Background: Aging increases milk fat globule EGF-VIII (MFG-E8) expression in the rat, nonhuman primate and human aortic walls, facilitating the invasion, proliferation, and proinflammation of vascular smooth muscle cells (VSMCs). The MFG-E8 C-terminal fragment medin has been reported to be involved in the necrosis of VSMCs at the inflammatory sites of aortic dissection, however, its cleavage processes, aortic wall levels with aging and bio-role in VSMCs remains to be determined.
Material and Methods: In this study, immunofluorescence, immunohistochemistry and western blot analysis demonstrate that MFG-E8 and its fragment medin (Figure), activated matrix metalloproteinase type II (MMP-2) and platelet derived growth factor receptor type-beta (PDGFR-β) protein levels were markedly increased in human grossly normal aortic walls from old (n=10, >50 yrs) vs. younger donors (n=10, <25 yrs). Importantly, exposure of medin peptide (20 to100nM) to primary cultured VSMCs isolated from young (8 mo) and old (30 mo) FXBN rat aortae significantly increased MMP-2 activation, PDGFR-β expression, and migratory capacity measured by a modified Boyden chamber in a dose-dependent manner in both also. Separate exposure of activated MMP-2 to recombinant human MFG-E8 protein and also to MFG-E8 enriched old human aortic protein markedly increased the cleavage product medin in both, which was substantially inhibited by an MMP inhibitor, GM6001. Exposure of medin to VSMCs did not significantly affect the expression of cell cycle related proteins. In addition, PDGF-BB treatment markedly activated MMP-2 in both young and old VSMCs, which was substantially reduced by a PDGFR-β inhibitor, RTK.
Conclusion: Taken together, targeting MFG-E8 or its cleavage product medin is a novel approach to the prevention or treatment of large arterial aging or age-associated disease.