TNF-α in vivo stimulates apoptosis in fibroblasts through caspase-8 activation and modulates the expression of pro-apoptotic genes

Apoptosis of matrix producing cells is common among many inflammatory diseases. The goal of the present study was to examine the apoptotic effects of tumor necrosis factor-α (TNF-α) on fibroblastic cells in vivo and to investigate the role of different caspases in this process. This was accomplished in vivo by subcutaneous injection of TNF-α in mice. The direct effects of TNF-α on fibroblast apoptosis were studied in vitro with normal diploid human fibroblasts. The results indicate that TNF-α in vivo induces apoptosis of fibroblasts. By RNase protection assay, we demonstrated that TNF-α stimulates expression of 12 apoptotic genes. Fluorometric studies demonstrated that TNF-α in vivo predominantly increased caspase-8 and -3 activity and by use of specific inhibitors, the activation of caspase-3 was shown to be initiated by caspase-8 with only a minor contribution from caspase-9. Thus, TNF-α acts to modulate the expression of many genes that favors apoptosis of fibroblastic cells, which is dependent mostly upon signaling through caspase-8.