Involvement of heparanase in atherosclerosis and other vessel wall pathologies

Abstract

Heparanase, the sole mammalian endoglycosidase degrading heparan sulfate, is causally involved in cancer metastasis, angiogenesis, inflammation and kidney dysfunction. Despite the wide occurrence and impact of heparan sulfate proteoglycans in vascular biology, the significance of heparanase in vessel wall disorders is underestimated. Blood vessels are highly active structures whose morphology rapidly adapts to maintain vascular function under altered systemic and local conditions. In some pathologies (restenosis, thrombosis, atherosclerosis) this normally beneficial adaptation may be detrimental to overall function. Enzymatic dependent and independent effects of heparanase on arterial structure mechanics and repair closely regulate arterial compliance and neointimal proliferation following endovascular stenting. Additionally, heparanase promotes thrombosis after vascular injury and contributes to a pro-coagulant state in human carotid atherosclerosis. Importantly, heparanase is closely associated with development and progression of atherosclerotic plaques, including stable to unstable plaque transition. Consequently, heparanase levels are markedly increased in the plasma of patients with acute myocardial infarction. Noteworthy, heparanase activates macrophages, resulting in marked induction of cytokine expression associated with plaque progression towards vulnerability. Together, heparanase emerges as a regulator of vulnerable lesion development and potential target for therapeutic intervention in atherosclerosis and related vessel wall complications.

Figure 1

Summary and proposed model for heparanase function in plaque vulnerability. A schematic illustration of blood vessel occluded with a vulnerable plaque is shown in the upper panel. A more detailed structure of the plaque is shown in the middle panel. A fibrous cap composed of endothelial cells (EC), smooth muscle cells (SMC) and collagen surrounds and stabilizes the plaque which is populated by macrophages, foam cells, monocytes, T cells and dendritic cells. Heparanase, secreted by EC, SMC or macrophages is abundantly present in the plaque and contributes to plaque rupture and vulnerability directly and indirectly. Heparanase activity, together with proteolytic activity exerted by MMP9 and other proteases weaken the fibrous cap making it more susceptible to rupture. Inactive heparanase activates macrophages and induces the expression of MMP9 and pro-inflammatory cytokines (i.e., TNF-α, MCP-1, IL-1), leading to the recruitment of more immune cells and plaque progression. Cytokine induction by heparanase involves the PI-3K and MAPK signaling pathways, NFκB, and TLR-2 and -4. The exact order of the complex sequence of events that lead to TLR activation and signaling cascade is not clear but is thought to be mediated by presently unidentified heparanase binding protein/receptor (HBP/R; lower panel). Adopted from Hansson and Libby, Nature Immunology 6:508–519, 2006; and Microangela (http://www5.pbrc.hawaii.edu/microangela).

Figure 2

A. Specimens of vulnerable plaques (VP) lesion were stained for CD163 (upper panel, red) and heparanase (middle panel, green). Merge image stained for CD 163 and heparanase is shown in the lower panel. Note that heparanase staining localizes, in part, to CD163-positive cells (appears yellow-orange in lower panel). Original magnification: x63. B. Elevation of heparanase levels in the plasma of atherosclerotic patients. Plasma samples of control healthy donors (Con) and patients exhibiting stable angina (SA) or acute myocardial infarction (AMI) were collected on admission and heparanase levels were quantified by ELISA. Note marked increase of heparanase levels in patients exhibiting SA and even more so in patients exhibiting AMI (p=0.0006). C. TLR-deficient macrophages. Macrophages were harvested from control wild type mice (WT, blue bars), and mice deficient for TLR2 (TLR2^−/−, red bars), TLR4 (TLR4⁻, yellow bars), or TLR2 and 4 (TLR2,4^−/−, green bars). Macrophages were left untreated (Con) or incubated with native (Hepa) or mutated heparanase (DM; 5 μg/ml). Medium was collected after 20 h and TNFα levels were quantified by ELISA.