Abstract. Butyrate is a short-chain fatty acid produced by the intestinal microflora and it not only induces apoptosis but also inhibits the proliferation of cancer cells. Recently, it has been reported that butyrate may cause resistance in colon cancer cells. Therefore, we investigated the effects of increased resistance to butyrate in HCT116 colon cancer cells. We established HCT116 cells resistant to butyrate (HCT116/ BR) by treating HCT116 parental cells (HCT116/PT) with increasing concentrations of butyrate to a maximum of 1.6 mM for 3 months. The butyrate concentrations that inhibited cell growth by 50% (IC 50 ) were 0.508 and 5.50 mM in HCT116/PT and HCT116/BR cells. The values after treatment with paclitaxel, 5-fluorouracil (5-FU), doxorubicin and trichostatin A (TSA) were 2.42, 2.36, 4.31 and 11.3-fold higher, respectively, in HCT116/BR cells compared with HCT116/PT cells. The protein expression of drug efflux pumps, such as P-glycoprotein (P-gp), breast cancer-resistant protein (BCRP) and the multidrug resistance associated protein 1 (MRP1), did not differ between HCT116/PT and HCT116/BR cells. The expression level of the anti-apoptotic Bcl-xL protein was increased while those of pro-apoptotic Bax and Bim proteins were reduced in HCT116/BR cells. There were no significant differences in cell motility and invasion. This study suggests that exposure of colon cancer cells to butyrate results in development of resistance to butyrate, which may play a role in the acquisition of chemoresistance in colon cancer. IntroductionColon cancer is one of the most common cancers and its prevalence is increasing gradually due to changes in lifestyle (1).Epidemiological studies have attempted to elucidate an association between a high-fiber diet and decreased incidence and progression of colon cancer. Butyrate is a short-chain fatty acid that is naturally produced by the intestinal microflora during dietary fiber fermentation and has been suggested as a target for colon cancer therapy because it inhibits cell proliferation and causes apoptosis in colon cancer cells (2). Butyrate suppresses cell proliferation by causing cell cycle arrest at the G 1 and G 2 phases, which involves cyclin D1 and retinoblastoma (Rb) signaling (3). In addition, butyrate is a histone deacetylase (HDAC) inhibitor, similar to trichostatin A(TSA), and results in apoptosis by increasing the expression of pro-apoptotic proteins, such as Bak, Bad and Bim, and decreasing that of anti-apoptotic proteins, such as Bcl-2 and Bcl-xL (4).Butyrate plays an important role in normal colonocytes as a major energy source and promotes their proliferation (5). However, most cancer cells prefer to utilize glucose as an energy source even under abundant oxygen conditions, so colon cancer cells cannot use butyrate efficiently, which enhances its anticancer effects (6).Recently, it has been reported that butyrate may trigger resistance in colon cancer cells (7). As mentioned above, butyrate is a natural product of dietary fiber fermentation to which colonocytes are c...