Objective-Activated mast cells (MCs) induce endothelial cell (EC) apoptosis in vitro and are present at sites of plaque erosions in vivo. To further elucidate the role of MCs in endothelial apoptosis and consequently in plaque erosion, we have studied the molecular mechanisms involved in MC-induced EC apoptosis. Methods and Results-Primary cultures of rat cardiac microvascular ECs (RCMECs) and human coronary artery ECs (HCAECs) were treated either with rat MC releasate (ie, mediators released on MC activation), rat chymase and tumor necrosis factor-␣ (TNF-␣), or with human chymase and TNF-␣, respectively. MC releasate induced RCMEC apoptosis by inactivating the focal adhesion kinase (FAK) and Akt-dependent survival signaling pathway, and apoptosis was partially inhibited by chymase and TNF-␣ inhibitors. Chymase avidly degraded both vitronectin (VN) and fibronectin (FN) produced by the cultured RCMECs. In addition, MC releasate inhibited the activation of NF-B (p65) and activated caspase-8 and -9. Moreover, in HCAECs, human chymase and TNF-␣ induced additive levels of apoptosis. Key Words: atherosclerosis Ⅲ mast cell Ⅲ apoptosis Ⅲ chymase Ⅲ plaque erosion T he most important pathological processes underlying the sudden onset of acute coronary syndromes, including unstable angina and acute myocardial infarction, are focal erosions and ruptures of a vulnerable coronary plaque. 1 Although erosions of coronary plaques account for as much as 30% of the fatal acute myocardial infarctions and sudden coronary deaths, 2 the molecular mechanisms of plaque erosion have remained enigmatic. One possible cause of plaque erosion is apoptosis of the involved endothelial cells (ECs). 3 Indeed, patients with unstable angina or myocardial infarction have increased levels of circulating EC-derived apoptotic microparticles, 4 and patients with symptomatic internal carotid artery plaques have increased numbers of apoptotic ECs. 5 EC apoptosis is especially seen in downstream areas of atherosclerotic plaques subjected to aberrant hemodynamic forces including turbulent blood flow. 6 The vulnerable sites of atherosclerotic plaques also contain an increased number of mast cells (MCs) that show signs of activation by degranulation, which is a prerequisite for the release of preformed mediators, both insoluble and soluble. 7,8 We have previously shown that activated rat serosal MCs induce apoptosis of rat cardiac microvascular ECs (RCMECs) in vitro, by a mechanism that involves the presence of both insoluble tumor necrosis factor-␣ (TNF-␣) in the granule-remnant fraction and soluble TNF-␣ in the granule-remnant free supernatant. 9 However, TNF-␣ alone could not explain the observed effects, 9 suggesting that other components of the MC releasate may have been involved. Chymase, the neutral serine protease of MCs, has been shown to induce apoptosis of smooth muscle cells (SMCs), 10 myocytes, 11 and epithelial cells. 12 Recently, we have shown that the MC-derived proteases, chymase and tryptase, when incubated with arterial segments in...

Degranulated mast cells are present in the subendothelial space of eroded (de-endothelialized) coronary atheromas. Upon degranulation, mast cells secrete into the surrounding tissue an array of preformed and newly synthesized mediators, including proapoptotic molecules, such as chymase and TNF-alpha. In a co-culture system involving rat serosal mast cells and rat cardiac (microvascular) endothelial cells, we could show, by means of competitive RT-PCR, immunoblotting, immunocytochemistry, annexin staining, flow cytometry, and DNA-laddering, that stimulation of mast cells with ensuing degranulation rapidly (within 30 min) down-regulated the expression of both bcl-2 mRNA and protein, with subsequent induction of apoptosis in the endothelial cells. The major effect of bcl-2 down-regulation resided in the exocytosed granule remnants, a minor effect also being present in the granule remnant-free supernatant. No significant changes were observed in the expression levels of the pro-apoptotic protein, bax. The mast cell-mediated apoptotic effect was partially (70%) dependent on the presence of TNF-alpha and involved the translocation of cytochrome C from mitochondria into cytoplasm. These results are the first to show that one of the cell types present in the atherosclerotic plaques, namely the mast cell, by releasing both granule-remnant-bound and soluble TNF-alpha, may contribute to the erosion of atherosclerotic plaques by inducing apoptosis in adjacent endothelial cells. Published 2003 Wiley-Liss, Inc.

No abstract

No abstract