PPARγ agonists upregulate sphingosine 1-phosphate (S1P) receptor 1 expression, which in turn reduces SIP-induced [Ca2+]i increases in renal mesangial cells

We previously identified peroxisome proliferator-activated receptor gamma (PPAR gamma) agonists (thiazolidinediones, TZDs) as modulators of the sphingolipid metabolism in renal mesangial cells. TZDs upregulated sphingosine kinase 1 (SK-1) and increased the formation of intracellular sphingosine 1-phosphate (SIP), which in turn reduced the expression of pro-fibrotic connective tissue growth factor. Since SW also acts as extracellular ligand at specific SIP receptors (S1PR, S1P(1-5)), we investigated here the effect of TZDs on S1PR expression in mesangial cells and evaluated the functional consequences by measuring SIP-induced increases in intracellular free Ca2+ concentration ([Ca2+](i). Treatment with two different TZDs, troglitazone and rosiglitazone, enhanced S1P(1) mRNA and protein expression in rat mesangial cells, whereas S1P(2-5) expression levels were not altered. Upregulation of S1P(1) mRNA upon TZD treatment was also detected in human mesangial cells and mouse glomeruli. PPAR gamma antagonism and promoter studies revealed that the TZD-dependent S1P(1) mRNA induction involved a functional PPAR response element in the S1P(1) promoter. Pharmacological approaches disclosed that SIP-induced [Ca2+](i) increases in rat mesangial cells were predominantly mediated by S1P(2) and S1P(3). Interestingly, the transcriptional upregulation of S1P(1) by TZDs resulted in a reduction of SIP-induced [Ca2+]; increases, which was reversed by the S1P(1/3) antagonist VPC-23019, the protein kinase C (PKC) inhibitor PKC-412, and by SW, siRNA. These data suggest that PPAR gamma-dependent upregulation of S1P(1) leads to an inhibition of SI P-induced Ca2+ signaling in a PKC-dependent manner. Overall, these results reveal that TZDs not only modulate intracellular SIP levels but also regulate S1PR signaling by increasing S1P(1) expression in mesangial cells. (C) 2013 Elsevier B.V. All rights reserved.