Gleevec suppresses p63 expression in head and neck squamous cell carcinoma despite p63 activation by DNA-damaging agents

Objectives: The objectives of this study were to determine the effects of Gleevec on p63 expression in head and neck squamous cell carcinoma (HNSCC) cell lines and to investigate the role of Gleevec in regulating p63 stabilization under DNA-damaging conditions. Methods: Immunohistochemical staining was performed to determine p63 expression in HNSCC tissue. Annexin V staining was used to assess the effects of p63 on early apoptosis. Immunoblotting was used to examine the effects of Gleevec on p63 protein levels in HNSCC cell lines in response to DNA damage. Immunofluorescence staining was performed to study the expression pattern of p63 and c-Abl. Results: In HNSCC, p63 protein levels are induced by DNA-damaging agents, including ionizing radiation, doxorubicin, and ultraviolet light. We demonstrate that Gleevec reduces p63/Delta Np63 expression in a dose-dependent manner in HNSCC and overrides the protein induction by DNA-damaging agents. Overexpression of c-Abl in the absence of Gleevec results in higher levels of p63 than those treated with Gleevec, implicating c-Abl kinase activity as a regulator of p63 protein stability. Conclusions: Gleevec downregulates p63/Delta Np63 levels in HNSCC in a dose-dependent manner under both normal and DNA-damaging conditions. This downregulation. can be explained by Gleevec's inhibition of c-Abl, which destabilizes p63. Based on our data, treating cancers with high expression of TAp63 with Gleevec may result in the unfavorable inhibition of a tumor suppressor, whereas downregulation of Delta Np63 would be advantageous. Further development of antibodies that can discriminate between TAp63 and Delta Np63 will be needed to determine the specific effects of Gleevec on p63 in HNSCC.