Chung et al. show that the myomitokine GDF15 can act to modulate oxidative and lipolytic function in a non–cell-autonomous manner, thereby regulating systemic energy homeostasis in skeletal muscle-specific Crif1-deficient mice. This pathway may be a potential therapeutic target for preventing the onset of obesity and insulin resistance.

Non-alcoholic fatty liver disease (NAFLD) is a dominant cause of chronic liver disease, but the exact mechanism of progression from simple steatosis to nonalcoholic steatohepatitis (NASH) remains unknown. Here, we investigated the role of exosomes in NAFLD progression. Exosomes were isolated from a human hepatoma cell line treated with palmitic acid (PA) and their miRNA profiles examined by microarray. The human hepatic stellate cell (HSC) line (LX-2) was then treated with exosome isolated from hepatocytes. Compared with controls, PA-treated hepatocytes displayed significantly increased CD36 and exosome production. The microarray analysis showed there to be distinctive miRNA expression patterns between exosomes from vehicle- and PA-treated hepatocytes. When LX-2 cells were cultured with exosomes from PA-treated hepatocytes, the expression of genes related to the development of fibrosis were significantly amplified compared to those treated with exosomes from vehicle-treated hepatocytes. In conclusion, PA treatment enhanced the production of exosomes in these hepatocytes and changed their exosomal miRNA profile. Moreover, exosomes derived from PA-treated hepatocytes caused an increase in the expression levels of fibrotic genes in HSCs. Therefore, exosomes may have important roles in the crosstalk between hepatocytes and HSCs in the progression from simple steatosis to NASH.

During liver injury, hepatocytes secrete exosomes that include diverse types of self-RNAs. Recently, self-noncoding RNA has been recognized as an activator of Toll-like receptor 3 (TLR3). However, the roles of hepatic exosomes and TLR3 in liver fibrosis are not yet fully understood. Following acute liver injury and early-stage liver fibrosis induced by a single or 2-week injection of carbon tetrachloride (CCl 4 ), increased interleukin (IL)-17A production was detected primarily in hepatic cd T cells in wild-type (WT) mice. However, liver fibrosis and IL-17A production by cd T cells were both significantly attenuated in TLR3 knockout (KO) mice compared with WT mice. More interestingly, IL-17A-producing cd T cells were in close contact with activated hepatic stellate cells (HSCs), suggesting a role for HSCs in IL-17A production by cd T cells. In vitro treatments with exosomes derived from CCl 4 -treated hepatocytes significantly increased the expression of IL-17A, IL-1b, and IL-23 in WT HSCs but not in TLR3 KO HSCs. Furthermore, IL-17A production by cd T cells was substantially increased upon coculturing with exosome-treated WT HSCs or conditioned medium from TLR3-activated WT HSCs. However, similar increases were not detected when cd T cells were cocultured with exosome-treated HSCs from IL-17A KO or TLR3 KO mice. Using reciprocal bone marrow transplantation between WT and TLR3 KO mice, we found that TLR3 deficiency in HSCs contributed to decreased IL-17A production by cd T cells, as well as liver fibrosis. Conclusion: In liver injury, the exosome-mediated activation of TLR3 in HSCs exacerbates liver fibrosis by enhancing IL-17A production by cd T cells, which might be associated with HSC stimulation by unknown self-TLR3 ligands from damaged hepatocytes. Therefore, TLR3 might be a novel therapeutic target for liver fibrosis. (HEPATOLOGY 2016;64:616-631) L iver fibrosis results from chronic liver injurymediated inflammation and activation of hepatic stellate cells (HSCs).(1,2) After liver injury, diverse factors such as cytokines, chemokines, self-ligands, and nonself-ligands (e.g., damage-and pathogen-associated molecular patterns) stimulate the expression of a-smooth muscle actin (a-SMA) and collagen fibers in HSCs.(1-3) In addition to the

Reactive oxygen species (ROS) contribute to the development of non-alcoholic fatty liver disease. ROS generation by infiltrating macrophages involves multiple mechanisms, including Toll-like receptor 4 (TLR4)-mediated NADPH oxidase (NOX) activation. Here, we show that palmitate-stimulated CD11b+F4/80low hepatic infiltrating macrophages, but not CD11b+F4/80high Kupffer cells, generate ROS via dynamin-mediated endocytosis of TLR4 and NOX2, independently from MyD88 and TRIF. We demonstrate that differently from LPS-mediated dimerization of the TLR4–MD2 complex, palmitate binds a monomeric TLR4–MD2 complex that triggers endocytosis, ROS generation and increases pro-interleukin-1β expression in macrophages. Palmitate-induced ROS generation in human CD68lowCD14high macrophages is strongly suppressed by inhibition of dynamin. Furthermore, Nox2-deficient mice are protected against high-fat diet-induced hepatic steatosis and insulin resistance. Therefore, endocytosis of TLR4 and NOX2 into macrophages might be a novel therapeutic target for non-alcoholic fatty liver disease.

Non-alcoholic fatty liver disease (NAFLD) is now considered to be the most common liver disease in the Western world and has no approved pharmacological therapy. PARP inhibitors given as a treatment in two different mouse models of NAFLD confer a protection against its development. PARP inhibitors may therefore represent a novel and practical pharmacological approach for treating NAFLD.