Angiotensin-mediated phosphatidylcholine hydrolysis and protein kinase C activation in mesangial cells

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Abstract
Angiotensin II (ANG II) in mesangial cells (MC) promotes phosphatidylinositol (PI) hydrolysis resulting in diacylglycerol (DAG)-mediated increases in protein kinase C (PKC) activity. The paucity of MC inositol lipid prompted us to consider whether phosphatidylcholine (PC) could sustain DAG formation. ANG II released choline and increased phosphatidylethanol (PEt) via PC-phospholipase D (PC-PLD). ANG II also stimulated phosphorylcholine consequent to PC-phospholipase C (PC-PLC) activation. ANG II-mediated PC hydrolysis augmented DAG for 30 min. PC breakdown was influenced by extracellular Ca2+, because Ni2+ partially inhibited ANG II-induced PEt and obliterated agonist-mediated DAG formation. The consequence of Ca2+ modulation of PC metabolism was investigated by measuring PKC activity. Ni2+ had no effect on early (PI-associated) activation by ANG II at 90 s but obviated translocation from cytosol to the membrane at 10 min. The pathway responsible for PC-associated DAG was studied in PKC downregulated cells. Whereas downregulation prevented PLD-mediated PEt elevation, ANG II-stimulated DAG formation in myristate-labeled cells was unaltered, indicating PC-PLC activation. In summary, ANG II stimulates PC-PLD and PC-PLC in MC. PC-PLD is tightly regulated by PKC, whereas PC-PLC is stringently controlled by extracellular Ca2+. ANG II mediated PC breakdown principally via PC-PLC provides a mechanism for maintaining elevated DAG levels and PKC activation.

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