Cytokines such as interleukin-6 induce tyrosine and serine phosphorylation of Stat3 that results in activation of Stat3-responsive genes. We provide evidence that Stat3 is present in the mitochondria of cultured cells and primary tissues, including the liver and heart. In Stat3−/− cells, the activities of complexes I and II of the electron transport chain (ETC) were significantly decreased. We identified Stat3 mutants that selectively restored the protein's function as a transcription factor or its functions within the ETC. In mice that do not express Stat3 in the heart, there were also selective defects in the activities of complexes I and II of the ETC. These data indicate that Stat3 is required for optimal function of the ETC, which may allow it to orchestrate responses to cellular homeostasis.

␣-Synuclein, a protein implicated in the pathogenesis of Parkinson disease (PD), is thought to affect mitochondrial functions, although the mechanisms of its action remain unclear. In this study we show that the N-terminal 32 amino acids of human ␣-synuclein contain cryptic mitochondrial targeting signal, which is important for mitochondrial targeting of ␣-synuclein. Mitochondrial imported ␣-synuclein is predominantly associated with the inner membrane. Accumulation of wild-type ␣-synuclein in the mitochondria of human dopaminergic neurons caused reduced mitochondrial complex I activity and increased production of reactive oxygen species. However, these defects occurred at an early time point in dopaminergic neurons expressing familial ␣-synuclein with A53T mutation as compared with wild-type ␣-synuclein. Importantly, ␣-synuclein that lacks mitochondrial targeting signal failed to target to the mitochondria and showed no detectable effect on complex I function. The PD relevance of these results was investigated using mitochondria of substantia nigra, striatum, and cerebellum of postmortem late-onset PD and normal human brains. Results showed the constitutive presence of ϳ14-kDa ␣-synuclein in the mitochondria of all three brain regions of normal subjects. Mitochondria of PD-vulnerable substantia nigra and striatum but not cerebellum from PD subjects showed significant accumulation of ␣-synuclein and decreased complex I activity. Analysis of mitochondria from PD brain and ␣-synuclein expressing dopaminergic neuronal cultures using blue native gel electrophoresis and immunocapture technique showed the association of ␣-synuclein with complex I. These results provide evidence that mitochondrial accumulated ␣-synuclein may interact with complex I and interfere with its functions. Parkinson disease (PD)2 is associated with the degeneration of dopaminergic neurons in the substantia nigra pars compacta.

Mitochondrial dysfunction is one of the major intracellular lesions of Alzheimer's disease (AD). However, the causative factors involved in the mitochondrial dysfunction in human AD are not well understood. Here we report that nonglycosylated full-length and C-terminal truncated amyloid precursor protein (APP) accumulates exclusively in the protein import channels of mitochondria of human AD brains but not in age-matched controls. Furthermore, in AD brains, mitochondrially associated APP formed stable ϳ480 kDa complexes with the translocase of the outer mitochondrial membrane 40 (TOM40) import channel and a super complex of ϳ620 kDa with both mitochondrial TOM40 and the translocase of the inner mitochondrial membrane 23 (TIM23) import channel TIM23 in an "N in mitochondria -C out cytoplasm " orientation. Accumulation of APP across mitochondrial import channels, which varied with the severity of AD, inhibited the entry of nuclearencoded cytochrome c oxidase subunits IV and Vb proteins, which was associated with decreased cytochrome c oxidase activity and increased levels of H 2 O 2 . Regional distribution of mitochondrial APP showed higher levels in AD-vulnerable brain regions, such as the frontal cortex, hippocampus, and amygdala. Mitochondrial accumulation of APP was also observed in the cholinergic, dopaminergic, GABAergic, and glutamatergic neuronal types in the category III AD brains. The levels of translocationally arrested mitochondrial APP directly correlated with mitochondrial dysfunction. Moreover, apolipoprotein genotype analysis revealed that AD subjects with the E3/E4 alleles had the highest content of mitochondrial APP. Collectively, these results suggest that abnormal accumulation of APP across mitochondrial import channels, causing mitochondrial dysfunction, is a hallmark of human AD pathology.

Alzheimer's amyloid precursor protein 695 (APP) is a plasma membrane protein, which is known to be the source of the toxic amyloid β (Aβ) peptide associated with the pathogenesis of Alzheimer's disease (AD). Here we demonstrate that by virtue of its chimeric NH2-terminal signal, APP is also targeted to mitochondria of cortical neuronal cells and select regions of the brain of a transgenic mouse model for AD. The positively charged residues at 40, 44, and 51 of APP are critical components of the mitochondrial-targeting signal. Chemical cross-linking together with immunoelectron microscopy show that the mitochondrial APP exists in NH2-terminal inside transmembrane orientation and in contact with mitochondrial translocase proteins. Mutational studies show that the acidic domain, which spans sequence 220–290 of APP, causes the transmembrane arrest with the COOH-terminal 73-kD portion of the protein facing the cytoplasmic side. Accumulation of full-length APP in the mitochondrial compartment in a transmembrane-arrested form, but not lacking the acidic domain, caused mitochondrial dysfunction and impaired energy metabolism. These results show, for the first time, that APP is targeted to neuronal mitochondria under some physiological and pathological conditions.

We have investigated the mechanism of mitochondrialnuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Δψ m ) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca 2⍣ level ([Ca 2⍣ ] i ). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca 2⍣ release in response to its modulator, caffeine; (ii) enhanced levels of Ca 2⍣ -responsive factors calineurin, calcineurindependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-κB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca 2⍣ ] i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca 2⍣ ] i changes, which are likely to be due to reduced ATP and Ca 2⍣ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Mitochondrial retrograde signaling is a pathway of communication from mitochondria to the nucleus that influences many cellular and organismal activities under both normal and pathophysiological conditions. In yeast it is used as a sensor of mitochondrial dysfunction that initiates readjustments of carbohydrate and nitrogen metabolism. In both yeast and animal cells, retrograde signaling is linked to TOR signaling, but the precise connections are unclear. In mammalian cells, mitochondrial dysfunction sets off signaling cascades through altered Ca(2+) dynamics, which activate factors such as NFkappaB, NFAT, and ATF. Retrograde signaling also induces invasive behavior in otherwise nontumorigenic cells implying a role in tumor progression.

Recently we showed that partial depletion of mitochondrial DNA (genetic stress) or treatment with mitochondrial-speci®c inhibitors (metabolic stress) induced a stress signaling that was associated with increased cytoplasmic-free Ca 2+ [Ca 2+ ] c . In the present study we show that the mitochondria-tonucleus stress signaling induces invasive phenotypes in otherwise non-invasive C2C12 myoblasts and human pulmonary carcinoma A549 cells. Tumor-speci®c markers cathepsin L and transforming growth factor b (TGFb) are overexpressed in cells subjected to mitochondrial genetic as well as metabolic stress. C2C12 myoblasts subjected to stress showed 4-to 6-fold higher invasion through reconstituted Matrigel membrane as well as rat tracheal xenotransplants in Scid mice. Activation of Ca 2+ -dependent protein kinase C (PKC) under both genetic and metabolic stress conditions was associated with increased cathepsin L gene expression, which contributes to increased invasive property of cells. Reverted cells with~70% of control cell mtDNA exhibited marker mRNA contents, cell morphology and invasive property closer to control cells. These results provide insights into a new pathway by which mitochondrial DNA and membrane damage can contribute to tumor progression and metastasis.

We have investigated mechanisms of mitochondrial stressinduced phenotypic changes and cell invasion in tumorigenic but poorly invasive human pulmonary carcinoma A549 cells that were partly depleted of mitochondrial DNA (mtDNA). Depletion of mtDNA (genetic stress) caused a markedly lower electron transport-coupled ATP synthesis, loss of mitochondrial membrane potential, elevation of steady state [Ca 2+ ] c , and notably induction of both glycolysis and gluconeogenic pathway enzymes. Markers of tumor invasion, cathepsin L and TGFb1, were overexpressed; calcium-dependent MAP kinases (ERK1 and ERK2) and calcineurin were activated. The levels of anti-apoptotic proteins Bcl2 and Bcl-X L were increased, and the cellular levels of pro-apoptotic proteins Bid and Bax were reduced. Both mtDNA-depleted cells (genetic stress) and control cells treated with carbonyl cyanide m-chlorophenylhydrazone (metabolic stress) exhibited higher invasive behavior than control cells in a Matrigel basement membrane matrix assay system. MtDNA-depleted cells stably expressing anti-sense cathepsin L RNA, TGFb1 RNA, or treated with specific inhibitors showed reduced invasion. Reverted cells with 80% of control cell mtDNA exhibited marker protein levels, cell morphology and invasive property closer to control cells. Our results suggest that the mitochondriato-nucleus signaling pathway operating through increased [Ca 2+ ] c plays an important role in cancer progression and metastasis. Oncogene (2002Oncogene ( ) 21, 7839 -7849. doi:10.1038 Keywords: mitochondria; calcium signaling; cathepsin L; TGFb; tumor invasion; MAP kinases IntroductionThe role of mitochondria in carcinogenesis was initially suggested by Warburg et al. (1926;Warburg, 1956), based on the observation that number of experimentally induced rodent tumors exhibit reduced respiration-coupled oxidative metabolism and increased glycolysis. Since then, altered mitochondrial morphology, as well as changes in mitochondrial enzyme patterns and membrane transport systems, have been described in several tumor types (Zafar et al., 1982). Cell fusion studies (Jonasson et al., 1977;Howell and Sager, 1978;Giguere and Morais, 1981) also suggest that unknown cytoplasmic elements may play roles in carcinogenesis. Fusion between normal cytoplasm and karyoplasts from malignant phenotypes resulted in the ablation of tumorigenicity (Israel and Schaeffer, 1987) and conversely, the cytoplasm of the malignant phenotype could transfer the malignancy to the karyoplasts from normal cells (Israel and Schaeffer, 1988). Studies using mitochondrial DNA (mtDNA)-depleted tumor cells to evaluate the role of mtDNA, and thus mitochondrial function in tumorigenicity have yielded mixed results (Giguere and Morais, 1981;Morais et al., 1994;Cavalli et al., 1997;Cavalli and Liang, 1998;Hofhaus and Gattermann, 1999). Cavalli et al. (1997) found diminished tumor formation by glioblastoma cells following depletion of mtDNA (r 0 cells) with ethidium bromide treatment, while Morais et al. (1994) found increased capacity to p...