Renal mesangial cells: moving on sphingosine kinase-1

Abstract

Sphingosine kinase-1 (SphK1) catalyses the phosphorylation of sphingosine to sphingosine-1-phosphate (S1P), which acts on at least five specific G-protein-coupled receptors and also intracellularly. SphK1 has been implicated in cell proliferation, cancer growth, chemoresistance, immune cell functions and cell migration. In this issue of the British Journal of Pharmacology, Klawitter et al. demonstrate that extracellular nucleotides stimulate the migration of renal mesangial cells. The nucleotides furthermore upregulated SphK1 expression and activity, and this enzyme was required for nucleotide-induced migration. Together with previous findings, these data raise exciting questions: by which mechanism does SphK1 regulate migration in mesangial cells, how is the interplay of purinoceptors and S1P receptors organized in these cells, and how would SphK1-deficient mice respond to kidney damage?

In renal mesangial cells, extracellular nucleotides acting on purine P2Y receptors stimulate several signalling pathways, thereby promoting proliferation (via p42/44 mitogen-activated protein kinase (MAPK)), stimulating prostaglandin release (via phospholipase A₂) and protecting from apoptosis (via Akt) (see Huwiler et al., 2002; Klawitter et al., 2007 and citations therein). In this issue of the British Journal of Pharmacology, Huwiler's group (Klawitter et al., 2007) demonstrates that extracellular adenosine triphosphate and uridine triphosphate stimulate the migration of renal mesangial cells, and they suggest that this might be an important event in the repair mechanism during inflammatory kidney diseases. Furthermore, the nucleotides upregulated expression and activity of sphingosine kinase-1 (SphK1), and this enzyme was essential for nucleotide-induced mesangial cell migration (Klawitter et al., 2007).

Sphingosine kinases catalyse the phosphorylation of sphingosine to sphingosine-1-phosphate (S1P), which is an ubiquitous lipid mediator, regulating cell growth and survival, migration and adhesion (for reviews, see Spiegel and Milstien, 2003; Sanchez and Hla, 2004; Chalfant and Spiegel, 2005). S1P acts on at least five specific G-protein-coupled receptors (S1P_1–5) and also intracellularly. Recently, the role of S1P in lymphocyte trafficking, angiogenesis and vascular permeability has gained particular attention as the novel immunosuppressant, FTY720, modulates these conditions by interfering with S1P receptors (Brinkmann et al., 2004; LaMontagne et al., 2006). Two sphingosine kinases, SphK1 and SphK2, and transcriptional variants thereof, have been identified in mammals so far. SphK1 and SphK2 are differentially expressed and appear to have opposing cellular functions, with SphK2 acting preferentially as a pro-apoptotic agent, whereas SphK1 has been implicated in cell proliferation, cancer growth, chemoresistance, immune cell functions and cell migration (for reviews, see Spiegel and Milstien, 2003; Taha et al., 2006).

The formation of S1P by sphingosine kinases is a highly regulated process. Both transcriptional and post-transcriptional regulation of SphK1 activity and/or protein have been reported in several cellular systems (for reviews, see Spiegel and Milstien, 2003; Taha et al., 2006). Acute stimulation of sphingosine kinase activity by P2Y₂ receptors, leading to S1P formation within a timescale of minutes, has been shown before (Alemany et al., 2000). Klawitter et al. now demonstrate a specific activation of SphK1, but not SphK2, by purine receptors in mesangial cells. Activation of the enzyme was biphasic, with a rapid phase, most likely during to post-transcriptional activation, and a delayed phase of transcriptional upregulation of SphK1 within hours, which was mediated by protein kinase C and MAPK. This was similar, for example, to epidermal growth factor-stimulated SphK1 expression in human breast cancer cells (Döll et al., 2005) and histamine- and hepatocyte growth factor-stimulated SphK1 expression in endothelial cells (Duan et al., 2004; Huwiler et al., 2006). SphK1 was essential for nucleotide-stimulated migration. This was demonstrated using (1) short interfering RNA (siRNA) to SphK1 and (2) mesangial cells from SphK1-deficient mice. Interestingly, mice deficient in SphK1 were viable and fertile with no obvious dysfunctions (Allende et al., 2004). Therefore, considering the present data, it would be important to study the response to renal damage in these mice, for example by streptozotocin treatment, which in a recent study was found to cause upregulation of sphingosine kinase activity and increased S1P levels within renal glomeruli (Geoffroy et al., 2005).

Another interesting observation was that deletion of SphK1 did not affect nucleotide-induced proliferation of mesangial cells. As discussed by the authors, mesangial cells differ in this regard from many other cell types in which SphK1 is essential for cell growth. SphK1 has furthermore been implicated in induction of cyclooxygenase-2 (reviewed in Taha et al., 2006); therefore, it would be interesting to determine whether purinoceptor-stimulated prostaglandin production is affected by SphK1 deletion.

The differential cellular response to SphK1 deletion raises the intriguing question, how does SphK1 mediate migration of mesangial cells in response to extracellular nucleotides? In several cellular systems, activation of SphK1 leads to autocrine S1P release and activation of G-protein-coupled S1P receptors. This was shown, for example, in mast cells, in which crosslinking of Fcɛ receptor-I stimulated SphK1 activity, S1P secretion and internalization of S1P₁ and S1P₂ receptors, with S1P₁ mediating migration and S1P₂ degranulation of the cells in response to antigen (Jolly et al., 2004). SphK1-siRNA had no effect on S1P-induced chemotaxis in this study. In PC12 cells and dorsal root ganglion cells, SphK1 was required for nerve growth factor-induced neurite extension, and SphK1 plasma membrane translocation and S1P₁ cross-activation were involved in this process (Toman et al., 2004). Remarkably, Klawitter et al. also showed that deletion of SphK1 abrogated cell migration induced by extracellular S1P. This highly interesting observation suggests that either the SphK1 protein or its catalytic product, S1P or a shift in the ceramide/sphingosine/S1P-balance acted intracellularly for signalling migration. In this scenario, extracellular S1P would then activate and induce SphK1. Indeed, G-protein-coupled S1P receptors can stimulate sphingosine kinase activity (Meyer zu Heringdorf et al., 2001). However, intracellular targets of S1P remain to be identified.

It has been found that G-protein-coupled S1P receptors differentially regulate migration, with S1P₁ and S1P₃ stimulating and S1P₂ inhibiting this response (Sanchez and Hla, 2004). Therefore, assuming that S1P inside-out signalling is involved in SphK1-dependent migration of mesangial cells, it is possible for the cells to modulate their response to SphK1 upregulation in a specific manner by up- and downregulating S1P receptors. Mesangial cells express the S1P receptors, S1P₁, S1P₂ and S1P₃ (Katsuma et al., 2002), which mediate increases in intracellular Ca²⁺ concentrations, activation of MAPK, p38, JNK and Akt kinases, cross-activation of transforming growth factor-β receptor and activation of the Smad signalling cascade (see Xin et al. (2004a) and citations therein). Furthermore, S1P stimulated proliferation (via MAPK activation), inhibited apoptosis (Katsuma et al., 2002) and induced migration (Klawitter et al., 2007) in these cells. Interestingly, purinoceptors are known to interfere with S1P receptor signalling in mesangial cells, causing a heterologous desensitization of S1P-stimulated MAPK activation (Xin et al., 2004b). As SphK1 deletion had no effect on purinoceptor-stimulated proliferation, it might be speculated that the heterologous desensitization serves to shut off certain S1P-receptor signalling pathways that otherwise would be activated by inside-out signalling, including MAPK activation and eventually inhibition of migration by S1P₂, thereby facilitating other pathways such as pro-migratory signalling by S1P₁ and/or S1P₃. Taken together, the data presented by Klawitter et al. demonstrate differential, cell type-specific signalling by SphK1. Together with previous findings, these data raise challenging questions regarding the mechanism of SphK1-mediated migration and the interplay of purine and G-protein-coupled S1P receptors in mesangial cells.

Abbreviations

MAPK

mitogen-activated protein kinase

S1P

sphingosine-1-phosphate

SphK

sphingosine kinase