Hypercholesterolemia induces oxidant stress that accelerates the ageing of hematopoietic stem cells

Abstract

Background: Clinical studies suggest that hypercholesterolemia may cause ageing in hematopoietic stem cells (HSCs) because ageing-associated alterations were found in peripheral blood cells and their bone marrow residing precursors in patients with advanced atherosclerosis. We hypothesized that hypercholesterolemia induces oxidant stress in hematopoietic stems cells that accelerates their ageing.

Methods and results: Here we show that HSCs from ApoE(-/-) mice, as well as HSCs from C57Bl/6 mice fed a high cholesterol diet (HCD) accumulated oxLDL and had greater ROS levels. In accordance, the expression pattern of the genes involved in ROS metabolism changed significantly in HSCs from ApoE(-/-) mice. Hypercholesterolemia caused a significant reduction in phenotypically defined long-term HSC compartment, telomere length, and repopulation capacity of KTLS cells, indicating accelerated ageing in these cells. Gene array analysis suggested abnormal cell cycle status, and the key cell cycle regulators including p19(ARF), p27(Kip1) and p21(Waf1) were upregulated in KTLS cells from hypercholesterolemic mice. These effects were p38-dependent and reversed in vivo by treatment of hypercholesterolemic mice with antioxidant N-acetylcysteine. The oxidant stress also caused aberrant expression of Notch1 that caused loss of quiescence and proliferation leading to the expansion of KTLS compartment in hypercholesterolemic mice.

Conclusion: Taken together, we provide evidence that hypercholesterolemia can cause oxidant stress that accelerates the ageing and impairs the reconstitution capacity of HSCs.

Keywords: ageing; hematopoietic stem cell; hypercholesterolemia; oxidant stress.

Figure 1.

Hypercholesterolemia accelerates the ageing of HSC. A, The number of KTLS cells (cKit⁺/Sca1⁺/CD90.1^lo/−/Lin⁻) are expressed as a percent of total bone marrow cells (M±SD, n=10, *P<0.05). B, The number of LT‐HSCs (phenotypically defined as CD34⁻Flk₂⁻KTLS) were expressed as a percentage of total KTLS (M±SD, n=6, *P<0.05). C, Telomere length in KTLS cells from WT and ApoE^−/− mice was expressed as relative telomere length (M±SD, n=8, *P<0.05). D, Side population cells (Hoechst^low KTLS) were detected in total KTLS cells from WT mice (18.8±4.4%) and ApoE^−/− mice (6.2±1.7% *) expressed as a percent of total KTLS cells (n=5, *P<0.05). E, The percent of CD45.2⁺ mononuclear cells in peripheral blood was determined 4, 8, and 12 weeks after transplantation (M±SD, n=6 to 7, **P<0.01 vs transplant with CD45.2 WT HSCs). F, The percent of CD45.2⁺ populations in total KTLS cells 12 weeks after transplantation were measured in the recipients with KTLS cells from WT mice (51.4±6.2%) or with KTLS cells from ApoE^−/− mice (20.3±9.5% *) (M±SD, n=6 to 7, *P<0.05 vs transplant with WT KTLS cells). KTLS indicates cKit⁺/Sca1⁺/CD90.1^lo/−/Lin⁻; LT‐HSC, long‐term hematopoietic stem cell; WT, wild type.

Figure 2.

High cholesterol diet‐induced hypercholesterolemia also accelerates the ageing of HSCs. A, KTLS were derived from WT mice or WT mice fed a high cholesterol diet (HCD). The number of KTLS cells is expressed as a percent of total bone marrow mononuclear cells (M±SD, n=15, **P<0.01). B, The number of phenotypically defined LT‐HSCs (CD34⁻Flk₂⁻KTLS) in WT and HCD mice were expressed as a percent of total KTLS cells (M±SD, n=6, *P<0.05 vs WT). C, Telomere length in KTLS cells from WT and HCD mice was expressed as relative telomere length (M±SD, n=8, *P<0.05). D, Side population of KTLS cells in normal chow mice (16.5±2.1%) and HCD mice (5.2±3.8% *), determined by the phenotype Hoechst^low KTLS, were expressed as a percent of total KTLS cells. The numbers indicate the M±SD (n=5, *P<0.05 vs normal chow mice). E, The percent of CD45.2⁺ populations in total KTLS cells 12 weeks after transplantation were measured in the recipients with KTLS cells from normal chow mice (50±3.2%) or with KTLS cells from HCD mice (15±11.5% *) (n=6, *P<0.05 vs transplant with KTLS cells from normal chow mice). LT‐HSC indicates long‐term hematopoietic stem cell; WT, wild type.

Figure 3.

Hypercholesterolemia‐induced ageing of HSCs is oxidant stress‐dependent. A, ROS level was measured by DCF staining in KTLS cells from WT (46.1±5.3%), N‐acetylcysteine (NAC)‐treated WT (40.7±3.3%), GSH‐treated WT (42.7±4.1%), ApoE^−/− (60.5±4.8% *), NAC‐treated ApoE^−/− (42.6±5.3% †) and GSH‐treated mice (49.4±2.5% †). The number represents the M±SD (n=8, *P<0.05 vs WT; †P<0.05 vs ApoE^−/−). B, H₂O₂ concentrations expressed as arbitrary fluorescent units in KTLS cells from WT, NAC‐treated WT, GSH‐treated WT, ApoE^−/−, NAC‐treated ApoE^−/− and GSH‐treated mice (M±SD, n=6, *P<0.05; †P<0.05 vs ApoE^−/−). C, ROS level did not show difference between total bone marrow cells from WT, ApoE^−/− and HCD mice. ROS level was detected by DCF staining and FACS (n=6). D, Gene expression profiling of oxidant stress responsive and antioxidant genes by PCR array. Log2 transformed gene expression values are shown for ApoE^−/− vs WT HSCs. Circles show upregulated (red) and downregulated (green) genes. (n=3). E, The frequency of KTLS cells from ApoE^−/−, NAC‐treated ApoE^−/− and GSH‐treated ApoE^−/− mice expressed as a percent of total bone marrow cells (M±SD, n=8 to 10, *P<0.05 vs ApoE^−/−). F, Relative telomere length in KTLS cells from ApoE^−/−, NAC‐treated ApoE^−/− and GSH‐treated ApoE^−/− mice (M±SD, n=8, *P<0.05, vs ApoE^−/−). G, The percent of CD45.2⁺ KTLS cells 12 weeks after competitive transplantation (M±SD, n=6 to 7, *P<0.05 vs transplant with ApoE^−/− KTLS). DCF indicates 2′,7′‐Dichlorofluorescin diacetate; HCD, high cholesterol diet; FACS, flowcytometry; GSH, L‐Glutamyl‐L‐Cysteinyl‐Glycine; HSC, hematopoietic stem cell; PCR, polymerase chain reaction; ROS, reactive oxygen species; WT, wild type.

Figure 4.

ROS level increases in KTLS cells isolated from HCD induced hypercholesterolemic mice. A, ROS level was measured by DCF staining in KTLS cells from WT mice (43±6.3%) and HCD mice (79±4.8% *). The number represents the M±SD (n=8, *P<0.05 vs normal chow). B and C, The accumulation of nitrotyrosine in KTLS cells from ApoE^−/− mice (B) (n=6, *P<0.05 vs WT) or HCD mice (C) (n=6, *P<0.05 vs normal chow) was shown by. The number represents the M±SD. HCD indicates high cholesterol diet; ROS, reactive oxygen species; WT, wild type.

Figure 5.

LOX‐1 and SRB‐2 mediates oxLDL uptake in KTLS cells. A, Accumulation of oxLDL in KTLS from ApoE^−/− mice was shown by immunostaining (Bars, 50 μm). The number represents the M±SD (n=6, *P<0.05 vs WT mice). B and C, LOX‐1 expression was detected by qRT‐PCR (B) and flowcytometry (C) in CD34⁻Flk₂⁻KTLS cells, CD34⁺Flk₂⁻KTLS cells (phenotypically defined short term HSCs), Lin⁻ bone marrow and Lin⁺ marrow cells from WT mice. LOX‐1 expression in the KTLS population was significantly lower than Lin⁻ and Lin⁺ bone marrow cells (M±SD, n=5, *P<0.05, vs Lin+ bone marrow cells; †P<0.05, vs total bone marrow cells). D and E, SRB‐2 expression was detected by qRT‐PCR (D) and Flowcytometry (E) in CD34⁻Flk₂⁻, CD34⁺Flk₂⁻KTLS cells, Lin⁻ and Lin⁺ marrow cells from ApoE^−/− mice. LOX‐1 expression in the KTLS population was significantly lower than Lin⁻ and Lin⁺ marrow cells (M±SD, n=5, *P<0.05, vs Lin⁻ bone marrow cells; †P<0.05, vs Lin⁺ bone marrow cells). F and G, The expression of LOX‐1 in KTLS cells isolated from ApoE^−/− mice was significantly greater than those in KTLS cells isolated from WT mice. No difference was detected in the expression of SRB‐2. The expression of LOX‐1 and SRB‐2 was measured by qRT‐PCR (F) and flowcytometry (G) (M±SD, n=5,*P<0.05, vs WT KTLS). H and I, The expression of LOX‐1 and SRB‐2 in KTLS cells from WT and ApoE^−/− mice after transfection of Lenti‐shLOX‐1 or Lenti‐shSRB‐2 was detected by qRT‐PCR (H) and flowcytometry (I) (M±SD, n=6,*P<0.05, vs WT KTLS transfected with Lenti‐GFP; †P<0.05, ‡P<0.01, vs ApoE^−/− KTLS transfected with Lenti‐GFP). J, in vitro oxLDL uptake was measured by Flowcytometry in Lenti‐GFP WT KTLS (1.26±0.1%), Lenti‐shLOX‐1 WT KTLS (0.65±0.1% *), Lenti‐shSRB‐2 WT KTLS (0.91±0.1% *), Lenti‐shLOX‐1/SRB‐2 WT KTLS (0.28±0.1% *), Lenti‐GFP ApoE^−/− KTLS (2.92±0.1%), Lenti‐shLOX‐1 ApoE^−/− KTLS (0.79±0.1% †), Lenti‐shSRB‐2 ApoE^−/− KTLS (1.21±0.1% †) and Lenti‐shLOX‐1/SRB‐2 ApoE^−/− KTLS (0.33±0.1% ‡). The number represents the M±SD (n=8,*P<0.05, vs Lenti‐GFP WT KTLS; ^†P<0.05, ^‡P<0.01, vs Lenti‐GFP ApoE^−/− KTLS). BM indicates bone marrow; FSC, forward side scatter; GFP, green fluorescent protein; LOX‐1, lectin‐like oxidized low‐density lipoprotein receptor 1; LT‐HSC, long‐term hematopoietic stem cell; MAPK, mitogen activated protein kinase; oxLDL, oxidized low‐density lipoprotein; qRT‐PCR, quantitative reverse transcription polymerase chain reaction; SRB‐2, scavenger receptors B2; ST‐HSC, short term hematopoietic stem cell; WT, wild type.

Figure 6.

Accumulation of oxLDL causes oxidant stress, decreases LT‐HSC compartment and impairs reconstitution capacity of KTLS cells. A, ROS level was measured by DCF staining in WT KTLS cells treated with 0 μg/mL nLDL (7.5±0.9%), 10 μg/mL nLDL (8.7±1.2%), 20 μg/mL nLDL (11.6±1.7%), 0.1 μg/mL oxLDL (9.8±0.6%), 1 μg/mL oxLDL (11.7±1.6%), 10 μg/mL oxLDL (28.5±.4% *) and 20 μg/mL oxLDL (39.8±5.7% **). The numbers represent M±SD (n=8, *P<0.05, **P<0.01 vs 0 μg/mL nLDL group). B, Phenotypically defined LT‐HSC population of WT KTLS cells in vitro culture was measured by Flowcytometry. The number of LT‐HSC was represented as percent of total KTLS cells (M±SD, n=8, *P<0.05, **P<0.01, vs control). C, Competitive HSC transplantation assay of WT KTLS cells treated with oxLDL. The percent of CD45.2⁺ cells in peripheral blood were represented as M±SD (n=8, *P<0.05, **P<0.01, vs control). LT‐HSC indicates long‐term hematopoietic stem cell; oxLDL, oxidized low‐density lipoprotein; ROS, reactive oxygen species; WT, wild type.

Figure 7.

High fat diet further increased expansion and ROS level in HSC compartment of ApoE^−/− mice. A, The frequency of KTLS cells are expressed as a percent of total bone marrow mononuclear cells (M±SD, n=15, *P<0.05 vs normal chow mice; †P<0.01 vs ApoE^−/− mice). B, ROS level was detected by DCF staining and Flowcytometry in KTLS cells. The number within each histogram represents the M±SD (n=8, *P<0.05 vs normal chow mice, †P<0.05 vs ApoE^−/− mice). HFD indicates high fat diet; HSC, hematopoietic stem cell; ROS, reactive oxygen species; WT, wild type.

Figure 8.

High fat diet induced monocytosis and spleen hypertrophy in ApoE^−/− mice. A through D, White blood cells (A), lymphocytes (B), granulocytes (C) and monocytes (D) in peripheral blood of WT, HFD+WT, ApoE^−/− and HFD ApoE^−/− mice at 6 and 12 weeks after HFD feeding (n=6, *P<0.05,**P<0.01 vs WT 6 weeks; ^†P<0.05, ^‡P<0.01 vs ApoE^−/− 6 weeks; •P<0.05, ••P<0.01 vs WT 12 weeks; ^#P<0.05, ^##P<0.01 vs ApoE^−/− 12 weeks). E, The spleen weight 12 weeks after HCD feeding (n=6, *P<0.05, vs HFD fed ApoE^−/− mice). F, Cholesterol level in peripheral blood was measured at 6 and 12 weeks after HFD feeding (n=6, *P<0.05,**P<0.01 vs WT 6 weeks; ^†P<0.05, ^‡P<0.01 vs ApoE^−/− 6 weeks; •P<0.05, ••P<0.01 vs WT 12 weeks; #P<0.05, ##P<0.01 vs ApoE^−/− 12 weeks). HCD indicates high cholesterol diet; HFD, high fat diet; WBC, white blood cells; WT, wild type.

Figure 9.

Hypercholesterolemia increases the activity of p38 MAPK in HSCs. A, Phosphorylated p38 expression was measured by Flowcytometry in KTLS cells from WT mice (2.7±0.9%), NAC treated WT mice (2.1±.7%), ApoE^−/− mice (8.4±0.6% *) and NAC treated ApoE^−/− mice (2.7±0.9% †). Total p38 did not show difference in KTLS cells from WT mice (59.7±2.4%), NAC treated WT mice (60.2±4.5%), ApoE^−/− mice (57±2.6%) and NAC treated ApoE^−/− mice (58.3±1.8%). The numbers represent M±SD (n=6, *P<0.05 vs WT; †P<0.05 vs ApoE^−/−). B and C, Phosphorylated and total p38 expression was measured by Western blot (B) in KTLS cells from WT mice, NAC treated WT, ApoE^−/−, and NAC treated ApoE^−/− mice. The results was quantified (C) (n=3, *P<0.05, vs WT; †P<0.05, vs ApoE^−/− mice). D, Phosphorylated p38 expression was measured by Flowcytometry in phenotypically defined LT (24.2±.8% *†) and ST‐HSC (3.9±.4% *) as well as Lin⁻ (0.7±0.2%) and Lin⁺ marrow cells (0.8±0.3%) from ApoE^−/− mice (n=6, *P<0.05 vs Lin⁻, †P<0.05 vs Lin⁺). E, Phosphorylated p38 expression was measured by Flowcytometry in KTLS cells from normal chow mice (2.5±0.3%) and HCD mice (9.8±1.9% *). Total p38 did not show difference between normal chow mice (55.2±4.5%) and HCD mice (53.6±3.9%). The numbers represent M±SD. (n=6, *P<0.05 vs WT). F, The frequency of phenotypically defined LT‐HSC in total KTLS population was measured by Flowcytometry in WT (19±0.8%) and SB203580 treated WT mice (22±2%). The frequency of KTLS cells in total bone marrow was measured by Flowcytometry in WT mice (0.053±0.01%) and SB203580 treated WT mice (0.051±0.006%). The numbers represent M±SD (n=6). G, The frequency of phenotypically defined LT‐HSC in total KTLS population was measured by Flowcytometry in ApoE^−/− (5.39±1.8%) and SB203580 treated ApoE^−/− mice (15.6±3.3% *). The frequency of KTLS cells in total bone marrow was measured by Flowcytometry in ApoE^−/− (0.102±0.01%) and SB203580 treated ApoE^−/− mice (0.067±0.008% *). (n=6, *P<0.05 vs ApoE^−/−). H, Competitive HSC transplantation assay of KTLS cells from WT (49.6±2.4% *), SB203580 treated WT (50.1±5.6% *), PD98059 treated WT(45.1±4.9% *), ApoE^−/− (19.3±2.4%), SB203580 treated ApoE^−/− (38.9±103% *), PD98059 treated ApoE^−/− mice (18.2±7.7%). The percent of CD45.2⁺ cells in total KTLS were represented as M±SD (n=8, *P<0.05, vs ApoE^−/−). BM indicates bone marrow; HCD, high cholesterol diet; HSCs, hematopoietic stem cells; LT, long‐term; NAC, N‐acetylcysteine; ST, short term; WT, wild type.

Figure 10.

The effect of hypercholesterolemia on HSC gene expression. A, Representative plot of log ratio vs log processed signal in ApoE^−/− vs WT HSC for all the 44 000 probes present on the microarray (n=3). B, The top 4 canonical pathways determined by Ingenuity Pathways analysis based on differentially expressed genes in HSC from ApoE^−/− vs WT mice. C, Ingenuity Pathway analysis showed increases in the expression of several important components in Notch signaling, including E2F1, MCAM, CDKN1A, and trypsin. CDKN1A indicates cyclin‐dependent kinase inhibitor 1; HSC, hematopoietic stem cell; MCAM, melanoma cell adhesion molecule; WT, wild type.

Figure 11.

Hypercholesterolemia causes a paradoxical increase in expression of cell cycle inhibitors via oxidant‐dependent increase in p38 activity. A, p19^Arf, p21^Waf1, and p27^Kip1 expression in HSCs from WT mice, NAC treated WT mice, ApoE^−/− mice, NAC treated ApoE^−/− mice, MEK1 inhibitor PD98059 treated ApoE^−/− mice, and p38 inhibitor SB203580 treated ApoE^−/− mice (n=6, *P<0.05 vs WT, ^†P<0.05 vs ApoE^−/−). B, p19^Arf, p21^Waf1, and p27^Kip1 expression in phenotypically defined LT‐ and ST‐HSC as well as Lin⁻ and Lin⁺ bone marrow cells from ApoE^−/− mice (n=6, **P<0.01 vs Lin⁻, ^‡P<0.01 vs Lin⁺, *P<0.05 vs Lin⁻, ^†P<0.05 vs Lin⁻). HSCs indicates hematopoietic stem cells; LT, long‐term; MEK, mitogen activated protein kinase kinase; NAC, N‐acetylcysteine; ST, short term; WT, wild type.

Figure 12.

Inhibition of p19^Arf, p21^Waf1, and p27^Kip1 increases phenotypically defined long term population in KTLS culture in vitro. A, Expression of p19^Arf in KTLS cells after Lenti‐shp19 transfection (n=6, *P<0.05 vs WT, ^‡P<0.01 vs ApoE^−/−). B, Expression of p21^Waf1 in KTLS cells after Lenti‐shp21 transfection (n=6, *P<0.05 vs WT, †P<0.05 vs ApoE^−/−). C, Expression of p27^Kip1 in KTLS cells after Lenti‐shp27 transfection (n=6, *P<0.05 vs WT, ^†P<0.05 vs ApoE^−/−). D, CD34⁻Flk₂⁻ cells in fresh KTLS or in total KTLS culture 2 weeks after Lentiviral transfection (M±SD, n=6, *P<0.05 vs WT GFP, ^†P<0.05 vs ApoE^−/− GFP). E, Ki67⁺ cells in fresh CD34⁻Flk₂⁻KTLS or in cultured CD34⁻Flk₂⁻KTLS 2 weeks after Lentiviral transfection (M±SD, n=6, *P<0.05 vs WT GFP, ^†P<0.05 vs ApoE^−/− GFP). F, Ki67⁺ cells in fresh CD34⁺Flk₂⁻KTLS or in cultured CD34⁺Flk₂⁻KTLS 2 weeks after Lentiviral transfection (M±SD, n=6, *P<0.05 vs WT GFP, ^†P<0.05 vs ApoE^−/− GFP). G, Annexin V⁺ cells in fresh CD34⁻Flk₂⁻KTLS or in cultured CD34⁻Flk₂⁻KTLS 2 weeks after Lentiviral transfection (M±SD, n=6, *P<0.05 vs WT GFP, ^†P<0.05 vs ApoE^−/− GFP). H, Annexin V⁺ cells in fresh CD34⁺Flk₂⁻KTLS or in cultured CD34⁺Flk₂⁻KTLS 2 weeks after Lentiviral transfection (M±SD, n=6, *P<0.05 vs WT GFP, ^†P<0.05 vs ApoE^−/− GFP). WT indicates wild type.

Figure 13.

Hypercholesterolemia‐induced oxidant stress increases Notch1 activity in a p38‐dependent manner that contributes to HSC ageing. A, Notch1 expression in KTLS cells from WT mice (37.7±5.1%), NAC‐treated mice (31.5±2.7%), ApoE^−/− (70.8±.6% *) and NAC‐treated ApoE^−/− mice (46.5±11.3% †). The numbers represent M±SD. (n=8, *P<0.05 vs WT, ^†P<0.05 vs ApoE^−/−). B, Notch1 expression in phenotypically defined LT‐HSC (7.3±1.8%) and ST‐HSC (39.4±4.5%) from WT mice as well as LT‐HSC (10.2±2.7%) and ST‐HSC (82.7±9.5% *) from ApoE^−/− mice (M±SD, n=8, *P<0.05 vs WT ST‐HSC). C, Notch1 expression in KTLS cells from WT (36.3±2.8%), SB203580 treated WT (33.4±3.7%), ApoE^−/− mice (72.9±6.3% *), and SB203580 treated ApoE^−/− mice (51.1±11% †) (n=8, *P<0.05 vs WT, ^†P<0.05 vs ApoE^−/−). D, Notch1, Sirt1, and Notch1 intracellular domain (NICD) expression in WT KTLS cells after in vitro SB203580 treatment (n=3). E, Notch1 intracellular domain and Sirt1 expression in WT KTLS cells after in vitro treatment with p38 inhibitor SB203580 and/or Sirt1 inhibitor MG132 (n=3). F, Frequency of KTLS in bone marrow of ApoE^−/− mice (0.11±0.01%) and DAPT treated ApoE^−/− mice (0.08±0.01†). Phenotypically defined LT‐HSC in KTLS population of ApoE^−/− mice (6.5±1.4%) and DAPT treated ApoE^−/− mice (10.8±1.9% †) (n=6, ^†P<0.05). G, The inhibition of KTLS expansion in vitro by DAPT. KTLS cells from WT and ApoE^−/− mice were treated with DAPT in vitro and HSC expansion was quantified (n=6, ^†P<0.05 vs WT, ^‡P<0.05 vs ApoE^−/−). DAPT indicates N‐[N‐(3,5‐Difluorophenacetyl)‐L‐alanyl]‐S‐phenylglycinet‐butyl ester; DMSO, dimethyl sulfoxide; HSC, hematopoietic stem cell; LT‐HSC, long‐term HSC; NAC, N‐acetylcysteine; ST‐HSC, short term HSC; WT, wild type.