Vascular aging: chronic oxidative stress and impairment of redox signaling-consequences for vascular homeostasis and disease

Abstract

Characteristic morphological and molecular alterations such as vessel wall thickening and reduction of nitric oxide occur in the aging vasculature leading to the gradual loss of vascular homeostasis. Consequently, the risk of developing acute and chronic cardiovascular diseases increases with age. Current research of the underlying molecular mechanisms of endothelial function demonstrates a duality of reactive oxygen and nitrogen species in contributing to vascular homeostasis or leading to detrimental effects when formed in excess. Furthermore, changes in function and redox status of vascular smooth muscle cells contribute to age-related vascular remodeling. The age-dependent increase in free radical formation causes deterioration of the nitric oxide signaling cascade, alters and activates prostaglandin metabolism, and promotes novel oxidative posttranslational protein modifications that interfere with vascular and cell signaling pathways. As a result, vascular dysfunction manifests. Compensatory mechanisms are initially activated to cope with age-induced oxidative stress, but become futile, which results in irreversible oxidative modifications of biological macromolecules. These findings support the 'free radical theory of aging' but also show that reactive oxygen and nitrogen species are essential signaling molecules, regulating vascular homeostasis.

Figure 1

General aspects of RNOS in aging. Young individuals have a higher stress tolerance due to a high antioxidant potential, and efficient repair mechanisms restore homeostatic conditions in response to damage. Under these physiological conditions redox regulation can normally proceed in special compartments but differs in a cell type-specific manner. Prostaglandin endoperoxide H₂ synthase (PGHS, COX) is an excellent example of a highly redox-controlled enzyme. In aging the stress tolerance to RNOS decreases. The molecular basis for the loss of balance between damage and repair is the enhanced RNOS generation which must be compensated by antioxidant systems. These exhibit a reduced efficiency in repair with increasing age. As a consequence, oxidatively altered macromolecules accumulate and burden physiological cellular processes. This attenuates redox regulation and also interferes with intercellular communication.

Figure 2

The vascular ^•NO signaling cascade and the antagonizing ^•O₂⁻. Agonist stimulation of endothelial cells, including angiotensin II, acetylcholine, serotonin, etc., or shear stress causes activation of endothelial NO synthase by a transient increase in intracellular calcium or phosphorylation. ^•NO as a freely diffusible molecule can easily cross cell membranes and activate soluble guanylyl cyclase in smooth muscle cells, increasing intracellular cGMP levels. Protein kinase G is activated and can cause a decrease in intracellular calcium [Ca²⁺]_i and mediates the dephosphorylation of myosin light chain leading to vasorelaxation. Superoxide can react with and reduce free ^•NO, impairing the ^•NO-cGMP signaling pathway. Inactivation of soluble guanylyl cyclase by superoxide has been reported, too. Various superoxide sources in the endothelium and vascular smooth muscle cells have been identified, including mitochondria, uncoupled NO synthase, xanthine oxidase, NADPH oxidases, and epoxygenase. Depending on the model and pathology, different sources of ^•O₂⁻ can be activated. (Ag = agonist; [Ca²⁺]_i = intracellular calcium; NOS-3 = endothelial NO synthase; sGC = soluble guanylyl cyclase; ^•NO = nitric oxide; cGMP = cyclic guanosine monophosphate; Rec = receptor; ^•O₂⁻ = superoxide anion)

Figure 3

The Yin – Yang system of prostanoids. Agonist stimulation of endothelial cells causes an increase in intracellular calcium levels [Ca²⁺]_i, which releases arachidonic acid via activation of phospholipase A₂ from membrane phospholipids. Alternatively, phospholipase A₂ is regulated by fluid or cyclic shear stress through phosphorylation. Arachidonic acid is converted by prostaglandin endoperoxide H₂ synthase (cyclooxygenase) into the intermediate prostaglandin endoperoxide H₂, which is further processed into prostacyclin by prostacyclin synthase. Prostacyclin mediates vasorelaxation by acting on the prostacyclin receptor, activating adenylyl cyclase, and elevating cyclic AMP, resulting in an increase in protein kinase A activity. Furthermore, prostacyclin inhibits platelet aggregation and leukocyte adhesion to the endothelium. Under conditions that cause inhibition of prostacyclin synthase, PGH₂ levels increase and PGH₂ exerts thromboxane-like effects via stimulation of the thromboxane A₂/PGH₂ receptor. Thromboxane A₂ stimulates platelet aggregation and enhances this process in an autocrine loop or promotes leukocyte adhesion. Activation of the thromboxane A₂/PGH₂ receptor leads to activation of phospholipase C and an increase in intracellular calcium levels. (AA = arachidonic acid; AC = adenylyl cyclase; Ag = agonist; [Ca²⁺]_i = intracellular calcium; G = G-protein; ONOO⁻ = peroxynitrite; ^•O₂⁻ = superoxide; PLA₂ = phospholipase A₂; ^•NO = nitric oxide; PGHS-1 = prostaglandin endoperoxide H₂ synthase; PGH₂ = prostaglandin endoperoxide H₂; PGIS = prostacyclin synthase; PLC = phospholipase C; PKA = protein kinase A; Rec = receptor; TxA₂ = thromboxane A₂; TxAS = thromboxane synthase)

Figure 4

Free radical-mediated platelet hyperreactivity in aging. Platelets use peroxynitrite at low concentrations (<21 nM) in order to provide the peroxide tone for modulating basal prostaglandin endoperoxide H₂ synthase (cyclooxygenase-1) activity after agonist-mediated platelet activation. NADPH oxidase and endothelial NO synthase concertedly release superoxide anion and ^•NO to form peroxynitrite. Cyclooxygenase-1 provides the substrate prostaglandin endoperoxide H₂ for thromboxane synthase, which is converted into thromboxane A₂. This potent eicosanoid further activates platelets in an autocrine loop or causes the recruitment of other platelets to the thrombus. However, aging leads to an elevated superoxide formation via uncoupling of NO synthase, conversion of xanthine dehydrogenase into an oxidase, activation of NADPH oxidase, or by electron leakage in the mitochondrial electron transport chain. This saturates the platelet peroxide tone, resulting in loss of the modulation of COX-1 activity, which subsequently leads to hyperreactivity of platelets. Therefore, platelet aggregation readily occurs at lower doses of agonists and proceeds faster. (Ag = agonist; Rec = receptor; PLA₂ = phospholipase A₂; PGHS-1 = prostaglandin endoperoxide H₂ synthase; AA = arachidonic acid; PGH₂ = prostaglandin endoperoxide H₂; TxAS = thromboxane synthase; ONOO⁻ = peroxynitrite; TxA₂ = thromboxane A₂; [Ca²⁺]_i = intracellular calcium; ^•NO = nitric oxide; ^•O₂⁻ = superoxide)