ERK-TSC2 signalling in constitutively-active HRAS mutant HNSCC cells promotes resistance to PI3K inhibition
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Objectives: The PI3K/AKT/mTOR pathway is frequently altered in head and neck squamous cell cancer (HNSCC), making this pathway a logical therapeutic target. However, PI3K targeting is not universally effective. Biomarkers of response are needed to stratify patients likely to derive benefit and exclude those unlikely to respond. Materials and methods: We examined the sensitivity of cell lines with constitutively-active (G12V mutant) HRAS and wild-type HRAS to PI3K inhibition using flow cytometry and cell viability assays. We then overexpressed and silenced HRAS and measured sensitivity to the PI3K inhibitor BYL719. Immunoblotting was used to determine activation of the PI3K pathway. MEK and mTOR inhibitors were then tested in HRAS mutant cells to determine their efficacy. Results: HRAS mutant cell lines were non-responsive to PI3K inhibition. Overexpression of HRAS led to reduced susceptibility to PI3K inhibition, while knockdown improved sensitivity. Immunoblotting revealed suppressed AKT phosphorylation upon PI3K inhibition in both wild-type and HRAS mutant cell lines, however mutant lines maintained phosphorylation of S6, downstream of mTOR. Targeting mTOR effectively reduced viability of HRAS mutant cells and we subsequently examined the ERK-TSC2-mTOR cascade as a mediator of resistance to PI3K inhibition. Conclusions: HRAS mutant cells are resistant to PI3K inhibition and our findings suggest the involvement of a signalling intersection of the MAPK and PI3K pathways at the level of ERK-TSC2, leading to persistent mTOR activity. mTOR inhibition alone or in combination with MAPK pathway inhibition may be a promising therapeutic strategy for this subset of HNSCC tumors.