Harnessing metabolism of hepatic macrophages to aid liver regeneration

Abstract

Liver regeneration is a dynamic and regulated process that involves inflammation, granulation, and tissue remodeling. Hepatic macrophages, abundantly distributed in the liver, are essential components that actively participate in each step to orchestrate liver regeneration. In the homeostatic liver, resident macrophages (Kupffer cells) acquire a tolerogenic phenotype and contribute to immunological tolerance. Following toxicity-induced damage or physical resection, Kupffer cells as well as monocyte-derived macrophages can be activated and promote an inflammatory process that supports the survival and activation of hepatic myofibroblasts and thus promotes scar tissue formation. Subsequently, these macrophages, in turn, exhibit the anti-inflammatory effects critical to extracellular matrix remodeling during the resolution stage. However, continuous damage-induced chronic inflammation generally leads to hepatic macrophage dysfunction, which exacerbates hepatocellular injury and triggers further liver fibrosis and even cirrhosis. Emerging macrophage-targeting strategies have shown efficacy in both preclinical and clinical studies. Increasing evidence indicates that metabolic rewiring provides substrates for epigenetic modification, which endows monocytes/macrophages with prolonged "innate immune memory". Therefore, it is reasonable to conceive novel therapeutic strategies for metabolically reprogramming macrophages and thus mediate a homeostatic or reparative process for hepatic inflammation management and liver regeneration.

Fig. 1. Spatiotemporal heterogeneity of hepatic macrophages.

A The liver is composed of hexagonal lobules with several portal triads, including the bile duct, portal vein and hepatic artery, as well as a central vein. Hepatic macrophages that have either a fetal origin or have been derived from progenitors in the peripheral circulatory system play a critical role in maintaining liver homeostasis and promoting liver regeneration. B In the homeostatic liver, liver resident Kupffer cells (KCs) are distributed in periportal (PP) regions, where they are critical for capturing bacteria exiting the gut. These KCs show self-renewal capacity and acquire a tolerogenic phenotype, whereas macrophages located in the central vein region are characterized by a pro-inflammatory phenotype. Monocyte-derived liver capsular macrophages (LCMs) can sense and control intrahepatic bacterial dissemination by recruiting neutrophils to the capsule. KCs and LCMs occupy and defend two different pathogen entry points and mediate immune responses in the liver. Moreover, lipid-associated macrophages (LAMs) are near bile ducts and are activated by local lipid exposure. They are also derived from peripheral monocytes, which have been broadly associated with the immune response. C After liver injury, the number of KCs is markedly decreased, and they promote the recruitment of immune cells into the liver. In this context, the monocyte-derived KC (moKC) population contributes to replenishment of the quantity and functionality of the KC pool. Pro-inflammatory macrophages as well as circulatory splenic macrophages infiltrate the damaged liver and generate an inflammatory microenvironment, which activates hepatic stellate cells (HSCs) and induces the formation of scar tissue. Then, pro-inflammatory macrophages can be reprogrammed into repair-promoting phenotypes to promote liver regeneration. LAMs have also been reported to be required for hepatic regeneration by forming crown-like structures (hCLSs), and loss of hCLSs promotes liver fibrosis. Recently, Gata6⁺ peritoneal macrophages (PMs) have been shown to directly invade the liver through a non-vessel pathway and enhance liver repair.

Fig. 2. Macrophages in metabolic zones.

Mitochondrial metabolism plays essential roles in macrophage function. Pro-inflammatory cytokines trigger glycolysis in macrophages and induce the immune response, while anti-inflammatory macrophages rely on oxidative phosphorylation (OXPHOS) and β-oxidation of fatty acids (FAO). A Metabolic zonation in the liver has been reported based on metabolic diversity in different regions of the liver. Blood circulation in the liver creates a series of gradients, such as oxygen, hormone, nutrient, and waste product gradients, which leads to enhanced OXPHOS and FAO in periportal regions and an increase in glycolysis around central veins. Accordingly, CD68⁺MARCO⁺ KCs distributed in periportal regions exhibit anti-inflammatory capacity. They express high PPARs and LIPA (encoding LAL), contributing to lipolysis and mitochondrial metabolism. LXR expression is also upregulated in these cells to regulate lipid metabolism and produce anti-inflammatory PUFAs. Notably, LXR is required for KC signature expression during the differentiation of monocytes into KCs. In contrast, CD68⁺MARCO^- macrophages are observed in central vein regions and are associated with inflammation. B During liver regeneration, KCs increase the uptake of glucose and trigger glycolytic metabolism. Ly6C^hi macrophages are recruited into the liver and accelerate inflammation. C Subsequently, they are further differentiated into resolutive Ly6C^lo macrophages to promote liver repair. Mitochondrial OXPHOS and β-oxidation of fatty acid occur in these macrophages, which contributes to the suppression of inflammation and ECM remodeling. The PPAR level is obviously increased in these cells compared with that in Ly6C^hi macrophages. This functional alteration is caused by phagocytosis. LAL lysosomal acid lipase, PUFA polyunsaturated fatty acid, PPARs peroxisome proliferator-activated receptors, LIPAs lipase A, LXR liver-x-receptor.

Fig. 3. Therapeutic approaches for liver regeneration.

Approaches targeting hepatic macrophages are shown. Metabolic reprogramming can be used to establish inflammation-resolving macrophages by reprogramming cells to undergo oxidative mitochondrial metabolism not proinflammatory glycolysis (left panel). An alternative strategy for boosting liver regeneration is macrophage-based cell therapy, which is being actively explored (right panel). See main text for details.

Fig. 4. Challenges of hepatic macrophage-targeted strategies.

Many key parameters need to be optimized in macrophage-based therapy designs. These parameters involve the sources and heterogeneity of macrophages, and an optimized protocol for macrophage pre-treatment, stability of macrophage function, dosage, are routes and timing for macrophage injection, which are crucial for the successful application of macrophages in clinical settings, need to be identified. Mϕ: macrophages.