Sphingosine Kinase Activity Is Not Required for Tumor Cell Viability

Abstract

Sphingosine kinases (SPHKs) are enzymes that phosphorylate the lipid sphingosine, leading to the formation of sphingosine-1-phosphate (S1P). In addition to the well established role of extracellular S1P as a mitogen and potent chemoattractant, SPHK activity has been postulated to be an important intracellular regulator of apoptosis. According to the proposed rheostat theory, SPHK activity shifts the intracellular balance from the pro-apoptotic sphingolipids ceramide and sphingosine to the mitogenic S1P, thereby determining the susceptibility of a cell to apoptotic stress. Despite numerous publications with supporting evidence, a clear experimental confirmation of the impact of this mechanism on tumor cell viability in vitro and in vivo has been hampered by the lack of suitable tool reagents. Utilizing a structure based design approach, we developed potent and specific SPHK1/2 inhibitors. These compounds completely inhibited intracellular S1P production in human cells and attenuated vascular permeability in mice, but did not lead to reduced tumor cell growth in vitro or in vivo. In addition, siRNA experiments targeting either SPHK1 or SPHK2 in a large panel of cell lines failed to demonstrate any statistically significant effects on cell viability. These results show that the SPHK rheostat does not play a major role in tumor cell viability, and that SPHKs might not be attractive targets for pharmacological intervention in the area of oncology.

A Metabolic Shift Favoring Sphingosine 1-Phosphate at the Expense of Ceramide Controls Glioblastoma Angiogenesis

“…both genetic ablation and SPHK inhibitors with micromolar inhibitory constants has described a role for SPHK1 in cancer cell proliferation and resistance to radiotherapy and chemotherapeutics (19,(51)(52)(53). In agreement with the results published by Kharel et al (36), Schnute et al (37), and Rex et al (38), we found that specific pharmacological inhibition of bulk S1P production by SPHK1 does not affect cancer cell proliferation and therapeutic resistance. However, SKI-1a did inhibit the angiogenic response of cocultured endothelial cells, in agreement with recent results showing an antiangiogenic effect of another SPHK1 inhibitor (41) and an inhibitory effect of SPHK1 silencing in cancer cells on the alignment of cocultured endothelial cells into tube-like structures in the two-dimensional Matrigel angiogenesis model (54).…”

Sphingosine Kinase 2 Inhibition and Blood Sphingosine 1-Phosphate Levels

“…In contrast, at the rat enzymes, SLC5111312 is a nonselective inhibitor. We and others have observed a similar differential in potency between mouse and human SphK1 over a variety of chemical scaffolds (Schnute et al, 2012;Rex et al, 2013;Zhang et al, 2014;Patwardhan et al, 2015). Importantly, SLC5111312 provides a tool that is simultaneously a SphK2 selective (mouse) and dual SphK (rat and human) inhibitor.…”

Emerging biology of sphingosine-1-phosphate: its role in pathogenesis and therapy

“…Earlier studies with genetic models of sphingosine kinase overexpression (107) and lower potency sphingosine kinase inhibitors have suggested a potentially important role for SPHKs in tumor cell proliferation and survival (105). However, results with novel, highly potent, and selective inhibitors that reduce S1P to undetectable levels in tumor cells did not affect their growth in vitro or in vivo, suggesting that tumor sphingosine kinases may not be efficacious therapeutic targets for cancer (108)(109)(110).…”