Background Emerging evidence has suggested that adventitia stem/progenitor cells (Ad-SPCs) migrate into intima in response to injury, where they differentiate toward smooth muscle cells (SMCs) and participate in neointima hyperplasia. Matrix metalloproteinase-8 (MMP-8) has potent proteolytic activity on various matrix proteins, and has been shown to be able to cleave many important chemokines/cytokines/growth factors. We have previously identified MMP-8 as a key determinant in atherosclerosis formation and progression, stem cell mobilisation to atherosclerotic plaques, atherosclerotic angiogenesis, angioplasty restenosis as well as macrophage differentiation/polarisation. However, little is known about the functional involvements of macrophage-derived MMP-8 in SMC differentiation from Ad-SPCs and its contribution to neointima hyperplasia.

Purpose In this study, we aimed to investigate the functional roles of macrophage-derived MMP-8 in Ad-SPC differentiation and injury-induced arterial remodelling.

Methods and results By using Ad-SPCs isolated from MMP-8 knockout (MMP_KO) and control wild-type (WT) mice, we observed no significant difference in terms of SMC differentiation induced by TGF-β between WT and MMP-8_KO Ad-SPCs. Further analysis showed that Ad-SPCs expressed a very low level of MMP-8. However, macrophages and Ad-SPCs co-culture studies showed that while WT macrophage could significantly promote MMP-8_KO Ad-SPC differentiation towards SMCs, such differentiation processes was dramatically impaired when the Ad-SPCs were co-cultured with MMP-8_KO macrophages. Moreover, we found that the MMP-8 protein secreted from macrophages was responsible for SMC differentiation from Ad-SPCs. Mechanistically, our data revealed that MMP-8 promoted TGF-β secretion from macrophages and increased its bioactivity, triggering SMC differentiation. Additionally, we showed that macrophages-derived MMP-8 activated Notch1 Signalling through increasing the cleavage of a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) in Ad-SPCs, and the activated Notch1 signalling promoted SMC differentiation from Ad-SPCs. We further demonstrated that the binding site for CBF1, Suppressor of Hairless, and Lag-1 (CSL) within SMC gene promoters is responsible for Notch1 mediated SMC differentiation from Ad-SPCs. Finally, by using a well-established arterial remodelling model and macrophage transplantation we demonstrated that macrophage-derived MMP-8 increased neointima SMC hyperplasia by promoting Ad-SPC differentiation towards SMCs in response to vascular injury.

Conclusions We have identified macrophage-derived MMP-8 as a critical regulator in SMC differentiation from Ad-SPCs and neointima SMC hyperplasia in response to injury. Our data provide new insight into the roles of MMP-8 in Ad-SPC differentiation and the pathogenesis of neointima formation and/or angiographic restenosis, and aid the development of novel therapeutic agents for the prevention of these diseases.