Angiocrine signaling in sinusoidal homeostasis and liver diseases

Abstract

The hepatic sinusoids are composed of liver sinusoidal endothelial cells (LSECs), which are surrounded by hepatic stellate cells (HSCs) and contain liver-resident macrophages called Kupffer cells, and other patrolling immune cells. All these cells communicate with each other and with hepatocytes to maintain sinusoidal homeostasis and a spectrum of hepatic functions under healthy conditions. Sinusoidal homeostasis is disrupted by metabolites, toxins, viruses, and other pathological factors, leading to liver injury, chronic liver diseases, and cirrhosis. Alterations in hepatic sinusoids are linked to fibrosis progression and portal hypertension. LSECs are crucial regulators of cellular crosstalk within their microenvironment via angiocrine signaling. This review discusses the mechanisms by which angiocrine signaling orchestrates sinusoidal homeostasis, as well as the development of liver diseases. Here, we summarise the crosstalk between LSECs, HSCs, hepatocytes, cholangiocytes, and immune cells in health and disease and comment on potential novel therapeutic methods for treating liver diseases.

Keywords: Kupffer cells; MASH; MASLD; NAFLD; NASH; alcoholic liver disease; hepatic stellate cells; liver fibrosis; liver regeneration; liver sinusoidal endothelial cells; metabolic dysfunction-associated steatotic liver disease; portal hypertension; sinusoidal capillarization.

Figure 1. Structures of normal and injured LSECs and zonation of LSECs across the hepatic sinusoid in healthy liver

(A) Upper left: typical scanning electron microscopy (SEM) image of normal rat LSECs. The fenestrae are organized in sieve plates. Upper right: typical SEM image of the normal rat liver sinusoid cast. Liver sinusoids start from the portal vein and have a well-organized distribution. Bottom left: typical SEM image of capillarized rat LSECs. The number and size of fenestrae are significantly decreased. Bottom right: typical SEM image of the liver sinusoid cast in a cirrhotic rat liver. The structure of the liver sinusoids is distorted and narrowed. Courtesy of our group. (B) LSECs are classified into periportal (zone 1), mid-zonal (zone 2), and pericentral (zone 3) LSECs according to their location and diverse gene expression patterns. Dll4, Cd45, Cd36, F8, and Efnb2 are highly expressed in periportal LSECs, Lyve-1 is highly expressed in mid-zonal LSECs, and Wnt2 and Kit are highly expressed in pericentral LSECs. Cd32b and Stab2 are highly expressed in both mid-zonal and pericentral LSECs. Arrows indicate the direction of the blood flow.

Figure 2. The central role of LSECs in sinusoidal homeostasis, the phenotypical and functional alterations of capillarized LSECs, and potential mechanisms.

(A) The sinusoidal niche includes LSECs, HSCs, hepatocytes, and immune cells. Sinusoidal homeostasis relies on orchestrated crosstalk among these cells. LSECs are the central cells in the sinusoidal niche and play a pivotal role in maintaining sinusoidal homeostasis. Healthy LSECs can maintain HSC quiescence and regulate hepatocyte proliferation, zonation, and function. Moreover, healthy LSECs regulate KC distribution and maintain hepatic immune tolerance by mitigating lymphocyte activation and function. However, upon liver injury, LSECs become capillarized and acquire profibrotic and proinflammatory phenotypes. Injured LSECs can induce HSC activation, migration, and contraction. Moreover, injured LSECs promote immune cell infiltration and activation. Furthermore, injured LSECs impair liver regeneration by inhibiting hepatocyte proliferation. (B) Phenotypical and functional alterations of capillarized LSECs compared to normal LSECs and potential mechanisms driving capillarization.

Figure 3. The role of LSEC angiocrine signaling in liver regeneration

Under conditions of acute or mild liver damage, LSECs promote hepatocyte proliferation by increasing the secretion of WNT2, IL-6, and HGF through the VEGFR-Id1 signaling pathway. Moreover, the secretion of Ang-2 is decreased, leading to decreased TGF-β production and increased hepatocyte proliferation. Id1 can be activated by CXCR7, which is upregulated in LSECs during liver injury. Platelets can also activate CXCR7. In contrast, during chronic liver injury, CXCR4 expression in LSECs predominates over CXCR7 expression, which is mediated by FGFR1. CXCR4 overexpression inhibits Id1 expression and results in the activation of HSCs and impaired hepatocyte proliferation. LSEC senescence and aging also impair liver regeneration. Bone marrow-derived LSEC progenitor cells can be recruited to the sinusoidal niche by VEGF secreted by injured hepatocytes. These LSEC progenitor cells can repopulate injured LSECs and secrete HGF in favor of hepatocyte proliferation.

Figure 4. The role of LSEC angiocrine signaling in MASLD

During sinusoidal homeostasis, normal LSECs contribute to the regulation of lipid synthesis and transportation. In MASLD, capillarized LSECs have decreased porosity and impeded triglyceride, cholesterol, and chylomicron remnant transportation through the sinusoid. Moreover, activation of the Notch signaling pathway in capillarized LSECs leads to decreased NO production. Reduced NO production increases lipogenesis and impairs lipid oxidation, leading to lipid accumulation in the liver. Furthermore, capillarized LSECs acquire proinflammatory phenotypes and enhance the secretion of chemokines and the expression of adhesion molecules. As a result, immune cells are recruited to the liver and activated, contributing to liver inflammation during MASLD.

Figure 5. The role of LSEC angiocrine signaling in liver fibrosis

During sinusoidal homeostasis, healthy LSECs sense shear stress and induce the expression of KLF2. KLF2 and TAZ can induce LSEC production of eNOS and NO. The NO-GC-cGMP axis maintains HSC quiescence. Moreover, healthy LSECs can reverse activated HSCs to a quiescent state. In contrast, during liver fibrosis, injured LSECs exhibit increased expression of CXCR4 and FGFR1 and can secrete a broad spectrum of profibrotic factors, such as A-FABP, PDGFB, SK1-containing exosomes, ET-1, and RANTES, which induce HSC activation, migration, and contraction. Injured LSECs also exhibit decreased expression of GATA4, ZEB2, ERK1/2, and JAM-A, which also facilitates liver fibrosis. Moreover, LSECs can undergo EndoMT and produce ECM, accompanied by impaired autophagy during liver fibrosis. LSECs also secrete chemokines such as CXCL1 and CCL2, which induce immune cell infiltration and lead to exaggerated liver inflammation and fibrosis. Injured hepatocytes secrete LECT2, which binds to the Tie2 receptor on LSECs and promotes HSC activation. VCAM expressed on LSECs also contributes to liver fibrosis.

Figure 6. The role of LSEC angiocrine signaling in portal hypertension

LSECs are involved in regulating the vascular tone of the hepatic microcirculatory system. On the one hand, injured LSECs overexpress vasoconstrictors, which induce HSC contraction. On the other hand, the release of the vasodilator nitric oxide (NO) was decreased in injured LSECs due to reduced production and increased scavenging. Consequently, intrahepatic vascular resistance and shear stress increase, leading to the upregulation of the vasoprotective factor KLF2. Increased tissue stiffness upregulates CXCL1 secretion in LSECs. CXCL1 contributes to neutrophil recruitment, which accelerates microvascular thrombosis together with LSEC overexpression of prothrombotic factors. Elevated shear stress also leads to CCL2 secretion and macrophage accumulation through the interaction of endothelial p300 with NFκB and BRD4. Decreased vasodilators, increased vasoconstrictors, and microvascular thrombosis together contribute to portal hypertension.