Hypoxia Enhances Sphingosine Kinase 2 Activity and Provokes Sphingosine-1-Phosphate-Mediated Chemoresistance in A549 Lung Cancer Cells

Abstract

Hypoxia and signaling via hypoxia-inducible factor-1 (HIF-1) is a key feature of solid tumors and is related to tumor progression as well as treatment failure. Although it is generally accepted that HIF-1 provokes tumor cell survival and induces chemoresistance under hypoxia, HIF-1-independent mechanisms operate as well. We present evidence that conditioned medium obtained from A549 cells, incubated for 24 h under hypoxia, protected naive A549 cells from etoposide-induced cell death. Lipid extracts generated from hypoxiaconditioned medium still rescued cells from apoptosis induced by etoposide. Specifically, the bioactive lipid sphingosine-1-phosphate (S1P) not only was essential for cell viability of A549 cells but also protected cells from apoptosis. We noticed an increase in sphingosine kinase 2 (SphK2) protein level and enzymatic activity under hypoxia, which correlated with the release of S1P into the medium. Knockdown of SphK2 using specific small interfering RNA relieved chemoresistance of A549 cells under hypoxia and conditioned medium obtained from SphK2 knockdown cells was only partially protective. Coincubations of conditioned medium with VPC23019, a S1P 1 /S1P 3 antagonist, reduced protection of conditioned medium, with the further notion that p42/ 44 mitogen-activated protein kinase transmits autocrine or paracrine survival signaling downstream of S1P 1 /S1P 3 receptors. Our data suggest that hypoxia activates SphK2 to promote the synthesis and release of S1P, which in turn binds to S1P 1 /S1P 3 receptors, thus activating p42/44 mitogen-activated protein kinase to convey autocrine or paracrine protection of A549 cells.

Junctional Protein Regulation by Sphingosine Kinase 2 Contributes to Blood–Brain Barrier Protection in Hypoxic Preconditioning-Induced Cerebral Ischemic Tolerance

“…Early reports from non-central nervous system tissues on the promotion of cell survival suggested S1P produced by SphK1 in the cytosol was antiapoptotic, while SphK2, due to targeting S1P to the endoplasmic reticulum, promoted apoptosis (Maceyka et al, 2005). However, evidence has since accumulated for a prosurvival role for SphK2 in some cell types (Schnitzer et al, 2009;Vessey et al, 2011), in agreement with both our previous pharmacologic data and our current genetic evidence for SphK2-mediated cytoprotection in the ischemic brain. With respect to elucidating the molecular basis of preconditioninginduced tolerance, our group documented increased cerebromicrovascular SphK2 protein expression and activity following HPC, attenuated HPC-induced ischemic tolerance with inhibition of both SphK isoforms, and efficacious pharmacological precondi-tioning by the S1P receptor agonist FTY720 (Wacker et al, 2009).…”

Regulation of cancer cell migration and invasion by sphingosine-1-phosphate

“…Ultimately, upregulation of SphK1 under hypoxic conditions could lead to induction of angiogenesis and enhanced survival of tumor cells under stressful conditions. In agreement with this idea, hypoxia induces SphK2 expression in A549 lung cancer cells, which leads to S1P release and resistance to chemotherapeutics [60] . Given the above evidence that S1P release from cancer cells can modulate tumor cell migration and invasiveness, it is tempting to speculate that SphK1 activation/induction and S1P release in response to hypoxia can also promote tumor cell invasion.…”

Prolonged Exposure to Sphingosine 1–Phosphate Receptor-1 Agonists Exacerbates Vascular Leak, Fibrosis, and Mortality after Lung Injury

“…Nevertheless, we cannot exclude the possibility that the increased overall inflammatory response or reduction in BAL T cells we observed with prolonged S1P 1 agonist treatment may have contributed to the exaggerated fibrotic response to lowdose bleomycin. S1P can also mediate other processes that may contribute to the development of lung fibrosis, including cellular apoptosis, fibroblast chemotaxis, angiogenesis, and TGF-b signaling (7,(67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77)(78). Therefore, further elucidation of the effects of S1P 1 agonists on these biological processes will be of interest for a better understanding of how S1P-S1P 1 signaling modifies the fibrotic response to injury.…”