Hyaluronan synthase-2-mediated hyaluronan overproduction in hepatic stellate cells contributes to liver fibrosis
AASLD LiverLearning®. Yang Y. Nov 14, 2016; 144575
Topic: Liver Fibrogenesis and Non-Parenchymal Cell Biology
Dr. Yoon Mee Yang
Dr. Yoon Mee Yang
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TITLE: Hyaluronan synthase-2-mediated hyaluronan overproduction in hepatic stellate cells contributes to liver fibrosis

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Liver cirrhosis is the advanced form of liver fibrosis, being the 12th leading cause of death in the US. Hyaluronan (HA), a major extracellular matrix protein, has been used as a biomarker of liver cirrhosis. HA is synthesized by hyaluronan synthase (HAS)1-3, of which HAS2 is involved in lung injury and fibrosis. However, the functional role of HA and its regulation by HAS2 in the development of liver fibrosis is still unknown. To determine whether HAS2 plays a crucial role in HA production during liver fibrosis, mice were subjected to bile duct ligation (BDL). We found that HA deposition was remarkable in fibrotic area of the liver and hepatic HAS2 expression was increased 3 weeks after BDL. Since HAS2 is mainly expressed in activated hepatic stellate cells (HSCs), we generated HSC-specific Has2 knockout mice by crossing Has2flox/flox mice with L-rat-Cre mice. Three weeks after BDL, HSC-specific Has2 knockout mice had a significant reduction in HA accumulation and collagen deposition as demonstrated by immunostaining for HA and Sirius Red staining. HA is initially produced as a high molecular weight (HMW) form (>2000kDa) and is then degraded into fragmented low molecular weight (LMW) HA (100-300kDa). HMW-HA did not trigger proinflammatory response whereas LMW-HA increased cytokine and chemokine production in liver macrophages, HSCs, and hepatocytes. TLR4 is one of LMW-HA receptors, and previously we showed that TLR4 on HSCs, but not liver macrophages, is required for liver fibrosis. We examined the role of TLR4 in LMW-HA-induced chemokine expression. We found that chemokine expression was repressed in TLR4-deficient HSCs and hepatocytes, but TLR4-deficient macrophages showed similar response to HA compared with wild-type cells, suggesting that HA-mediated chemokine production is TLR4-dependent in HSCs and hepatocytes, whereas it is TLR4-independent in liver macrophages. Finally, we investigated the potential intervention targeting HAS2 using a HAS2 inhibitor, 4-methylumbelliferone (4-MU). Oral administration of 4-MU diminished the expression of fibrogenic genes including Timp1 and a-SMA, induced by BDL or feeding of choline-deficient amino acid-defined diet. In conclusion, our results suggest that HAS2-mediated HA has a crucial role in the pathogenesis of liver fibrosis, and HAS2 may be a therapeutic target for liver fibrosis by controlling endogenous HA production.
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