Mechanisms of Berberine for the Treatment of Non-Alcoholic Fatty Liver Disease

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Nonalcoholic fatty liver disease (NAFLD) has emerged as the most prevalent chronic liver disease, especially among US veterans. NAFLD is critically linked to inflammation, insulin resistance, dyslipidemia, and the metabolic syndrome. Recent advances in NAFLD and non-alcoholic steatohepatitis (NASH) studies indicate that gut microbiota exerts a significant role in the disease progression by affecting host metabolic balance and immune response. The “metabolic endotoxemia” and elevated circulating levels of lipopolysaccharide (LPS) due to disruption of intestinal barrier function and free long chain fatty acids (FFA) are implicated in the stimulation of systemic inflammation and insulin resistance, both of which are positively correlated with the development and progression of NAFLD. Although the mechanism by which LPS/FFA induce hepatic lipotoxicity is still not fully understood, LPS/FFA-induced expression of inflammatory cytokines such as TNF-  and activation of the endoplasmic reticulum (ER) stress signaling pathway, known as the unfolded protein response (UPR), are major contributors. Furthermore, micro-RNAs (miR), small noncoding RNAs (~22 nucleotides), have recently been found to be potent regulators of lipid and cholesterol metabolism. Alterations in miR expression have also been linked to LPS/FFA-induced inflammatory response, hepatic lipotoxicity and insulin resistance as well as disease progression of NAFLD in the clinic. Berberine (BBR), an isoquinoline alkaloid isolated from many medicinal herbs, has been used to treat various infectious disorders for more than 3,000 years in Asia. Numerous studies have shown that BBR has various pharmacological activities including anti-inflammatory, hypoglycemic and lipid-lowering effects. Importantly, BBR represents a novel cholesterol- lowering drug through a unique mechanism distinct from the current statin therapy. These studies strongly indicate that BBR is a promising natural therapeutic agent for NAFLD/NASH. However, the molecular mechanisms underlying BBR’s anti-inflammatory and lipid-lowering properties remain to be fully identified. We have demonstrated that BBR inhibits ER stress-mediated TNF- and IL-6 expression through regulating the RNA-binding protein (RBP) HuR in macrophages. Recent advances in miR research have identified specific panels of miRs as major regulators of inflammation, lipid metabolism and metabolic disorders, such as miR-155, miR-125a-5p, miR-33, miR-34a, and miR-122. In addition, it has been reported that BBR exerted protective effects against high fat Western diet (WD)-induced NAFLD/NASH via modulating gut microbiota and bile acid metabolism. Our preliminary studies also indicate that 1) BBR significantly inhibited WD-induced hepatic lipid accumulation and systemic inflammation in animal models; 2) BBR inhibited FFA-induced ER stress and lipid accumulation in hepatocytes; 3) BBR inhibited FFA-induced miR-34a expression in hepatocytes and LPS-induced miR-125a-5p expression in macrophages; 4) The expression of miR-34a was significantly down-regulated in C/EBP homologous protein (CHOP, a major player of ER stress-mediated apoptosis) knock out (CHOP-/-) primary hepatocytes; 5) BBR significantly reduced WD-induced increase of intestinal permeability; 6) ER stress- and LPS-induced suppression of intestinal epithelial cell renewal was rescued by deletion of the CHOP. Based on these observations, we HYPOTHESIZE that BBR inhibits WD- induced hepatic lipotoxicity by inhibiting ER stress and inflammatory response. The following two specific aims are proposed to test this hypothesis. Aim #1:To determine the mechanism by which BBR inhibits LPS/FFA-induced ER stress and the inflammatory response in hepatocytes and macrophages; Aim #2: To identify the mechanism by which BBR inhibits WD-induced hepatic lipotoxicity and further test the therapeutic effect of BBR on WD-induced hepatic lipotoxicity in a novel in vivo NAFLD/NASH model.