Mixed lineage kinase 3 connects hepatocellular lipotoxicity with macrophage chemotaxis
Authors: Jan‐Peter Sowa, Christian Dominik Fingas, Ali Canbay
Journal: Hepatology (2015)

Potential conflict of interest: Nothing to report. See Article On Page 731 Nonalcoholic fatty liver disease (NAFLD) will become a major health care problem all over the world in the near future. Despite ongoing efforts in basic and clinical research, many open questions about this common hepatological disorder remain. Though it is by now generally accepted that NAFLD is the liver manifestation of metabolic syndrome, it is less clear how accumulation of fat in the liver (steatosis hepatis or nonalcoholic fatty liver = NAFL) can progress to a more severe form: nonalcoholic steatohepatitis (NASH). The latter type presents with hepatocyte injury, liver inflammation, and varying degrees of fibrosis. NASH comprises a significant risk factor for end‐stage liver disease, hepatocellular carcinoma (HCC), and cardiovascular diseases,1 though detailed mechanisms of development from NAFL to NASH are unclear. NAFL and NASH could even be separate entities, where development of either form depends on individual genetic and environmental factors. Thus, the focus of basic research in recent years was to determine exactly how lipid accumulation leads to NASH. Lipotoxicity is the commonly accepted concept describing cellular damage resulting from excess storage of lipid components. Whereas adipocytes are well adapted to store large amounts of lipids, other cell types forced to manage increased lipid uptake because of overflow of adipocytes will eventually undergo injury by lipotoxicity. Extent of lipotoxicity depends on the affected cell type, amount of lipids, and composition of lipid components. Some specific fatty acids (FAs) seem to be responsible for lipotoxic effects, whereas other FAs do not lead to lipotoxicity in hepatocytes.3 Continuing lipotoxicity will elicit an inflammatory response and culminate in cell death. For NAFLD, we and other groups demonstrated lipotoxicity resulting from specific (long‐chain) FA as an important mechanism of disease development.4 But the exact pathways from lipotoxicity to the strong inflammatory reaction observed in NASH have been elusive until now. In a well‐thought‐out and elaborate study, Ibrahim et al.6 may have identified the core connection between lipotoxicity and recruitment of macrophages to the liver, promoting inflammatory processes in NASH. The researchers investigated the mixed lineage kinase 3 (MLK3) and the chemokine (C‐X‐C motif) ligand 10 (CXCL10) in hepatocytes stimulated with lysophosphatidylcholine (LPC) or palmitic acid and in an in vivo model of NASH. Treated cells and mice fed a high‐calorie (saturated fat, cholesterol, and fructose; FFC) diet exhibited an increase of extracellular vesicles (EVs) containing CXCL10. CXCL10 bound to EVs exerted a strong chemotactic effect on macrophages in vitro, which surpassed the effect of unbound CXCL10. By knockout or pharmacological inhibition of MLK3, two important mechanisms regulated by MLK3 were exposed: First, blocking of MLK3 reduces the LPC‐induced production of CXCL10, probably through a p38/signal transducer and activator of transcription 1–dependent pathway; second, MLK3 inhibition diminished trafficking of CXCL10 into EVs (with increased intracellular amounts of CXCL10 in hepatocytes) and LPC‐induced EV release, which may depend on c‐Jun N‐terminal kinase activation. In summary, MLK3‐dependent release of CXCL10‐containing EVs from hepatocytes may represent the main link between lipotoxicity and inflammation promoted by macrophages recruited into the liver. Finally, this study not only highlights the role of CXCL‐10 in promoting hepatic inflammation in NASH, but it also implicates the role of EVs in hepatic inflammation, an emerging area of cell biology that warrants further study.6 Whereas these findings are very important to identify, on the one hand, a general mechanism directly connecting lipotoxicity and liver inflammation, which has been long sought after, and, on the other hand, a potential target to avoid progression to NASH, they have to be taken with care regarding clinical implementation. Indeed, liver inflammation comprises an important problem in NASH,7 but unfortunately not the only one8 (Fig. 1). The root of NAFLD—obesity and subsequent development of metabolic syndrome—is not addressed. Weight loss and/or treatment of insulin resistance are probably of similar importance to avoid not only continuing hepatic injury, but also consequences of an altered fatty acid metabolism in the liver on other organ systems.9 Another downside of approaches targeting the inflammatory component of NASH is the timing and duration of treatment. The researchers of the presented study very nicely show that MLK3–/– mice under the FFC diet are protected from development of NASH (but not steatosis),6 though it remains unclear whether MLK3 inhibition would improve NASH after it is established or for how long a potential treatment would need to continue to avoid relapse (given unchanged lifestyle habits). Because no pharmacological treatment of NASH is available at all, these potential downsides in targeting liver inflammation may outweigh the option to help patients with continuous NASH progression and high risk for cirrhosis or HCC. Still, the causative factors for NAFLD itself should be targeted at the same time. At present, the first‐line therapy to counter obesity and insulin resistance is still (significant) lifestyle change with reduced calorie intake and sufficiently intense exercise.11Figure 1: Hepatocytes integrate a host of signals and effects, which may cumulate in disease states such as NASH. A multitude of factors1 affects the liver, with surplus calories and free fatty acids (FFAs) leading to fat accumulation and steatosis. Depending on the various displayed conditions, lipotoxicity may occur, leading to hepatocellular cell death and—as demonstrated by Ibrahim et al.—release of CXCR10‐rich EVs. CXCR10‐EVs function as chemotactic signal for macrophages, connecting the lipotoxic injury and inflammation observed in NASH. Depiction of digestive system from: Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. doi: 10.15347/wjm/2014.010. ISSN 20018762. 1Listed factors are not exhaustive. Abbreviation: CXCR10, CXC chemokine receptor 10.The study by Ibrahim et al. demonstrates, in a clear and meaningful approach, a highly probable mechanism connecting hepatocyte lipotoxicity with chemotaxis of macrophages. Whereas the focus of current therapeutical approaches for NAFLD should remain on weight loss and reduction of insulin resistance, these findings are crucial to understanding the pathology of NASH. Author names in bold designate shared co‐first authorship.