Mitochondrial Dysfunction Induces Senescence with a Distinct Secretory Phenotype

Abstract

Cellular senescence permanently arrests cell proliferation, often accompanied by a multi-faceted senescence-associated secretory phenotype (SASP). Loss of mitochondrial function can drive age-related declines in the function of many post-mitotic tissues, but little is known about how mitochondrial dysfunction affects mitotic tissues. We show here that several manipulations that compromise mitochondrial function in proliferating human cells induce a senescence growth arrest with a modified SASP that lacks the IL-1-dependent inflammatory arm. Cells that underwent mitochondrial dysfunction-associated senescence (MiDAS) had lower NAD+/NADH ratios, which caused both the growth arrest and prevented the IL-1-associated SASP through AMPK-mediated p53 activation. Progeroid mice that rapidly accrue mtDNA mutations accumulated senescent cells with a MiDAS SASP in vivo, which suppressed adipogenesis and stimulated keratinocyte differentiation in cell culture. Our data identify a distinct senescence response and provide a mechanism by which mitochondrial dysfunction can drive aging phenotypes.

Figure 1. Loss of Mitochondrial SIRTs Induce Senescence

(A) (Left) SA-Bgal and BrdU indices 10 days after IMR-90 cells were infected with control (scrambled sequence), shSIRT3-, or shSIRT5-expressing lentiviruses. (Right) SIRT3, SIRT5, and actin (control) protein levels after infection. (B) Representative image of cells 10 days after senescence induction by shSIRT3, shSIRT5, and/or 10 Gy IR, stained for SA-Bgal. (C) Intracellular LMNB1, p21, and HMGB1 protein levels 10 days after IR or infection with control (−) or shSIRT3 (+) lentiviruses. (D) (Left) Representative image of EdU and HMGB1 staining in control and shSIRT3-expressing cells. (Right) Quantification of nuclear HMGB1 staining intensity. (E) IL-6, measured by ELISA, in conditioned media from control, shSIRT3-, or shSIRT5-expressing cells 10 days after mock irradiation or 10 Gy IR. (F) Heatmap of the indicated mRNA levels detected by qPCR in non-senescent (mock) or senescent (IR) cells infected with control or shSIRT3 lentiviruses, normalized to mock cells. (G) Heatmap of secreted protein levels by above cells, detected by antibody array. (H and I) Secreted IL-6 levels in response to senescence inducers, normalized to control shRNA cells: IR, NaBu, Bleo, and expression of constitutively active MKK6 (MKK6EE) or oncogenic RAS. Bar graphs indicate mean + SEM.

Figure 2. Mitochondrial Dysfunction-Induced Senescence

(A) (Left) IMR-90 cells were cultured in DMSO (control), ethidium bromide (rho0), rotenone (Rot), or antimycin A (Anti A) or (right) infected with control, shSIRT3-, or shHSPA9-expressing lentiviruses and analyzed for SA-Bgal and EdU 10 days later. (B) Cells from (A) were mock irradiated or irradiated (10 Gy) and analyzed for IL-6 mRNA by qPCR. (C) Cells cultured in control DMSO, EtBr (rho0), or rotenone (Rot) for 14 days were analyzed for EdU incorporation and nuclear HMGB1 by immunofluorescence and cellular HMGB1 by immunoblotting. (D) Rho0 IMR-90 (left) or MSC-1 (right) were generated in the presence of pyruvate and uridine. WT and rho0 cells cultured with (Pyr+) or without (Pyr−) pyruvate for 7 days were analyzed for EdU incorporation. (E) SA-Bgal activity in cells from (D). (F) WT cells were mock or irradiated to senesce by IR (IR) and compared to rho0 cells cultured + or −pyruvate. Indicated mRNA levels were determined by qPCR. Bar graphs indicate mean + SEM.

Figure 3. MiDAS Is Mediated by NAD+/NADH Ratios

(A) NAD+/NADH ratios in WT and rho0 cells cultured + or −pyruvate for 10 days. (B) Control (WT or DMSO), rho0, or rotenone-treated (Rot) cells cultured without pyruvate and + or −K3[Fe(CN)6], analyzed for SA-Bgal. (C) Cells cultured −or + K3[Fe(CN)6], and either DMSO or rotenone (Rot), for 8 days, were fixed and stained with crystal violet. (D–G) WT cells cultured in PBS or AOA for 10 days: (D) SA-Bgal activity and EdU labeling; (E) (left) representative image of HMGB1 staining and EdU labeling in PBS or AOA-treated cells, (middle) quantification of nuclear HMGB1 staining, (right) LMNB1 and HMGB1 protein in cells treated for 7 days with PBS or AOA; (F) IL-6 ELISAs of CM from mock or irradiated cells 10 days after IR and AOA treatment; (G) cells given PBS or AOA + or −pyruvate (Pyr) for 10 days were stained with crystal violet. (H) Cells cultured for 21 days + or −20 nM FK866 were analyzed for SA-Bgal and EdU labeling. (I and J) Cells treated with 40 nM FK866 for 3 days following IR were analyzed for IL-6 secretion by ELISA (I) and NAD (J). Bar graphs indicate mean + SEM.

Figure 4. MiDAS Depends on AMPK Activation

(A) Cells infected with control or shSIRT3-expressing lentiviruses (upper panel), or treated with DMSO, Rot, Anti A, or ethidium bromide (EtBr) (lower panel) for 10 days were analyzed for ADP:ATP ratios. (B) WT or rho0 cells cultured + or −pyruvate and/or 2-DG for 10 days were analyzed for EdU labeling (upper panel) or SA-Bgal (lower panel). (C) Control or shSIRT3-expressing cells were cultured for 10 days after infection. WT (mtDNA⁺) and rho0 (mtDNA−) cells were cultured + or − 1 mM pyruvate for 10 days, lysed, and analyzed by immunoblotting for total and phosphorylated AMPK and p53 and intracellular LMNB1, HMGB1, p21^WAF1, p16^INK4a, SIRT3, COX2, and actin. (D) Control (siSIRT3/AMPK−), SIRT3 (siSIRT3+), and AMPKα-1/2 (siAMPK+) siRNAs were transfected into cells, which were analyzed by immunoblotting 4 days later for the indicated proteins. (E and F) Cells infected with control or dominant-negative AMPK (AMPK-DN)-expressing lentiviruses, then control or shSIRT3-expressing lentiviruses were analyzed 7 days later for (E) SA-Bgal and (F) EdU incorporation. Bar graphs indicate mean + SEM.

Figure 5. p53 Suppresses SASP Factors in MiDAS

(A–C) Cells were infected with lentiviruses expressing control, SIRT3 or HSPA9 shRNAs, then a p53 shRNA (shp53) or control virus, were assayed 10 days later for HSPA9, SIRT3, p53 and p21 protein (A), SA-Bgal activity (B), and BrdU incorporation (C). (D) RNA from WT and rho0 cells, infected with shp53 or control lentiviruses and cultured + or − pyruvate for 10 days was analyzed for IL-1A (upper panel) and IL-1B (lower panel) mRNA by qPCR. (E) RNA from WT or rho0 cells, mock- or IR-treated, and cultured + or − pyruvate for 10 days was analyzed as in (D). (F) NF-κB reporter activity in cells cultured as in (E). (G) ChIP assays for RelA and CBP binding at the IL1A promoter in WT and rho0 cells cultured + or − pyruvate for 7 days. (H) WT and rho0 cells were cultured + or − pyruvate and infected with control or shRelA-expressing lentiviruses, and mRNA levels were measured by qPCR. (I) WT and rho0 cells cultured + or − pyruvate were analyzed for IL-10 and TNF-α mRNAs by qPCR. Bar graphs indicate mean + SEM.

Figure 6. MiDAS in POLG^D257A Mouse Tissues

(A) Representative image of SA-Bgal in IAT from 8-month-old WT and POLG^D257A mice. (B) Tissues from (A) were sectioned and immunostained for HMGB1 (representative image). Black arrows indicate nuclei; blue arrow indicates SA-Bgal. (C and D) HMGB1-positive nuclei and SA-Bgal-positive cells were quantified in tissues from (A). (E) qPCR analysis for p16^INK4a mRNA in WT and POLG^D257A tissue. (F) NAD+/NADH ratios in IAT from WT and POLG^D257A tissues. (G) qPCR analysis for the indicated mRNAs in tissues from (E). (H) 3T3-L1 cells were induced to differentiate in the presence of conditioned media from WT or rho0 IMR-90 fibroblasts. (Left panel) Representative images of cells stained with oil red O 6 days after treatment. (Right panel) quantitation of cells with <25% or >75% of cell area positive for oil red O. (I) qPCR for markers of adipocyte differentiation in cells described in (H). Bar graphs indicate mean + SEM.