Molecular Mechanisms and Treatment Strategies for Obesity-Associated Coronary Artery Disease, an Imminent Military Epidemic

TECHNICAL ABSTRACT

Background: Obesity-associated atherosclerotic coronary artery disease (CAD) is a major problem among military personnel and their dependents. Although there is intensive research on the pro-atherogenic mechanisms of obesity, there is a large gap in our knowledge of the molecular and cellular mechanisms at the level of the advanced atherosclerotic plaque. The Principal Investigator's (PI's) laboratory has a long-standing interest in how macrophages (Mfs) (the major cell type in atherosclerosis) lead to advanced atherosclerosis and CAD. Acute atherosclerotic events are caused by changes in plaque composition that eventually lead to plaque rupture or erosion. Two major events that promote this so-called "plaque vulnerability" are Mf inflammation and death (apoptosis) that occur when advanced plaque Mfs accumulate excess unesterified, or "free," cholesterol (FC). The FC triggers these events in Mfs by inducing an endoplasmic reticulum-based signal transduction pathway called the unfolded protein Response (UPR) as well as mitogen-activated protein kinase (MAPK) pathways. The PI's laboratory recently obtained exciting new data indicating that angiotensin II (AngII), a hormone that is activated in obesity, triggers apoptosis in FC-loaded, UPR-activated Mfs. Preliminary evidence suggests that UPR activation "primes" the Mfs for death, but the final death pathways are prevented by a variety of compensatory survival pathways. AngII appears to trigger death by blocking the compensatory survival pathways and/or by inducing complementary apoptotic signals.

Hypotheses: Based on the background outlined above, we propose that an important cause of obesity-induced CAD is precipitation of late lesional Mf inflammation and apoptosis by obesity-associated activation of the AngII pathway. This hypothesis predicts that drugs that block AngII synthesis or AngII action will help prevent obesity-associated CAD. Moreover, because a class of drugs that target the nuclear transcription factor PPARg oppose the cellular actions of AngII, we propose that these drugs also will be very valuable in preventing obesity-associated CAD.

Specific Aims: (1) To elucidate the cellular and molecular mechanisms of AngII-induced triggering of apoptosis and inflammation in UPR-activated Mfs, and how PPARg activation may modulate these effects. (2) To investigate the relationships among obesity/insulin resistance, UPR, angiotensin II, PPARg, and plaque vulnerability in mouse models of atherosclerosis and human atherosclerotic lesions.

Study Design: For the first aim, cultured mouse peritoneal Mfs will be used to elucidate how AngII triggers apoptosis in FC-loaded Mf and to determine whether AngII also enhances FC-induced Mf inflammation. We will emphasize AngII-mediated antagonism of compensatory survival pathways and induction of complementary death pathways. We also will determine whether AngII enhances FC-induced inflammation in Mfs, which we hypothesize occurs by enhancement of MAPK signaling by AngII. These mechanistic studies will be expanded to test the hypothesis that PPARg ligands block these AngII-mediated effects on apoptosis and inflammation by interfering with AngII cellular signaling. In the second aim, a model of obesity-induced advanced atherosclerosis will be used to test the hypothesis that obesity-associated activation of AngII is important in FC-induced Mf apoptosis and inflammation in advanced atherosclerotic lesions and vulnerable plaque development. This model also will be used to determine whether PPARg ligands mitigate these effects and whether RAS-induced vulnerable plaque development in obesity is due to increased adiposity itself or to obesity-associated insulin resistance. Another set of mechanistic studies and in vivo experiments will test the hypothesis that the decrease in the adipocyte hormone adiponectin, which is known to occur in obesity, promotes late lesional Mf death, inflammation, and plaque vulnerability. Finally, we will collaborate with the recent Director of Cardiovascular Pathology at the Armed Forces Institute of Pathology to determine whether vulnerable plaque specimens obtained from the AFIP show evidence of UPR activation that occurs in FC- and Mf-rich areas near apoptotic Mfs and necrotic cores. We then will determine whether these findings correlate with specific patient characteristics, notably the presence or absence of obesity/insulin resistance and treatment with ACE inhibitors/ARB and/or TZDs.

Relevance: CAD caused by obesity/metabolic syndrome represents an enormous burden to military personnel and their families as well as to the general population. The studies in this proposal will explore a novel mechanism of how obesity and metabolic syndrome leads to acute cardiovascular events. The proposed mechanism is grounded in solid background information and preliminary data and is one of the first to explain how specific molecular and cellular events at the level of the advanced atherosclerotic plaque link obesity to CAD. Most importantly, these studies will provide the mechanism and rationale for the clinical testing of specific drugs to prevent obesity-associated CAD. Given the rapidly expanding epidemic of obesity and metabolic syndrome in the military and general populations and the associated rapid increase in obesity-associated CAD in these populations, achieving the goals of this proposal could have a major impact on the morbidity, mortality, and health care costs associated with this growing epidemic.