PRSS contributes to cetuximab resistance in colorectal cancer

Tan, Zhaoli; Gao, Lihua; Wang, Yan; Yin, Huihui; Xi, Yongyi; Wu, Xiaojie; Shao, Yong; Qiu, Weiyi; Du, Peng; Shen, Wenlong; Fu, Ling; Jia, Ru; Zhao, Chuanhua; Zhang, Yun; Zhao, Zhihu; Sun, Zhiwei; Chen, Hongxing; Hu, Xianwen; Xu, Jianming; Wang, Youliang

As PRSS1 is a serine proteinase, we considered whether PRSS1 causes cellular resistance to cetuximab by cleaving cetuximab. We used recombinant human PRSS1 and the culture supernatant of colon cancer cells to resolve this question. Both the recombinant human PRSS1 and the HT-29 cell culture supernatant cleaved cetuximab (Fig. 3A) . We also noted that the colon cancer cell culture supernatant and recombinant human PRSS1 had the same pattern of cetuximab cleavage, indicating that PRSS1 in the cell culture supernatant cleaved the mAb (Fig. 3A) . As trypsin is a protein homologous to PRSS1, we considered whether the residual trypsin in the cell culture supernatant cleaved cetuximab after the cells underwent trypsin digestion. Accordingly, we used an ELISA to detect PRSS1 levels in the cell culture supernatant. PRSS1 levels in the cell culture supernatant at 0 hours were too low to be detected but increased over time ( fig. S2A ), suggesting that the PRSS1 expressed in the cell culture supernatant and not the residual 0.25% trypsin cleaves cetuximab.