Loss of LGR5 through plasticity or gene ablation is associated with therapy resistance and enhanced MET-STAT3 signaling in colorectal cancer cells

Plasticity plays a significant role in colorectal tumor initiation, progression, and drug resistance. LGR5 is highly expressed in colorectal cancer (CRC) and marks functional cancer stem cells (CSCs). While LGR5+ CSCs are tumor-initiating, the majority of CRC cells that disseminate to seed metastases are LGR5-; however, reemergence of LGR5+ CSCs is required to drive metastatic outgrowth. LGR5+ CSCs have been shown to convert to LGR5- CRC cells in response to chemotherapies and this loss of LGR5 promotes a more drug-resistant phenotype. However, the molecular mechanisms that mediate plasticity remain elusive. In this study, we demonstrate conversion of LGR5+ CRC cells to an LGR5- state in response to chemotherapy, LGR5-targeted antibody-drug conjugates (ADCs), or LGR5 gene ablation, led to activation of STAT3. Further investigation revealed increased STAT3 activation occurred a result of increased MET activity. LGR5 overexpression decreased MET-STAT3 activity and sensitized CRC cells to therapy. STAT3 inhibition suppressed MET phosphorylation, while constitutively active STAT3 reduced LGR5 levels and increased MET activity, suggesting a potential feedback mechanism. Combination treatment of MET-STAT3 inhibitors with irinotecan or ADCs substantiated synergistic effects in vitro. In CRC xenografts, STAT3 inhibition combined with irinotecan enhanced tumor growth suppression and prolonged survival. These findings suggest a mechanism by which drug-resistant LGR5- CRC cells acquire a survival advantage through activation of MET-STAT3 and provide rationale for new treatment strategies that target CRC cell plasticity. Significance This study reveals that transition of highly plastic LGR5+ CRC cells to a more drug-resistant LGR5- state involves activation of MET-STAT3 signaling and provides new insight into therapeutic strategies to combat plasticity. ### Competing Interest Statement The authors have declared no competing interest.