Pharmacological inhibition of serine palmitoyl transferase and sphingosine kinase-1/-2 inhibits Merkel Cell Carcinoma cell proliferation.

The majority of Merkel cell carcinoma (MCC), a highly aggressive neuroendocrine cancer of the skin, is associated with Merkel cell polyomavirus (MCPyV) infection. Polyomavirus binding, internalization and infection is mediated by glycosphingolipids (GSL). Besides receptor function, bioactive sphingolipids (SL) are increasingly recognized as potent regulators of several hallmarks of cancer. MCPyV and MCPyV cells express serine palmitoyltransferase (SPT) subunits and sphingosine kinase (SK)1/2 mRNA. Induced expression of MCPyV-large tumor antigen in human lung fibroblasts resulted in upregulation of SPTLC1-3 and SK1/2 expression. Therefore we exploited pharmacological inhibition of SL metabolism as an option to interfere with proliferation of MCPyV MCC cell lines. We used myriocin (a SPT antagonist) and two SK inhibitors (SKI-II and ABC294640). In MKL-1 and WaGa cells myriocin decreased cellular ceramide, sphingomyelin, and sphingosine-1-phosphate (S1P) content. SKI-II increased ceramide species but decreased sphingomyelin and S1P concentrations. Aberrant SL homeostasis was associated with reduced cell viability, increased necrosis, procaspase-3 and PARP processing, caspase-3 activity, and decreased AKT phosphorylation. Myriocin and SKI-II decreased tumor size and Ki-67 staining of xenografted MKL-1 and WaGa tumors on the chorioallantoic membrane. Our data suggest that pharmacological inhibition of SL synthesis could represent a potential therapeutic approach in MCC.