Atherothrombosis, resulting from rupture or erosion of unstable atherosclerotic plaques, is the leading cause of death worldwide. However, the mechanisms that regulate the stability of late stage atherosclerotic lesions remain poorly understood. The general dogma based on extensive human histopathology studies is that: 1) plaque composition not size is a critical determinant of late stage lesion stability and the probability of rupture or erosion and a possible heart attack or stroke; 2) plaques containing a large necrotic core, a thin fibrous cap, and large numbers of CD68+ cells relative to Acta2+ cells [presumed to be macrophages (MФ) and smooth muscle cells (SMC) respectively] are more prone to rupture or erosion; and 3) the primary role of the SMC is athero-protective by virtue of them being the primary cell type responsible for formation of a protective fibrous cap. However, several recent Nature Medicine studies1, 2 by our lab involving simultaneous SMC lineage tracing and SMC-specific knockout (KO) of the stem cell pluripotency genes Oct4 or Klf4, have provided compelling evidence challenging this dogma and showing that SMC play a much greater role in lesion pathogenesis than has been generally appreciated [see our recent review3]. For example, we showed that >80% of SMC-derived cells within advanced lesions of ApoE-/- mice fed a Western diet (WD) for 18 weeks lacked detectable expression of SMC markers such as Acta2 typically used to identify them meaning that previous studies have grossly under-estimated the number of SMC-derived cells within advanced lesions. Moreover, >30% of cells previously identified as MФ within advanced mouse brachiocephalic artery (BCA) lesions and human advanced coronary artery lesions were found to be of SMC not myeloid origin meaning that previous estimates of SMC/ MФ ratios are highly inaccurate. Even more importantly, we found that SMC can play either a beneficial or detrimental role in lesion pathogenesis depending on the nature of their phenotypic/functional transitions. For example, Klf4-dependent transitions, including formation of SMC-derived MФ-marker+ foam cells1 exacerbated lesion pathogenesis whereas Oct4-dependent transitions2 were atheroprotective including being critical for migration and investment of SMC into a protective fibrous cap. In addition, results of RNAseq and Oct4/Klf4 CHIPseq analyses of advanced brachiocephalic lesions from SMC Klf4 versus SMC Oct4 knockout mice showed that >40% of GWAS variants linked to increased coronary artery disease (CAD) in man represent SMC Klf4 or Oct4 target genes. Taken together, results show that SMC play a critical, even dominant role, in late stage lesion pathogenesis and contribute substantially to genetic variation associated with CAD [see our recent review3].
A major focus of current studies is to identify factors, mechanisms, and potential therapeutic targets that can promote beneficial, and/or inhibit detrimental SMC phenotypic transitions within advanced lesions and thereby promote increased plaque stability. To this end, we treated our SMC lineage-tracing ApoE-/- mice with an anti-mouse IL1β antibody between 18-26 or 18-21 weeks of Western diet feeding to mimic treatment of hyperlipidemic patients with advanced atherosclerosis [2018 Nature Medicine]4. Completely contrary to expectations, late stage IL1β antibody treatment resulted in multiple detrimental changes including: 1) rapid and nearly complete loss of a SMC-rich protective fibrous cap; 2) reduced collagen content; 3) a large increase in MФ number in the fibrous cap driven by increased local MФ proliferation not increased trafficking; and 4) inhibition of beneficial outward remodeling. Equally surprising, SMC-specific conditional KO of the IL1R1 within ApoE-/- mice resulted in nearly complete impairment of formation of a SMC-rich fibrous cap and lesions enriched in MФs. Results suggest that global suppression of inflammation in the context of sustained hyperlipidemia can induce detrimental rather than beneficial effects on late stage lesion pathogenesis by suppressing IL1β signaling processes in SMC necessary for formation and maintenance of the protective fibrous cap. We are currently attempting to identify alternative therapeutic approaches for promoting beneficial (plaque stabilizing) changes in SMC phenotype, including those customized to the individual patient based on genetic and genomic profiling.
Finally, we are investigating the potential role of SMC and pericyte phenotypic transitions in the pathogenesis of microvascular disease associated with Type II diabetes/metabolic disease, and in regulation of tumor cell growth and metastasis. An example of the latter studies were published in Nature Medicine5, and done in collaboration with Dr. Rosey Kaplan’s lab at NIH. Results showed that highly metastatic tumor cells secrete factors that circulate in blood and induce Klf4-dependent reprogramming of SMC and pericytes within metastatic niches that make them permissive for tumor cell invasion and survival. Remarkably, we found that SMC-pericyte specific knockout of the stem cell pluripotency gene Klf4 dramatically reduced tumor metastasis by >70%.
For the most current list of Dr. Owen's publications, please click here.
(1) Shankman LS, Gomez D, Cherepanova OA, Salmon M, Alencar GF, Haskins RM, Swiatlowska P, Newman AA, Greene ES, Straub AC, Isakson B, Randolph GJ, Owens GK. KLF4-dependent phenotypic modulation of smooth muscle cells has a key role in atherosclerotic plaque pathogenesis. Nat Med 2015 May 18;21(6):628-37. PMCID: PMC4552085.
(2) Cherepanova OA, Gomez D, Shankman LS, Swiatlowska P, Williams J, Sarmento OF, Alencar GF, Hess DL, Bevard MH, Greene ES, Murgai M, Turner SD, Geng YJ, Bekiranov S, Connelly JJ, Tomilin A, Owens GK. Activation of the pluripotency factor OCT4 in smooth muscle cells is atheroprotective. Nat Med 2016 May 16;22(6):657-65. PMCID: PMC4899256.
(3) Bennett MR, Sinha S, Owens GK. Vascular Smooth Muscle Cells in Atherosclerosis. Circ Res 2016 February 19;118(4):692-702. PMCID: PMC4762053.
(4) Gomez D, Baylis RA, Durgin BG, Newman AAC, Alencar GF, Mahan S, St HC, Muller W, Waisman A, Francis SE, Pinteaux E, Randolph GJ, Gram H, Owens GK. Interleukin-1b has atheroprotective effects in advanced atherosclerotic lesions of mice. Nat Med 2018 July 23. PMID: 30038218
(5) Murgai M, Ju W, Eason M, Kline J, Beury DW, Kaczanowska S, Miettinen MM, Kruhlak M, Lei H, Shern JF, Cherepanova OA, Owens GK, Kaplan RN. KLF4-dependent perivascular cell plasticity mediates pre-metastatic niche formation and metastasis. Nat Med 2017 October;23(10):1176-90. PMCID: PMC5724390.
Washington University of St. Louis
Dr. Gary Owens has established a collaborative partnership with Dr. Gwen Randolph of Washington University where she will commit effort toward experiments related to his recent RO1 titled, “Role of IL1B in Regulating SMC and Macrophage Differentiation in Atherosclerosis”. The central focus of this grant is to determine the effects of genetic or pharmacological inhibition of IL1β and IL1R1 signaling on phenotypic transitions of SMC and macrophages, as well as on the overall size and stability of late stage atherosclerotic lesions. Whereas there is good evidence that disruption of IL1β signaling inhibits formation of fatty streaks and early stage lesions, the role of IL1 in late stage lesions is unclear. Aim 1a will use novel utilize SMC and myeloid specific lineage tracing IL1R1 knockout mouse lines generated by our lab to test the hypothesis that IL1R1-dependent transitions in phenotype of SMC and macrophages within advanced atherosclerotic lesions play a critical role in determining overall plaque and lumen size, as well as lesion composition including multiple indices of plaque stability.
In collaboration and partnership, Dr. Randolph will design and refine experimental plans in regards to mouse and human macrophage and monocyte analyses in atherosclerotic plaques, while sharing and refining protocols to help us troubleshoot approaches based on her expert knowledge of macrophages, monocytes, and how these cells are best studied.