Annual Report 2017

Hirofumi Arakawa, Yasuyuki Nakamura, Akiko Kuma, Masayuki Tsuneki, Yoko Sagami, Katsuko Honjo, Makoto Yamamoto, Katsuya Chinen, Naoki Tsukimata

Research activities

1. Mieap-regulated mitochondrial quality control

 Mieap controls mitochondrial quality via two distinct novel mechanisms. One of the mechanisms has been designated MALM for Mieap-

induced accumulation of lysosome-like organelles

within mitochondria (PLoS ONE 6: e16054, 2011). In this mechanism, Mieap induces the accumulation of intramitochondrial lysosomal proteins in order to eliminate oxidized mitochondrial proteins in response to mitochondrial damage. This leads to a decrease in reactive oxygen species generation and an increase in mitochondrial ATP synthesis activity, implying MALM plays a role in repairing unhealthy mitochondria.

 BNIP3 and NIX, mitochondrial outer membrane proteins, two Mieap-interacting proteins mediate the translocation of lysosomal proteins from cytosol into mitochondria during MALM by forming an unknown pore in the mitochondrial double membrane (PLoS ONE 7: e30767, 2012). 14-3-3g mediates the degradation of oxidized mitochondrial proteins within mitochondria during MALM (Scientific Reports 2: 379, 2012).

 Alternatively, the other mechanism has been designated MIV for Mieap-induced vacuole (PLoS ONE 6: e16060, 2011). When MALM is inhibited, Mieap induces a vacuole-like structure, MIV. The MIV engulfs the damaged mitochondria and accumulates lysosomes, leading to the degradation of unhealthy mitochondria. MIV likely represents a novel mechanism for mitochondrial autophagy, also called "mitophagy". Therefore, Mieap controls mitochondrial quality by repairing or eliminating unhealthy mitochondria via MALM or MIV generation, respectively (Figure 1).

Figure 1. Mieap-regulated mitochondrial quality control

Figure 1. Mieap-regulated mitochondrial quality control

2. Frequent inactivation of Mieap-regulated mitochondrial quality control in human cancer

 The accumulation of unhealthy mitochondria results in mitochondrial dysfunction, which has been implicated in aging, degenerative diseases and cancer. The Mieap-regulated mitochondrial quality control (MQC) was found to be frequently inactivated by p53 mutations or Mieap/BNIP3 promoter methylation in more than 70% of primary cancer tissues of colorectal cancer patients, leading to accumulation of unhealthy mitochondria and high levels of mitochondria reactive oxygen species (ROS) generation (Oncogenesis 5: e181, 2016).

 The elevated mitochondrial ROS causes oxidative damage to DNA, RNA, protein, lipid, and so on. This induces genomic instability. The mitochondrial ROS contribute to tumor growth,

epithelial-to-mesenchymal transition, cancer inva-

sion, cancer metastasis, and tumor angiogenesis through the activation of HIF-1, NF-kB, MMPs, AKT, Erk1/2, JNK, and so on. Therefore, the Mieap-regulated MQC is a tumor suppressor for colorectal cancer (Figure 2).

Figure 2. Alteration of Mieap-regulated mitochondrial quality control in cancer

Figure 2. Alteration of Mieap-regulated mitochondrial quality control in cancer

3. Mieap-deficient cancer animal model

 To clarify the in vivo role of the Mieap-

regulated MQC in tumorignenesis, the Mieap knockout mice were generated in our division. Using the Mieap knockout mice, the Mieap-

deficient ApcMin/+ mice were also generated and analyzed in order to elucidate the role of Mieap in colorectal cancer tumorigenesis (Scientific Reports 5: 12472, 2015).

 Interestingly, the Mieap-deficient ApcMin/+ mice exhibited remarkably reduced lifespans compared with those of ApcMin/+ mice. Furthermore, a substantial increase in the number and size of intestinal polyps was found in the Mieap-deficient ApcMin/+ mice. Histopathologically, intestinal tumors in the Mieap-deficient ApcMin/+ mice clearly exhibited advanced grades of adenoma and adenocarcinomas. Unhealthy mitochondria dramatically accumulated in the tumor cells and generated high levels of ROS in the Mieap-deficient ApcMin/+ mice. These results suggest that the Mieap-regulated MQC pathway has a critical role in the suppression of intestinal tumor in vivo.

4. Non-canonical mitophagy induced by Mieap and its role in tumor suppression via cell death

 Parkin/Pink1-mediated mitophagy plays a critical role in MQC, in which the damaged mitochondria are sequestered by autophagosomes, and degraded by fusion between autophagosomes and lysosomes. We found that the Mieap-

mediated non-canonical mitophagy as a new function of tumor suppressor p53. Mieap induces large vacuoles in cancer cells. The Mieap-induced vacuoles are generated from the mitochondria, and directly eat and degrade the cancer mitochondria. UVRAG regulates the Mieap-mediated non-canonical mitophagy. The Mieap-mediated

non-canonical mitophagy induces cell death via the iron-dependent production of reactive oxygen species (ROS). The Mieap-mediated non-

canonical mitophagy plays a critical role in tumor suppression via iron-dependent cell death.

Future prospects

 Analyses of cancer cell lines, clinical cancer tissues, and cancer-mouse models enable us to understand the actual role of the Mieap-regulated MQC in human cancer initiation, progression, invasion, and metastasis. Finally, we will be able to establish a solid foundation for the development of new strategies for cancer prevention, diagnosis, and therapy in the future.