Abstract
TP53 mutation is one of the most frequent gene mutations in head and neck squamous cell carcinoma (HNSCC) and could be a potential therapeutic target. Recently, the WEE1 G2 checkpoint kinase (WEE1) inhibitor adavosertib (Adv) has attracted attention because of its selective cytotoxicity against TP53‑mutated cells and has shown promising activity in early phase clinical trials. In the present study, it was demonstrated that combined treatment with Adv and a selective histone deacetylase 6 (HDAC6) inhibitor, ricolinostat (RCS), synergistically enhanced cell death induction in four out of five HNSCC cell lines with TP53 mutation (CAL27, SAS, HSC‑3, and OSC‑19), one HNSCC cell line with impaired TP53 function by HPV‑infection (UPCI‑SCC154), and TP53‑knockout human lung cancer cell line (A549 TP53‑KO), but not in TP53 wild‑type A549 cells. Time‑lapse imaging showed that RCS enhanced the Adv‑induced mitotic catastrophe. Consistent with this, RCS treatment suppressed checkpoint kinase 1 (Chk1) (Ser345) phosphorylation and co‑administration of RCS with Adv suppressed cyclin‑dependent kinase 1 (Tyr15) phosphorylation along with increased expression of γ‑H2A.X, a marker of DNA double‑strand breaks in CAL27 cells. These data showed that RCS enhanced Adv‑induced premature mitotic entry and cell death induction in the mitotic phase. However, although HDAC6 knockdown enhanced Adv‑induced cell death with γ‑H2A.X elevation, HDAC6 knockdown did not repress Chk1 phosphorylation in CAL27 cells. Our data demonstrated that the co‑administration of RCS with Adv in HNSCC cells resulted in the suppression of Chk1 activity, leading to synergistically enhanced apoptosis via mitotic catastrophe in a p53‑dependent manner. This enhanced cell death appeared to be partially mediated by the inhibition of HDAC6 activity by RCS.