Reg4 interacts with CD44 to regulate proliferation and stemness of colorectal and pancreatic cancer cells.

Regenerating Gene 4 (Reg4) is highly upregulated in gastrointestinal (GI) malignancies including colorectal and pancreatic cancers. Numerous studies demonstrated an association between higher Reg4 expression and tumor aggressiveness, intrinsic resistance to apoptotic death, and poor outcomes from GI malignancies. However, the precise receptor and underlying signaling mechanism have remained unknown. Although we previously reported a Reg4-mediated induction of epidermal growth factor receptor (EGFR) activity in colorectal cancer (CRC) cells, a direct interaction between Reg4 and EGFR was not observed. The present study is focused on identifying the cell surface binding partner of Reg4 and dissecting its role in CRC and pancreatic cancer (PC) growth and stem cell survival. In vitro models of human CRC and PC were used to evaluate the results. Results of this study find: i) Reg4 interacts with CD44, a transmembrane protein expressed by a population of CRC and PC cells, ii) Reg4 activates regulated intramembrane proteolysis (RIP) of CD44 resulting in γ-Secretase-mediated cleavage and release of the CD44 intracytoplasmic domain (CD44ICD) that functions as a transcriptional activator of D-type cyclins involved in the regulation of cancer cell proliferation and Klf4 and Sox2 expression involved in regulating pluripotency of cancer stem cells; and iii) Reg4 significantly increases CRC and PC cell proliferation and their clonogenic potential in stem cell assays. Implications: These results suggest that pro-proliferative and pro-stemness effects of Reg4 are mediated through γ-Secretase-mediated CD44/CD44ICD signaling, hence strategies to disrupt Reg4-CD44-γ-Secretase-CD44ICD signaling axis may increase cancer cell susceptibility to chemo and radiotherapeutics.