Background: Oxidatively modified low density lipoprotein (OxLDL) induces apoptosis in vascular cells including macrophages, while NO exerts antiapoptotic effects. Here we studied the impact of nitric oxide (NO) on OxLDL-induced cytochrome c release, apoptosis, and expression of the proapoptotic p53 in macrophages.
Methods: Human LDL was oxidised by Cu(2+), and monocytes were prepared from human buffy coats. Differentiation to macrophages was achieved by culturing cells in the presence of human serum and was followed by detecting monocyte chemoattractant protein 1 (MCP-1) expression (RT-PCR). Cytochrome c release and p53 expression of macrophages were detected by immunoblotting, and apoptosis by visualisation of nuclear condensation.
Results: OxLDL dose-dependently (50-200 microg/ml) induced cytochrome c release that was prevented by preincubation with the NO-donor S-nitrosoglutathione (GSNO) (100 microM) or with the cGMP analogue 8-br-cGMP (100 microM) for 15 h. In cells co-treated with GSNO and the soluble guanylate cyclase (sGC) inhibitor oxadialoquinoxalione (ODQ, 10 microM, 15 h), OxLDL-evoked cytochrome c release remained effective, indicating that NO acted via sGC-dependent cGMP formation. Parallel incubation of macrophages with 8-br-cGMP (100 microM) and ODQ (10 microM) for 15 h left the protective effect of 8-br-cGMP unaltered. Short pre-incubation (30 min) with GSNO or 8-br-cGMP was ineffective in preventing OxLDL-elicited cytochrome c release. Initiation of cytochrome c release in macrophages was paralleled by a dose-dependent accumulation of the proapoptotic factor p53, and by enhanced rate of nuclear condensation. Stabilisation of p53 was prevented by preincubation with the NO-donor GSNO or 8-br-cGMP, thus implying a downmodulatory effect of cGMP on pathways that upregulate the tumor suppressor p53.
Conclusions: OxLDL induces cytochrome c release and apoptosis in human macrophages in close association with p53 accumulation. NO attenuates OxLDL-induced cytochrome c release and p53 accumulation via activation of sGC and cGMP formation. These effects may be of particular importance in arterial tissue with reduced NO activity.