The role of immune system in liver pathophysiology

Resumen

Obesity is characterized by a low-grade inflammatory state that impairs insulin signaling and contributes to excess hepatic lipid accumulation, promoting the development of non-alcoholic fatty liver disease (NAFLD). In this scenario, neutrophils are one of the first immune cells contributing to the onset of liver inflammation and pathology. On the other side, neutrophils rhythmically infiltrate into certain tissues affecting their physiological circadian function. Keeping liver daily rhythms is essential for hepatic physiology. However, whether neutrophils can control liver daily rhythms remains unknown. Liver resident macrophages, also known as Kupffer cells (KCs), are also crucial for the progression of NAFLD and many studies demonstrated that depletion of these macrophages protects against hepatic disease. However, detailed mechanisms for this regulation are largely unknown. The family of stress activated kinases (SAPKs) are comprised of c-Jun NH2-terminal kinase (JNK) and p38 protein kinases. These kinases are essential in the transduction of stress signals and many studies demonstrated their critical role in the progression of obesity and fatty liver. Still, little is known about the impact of p38 activation on myeloid or Kupffer cells and its contribution to liver diseases. The main aim in this thesis is to address the role of neutrophils and liver macrophages in liver pathophysiology and explore the contribution of p38 activation in macrophages to this process. Using mice lacking neutrophils or deficient for neutrophil elastase (NE) we demonstrated that neutrophils were necessary for keeping the liver clock. Importantly, these mice were protected against western diet or JetLag induced liver circadian disruption. This protection was lost when mice lacking neutrophils were infused with neutrophils from control mice, but not from mice lacking NE, corroborating that NE secreted by neutrophils is the mediator of this modulation. At the molecular level, we deciphered that NE in the liver signals through JNK controlling the expression of liver clock genes and repressing the levels of Fibroblast Growth Factor 21 (FGF21), the master regulator of lipid metabolism in the liver. Secondly, we explored the role of p38 activation in myeloid cells during obesity. Using a conditional mouse for the upstream activators of p38s, MKK3 and MKK6, in myeloid cells, we observed that these mice were more obese, diabetic and presented marked energy imbalance after High-fat diet (HFD) feeding. We uncovered that myeloid p38 activation was modulating whole-body metabolism through the regulation of brown adipose tissue thermogenesis and energy expenditure by directly affecting hepatic FGF21. At the molecular level, we found that activation of myeloid p38 in liver infiltrating macrophages was modulating hepatic FGF21 through the control of IL-12 release. To further explore the role of p38 pathway activation specifically in KCs during obesity, we generated mice deficient in MKK3 and MKK6 in this compartment. We found that activation of p38 in KCs was necessary for the expression of the lipid scavenger receptor CD36 and lipid accumulation by these cells during obesity. Intriguingly, activation of this pathways was also essential for the expansion of KC2, a subpopulation of KCs that is actively implicated in the development of obesity and NAFLD. Moreover, we demonstrated that control of KC2 expansion during obesity by KC p38 activation was through p38 regulation of CD36 in KCs. Finally, we addressed the impact of KC depletion during the progression of NAFLD and obesity using a mice model that specifically express the inducible receptor for diphtheria toxin (DT) in KCs. We found that chronic depletion of KCs protected against HFD induced hepatic steatosis and inflammation and that replenishment of KCs by macrophages from the bone-marrow also improved mice metabolic profile and protected against NAFLD. The results from this thesis provide novel insights into the control of liver daily rhythms by neutrophil elastase and highlight the importance of p38 activation in liver macrophages and KCs during the development of hepatic steatosis and obesity, providing potential new therapeutic targets for the treatment of liver diseases