ABSTRACT
Objective To explore the effects of Na+ H-exchanger 1(NHE1) knockdown on ATP binding cassette transporter A1 (ABCA1) protein expression levels and cholesterol efflux in the hypoxic RAW264.7 cells.Methods The RAW264.7 cells were infected with lentiviral vectors expressing shRNA specific for NHE1(siNHE1) or scramble RNA (siNC).The expression of NHE1 at mRNA or protein level was detected by qRT-PCR and Western blotting respectively in the infected cells after 24 h in a hypoxia condition.In the meantime,the methods of SNARF-1,Fluo-4 NW andSuc-LLVY-aminoluciferin were employed to determine NHE1 activity,intracellular Ca2+ ([Ca2+]i) and calpain activity,respectively.Furthermore,ABCA1 protein levels were detected by Western blotting in the 24 h hypoxic cells.In parallel,the intracellular cholesterol content and cholesterol efflux were analyzed by the methods of combined enzymatic HLPC and 3 H-cholesterol.Results The hypoxia condition versus the normoxia condition up-regulated NHE1 mRNA and protein expression level and activity by 2.48 folds,1.28 folds and 61.96％ (all P＜0.05),and increased[Ca2+]i and calpain activity by 4.51 folds and 2.41 folds(all P＜0.05).Whereas the NHE1 mRNA and protein expression and activity at the presence of hypoxia were inhibited by siNHE1 with the inhibition ratio of 84.95％,60.75％ and 66.44％,respectively (all P＜0.05)and[Ca2+]i and calpain activity were reduced by 59.23％ and 54.66％ (P＜0.05).Furthermore,the ABCA1 protein level was 61.67％ lower in the hypoxic cells than in the normoxic cells (P＜0.05),and siNHE1 was increased by 56.52％ after treatment of Hypoxia.Hypoxia elevated intracellular total cholesterol and cholesterol ester by 74.57 ％ and 101.81％ (all P＜0.05).Treatment with siNHE1 in the hypoxia condition can reduce total cholesterol and cholesterol ester by 34.24 ％ 及 49.66 ％ (all P＜0.05).Hypoxia reduced the cholesterol efflux by 34.79％(P＜0.05),which were partially reversed by siNHE1.Conclusions NHE1 might play an important role in hypoxia-induced ABCA1 protein attenuation and reverse cholesterol transport dysfunction through[Ca2+]i/calpain pathway.
ABSTRACT
AIM:To examine the effects of hypoxia on sodium-hydrogen exchange 1 (NHE1) expression, in-tracellular Ca2+concentration ( [ Ca2+] i ) and calpain activity, and to explore the effect of amiloride on adenosine triphos-phate-binding cassette transporter A1 (ABCA1) degradation and its calpain-related mechanism.METHODS:RAW264.7 cells were exposed to hypoxia for 0 h, 12 h, 24 h and 48 h.The cell viability was measured by MTT assay and the expres-sion of NHE1 at mRNA and protein levels was detected by real-time PCR and Western blot.[ Ca2+] i was analyzed by flow cytometry.Calpain activity was assessed by the method of Suc-LLVY-aminoluciferin.Furthermore, the protein levels of ABCA1 in the RAW264.7 cells exposed to hypoxia for 24 h were determined after 6 h or 12 h treatment with NHE1 inhibi-tor amiloride in the presence of cycloheximide.ABCA1 protein levels and calpain activity were detected after 12 h incuba-tion with calpain inhibitor ALLN or intracellular calcium-chelating agent BAPTA.RESULTS: Hypoxia inhibited the cell viability in a time-dependent manner.Hypoxia up-regulated the mRNA and protein expression of NHE1, and increased [ Ca2+] i and calpain activity.Hypoxia increased the degradation of ABCA1 and amiloride slowed down the ABCA1 degra-dation.ALLN or BAPTA increased ABCA1 protein level and decreased calpain activity.CONCLUSION:NHE1 inhibitor amiloride attenuates the calpain-mediated degradation of ABCA1, indicating that hypoxia-induced NHE1 might, at least in part, participate in the ABCA1 degradation.