Overexpression of Tumor Necrosis Factor- α Activates Both Anti- and Pro-Apoptotic Pathways in the Myocardium

We have previously reported that mice with cardiac-specific overexpression of tumor necrosis factor (TNF)- α develop myocardial inflammation, cardiac hypertrophy, and dilated cardiomyopathy. TNF- α is reported to induce apoptosis in cultured cardiac myocytes. To investigate the role of apoptosis in this transgenic model, wild-type controls (WT) and transgenic mice (TG) at the age of 1, 8, and 40 weeks were analyzed. Increased incidence of apoptosis in TG was indicated by DNA laddering. TUNEL assays revealed that the frequencies of apoptotic cells were increased in the TG myocardium at all ages. However, as revealed by histochemical and immunofluorescent methods, most of the apoptotic cells appeared to be non-myocytes even in the mice with overt congestive heart failure. To elucidate the signaling pathways responsible for TNF-α induced apoptosis, expression of apoptosis-related genes were evaluated by multi-probe RNase protection assays. Transcripts for death-domain-related proteins, including TNFR1, Fas, FADD, TRADD, and RIP, were constitutively expressed in WT and upregulated in the TG myocardium. Expression of caspase-1 through -8 was also enhanced in TG. While both anti- and pro-apoptotic Bcl-2 family genes were constitutively expressed in WT, TNF- α overexpression strongly induced anti-apoptotic A1 in the myocardium. Furthermore, TNF- α overexpression activated NF- κ B, a mediator of anti-apoptotic pathways, in the myocardium. Thus, overexpression of TNF- α activated both anti- and pro-apoptotic pathways in the myocardium, resulting in an increase of apoptosis, primarily in non-myocytes. These results suggest that TNF- α by itself is not sufficient to induce apoptosis in cardiac myocytes in vivo.