Function of Mitogen-Activated Protein Kinases in Hepatic Inflammation.

The western diet and overuse of anti-inflammatory medication have caused a great deal of stress on the liver. Obesity and the associated inflammatory state in insulin-responsive tissues result in the release of pro-inflammatory cytokine that activates the stress-responsive MAPKs, p38 MAPK, and JNK. These MAPKs have figured prominently as critical effectors in physiological and pathophysiological hepatic inflammation. In contrast, evidence for a role for ERK1/2 in hepatic inflammation has been less well developed. In this review article, we describe recent insights into the physiology and pathophysiology of the role of stress-responsive MAPKs in hepatic inflammation during obesity and liver injury with a focus on macrophages, hepatocytes and hepatic stellate cells. In response to metabolic stress and liver injury, JNK activation in macrophages and hepatocytes promotes the secretion of inflammatory cytokines and macrophage and neutrophil infiltration. p38 MAPK plays an important role in contributing to the progression of hepatic inflammation in response to various hepatic cellular stresses, although the precise substrates mediating these effects in hepatocytes and hepatic stellate cells remain to be identified. Both JNK and p38 MAPK promotes profibrotic behavior in hepatic stellate cells.

Fasting-Induced Upregulation of MKP-1 Modulates the Hepatic Response to Feeding.

The liver plays a key role in whole-body, glucose and lipid homeostasis. Nutritional signals in response to fasting and refeeding regulate hepatic lipid synthesis. It is established that activation of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) in response to overnutrition regulates MAPK-dependent pathways that control lipid metabolism in the liver. However, the regulatory mechanisms and the impact of the actions of MKP-1 in hepatic response to fasting remains unclear. We investigated the effect of fasting on the expression of MKP-1 and the impact on hepatic response to feeding. In this study, we demonstrate that fasting stress induced upregulation of hepatic MKP-1 protein levels with a corresponding downregulation of p38 MAPK and JNK phosphorylation in mouse livers. We found that MKP-1-deficient livers are resistant to fasting-induced hepatic steatosis. Hepatic MKP-1 deficiency impaired fasting-induced changes in the levels of key transcription factors involved in the regulation of fatty acid and cholesterol metabolism including Srebf2 and Srebf1c. Mechanistically, MKP-1 negatively regulates Srebf2 expression by attenuating p38 MAPK pathway, suggesting its contribution to the metabolic effects of MKP-1 deficiency in the fasting liver. These findings support the hypothesis that upregulation of MKP-1 is a physiological relevant response and might be beneficial in hepatic lipid utilization during fasting in the liver. Collectively, these data unravel some of the complexity and tissue specific interaction of MKP-1 action in response to changes in nutritional cues, including fasting and excess nutrients.