Differential effects of lipoprotein lipase on tumor necrosis factor-alpha and interferon-gamma-mediated gene expression in human endothelial cells

Lipoprotein lipase (LPL) is a key enzyme in the hydrolysis of triglyceride-rich lipoproteins. In vascular diseases, such as atherosclerosis, inflammation plays an important role in the pathogenesis of the disease. We examined the role of LPL in modulating tumor necrosis factor-alpha (TNF-alpha)- and interferon-gamma (IFN-gamma)-mediated inflammatory cytokine signal transduction pathways in human aortic endothelial cells (HAECs). LPL significantly suppressed TNF-alpha-induced gene expression, and this suppression was reversed by tetrahydrolipstatin and heparinase. In contrast, LPL synergistically enhanced IFN-gamma-induced gene expression in HAECs. To elucidate the molecular mechanisms of LPL action, we investigated the role of transcription factors nuclear factor kappa B (NF-kappaB) and signal transducer and activator of transcription factor 1 (Stat1). The anti-inflammatory response of LPL in suppressing TNF-alpha-induced gene expression was a result of its inhibition of NF-kappaB activity by the abrogation of IkappaB-alpha degradation and phosphorylation of the p65 subunit. Although LPL alone had no effect on Stat1 activation, LPL enhanced IFN-gamma-induced phosphorylation of Stat1 on tyrosine 701 and serine 727, as well as Stat1-mediated transactivation. The synergistic effect of LPL on IFN-gamma-induced Stat1 activation was mediated by enhanced activation of the tyrosine kinase JAK2 and was abrogated by LY294002, a specific inhibitor of the phosphatidylinositol 3'-kinase pathway. Our studies indicate that LPL has differential effects on several inflammatory pathways known to be important in atherosclerosis.