MLK3 as a regulator of disease progression in Non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is one of the most common chronic liver diseases worldwide (1). Despite the large number of studies published in the field, the molecular signals triggering the progression of NASH from simple steatosis to necroinflammation are still poorly understood. One of the most important and early features of progressive NASH is lipoapoptosis of hepatocytes that creates a proinflammatory and fibrogenic environment (2, 3). The activation of c-jun N-terminal kinase 1(JNK1) in both hepatocytes and macrophages has been described as a key event in NASH (4, 5), thus the signals governing its induction need to be better elucidated. The article published by Ibrahim et al. (6) has focused on the role of the mixed-lineage kinase 3 (MLK3) as an important proximal JNK activator that has a major role in progressive liver injury in diet-induced NASH. Several studies demonstrated that MLK3/JNK activation plays an essential role in saturated fatty acid-induced insulin resistance and hepatocyte lipotoxicity (7–9). Exposure to free fatty acid induced MLK3/JNK in mouse embryonic fibroblasts, and the MLK3^-/- cells displayed increased insulin sensitivity (8); the same trend was found in the MLK3^-/- mice on high-fat diet (8, 10). In hepatocytes, palmitate induced the recruitment of cdc42/Rac1 and the activation of MLK3/JNK, leading to downstream ER stress signalling (7). In a different study, the palmitate-induced M1 macrophage polarization was diminished in the MLK3^-/- cells (10). All of these suggest that MLK3/JNK signalling plays an important role in the progression of NASH.

MLKs are MAPK-kinase kinases (MKKKs) that activate JNK and p38 signalling cascades via the MAPK kinases (MKKs) by either forming a homodimer or associating with Rho GTPases (11–13). The selective activation of MKKs and the downstream induction of JNK1/2 or p38 by MLKs are mediated by the MAPK scaffold proteins such as JNK-interacting proteins (JIPs) (13, 14). How these JIPs direct selective activation of their targets is still under investigation. JIP3 was shown to bind to MLK3/MKK7 inducing JNK1 activation in neurons (14–16). It is however, not clear which JIPs are predominant in the liver and whether they selectively mediate JNK1 and 2 activation.

c-Jun N-terminal kinase activation plays a key role in saturated fatty acid-induced hepatocyte apoptosis (5), both in humans and in animal models (17–19). In HSC, phosphorylated-JNK1 directly mediates transdifferentiation contributing to fibrosis (20). MLK3/JNK/P38 activation in LX-2 cells has been observed when cells were stimulated with a PPARβ/δ ligand, leading to cell proliferation (21). JNK1 and JNK2 have differential effects in NASH: JNK1 mediates steatohepatitis and lipotoxicity, whereas JNK2 activation is more protective (22, 23). It is not yet known whether MLK3 has differential effects on JNK1 or JNK2 in the liver, and if so, how the differential induction is mediated. This study is in agreement with previous findings demonstrating that the MLK3^-/- mice developed less hepatic steatosis, decreased liver injury, inflammation and fibrosis. The diet used in this study mimics the fast food diet consumed by humans inducing insulin resistance, steatohepatitis and fibrosis (24). It has been recently reported that compared to the WT mice, the global MLK3-deficient mice on high-fat diet displayed significantly less weight gain, and reduced macrophage infiltration in the adipose tissue and also decreased systemic inflammation (10). Interestingly, these mice had increased energy expenditure that could have accounted for the slower weight gain. The improved inflammation in the liver could be explained by the decreased macrophage recruitment and JNK inactivation in the MLK3^-/- mice (10). Using the MLK2/3 double knockout mice on the high-fat diet model, Davis and colleagues showed that the obesity-resistant phenotype of these animals was related to the upregulation of the sympatho-adrenal system, as using a selective antagonist to the β3-adrenergic receptor prevented the increase in body temperature and decreased the expression of the adrenergic target genes (25). Distinct from the studies described above, the authors in this study chose high-fat diet combined with high fructose consumption. The mice in this study gained similar amount of weight in both treatment arms, and this could be attributed to the high fructose intake. As fibrosis was diminished in the MLK3^-/- mice, it is likely that MLK3 in stellate cells was involved in their transdifferentiation, which has been observed in lung fibroblasts by Lin et al. (26). Macrophage polarization could also be affected by MLK3 (10); and there is evidence showing that MLK3 interacts with TLR signalling by directly binding to Myd88 (27). The dominant cell type and mechanisms for the pro-inflammatory and fibrogenic activity of MLK3 in the liver would require further investigation, and could be addressed in the future by the generation of conditional knockout mice.