Arsenic Stimulates Sinusoidal Endothelial Cell Capillarization and Vessel Remodeling in Mouse Liver

The vascular effects of arsenic are a global public health concern that contribute to disease in tens of millions of people worldwide.1 Whereas the role of environmental contaminants in the etiology of vascular diseases and in the vascular contributions to organ dysfunction remains poorly defined, epidemiological studies have associated As(III) exposures to increased risk of cardiovascular diseases1 and vascular contributions to liver disease.2 Liver effects associated with arsenic in drinking water include noncirrhotic portal fibrosis and, to a lesser extent, portal hypertension.2,3 These pathologic conditions involve increased vascular channels in the portal regions of the liver. Higher levels of chronic As(III) consumption increase urinary levels of porphyrins, a biomarker for liver injury, which are more pronounced in people under 20 years of age.4 In addition, cardiac and liver disorders are the major side effects of therapeutic As(III) regimes that treat leukemias.5 Despite epidemiological evidence that the liver vasculature is a pathogenic target of chronic As(III) ingestion,2 the direct effects of As(III) on liver vascular cells are unknown. In other vascular beds and isolated cell cultures, As(III) affects both endothelial and smooth muscle cell physiology. Arsenic(III) stimulates angiogenic processes in cultured endothelial cells and neovascularization in intact mouse and avian models.6–9 These angiogenic effects promote tumorigenesis in mice, as well as vascular remodeling.10,11 Stimulation of endothelial cell proliferation occurs at concentrations of As(III) that are not cytotoxic, whereas higher concentrations are cytotoxic and inhibit angiogenesis in tumors.10,11 Arsenic(III) stimulates cultured smooth muscle cells to proliferate and express vascular endothelial cell growth factor (VEGF), a primary mediator of angiogenesis.12 Although these effects of As(III) on cells from systemic blood vessels are well recognized, there is little data on the effects of As(III) exposures on liver vascular remodeling or specifically on the fenestrated sinusoidal endothelium. The liver sinusoidal endothelial cells (SECs) are highly specialized fenestrated cells that are unique in the extensive heterogeneity of vascular endothelium.13 Early in development, these cells differentiate to lose markers of a continuous endothelium, such as junctional expression of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) and a basement membrane containing the matrix protein laminin-1.14 In the differentiation process, the SECs become fenestrated to allow sieving of circulating nutrients, lipids, and lipoproteins for normal liver metabolism.13 The SEC angiogenic process is different from angiogenesis in endothelial cells of systemic vessels, because there is no increase in sinusoidal vessel number or density. Instead, SEC angiogenesis is a dedifferentiation and maturation process called capillarization with diagnostic hallmarks of SEC defenestration and renewed surface expression of PECAM-1 and laminin-1 proteins.13,15–18 SEC defenestration and formation of tight intercellular junctions limits transendothelial cell transport.13,16,19,20 Capillarization precedes vascular remodeling of other liver vessels, such as hepatic arterioles and PBVP, causing blood flow shunting, vascular channel formation, and eventually liver fibrosis.15,16,21 Liver angiogenesis in general is recognized as an important pathogenic process not only in portal fibrosis, but also in portal hypertension and progression of HCCs.18,22–25 Finally, animal models have been used to demonstrate that liver capillarization affects the systemic vasculature by decreasing liver metabolism of lipids, lipoproteins, and glucose to promote atherogenesis in response to environmental stresses and aging.13,26,27 In mouse tumors or surrogate in vivo neovascularization assays, low to moderate levels of As(III) (5–250 ppb) in drinking water are angiogenic.7,28 However, As(III) effects on the liver endothelium or pathogenic angiogenic processes in the liver are unknown. The aim of this study was to investigate whether a chronic, human-relevant, environmental exposure to As(III) causes SEC dedifferentiation and dysfunctional capillarization in intact mice. The data indicate that prolonged As(III) exposure causes SEC maturation and sustained functional changes in SEC cell signaling.