Abstract
BACKGROUND: Adenosine deaminase (ADA) and osteopontin (OPN) may play opposing roles in the pathogenesis of COPD. Deficiency of ADA results in enhanced adenosine signaling which up-regulates OPN expression. Although statins suppress OPN in cancer cells, little is known about their effects on ADA and OPN in COPD patients. METHODS: We extended a previous randomized double-blind placebo crossover study to investigate the effects of simvastatin (20 mg/day) on sputum ADA and OPN expression and explored the underlying signaling pathways involved by conducting in vitro experiments with cigarette smoke extract (CSE)-treated monocyte-derived macrophages (MDM) from COPD patients and healthy subjects. RESULTS: Simvastatin decreased sputum IL-13, OPN and CD73, while increasing ADA expression, irrespective of inhaled corticosteroid treatment and smoking status in parallel to increased inosine levels. The degree of simvastatin-restored ADA activity was significantly correlated with the magnitude of changes in pre-bronchodilator FEV1. Mechanistic exploration showed that CSE enhanced the expression of IL-13, which induced an increase in OPN and inhibited ADA mRNA accumulation in MDM from COPD patients but not healthy subjects through a STAT6-dependent mechanism. Simvastatin treatment inhibited IL-13 transcription in a dose-dependent manner, and therefore diminished the IL-13-induced increase in OPN and restored IL-13-suppressed ADA. There was no effect of simvastatin on adenosine receptors in CSE-stimulated MDM, indicating that its effects were on the adenosine pathway. CONCLUSION: Simvastatin reversed IL-13-suppressed ADA activity that leads to the down-regulation of adenosine signaling and therefore inhibits OPN expression through the direct inhibition of IL-13-activated STAT6 pathway. Inhibition of IL-13 may reverse the imbalance between ADA and OPN in COPD and therefore may prevent COPD progression.