Upregulation of sphingosine kinase 1 in response to doxorubicin generates an angiogenic response via stabilization of Snail.

Sphingosine kinase 1 (SK1) converts the pro-death lipid sphingosine to the pro-survival sphingosine-1-phosphate (S1P) and is upregulated in several cancers. DNA damaging agents, such as the chemotherapeutic doxorubicin (Dox), have been shown to degrade SK1 protein in cancer cells, a process dependent on wild-type p53. As mutations in p53 are very common across several types of cancer, we evaluated the effects of Dox on SK1 in p53 mutant cancer cells. In the p53 mutant breast cancer cell line MDA-MB-231, we show that Dox treatment significantly increases SK1 protein and S1P. Using MDA-MB-231 cells with CRISPR-mediated knockout of SK1 or the selective SK1 inhibitor PF-543, we implicated SK1 in both Dox-induced migration and in a newly uncovered proangiogenic program induced by Dox. Mechanistically, inhibition of SK1 suppressed the induction of the cytokine BMP4 and of the EMT transcription factor Snail in response to Dox. Interestingly, induction of BMP4 by SK1 increased Snail levels following Dox treatment by stabilizing Snail protein. Furthermore, we found that SK1 was required for Dox-induced p38 MAP kinase phosphorylation and that active p38 MAPK in turn upregulated BMP4 and Snail, positioning p38 downstream of SK1 and upstream of BMP4/Snail. Modulating production of S1P by inhibition of de novo sphingolipid synthesis or knockdown of the S1P-degrading enzyme S1P lyase identified S1P as the sphingolipid activator of p38 in this model. This work establishes a novel angiogenic pathway in response to a commonly utilized chemotherapeutic and highlights the potential of SK1 as a secondary drug target for patients with p53 mutant cancer.

Bioactive sphingolipids: Advancements and contributions from the laboratory of Dr. Lina M. Obeid.

Sphingolipids and their synthetic enzymes have emerged as critical mediators in numerous diseases including inflammation, aging, and cancer. One enzyme in particular, sphingosine kinase (SK) and its product sphingosine-1-phosphate (S1P), has been extensively implicated in these processes. SK catalyzes the phosphorylation of sphingosine to S1P and exists as two isoforms, SK1 and SK2. In this review, we will discuss the contributions from the laboratory of Dr. Lina M. Obeid that have defined the roles for several bioactive sphingolipids in signaling and disease with an emphasis on her work defining SK1 in cellular fates and pathobiologies including proliferation, senescence, apoptosis, and inflammation.

Transcriptional Regulation of Sphingosine Kinase 1.

Once thought to be primarily structural in nature, sphingolipids have become increasingly appreciated as second messengers in a wide array of signaling pathways. Sphingosine kinase 1, or SK1, is one of two sphingosine kinases that phosphorylate sphingosine into sphingosine-1-phosphate (S1P). S1P is generally pro-inflammatory, pro-angiogenic, immunomodulatory, and pro-survival; therefore, high SK1 expression and activity have been associated with certain inflammatory diseases and cancer. It is thus important to develop an understanding of the regulation of SK1 expression and activity. In this review, we explore the current literature on SK1 transcriptional regulation, illustrating a complex system of transcription factors, cytokines, and even micro-RNAs (miRNAs) on the post transcriptional level.

A role for caspase-2 in sphingosine kinase 1 proteolysis in response to doxorubicin in breast cancer cells - implications for the CHK1-suppressed pathway.

Sphingosine kinase 1 (SK1) is a lipid kinase whose activity produces sphingosine 1-phosphate, a prosurvival lipid associated with proliferation, angiogenesis, and invasion. SK1 overexpression has been observed in numerous cancers. Recent studies have demonstrated that SK1 proteolysis occurs downstream of the tumor suppressor p53 in response to several DNA-damaging agents. Moreover, loss of SK1 in p53-knockout mice resulted in complete protection from thymic lymphoma, providing evidence that regulation of SK1 constitutes a major tumor suppressor function of p53. Given this profound phenotype, this study aimed to investigate the mechanism by which wild-type p53 regulates proteolysis of SK1 in response to the DNA-damaging agent doxorubicin in breast cancer cells. We find that p53-mediated activation of caspase-2 was required for SK1 proteolysis and that caspase-2 activity significantly alters the levels of endogenous sphingolipids. As p53 is mutated in 50% of all cancers, we extended our studies to investigate whether SK1 is deregulated in the context of triple-negative breast cancer cells (TNBC) harboring a mutation in p53. Indeed, caspase-2 was not activated in these cells and SK1 was not degraded. Moreover, caspase-2 activation was recently shown to be downstream of the CHK1-suppressed pathway in p53-mutant cells, whereby inhibition of the cell cycle kinase CHK1 leads to caspase-2 activation and apoptosis. Indeed, knockdown and inhibition of CHK1 led to the loss of SK1 in p53-mutant TNBC cells, providing evidence that SK1 may be the first identified effector of the CHK1-suppressed pathway.