Increased Expression of the Transforming Growth Factor-β Signaling Pathway, Endoglin, and Early Growth Response-1 in Stable Plaques

Background and Purpose-Unstable atherosclerotic plaques are characterized by increased macrophages and reduced smooth muscle cells (SMCs) and collagen content. Endoglin, an accessory transforming growth factor-beta (TGF beta) receptor, is a modulator of TGF beta signaling recently found to be expressed on SMCs in atherosclerotic plaques. Its function in plaque SMCs and plaque development is unknown. Early growth response-1 (EGR-1), a transcription factor downstream of TGF beta, stimulates SMC proliferation and collagen synthesis. In atherosclerotic lesions, it is mainly expressed by SMCs. Therefore, we studied the TGF beta, endoglin, and EGR-1 pathway in advanced atherosclerotic plaques in relation to plaque phenotype. Methods-Human carotid atherosclerotic plaques (n=103) were collected from patients undergoing carotid endarterectomy. Histologically, plaques were analyzed for plaque characteristics, ie, collagen, macrophage and SMC content, and intraplaque thrombus. Intraplaque endoglin, pSmad (indicative for TGF beta signaling), EGR-1, and TGF beta levels were analyzed using Western blots and enzyme-linked immunosorbent assays, respectively. Results-Higher endoglin and EGR-1 protein levels correlated positively with increased plaque collagen levels, increased smooth muscle cell content, and decreased intraplaque thrombi as well as TGF beta signaling (pSmad). Although EGR-1 overexpression in vitro stimulated collagen synthesis, inhibiting endoglin resulted in lower EGR-1 levels, decreased SMC proliferation, and decreased collagen content. Conclusions-TGF beta in human atherosclerotic plaques is active and signals through the TGF beta/Smad pathway. For the first time, we show a strong association between endoglin and EGR-1, increased collagen and SMCs expression, decreased levels of intraplaque thrombosis, and a stable plaque phenotype. (Stroke. 2009; 40: 439-447.)