The effects of acid sphingomyelinase on endothelial cell function and its role in chronic inflammatory disease

• Main Author: Kiprianos, Allan Panagiotis
• Published: University of Birmingham 2011
• Subjects: 616.079; RC Internal medicine
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.535364

Accelerated atherosclerosis and cardiovascular disease are seen in patients with primary vasculitis (PSV) and rheumatoid arthritis (RA). The mechanisms behind this remain unclear, but recent studies have linked it to endothelial cell dysfunction (ECD), which may result from the vasculitis also seen in RA, secondary to the joint disease. TNFα has been shown to induce ECD in vitro and in vivo through poorly understood mechanisms, but other pro‐inflammatory cytokines can induce secretion of acid sphingomyelinase (ASM) from EC. ASM has been shown to induce reduced intracellular Ca\(^{2+}\)(\(_i\)Ca\(^{2+}\)) signalling responses in lymphocytes through ceramide generation. Since NO production in EC is tightly coupled to agonist‐induced \(_i\)Ca\(^{2+}\) responses, this thesis set out to investigate whether inflammatory mediator‐induced secretion of ASM (S‐SMase) could regulate EC function through changes in \(_i\)Ca\(^{2+}\) responses. The activity of S‐SMase in blood from patients with RA and primary vasculitis was also assessed to underline the clinical relevance of the hypothesis. Patients (n=20) with ANCA‐associated systemic vasculitis (AASV) had high levels of S‐SMase (114.8 ± 41 pmol/mL/h\(^{‐1}\)) in their blood compared to 13 healthy controls (52.3 ± 35 pmol/mL/h\(^{‐1}\)). These levels fell following induction of remission by 14 weeks (96.7 ± 41 pmol/mL/h\(^{‐1}\)) and remained low at 6 months (33.7 ± 14 pmol/mL/h\(^{‐1}\)). Raised active blood S‐SMase was also confirmed in 35 patients with RA (84.1 ± 35 pmol/mL/h\(^{‐1}\)) where enzyme activity correlated with several CV risk factors. TNFαdirectly induced the secretion of a redox‐sensitive and enzymatically active S‐SMase from human umbilical vein EC (HUVEC), a process blocked by antibody‐mediated inhibition of TNFα. Exposure of HUVEC to ASM inhibited \(_i\)Ca\(^{2+}\) signals in response to bradykinin. Subsequent studies on endothelial nitric oxide synthase (eNOS) suggested that exogenous ASM could alter eNOS phosphorylation at Ser\(^{1177}\) responsible for its activation, while ASM may also decrease NO production in HUVEC. We propose that TNFα and the prevalent oxidative environment at sites of vascular inflammation could promote increased activation and secretion of S‐SMase that would act systemically on EC causing diffuse, widespread ECD promoting the development of atherosclerosis in inflammatory disease.