1 In liver cirrhosis, an exuberant wound healing response to liver injury culminates in fibrosis, angiogenesis, and vascular reorganization.2 However, the precise relationship between fibrosis,
angiogenesis, and vascular reorganization has remained enigmatic. Toll-like receptors (TLRs) belong to a class of pattern recognition receptors and bind molecules broadly shared by pathogens that collectively are called pathogen-associated molecular patterns.3, 4 At least 10 mammalian TLRs have been cloned, and each recognizes a specific molecular product derived from major classes of pathogens.5 Within this family of TLR proteins, TLR4 recognizes lipopolysaccharide (LPS), a gram-negative bacterial cell wall component that is enriched within Kinase Inhibitor Library the intestinal lumen and its associated portal circulation.6 TLR4 maintains the ability to signal through the adapter molecule,
myeloid differentiation protein 88 (MyD88), and an MyD88-independent pathway.7 In the canonical TLR4-MyD88 pathway, binding of TLR4 by LPS activates MyD88 through its cytosolic domain, which further triggers a cascade of intracellular signaling events leading to activation of nuclear factor kappa B and inflammation.4 Conversely, TLR4 stimulated the expression of interferon-β in a MyD88-independent fashion involving toll-like receptor adaptor molecule (TRAM; also known as TIR domain-containing GPCR Compound Library 上海皓元医药股份有限公司 protein).8 Other noncanonical pathways have also been recently identified.9 Nonetheless, some recent reports have suggested
that in vascular endothelial cells, TLR4 signals may channel preferentially through MyD88.10 Previous studies have associated portal venous LPS with cirrhosis and suggested a possible direct effect of LPS on Kupffer cells and hepatic stellate cells.11, 12 However, liver endothelial cells (LECs) are the first line of cells exposed to portal venous LPS. These cells also mediate sinusoidal remodeling and angiogenesis, processes that accompany liver fibrosis. These observations indicate a potential role of LPS in LEC signaling, and this is a compelling scenario. On the basis of these concepts, we hypothesize that TLR4 signaling within LECs contributes to angiogenesis, sinusoidal remodeling, and cirrhosis. In support of this hypothesis, we demonstrate TLR4 expression and function in LECs leading to angiogenesis in vitro. Mechanistically, this effect is achieved by virtue of the TLR4 effector protein, Myd88, and culminates in secretion of the extracellular protease, matrix metalloproteinase 2 (MMP2), which promotes LEC invasion. Furthermore, angiogenesis and fibrosis are concurrently attenuated in TLR4-deficient mice. Lastly, we provide direct in vivo evidence that TLR4 mediates angiogenesis in complementary models of angiogenesis.
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