In this study, we have performed a comprehensive quantitative proteomic study to analyze aortic proteins from young (8 mo) and old (30 mo) rats. Using 2-D DIGE, we have obtained 2-D gel maps of 301 identified non-redundant proteins from rat aortas and observed 18 proteins that significantly change abundance with aging. Utilizing iTRAQ, 921 proteins were quantified and between both methods, 50 proteins were shown to have significantly different age-associated abundance. The two methods have complemented each other and therefore, increased the confidence of the results. Totally, 981 proteins were detected by two methods. The majority (240, 80%) of the proteins detected by 2DE are also quantified by iTRAQ. Approximately 5% of the proteins identified have significantly different abundance with aging. Of the proteins found to differ with aging, 5 proteins had post-translation modifications (PTM). Acidic calponin3 is a phosphoprotein, while the milk fat globule protein epidermal growth factor 8 (MFG-E8), Alpha-1-inhibitor III, kininogen 1 and periostin were detected as N-linked glycoproteins. Furthermore, proteomic analysis shows that one protein of interest, MFG-E8, significantly increases in abundance in old rat aortas. Transcription and translation analysis demonstrated that aortic MFG-E8 mRNA and protein levels increase with aging in several mammalian species, including humans. Dual immunolabeling shows that MFG-E8 co-localizes with both angiotensin II (Ang II) and monocyte chemo-attractant protein-1 (MCP-1) within vascular smooth muscle cells (VSMC) of the thickened aged aortic wall. Exposure of early passage VMSC from young aorta to Ang II markedly increases MFG-E8 and enhances invasive capacity to levels observed in VSMC from old rats. Treatment of VSMC with MFG-E8 increases MCP-1 and VSMC invasion that are inhibited by the MCP-1 receptor blocker, vCCI. Silencing MFG-E8 RNA substantially reduces MFG-E8 expression and VSMC invasion capacity. Taken together, the proteome changes observed in this study, including cytoskeletal remodeling, apoptosis/cell death and cell cycle/proliferation, and extracellular-related matrix, reveal that the aged aorta is affected in concert by numerous signaling proteins, including MFG-E8. Additional findings indicate that arterial MFG-E8 significantly increases with aging and is a pivotal relay element within the Ang II MCP-1/VSMC invasion signaling cascade. Importantly, we have identified that aging arterial MFG-E8-enriched VSMC are activated and proliferating both in vivo and in vitro. Increased MFG-E8 in VSMC triggers phosphorylation of ERK1/2, augments levels of PCNA and CDK4, increases BrdU incorporation and promotes growth. The knockdown of MFG-E8 reduces levels of cell cycle, accelerating signaling molecules PCNA and CDK4 expression, and raises levels of cell cycle, decelerating signaling molecules ATM, p53, and p21, facilitating cell entry into a growth-arrested state. Furthermore, we find that avb3/5 is upregulated with MFG-E8 and is essential to relay proliferative signals to aging VSMC. In addition, transcription and translation of MFG-E8 are markedly upregulated in the thoracic aortae of monkeys with a high-fat/cholesterol diet, which are substantially reduced by resveratrol treatment. Furthermore, in vitro studies indicate that MFG-E8 facilitates the uptake of oxLDL in VSMC. In contrast, MFG-E8 silencing markedly inhibits the uptake of lipoproteins in these cells. Thus, targeting of MFG-E8 within this signaling axis pathway is a potential novel therapy for prevention and treatment of the quintessential age-associated diseases, i.e., arthrosclerosis and diseases involved in inflammation and smooth muscle cell invasion.
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