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Under various conditions, noxious stimuli damage mitochondria, resulting in mitochondrial fragmentation; however, the mechanisms by which fragmented mitochondria are eliminated from the cells remain largely unknown. Here we show that cytoplasmic vacuoles originating from the plasma membrane engulfed fragmented mitochondria and subsequently extruded them into the extracellular spaces in undergoing acute tumor necrosis factor α-induced cell death in a caspase-dependent fashion. Notably, upon fusion of the membrane encapsulating mitochondria to the plasma membrane, naked mitochondria were released into the extracellular spaces in an exocytotic manner. Mitochondrial extrusion was specific to tumor necrosis factor α-induced cell death, because a genotoxic stress-inducing agent such as cisplatin did not elicit mitochondrial extrusion. Moreover, intact actin and tubulin cytoskeletons were required for mitochondrial extrusion as well as membrane blebbing. Furthermore, fragmented mitochondria were engulfed by cytoplasmic vacuoles and extruded from hepatocytes of mice injected with anti-Fas antibody, suggesting that mitochondrial extrusion can be observed in vivo under pathological conditions. Mitochondria are eliminated during erythrocyte maturation under physiological conditions, and anti-mitochondrial antibody is detected in some autoimmune diseases. Thus, elucidating the mechanism underlying mitochondrial extrusion will open a novel avenue leading to better understanding of various diseases caused by mitochondrial malfunction as well as mitochondrial biology.

Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) is oncogenic and indispensable for EBV-mediated B cell transformation. LMP1 is capable of activating several intracellular signaling pathways including the NF-kappaB pathway, which contributes to the EBV-mediated cell transformation. Two regions in the cytoplasmic carboxyl tail of LMP1, namely C-terminal activating regions 1 and 2 (CTAR1 and CTAR2), are responsible for NF-kappaB activation, with CTAR2 being the main NF-kappaB activator. Although the CTAR1-mediated NF-kappaB activation was previously shown to be TRAF3-dependent, we showed here that the CTAR2-mediated NF-kappaB activation is mainly TRAF6-dependent but TRAF2/5-independent. In contrast to the interleukin-1 receptor/toll-like receptor-mediated NF-kappaB pathways, the CTAR2-mediated NF-kappaB pathway does not require MyD88, IRAK1, or IRAK4 for TRAF6 engagement. Furthermore, we showed that TAK1 is required for NF-kappaB activation by LMP1. Thus, LMP1 utilizes two distinct pathways to activate NF-kappaB: a major one through CTAR2/TRAF6/TAK1/IKKbeta (canonical pathway) and a minor one through CTAR1/TRAF3/NIK/IKKalpha (noncanonical pathway).

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER overload, resulting in ER stress. To cope with ER stress, mammalian cells trigger a specific response known as the unfolded protein response (UPR). Although recent studies have indicated cross-talk between ER stress and oxidative stress, the mechanistic link is not fully understood. By using murine fibrosarcoma L929 cells, in which tumor necrosis factor (TNF) alpha induces accumulation of reactive oxygen species (ROS) and cell death, we show that TNFalpha induces the UPR in a ROS-dependent fashion. In contrast to TNFalpha, oxidative stresses by H2O2 or arsenite only induce eukaroytic initiation factor 2alpha phosphorylation, but not activation of PERK- or IRE1-dependent pathways, indicating the specificity of downstream signaling induced by various oxidative stresses. Conversely, the UPR induced by tunicamycin substantially suppresses TNFalpha-induced ROS accumulation and cell death by inhibiting reduction of cellular glutathione levels. Collectively, some, but not all, oxidative stresses induce the UPR, and pre-emptive UPR counteracts TNFalpha-induced ROS accumulation.

The activation of NF-kappaB has been shown to be regulated by multiple phosphorylations of IkappaBs and the NF-kappaB p65 subunit. Here, we characterized the intracellular signaling pathway leading to phosphorylation of p65 on Ser-536 using a novel anti-phospho-p65 (Ser-536) antibody. The Ser-536 of endogenous p65 was rapidly phosphorylated in response to a wide variety of NF-kappaB stimulants including TNF-alpha in the cytoplasm and rapidly dephosphorylated in the nucleus. The TNF-alpha-but not IL-1beta-induced Ser-536 phosphorylation was severely impaired in murine embryonic fibroblasts derived from traf2-/-traf5-/- mice. Bay 11-7082, an inhibitor of IkappaB phosphorylation, inhibited the TNF-alpha-induced phosphorylation in vivo. In addition, overexpression of TGF-beta-activated kinase 1 (TAK1), IKKalpha and IKKbeta stimulated the phosphorylation, and their dominant negative mutants blocked the TNF-alpha-induced phosphorylation. Moreover, small interfering RNAs (siRNAs) against TAK1, IKKalpha and IKKbeta blocked the phosphorylation of endogenous p65. On the other hand, calyculin-A, a protein phosphatase inhibitor, blocked the dephosphorylation in the nucleus in vivo. These results indicate that similar signaling pathways were utilized for the phosphorylations of IkappaBalpha and p65, which further support the idea that both IkappaB and NF-kappaB are substrates for the IKK complex in the activation of NF-kappaB.

Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) is a member of the TNF superfamily that has been shown to induce angiogenesis, apoptosis in tumor cells, and NF-kappaB activation through binding to its receptor, fibroblast growth factor-inducible 14. We have identified TWEAK as an inducer of constitutive NF-kappaB activation by expression cloning, and we report here sequential regulation by TWEAK of two separate signaling cascades for NF-kappaB activation, the NF-kappaB essential modulator-dependent and -independent signaling pathways. Upon TWEAK stimulation, IkappaBalpha is rapidly phosphorylated, generating NF-kappaB DNA-binding complexes containing p50 and RelA in a manner dependent on the canonical IkappaB kinase complex. Unlike TNF-alpha, TWEAK stimulation results in prolonged NF-kappaB activation with a transition of the DNA-binding NF-kappaB components from RelA- to RelB-containing complexes by 8 h, and the latter remained active in binding at least until 24 h post-stimulation. This long lasting activation is accompanied by the proteasome-mediated processing of NF-kappaB2/p100, which does not depend on the NF-kappaB essential modulator but requires IkappaB kinase 1 and functional NF-kappaB-inducing kinase activity. Finally, we show that fibroblast growth factor-inducible 14 with a mutation at its TNF receptor-associated factor (TRAF)-binding site cannot activate NF-kappaB and that TWEAK fails to induce the p100 processing and IkappaBalpha phosphorylation in cells deficient for TRAF2 and TRAF5. Our results thus identify TWEAK as a novel physiological regulator of the non-canonical pathway for NF-kappaB activation.

Signals emanated from CD30 can activate the nuclear factor kappaB (NFkappaB). The two conserved subdomains, D1 and D2, in the C-terminal cytoplasmic region of CD30 were tested for interaction with two tumor necrosis factor receptor-associated factor (TRAF) proteins with NFkappaB activating capacity, TRAF2 and TRAF5. TRAF5 is the newest member of the TRAF family that binds to lymphotoxin beta receptor and CD40. TRAF5, as well as TRAF2, interacted with the D2 subdomain of CD30 in vitro and in vivo. Deletion analysis by the yeast two-hybrid system revealed that the C-terminal 22 and 30 amino acid residues are dispensable for interaction of TRAF5 and TRAF2 with CD30, respectively. Substitution of alanine for threonine at 463 abolished the interaction with TRAF2. Overexpression of the TRAF domain of TRAF2 or TRAF5 showed a dominant negative effect on CD30-mediated NFkappaB activation. Simultaneous expression of these TRAF domains further suppressed the NFkappaB activation, suggesting an interplay of these TRAF proteins. Expression of TRAF2 and TRAF5 mRNA was demonstrated in T- and B-cell lines that express CD30. Taken together, our results indicate that TRAF2 and TRAF5 directly interact with CD30 and are involved in NFkappaB activation by CD30 signaling.