The Role of Sinusoidal Endothelial Cells in the Axis of Inflammation and Cancer Within the Liver

Abstract

Liver sinusoidal endothelial cells (LSEC) form a unique barrier between the liver sinusoids and the underlying parenchyma, and thus play a crucial role in maintaining metabolic and immune homeostasis, as well as actively contributing to disease pathophysiology. Whilst their endocytic and scavenging function is integral for nutrient exchange and clearance of waste products, their capillarisation and dysfunction precedes fibrogenesis. Furthermore, their ability to promote immune tolerance and recruit distinct immunosuppressive leukocyte subsets can allow persistence of chronic viral infections and facilitate tumour development. In this review, we present the immunological and barrier functions of LSEC, along with their role in orchestrating fibrotic processes which precede tumourigenesis. We also summarise the role of LSEC in modulating the tumour microenvironment, and promoting development of a pre-metastatic niche, which can drive formation of secondary liver tumours. Finally, we summarise closely inter-linked disease pathways which collectively perpetuate pathogenesis, highlighting LSEC as novel targets for therapeutic intervention.

Keywords: capillarisation; endothelial dysfunction; fibrosis; hepatocellular carcinoma; inflammation; leukocyte recruitment; liver sinusoidal endothelial cell; metastasis.

FIGURE 1

The role of LSEC in maintaining homeostasis and disease pathology following capillarisation and endothelial dysfunction. Left: LSEC have a distinct morphology which facilitates their homeostatic function. (1) Lack of basement membrane and fenestrations arranged in sieve plates permit relatively free movement of macromolecules, such as lipoproteins, towards hepatocytes within the parenchyma. Lipoproteins can also be endocytosed by scavenger receptors SR-B1 and Stab1. (2) Scavenger receptors also facilitate uptake and clearance of waste products including apoptotic cell bodies (SCARF1), IgG immune complexes (CD32b), lysosomal enzymes (MR), and collagen α chains (MR). (3) LSEC remain in close proximity with HSC within the space of Disse via CXCR4-SDF1α and PDGF-β PDGFR-β interactions. (4) LSEC maintain HSC quiescence in response to shear stress through eNOS-dependent NO production, and inhibition of ET-1, via transcription factor KFL2. (5) The differentiated LSEC phenotype maintains vasodilation of the sinusoids. (6) VEGF production by LSEC, HSC, hepatocytes and cholangiocytes also maintain HSC quiescence and prevent LSEC capillarisation. Right: (1) Capillarisation is associated with upregulation of VCAM-1 and CD31, loss of GATA4 signalling, reduced fenestrations, and basement membrane synthesis, leading to hyperlipoproteinaemia. This can be prevented by BMP9. (2) Endothelial dysfunction is the inability to produce NO in response to shear stress, and paired with ET-1 synthesis, results in HSC activation. (3) Additional angiocrine signals release from capillarised LSEC also perpetuate HSC activation, such as excess VEGF, Hh signals and TGFβ (4) Activated HSC begin to deposit ECM which increases tissue stiffness, further stimulating HSC activation. (5) HSC respond by causing vasoconstriction which increases vascular resistance and shear stress. (6) It is thought that LSEC respond to these mechanocrine signals via PIEZO channels, notch-dependent HEY1 and HES1 translocation and subsequent CXCL1 secretion. (7) This leads to neutrophil recruitment, and NETosis induces microvessel thrombosis which perpetuates increased vascular resistance resulting in portal hypertension. (8) Ultimately, capillarisation and endothelial dysfunction precede angiogenesis and fibrosis, which increase the risk of cirrhosis and HCC. LSEC, liver sinusoidal endothelial cells; SR-B1, scavenger receptor class B type 1; Stab1, stabilin-1; SCARF1, scavenger receptor class F member 1; CD32b, Fcγ receptor 2b; MR, mannose receptor; RME, receptor-mediated endocytosis; HSC, hepatic stellate cell; CXCR4, C-X-C chemokine receptor type 4; SDF1α, stromal-derived factor 1α; PDGFβ platelet-derived growth factor β; eNOS, endothelial nitric oxide synthase; NO, nitric oxide; ET-1, endothelin-1; KLF2, Krüppel-like factor 2; VEGF, vascular endothelial growth factor; VCAM-1, vascular cell adhesion molecule 1; BMP9, bone morphogenic protein 9; Hh, hedgehog; TGFβ, transforming growth factorβ ECM, extracellular matrix; NETs, neutrophil extracellular traps; HCC, hepatocellular carcinoma.

FIGURE 2

LSEC maintain immune tolerance and facilitate immune surveillance by several mechanisms. (1) Viral particles gain access to the parenchyma through fenestrations and HBV/HCV can then go on to infect hepatocytes. (2) CD8⁺ T cells extend protrusions through fenestrations and can probe for viral antigens presented by infected hepatocytes via MHCI. (3) LSEC can also take up viral particles via transcytosis, and RNA sensing by intracellular TLRs leads to production of IFN-rich exosomes which inhibit viral replication. (4) LSEC express pathogen recognition receptors, including TLR4 and NOD1/2, which signal via NFκB leading to cytokine production. (5) Clearance of dietary LPS via TLR4 induces tolerogenic responses by inhibiting NFκB translocation and antigen presentation. (6) MR-mediated antigen uptake and presentation by MHCI induces tolerogenic CD8⁺ T cell responses in the presence of PDL1, which can be overcome by excess TCR signalling in response to high antigen concentrations. (7) MR-mediated uptake also precedes antigen presentation to CD4⁺ T cells via MHCII, leading to T_reg induction in the presence of PDL1 and absence of co-stimulation. (8) Typically, classic adhesion molecules VCAM-1 and ICAM-1, as well as VAP-1, are involved in T_eff recruitment. VCAM-1 is often arranged in microdomains, forming endothelial adhesive platforms in associated with tetraspanin CD151. Hepatocytes can mediate leukocyte recruitment indirectly by modulating expression of adhesion molecules. LSEC are also involved in recruitment of distinct leukocyte subsets via atypical adhesion molecules and chemokines, such as (9) CD4⁺ and T_reg cells by SCARF1 and Stab1, respectively, and (10) production of CXCL16 which promotes retention of CXCR6⁺ NK and NKT cells. (11) LSEC also contribute to immune tolerance by inhibiting DCs and promoting apoptosis of CD4⁺ autoreactive thymic emigrants. (12) LSEC recruit CXCR7⁺ BM SPCs via SDF1α, which mediate hepatocyte proliferation via HGF production and thus, liver regeneration. LSEC, liver sinusoidal endothelial cells; HBV, hepatitis B virus; HCV, hepatitis C virus; TCR, T cell receptor; MHCI, major histocompatibility complex class I; TLR, toll-like receptor; IFN, interferon; NOD, nucleotide-binding oligomerisation domain; NFκB, nuclear factor κ-light-chain-enhancer of activated B cells; LPS, lipopolysaccharide; MR, mannose receptor; Ag, antigen; PD-1, programmed cell death protein 1; PDL1, programmed death ligand 1; T_eff, effector T cell; IL-2, interleukin-2; MHCII, major histocompatibility complex class II; T_reg, regulatory T cell; VCAM-1, vascular cell adhesion molecule 1; ICAM-1, intercellular adhesion molecule-1; VAP-1, vascular adhesion protein 1; Stab1, stabilin-1; SCARF1, scavenger receptor class F member 1; CXCL16, C-X-C chemokine ligand 16; CXCR6, C-X-C chemokine receptor 6; NK, natural killer; NKT natural killer T; DC, dendritic cell; BM SPCs, bone marrow sinusoidal precursor cells; SDF1α, stromal-derived factor 1α; HGF, hepatocyte growth factor; HSC, hepatic stellate cell.

FIGURE 3

LSEC orchestrate the immune microenvironment during chronic inflammation. (1) During chronic inflammation, repeated hepatocyte injury results in release of DAMPs which are sensed by KC, resulting in their activation and subsequent production of pro-inflammatory cytokines. These DAMPs also trigger cytokine release from LSEC via NFκB and inflammasome signalling, further perpetuating LSEC and KC activation. This is exacerbated by endothelial dysfunction. (2) Production of BMP4 by LSEC can also promote viral replication which can worsen hepatocyte damage during chronic viral infection. Activated LSEC (3) secrete chemokines and (4) upregulate their expression of adhesion molecules, which facilitates leukocyte recruitment, adhesion and transmigration. (5) Leukocytes can be retained within the space of Disse due to VAP-1 expression by HSC. (6) Following SCARF1-mediated adhesion, CD4⁺ T cells have been shown to perform lateral intracellular crawling between LSEC, which is mediated by ICAM-1 and Stab1. LSEC are also important for recruiting distinct pro-inflammatory leukocyte subsets during diseases states, including (7) gut-homing lymphocytes via α4β7-MAdCAM interactions, and (8) CD16⁺ Mo via secretion of CX₃CL1. LSEC, liver sinusoidal endothelial cells; DAMPs, danger-associated molecular patterns; KC, Kupffer cell; TNFα, tumour necrosis factor α; MCP-1, monocyte chemoattractant protein 1; IL-6, interleukin-6; NFκB, nuclear factor κ-light-chain-enhancer of activated B cells; NLRP, nucleotide-binding oligomerisation domain, leucine-rich repeat and pyrin domain; NO, nitric oxide; BMP4, bone morphogenic protein 4; ICAM-1, intercellular adhesion molecule-1; VAP-1, vascular adhesion protein 1; VCAM-1, vascular cell adhesion molecule 1; SCARF1, scavenger receptor class F member 1; Stab1, stabilin-1; MAdCAM, mucosal addressin cell adhesion molecule 1; Mo, monocyte; CX₃CL1, fractalkine; HSC, hepatic stellate cell.

FIGURE 4

LSEC actively contribute to the tumour microenvironment during HCC and liver metastasis. Left: LSEC promote an immunosuppressive microenvironment and thus, HCC development and progression. (1) LSEC presentation of tumour antigens to CD8⁺ T cells via MHCI induces tolerogenic responses, in the presence of PDL1, which is upregulated in HCC tumours. (2) Production of IL-10 by LSEC induces T_reg, which are recruited via Stab1, undergoing transmigration and inhibiting T_eff responses. (3) T_reg are also induced by MDSCs, which accumulate in HCC due to LSEC production of CXCL1 and CXCL2. MDSCs elicit pro-tumourigenic effects including inhibition of T cell activation, NK cell inhibition, and stimulation of angiogenesis. (4) Transdifferentiated LSEC lose expression of LSEC markers and upregulate expression of adhesion molecules VCAM-1, CD151, VAP-1 and ICAM-1, which facilitates leukocyte recruitment. (5) Transformed malignant hepatocytes enhance CCL2, CCL3, and CXCL10 secretion, further promoting leukocyte recruitment. (6) Hypoxia-induced production of CCL20 by hepatomas inhibits T cell proliferation. (7) LSEC production of adipokines, such as FABP4, in response to hypoxia and VEGF exert pro-oncogenic effects by inducing hepatocyte proliferation. These effects can be attenuated with FABP4-specific inhibitor BMS309403. (8) LSEC also foster conditions which promote pro-tumourigenic angiogenesis, including recruitment of pro-angiogenic BM EPC, AEP production and expression of T-cadherin in response to hepatoma- and MDSC-derived FGF. (9) Anti-inflammatory TAM also promote immunosuppression and angiogenesis. Right: LSEC orchestrate formation of a pre-metastatic niche which promotes development of secondary liver tumours. (1) Blood-borne cancer cells can become entrapped within LSEC fenestrae and accumulate in the sinusoidal lumen. (2) Tumour cells promote LSEC secretion of pro-metastatic mediators such as MIF, IL-1 and CXCL12, as well as (3) activation of KC which produce pro-inflammatory cytokines that in turn activate LSEC. TNFR inhibition has been shown to prevent liver metastasis in mice. (4) Activated LSEC upregulate expression of adhesion molecules which promotes binding and invasion of cancer cells to the space of Disse, where they are generally protected from KC and NK cells within the sinusoids. Wnt-independent Notch activation has been shown to inhibit tumour cell adhesion. (5) LSEC secrete FN which interact with α9β1 integrin on cancer cells, initiating EMT and promoting metastatic spread. (6) LSEC also express L-SIGN and LSECtin, which are upregulated in liver metastasis and mediate adhesion and migration of cancer cells. L-SIGN blockade reduces colon cancer metastasis in murine models. LSEC, liver sinusoidal endothelial cells; HCC, hepatocellular carcinoma; MHCI, major histocompatibility complex I; PD-1, programmed cell death protein 1; PDL1, programmed death ligand 1; IL-10, interleukin 10; T_reg, regulatory T cell; Stab1, stabilin-1; T_eff, effector T cell; MDSC, myeloid-derived suppressor cell; CXCL1, C-X-C chemokine ligand type 1; NK, natural killer cell; VCAM-1, vascular cell adhesion molecule 1; VAP-1, vascular adhesion protein 1; ICAM-1, intercellular adhesion molecule 1; CCL2, C-C chemokine ligand type 2; FABP4, fatty acid binding protein 4; VEGF, vascular endothelial growth factor; BM EPC, bone marrow erythroid progenitor cells; AEP, asparaginyl endopeptidase; FGF, fibroblast growth factor; TAM, tumour-associated macrophage; HSC, hepatic stellate cell; TGFβ, transforming growth factor β; MIF, macrophage migration inhibitory factor; KC, Kupffer cell; TNFα, tumour necrosis factor α; TNFR, tumour necrosis factor receptor; FN, fibronectin; EMT, epithelial-to-mesenchymal transition; L-SIGN, lymph node-specific ICAM-3 grabbing non-integrin; LSECtin, lymph node sinusoidal endothelial cell C-type lectin.

FIGURE 5

Liver disease follows a common pathway of progression which results in fibrogenesis that is both preceded and driven by LSEC capillarisation and dysfunction. This figure summarises the common disease pathways discussed in this review, highlighting various approaches for potential therapeutic intervention. These include: (1) molecules which maintain LSEC homeostasis, such as BMP9, statins and phosphodiesterase inhibitors; (2) anti-angiogenics which are also anti-inflammatory and anti-fibrotic, such as L1-10, AT-II inhibition and sorafenib; (3) anti-fibrotics, including anti-VAP1 and Hh inhibition; (4) and targets involved in leukocyte recruitment. LSEC, liver sinusoidal endothelial cells; HCC, hepatocellular carcinoma; BMP9, bone morphogenic protein 9; VEGF, vascular endothelial growth factor; NO, nitric oxide; sGC, soluble guanylate cyclase; PDE, phosphodiesterase; AT-II, angiotensin II; VAP-1, vascular adhesion protein 1; Hh, hedgehog; HSC, hepatic stellate cell.