No abstract

In the very early stages of reperfusion, apoptosis is first seen in the endothelial cells from small coronary vessels. The radial spread of apoptosis to surrounding cardiac myocytes suggests that reperfusion induces the release of soluble pro-apoptotic mediators from endothelial cells that promote myocyte apoptosis.

The STAT-1 transcription factor has been implicated as a tumor suppressor by virtue of its ability to inhibit cell growth and promoting apoptosis. However, the mechanisms by which STAT-1 mediates these effects remain unclear. Using human and mouse STAT-1-deficient cells, we show here that STAT-1 is required for optimal DNA damage-induced apoptosis. The basal level of the p53 inhibitor Mdm2 is increased in STAT-1(؊/؊) cells, suggesting that STAT-1 is a negative regulator of Mdm2 expression. Correspondingly, both basal p53 levels, and those induced by DNA damage were lower in STAT-1(؊/؊) cells. In agreement with this lower p53 response to DNA damage in cells lacking STAT-1, the induction of p53 responsive genes, such as Bax, Noxa, and Fas, was reduced in STAT-1-deficient cells. Conversely, STAT-1 overexpression enhances transcription of these genes, an effect that is abolished if the p53 response element in their promoters is mutated. Moreover, STAT-1 interacts directly with p53, an association, which is enhanced following DNA damage. Therefore, in addition to negatively regulating Mdm2, STAT-1 also acts as a coactivator for p53. Hence STAT-1 is another member of a growing family of protein partners able to modulate the p53-activated apoptotic pathway.

We show here that exposure of cardiac cells to simulated ischemia results in apoptosis and is accompanied by phosphorylation and increased expression and transcriptional activity of STAT-1. Similarly, interferon-␥, which is known to induce STAT-1 activation, also induced apoptosis in cardiac cells. STAT-1-transfected cells were more susceptible to ischemia-induced cell death than cells transfected with a control plasmid lacking the STAT-1 coding sequence. Furthermore, an antisense STAT-1 vector reduced both ischemia-and overexpressed STAT-1-induced cell death in cardiac cells. Both STAT-1 overexpression and interferon-␥ treatment or exposure to ischemia activated the promoter of the pro-apoptotic caspase-1 gene in cardiomyocytes. Finally, ischemia/reperfusion also induced STAT-1 activation and caspase-1 processing in ventricular myocytes in the intact heart ex vivo. Immunofluorescent staining demonstrated an increase in STAT-1-positive staining in cardiomyocytes in response to ischemia/reperfusion that co-localized with terminal deoxynucleotidyl transferase dVTP nick end-labeling-positive apoptotic cells. These results suggest that STAT-1 plays a critical role in the regulation of ischemia/reperfusion-induced apoptosis in cardiac cells, acting at least in part via a caspase-1 activation-dependent pathway.Loss of cardiomyocytes by programmed cell death (apoptosis) is an important mechanism in the development of cardiac failure during injury due to ischemia/reperfusion and myocardial infarction (1, 2). Recent studies have indicated that apoptotic death occurs in cardiac cells exposed to a variety of damaging stimuli both in vitro and in the intact heart in vivo (3-6). Thus, cardiac cells exposed to a hypoxic/ischemic insult followed by reperfusion undergo apoptotic cell death in vitro (3, 6). Similarly, apoptotic cell death is also observed in the intact heart following ischemia in vivo (4, 5). Despite the convincing evidence that apoptosis occurs, the mechanism and signaling pathway that leads to hypoxic/ischemic stimuli resulting in apoptosis in cardiac cells is as yet unknown. However, as in other cell types, caspases have been implicated in apoptotic cell death in cardiomyocytes (7). For example, the infarct size following ischemia/reperfusion of the intact heart in vivo can be reduced by priming animals with the nonspecific caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone, directly demonstrating the role of caspases in mediating cell death in cardiac cells (8).Cytokines and growth factors are known to modulate growth, differentiation, and death in many cell types. For example, interferons (IFNs) 1 have been shown to trigger cell cycle arrest and death in non-cardiac cells (9, 10), whereas many interleukins stimulate growth and protect cells from apoptosis (11, 12). These pro-and anti-apoptotic effects are mediated, at least in part, by signaling through a family of transcription factors called STATs. Six STATs have been cloned, some of which exist in different isomeric forms, and all shar...