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This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/liv.12556

This article is protected by copyright. All rights reserved.

Received Date : 11-Feb-2014

Accepted Date : 26-Mar-2014

Article type : Editorials

MLK3 as a regulator of disease progression in NASH

Joy X. Jiang and Natalie J. Török

Department of Internal Medicine, Division of Gastroenterology and Hepatology UC Davis

Medical Center, Sacramento, CA

Address for correspondence:

Natalie J. Török MD., UC Davis Medical Center, 4150 V Street, Suite 3500, Sacramento, CA

95817. Email: Natalie.Torok@ucdmc.ucdavis.edu; fax: 916-734-7908.

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

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elucidated. The paper 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 signaling (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 signaling plays an

important role in the progression of NASH.

MLKs are MAPK-kinase kinases (MKKKs) thatactivate JNK and p38 signaling 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.

JNK activationplays 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

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MLK3 has differential effects on JNK1 or JNK2 in the liver, and if so, how the differential

induction is mediated. The current 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

the current 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 signaling by directly binding to Myd88

(27). The dominant cell type and mechanisms for the proinflammatory 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.

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