The tumor suppressor p53 plays a crucial role in the cellular response to DNA damage by transcriptional activation of numerous downstream genes. Although a considerable number of p53 target genes have been reported, the precise mechanism of p53-regulated tumor suppression still remains to be elucidated. Here, we report a novel role of the DFNA5 gene in p53-mediated etoposide-induced cell death. The DFNA5 gene has been previously reported to be responsible for autosomal-dominant, nonsyndromic hearing impairment. The expression of the DFNA5 gene was strongly induced by exogenous and endogenous p53. The chromatin immunoprecipitation assay indicated that a potential p53-binding sequence is located in intron 1 of the DFNA5 gene. Furthermore, the reporter gene assay revealed that the sequence displays p53-dependent transcriptional activity. The ectopic expression of DFNA5 enhanced etoposide-induced cell death in the presence of p53; however, it was inhibited in the absence of p53. Finally, the expression of DFNA5 mRNA was remarkably induced by gammaray irradiation in the colon of p53(+/+) mice but not in that of p53(-/-) mice. These results suggest that DFNA5 plays a role in the p53-regulated cellular response to genotoxic stress probably by cooperating with p53.
Although the regulation of tumor angiogenesis is believed to be one of the core functions of p53, the mechanism still remains to be elucidated. Here, we report that semaphorin 3F (SEMA3F), an axon guidance molecule, is involved in p53-regulated antiangiogenesis. The expression level of SEMA3F mRNA was increased by both exogenous and endogenous p53. Chromatin immunoprecipitation assay indicated that a potent p53-binding sequence in intron 1 of SEMA3F interacts with p53 and that it has a p53-responsive transcriptional activity. Overexpression of SEMA3F inhibited in vitro cell growth of the lung cancer cell line H1299. In nude mice assay, the size of the H1299 tumors expressing SEMA3F was much smaller, and they showed lesser number of blood vessels as compared with the control tumors. Moreover, tumors derived from the p53-knockdown colorectal cancer cell line LS174T displayed a remarkable enhancement of tumor vessel formation as compared with control tumors containing normal levels of p53. The expression levels of SEMA3F and neuropilin-2 (NRP2), the functional receptor for SEMA3F, in p53-knockdown LS174T tumors were lower than those in the control tumors. Adenovirus-mediated SEMA3F gene transfer induced the remarkable in vitro growth suppression of the stable transformant of H1299 cells, which express high levels of NRP2. These results suggest that p53 negatively regulates tumor vessel formation and cell growth via the SEMA3F-NRP2 pathway.
Mieap, a p53-inducible protein, controls mitochondrial quality by repairing unhealthy mitochondria. During repair, Mieap induces the accumulation of intramitochondrial lysosomal proteins (designated MALM for Mieap-induced accumulation of lysosome-like organelles within mitochondria) by interacting with NIX, leading to the elimination of oxidized mitochondrial proteins. Here, we report that an additional mitochondrial outer membrane protein, BNIP3, is also involved in MALM. BNIP3 interacts with Mieap in a reactive oxygen species (ROS)-dependent manner via the BH3 domain of BNIP3 and the coiled-coil domains of Mieap. The knockdown of endogenous BNIP3 expression severely inhibited MALM. Although the overexpression of either BNIP3 or NIX did not cause a remarkable change in the mitochondrial membrane potential (MMP), the co-expression of all three exogenous proteins, Mieap, BNIP3 and NIX, caused a dramatic reduction in MMP, implying that the physical interaction of Mieap, BNIP3 and NIX at the mitochondrial outer membrane may regulate the opening of a pore in the mitochondrial double membrane. This effect was not related to cell death. These results suggest that two mitochondrial outer membrane proteins, BNIP3 and NIX, mediate MALM in order to maintain mitochondrial integrity. The physical interaction of Mieap, BNIP3 and NIX at the mitochondrial outer membrane may play a critical role in the translocation of lysosomal proteins from the cytoplasm to the mitochondrial matrix.
Mieap, a p53-inducible protein, controls mitochondrial quality by repairing or eliminating unhealthy mitochondria. BNIP3 and NIX are critical mediators for the Mieap-regulated mitochondrial quality control. Mieap suppresses murine intestinal tumor via its mitochondrial quality control function. To explore the role of the Mieap-regulated mitochondria quality control function in colorectal cancer patients, we examined the statuses of p53, Mieap, BNIP3 and NIX in 57 primary colorectal cancer tissues. Promoter methylation of the Mieap and BNIP3 genes was found in 9% and 47% of colorectal cancer cases, respectively, whereas p53 mutation was found in more than 50% of colorectal cancer tissues lacking methylation of the Mieap and BNIP3 promoters, implying that the p53/Mieap/BNIP3-regulated mitochondria quality control pathway is inactivated in more than 70% of colorectal cancer patients. In LS174T colorectal cancer cells, hypoxia activated the Mieap-regulated mitochondria quality control function. Knockdown of p53, Mieap or BNIP3 in LS174T cells severely impaired the hypoxia-activated function, leading to the accumulation of unhealthy mitochondria and increase of mitochondrial reactive oxygen species generation. The mitochondrial reactive oxygen species generated by unhealthy mitochondria in the p53/Mieap/BNIP3-deficient cells remarkably enhanced cancer cell migration and invasion under hypoxic condition. These results suggest that the Mieap-regulated mitochondria quality control has a critical role in colorectal cancer suppression in the in vivo hypoxic tumor microenvironment.
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