Adaptive c-Met-PLXDC2 Signaling Axis Mediates Cancer Stem Cell Plasticity to Confer Radioresistance-associated Aggressiveness in Head and Neck Cancer

Radiotherapy plays an essential role in the treatment of head and neck squamous cell carcinoma (HNSCC), yet radioresistance remains a major barrier to therapeutic efficacy. A better understanding of the predom-inant pathways determining radiotherapy response could help develop mechanism-informed therapies to improve cancer management. Here we report that radioresistant HNSCC cells exhibit increased tumor aggressive-ness. Using unbiased proteome profiler antibody arrays, we identify that upregulation of c-Met phosphorylation is one of the critical mechanisms for radioresistance in HNSCC cells. We further uncover that radioresistance-associated HNSCC aggressiveness is effectively exacerbated by c-Met but is suppressed by its genetic knockdown and pharmacologic inactivation. Mechanistically, the resulting upregulation of c-Met promotes elevated expression of plexin domain containing 2 (PLXDC2) through activating ERK1/2-ELK1 signaling, which in turn modulates cancer cell plasticity by epithelial-mesenchymal transition (EMT) induction and enrichment of the cancer stem cell (CSC) subpopulation, leading to resistance of HNSCC cells to radiotherapy. Depletion of PLXDC2 overcomes c-Met-mediated radioresistance through reversing the EMT progress and blunting the self -renewal capacity of CSCs. Therapeutically, the addition of SU11274, a selective and potent c-Met inhibitor, to radiation induces tumor shrink-age and limits tumor metastasis to lymph nodes in an orthotopic mouse model. Collectively, these significant findings not only demonstrate a novel mechanism underpinning radioresistance-associated aggressiveness but also provide a possible therapeutic strategy to target radioresistance in patients with HNSCC.Significance: This work provides novel insights into c-Met-PLXDC2 signaling in radioresistance-associated aggressiveness and suggests a new mechanism-informed therapeutic strategy to overcome failure of radiotherapy in patients with HNSCC.

Automated summary

Radiotherapy is essential for the treatment of head and neck squamous cell carcinoma (HNSCC), but radioresistance remains a major obstacle. This study shows that radioresistant HNSCC cells exhibit increased aggressiveness and that upregulation of c-Met phosphorylation is a critical mechanism for radioresistance. The study also reveals that c-Met exacerbates radioresistance-associated aggressiveness, but its genetic knockdown and pharmacologic inactivation suppress it. Mechanistically, c-Met upregulation promotes the expression of PLXDC2, which leads to epithelial-mesenchymal transition (EMT) induction and enrichment of cancer stem cells (CSCs), resulting in radioresistance. Depletion of PLXDC2 overcomes c-Met-mediated radioresistance. Therapeutically, the addition of a selective c-Met inhibitor to radiation induces tumor shrinkage and limits tumor metastasis in a mouse model. These findings provide insights into radioresistance-associated aggressiveness and suggest a potential therapeutic strategy for HNSCC patients.