Cellular and molecular biology of aging endothelial cells

Abstract

Cardiovascular disease (CVD) is the leading cause of death in the United States and aging is a major risk factor for CVD development. One of the major age-related arterial phenotypes thought to be responsible for the development of CVD in older adults is endothelial dysfunction. Endothelial function is modulated by traditional CVD risk factors in young adults, but advancing age is independently associated with the development of vascular endothelial dysfunction. This endothelial dysfunction results from a reduction in nitric oxide bioavailability downstream of endothelial oxidative stress and inflammation that can be further modulated by traditional CVD risk factors in older adults. Greater endothelial oxidative stress with aging is a result of augmented production from the intracellular enzymes NADPH oxidase and uncoupled eNOS, as well as from mitochondrial respiration in the absence of appropriate increases in antioxidant defenses as regulated by relevant transcription factors, such as FOXO. Interestingly, it appears that NFkB, a critical inflammatory transcription factor, is sensitive to this age-related endothelial redox change and its activation induces transcription of pro-inflammatory cytokines that can further suppress endothelial function, thus creating a vicious feed-forward cycle. This review will discuss the two macro-mechanistic processes, oxidative stress and inflammation, that contribute to endothelial dysfunction with advancing age as well as the cellular and molecular events that lead to the vicious cycle of inflammation and oxidative stress in the aged endothelium. Other potential mediators of this pro-inflammatory endothelial phenotype are increases in immune or senescent cells in the vasculature. Of note, genomic instability, telomere dysfunction or DNA damage has been shown to trigger cell senescence via the p53/p21 pathway and result in increased inflammatory signaling in arteries from older adults. This review will discuss the current state of knowledge regarding the emerging concepts of senescence and genomic instability as mechanisms underlying oxidative stress and inflammation in the aged endothelium. Lastly, energy sensitive/stress resistance pathways (SIRT-1, AMPK, mTOR) are altered in endothelial cells and/or arteries with aging and these pathways may modulate endothelial function via key oxidative stress and inflammation-related transcription factors. This review will also discuss what is known about the role of "energy sensing" longevity pathways in modulating endothelial function with advancing age. With the growing population of older adults, elucidating the cellular and molecular mechanisms of endothelial dysfunction with age is critical to establishing appropriate and measured strategies to utilize pharmacological and lifestyle interventions aimed at alleviating CVD risk. This article is part of a Special Issue entitled "SI: CV Aging".

Keywords: Aging; Endothelium; Genomic instability; Inflammation; Oxidative stress; Senescence.

Figure 1. Age-associated Endothelial Oxidative Stress and Impaired NO Bioavailability

In younger endothelial cells (Upper Panel), endothelial nitric oxide synthase (eNOS) has adequate cofactor availability, e.g., tetrahydrobiopterin (BH₄), and produces nitric oxide (NO) through the conversion of L-arginine to L-citrulline. Reactive oxygen species (ROS), e.g., superoxide (O₂⁻) and hydrogen peroxide (H₂O₂), produced by the mitochondrial electron transport chain (ETC) or cytosolic oxidant enzymes, such as NADPH oxidase (NOX), are quenched by endogenous antioxidant enzymes (superoxide dismutase [SOD] and catalase). In older endothelial cells (Lower Panel), ROS produced in the mitochondria increase NOX mediated O₂⁻, this quenches NO bioavailability, through its conversion to peroxynitrite (ONOO⁻), as well as uncouple eNOS by reducing BH₄ availability. In the face of unchanged antioxidant defenses, these effects lead to a reduction in NO bioavailability and a pro-oxidant phenotype in the aged endothelium.

Figure 2. Inflammation and Oxidative Stress in the Aged Endothelium: A Vicious Cycle

The pro-inflammatory transcription factor, nuclear factor kappa B (NFκB) normally resides in the cytosol, where it is inactive, and endothelial nitric oxide synthase (eNOS) produces nitric oxide (NO) that is released from the endothelium and acts on the vascular smooth muscle to cause relaxation. With aging, the endothelial environment is perturbed by increases in cytokines and reactive oxygen species (ROS; e.g., superoxide [O₂⁻]), that can both be produced within the endothelium or by neighboring immune cells. These cytokines exacerbate oxidative stress and inflammation in the endothelium by activating oxidant enzymes, such as NADPH oxidase (NOX) increasing O₂⁻ production, as well as by acting in a feed forward manner to increase pro-inflammatory NFκB transcription. Likewise, the oxidative stress produced in the microenvironment of aged arteries, also acts in a feed forward manner to increase pro-inflammatory NFκB activity and activate neighboring immune cells as well as contributes directly to impaired NO by reducing eNOS activity via decreased BH₄ availability as well as by quenching NO, leading to impaired endothelium dependent dilation in aged arteries. Thus, with aging there is a vicious cycle in aged arteries, in which inflammation and oxidative stress exacerbate one another impairing NO bioavailability and endothelial function.

Figure 3. Endothelial Senescence and Aging

With advancing age, reactive oxygen species (ROS), as well as genotoxic stressors and telomere dysfunction, lead to double strand DNA breaks and genomic instability (Upper Panel). This genomic instability induces the DNA damage response, leading to the activation, p53 and nuclear factor kappa B (NFκB) that then transcribe genes that contribute to cell senescence such as the cyclin dependent kinase inhibitor, p21; oxidative stress, e.g., NADPH oxidase (NOX); and inflammatory cytokines. The cytokines and ROS act in a paracrine manner to impair function in neighboring endothelial cells (Lower Panel).