Concomitant inhibition of AKT and autophagy is required for efficient cisplatin-induced apoptosis of metastatic skin carcinoma.

Cutaneous squamous cell carcinoma (cSCC) is one of the most common cancers in the Caucasian population. Although early stages of skin cancer have a high curability and excellent prognosis, advanced cSCC shows resistance to chemotherapy, including cisplatin. The PI3-K/AKT pathway is known to have a role in both skin cancer development and resistance to therapeutic drugs. In this study, we used isogenic cell lines representing different stages of malignant transformation of the keratinocytes that were derived from dysplastic forehead skin (PM1), primary cutaneous SCC (MET1) and its lymph node metastasis (MET4) of an immunosuppressed patient. We show that skin tumor progression parallels enhanced AKT activation and increased resistance to cisplatin-induced apoptosis. Pharmacological AKT inhibition, or specific AKT1 knock down, sensitizes the apoptosis-resistant MET1 and, to a lesser extent, MET4 cells to cisplatin-mediated cell death. Concomitantly autophagy induction was observed in MET4, as demonstrated by accumulation of the autophagic protein marker LC3-II, by analysis of full autophagosome maturation process using tandem mRFP-GFP fluorescence microscopy and by electron microscopy. Counteracting the autophagic process by 3-methyladenine or specific ATG5 knock down enhanced cytotoxicity of cisplatin combined with AKT inhibitor, thus revealing a key role for autophagy in chemoresistance. Taken together, these results indicate that concomitant inhibition of autophagy is required to increase the therapeutic benefit of AKT inhibition for combination therapy with the standard chemotherapeutic agent cisplatin in advanced skin carcinoma.