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Heat shock and tumor necrosis factor-alpha (TNF-alpha) induce apoptosis through different mechanisms, with heat shock acting to cause mitochondrial depolarization and caspase-9 activation, while TNF-alpha acts through a receptor-mediated process to activate caspase-8. In some cells, however, TNF-alpha can also cause mitochondrial depolarization and caspase-9 activation. In the present study, we tested the hypothesis that heat shock at 41 degrees C and TNF-alpha induce apoptosis in bovine preimplantation embryos through a caspase-9-dependent mechanism. Treatment of embryos with either heat shock (41 degrees C) or TNF-alpha increased the proportion of blastomeres that were TUNEL positive and the proportion of embryos exhibiting elevated caspase-9 activity. Furthermore, the caspase-9 inhibitor, z-LEHD-fmk, blocked the increase in TUNEL-positive nuclei caused by both heat shock and TNF-alpha. For embryos at day 6 after insemination, for example, the percent of blastomeres positive for TUNEL was 3.6% for control embryos, 11.1% for embryos cultured at 41 degrees C, and 15.1% for embryos cultured with 10 ng/ml TNF-alpha. In the presence of z-LEHD-fmk, the percent of cells positive for TUNEL was 3.7% for control embryos, 6.1% for embryos cultured at 41 degrees C, and 8% for embryos cultured with 10 ng/ml TNF-alpha. Although TNF-alpha did not cause a measurable increase in caspase-8 activity, there was a tendency (P = 0.07) for treatment of embryos with z-IETD-fmk, an inhibitor of caspase-8, to partly reduce the magnitude of the increase in TUNEL-positive cells caused by TNF-alpha. The percent of cells that were TUNEL positive was increased by TNF-alpha from 9.7 to 19.7% in the absence of inhibitor and from 13.0 to 15.6% in the presence of z-IETD-fmk. Results indicate that induction of apoptosis by both heat shock and TNF-alpha involve activation of caspase-9-dependent pathways. It is likely that TNF-alpha also activates apoptotic pathways involving caspase-8 but that the degree of activation is small and caspase-9-dependent pathways are required for full activation of apoptosis.

Heat shock and tumor necrosis factor-α induce apoptosis in bovine preimplantation embryos through a caspase-9-dependent mechanism