Differential effects of JNK1 and JNK2 inhibition on murine steatohepatitis and insulin resistance.

The hepatocellular mechanisms underlying the development of steatosis and the progression to steatohepatitis in nonalcoholic fatty liver disease (NAFLD) remain unclear.1 A critical factor in the pathogenesis of this disease is thought to be the existence of insulin resistance, and NAFLD can be considered a component of the metabolic syndrome whose manifestations also include obesity and diabetes.2,3 Recent investigations have suggested that activation of the mitogen-activated protein kinase c-Jun N-terminal kinase (JNK) is a central mechanism for the development of obesity and insulin resistance as well as for steatohepatitis.4-7 JNK may therefore represent a unique common target for the therapy of a variety of diseases that comprise the metabolic syndrome, but whether an inhibition of JNK function will decrease or prevent further progression of established steatohepatitis is currently unknown. Most cells including hepatocytes express two JNK genes, jnk1 and jnk2, which are alternatively spliced to yield α and β forms of both a p54 and p46 protein.8 Recent studies have demonstrated that the JNK isoforms differ in function as exemplified by the ability of JNK1 to phosphorylate and activate the transcription factor c-Jun, whereas JNK2 lacks this activity and may even oppose this action of JNK1.9 Our prior studies have implicated JNK signaling in the development of steatohepatitis. First, it was demonstrated that hepatocyte overexpression of the prooxidant enzyme cytochrome P450 2E1 as occurs in human NAFLD induced sustained JNK activation and insulin resistance.10 Second, the development of murine steatohepatitis induced by a methionine- and choline-deficient diet was associated with increased JNK activation.6 The development of steatosis and liver injury was prevented in jnk1 but not jnk2 null mice, indicating that JNK1 specifically functions in the development of this disease.6 JNK1 has also been demonstrated to mediate the development of obesity and insulin resistance in both high fat diet (HFD)-fed and genetically obese mouse models.4 JNK2 was reportedly not involved in these processes, but subsequent studies in mice haploinsufficient for jnk1 and null for jnk2 suggested that JNK2 may also promote the development of obesity and insulin resistance.7 These mice and jnk1-/- mice also failed to develop steatosis from a HFD.7 Thus, JNK1 function is clearly linked to the development of steatohepatitis, obesity and insulin resistance whereas the function of JNK2 in these conditions is less clear. To more clearly establish the function of JNK1 and JNK2 in steatohepatitis occurring in the setting of obesity and insulin resistance, the effect of JNK1 or JNK2 ablation on the development of steatohepatitis was examined in HFD-fed mice. Although such studies are important to our understanding of the pathophysiology of NAFLD, findings derived from investigations of developing disease do not define JNK function in the progression or perpetuation of established steatohepatitis. Studies to examine the role of JNK in existing steatohepatitis have not been performed and are critical to determine whether JNK is a viable therapeutic target in this disease. The development of antisense oligonucleotides (ASO) that are effective in inhibiting JNK1 or JNK2 expression in the liver,11 led us to examine the effects of JNK inhibition in established as well as developing steatohepatitis in HFD-fed mice. The studies demonstrate that JNK1 mediates insulin resistance and both the development and progression of steatohepatitis. In contrast, although JNK2 promoted insulin resistance, this JNK isoform did not mediate the development or progression of steatohepatitis. These findings indicate that the JNK isoforms have distinct functions that differ among the various manifestations of the metabolic syndrome and have important implications for JNK as a therapeutic target in this disorder.