P21‐Activated Kinase 1 (PAK1) Modulates Therapeutic Response to Ionizing Radiation in Head and Neck Squamous Cell Carcinoma Cells

ABSTRACTHead and neck squamous cell carcinoma (HNSCC) continues to be a formidable epithelial malignancy characterized by late‐stage detection and recurrence impacting survival. P21‐activated kinase‐1 (PAK1) was reported to be overexpressed in head and neck cancers and activated by ionizing radiation (IR), affecting treatment outcomes. Present investigations revealed that PAK1 silencing on HNSCC cells reverted the aggressive phenotype and showed impaired DNA damage repair upon IR exposure. Further HNSCC cells were resistant to IR up to 30 Gy with elevated pPAK1 levels. Radiation‐resistant (RR) HNSCC cells expressed radiation‐resistant markers, namely MRE‐11 and NME‐1; stemness markers—OCT4 and SOX2; and EMT & metastasis markers—vimentin, snail, and α‐smooth muscle actin (α‐SMA). In addition, HNSCC RR cells showed increased levels of DNA damage response protein H2AX, indicative of an aggressive phenotype with an augmented DNA repair machinery and a potential target for inhibition. Since H2AX appears to be a mechanistic hub for PAK1‐induced radiation resistance, using in silico methods, peptides were designed, and the PL‐8 peptide was chosen to target the phosphorylation of H2AX, which could enhance the sensitivity to IR and push the cells to radiation‐induced cell death. PL‐8 peptide inhibited H2AX phosphorylation on HNSCC cells and triggered radiation‐induced cell death as determined by functional assays. The present study reveals PAK1 induced in HNSCC cells by IR and causes resistance by enhancing DNA damage response mediated through γH2AX. To counteract this complex molecular interplay, we propose inhibiting γH2AX formation & silencing PAK1 appears to be a probable way forward in HNSCC.