The Histone Deacetylase Inhibitor Valproic Acid Sensitizes Gemcitabine-Induced Cytotoxicity in Gemcitabine-Resistant Pancreatic Cancer Cells Possibly Through Inhibition of the DNA Repair Protein Gamma-H2AX

Gemcitabine (GEM) remains a major chemotherapeutic drug for pancreatic cancer, but resistance to GEM has been a big problem, as its response rate has been decreasing year by year. The effect of the histone deacetylase inhibitor (HDAI) valproic acid (VPA) was compared with tranilast and RI-1 as a combinatorial treatment with GEM in four pancreatic cancer cell lines, BxPC-3, PK45p, MiaPaCa-2 and PK59. Cell viability assays were carried out to check the cytotoxic effects, western blotting was carried out for DNA repair mechanisms, and localization was determined by immunofluorescence. The sensitization factors (i.e., the fold ratio of cell viability for GEM/GEM plus drug) reveal that VPA increases the cytotoxic sensitization to GEM at approximately 2.7-fold, 1.2-fold, 1.5-fold and 2.2-fold in BxPC-3, MiaPaCa-2, PK-45p and PK-59 cell lines, respectively. Moreover, GEM induces activation of the DNA repair protein H2AX proportional to the dosage. Interestingly, however, this effect can be abrogated by VPA. These results indicate that VPA enhances GEM-induced cytotoxicity in GEM-resistant pancreatic cancer cells, possibly through inhibition of DNA damage signaling and repair. Our study suggests VPA as a potential therapeutic agent for combinatorial treatment with GEM in pancreatic cancer.