Chloroquine blocks the autophagic process in cisplatin-resistant osteosarcoma cells by regulating the expression of p62/SQSTM1

Cisplatin (DDP) is one of the most effective chemotherapeutic drugs against osteosarcoma (OS), the most common malignant bone-specific tumor. However, the acquired resistance to DDP limits its effectiveness in tumor treatment. In this study, in order to elucidate the mechanisms of drug resistance in cancer cells, we investigated cell death induced by DDP in OS cells. We evaluated the contribution of autophagy in the process of drug resistance in a panel of four OS cell lines, MG-63, U-2OS, MNNG/HOS and Saos-2. The cells were treated with DDP (0-50 mu M) for 48 h and then cell vaibility was assessed using the Cell Counting kit-8 (CCK-8). Apoptosis was detected by flow cytometry and the green fluorescent protein (GFP)-microtubule-associated protein 1 light chain 3 (LC3) expression vector was used to visualize the formation of autophagic vesicles. Our results demonstrated that autophagy was induced by DDP in the drug-resistant cell line, Saos-2, which does not respond to DDP with apoptosis. DDP-induced autophagy protected the Saos-2 cells from apoptotic cell death. Moreover, the inhibition of autophagy by chloroquine, an inhibitor of lysosomal proteases, accelerated the DDP-induced cell death in Saos-2 cells. We also found that during DDP treatment, the protein expression level of the autophagic regulator, p62/sequestosome 1 (SQSTM1), decreased during the first hour of treatment, followed by a rapid recovery. Therefore, our data suggest a potential clinical therapy by targeting autophagy with chloroquine or monoclonal antibodies for the treatment of drug-resistant OS.