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...
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