NHE8 Deficiency Promotes Colitis-Associated Cancer in Mice via Expansion of Lgr5-Expressing Cells

We discovered that Na+/H+ exchanger isoform 8 (NHE8) expression is strongly inhibited in human colorectal cancer and that loss of NHE8 function resulted in increased tumor burden in mice. Furthermore, we found that NHE8 is expressed in intestinal stem cells. Thus, NHE8 likely is involved in Wnt/beta-catenin pathway regulation. BACKGROUND & AIMS: Lgr5 overexpression has been detected in colorectal cancers (CRCs), including some cases of colitis-associated CRCs. In colitis-associated CRCs, chronic inflammation is a contributing factor in carcinogenesis. We recently reported that intestinal Na+/H+ exchanger isoform 8 (NHE8) plays an important role in intestinal mucosal protection and that loss of NHE8 expression results in an ulcerative colitis-like condition. Therefore, we hypothesized that NHE8 may be involved in the development of intestinal tumors. METHODS: We assessed NHE8 expression in human CRCs by immunohistochemistry and studied tumor burden in NHE8 knockout (KO) mice using an azoxymethane/dextran sodium sulfate colon cancer model. We also evaluated cell proliferation in HT29(NHE8KO) cells and assessed tumor growth in NOD scid gamma (NSG) mice xenografted with HT29(NHE8KO) cells. To verify if a relationship exists between Lgr5 and NHE8 expression, we analyzed Lgr5 expression in NHE8KO mice by polymerase chain reaction and in situ hybridization. Lgr5 expression and cell proliferation in the absence of NHE8 were confirmed in colonic organoid cultures. The expression of beta-catenin and c-Myc also were analyzed to evaluate Wnt/beta-catenin activation. RESULTS: NHE8 was undetectable in human CRC tissues. Although only 9% of NHE8 wild-type mice showed tumorigenesis in the azoxymethane/dextran sodium sulfate colon cancer model, almost 10 times more NHE8KO mice (89%) developed tumors. In the absence of NHE8, a higher colony formation unit was discovered in HT29(NHE8KO) cells. In NSG mice, larger tumors developed at the site where HT29(NHE8KO) cells were injected compared with HT29(NHE8 wild type) cells. Furthermore, NHE8 deficiency resulted in increased Lgr5 expression in the colon, in HT29-derived tumors, and in colonoids. The absence of NHE8 also increased Wnt/beta-catenin activation. CONCLUSIONS: NHE8 might be an intrinsic factor that regulates Wnt/beta-catenin in the intestine.