Repression of sphingosine kinase (SK)-interacting protein (SKIP) in acute myeloid leukemia diminishes SK activity and its re-expression restores SK function

Abstract

Previous studies have shown that sphingosine kinase interacting protein (SKIP) inhibits sphingosine kinase (SK) function in fibroblasts. SK phosphorylates sphingosine producing the potent signaling molecule sphingosine-1-phosphate (S1P). SKIP gene (SPHKAP) expression is silenced by hypermethylation of its promoter in acute myeloid leukemia (AML). However, why SKIP activity is silenced in primary AML cells is unclear. Here, we investigated the consequences of SKIP down-regulation in AML primary cells and the effects of SKIP re-expression in leukemic cell lines. Using targeted ultra-HPLC-tandem MS (UPLC-MS/MS), we measured sphingolipids (including S1P and ceramides) in AML and control cells. Primary AML cells had significantly lower SK activity and intracellular S1P concentrations than control cells, and SKIP-transfected leukemia cell lines exhibited increased SK activity. These findings show that SKIP re-expression enhances SK activity in leukemia cells. Furthermore, other bioactive sphingolipids such as ceramide were also down-regulated in primary AML cells. Of note, SKIP re-expression in leukemia cells increased ceramide levels 2-fold, inactivated the key signaling protein extracellular signal-regulated kinase, and increased apoptosis following serum deprivation or chemotherapy. These results indicate that SKIP down-regulation in AML reduces SK activity and ceramide levels, an effect that ultimately inhibits apoptosis in leukemia cells. The findings of our study contrast with previous results indicating that SKIP inhibits SK function in fibroblasts and therefore challenge the notion that SKIP always inhibits SK activity.

Recent advances in the role of sphingosine 1‐phosphate in cancer

“…Primary AML cells have lower levels of SK1 and intracellular S1P and this can be reversed by re-expression of SKIP, concomitant with increased ceramide levels, and reduced ERK and increased apoptosis. Therefore, contrary to previous findings the downregulation of SKIP reduces SK1 activity in AML [116].…”

Exploring the Metabolic Landscape of AML: From Haematopoietic Stem Cells to Myeloblasts and Leukaemic Stem Cells

“…In addition to FAs and related metabolic pathways, sphingolipids also play a critical role in leukemogenesis by regulating the balance between cell proliferation and cell death ( 97 , 98 ). The formation and functionality of sphingolipids rely on oncogenic proteins including sphingosine kinases and acid ceramidases in AML cells.…”

Advancements on the Multifaceted Roles of Sphingolipids in Hematological Malignancies

“…Additionally, in line with potentially enhanced activity of SPHK1 in AML is the reduced expression of SKIP , a SPHK1 inhibitor, in primary AML cells [ 152 ]. However, while SKIP has been shown to inhibit SPHK function in fibroblasts [ 153 , 154 ], a study in K562 Chronic Myelogenous Leukemia cells reported that SKIP acts more as a positive, rather than negative, regulator of SPHK1/S1P [ 155 ]. Moreover, it was also found that primary AML cells showed decreased SPHK1 and SPHK2 activity, as measured by the conversion of d17-Sph into d17-S1P [ 155 ].…”

“…However, while SKIP has been shown to inhibit SPHK function in fibroblasts [ 153 , 154 ], a study in K562 Chronic Myelogenous Leukemia cells reported that SKIP acts more as a positive, rather than negative, regulator of SPHK1/S1P [ 155 ]. Moreover, it was also found that primary AML cells showed decreased SPHK1 and SPHK2 activity, as measured by the conversion of d17-Sph into d17-S1P [ 155 ]. Ultimately, additional studies are needed to conclusively address whether the levels of SPHK1/S1P are different in AML cells, perhaps by stratification of AML types and/or level/types of resistance.…”