Abstract Atherosclerotic plaque rupture is the cause of acute coronary syndromes and symptomatic carotid artery disease in 75% of cases with high mortality. Most studies in animal models have focused on investigation of early stage lesions, not advanced lesions which in humans contribute to late stage clinical events. As such, our understanding of mechanisms and factors that control stability of late stage lesions is relatively poor. In many vascular diseases such as post-angioplasty restenosis and atherosclerosis, contractile vascular smooth muscle cells (VSMCs) undergo phenotypic modulation to a synthetic phenotypic in which VSMC exhibit increased proliferation and migration and decreased differentiation. Whether or not VSMC phenotypes controls plaque stability hasn't been well studied and, further, the factors controlling VSMC phenotype switch remain unknown. Our published and preliminary data including in this proposal have demonstrated that AMP-activated protein kinase (AMPK) α2 deficiency results in decreased expression of contractile proteins and increased synthetic proteins in the plaque and promotes the features of unstable plaques in vivo. Mechanistically, we have found that AMPKα2 deficiency upregulates the expression of Krüppel-like factor 4 (KLF4), which is a key regulator of VSMC phenotypic switch. Based on this exciting preliminary data, we hypothesize that AMPKα2 is essential in suppressing atherosclerotic plaque growth and vulnerability by inhibiting the VSMC phenotypic switching in a KLF4-dependent mechanism. This hypothesis will be tested in three aims. Aim 1 is to establish the essential role of AMPKα2 in regulating VSMC phenotypical switch, atherosclerosis, and the instability of atherosclerotic plaques in VSMC-specific ApoE-/- /AMPKα2-/- double knockout mice (Apoe−/−AMPKα2sm-/-) with or without metformin, a potent AMPK activator; After that, we will determine if phenotypic transitions observed in our mouse studies also occur in human atherosclerotic lesions (from autopsy samples). Aim 2 is to demonstrate that KLF4 is required for AMPKα2-deficiency-induced VSMC phenotypic switching. We fully anticipate the completion of the proposed studies will provide the rationales for developing AMPKα2-specific agonists as novel strategies for the prevention and efficient treatment of plaque destabilization and rupture that contribute to end stage clinical events leading to myocardial infarction and stroke.