Rationale:: The vascular adventitia is a complex layer of the vessel wall consisting of vasa vasorum microvessels, nerves, fibroblasts, immune cells, and resident progenitor cells. Adventitial progenitors express the stem cell markers, Sca1 and CD34 (adventitial sca1-positive progenitor cells [AdvSca1]), have the potential to differentiate in vitro into multiple lineages, and potentially contribute to intimal lesions in vivo. Objective:: Although emerging data support the existence of AdvSca1 cells, the goal of this study was to determine their origin, degree of multipotency and heterogeneity, and contribution to vessel remodeling. Methods and Results:: Using 2 in vivo fate-mapping approaches combined with a smooth muscle cell (SMC) epigenetic lineage mark, we report that a subpopulation of AdvSca1 cells is generated in situ from differentiated SMCs. Our data establish that the vascular adventitia contains phenotypically distinct subpopulations of progenitor cells expressing SMC, myeloid, and hematopoietic progenitor-like properties and that differentiated SMCs are a source to varying degrees of each subpopulation. SMC-derived AdvSca1 cells exhibit a multipotent phenotype capable of differentiating in vivo into mature SMCs, resident macrophages, and endothelial-like cells. After vascular injury, SMC-derived AdvSca1 cells expand in number and are major contributors to adventitial remodeling. Induction of the transcription factor Klf4 in differentiated SMCs is essential for SMC reprogramming in vivo, whereas in vitro approaches demonstrate that Klf4 is essential for the maintenance of the AdvSca1 progenitor phenotype. Conclusions:: We propose that generation of resident vascular progenitor cells from differentiated SMCs is a normal physiological process that contributes to the vascular stem cell pool and plays important roles in arterial homeostasis and disease.

Resident vascular adventitial SCA1 ⁺ progenitor (AdvSca1) cells are essential in vascular development and injury. However, the heterogeneity of AdvSca1 cells presents a unique challenge in understanding signaling pathways orchestrating their behavior in homeostasis and injury responses. Using smooth muscle cell (SMC) lineage-tracing models, we identified a subpopulation of AdvSca1 cells (AdvSca1-SM) originating from mature SMCs that undergo reprogramming in situ and exhibit a multipotent phenotype. Here we employed lineage tracing and RNA-sequencing to define the signaling pathways regulating SMC-to-AdvSca1-SM cell reprogramming and AdvSca1-SM progenitor cell phenotype. Unbiased hierarchical clustering revealed that genes related to hedgehog/WNT/beta-catenin signaling were significantly enriched in AdvSca1-SM cells, emphasizing the importance of this signaling axis in the reprogramming event. Leveraging AdvSca1-SM–specific expression of GLI-Kruppel family member GLI1 ( Gli1 ), we generated Gli1 -CreER ^T2 -ROSA26-YFP reporter mice to selectively track AdvSca1-SM cells. We demonstrated that physiologically relevant vascular injury or AdvSca1-SM cell–specific Kruppel-like factor 4 ( Klf4 ) depletion facilitated the proliferation and differentiation of AdvSca1-SM cells to a profibrotic myofibroblast phenotype rather than macrophages. Surprisingly, AdvSca1-SM cells selectively contributed to adventitial remodeling and fibrosis but little to neointima formation. Together, these findings strongly support therapeutics aimed at preserving the AdvSca1-SM cell phenotype as a viable antifibrotic approach.

Hepcidin, a key regulator of iron metabolism, is activated by bone morphogenetic proteins (BMPs). Mice pair-fed with regular and ethanol-containing L. De Carli diets were employed to study the effect of alcohol on BMP signaling and hepcidin transcription in the liver. Alcohol induced steatosis and TGF-beta expression. Liver BMP2, but not BMP4 or BMP6, expression was significantly elevated. Despite increased BMP expression, the BMP receptor, and transcription factors, Smad1 and Smad5, were not activated. In contrast, alcohol stimulated Smad2 phosphorylation. However, Smad4 DNA-binding activity and the binding of Smad4 to hepcidin promoter were attenuated. In summary, alcohol stimulates TGF-beta and BMP2 expression, and Smad2 phosphorylation but inhibits BMP receptor, and Smad1 and Smad5 activation. Smad signaling pathway in the liver may therefore be involved in the regulation of hepcidin transcription and iron metabolism by alcohol. These findings may help to further understand the mechanisms of alcohol and iron-induced liver injury.

Cardiovascular diseases are characterized by chronic vascular dysfunction and provoke pathological remodelling events, such as neointima formation, atherosclerotic lesion development, and adventitial fibrosis. While lineage-tracing studies have shown that phenotypically modulated smooth muscle cells (SMCs) are the major cellular component of neointimal lesions, the cellular origins and microenvironmental signalling mechanisms that underlie remodelling along the adventitial vascular layer are not fully understood. However, a growing body of evidence supports a unique population of adventitial lineage-restricted progenitor cells expressing the stem cell marker, stem cell antigen-1 (Sca1; AdvSca1 cells) as important effectors of adventitial remodelling and suggests that they are at least partially responsible for subsequent pathological changes that occur in the media and intima. AdvSca1 cells are being studied in murine models of atherosclerosis, perivascular fibrosis, and neointima formation in response to acute vascular injury. Depending on the experimental conditions, AdvSca1 cells exhibit the capacity to differentiate into SMCs, endothelial cells, chondrocytes, adipocytes, and pro-remodelling cells, such as myofibroblasts and macrophages. These data indicate that AdvSca1 cells may be a targetable cell population to influence the outcomes of pathologic vascular remodelling. Important questions remain regarding the origins of AdvSca1 cells and the essential signalling mechanisms and microenvironmental factors that regulate both maintenance of their stem-like, progenitor phenotype and their differentiation into lineage-specified cell types. Adding complexity to the story, recent data indicate that the collective population of adventitial progenitor cells is likely composed of several smaller, lineage-restricted subpopulations, which are not fully defined by their transcriptomic profile and differentiation capabilities. The aim of this review is to outline the heterogeneity of Sca1+ adventitial progenitor cells, summarize their role in vascular homeostasis and remodelling, and comment on their translational relevance in humans.

Phosphatase and tensin homolog (PTEN) is an essential regulator of the differentiated vascular smooth muscle cell (SMC) phenotype. Our goal was to establish that PTEN loss promotes SMC dedifferentiation and pathological vascular remodeling in human atherosclerotic coronary arteries and nonatherosclerotic coronary arteries exposed to continuous-flow left ventricular assist devices (CF-LVADs). Arteries were categorized as nonatherosclerotic hyperplasia (NAH), atherosclerotic hyperplasia (AH), or complex plaque (CP). NAH coronary arteries from CF-LVAD patients were compared to NAH coronaries from non-LVAD patients. Intimal PTEN and SMC contractile protein expression was reduced compared with the media in arteries with NAH, AH, or CP. Compared with NAH, PTEN and SMC contractile protein expression was reduced in the media and intima of arteries with AH and CP. NAH arteries from CF-LVAD patients showed marked vascular remodeling and reduced PTEN and α-smooth muscle actin (αSMA) in medial SMCs compared with arteries from non-LVAD patients; this correlated with increased medial collagen deposition. Mechanistically, compared with ApoE ^–/– mice, SMC-specific PTEN-null/ApoE ^–/– double-knockout mice exhibited accelerated atherosclerosis progression and increased vascular fibrosis. By microarray and validated quantitative RT-PCR analysis, SMC PTEN deficiency promotes a global upregulation of proinflammatory and profibrotic genes. We propose that PTEN is an antiinflammatory, antifibrotic target that functions to maintain SMC differentiation. SMC loss of PTEN results in pathological vascular remodeling of human arteries.

Sathish Kumar Natarajan, Bailey A. Stringham, Ashley M. Mohr, Cody J. Wehrkamp, Sizhao Lu, Mary Anne Phillippi, Dee Harrison-Findik, Justin L. Mott