treatment of prostate cancer, we previously demonstrated that both romidepsin (also known as depsipeptide) and MS-275 enhanced the in vitro efficacy of adenoviral TRAIL gene therapy in castration-sensitive LNCaP prostate cancer cells [3]. This effect was selective for the malignant cells as primary cultures of prostate epithelial cells were not adversely affected [3]. TRAIL gene therapy has been evaluated for safety in prostate cancer patients with locally confined disease scheduled for prostatectomy [4]. Although