Fibroblast activation protein is induced by inflammation and degrades type I collagen in thin-cap fibroatheromata

Aims Collagen degradation in atherosclerotic plaques with thin fibrous caps renders them more prone to rupture. Fibroblast activation protein (FAP) plays a role in arthritis and tumour formation through its collagenase activity. However, the significance of FAP in thin-cap human fibroatheromata remains unknown. Methods and results We detected enhanced FAPexpression in type IV-V human aortic atheromata (n = 12), compared with type II-III lesions (n = 9; P < 0.01) and healthy aortae (n = 8; P< 0.01) by immunostaining and western blot analyses. Fibroblast activation protein was also increased in thin-cap (< 65 mu m) vs. thick-cap (>= 65 mu m) human coronary fibroatheromata (n = 12; P< 0.01). Fibroblast activation protein was expressed by human aortic smooth muscle cells (HASMC) as shown by colocalization on immunofluorescent aortic plaque stainings (n = 10; P < 0.01) and by flowcytometry in cell culture. Although macrophages did not express FAP, macrophage burden in human aortic plaques correlated with FAP expression (n = 12; R-2 = 0.763; P< 0.05). Enzyme-linked immunosorbent assays showed a time-and dose-dependent up-regulation of FAP in response to human tumour necrosis factor alpha (TNF alpha) inHASMC(n = 6; P< 0.01). Moreover, supernatants from peripheral blood-derived macrophages induced FAP expression in cultured HASMC (n = 6; P < 0.01), an effect abolished by blocking TNF alpha (n = 6; P < 0.01). Fibroblast activation protein associated with collagen-poor regions in human coronary fibrous caps and digested type I collagen and gelatin in vitro (n = 6; P< 0.01). Zymography revealed that FAP-mediated collagenase activity was neutralized by an antibody directed against the FAP catalytic domain both in HASMC (n = 6; P< 0.01) and in fibrous caps of atherosclerotic plaques (n = 10; P< 0.01). Conclusion Fibroblast activation protein expression in HASMC is induced by macrophage-derived TNF alpha. Fibroblast activation protein associates with thin-cap human coronary fibroatheromata and contributes to type I collagen breakdown in fibrous caps.