Reversal of vasohibin-driven negative feedback loop of vascular endothelial growth factor/angiogenesis axis promises a novel antifibrotic therapeutic strategy for liver diseases

Abstract

Pubmed search with key words “vasohibin (VASH) and liver” extracted only eight publications since 2004 when vasohibin was reported for the first time. Among them, only two works1, 2 have dealt with hepatocellular carcinoma (HCC), whereas another work explored the VASH implications in liver regeneration.3 At this point of time, the work of Coch et al. has significantly extended and strengthened the understanding of VASH1 implications in remodeling of hepatic angiogenesis pertaining to development of liver diseases.4 Discovery of VASH and its selective induction in endothelial cells (ECs) by VEGF by Sato's group in 2004 is an important addition to the list of antiangiogenic proteins.5 VASH appears to operate as an intrinsic, highly specific feedback inhibitor of activated ECs engaged in the process of angiogenesis. This virtue of VASH could be exploited in developing a therapeutic strategy to curb liver fibrosis because fibrotic scar formation is associated with elevated proangiogenic activity in the sensitized liver.4 Based on this proposition, the work of Coch et al. elaborated on the antiangiogenic effects of VASH1 that contained liver fibrosis and cirrhosis. This was a significant step forward in formulating a therapeutic strategy for liver pathologies associated with fibrosis. Although the work convincingly demonstrated antiangiogenic effects of VASH1 on VEGF-dominated liver fibrosis, it could not offer the mechanistic insight of the VASH1-associated inhibition of fibrosis by studying primary fibrogenic pathways. In this connection, a previous work by Saito et al. offered a hint that overexpression of VASH1 in diabetic animals suppressed renal levels of transforming growth factor beta 1 (TGF-β1).6 The dynamic process of fibrosis forges the liver matrix, becoming heterogeneous with respect to various types of collagen, combinations within different tissue regions, and age-associated alterations.7 This progressive condition ultimately leads to portal hypertension (PH) at the end stage of the disease and makes clinical management of liver fibrosis and cirrhosis difficult. Coch et al. demonstrated that overexpression of VASH1 by adenoviral gene transfer exerts multifold beneficial effects in PH and significantly decreases portohepatic resistance and portal pressure.4 Therefore, VASH1-mediated beneficial effects in PH strongly suggests that supplementation with VASH1 might be a novel, promising therapeutic strategy for halting chronic liver disease progression. Hepatic stellate cells (HSCs) play a critical role in defining fibrosis in the liver.7 During resolution of liver fibrosis, activated HSCs undergo either apoptosis or inactivation of sensitized HSCs.7 Many pathways associated with fibrosis, such as TGF-β17 and lysyl oxidase-like 2,8 have been targeted to develop antifibrosis drugs, however with limited success.9 Although there is evidence of reversibility of liver fibrosis in animal models, no specific antifibrotic drug has been developed so far for treatments of liver fibrosis. Pathological angiogenesis in the liver emerged as a target module for antifibrotic drug development because “vascular remodeling” is a core contributing factor in progression of fibrosis.9 DNA array analysis of human cirrhotic livers showed overexpression of key angiogenesis genes, such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF)-8, FGF receptor 1, platelet-derived growth factor (PDGF), and their corresponding receptors, cell-cell and cell-matrix adhesion molecules, and matrix remodeling molecules.10 However, the antiangiogenesis therapy for tumors as a whole could not achieve much because of the lack of understanding of mechanistic insight of the neovascularization process. At one point, bevacizumab was approved for breast cancer by the U.S. Food and Drug Administration, but the approval was revoked on November 18, 201111 because of collateral interference with vascular functions and less effectiveness in increasing the lifespan of patients. Recent studies suggest that antiangiogenic therapies can prevent liver fibrosis.12 In relation to the use of antiangiogenesis drugs in liver fibrosis, the world is in search of a drug that would ideally attenuate pathological neoangiogenesis, dampening collagen synthesis and restricting transformation of the liver from fibrotic to tumorogenic. Discovering VASH1 as an antifibrotic and antiangiogenic natural protein holds promise for developing an effective antifibrotic and liver-protective drug. The scope of such propositions appeared optimistic, particularly after observing antifibrosis functions of kinase and multikinase inhibitors, such as bevacizumab, sorafenib, and sunitinib, as well as their increasing uses in liver pathologies.13 Sunitinib, a PDGF and VEGF receptor tyrosine kinase inhibitor, could potentially block both activated HSCs and angiogenesis and thus prevent progression of the cirrhotic liver to HCC. Tugues et al. demonstrated that sunitinib decreased hepatic vascular density, collagen expression, and portal pressure in a rodent model of cirrhosis.14 Sorafenib, a Raf kinase and VEGF receptor type 2 kinase inhibitor, was the first drug of this kind to prolongate the survival of patients with HCC.15 A recent work by Huang et al.16 revealed that bevacizumab was also found to down-regulate expression of alpha-smooth muscle actin and TGF-β1, which have been reported to be profibrogenic genes in vivo. In a similar fashion, VASH1 can be proposed as a “dual effecter” to inhibit angiogenesis and fibrosis simultaneously with relatively lower doses of VASH1. The first ever effort, by Coch et al., in overexpressing VASH1 protein in vivo could efficiently suppress pathological neovascularization and reduce the formation of portosystemic collateral vessels in cirrhotic rats. The encouraging results of the study confirm that overexpression of VASH1 in bile-duct–ligated liver has dual actions: (1) inhibition of angiogenesis and (2) attenuation of intrahepatic fibrogenesis through suppression of HSC activation. A recent work by Xue et al. demonstrated that activation of VASH2 in the HCC milieu promotes angiogenesis.1 Therefore, it would be very relevant and significant to study whether differential expression of VASH1 and VASH2 define the transition of the fibrotic liver to HCC. The challenges lying ahead are formulating a strategy of keeping precise balance between antiangiogenic and other vascular functions in the liver, particularly when the antiangiogenic drugs perturb critical signaling networks in both pathological angiogenesis and vascular functions. The main side effects of bevacizumab are hypertension and heightened risk of bleeding.8 Sunitinib showed some effects against HCC in phase II studies, but a large phase III study was interrupted because of the unfavorable safety profile associated with vascular health and shorter survival, in comparison with sorafenib. Therefore, therapeutic use of VASH1 in curbing liver fibrosis should be studied in parallel with bevacizumab, sunitininb, and sorafenib in in vitro and in vivo models to understand the relative effectiveness of the approach in reversing liver fibrosis. Factually, the observation by Coch et al. is convincing that disruption of the VEGF-VASH1 negative-feedback loop induced local down-regulation of excessive expression of the proangiogenic growth factor, VEGF.4 However, the paradox is that overexpression or ectopic delivery of VASH1, which was already overexpressed in cirrhotic livers, could reverse liver pathology, in part (Fig. 1). As with VASH, similar observations were made previously with secreted frizzled-related protein 4 (SFRP4), a natural antiangiogenesis protein.17 Ectopic delivery of sFRP4 could effectively inhibit tumor neovascularization and restricted tumor size in BALB/C nude mice, whereas endogenous expression of sFRP4 always remained high in solid tumors.17 The study of Coch et al. hinted that ectopic and VEGF independent overexpression of VASH1 disrupts the circuit but enables to keep constant and lower levels of VEGF. This situation is sufficient to maintain vascular homeostasis but not pathological angiogenesis.4 This proposition only could be verified by thorough study of crosstalk between VASH1 and VEGF signaling that would enable the paradox to be explained. A comprehensive study of receptor biology of VEGF under ectopic application of VASH1 would be able to explain the paradox. This study would also elaborate on the VEGF-VASH1 negative-feedback loop, because interplay between VEGF receptor types and VEGF splice variants is key to the final vasculature architecture of the liver. Studying VASH1 signaling under a low oxygen environment in the sensitized liver is also critical because hypoxia and hypoxia-inducible factor (HIF) play a decisive role in fibrotic processes in the liver.18 A recent work by Kozako et al. described that VASH1 induces prolyl hydroxylase-mediated degradation of HIF-1α in human umbilical vein ECs under oxidative stress.19 The heterogeneous nature of liver fibrosis and its consequences throw challenges at different levels. Therefore, the research might be directed to understand the following layers of events in the future: (1) Does VASH1 act only through the VEGF axis? Does it have transcriptional or post-translational regulation of other signaling pathways pertaining to liver fibrosis? (2) VASH1 implications in the activation and apoptosis of HSCs; (3) profiling transcriptional regulation, post-translational events, and protein expression of VASH1 under routine therapeutic regime of liver diseases; and (4) synthesizing cell-permeable and targeted “VASH1-mimicking” small peptide to have better delivery and control over VASH functions. The work of Coch et al.4 holds promise for developing a novel antifibrotic therapeutic strategy for liver pathologies because the study hinted at a reversal of the VASH-driven negative feedback loop of the VEGF-angiogenesis axis in the fibrotic liver (Fig. 1). Suvro Chatterjee, Ph.D.1,2 1Vascular Biology Laboratory AU-KBC Research Center Chennai, India 2Department of Biotechnology Anna University Chennai, India The author acknowledges Dr. Syamantak Majumder, Aab Cardiovascular Research Institute, University of Rochester (Rochester, NY), and Saranya Rajendran, AU-KBC Research Centre, Anna University, Chennai, India for helping in writing and editing the manuscript.