Abstract:
Introduction: The PI3K pathway is frequently dysregulated in various cancers including head and neck squamous cell carcinoma (HNSCC). Tumour hypoxia is another prominent feature of HNSCC. A number of PI3K inhibitors have been designed to target the dysregulated PI3K pathway but the impact of hypoxia on PI3K signalling and the anti-proliferative activity of PI3K inhibitors is not clearly understood. This study is directed at testing the hypothesis that tumour hypoxia can alter the activity of PI3K inhibitors by increasing the activity of the PI3K pathway. Methods: Eight PI3K inhibitors were tested in 7 HNSCC cell lines. The anti-proliferative activity of the compounds was assessed by sulforhodamine B (SRB) cell proliferation assay under normoxic and hypoxic (0.2% and 2% O2) conditions. The inhibition of phosphorylated AKT (pAKT) expression by PI3K inhibitors, and pAKT expression under hypoxia was determined through Western blotting. Results: The pan PI3K inhibitors were more potent than isoform selective inhibitors in inhibiting cell proliferation and pAKT expression in all the cell lines. Hypoxia had a variable effect on the pAKT expression. Some cell lines showed a significant increase, some showed a significant decrease, while others showed a small rise and fall in pAKT expression under 0.2% and 2% O2 over a 24 h period. Overall, an increase in pAKT expression appeared to be more common under 0.2% O2, while under 2% O2 pAKT expression seemed to be more commonly decreased. Hypoxia had little effect on the anti-proliferative activity of the PI3K inhibitors, though some differences were observed with BEZ235. The anti-proliferative activity of BEZ235 appeared to decrease in many cell lines under both 0.2% and 2% O2 yet was significant for 110B only. Conclusion: Hypoxia had a variable impact on the activity of PI3K signalling and did not alter the anti-proliferative activity of PI3K inhibitors to much extent. Hence, the hypothesis that tumour hypoxia can alter the activity of PI3K inhibitors by increasing the activity of the PI3K pathway was not proved from this study.