ENDOTHELIN STIMULATES PHOSPHATIDYLCHOLINE HYDROLYSIS THROUGH BOTH PLC AND PLD PATHWAYS IN MESANGIAL CELLS

Endothelin (ET) is a recently characterized vasoconstrictor hormone that has potent effects on glomerular function. Many vasoconstrictors, like ET, that stimulate phospholipase C (PLC) hydrolysis of polyphosphoinositides also stimulate phosphatidylcholine (PtdCho) hydrolysis via both PLC and phospholipase D (PLD) pathways. We have previously reported that ET stimulates a protein kinase C (PKC)regulated, intracellular calcium-insensitive PLD activity that forms phosphatidic acid (PA) in rat mesangial cells (MC). We now ask whether ET-induced diglyceride (DG) production is also, in part, a result of either PLC- or PLD-induced hydrolysis of PtdCho. ET induced both a time- and dose-dependent stimulation in DG as measured by radioflux and mass assays. ET-stimulated DG production was still elevated even at time points where inositol polyphosphates had returned to basal levels. In addition, using [H-3]choline-labeled cells, ET stimulated [H-3]phosphocholine accumulation, suggesting a PLC-mediated hydrolysis of PtdCho. Stimulation of DG was unaffected by the presence of ethanol or propranolol, suggesting that ET-stimulated DG were not a result of a sequential PLD/PA phosphohydrolase activity. We further dissociated PtdCho-dependent PLC and PLD activities because, in contrast to ET-induced stimulation of PLD, the effect of ET on DG formation was mimicked with ionomycin and was inhibited with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid but not ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid. ET stimulation of DG could not be mimicked by phorbol myristate acetate and was not blocked by PKC inhibition or depletion. Together, these data suggest that ET stimulates multiple signaling pathways in MC that hydrolyze PtdCho via separate PLC and PLD mechanisms. The potential significance of ET-stimulated PtdCho hydrolysis includes the generation of diverse species of DG and PA, lipid second messengers that may augment an activated mesangial cell phenotype.