PROTECTION AGAINST ENDOTHELIAL INFLAMMATION BY GREEN TEA FLAVONOIDS

Endothelial inflammation is a pivotal early event in the development of atherosclerosis. Long term exposure to cardiovascular risk factors will ultimately exhaust those protective anti-inflammatory factors such as the heme oxygenase (HO) system. The HO system plays a critical role in cellular and tissue self-defense against oxidative stress and inflammation. Caveolae are membrane domains and are particularly abundant in endothelial cells, where they are believed to play a major role in the regulation of endothelial vesicular trafficking as well as the uptake of lipids and related lipophilic compounds, possibly including bioactive food components such as flavonoids. Research in this dissertation addresses the role of HO-1 and caveolae on dietary flavonoid epigallocatechin gallate (EGCG) mediated protection against pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) and linoleic acid-induced activation of endothelial cells. The data support the hypothesis that EGCG protects against TNF-α-induced monocyte recruitment and adhesion partially through the induction of HO-1 and bilirubin. The observed anti-inflammatory effects of EGCG are mimicked by the HO-1 inducer cobalt protoporphyrin (CoPP) and abolished by HO-1 gene silencing. Nrf2 is the major transcription factor of phase II antioxidant enzymes including HO-1. Results clearly show that EGCG-induced HO-1 expression and subsequent bilirubin productions are dependent on functional Nrf2. EGCG also can down-regulate the base-line level of caveolin-1. Furthermore, silencing of the caveolin-1 gene can markedly down-regulate linoleic acid-induced COX-2 and MCP-1, indicating that caveolae may be a critical platform regulating inflammatory signaling pathways. Similar to EGCG treatment, silencing of caveolin-1 can also result in the activation of Nrf2, up-regulation of HO-1 and bilirubin. This may be one of the mechanisms to explain the protection effect of caveolin-1 gene silencing against endothelial inflammation. Moreover, EGCG rapidly accumulates in caveolae, which is associated with caveolin-1 displacement from the plasma membrane towards the cytosol. Caveolin-1 gene silencing can significantly reduce the uptake of EGCG in endothelial cells within 30 min. These data suggest that caveolae may play a role in the uptake and transport of EGCG in endothelial cells. These studies provide a novel target through which EGCG functions to protect against inflammatory diseases such as atherosclerosis