Abstract 4106: Sirt3 controls the switch between mitophagy and apoptosis in tumor cells
Authors: Cheng Yan , Huber-keener Kathryn , Zhang Yi , Ren xingcong , Yang Jin-ming 
Published: August 2012

Abstract
Sirt3, a member of the family of nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylases, mainly localizes at mitochondria and participates in regulation of multiple cellular processes through its effect on protein acetylation. In recent years, the role of protein acetylation in controlling autophagy, a process of self-degradation of cellular components in response to various stresses such as starvation and oxidative stress, has begun to emerge. The objective of this study was to determine whether Sirt3, as a mitochondrial deacetylase, affects the induction of mitochondrial autophagy (mitophagy), a process of the specific autophagic elimination of damaged mitochondria. In human glioma cell lines T98G and LN229 and breast cancer cell lines MCF-7 and MDAMB468, we showed that increased production of reactive oxygen species (ROS) and damaged mitochondria (decreased mitochondrial membrane potential, MMP) caused by hypoxia were accompanied by activation of autophagy, as evidenced by elevated amounts of LC3 II, Atg5 and Atg12-5 complex that were detected by western blot. Silencing of Sirt3 expression by RNA interference blunted the hypoxia-induced autophagic response, blocked the degradation of mitochondria marker COX IV, and further increased the accumulation of ROS, suggesting an involvement of this protein deacetylase in the induction of mitophagy. Silencing of Sirt3 expression also promoted apoptosis, as demonstrated by an increase in the population of tumor cells with Annexing V staining and by a reduction in MMP. Furthermore, our experiments showed that silencing of Sirt3 expression led to a down-regulation of the anti-apoptotic proteins mcl-1 and survivin, and MG132, a proteasome inhibitor, could rescue the down-regulation of these apoptosis-regulated proteins in the cells with silencing of Sirt3 expression. These results suggest that Sirt3 plays a role in stabilizing mcl-1 and survivin proteins, and that the down-regulation of mcl-1 and survivin by Sirt3 knockdown may account for the promotion of apoptosis in the cells treated with Sirt3-targeted siRNA. Similarly observations were made when these cell lines were subjected to starvation and oxidative stress (H2O2 treatment). Taken together, the results of the current study reveal Sirt3 as a novel regulator coupling mitophagy and apoptosis, two important cellular processes that determine survival and death of tumor cells. As alterations of mitophagy and apoptosis have been known to be associated with tumor development, progression, and therapy, our study on the role of Sirt3 in regulating these two cellular processes provides evidence for exploiting Sirt3 as a new drug target for prevention and treatment of cancer.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4106. doi:1538-7445.AM2012-4106