Calpain regulates thymidylate synthase–5‐fluoro‐dUMP complex levels associated with response to 5‐fluorouracil in gastric cancer cells

Thymidylate synthase (TS) plays a major role in the response to 5‐fluorouracil (5‐FU) by binding directly to the 5‐FU metabolite, 5‐fluoro‐dUMP (FdUMP). The change in the TS expression levels after 5‐FU administration was examined in parallel to 5‐FU responsiveness in six human gastric adenocarcinoma cell lines to elucidate the source of variability of 5‐FU sensitivity. MKN‐1, SH‐10‐TC and MKN‐74 cells were more resistant to 5‐FU than MKN‐28, KATO III and MKN‐45 cells. Western blotting analysis revealed that the 5‐FU sensitivity of these cells did not correlate with the basal TS expression levels but did correlate with rapid detection of the TS‐FdUMP complex after exposure to 5‐FU. In 5‐FU‐resistant cells, very low levels of the TS‐FdUMP complex early after 5‐FU exposure were elevated by pretreatment with calpain inhibitors such as benzyloxycarbonyl‐leucyl‐leucinal (ZLLH), benzyloxycarbonyl‐leucyl‐leucyl‐leucinal (ZLLLH) and ALLN, but not by other protease inhibitors. In contrast, ONO‐3403, which causes calpain activation, stimulated downregulation of the TS‐FdUMP complex in 5‐FU‐sensitive cells. The expression levels of calpastatin, an endogenous calpain inhibitor, were higher in 5‐FU‐sensitive cells than in 5‐FU‐resistant cells. ZLLH increased the 5‐FU sensitivity of 5‐FU‐resistant cells, whereas ONO‐3403 decreased the sensitivity of 5‐FU‐sensitive cells. In addition, knockdown of m‐calpain by siRNA increased the 5‐FU sensitivity in 5‐FU‐resistant cells, while knockdown of calpastatin reduced the sensitivity in 5‐FU‐sensitive cells. These results suggest that calpain might reduce the chemosensitivity of human gastric cancer cells to 5‐FU possibly by rapid degradation of the TS‐FdUMP complex, a finding that is considered to have novel therapeutic implications. (Cancer Sci 2011; 102: 1509–1515)