17-(Allylamino)-17-demethoxygeldanamycin Enhances Etoposide-Induced Cytotoxicity Via the Downregulation of Xeroderma Pigmentosum Complementation Group C Expression in Human Lung Squamous Cell Carcinoma Cells

Etoposide (VP16) is a topoisomerase II inhibitor and has been used for the treatment of non-small cell lung cancer (NSCLC). Xeroderma pigmentosum complementation group C (XPC) protein is a DNA damage recognition factor in nucleotide excision repair and involved in regulating NSCLC cell proliferation and viability. Heat shock protein 90 (Hsp90) is a ubiquitous molecular chaperone that is responsible for the stabilization and maturation of many oncogenic proteins. In this study, we report whether Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) enhanced etoposide-induced cytotoxicity in NSCLC cells through modulating the XPC expression. We found that etoposide increased XPC expression in an AKT activation manner in 2 squamous cell carcinoma H1703 and H520 cells. Knockdown of XPC using siRNA or inactivation of AKT by pharmacological inhibitor PI3K inhibitor (LY294002) enhanced the cytotoxic effects of etoposide. In contrast, enforced expression of XPC cDNA or AKT-CA (a constitutively active form of AKT) reduced the cytotoxicity and cell growth inhibition of etoposide. Hsp90 inhibitor 17-AAG enhanced cytotoxicity and cell growth inhibition of etoposide in NSCLC cells, which were associated with the downregulation of XPC expression and inactivation of AKT. Our findings suggested that the Hsp90 inhibition induced XPC downregulation involved in enhancing the etoposide-induced cytotoxicity in H1703 and H520 cells.