Cetuximab promotes SN38 sensitivity via suppression of heat shock protein 27 in colorectal cancer cells with wild-type RAS.

Combination treatment with cetuximab and CPT-11 produces beneficial and synergistic effects in wild-type RAS metastatic colorectal cancer (mCRC) patients. However, the mechanism underlying this synergism is not yet understood. We examined whether cetuximab had a synergistic effect with CPT-11 and its active metabolite, SN38, and examined the molecular mechanism of the synergism between cetuximab and SN38 in CRC cells with various mutational status. We hypothesized that cetuximab promotes sensitivity to SN38 via suppression of heat shock protein 27 (HSP27), a protein involved in multidrug resistance through blocking the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathway, which is associated with chemosensitivity. Four human CRC cell lines with different RAS and BRAF mutational status were used. Expression levels of HSP27 protein correlated with SN38 sensitivity in these cell lines (R=0.841, p=0.159). Exposure to cetuximab and various concentration of AG490, an inhibitor of JAK2, STAT3 and HSP27 protein levels, except in the KRAS G12V mutant line, SW620. A synergistic effect of cetuximab in combination with SN38 was observed in RAS and BRAF wild-type cells (here, Caco2), but not in the three other RAS- or BRAF-mutated cell lines. These results indicate that cetuximab may promote sensitivity to SN38 via suppression of HSP27 through blocking the JAK/STAT pathway in Caco2 cells. The mutational status of numerous downstream effectors, such as RAS and BRAF, is important in mono- or combination therapy with cetuximab. In conclusion, cetuximab may promote SN38 sensitivity via suppression of HSP27, through blocking the JAK/STAT signaling pathway, and shows synergistic effects when combined with SN38 in wild-type RAS CRC cells.