Abstract SY41-03: Vascular normalization by histidine-rich glycoprotein

Abstract

Abstract Histidine-rich glycoprotein (HRG) is a 75 KDa heparin-binding plasma protein exclusively synthesized by hepatocytes. Structurally, HRG belongs to the family of type 3 secreted cystatins and is composed of an NH2-terminal cystatin-like region, a central Zn2+-binding domain rich in histidine and proline residues, and a C-terminal tail. HRG is stored in platelet alpha-granules but is also present free in plasma at relatively high concentrations (0.1-0.15 mg/ml in human plasma). Several reports have shown potent inhibition of tumor angiogenesis by HRG and peptides derived from its central His/Pro-rich domain, leading to reduced vascularization and growth of different syngeneic tumor models (1). HRG inhibits adhesion and chemotaxis of primary endothelial cells in vitro, which provides at least a partial mechanism for the effect of HRG in attenuation of tumor angiogenesis (2). However, HRG interacts with a wide range of ligands, at least in vitro, and exerts diverse biological functions (3). Thus, HRG binds to different components of both the coagulation and fibrinolytic systems, components of the immune system such as complement proteins and Fcg receptors, immunoglobulins (IgGs), immunocomplexes and glycosaminoglycans. Through some of these interactions, HRG modulates immune responsiveness. For example, HRG facilitates the clearance of immune complexes and dying/dead cells via an FcgR-I dependent mechanism HRG also enhances the ability of immune complexes to activate complement, allowing faster clearance of necrotic material (4), and it has been implicated in the defense at the local site of bacterial infection (5). We have asked how this wide range of effects could be amalgamated. In a recent study, we reported that HRG inhibits tumor growth and metastasis by polarizing tumor-associated macrophages (TAMs) to the M1 phenotype, leading to anti-tumor immunity and vessel normalization (6). Infiltration of TAMs into the tumor tissue is associated with an unfavorable prognosis. However, not only the number of infiltrating TAMs but also their phenotype regulates tumorigenesis (7, 8). In non-progressing or regressing tumors, TAMs are biased to a classic macrophage activation M1-like program, characterized by pro-inflammatory activity, antigen presentation and tumor lysis. In malignant tumors, TAMs resemble alternatively activated macrophages (M2-type), which increase angiogenesis and tumor cell intra/extra-vasation and growth; they suppress anti-tumor immunity by preventing activation of dendritic cells, cytotoxic T lymphocytes and natural killer cells. Using an hrg gene-inactivated mouse model, we now provide genetic evidence for HRGs effect on macrophage polarization. HRG ablation leads to increased tumor growth rate and metastatic dissemination, enhanced tumor vessel abnormalization, critical changes in the activation state of macrophages that lead to a tumor-promoting immune response, and a constitutive M2-like polarity of peritoneal macrophages. Furthermore, microarray analyses of the HRG-induced gene regulation profile in peritoneal macrophages ex vivo, show a pattern in excellent agreement with the inhibitory immune profile in HRG-deficient tumors. Combined our data show that HRG regulates TAM polarization in tumors, leading to reduced angiogenesis and induction of anti-tumor immunity, moreover, constitutive loss of HRG has a clear impact on the expression profile of circulating macrophages. Macrophage polarization by HRG offers new therapeutic opportunities for anti-cancer and anti-angiogenic treatment.