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 integrity by inducing the accumulation of lysosomal proteins within mitochondria. This phenomenon is designated MALM, for Mieap-induced accumulation of lysosome-like organelles within mitochondria. To identify this novel Mieap-interacting protein(s), we performed a two-dimensional image-converted analysis of liquid chromatography and mass spectrometry (2DICAL) on the proteins immunoprecipitated by an anti-Mieap antibody. We indentified 14-3-3γ as one of the proteins that was included in the Mieap-binding protein complex when MALM was induced. The interaction between Mieap and 14-3-3γ was confirmed on the exogenous and endogenous proteins. Interestingly, 14-3-3γ was localized within mitochondria when MALM occurred. A 14-3-3γ deficiency did not affect the accumulation of Mieap and lysosomal proteins within mitochondria, but dramatically inhibited the elimination of oxidized mitochondrial proteins. These results suggest that 14-3-3γ plays a critical role in eliminating oxidized mitochondrial proteins during the MALM process by interacting with Mieap within mitochondria.

AbstrAct:The membrane-anchored matrix metalloproteinase-regulator RECK is often downregulated in various types of cancers; the levels of residual RECK in resected tumors often correlate with better prognosis. Forced expression of RECK in cancer cells suppresses tumor angiogenesis, invasion, and metastasis in xenograft models. RECK is therefore a promising marker for benignancy and a potential effector in cancer therapy. We established a cell line containing two transgene systems: (1) the secreted alkaline phosphatase (SEAP) gene fused to Reck promoter and (2) the HRAS 12V oncogene driven by the Tet-off promoter system. This cell line exhibits transformed phenotype in regular medium and flat morphology with increased SEAP activity in the presence of doxycycline, allowing the assessment of RECK-inducing activity of chemicals in the contexts of both transformed and untransformed cells. Our pilot experiments with 880 known bioactive compounds detected 34 compounds that activate RECK promoter; among these, 10 were authentic anticancer drugs. Four selected compounds up-regulated endogenous RECK protein in several human cancer cell lines. The top-ranking compound, disulfiram, strongly suppressed spontaneous lung-metastasis of human fibrosarcoma cells in nude mice. Our data demonstrate the value of this screen in discovering effective cancer therapeutics.

Mitochondrial quality control (MQC) plays a critical role in maintaining a healthy status of mitochondria, preventing aging, cancer, and a variety of degenerative diseases. However, the mechanism largely remains unclear. Recently, we discovered a novel MQC mechanism regulated by Mieap, a p53-inducible protein, in which Mieap induces intramitochondrial lysosome-like organella that plays a critical role in the elimination of oxidized mitochondrial proteins (designated MALM for Mieap-induced accumulation of lysosome-like organelles within mitochondria) in response to mitochondrial damage (1-4). Here, we report a physiological role of the MALM function. Hypoxia dramatically induced accumulation of lysosomal proteins within mitochondria without the destruction of mitochondrial structure in both human normal mammary epithelial cells (HMEC4s) and mouse embryonic fibroblasts cells (MEFs). In response to the hypoxic stress, the MALM was inducible in ATG5-KO, ATG7-KO, and PINK1-KO MEFs, but not in p53-KO and Mieap-KO MEFs, suggesting that the phenomenon is completely different from canonical autophagy. In the MALM-deficient cells, hypoxia caused an increase of mitochondrial reactive oxygen species (ROS) generation and accumulation of oxidized proteins within the mitochondria. Surprisingly, the MALM-deficient cells in the hypoxic condition revealed enhanced activities of cell migration and invasion. This phenotype was totally cancelled by a ROS scavenger reagent, Ebselen. These results suggest that the MALM is a universal and fundamental function in the cell (at least from mouse to human), which plays a critical role in MQC under hypoxia. Alteration of the MALM function in cancer cells could facilitate cancer invasion and metastasis via mitochondrial ROS generated by unhealthy mitochondria. 1. Miyamoto Y et al. PLoS ONE 6: e16054, 2011 2. Kitamura N et al. PLoS ONE 6: e16060, 2011 3. Nakamura Y et al. PLoS ONE 7: e30767, 2012 4. Miyamoto T et al. Sci Rep 2: 379, 2012 Citation Format: Yasuyuki Nakamura, Masaki Yoshida, Noriaki Kitamura, Hiroki Kamino, Ryuya Murai, Yuri Saito, Hitoya Sano, Hirofumi Arakawa. Hypoxia induces accumulation of lysosomal proteins within mitochondria. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1900. doi:10.1158/1538-7445.AM2013-1900

Recently, we discovered a novel mechanism for mitochondrial quality control (MQC), in which Mieap, a p53-inducible protein, controls MQC by repairing or eliminating unhealthy mitochondria in a process known as MALM (Mieap-induced Accumulation of Lysosome-like organelles within Mitochondria) or MIV (Mieap-Induced Vacuole), respectively (1, 2). We also reported that two mitochondrial outer membrane proteins, BNIP3 and NIX, are essential mediators of the MALM (2, 3). To examine the role of Mieap-regulated MQC pathway in tumorigenesis, we analyzed the status of p53, Mieap, BNIP3, and NIX in 57 cases of primary colorectal cancer. Of the 57 patients, Mieap and BNIP3 promoter methylation was observed in 5 (8.8%) and 26 (45.6%) patients, respectively. A p53 mutation was detected in 4 (18%) of the 22 patients with either Mieap or BNIP3 promoter methylation whereas it was found in 9 (52%) of the 17 patients with neither Mieap nor BNIP3 promoter methylation, implying that the p53/Mieap/BNIP3 pathway in the Mieap-regulated MQC is inactivated in at least 80% of colorectal cancer cases. Hypoxia induced MALM in the colorectal cancer cell line LS174T (wild-type p53, Mieap-positive, BNIP3-positive). In vitro knockdown of p53, Mieap, or BNIP3 in the LS174T cells severely inhibited hypoxia-induced MALM, leading to the accumulation of unhealthy mitochondria and increased mitochondrial reactive oxygen species (mtROS) generation. Furthermore, the mtROS dramatically enhanced cancer migration and invasion under hypoxic conditions. These results suggest that the Mieap-regulated MQC function plays a critical role in tumor suppression in colorectal cancer. 1. Miyamoto Y et al. PLoS ONE 6: e16054, 2011 2. Kitamura N et al. PLoS ONE 6: e16060, 2011 3. Nakamura Y et al. PLoS ONE 7: e30767, 2012 Citation Format: Hiroki Kamino, Yasuyuki Nakamura, Noriaki Kitamura, Manabu Futamura, Masaki Yoshida, Ryuya Murai, Yuri Saito, Hitoya Sano, Yae Kanai, Yoshihiro Moriya, Hirofumi Arakawa. Frequent inactivation of the Mieap-regulated mitochondrial quality control in colorectal cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1687. doi:10.1158/1538-7445.AM2013-1687