Sphingosine Kinase1 Inhibitor SK178 Preferentially Hinders Growth of KIT D816V Mast Cells In Vitro and In Vivo.

The lipid mediator sphingosine-1-phosphate (S1P) promotes cell proliferation and prevents apoptosis in many cell types and is linked to oncogenesis. Mastocytosis is characterized by abnormal mast cell (MC) proliferation and accumulation in tissues. Therapies required to treat aggressive forms of mastocytosis have been unsuccessful, thus exploration of pharmacological targets is desirable. We studied the involvement of the sphingosine kinase isoforms (SphK1 and SphK2) and generation of S1P in the regulation of neoplastic MC growth. Growth of human neoplastic MCs (LAD2, HMC1.1, HMC1.2) and murine P815 treated with inhibitors for SphK1 and SphK2 (SKI-178 and ABC294640, respectively) was followed by proliferation assays and cell cycle analysis and apoptosis by FACS. RT2 Profiler PCR cell cycle array and immunoblotting were used to explore the mechanisms of action. A mouse model of aggressive mastocytosis was used to investigate the effectiveness of these inhibitors in vivo. While SphK2 inhibition resulted in cell cycle arrest in normal and neoplastic MCs with minimal effect on cell survival, SphK1 inhibition caused mitotic catastrophe and apoptosis particularly in aggressive MCs with D816V-KIT mutations. This was caused by an alteration in the damage response cascade, namely activation of checkpoint kinase 2 (chk2) combined with a depletion of chk1, leading respectively to p53- and caspase2-induced apoptosis and cell cycle arrest in G2/M. SphK inhibitors also reduced MC accumulation in a mouse model of aggressive mastocytosis. Our results support the use of SphK inhibitors as a possible treatment option for mastocytosis and possibly other hematological malignancies associated with the KIT-D816V mutation.