Life without histidine‐rich glycoprotein: modulation of the hemostatic balance revisited

Histidine-rich glycoprotein (HRG), also known as histidineproline-rich glycoprotein, is a relatively abundant plasma protein that belongs to the cystatin superfamily. A clear picture of the physiological role(s) of HRG is still emerging. In vitro studies of the molecular interactions of HRG and its effects on cells have suggested a function in host defense, as a modulator of the coagulation and fibrinolytic systems, as well as of innate immunity. Classic HRG ligands include heparin, plasminogen, fibrinogen, and thrombospondin. New activities for this protein are continuously unraveled. HRG was recently found to exert antiangiogenic properties in vitro [1] and antitumor effects in vivo [2]. It acts as an opsonin by bridging FccRI receptors on macrophages to DNA on apoptotic cells, stimulating phagocytosis [3], and modulates the binding of IgG and immune complexes to FccRI [4]. The hallmark feature of HRG is a central domain rich in histidine and proline, which is composed of tandem pentapeptide repeats with the consensus sequence GHHPH . This conserved motif has no homology to other protein sequences and may exhibit a characteristic new structural fold. Owing to an abundance of histidine residues, the histidine domain binds transitional metal cations, such as zinc(II) or copper(II), which promotes HRG interactions with negatively charged ligands, particularly sulfated glycosaminoglycans. The histidineproline-rich domain acquires a large positive charge upon protonation of histidine residues [5]. Consequently, a drop in pH values below the physiological range facilitates HRG binding to heparin, heparan sulfate, or dermatan sulfate [6]. This exquisite sensitivity to pH changes and the presence of transitional metal ions was postulated to regulate the localization and activity of HRG [6]. Themodular domain structure of HRG allows simultaneous interactions with multiple ligands, suggesting that HRG may functionas anadaptermolecule.For instance,HRGinteraction with plasminogen is mediated by lysine residues in its amino and/or carboxyl-terminal domains, and can occur concomitantly with binding to thrombospondin [7] or glycosaminoglycans [8]. Under conditions of low pH or elevated Zn, HRG associates with cell-surface heparan sulfates [9] and acts as a high affinity receptor for plasminogen [10]. Because generation of plasmin on cell surfaces has been implicated in cell migration associated with wound healing, angiogenesis, and tumor metastasis, it is of interest that HRG strongly stimulates plasminogen activation when bound to a surface [8,11]. In contrast, HRG in solution weakly inhibits plasminogen activation. This may provide a regulatory mechanism for HRG activity, analogous to surface-dependent activation of many components of the coagulation and fibrinolytic systems. However intriguing or appealing, all biological functions of HRG inferred from in vitro studiesmust ultimately be validated by in vivo findings. To date, the lack of an HRG-knockout animal model has hampered a direct experimental approach. Nevertheless, several families with congenital HRG deficiency have been described, and studies of their clinical phenotype may provide clues about the physiological role of HRG. In some patients, reduced levels of HRG are associated with a thrombophilic phenotype; however, a causal connection has not been proved. Although such observations suggest that HRG may modulate the hemostatic balance, the evidence remains circumstantial. Furthermore, as patients still have 20%–50% of the normal levels of HRG, which may be sufficient to provide adequate function, the phenotype in the complete absence of HRG remains unknown. In this issue of the journal, Tsuchida-Straten et al. [12] report the long-awaited generation of a transgenic mouse with a targeted deletion of HRG, and provide an initial characterization of its phenotype. The HRG-knockout mice were viable and fertile with no apparent abnormalities. Changes in hemostatic parameters pointed to a mild anticoagulant activity of HRG in vivo. Plasma from HRG-deficient mice had higher Correspondence: D-B. Borza, Assistant Professor of Medicine, Division of Nephrology and Hypertension, Vanderbilt University School of Medicine, Nashville, TN, 37232-2372, USA. Tel. : +1 (615) 322 2089; fax: +1 (615) 343 7156; e-mail: DorinBogdan.Borza@vanderbilt.edu Journal of Thrombosis and Haemostasis, 3: 863–864