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 studies3, 4 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 review2]. 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 in the field 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 cells4 exacerbated lesion pathogenesis whereas Oct4-dependent transitions3 were atheroprotective including being critical for migration and investment of SMC into a protective fibrous cap. Indeed, remarkably, results of RNAseq and Oct4/Klf4 CHIPseq analyses of advanced brachiocephalic lesions from SMC Klf4 versus SMC Oct4 knockout mice showed virtually completely opposite genomic signatures. Taken together, results show that SMC play an absolutely critical, even dominant role, in late stage lesion pathogenesis in that conditional loss of a single gene in SMC can completely alter lesion pathogenesis [see our recent review2].
A major focus of our 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. In addition, we are determining if mutations or gene polymorphisms that are linked to increased cardiovascular disease in humans may function, at least in part, by promoting detrimental changes in SMC phenotype and their associated functions. Finally, we have initiated additional studies 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. The latter studies, which were published in Nature Medicine1 and done in collaboration with Dr. Kaplan’s lab at NIH, 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.
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.
(1) KLF4-dependent perivascular cell plasticity mediates pre-metastatic niche formation and metastasis.
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. Nat Med 2017 October;23(10):1176-90. DOI: 10.1038/nm.4400. Epub 2017 Sep 18. PMID: 28920957.
(2) Vascular Smooth Muscle Cells in Atherosclerosis.
Bennett MR, Sinha S, Owens GK. Circ Res 2016 February 19;118(4):692-702. doi: 10.1161/CIRCRESAHA.115.306361. Epub 2017 Feb 19. PMCID: PMC4762053.
(3) Activation of the pluripotency factor OCT4 in smooth muscle cells is atheroprotective.
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. Nat Med. 2016 Jun;22(6):657-65. doi: 10.1038/nm.4109. Epub 2016 May 16. PMCID: PMC4899256
Link to paper
(4) KLF4-dependent phenotypic modulation of smooth muscle cells has a key role in atherosclerotic plaque pathogenesis.
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. Nat Med. 2015 Jun;21(6):628-37. doi: 10.1038/nm.3866. Epub 2015 May 18. PMID: 25985364
(5) Alexander MR, Owens GK. Epigenetic Control of Smooth Muscle Cell Differentiation and Phenotypic Switching in Vascular Development and Disease. Annu Rev Physiol 2012 February 15;74:13-40.
(6) Wamhoff BR, Hoofnagle MH, Burns A, Sinha S, McDonald OG, Owens GK. A G/C Element Mediates Repression of the SM22a Promoter Within Phenotypically Modulated Smooth Muscle Cells in Experimental Atherosclerosis. Circ Res 2004 November 12;95(10):981-8.
(7) Yoshida T, Kaestner KH, Owens GK. Conditional Deletion of Kruppel-Like Factor 4 Delays Downregulation of Smooth Muscle Cell Differentiation Markers but Accelerates Neointimal Formation Following Vascular Injury. Circ Res 2008 June 20;102(12):1548-57.
(8) Gomez D, Shankman LS, Nguyen AT, Owens GK. Detection of histone modifications at specific gene loci in single cells in histological sections. Nat Methods 2013 January 13;10:171-7.
(9) Salmon M, Gomez D, Greene E, Shankman L, Owens GK. Cooperative Binding of KLF4, pELK-1, and HDAC2 to a G/C Repressor Element in the SM22alpha Promoter Mediates Transcriptional Silencing During SMC Phenotypic Switching In Vivo. Circ Res 2012 August 31;111(6):685-96.
(10) McDonald OG, Wamhoff BR, Hoofnagle MH, Owens GK. Control of SRF binding to CARG-box chromatin regulates smooth muscle gene expression in vivo. J Clin Invest 2006;116:36-48.
(11) Gan Q, Yoshida T, McDonald OG, Owens GK. Concise review: epigenetic mechanisms contribute to pluripotency and cell lineage determination of embryonic stem cells. Stem Cells 2007 January;25(1):2-9.