Thrombospondin-1, Free Radicals, and the Coronary Microcirculation: The Aging Conundrum

Abstract

Significance: Successful matching of cardiac metabolism to perfusion is accomplished primarily through vasodilation of the coronary resistance arterioles, but the mechanism that achieves this effect changes significantly as aging progresses and involves the contribution of reactive oxygen species (ROS). Recent Advances: A matricellular protein, thrombospondin-1 (Thbs-1), has been shown to be a prolific contributor to the production and modulation of ROS in large conductance vessels and in the peripheral circulation. Recently, the presence of physiologically relevant circulating Thbs-1 levels was proven to also disrupt vasodilation to nitric oxide (NO) in coronary arterioles from aged animals, negatively impacting coronary blood flow reserve.

Critical issues: This review seeks to reconcile how ROS can be successfully utilized as a substrate to mediate vasoreactivity in the coronary microcirculation as "normal" aging progresses, but will also examine how Thbs-1-induced ROS production leads to dysfunctional perfusion and eventual ischemia and why this is more of a concern in advancing age.

Future directions: Current therapies that may effectively disrupt Thbs-1 and its receptor CD47 in the vascular wall and areas for future exploration will be discussed. Antioxid. Redox Signal. 27, 785-801.

Keywords: CD47; age; blood flow; cardiac; microvessel; reactive oxygen species.

FIG. 1.

Schematic depicting redox-mediated vasoreactivity effects in the myocardium. (A) Redox-mediated cardiomyocyte-dependent vasodilation: Superoxide (O₂^•−) is produced in proportion to cardiac metabolism by mitochondrial electron transport in cardiomyocytes. Dismutation of O₂^•− by SOD forms easily diffusible vascular vasodilator metabolite, H₂O₂, which diffuses to VSMCs and increases activity of 4-aminopyridine (4-AP) sensitivity in Kv channels. Kv channels inhibit Ca²⁺ influx into the VSMC, which prevents contraction and allows for VSMC relaxation. Endothelium-dependent vasodilation is also mediated by PGE₂, which is stimulated by H₂O₂-induced COX-2 production in ECs. (B) Endothelial-dependent vasoconstriction (left): O₂^•− formed in ECs are converted into H₂O₂, which elicits COX-2 dependent release of thromboxane-A2 (TxA₂). Thromboxane receptors increase Ca²⁺ sensitivity into VSMC, leading to vasoconstriction. Endothelial-independent vasoconstriction (right): increased oxidative stress increases H₂O₂ production in VSMC, which can stimulate either COX-2 or NADPH oxidases leading to O₂^•−-mediated vasoconstriction. COX, cyclooxygenase; ECs, endothelial cells; NADPH, nicotinamide adenine dinucleotide phosphate; SOD, superoxide dismutase; VSMCs, vascular smooth muscle cells.

FIG. 2.

The age-related balance between redox signaling and endogenous buffering. In healthy, young myocardium, a balance between deleterious insults and protective systems exists. With advancing age, however, this balance is progressively lowered, which leads to endothelial dysfunction and deranged vascular reactivity.

FIG. 3.

Factors contributing to resting and maximal coronary blood flow change with advancing age. (A) Coronary blood flow during the rest or baseline conditions is influenced by metabolic and endothelial factors, such as NO, H₂O₂, adenosine, and prostaglandins. With advancing age, NO contribution to mediating blood flow (red line) decreases while H₂O₂ contribution increases (blue line). Conversely, a small contribution to resting coronary blood flow comes from prostaglandins and adenosine at a young age, and this decreases prior to middle age (purple line). The level of resting blood flow (green dotted line) throughout the myocardium remains relatively the same throughout the life span due to compensatory structural and functional changes in the ventricle and circulation. (B) Even though H₂O₂ contribution to maintain resting blood flow (green dotted line) is increasing with age, increased H₂O₂ and other unregulated ROS in the vascular wall lead to impaired vasodilation and coronary vasoconstriction, which lowers maximal blood flow (black dashed line) that can be achieved in the aged population. H₂O₂, hydrogen peroxide; NO, nitric oxide; ROS, reactive oxygen species.

FIG. 4.

Thrombospondin-1 (Thbs-1) effects on vasoreactivity in endothelial and VSMCs. Red lines and arrows indicate deleterious effects of Thbs-1, which may lead to microvascular vasoconstriction and coronary perfusion deficits. Pathways downstream of CD47 are implicated in the exacerbated response to Thbs-1 observed in coronary arterioles from aged rats. Downstream effects of CD47 include activation of NADPH oxidase (Nox1), uncoupling of eNOS, inhibition of sGC, activation of SIRPα, and promoting MLC phosphorylation. eNOS, endothelial nitric oxide synthase. MLC, myosin light chain; sGC, soluble guanylyl cyclase.

FIG. 5.

Concentration-dependent control of Thbs-1-mediated angiogenic and vascular signaling by NO. At intermediate (physiological) up to very high levels of NO, Thbs-1 does not have a large vasoreactive role, but stimulates apoptosis, proliferation, and chemotaxis. These levels of NO correspond to those typically reported in young, healthy, and/or exercise-trained populations. Furthermore, balanced vasodilation and vasoconstriction in tissues and ROS buffering allow for efficient blood flow and perfusion. Conversely, very low concentrations of NO are typically reported in advanced age, chronic diseases, and during acute hemostasis, and correspond to Thbs-1-induced impairment of NO signaling (through inhibition of sGC), exaggerated vasoconstriction, excess ROS, and resultant perfusion deficits. Figure adapted from references (63, 71, 118, 122, 137).

FIG. 6.

CD47 blockade improves coronary blood flow reserve in advanced age. (A) Coronary blood flow reserve (CFR) was calculated from measurements of blood flow at baseline and with dobutamine stimulation. Hearts from control IgG-treated (n = 5) and αCD47-treated (n = 5) 24-month-old female Fischer-344 rats were sectioned to allow for analysis of regional perfusion changes. Color for each section corresponds to percent increase in blood flow with dobutamine stimulation over baseline. *Indicates significant difference between αCD47 and control IgG sections. Figure from Nevitt et al. (111). (B) CFR was calculated from measurements of hyperemic (post-dobutamine) and baseline blood flow (hyperemic/baseline flow). Hearts from young rats (female Fischer-344 rats aged 3–6 months, n = 3) are included as reference, along with old rats treated for 45 min with control IgG (n = 5) or with αCD47 (n = 5). *Indicates significant difference between αCD47 and old control IgG. Graph adapted from data in Nevitt et al. (111) and from unpublished data (young rats).