Abnormal nitric oxide metabolism in systemic sclerosis: increased levels of nitrated proteins and asymmetric dimethylarginine

Objectives. Endothetial dysfunction is a primary event in systemic sclerosis; however, the aetiology of events and the role of nitric oxide (NO) is still unclear. The aim of the present study is to investigate whether there are abnormalities in NO metabolism in plasma from patients with primary Raynaud's phenomenon (RP) and in the pathogenesis of systemic sclerosis (SSc): limited SSc (ISSc) and diffuse (dSSe). We also wanted to investigate the effect of factors within patients' SSc serum on NO metabolism in human microvascular endothelial cells (HMECs). Methods. Plasma (n = 89) or serum (n = 80) was assayed for total nitrate and nitrite (NOx), nitration of proteins and the NO inhibitor asymmetric dimethylarginine (ADMA). HMECs were treated with patients' SSc serum and assayed for indicators of NO metabolism. Results. Plasma NOx was elevated in patients with RP or ISSc (P < 0.002), but not in patients with dSSc, compared with controls. Nitrated proteins in plasma, however, were found to be very high in dSSc patients (P < 0.03), compared with RP, ISSc or controls. Patients with dSSc also showed increased levels of serum ADMA (P < 0.05). The high level of nitrated proteins in dSSc was strongly associated with the severity and duration of dSSc disease. Skin biopsy sections from dSSc patients also showed enhanced nitrotyrosine staining compared with controls. In HMECs, pre-incubation with SSc serum impaired the activity of nitric oxide synthase (NOS) but not the expression of inducible or endothelial NOS. SSc serum also induced a reduction in intracellular cGMP synthesis, and NOx production in the cell culture medium, but was not associated with increased cell cytotoxicity. Conclusions. NO formation is increased in patients with primary RP or ISSc, but nitration of proteins and elevated ADMA is a particular feature of dSSc and may reflect abnormal NO regulation and/or contribute to endothelial dysfunction in SSc.