Regulation of Head and Neck Squamous Cancer Stem Cells by PI3K and SOX2

Background: We have an incomplete understanding of the differences between cancer stem cells (CSCs) in human papillomavirus–positive (HPV-positive) and –negative (HPV-negative) head and neck squamous cell cancer (HNSCC). The PI3K pathway has the most frequent activating genetic events in HNSCC (especially HPV-positive driven), but the differential signaling between CSCs and non-CSCs is also unknown.

Methods: We addressed these unresolved questions using CSCs identified from 10 HNSCC patient-derived xenografts (PDXs). Sored populations were serially passaged in nude mice to evaluate tumorigenicity and tumor recapitulation. The transcription profile of HNSCC CSCs was characterized by mRNA sequencing, and the susceptibility of CSCs to therapy was investigated using an in vivo model. SOX2 transcriptional activity was used to follow the asymmetric division of PDX-derived CSCs. All statistical tests were two-sided.

Results: CSCs were enriched by high aldehyde dehydrogenase (ALDH) activity and CD44 expression and were similar between HPV-positive and HPV-negative cases (percent tumor formation injecting ≤ 1x10³ cells: ALDH⁺CD44^high = 65.8%, ALDH^-CD44^high = 33.1%, ALDH⁺CD44^high = 20.0%; and injecting 1x10⁵ cells: ALDH^-CD44^low = 4.4%). CSCs were resistant to conventional therapy and had PI3K/mTOR pathway overexpression (GSEA pathway enrichment, P < .001), and PI3K inhibition in vivo decreased their tumorigenicity (40.0%–100.0% across cases). PI3K/mTOR directly regulated SOX2 protein levels, and SOX2 in turn activated ALDH1A1 (P < .001 013C and 067C) expression and ALDH activity (ALDH⁺ [%] empty-control vs SOX2, 0.4% ± 0.4% vs 14.5% ± 9.8%, P = .03 for 013C and 1.7% ± 1.3% vs 3.6% ± 3.4%, P = .04 for 067C) in 013C and 067 cells. SOX2 enhanced sphere and tumor growth (spheres/well, 013C P < .001 and 067C P = .04) and therapy resistance. SOX2 expression prompted mesenchymal-to-epithelial transition (MET) by inducing CDH1 (013C P = .002, 067C P = .01), followed by asymmetric division and proliferation, which contributed to tumor formation.

Conclusions: The molecular link between PI3K activation and CSC properties found in this study provides insights into therapeutic strategies for HNSCC. Constitutive expression of SOX2 in HNSCC cells generates a CSC-like population that enables CSC studies.

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