Adventitial dysfunction: an evolutionary model for understanding atherosclerosis.

Abstract

Endothelial and smooth muscle dysfunctions are widely implicated in the pathogenesis of atherosclerosis. Modern mechanical and pharmacologic treatments aim to remodel abnormalities of the vessel intima and media. We hypothesize that adventitial dysfunction comprises the dominant source of atherosclerosis by originating many endothelial and smooth muscle abnormalities. The autonomic nervous system innervates the adventitia, and autonomic dysfunction induces many end-organ dysfunctions including inflammation and thrombosis. The link between diabetes and atherosclerosis may operate through adventitial autonomic neuropathy. Smoking may promote atherosclerosis by inducing adventitial autonomic dysfunction related to nicotine-mediated compensatory upregulation of sympathetic bias independent of endothelial injury induced by purported tobacco toxins. While hypertension is thought to cause atherosclerosis, the two conditions may instead represent independent consequences of autonomic dysfunction. The link between aging and atherosclerosis may operate through adventitial dysfunction induced by autonomic dysregulations. Exercise may ameliorate atherosclerosis by restoring adventitial autonomic function, thereby normalizing adventitial regulation of medial and intimal biology. Feed-forward adventitial vascular baroreceptor and chemoreceptor dysregulation may further exacerbate atherosclerosis as intimal plaque interferes with these sensors. Since penetrating external physical injury likely represented a dominant selective force during evolution, the adventitia may be preferentially equipped with sensors and response systems for vessel trauma. The convergent response of adrenergia, inflammation, and coagulation, which is adaptive for physical trauma, may be maladaptive today when different stressors trigger the cascade. Endoluminal therapies including atherectomy, angioplasty, and stent deployment involve balloon expansion that traumatizes all layers of the vessel wall. These interventions may paradoxically reinitiate the cascade of atherogenesis that begins with adventitial dysfunction and leads to restenosis. Methods to reduce adventitial trauma, a maladaptive trigger of adventitial dysfunction, may reduce the risk of restenosis. We envision novel mechanical and biopharmaceutical solutions that target the adventitia to prevent or treat atherosclerosis including novel drug delivery strategies, exo-stents that wrap vessels, and neuromodulation of vessels.