In this study we first determined
In this study, we first determined whether AIF in bovine LT muscle is expressed and the mitochondria released AIF-mediated apoptosis during postmortem aging. For apoptotic issues, the mitochondrial outer membrane is permeabilized, and AIF translocates to the cytosol and to the nucleus, where it induces apoptosis of nucleus (Cande et al., 2002). Researches have showed that AIF is considered a major mediator of cancer cell death induced by several death stimuli. The expression of AIF has been studied in several organs and tissues (Lee et al., 2006). In agreement with this hypothesis, the present study revealed that AIF in bovine LT muscle is released from the mitochondria to nucleus. In brief, mitochondrial released AIF can induce cell apoptosis during postmortem aging. Numerous data suggest that AIF is a novel death cysteine protease inhibitor released from mitochondria during apoptosis (Wang, Yang, Chai, Shi & Xue, 2002). However, whether mitochondrial release of AIF mediate apoptosis is related to caspase pathway during postmortem aging is limited. AIF is the first identified protein involved in the caspase-independent pathway. Sun et al. (2016) demonstrated that the caspase inhibitor called Z-DEVE-FMK have no effect on the AIF translocation, suggesting that caspase activation is not related to AIF release in the HepG2 cell death. However, Zamzami et al. (2000) reported that AIF is released from mitochondria to the cytosol and to the nucleus by activated caspases, suggesting that AIF nuclear translocation could be triggered by caspase activation. Moreover, subsequent researches by Arnoult, Karbowski & Youle (2003) found that the caspase inhibitor, Z-DEVE-FMK, could block AIF release. These studies suggested that AIF release depends upon caspase activation under certain stimulation of certain cells. However, whether AIF released from mitochondria depends on caspase pathway in postmortem bovine muscles remains unclear. Therefore, we investigated the AIF release of bovine LT muscle after treatment with caspase inhibitor. Z-DEVE-FMK caspase inhibitor could inhibit the AIF release, suggesting that AIF release is related to caspase-dependent pathway, at least, in the bovine LT muscle during postmortem aging. Furthermore, the recent article demonstrated that AIF can trigger the release of cytochrome c from isolated mitochondria in vitro, suggesting that AIF is released from the mitochondria before cytochrome c (Cande et al., 2002). AIF is a known upstream molecule of cytochrome c-mediated caspase-dependent apoptosis, but its exact mechanism requires further study. We confirmed the expression of AIF in postmortem bovine muscle. AIF was proven to be involved in the caspase-dependent cellular apoptosis from our experiments. As a basic component of the mitochondrial respiratory chain, AIF is a key factor in apoptosis initiated by mitochondria and is crucial in energy metabolism. Moreover, mitochondrial metabolism can produce free radicals. The formation of free radicals plays a positive role in AIF expression (Milasta et al., 2016). Thus, certain internal factors stimulate the release of AIF from mitochondria. Mitochondria are essential to the intrinsic apoptosis pathway and determine whether cells survive or undergo apoptosis. The intrinsic pathway involves an increase in outer mitochondrial membrane permeability that leads to the release of various proteins from the intermembrane space into the cytoplasm, including apoptotic molecules, such as cytochrome c, Smac/DIABLO, Bcl-2, and AIF (Wang, 2001). AIF is normally confined to the mitochondrial intermembrane space. Accordingly, mitochondrial permeability has long been considered as the prime mechanism responsible for AIF release. Majority of the existing studies focuses on the effect of inducers on the change in mitochondrial membrane permeability (Nakagawa et al., 2005), whereas limited information is available on the effects of postmortem aging on mitochondrial membrane permeability. Thus, the present study mainly focuses on the changes in mitochondrial membrane permeability with the extension of aging time. Current research shows that mitochondrial swelling increases with the extension in postmortem aging. Mitochondrial swelling reflects the degree of openness of the pores of the mitochondrial membranes (Zorov, Juhaszova & Sollott, 2006). Postmortem aging creates major changes to the mitochondrial membrane permeability and aggravates mitochondrial swelling. This change may lead to the rupture of the mitochondrial outer membrane, apoptotic protein AIF translocation through the outer mitochondrial membrane to the cytosol and to the nucleus, and mediation of apoptosis in postmortem bovine muscle. Thus, we considered that increased mitochondrial swelling is necessary conditions for AIF release during postmortem aging.