Adventitial Stem Cells in Vein Grafts Display Multilineage Potential That Contributes to Neointimal Formation

Abstract

Interestingly, these resident progenitor cells have been demonstrated to differentiate into a variety of cell types, including those of the vascular lineage, in response to specific culture conditions in vitro. 13 Although seminal studies implicate the © 2013 American Heart Association, Inc. Objective-This study was designed to carry out the characterization of stem cells within the adventitia and to elucidate their functional role in the pathogenesis of vein graft atherosclerosis. Approach and Results-A mouse vein graft model was used to investigate the functional role of adventitial stem/progenitor cells on atherosclerosis. The adventitia of vein grafts underwent significant remodeling during early stages of vessel grafting and displayed markedly heterogeneous cell compositions. Immunofluorescence staining indicated a significant number of stem cell antigen-1-positive cells that were closely located to vasa vasorum. In vitro clonogenic assays demonstrated 1% to 11% of growing rates from adventitial cell cultures, most of which could be differentiated into smooth muscle cells (SMCs). These stem cell antigen-1-positive cells also displayed a potential to differentiate into adipogenic, osteogenic, or chondrogenic lineages in vitro. In light of the proatherogenic roles of SMCs in atherosclerosis, we focused on the functional roles of progenitor-SMC differentiation, in which we subsequently demonstrated that it was driven by direct interaction of the integrin/collagen IV axis. The ex vivo bioreactor system revealed the migratory capacity of stem cell antigen-1-positive progenitor cells into the vessel wall in response to stromal cell-derived factor-1. Stem cell antigen-1-positive cells that were applied to the outer layer of vein grafts showed enhanced atherosclerosis in apolipoprotein E-deficient mice, which contributed to ≈30% of neointimal SMCs. Conclusions-We demonstrate that during pathological conditions in vein grafting, the adventitia harbors stem/ progenitor cells that can actively participate in the pathogenesis of vascular disease via differentiation into SMCs.

Vascular Wall Progenitor Cells in Health and Disease

“…Further analyses of their mouse vein graft model showed that remodeling adventitia contained a significant number of adventitial Sca-1 + cells in close proximity to vasa vasorum, ≈5% to 10% of which could form primary colonies in culture and were heterogeneous in size and morphology, suggesting different subpopulations of Sca-1 + progenitor cells. 62 These adventitia-derived cells possessed multipotent differentiation capacity giving rise to adipocytes, osteoblasts, chondrocytes, and SMCs under specific culture conditions, with the latter found to be dependent on integrin/collagen IV interactions. Moreover, Sca-1 + cells were also isolated from neointimal lesions and these too could be induced toward EC and SMC lineages in culture.…”

“…60 Evaluation of adventitial Sca-1 + cells seeded onto decellularized vascular scaffolds in a bioreactor system revealed that their migratory ability was mediated by the stromal cell-derived factor-1/CXCR4 chemokine axis and their SMC differentiation potential via signaling through epidermal growth factor receptor, followed by activation of extracellular signal-regulated kinase 1/2 and nuclear translocation of β-catenin. 61,62 Interestingly, the drug sirolimus was shown to augment both progenitor cell migration and differentiation through these respective pathways, raising a new potential mechanism for restenosis formation after drug-eluting stent treatment. 61 In addition to adventitial progenitor cells, distinct subpopulations of mural SPCs have also been identified in the media of blood vessels.…”

“…Some of the known mechanisms that regulate VW-SPC biology include (1) integrin/collagen IV interactions, (2) growth factor and chemokine signaling (eg, PDGF, TGF-β, and stromal cell-derived factor-1), and (3) transcription factor-mediated expression of SMC-specific genes via binding of the myocardin/serum response factor complex to the CArG (CC(AT) 6 GG) element of their promoter regions. 62,66 An excellent review by Majesky et al 52 clarifies the critical role of coactivators (myocardin family, eg, myocardin-related transcription factors) and corepressors (eg, Msx1, KLF4, and FoxO4) of serum response factor-CArG binding in SMC promoter/enhancer regions, along with its epigenetic regulation by histone acetylation and methylation, and ATPase-dependent chromatin remodeling complexes, and the influence of miRs (eg, miR-143, miR-145, miR-221, and miR-222) and other intracellular signaling pathways (eg, RhoA/ROCK1/LIMK1 and Nrf3/Nox4).…”

“…Each of these soluble factors may influence VW-PC responses, with one such example being the effect of stromal cell-derived factor-1 on the migration of adventitial Sca-1 + SPCs. 62 Other adventitial stimuli that may also be relevant to progenitor cell biology include the augmentation of reactive oxygen species and extracellular purines and pyrimidines in response to hypoxia and inflammation, 125 and the effect of biomechanical forces caused by vascular pulsations and adventitial stretch. 122 Research efforts must also focus on the developmental origins of postnatal VW-PCs, addressing unanswered questions relating to the embryonic formation of the adventitia and its interface with the media, and the selective and architectural seeding of this territory with progenitor cells.…”

“…However, the authors also concluded that cell migration more than proliferation contributed to neointimal hyperplasia. Although early reports indicated that adventitial fibroblasts were the probable candidate for this migratory phenomenon, 64 subsequent studies have largely rejected this notion on the way to demonstrating roles for adventitial SPCs, 5,62 medial MVSCs, 7 and intimal and adventitial pericytes 134,135 in mediating neointimal thickening. The mechanisms by which adventitial VW-PCs are released from their niches and migrate to the other mural layers are far from resolved.…”