Arterial "inflammaging" drives vascular calcification in children on dialysis

Abstract

Children on dialysis have a cardiovascular mortality risk equivalent to older adults in the general population, and rapidly develop medial vascular calcification, an age-associated pathology. We hypothesized that premature vascular ageing contributes to calcification in children with advanced chronic kidney disease (CKD). Vessels from children with Stage 5 CKD with and without dialysis had evidence of increased oxidative DNA damage. The senescence markers p16 and p21 were also increased in vessels from children on dialysis. Treatment of vessel rings ex vivo with calcifying media increased oxidative DNA damage in vessels from children with Stage 5 CKD, but not in those from healthy controls. Vascular smooth muscle cells cultured from children on dialysis exhibited persistent DNA damage, impaired DNA damage repair, and accelerated senescence. Under calcifying conditions vascular smooth muscle cells from children on dialysis showed increased osteogenic differentiation and calcification. These changes correlated with activation of the senescence-associated secretory phenotype (SASP), an inflammatory phenotype characterized by the secretion of proinflammatory cytokines and growth factors. Blockade of ataxia-telangiectasia mutated (ATM)-mediated DNA damage signaling reduced both inflammation and calcification. Clinically, children on dialysis had elevated circulating levels of osteogenic SASP factors that correlated with increased vascular stiffness and coronary artery calcification. These data imply that dysregulated mineral metabolism drives vascular "inflammaging" by promoting oxidative DNA damage, premature senescence, and activation of a pro-inflammatory SASP. Drugs that target DNA damage signaling or eliminate senescent cells may have the potential to prevent vascular calcification in patients with advanced CKD.

Keywords: aging; calcification; dialysis; senescence; vascular smooth muscle cells.

Figure 1. Vessels from children with chronic kidney disease (CKD) show elevated levels of oxidative DNA damage and senescence markers.

(a) Immunohistochemistry and quantification for 8-oxo-dG showed a significantly increased percentage of vascular smooth muscle cells with oxidative DNA damage in vessels from predialysis (CKD5) and dialysis (CKD5D) patients compared with control subjects. Positive nuclei are indicated by arrows, and the boxed inset shows enlargement. Bar = 100 μm. (b) Immunohistochemistry showing increased p21 and p16 nuclear staining in CKD stage 5 predialysis (CKD5) and dialysis (CKD5D) vessels compared with control vessels. Note the nuclear staining (arrowed and insets). Bar = 50 μm. (c) The percentage of positively stained cells and nuclei for p21 was significantly higher in both CKD5 and CKD5D when compared with control vessels, while p16 was only significantly elevated in dialysis vessels. Graphs show mean ± SE (analysis of variance.) Ad, adventitia; M, media. To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.

Figure 2. Vascular smooth muscle cells (VSMCs) from dialysis vessels senesce early and show elevated levels of DNA damage in vitro.

(a–c) VSMCs grown from dialysis patients showed premature senescence compared with cells from control subjects as shown by (a) slower growth rates, (b) senescence-associated β galactosidase staining, and (c) elevated levels of p16 at equivalent passage number (P). Representative data from 3 control and 3 dialysis isolates are shown. Bar = 50 μm. (d) Immunofluorescence staining for γH₂AX and phosphorylated ataxia-telangiectasia mutated (pATM) and ataxia telangiectasia and Rad3-related protein (ATR) substrate shows dialysis VSMCs have increased DNA damage signaling when compared with control cells at equivalent early (P6) passage (positive nuclei are indicated by arrows). Bar = 10 μm. (e) The percentage of γH₂AX– and pATM/ATR–positive nuclei was significantly higher in dialysis VSMCs compared with both control and predialysis (CKD5) cells. Each bar represents mean ± SE (***P < 0.001, dialysis vs. control and dialysis vs. predialysis cells, analysis of variance). (f) Western blot showing increased γH₂AX and pATM/ATR protein levels in dialysis (n = 3, patients [pt] 04, 34, 51) VSMCs compared with chronic kidney disease stage 5 predialysis (n = 2, patients 07, 40) and control (n = 2, patients 20, 39) VSMCs at equivalent passage number. (g) Comet assays confirm increased DNA damage in dialysis VSMCs with an increased percentage of cells with comet tails (indicated by arrow in inset showing dialysis VSMCs) compared with control cells at equivalent passage (****P < 0.001, n = 3, Student’s t-test). Bar = 10 μm. DAPI, 4′,6-diamidino-2-phenylindole. To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.

Figure 3. Elevated calcium and phosphate (CaP) caused oxidative stress and increased oxidative DNA damage in chronic kidney disease (CKD) vessels.

(a) Immunofluorescence staining for γH₂AX and phosphorylated ataxia-telangiectasia mutated (pATM) or ataxia telangiectasia and Rad3-related protein (ATR) substrate or both indicates that CaP promoted DNA damage (arrows indicate positive nuclei). Bar = 10 μm. (b) The percentage of γH₂AX– and pATM/ATR–positive nuclei in response to CaP media is similar to doxorubicin and hydrogen peroxide treatments and significantly higher compared with baseline vascular smooth muscle cells. Each bar represents mean ± SE (****P < 0.001 compared with baseline). Bar = 10 μm. (c) Western blotting shows an increase in γH₂AX and pATM/ATR protein levels in calcifying media equivalent to doxorubicin (Dox) and hydrogen peroxide treatments. (d) Immunohistochemistry for 8-oxo-dG shows that calcifying media promotes an increase in 8-oxo-dG staining in CKD stage 5 predialysis (CKD5) and dialysis (CKD5D) vessels. (e) Quantification shows that calcifying media significantly increases 8-oxo-dG positive nuclei in CKD5 (P = 0.04; n = 4) and dialysis (P = 0.001; n = 4) vessels with a greater increase in dialysis vessels despite similar baseline levels of oxidative DNA damage. The effect of CaP on control vascular smooth muscle cells was not significant (P = 0.08; n = 4). M, media. To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.

Figure 4. Dialysis vascular smooth muscle cells (VSMCs) showed impaired DNA damage repair.

(a,b) Immunofluorescence showing elevated levels of DNA damage in dialysis VSMCs (arrows) 24 hours after washout of the DNA damage agent doxorubicin (Dox). In contrast, control VSMCs are mostly repaired at this stage. Quantification shows both γH₂AX and 53BP1 foci are elevated. A high percentage of cells with >5 53BP1 foci is indicative of persistent DNA damage and failure of repair. n = 3 isolates/group. Analysis of variance (*P < 0.05, **P < 0.01). Bar = 10 μm. DMSO, dimethylsulfoxide. To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.

Figure 5. Vascular smooth muscle cells (VSMCs) from dialysis patients show increased osteogenic differentiation and calcification.

(a) Quantitative reverse transcriptase–polymerase chain reaction shows that VSMCs from dialysis patients have reduced expression of SM markers and increased expression of osteogenic markers when compared with control VSMCs. Each bar represents mean ± SE, Student’s t-test. (b) Alizarin red staining shows VSMCs from dialysis vessels are more prone to calcify than VSMCs from control vessels in response to calcifying medium for 7 days. Each bar represents mean ± SE (*P < 0.05, analysis of variance). (c) Quantitative reverse transcriptase–polymerase chain reaction shows that VSMCs from dialysis patients have significantly elevated mRNA expression of the osteogenic markers Runx2 and bone morphogenetic protein-2 (BMP2) in response to calcifying media when compared with control media. Each bar represents mean ± SE (*P < 0.05, **P < 0.01, ***P < 0.001). Ca, calcium; CaP, calcium and phosphate; P, phosphate; SMA, smooth muscle actin.

Figure 6. Dialysis vascular smooth muscle cells (VSMCs) show increased senescence in response to calcifying media.

(a,b) VSMCs were incubated in control (Con), calcium (Ca) (2.7 mmol/l), phosphate (P) (2.5 mmol/l), or CaP supplemented serum-free medium for 16 hours and stained for senescence-associated β galactosidase (SA-βG). Increased staining was observed in response to elevated CaP or P alone in dialysis VSMCs (***P < 0.001, analysis of variance). Bar = 100 μm. (c) Western blot shows that in response to elevated calcium and phosphate (CaP) p16 is increased in dialysis VSMCs but not in control VSMCs. The p21 levels did not consistently change in either group. Blots representative of n = 3 isolates/group. To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.

Figure 7. Inhibition of DNA damage signaling reduces calcification and expression of senescence-associated secretory phenotype (SASP) factors in dialysis vascular smooth muscle cells (VSMCs).

(a,b) Control and dialysis VSMCs were treated with osteogenic media in the presence or absence of the ataxia-telangiectasia mutated inhibitor (ATMi) Ku55933. Alizarin red staining shows that after 5 days, dialysis but not control VSMCs had calcified and calcification of dialysis cells was decreased by ATM inhibition. (*P < 0.05, analysis of variance.) (c) Quantitative reverse transcriptase–polymerase chain reaction showing levels of expression of the SASP factors bone morphogenetic protein-2 (BMP2), interleukin-6 (IL-6), and osteoprotegerin (OPG) in control and dialysis VSMCs. Note the higher levels of these factors in dialysis VSMCs at baseline. ATM inhibition using small, interfering RNA (siRNA) decreased elevated levels of the SASP factors BMP2, OPG, and IL-6 in dialysis VSMCs (pink) with little or no effect on control VSMCs (green). Effectiveness of ATM siRNA knockdown compared with control siRNA was equivalent for both control and dialysis VSMCs. (Mean ± SE, analysis of variance, *P < 0.05.) CaP, calcium and phosphate. To optimize viewing of this image, please see the online version of this article at www.kidney-international.org.

Figure 8. Children with chronic kidney disease (CKD) show elevated senescence-associated secretory phenotype factors correlating with vascular stiffness.

(a,b) Serum levels of bone morphogenetic protein-2 (BMP2) and interleukin-6 (IL-6) in children with CKD stage 5 predialysis (n = 11) and those on dialysis (n = 24) showing significantly higher levels of both markers in children on dialysis. In children with CKD stage 5 and those on dialysis who were older than 5 years of age (n = 31), BMP2 and osteoprotegerin (OPG) correlated with aortic pulse wave velocity (PWV) measured by applanation tonometry (c,d), showing significant correlation between these markers and PWV. (e) Coronary artery calcification (CAC) was measured by multislice computed tomography scan and patients were divided into 2 groups: those with calcification (n = 8) and those who did not have calcification (n = 16).The median (range) CAC score was 149 (43 to 2019) in the calcification group and 7 of 8 calcified patients were on dialysis. Children with CAC had significantly higher levels of BMP2, OPG, and IL-6 compared with those without calcification (P < 0.0001; multiple analysis of variance). Comparing individual factors both BMP2 and IL-6 were significantly elevated in the calcified group while OPG was not (P values for unpaired t-test are indicated on legend).