Proteasome Inhibitors Counteract the Effect of Cisplatin in HPV-Positive Squamous Cell Carcinoma in Vitro

Background A rapid increase in human papilloma virus (HPV)-positive oropharyngeal squamous cell carcinoma (OPSCC) is a global trend. Although HPV-positive patients have a more favorable prognosis, distant metastases occur, warranting new, systemic treatment options. The aim of this study was to investigate the effect of combining proteasome or MDM2 inhibitors with cisplatin on an HPV-positive oropharyngeal squamous cell carcinoma cell line (LU-HNSCC-26).Methods The LU-HNSCC-26 cells were treated with proteasome inhibitor (bortezomib, carfilzomib or ixazomib) or MDM2 inhibitor (RG7112) in combination with cisplatin. Combinatorial effects were analyzed by isobolograms. Protein expression was investigated by Western blotting and cell cycle phase distribution by flow cytometry. Results There was no synergy between the substances and cisplatin. All proteasome inhibitors displayed antagonistic effects while the MDM2 inhibitor was additive in combination with cisplatin. The expression of p53 was only marginally affected and apoptosis was not detected. The cell cycle progression was halted in G0/G1 with all inhibitors and in S phase with cisplatin. The expression of p21 increased by bortezomib or carfilzomib, ixazomib increased p21 in combination with cisplatin while RG7112 did not affect p21. There was no effect on ERCC1 with any of the substances.Conclusions In the investigated HPV16-positive OPSCC cell line, proteasome inhibition decreased the effect of cisplatin. A possible mechanism for this includes low effects on p53 expression with concomitant increase in p21 expression and blocking of cell cycle progression in G0/G1 with preserved DNA damage repair. The combination of proteasome inhibition with ordinary cytotoxic treatment for HPV-positive OPSCC patients is thus questionable, and clinical trials should be preceded by thorough testing in adequate models.