Merkel cell carcinoma (MCC) is a rare and highly aggressive skin cancer with frequent viral etiology. Indeed, in about 80% of cases, there is an association with Merkel cell polyomavirus (MCPyV); the expression of viral T antigens is crucial for growth of virus-positive tumor cells. Since artesunate-a drug used to treat malaria-has been reported to possess additional anti-tumor as well as anti-viral activity, we sought to evaluate pre-clinically the effect of artesunate on MCC. We found that artesunate repressed growth and survival of MCPyV-positive MCC cells in vitro. This effect was accompanied by reduced large T antigen (LT) expression. Notably, however, it was even more efficient than shRNA-mediated downregulation of LT expression. Interestingly, in one MCC cell line (WaGa), T antigen knockdown rendered cells less sensitive to artesunate, while for two other MCC cell lines, we could not substantiate such a relation. Mechanistically, artesunate predominantly induces ferroptosis in MCPyV-positive MCC cells since known ferroptosis-inhibitors like DFO, BAF-A1, Fer-1 and β-mercaptoethanol reduced artesunate-induced death. Finally, application of artesunate in xenotransplanted mice demonstrated that growth of established MCC tumors can be significantly suppressed in vivo. In conclusion, our results revealed a highly anti-proliferative effect of the approved and generally well-tolerated anti-malaria compound artesunate on MCPyV-positive MCC cells, suggesting its potential usage for MCC therapy.Cancers 2020, 12, 919 2 of 15 cells [6], and subsequent studies confirmed that approximately 80% of all MCC cases are associated with MCPyV [7]. Importantly, the integration patterns suggest that clonal expansion of the tumor cells occurs after MCPyV integration sustaining the assumption that viral proteins are causal for tumorigenesis [6,8,9]. Moreover, in MCPyV-positive MCC cells, expression of the viral oncoproteins small and Large T-antigen (sT and LT) can be detected, and these proteins are essential for growth of the tumor cells [10,11] qualifying them as potential therapeutic targets.The five-year overall survival rate for patients with MCC is only about 40%, although the relative survival rate (compared to an age-and sex-matched population) is 54% [12]. Primary MCCs are excised by surgery, and adjuvant radiotherapy of the primary tumor location and the lymph node region is recommended [13]. Until recently, the metastatic disease was treated preferentially with various, not-standardized chemotherapeutic regimens, all of which could not improve survival of the patients significantly [14]. Recently, however, antibodies targeting the immune suppressive protein programmed cell death protein 1 (PD-1) or its ligand PD-L1 have demonstrated high response rates of 56 in first-line and 32% in second-line treatment, respectively for patients with stage IV disease [15,16]. Indeed, the PD-L1 targeting antibody Avelumab was the first treatment for metastatic MCC approved both in the US and European Union [17]. Importantly, data avail...
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