Activation of IL-6 ERK c-MYC signaling confers resistance to NVP-BKM120, a pan-PI3K inhibitor, in head and neck squamous cell carcinoma

Abstract

Although, the phosphatidylinositol 3-kinase (PI3K) signaling axis is the most frequentlyderegulated pathways in head and neck squamous cell carcinoma (HNSCC), therapeuticstrategies targeting this pathway have been underwhelming to date. The purpose of thisstudy is to elucidate potential mechanisms of resistance to PI3K inhibitor in HNSCC. Theinhibitory effect of NVP-BKM120, a pan PI3K inhibitor, on cellular growth in 10 humanHNSCC cell lines was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) assay and colony formation. Functional and mechanistic studies wereanalyzed by means of ELISA, RT-PCR, and western blot in CAL27 and SCC15 cells amongHNSCC cell lines. Treatment of NVP-BKM120 showed the limited anti-proliferative effect with IC50s in 0.5 to 1.5 micromole ranges against 10 HNSCC cell lines. In both CAL27 andSCC15 cells, the activation of PI3K downstream signaling pathways includingphosphorylated-AKT and S6K was initially blocked, but the phosphorylation of AKT andS6K was restored after 24h of NVP-BKM120 treatment. Moreover, NVP-BKM120significantly induced c-MYC expression, ERK activation, and IL-6 secretion. Theknockdown of c-MYC with siRNA transfection in two HNSCC cell lines led to theincreased sensitivity to NVP-BKM120. Furthermore, IL-6 receptor neutralizing antibodycompletely abolished NVP-BKM120-induced c-MYC expression by down-regulating ERKactivation, whereas inhibition of c-MYC and ERK failed to block the increased levels of IL-6 secretion. Collectively, these results suggest that IL-6/ERK/c-MYC axis contributing tointrinsic and adaptive resistance to NVP-BKM120 may ultimately limit its efficiency ofNVP-BKM120 in HNSCC cell lines. Our preclinical study provides a rationale forcombination therapy of IL-6/ERK/c-MYC inhibition and NVP-BKM120.