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, 17 (1), 41-52

SIRT3 Is a Mitochondria-Localized Tumor Suppressor Required for Maintenance of Mitochondrial Integrity and Metabolism During Stress

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SIRT3 Is a Mitochondria-Localized Tumor Suppressor Required for Maintenance of Mitochondrial Integrity and Metabolism During Stress

Hyun-Seok Kim et al. Cancer Cell.

Abstract

The sirtuin gene family (SIRT) is hypothesized to regulate the aging process and play a role in cellular repair. This work demonstrates that SIRT3(-/-) mouse embryonic fibroblasts (MEFs) exhibit abnormal mitochondrial physiology as well as increases in stress-induced superoxide levels and genomic instability. Expression of a single oncogene (Myc or Ras) in SIRT3(-/-) MEFs results in in vitro transformation and altered intracellular metabolism. Superoxide dismutase prevents transformation by a single oncogene in SIRT3(-/-) MEFs and reverses the tumor-permissive phenotype as well as stress-induced genomic instability. In addition, SIRT3(-/-) mice develop ER/PR-positive mammary tumors. Finally, human breast and other human cancer specimens exhibit reduced SIRT3 levels. These results identify SIRT3 as a genomically expressed, mitochondria-localized tumor suppressor.

Figures

Figure 1
Figure 1. SIRT3 knockout MEFs exhibit increased superoxide levels, aneuploidy in response to exogenous stress, and decreased mitochondrial integrity with increasing age
(A) Superoxide levels were elevated in SIRT3 knockout cells exposed to agents that induce mitochondrial damage. SIRT3+/+ and SIRT3−/− MEFs were cultured in 6% oxygen and exposed to either 5 Gy of IR or 5 µM of antimycin A for 3 hours, and superoxide levels were monitored by DHE oxidation as compared to control, untreated cells (Cont). For all DHE oxidation experiments the results were the normalized MFI for three independent replicates. (B) SIRT3−/− superoxide levels were elevated when cultured in 21% oxygen. SIRT3+/+ and SIRT3−/− MEF cells were cultured at 21% O2 for 6 hrs and superoxide levels were monitored by DHE oxidation, as compared to control cells grown at 6% O2. (C) Mitochondrial superoxide levels are elevated in SIRT3 knockout MEFs and increase following exogenous stress. Mitochondrial superoxide levels were determined by the addition of Mito-SOX (3 µM) to the culture medium and cells were incubated for an additional 10 minutes before being trypsinized and resuspended. Fluorescence was measured via flow cytometry, and 20,000 and 40,000 cells were counted for each sample. (D) SIRT3 knockout MEFs exhibited aneuploidy following exposure to IR. SIRT3+/+ and SIRT3−/− MEFs were exposed to either 2 or 5 Gy. Whole-mount chromosomes were counted in a blinded fashion. Bars show the mean chromosome number per cell from 100 separate counts. (E) Livers from SIRT3 knockout mice have increased mtDNA damage with age. DNA was isolated from the livers of SIRT3 wild-type and knockout mice at 20, 36, and 58 weeks, and mtDNA primers that amplify either the 10 kb Amplicon or a 117 bp region (Supplemental Fig. S4A) were used for PCR. Primers to the genomic p-globin gene were used as a control. (F) SIRT3 knockout MEFs have decreased mtDNA integrity. DNA was isolated from SIRT3+/+ and SIRT3−/− MEFs at passage 2, 6, and 10, and mtDNA primers that amplify either the 10 kb Amplicon used for PCR. All the results in this figure are from at least three separate experiments. Data are presented as the average +/− SD. * indicates P < 0.05 by t-test. See also Figure S1.
Figure 2
Figure 2. SIRT3 knockout MEFs expressing a single oncogene exhibit an in vitro transformation-permissive phenotype
(A) Long-term culture (28 days) of confluent SIRT3 knockout MEFs results in decreased contact inhibition as shown by spontaneous colony formation. SIRT3+/+ and SIRT3−/− MEFs were plated at 1 × 106/100 mm dish and fed with fresh media every 3–4 days for a total of 28 days. Colonies were evident by both phase-contrast microscopy and H&E stain. (B) SIRT3−/− MEFs infected with Myc, Ras, or both demonstrated decreased contact inhibition. SIRT3+/+, SIRT3−/−, SIRT3+/+ Myc/Ras, SIRT3−/− Myc, SIRT3−/− Ras, and SIRT3−/− Myc/Ras cells were plated as above and medium was replaced every 2 days for 28 days. Cells were then stained with crystal violet and counted. (C) SIRT3 knockout Myc, Ras, and Myc/Ras MEFs exhibit an increased pro-proliferative growth phenotype when plated at very low densities. SIRT3+/+, SIRT3+/+ Myc/Ras, SIRT3−/−, SIRT3−/− Myc, SIRT3−/− Ras, and SIRT3−/− Myc/Ras MEFs were plated at either 100 or 250 cells per plate (6-well plates), stained with crystal. For B–C, all results are from at least three separate experiments. Data are presented as the average +/− SD. * indicates P < 0.01 by t-test. Scale bar = 3 mm in (A). See also Figure S2.
Figure 3
Figure 3. Loss of SIRT3 results in an invasive and tumorigenic phenotype
(A) SIRT3 knockout MEFs expressing Myc and/or Ras exhibit polyploidy. Transformed MEF cells were harvested and analyzed by FACS. The percentage of cells containing greater than 4n is shown. (B) SIRT3−/− Myc and/or Ras MEFs exhibit increased chromosomal aberrations. Whole-mount chromosomes were counted in a blinded fashion. Columns are the mean chromosome number per cell from 100 separate counts. (C) SIRT3 knockout MEFs expressing Ras or Myc or both display anchorage independent growth in soft agar. SIRT3+/+, SIRT3−/−, SIRT3+/+ Myc/Ras, SIRT3−/− Myc, SIRT3−/− Ras, and SIRT3−/− Myc/Ras cells were seeded and colonies were stained with methylene blue after 12 days and counted. (D) SIRT3+/+, SIRT3−/−, SIRT3+/+ Myc/Ras, SIRT3−/− Myc, SIRT3−/− Myc/Ras, and SIRT3−/− Ras cells were implanted into both hind limbs of nude mice. Photographs of the hind limbs of nude mice injected with the cells are shown. (E) Histological examination of SIRT3−/− Myc/Ras allograft tumors stained with H&E. Results in this figure are the mean of at least three separate experiments. Error bars represent one standard deviation about the arithmetic mean. * indicates P < 0.05 by t-test. Scale bar = 80 µm in (E).
Figure 4
Figure 4. SIRT3 knockout MEFs expressing Myc and/or Ras have altered biochemical and metabolic properties and exhibit decreased complex I and III activity
(A) Superoxide levels in SIRT3 wild-type and knockout Myc- and/or Ras- infected MEF cells, monitored by DHE oxidation. MFI of three independent replicates was plotted. (B) Total cellular ATP levels in SIRT3+/+ Myc/Ras, SIRT3−/− Myc, SIRT3−/− Ras, and SIRT3−/− Myc/Ras. Cells were lysed and ATP levels were measured using chemiluminescence. (C) Mitochondrial ATP levels are decreased in SIRT3 knockout cells expressing either Ras or Myc and Ras. SIRT3+/+ Myc/Ras, SIRT3−/− Myc, SIRT3−/− Ras, and SIRT3−/− Myc/Ras cells were treated with 20 mM 2DG for 4 hours and mtATP levels were measured using chemiluminescence. Data are presented as relative mtATP levels as a percentage relative to the SIRT3+/+ Myc/Ras (Control) cells. (D) Analysis of glucose consumption in SIRT3 wild-type and knockout cells infected with Myc and/or Ras. Cells were counted and medium samples were obtained at 48 hours and analyzed using an YSI glucometer. Glucose consumption was determined by subtracting glucose content at the 48-hour point from the time zero sample and dividing by the number of cells. (E) SIRT3−/− Ras and SIRT3−/− Myc/Ras cells exhibit decreased complex I and III activity. Oxidative phosphorylation enzyme activities were measured on total cellular protein. Complex I activity was measured as the rotenone inhibitable rate of NADH oxidation. Complex II activity was measured by the succinate induced rate of reduction of DCIP. Complex III activity was measured as the rate reduction of cytochrome c (III) when stimulated with CoQ2H2. Complex IV activity was measured as the rate of cytochrome c (II) oxidation. All enzyme complex activities are expressed relative to SIRT3+/+ Myc/Ras. Results are from three separate experiments. Data are presented as the average +/− SD. * indicates P < 0.05 by t-test. See also Figure S3.
Figure 5
Figure 5. The transformative and growth properties of transformed SIRT3 knockout cells are decreased by SOD
(A) SIRT3 knockout cells exhibited an increased growth rate. SIRT3+/+, SIRT3−/−, SIRT3−/− Myc, SIRT3−/− Ras, SIRT3+/+ Myc/Ras, and SIRT3−/− Myc/Ras MEFs were plated at 2 × 104 cells per plate and harvested at 2, 3, and 4 days. The number of cells per plate was plotted as a function of days to determine growth rate and doubling times. (B) Infection with a MnSOD-expressing adenovirus decreases the growth rate of SIRT3 knockout cells. SIRT3+/+ Myc/Ras, SIRT3−/− Myc/Ras, and SIRT3−/− Ras cells were infected with Adeno-MnSOD and cells were isolated at 72 and 90 hours to determine cell growth rates. (C) SIRT3−/− MEFs exhibit increased in vitro colony formation at 21% O2. 1 × 106 SIRT3+/+ and SIRT3−/− MEFs were plated on a 10 cm plate and cultured at either 6% or 21% O2. Media was replaced every 2 days and after 28 days the MEFs were subsequently stained with crystal violet and counted. (D) The addition of MnSOD reverses the increase in contact-inhibited growth in SIRT3−/− cells at 21% O2. SIRT3−/− MEFs were plated and cultured at either 6% or 21% O2 with infection with either 5 MOI of either a control lentivirus or a lentivirus containing MnSOD. Cells were subsequently stained with crystal violet and counted as above. (E) MnSOD prevents aneuploidy in SIRT3 knockout MEFs exposed to IR. SIRT3−/− MEFs were infected with either a control lentivirus or lenti-MnSOD 24 hours prior to exposure to 2 or 5 Gy. Whole-mount chromosomes were counted in a blinded fashion. Bars show the mean chromosome number per cell from 100 separate counts. (F) MnSOD expression in wild-type and SIRT3 knockout mouse livers at 5, 9, and 13 months. RNA was harvested from four age matched SIRT3+/+ and SIRT3−/− mouse livers and MnSOD expression was determined by qRT-PCR using MnSOD and β-actin Taqman probes (ABI). (G) Infection of lenti-SIRT3-wt but not lenti-SIRT3-dn (deacetylation null mutant) increases MnSOD protein levels in SIRT3−/− Myc/Ras transformed MEFs. SIRT3−/− Myc/Ras cells were infected with virus, and 48 hours later cells were harvested and extracts were isolated and 20 µg of protein were separated by SDS-PAGE, transferred onto nitrocellulose, and immunoblotted using an anti-MnSOD antibody (Cell Signaling Technology, Inc). (H) Infection of lenti-SIRT3-wt but not lenti-SIRT3-dn in SIRT3−/− Myc/Ras transformed MEFs reverses the increase in mitochondrial superoxide levels. Mitochondrial superoxide levels were determined by the addition of Mito-SOX (3 µM) to the culture medium and cells were incubated for an additional 10 minutes before being trypsinized and resuspended. Fluorescence was measured via flow cytometry, and 20,000 and 40,000 cells were counted for each sample. Results are from at least three separate experiments. Data are presented as the average +/− SD. * indicates P < 0.05 and ** indicates P < 0.01 by t-test. See also Figure S4.
Figure 6
Figure 6. SIRT3 is a mitochondrial localized murine tumor suppressor
(A) SIRT3 knockout mice develop mammary tumors. The total number of mammary tumors at 24 months in SIRT3 wild-type and knockout mice is shown. Data are presented as the average +/− SD. * indicates P < 0.05 by t-test. (B) Plot of the number of tumor-free SIRT3−/− (n = 10 × 2) and wild-type mice (n = 12 × 2) over 24 months. (C) Representative H & E slides from mammary tissue from a SIRT3+/+ and a SIRT3−/− mouse that developed a mammary tumor. (D) IHC staining of SIRT3−/− murine mammary tumors with DAPI/CK14 (left panel), DAPI/CK18 (middle panel), and merged (right panel). (E) IHC staining for ER and PR status was performed on paraffin sections from the seven SIRT3−/− mice that developed mammary tumors. ER/PR levels were characterized as absent (−), intermediate (+), or strongly present throughout the sample (++). (F) SIRT3 knockout mice mammary ductal cells exhibit increased anti-nitrotyrosine IHC staining. Mammary tissue from four SIRT3+/+ and SIRT3−/− mice at age 12 months was stained with an anti-nitrotyrosine antibody (StressMarq Biosciences Inc.). A representative micrograph is shown. Scale bar = 160 µm in (C, left panel), 80 µm in (C, right panel) and (F), and 40 µm in (D). See also Figure S5.
Figure 7
Figure 7. SIRT3 is a potential human tumor suppressor
(A) SIRT3 protein levels are decreased in human breast tumors; 38 normal and 36 breast cancer (US Biomax, Inc) samples were measured by IHC on a tissue array. Tissue arrays were stained with a human SIRT3 antibody (Cell Signaling Technology, Inc) and slides were read by two independent researchers. Levels of SIRT3 staining were classified as absent, medium, or high. (B) IHC images from breast tumor and normal samples demonstrating SIRT3 staining. The boxed regions (enlarged in the bottom row) show SIRT3 in normal epithelium (left six images) and breast tumors (right four images). (C) SIRT3 expression in RNA samples from normal breast (NB) and Stage I, IIA, IIB, and III breast malignancies (TissueScan Breast Cancer Panel 1 Origene). SIRT3 expression was determined by qRT-PCR using SIRT3 and β-actin Taqman probes (ABI). (D-F) SIRT3 expression is decreased in human breast cancer and as a function of pathological classification. The Oncomine cancer microarray database (http://www.oncomine.org) was used to determine SIRT3 expression in (D) normal versus breast cancers (Richardson et al., 2006), as a function of (E) Elston (G-1, G-2, or G-3) Grade (Ivshina et al., 2006), or (F) pathological (well, moderately, or poorly differentiated) differentiation (Sotiriou et al., 2006). The y-axis represents normalized expression units. Shaded boxes represent the interquartile range; whiskers represent the 10th-90th percentile range; bars represent the median. (G) Schema for the development of increased mtROS and ER/PR positive mammary tumors in ductal cells lacking SIRT3. Scale bar = 80 µm in (B). See also Figure S6.

Comment in

  • A Tumor Suppressor SIRTainty
    PT Schumacker. Cancer Cell 17 (1), 5-6. PMID 20129243. - Review
    Sirtuin deacetylases are linked to longevity, aging, and stress responses. In this issue of Cancer Cell, Kim et al. show that SIRT3 functions as a tumor suppressor by enh …

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