Table 2.

General Mechanisms of Large Artery Stiffening

Mechanism	General Description	Key Molecular Mediators
Elastin Fragmentation	Elastin deposition in the arterial is limited to the fetal development period and infancy and is subsequently turned off; therefore, in adulthood, elastin fiber damage is essentially irreparable. Progressive elastin fragmentation/loss occurs due to: (a) Mechanical fatigue over the lifetime (cyclic stress); (b) Elastase-mediated proteolysis.	MMP-1, MMP-2, MMP-9, MMP-12, MMP-14, cathepsins, neutrophil elastase, other elastases. Elastin glycation, carbamylation and peroxidation may increase its mechanical fragility and/or susceptibility to proteolysis. Elastin degradation products (elastin-related peptides) may promote arterial damage, calcification and systemic metabolic dysregulation.
Collagen deposition	Collagen deposits occur at the arterial wall (predominantly the media), including sites of elastin breakdown	VSMC angiotensin-ll type 1 receptor activation VSMC mineralocorticoid receptor activation
AGE-mediated collagen and elastin cross-linking	Glucose cross-links develop and AGEs form between proteins with a long half-life (such as collagen). Cross-linked collagen is more resistant to enzymatic proteolysis. Glycated elastin is more susceptible to degradation. Hyperglycemia leads to AGE formation, whereas renal dysfunction leads to reduced AGE elimination	Glucose, uremic toxins AGEs can activate the receptor of AGEs (RAGE), inducing inflammation, oxidative stress and calcification pathways (via sodium phosphate co-transporter PiT-1 expression). The functional significance of AGE-RAGE interactions in vivo is unclear.
VSMC Stiffening/tone	Alterations in the VSMC cytoskeleton and integrin interactions with the extracellular matrix. Likely to be more important in distal aortic segments, which are richer in VSMCs.	Increased α-smooth muscle actin and β1-integrin expression Increased adhesiveness to fibronectin (primate models)
Calcification	Medial calcification is predominantly mediated by the osteochondrogenic differentiation of VSMCs. This process is different from the intimal calcification associated with atherosclerosis, which is predominantly mediated by chondrocyte-like cells of bone marrow origin. Adventitial calcification may also occur, which involves myofibroblasts and/or microvascular pericytes.	Imbalance between calcification inhibitors (MGP, fetuin A, pyrophosphate, osteopontin, osteoprotegerin, Klotho) and activators (FGF-23, inflammatory cytokines, vitamin D). BMP-2 signaling ALP activation (degrades pyrophosphate, a calcification inhibitor) Osteogenic transcription factors (Msx2-Wnt signaling)
Endothelial dysfunction	Reduced NO production by the endothelium may play a role in regulating VSMC stiffness/tone in distal aortic segments like the abdominal aorta. Increased stiffness of the endothelial cell cytoskeleton that is in close to the membrane (cortical cytoskeleton) may also play a role in modulating stiffness	Oxidative stress Nitric oxide deficiency Proinflammatory pathways
Inflammation	There are interconnections between inflammatory pathways and various mechanisms of large artery stiffening (elastin degradation, medial calcification, calcification, endothelial dysfunction)	TNF-alpha (can activate calcification pathways via Msx2-Wnt signaling and release of BMP-2 within endothelial microparticles) Various elastases are produced by inflammatory cells Inflammatory cascades lead to endothelial dysfunction

VSMC=vascular smooth muscle cell; MMP=matrix metalloproteinase; MGP=matrix GIa protein; FGF-23=fibroblast growth factor 23; BMP-2=Bone morphogenetic protein 2; Msx2=Homeobox Protein Hox-8; Wnt=willingness Int; PiT-1=phosphate transporter 1; ALP=alkaline phosphatase; AGEs=advanced glycation end-products; RAGE=receptor of AGEs; NO=nitric oxide.