The role of p53 and Bcl -xL in cisplatin resistance in head and neck cancer.

Abstract

Cisplatin-based chemotherapy and radiation for advanced head and neck squamous cell carcinoma (HNSCC) provides an alternative to conventional surgery and radiation. These alternative strategies offer preservation of organ structure and function that improve quality of life. However, tumor cell resistance limits the use of chemotherapy in HNSCC. Wild-type (WT) p53 status and high expression of Bcl-x_L is associated with poor tumor response to cisplatin-based chemotherapy in HNSCC patients. The primary goal of our studies is to elucidate the primary mechanisms of resistance of HNSCC cells to cisplatin. The secondary goal is to overcome resistance with novel therapeutic agent(s) that target these mechanisms. Using an <italic>in vitro</italic> model of cisplatin resistance in HNSCC cells, we discovered the cisplatin-resistant phenotype to be WT p53 and high Bcl-x_L expression. To test the validity of this concept we used transduction studies and HNSCC cells to show that cells with non-functional p53 (mutated or silenced by RNA inhibition) display mitotic damage following cisplatin exposure. This is independent of Bcl-x_L expression and secondary to failure to arrest and repair cisplatin-induced damage. In contrast, WT p53 cells are resistant to cisplatin-induced apoptosis only when Bcl-x_L is overexpressed, in which case the cells undergo p53-dependent cell cycle arrest and DNA repair that protects from mitotic damage. Thus, p53-dependent cell cycle arrest and induction of DNA repair in the presence of high Bcl-x_L expression is sufficient for resistance of HNSCC cells to cisplatin. Additionally, we show that Bcl-x_L is overexpressed in the majority of HNSCC cell lines. Bcl-x_L RNA inhibition induces spontaneous cell death and sensitizes highly resistant HNSCC cells to cisplatin. (-)-Gossypol, a novel small molecule that targets Bcl-x_L, displays selective growth inhibition of HNSCC cells over normal cells. (-)-Gossypol induces high levels of apoptosis in HNSCC cells most resistant to cisplatin, i.e. those that contain WT p53 and high Bcl-x_L. Finally, we show (-)-gossypol acts through the disruption of Bcl-x_L/Bim heterodimerization and induction of the intrinsic mitochondrial apoptosis pathway. These studies have characterized an important mechanism of cisplatin resistance in HNSCC cells and identified strategies to overcome resistance.