Smooth muscle cell phenotypic switching in atherosclerosis

Abstract

Smooth muscle cells (SMCs) possess remarkable phenotypic plasticity that allows rapid adaptation to fluctuating environmental cues, including during development and progression of vascular diseases such as atherosclerosis. Although much is known regarding factors and mechanisms that control SMC phenotypic plasticity in cultured cells, our knowledge of the mechanisms controlling SMC phenotypic switching in vivo is far from complete. Indeed, the lack of definitive SMC lineage-tracing studies in the context of atherosclerosis, and difficulties in identifying phenotypically modulated SMCs within lesions that have down-regulated typical SMC marker genes, and/or activated expression of markers of alternative cell types including macrophages, raise major questions regarding the contributions of SMCs at all stages of atherogenesis. The goal of this review is to rigorously evaluate the current state of our knowledge regarding possible phenotypes exhibited by SMCs within atherosclerotic lesions and the factors and mechanisms that may control these phenotypic transitions.

Figure 1

Hypothetical origins of SMC- and macrophage-like cells within atherosclerotic lesions. The lack of definitive SMC lineage-tracing studies in the context of atherosclerosis, and difficulties in identifying phenotypically modulated SMCs within lesions that have down-regulated typical SMC marker genes, and/or activated expression of markers of alternative cell types including macrophages, raise major questions regarding the contributions of SMC within atherosclerotic lesions. Similarly, there is evidence showing that macrophages can activate at least some SMC markers. The net result is that there are major ambiguities regarding the origins of many of the principal cell types present within atherosclerotic lesions and their roles in plaque development and stability. For the purpose of this figure and review, we define SM-like lesion cells as being positive for at least some SMC marker genes such as SM α-actin, whereas macrophage-like cells are those expressing at least some macrophage marker genes but negative for SMC markers. Interestingly, Andreeva et al. reported the presence of cells that express both SM α-actin and the macrophage marker CD68 within human atherosclerotic lesions. What is unclear is whether these are SMCs that have activated macrophage markers or macrophages that have activated SMC markers? Numbers within parentheses are the references of studies summarized in this figure.

Figure 2

Molecular mechanisms of decreased SM marker gene expression in vivo within atherosclerotic lesions of ApoE−/− Western diet fed mice (reprinted from Wamhoff et al., Circ Res 2004;95:981–988; used with permission). Mutation of the G/C repressor virtually abolished repression of the SM22α transgene in intimal SMC in atherosclerotic lesions within the aortic arch region in ApoE−/− mice fed a Western diet for 18 weeks (compare lacZ transgene staining in the upper panels in mice containing the wild-type SM22α lacZ transgene with that in the lower panels in mice with the G/C repressor mutant SM22α lacZ transgene). The LacZ-positive cells within the intima in SM22α G/C mutant-lacZ mice in the lower panels represent putative phenotypically modulated SMCs unidentifiable as being SMCs based on expression of their endogenous SMC marker genes such as SM α-actin or SM22α. However, given that macrophages can activate expression of SMC markers including SM22α, we cannot unambiguously identify the G/C repressor mutant SM22α lacZ-positive cells as being of SMC origin since they could possibly be of macrophage origin. Taken together, studies illustrate why there is major ambiguity regarding identification of SMC-derived cells in lesions and their possible contributions to lesion development and plaque stability.

Figure 3

Epigenetic mechanisms play a key role in SMC differentiation and phenotypic switching. During SMC differentiation, epigenetic modifications including histone acetylation and H3K4dime appear on promoters of SM marker genes such as SM α-actin and SM MHC.^, These modifications are thought to induce chromatin relaxation making CArG box regions accessible for binding of SRF/myocardin and other transcriptional activators. SMC phenotypic switching, in response to treatment of cultured SMCs with PDGF-BB, was associated with profound repression of expression of SMC marker genes and loss of H4 acetylation. In contrast, phenotypically modulated SMCs did not show loss of H3K4dime at SMC gene loci, suggesting that this particular epigenetic modification at SMC gene loci is a stable epigenetic signature of the SMC lineage. Although these mechanisms have been well described in vitro, there is so far no definitive evidence that similar processes occur in vivo during phenotypic switching of SMC in the context of atherosclerosis or vascular injury in which there are far more prolonged states of SMC phenotypic modulation when compared with PDGF-BB treatment models in vitro.