Mechanisms of hepatocyte protection against hypoxic injury by atrial natriuretic peptide

Atrial natriuretic peptide (ANP) reduces ischemia and/or reperfusion damage in several organs, but the mechanisms involved are largely unknown. We used freshly isolated rat hepatocytes to investigate the mechanisms by which ANP enhances hepatocyte resistance to hypoxia. The addition of ANP (1 μmol/L) reduced the killing of hypoxic hepatocytes by interfering with intracellular Na⁺ accumulation without ameliorating adenosine triphosphate (ATP) depletion and pH decrease caused by hypoxia. The effects of ANP were mimicked by 8-bromo-guanosine 3′, 5′-cyclic monophosphate (cGMP) and were associated with the activation of cGMP-dependent kinase (cGK), suggesting the involvement of guanylate cyclase-coupled natriuretic peptide receptor (NPR)-A/B ANP receptors. However, stimulating NPR-C receptor with des-(Gln¹⁸, Ser¹⁹,Gly²⁰,Leu²¹,Gly²²)-ANP fragment 4-23 amide (C-ANP) also increased hepatocyte tolerance to hypoxia. C-ANP protection did not involve cGK activation but was instead linked to the stimulation of protein kinase C (PKC)-δ through G_i protein- and phospholipase C-mediated signals. PKC-δ activation was also observed in hepatocytes receiving ANP. The inhibition of phospholipase C or PKC by U73122 and chelerythrine, respectively, significantly reduced ANP cytoprotection, indicating that ANP interaction with NPR-C receptors also contributed to cytoprotection. In ANP-treated hepatocytes, the stimulation of both cGK and PKC-δ was coupled with dual phosphorylation of p38 mitogen-activated protein kinase (MAPK). The p38 MAPK inhibitor SB203580 abolished ANP protection by reverting p38 MAPK-mediated regulation of Na⁺ influx by the Na⁺/H⁺ exchanger. In conclusion, ANP recruits 2 independent signal pathways, one mediated by cGMP and cGK and the other associated with G_i proteins, phospholipase C, and PKC-δ. Both cGK and PKC-δ further transduce ANP signals to p38 MAPK that, by maintaining Na⁺ homeostasis, are responsible for ANP protection against hypoxic injury.