Cell cycle arrest is accompanied by a decrease in serum prostate-specific antigen (PSA) and the expression of androgen receptor (AR) in AR positive xenografts upon chronic administration of valproic acid (VPA).

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VPA is commonly used for treating seizure and bipolar disorders, but recently has also been found to have Histone Deacetylase Inhibitor (HDACI) activity. HDACIs are being investigated as anticancer agents that are thought to induce growth arrest. VPA inhibited the growth of both AR positive (LNCaP and C4-2) and AR negative (DU145 and PC3) prostate cancer cell lines in vitro and in vivo. Previously we have shown that chronic treatment with VPA resulted in a significant reduction in tumor volume. In order to better understand the mechanism by which VPA affected xenograft growth, we generated a tissue microarray (TMA) of the treated and untreated xenograft tumors, and analyzed the TMA for expression of cell-cycle associated markers, AR and other markers of cellular growth thought to be important in prostate cancer progression. LNCaP and C4-2 cells were injected into the lateral flanks of male athymic mice. Once palpable tumors were established, animals were randomized into control and treatment arms with the latter receiving 0.4% VPA in drinking water. Animals were weighed and tumors harvested on day 35 for generating the TMA. Blood samples were also taken for measuring serum PSA and toxicity studies. Immunohistochemical (IHC) analysis was carried out for AR, PSA, p21, p27, cyclin D1, and cytokeratin 18 (CK18) on the TMA. Weight of animals with xenografts after 35 days of treatment and toxicity studies revealed no difference between treated and control animals. Chronic VPA treatment of animals with LNCaP xenograft significantly decreased serum PSA 2.39 fold (p=0.0008), but increased the PSA index (PSA:tumor volume ratio) by 1.43 fold (p=0.0371). Animals with C4-2 xenografts had the same trend. IHC analysis of LNCaP Xenografts in the TMA showed a reduction of AR and cyclin D1 (1.3 & 1.2 fold) (p=0.0279 & p=0.0298), and p21 was up-regulated 3.5 fold (p=0.0461). There was no significant difference of the expression of p27 and CK18. But in C4-2 xenografts, the expression of CK18 was increased 2.72 fold (p=0.0418). Taken together, our results indicate that chronic administration of VPA induces cell cycle arrest by down-modulation of the protein expression of cyclin D1 and up-regulation of p21 in human prostate cancer xenograft tested, accompanied by a decrease in serum PSA and cellular AR.