Atrial Natriuretic Peptide Prevented Lipid‐Induced Kidney Injuries by Inhibiting Endoplasmic Reticulum Stress

Hyperlipidemia plays an important role in the progression of kidney injury, likely due to renal lipotoxicity. The purpose of the present study was to investigate whether atrial natriuretic peptide (ANP) prevented lipid‐induced injury through inhibiting endoplasmic reticulum (ER) stress and apoptosis in the kidney. In human proximal tubule HK2 cells, saturated fatty acid palmitic acid (PA) treatment (0.4mM) for 24h markedly increased protein abundance of two ER stress makers BiP (3.7‐folds of controls) and CHOP (5.2‐folds of controls), which was associated with upregulated cleaved‐caspase 3 expression (3.2‐folds of controls) and decreased ratio of Bcl2/Bax (0.76‐folds of controls), two markers of apoptosis. ANP treatment (10pM) markedly decreased protein abundance of BiP (2.8‐folds of controls), CHOP (3.7‐folds of controls), and cleaved‐caspase 3 (1.7‐folds of controls), and increased the ratio of Bcl2/Bax (1.2‐folds of controls) in HK2 cells treated by PA. Co‐treatment with ANP and LBQ657, a neprilysin inhibitor preventing hydrolysis of ANP, showed better protective effects than ANP treatment alone in HK2 cells treated with PA. ANP is known to stimulate intracellular cGMP‐PKG signaling pathway, the role of cGMP and PKG in ANP‐induced protection after PA treatment in HK2 cells was thus investigated. Sildenafil, an inhibitor of phosphodiesterase‐5 (PDE5), and exogenous 8‐Br‐cGMP markedly suppressed PA‐induced ER stress and apoptosis, whereas KT‐5823, a selective cGMP‐dependent PKG inhibitor, blocked protection of ANP or cGMP in HK2 cells treated with PA. A synthesized, specific PDE5 inhibitor LW1646 showed slightly better prevention than sildenafil on PA‐induced HK2 cell injury. ANP attenuated PA‐induced upregulation of inositol 1,4,5‐triphate receptor (IP3R) that is involved in calcium efflux from ER to cytoplasm. ANP decreased PA‐induced intracellular calcium oscillation, prevented PA‐induced mitochondrial depolarization, and promoted mitochondrial biogenesis in HK2 cells. Compared with controls, in mice fed with high‐fat diet for 8wk, the protein expression of BiP and CHOP in the kidney cortex showed 2.8‐fold and 2.3‐fold increases, respectively, which was markedly prevented by sildenafil (15mg/kg·BW/day, gavage). In conclusion, these findings demonstrated that ANP inhibited lipid‐induced ER stress and apoptosis, likely via activating the cGMP‐PKG signaling pathway and maintaining mitochondrial function, it may be a potentially therapeutic target for lipid‐induced kidney injury.