Protein targets of inflammatory serine proteases and cardiovascular disease

Abstract

Serine proteases are a key component of the inflammatory response as they are discharged from activated leukocytes and mast cells or generated through the coagulation cascade. Their enzymatic activity plays a major role in the body's defense mechanisms but it has also an impact on vascular homeostasis and tissue remodeling. Here we focus on the biological role of serine proteases in the context of cardiovascular disease and their mechanism(s) of action in determining specific vascular and tissue phenotypes. Protease-activated receptors (PARs) mediate serine protease effects; however, these proteases also exert a number of biological activities independent of PARs as they target specific protein substrates implicated in vascular remodeling and the development of cardiovascular disease thus controlling their activities. In this review both PAR-dependent and -independent mechanisms of action of serine proteases are discussed for their relevance to vascular homeostasis and structural/functional alterations of the cardiovascular system. The elucidation of these mechanisms will lead to a better understanding of the molecular forces that control vascular and tissue homeostasis and to effective preventative and therapeutic approaches.

Figure 1

Western blotting analysis of FGF-2 expression in canine arterialized vein grafts (AVG) or in control femoral veins (C) harvested at the indicated times after grafting showed disappearance of HMW FGF-2 at 8 h and the presence of a band in the 18 kDa range.

Figure 2

Schematic of the different human FGF-2 forms: high molecular weight (HMW, 24, 22.5, 22 kDA) or low molecular weight (LMW, 18 kDa) expressed upon alternative translation from a unique mRNA. The HMW forms (black plus grey bars) are colinear extensions of LMW FGF-2 (black bar only), and are inactive or inhibitory on vascular cell migration. Upon tissue injury and cellular damage, thrombin cleaves all HMW FGF-2 forms exported in the extracellular environment in at least three different bonds (indicated as a unique white dotted line) upstream of the initiating methionine of 18 kDa FGF-2 thus generating an eighteen kDa-like FGF-2 form (ELF-2) that induces vascular cell proliferation and migration.

Figure 3

Thrombin can act through PAR activation and/or by targeting specific protein substrates present in the vasculature such as osteopontin and HMW FGF-2. PAR activation occurs when thrombin cleaves the seven transmembrane GPCR generating a tethered ligand (TL, small gray bar) that triggers a cascade of intracellular signaling events. Thrombin can also cleave a number of alternative substrates. Osteopontin cleavage exposes its functional domains: integrin-binding sites with RGD and SVVYGLR domains (grey bar) plus the C-terminal CD44-binding domain (marbled grey bar) critical for cellular recognition/interaction. HMW FGF-2 cleavage by thrombin leads to generation of vasoactive ELF-2 (see text and Figs. 1 and 2). Cleavage of either protein results in modifications of their biological activity which may have profound repercussions on cardiovascular homeostasis. Elucidation of these mechanisms may lead to specifically inhibit and/or control thrombin activity on specific protein substrates without affecting its coagulation properties.