Abstract.Although the therapeutic efficacy of valproic acid (VPA) has been observed in patients with solid tumors, the very high concentration required to induce antitumor activity limits its clinical utility. The present study focused on the development of combined molecular targeted therapies using VPA and proteasome inhibitors (PIs: MG132, PI-1 and PR-39) to determine whether this combination of treatments has synergistic anticancer and chemosensitizing effects against colorectal cancer. Furthermore, the potential molecular mechanisms of action of the VPA/PI combinations were evaluated. The effects of VPA in combination with PIs on the growth of colorectal cancer cells were assessed with regard to proliferation, cell cycle, apoptosis, reactive oxygen species (ROS) generation and the expression of genes that control the cell cycle, apoptosis and pro-survival/stress-related pathways. Treatment with combinations of VPA and PIs resulted in an additive/synergistic decrease in colorectal cancer cell proliferation compared to treatment with VPA or PIs alone. The combination treatment was associated with a synergistic increase in apoptosis and in the number of cells arrested in the S phase of the cell cycle. These events were associated with increased ROS generation, pro-apoptotic gene expression and stress-related gene expression. These events were also associated with the decreased expression of anti-apoptotic genes and pro-survival genes. The combination of VPA with MG132 or PI-1 enhanced the chemosensitivity of the SW1116 (29-185-fold) and SW837 (50-620-fold) colorectal cancer cells. By contrast, the combination of VPA/PR-39 induced a pronounced increase in the chemosensitivity of the SW837 (16-54-fold) colorectal cancer cells. These data provide a rational basis for the clinical use of this combination therapy for the treatment of colorectal cancer.
IntroductionHistone deacetylase (HDAC) activity has been shown to be upregulated in cancer cells. It is considered that this upregulation results in the repression of tumor suppressor gene products, such as p53, rendering HDACs an attractive drug target. In cell culture models, HDAC inhibitors (HDACIs) have been shown to decrease proliferation rates, induce apoptosis and induce autophagy-related cell death in several cancer cell lines. Due to their relative specificity toward cancer cells, HDACIs represent a new class of cancer treatment agents that are generally well tolerated (1).Short-chain fatty acids, such as butyric and valproic acid (VPA) were the first HDACIs to be identified as tumor growth inhibitors and inducers of apoptosis both in vitro and in vivo. However, they were found to have low potency (2). Despite such weak in vitro activity, the anticancer effects of VPA have been investigated in preclinical models of skin, breast, colon, prostate and small cell lung cancer, and the drug is currently being used in phase I-III clinical trials (3). However, the therapeutic doses of VPA are very high and cause side-effects severe enough to limit its usage.Despite ...