Inhibition of acetylation of histones 3 and 4 attenuates aortic valve calcification.

Aortic valve calcification develops in patients with chronic kidney disease who have calcium and phosphate metabolic disorders and poor prognoses. There is no effective treatment except valve replacement. However, metabolic disorders put patients at high risk for surgery. Increased acetylation of histones 3 and 4 is present in interstitial cells from human calcific aortic valves, but whether it is involved in aortic valve calcification has not been studied. In this study, we found that treating cultured porcine aortic valve interstitial cells with a high-calcium/high-phosphate medium induced calcium deposition, apoptosis, and expression of osteogenic marker genes, producing a phenotype resembling valve calcification in vivo. These phenotypic changes were attenuated by the histone acetyltransferase inhibitor C646. C646 treatment increased the levels of class I histone deacetylase members and decreased the acetylation of histones 3 and 4 induced by the high-calcium/high-phosphate treatment. Conversely, the histone deacetylase inhibitor suberoylanilide hydroxamic acid promoted valve interstitial cell calcification. In a mouse model of aortic valve calcification induced by adenine and vitamin D treatment, the levels of acetylated histones 3 and 4 were increased in the calcified aortic valves. Treatment of the models with C646 attenuated aortic valve calcification by restoring the levels of acetylated histones 3 and 4. These observations suggest that increased acetylation of histones 3 and 4 is part of the pathogenesis of aortic valve calcification associated with calcium and phosphate metabolic disorders. Targeting acetylated histones 3 and 4 may be a potential therapy for inoperable aortic valve calcification in chronic kidney disease patients.

Introduction

Cardiovascular complications contribute to the prognosis of chronic kidney disease (CKD). Cardiac valve calcification, one of most common disorders in patients with end-stage renal disease (ESRD) who require dialysis therapy, is correlated with an increased risk of cardiovascular events[1-5]. Calcification occurs at a younger age and progresses more rapidly in patients with CKD than in the general population[6]. Eventually, the leaflets of the aortic valve (AV) restrict the opening of the outflow tract of the left ventricle, which leads to congestive heart failure or sudden cardiac death[7]. The association between mineral metabolic disorders and cardiac valve calcification has been extensively evaluated in patients with ESRD[8-10].

Normal heart valve function requires the organization of differentiated cell types and specialized extracellular matrices within the valve leaflet that are arranged according to blood flow[11]. This defined tissue architecture provides the mechanical resilience and compressibility required to open and close the valve orifices effectively during the cardiac cycle[12]. Although the mechanisms that promote the onset and progression of heart valve calcification have yet to be fully elucidated, recent reports suggest a complex process involving molecular and cellular phenotypes, bone formation, and chronic inflammation. Impaired calcium and phosphate homeostasis, increased oxidative stress, and loss of calcification inhibitors have been linked to valve calcification in patients with CKD. Bone-associated genes—including Runt-related transcription factor 2 (Runx2), a transcription factor required for the osteoblast gene program, and the osteogenic gene markers osteocalcin (OCN), osteopontin (OPN), and alkaline phosphatase (ALP)—are overexpressed in calcified lesions of human heart valves[12,13].

The acetylation of histones and transcription factors promotes transcription by decondensing chromatin and facilitating protein–DNA interactions[14]. Nuclear proteins can be acetylated by one of several histone acetyltransferases (HATs), including E1A-associated nuclear protein p300 (p300), CREB-binding protein (CBP), and p300/CBP-associated factor (P/CAF)[15]. The reverse reaction inhibits transcription and is performed by class I histone deacetylase complexes (HDACs), a large protein family with at least 18 members and 3 distinct structural classes[16]. HATs and HDACs play critical roles in cell proliferation, survival, growth, and differentiation and participate in AV calcification[7,17]. P300, a well-studied HAT, is a transcriptional coactivator containing a domain with intrinsic histone acetyltransferase activity and has been implicated in chromatin structure alterations associated with gene expression[18,19]. P300 regulates gene expression through scaffolding, bridging, or intrinsic HAT activity[7,15]. In addition, P300 is critical for regulating osteoblast-specific genes as a coregulator of the key osteogenic factor Runx2[20,21]. The purposes of this study were to investigate the effects of acetylated histones 3 (AcH3) and 4 (AcH4) on aortic valve calcification and to evaluate the clinical potential of AcH3 and AcH4 inhibition for CKD-associated AV calcification.

Materials and methods

Human aortic valves

Normal human AV leaflets were collected from three aortic insufficiency patients undergoing AV replacement, and calcific aortic valve (CAV) leaflets were obtained from three patients with calcific aortic valve disease undergoing AV replacement. All patients gave informed consent for the use of their valves for this study. This study was approved by the Institutional Review Board of the University of Nanjing Medical University.

Cell isolation and culture

Porcine aortic valve interstitial cells (pAVICs) were isolated by collagenase digestion, as previously described[22] and cultured in Dulbecco’s modified Eagle’s medium (Gibco; Thermo Fisher Scientific, Inc., Waltham, MA, USA) supplemented with 10% fetal bovine serum (ScienCell Research Laboratories, Inc., San Diego, CA, USA) and 1% penicillin-streptomycin. Normal cells from passages 3 to 5 were used for later experiments. To induce calcification, pAVICs were cultured in calcification medium containing 1.5 mM calcium and 2 mM phosphate for 3 days. To identify the role of histone acetylation in pAVIC calcification, pAVICs were treated with a HAT p300 inhibitor (C646, 10 μmol/l; cat. no. S7152; Selleckchem; Houston, TX, USA) and suberoylanilide hydroxamic acid (SAHA, 5 μmol/l; Sigma-Aldrich; Merck KGaA, Darmstadt, Germany) with or without calcification medium for 3 days and then harvested for further measurements.

Animal experiments

All experimental procedures were approved by the Institutional Committee for the Care and Use of Laboratory Animals and were performed according to animal protection guidelines. A total of 30 male C57BL/6 mice (aged 6 weeks) were obtained from the model animal research center of Nanjing University. CKD was induced with adenine-containing chow (0.2% adenine in chow). The mice were randomly divided into three groups and fed either adenine-containing or normal chow. After 3 weeks of treatment, renal function was assessed with biochemical analyses of serum creatinine and BUN. To induce valvular calcification, the animals were then intraperitoneally injected with vitamin D (VitD) (8.75 mg/kg/day) or olive oil for 10 days. For C646 treatment, C646 (1 mg/kg/day) or vehicle was subcutaneously injected for 14 days starting from the day of VitD injection. The mice were housed in plastic cages with a 12/12 h light/dark cycle and had free access to water and a basal diet. Under gas anesthesia with 2% isoflurane, blood was collected for serum calcium assays. Following phosphate-buffered saline (PBS) perfusion, the mice were perfused with 4% buffered paraformaldehyde. The hearts were fixed in 4% buffered paraformaldehyde overnight and embedded in optimal cutting temperature compound. Serial sections (8 μm) were cut from the entire murine AV area.

Flow cytometry

AVICs were seeded in 6-well culture plates at a density of 20,000 cells per well. Four days after being seeded, the cells were treated with pro-mineralizing medium and the HAT inhibitor C646 for 3 days and then centrifuged at 300 × g for 5 min at 4 °C. After being washed twice with PBS, the cell pellet was suspended in 100 μl of binding buffer. The cells were incubated with 5 μl of Annexin V/Alexa Fluor 647 and 10 μl of 20 µg/ml propidium iodide (PI) for 15 min at room temperature away from light before the addition of 400 μl of PBS. The cell suspension was analyzed using a FACScan flow cytometer (BD Biosciences; USA). Early apoptotic cells (Annexin V positive and PI negative) were located in the lower-right quadrant. Late apoptotic or necrotic cells (Annexin V positive and PI positive) were located in the upper-right quadrant. Healthy cells (negative for both probes) were located in the lower-left quadrant. The results are expressed as the percentage of positively stained cells among the total cells.

Real-time polymerase chain reaction

RNA was extracted using TRIzol reagent (cat. no. 15596-018; Invitrogen; Thermo Fisher Scientific, Inc.) according to the manufacturer’s recommendations. Reverse transcriptase reactions were performed using PrimeScriptTM RT Master Mix (cat. no. RR036A; Takara Bio, Inc., Otsu, Japan). Real-time PCR was performed on an ABI Prism 7900 system. The TaqMan primers and probes used for real-time PCR to detect Runx2 (sense primer, 5′-CTTTTGGGATCCGAGCAC-3′; antisense primer, 5′-GGCTCACGTCGCTCATCT-3′; probe, Roche UPL #66), OPN (sense primer, 5′-AATCTAAGAAGTTCCGCAGATCC-3′; antisense primer, 5′-CCACATGTGACGTGAGGTCT-3′; probe, Roche UPL #3), and OCN (sense primer, 5′-CCTATCGGCGCTTCTATGG-3′; antisense primer, 5′-GCAGGGAAGTCAGGGTAGG-3′; probe, Roche UPL #82) were purchased from Roche. Each sample was analyzed in triplicate, and the expression of the target genes was normalized to that of the reference housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPDH; sense primer, 5′-ACAGACAGCCGTGTGTTCC-3′; antisense primer, 5′-ACCTTCACCATCGTGTCTCA-3′; probe, Roche UPL #28). The fold differences were then calculated for each treatment group using CT values normalized to those of the control group.

Immunofluorescence

Immunofluorescence staining was performed on frozen tissue sections. Prior to immunostaining, the tissue sections were fixed with 4% paraformaldehyde for 15 min and permeabilized for 15 min using 0.5% Triton X-100. The tissue sections were incubated for 1 h at room temperature with blocking solution (5% bovine serum albumin, 10% horse serum, and 0.05% Triton X-100) to block nonspecific binding and then incubated at 4 °C overnight with primary antibodies before being incubated for 1 h with the appropriate secondary antibody conjugated to Alexa Fluor 488 or CyTM3 (Jackson ImmunoResearch; West Grove, PA, USA) in the dark. DAPI was used to counterstain the nuclei. The samples were covered with mounting medium (cat. no. P36935; Invitrogen; Thermo Fisher Scientific), overlaid with cover slips, and examined under a confocal laser scanning microscope (Carl Zeiss MicroImaging). The primary antibodies used were as follows: Runx2 (1:100; cat. no. 20700-1-AP; Proteintech; Wuhan, China), histone deacetylase 1 (HDAC1; 1:100; cat. no. 5356; Cell Signaling Technology; Danvers, MA, USA), acetylated histone 3 (AcH3; 1:100; cat. no. 06–599; Millipore; USA), acetylated histone 4 (AcH4; 1:100; cat. no. 06–598; Millipore), and vimentin (1:100; cat. no. 92547; Abcam; Cambridge, MA, USA).

Alizarin red staining

Cells were washed with Ca2+-free PBS three times, fixed with 4% paraformaldehyde for 10 min, and then fixed with 95% ethanol for 20 min. Next, the cells were stained in 1% alizarin red solution (pH 4.2; cat. no. BM1853; HeFei BoMei Biotechnology; Hefei, China) for 1 min to visualize matrix calcium deposition. The remaining dye was washed out with several washes of distilled water, and the stained cells were photographed.

Calcium quantitation

Calcium content was determined colorimetrically by the o-cresolphthalein complexone method using a QuantiChromTM calcium assay kit (cat. no. DICA-500; BioAssay Systems) in 0.6 M HCl extracts from cultured cells overnight at 4 °C. Briefly, 5 μl of sample was transferred to a 96-well plate. Working reagent (200 μl) was added, and the absorbance was then measured at 612 nm using a SynergyTM 2 microplate reader from BioTek (Winooski). After decalcification, the cells were washed three times with PBS and lysed with lysis buffer. The protein content was measured with a PierceTM BCA protein assay kit (cat. no. 23225; Thermo Fisher Scientific). The calcium content of AVICs was then normalized to the protein content.

Alkaline phosphatase activity assay

ALP activity was determined using a LabAssayTM ALP kit (cat. no. 291–58601; Wako Chemicals Gmbh) according to the manufacturer’s protocol. This measurement was used to determine the quantity of p-nitrophenyl phosphate released from the substrate. The cell layers were lysed with 250 μl of ice-cold 0.05% Triton X-100 in PBS. The samples were then frozen and thawed three times, and the cell lysates were collected. Samples (20 μl) were combined with 100 μl of ALP reagent, and the activity was measured after an incubation of 15 min at 37 °C. The absorbance was measured immediately at 405 nm, and the amount of p-nitrophenyl phosphate was determined by comparison with a standard curve. The ALP activity (U, µmol p-nitrophenyl phosphate released per min) was normalized to the protein content.

Von Kossa staining

For von Kossa staining, frozen tissue sections were fixed with 4% paraformaldehyde for 15 min and incubated with silver nitrate solution (cat. no. BM1853; HeFei BoMei Biotechnology; Hefei, China) and then exposed to ultraviolet light for more than 10 min according to the manufacturer’s protocol. Next, the tissue sections were washed, treated with sodium thiosulfate for 2 min, washed again, and stained with eosin.

Western blot analysis

Proteins were extracted with a whole-cell lysis assay kit (cat. no. KGP250/KGP2100; Keygen Biotech) or a NucBusterTM protein extraction kit (cat. no. 71183-3; Millipore) according to the manufacturer’s instructions. Then, protein samples (30 µg) were separated on 10~15% SDS–polyacrylamide gels by electrophoresis and transferred onto polyvinylidene difluoride membranes (Millipore). After blocking with Tris-buffered saline containing 0.1% Tween 20 and 5% bovine serum albumin for 2 h at room temperature, the proteins of interest were detected using primary antibodies overnight at 4 °C. The membranes were then washed and incubated with corresponding horseradish peroxidase-labeled secondary antibodies (1:5,000 dilution) for 2 h at room temperature. The following primary antibodies purchased from Cell Signaling Technology were used: anti-Runx2 (1:1,000; cat. no. 12556), anti-GAPDH (1:1,000; cat. no. 5174), anti-caspase 3 (1:1,000; cat. no. 9662), anti-Bcl-2 (1:1,000; cat. no. 2876), anti-HDAC1 (1:1,000; cat. no. 5356), anti-HDAC2 (1:1,000; cat. no. 5113), anti-HDAC3 (1:1,000; cat. no. 3949), anti-HDAC4 (1:1,000; cat. no. 7628), anti-histone 3 (1:1,000; cat. no. 4499), anti-acetyl histone 3 (Lys9) (1:1,000; cat. no. 9649), anti-acetyl histone 3 (Lys14) (1:1,000; cat. no. 7627), anti-acetyl histone 3 (Lys18) (1:1,000; cat. no. 13998), anti-acetyl histone 3 (Lys27) (1:1,000; cat. no. 8173), anti-acetyl histone 3 (Lys56) (1:1,000; cat. no. 4243), anti-acetyl histone 4 (Lys5) (1:1,000; cat. no. 8647), and anti-acetyl histone 4 (Lys8) (1:1,000; cat. no. 2594). Antibodies targeting acetyl histone 3 (1:1,000; cat. no. 06–599) and acetyl histone 4 (1:1,000; cat. no. 06–598) were purchased from Millipore, and an antibody targeting OPN (1:1,000; cat. no. BS1264) was purchased from Bioworld Technology (Nanjing, China). The blots were developed with enhanced chemiluminescence reagent (cat. no. 34096; Thermo Fisher Scientific) and exposed on a ChemiDoc MP imager (Bio-Rad). Image Lab™ software was used to quantify band density.

Statistical analysis

All measurement data are expressed as the means ± standard errors of the means. Treatment group values were compared with corresponding control values using GraphPad Prism 5.0 (GraphPad Software). Significant differences among three or more groups were determined by one-way analysis of variance (ANOVA) followed by Bonferroni’s multiple comparison test. P values less than 0.05 were considered to indicate statistical significance.

Results

The levels of histones 3 and 4 acetylation are elevated in human calcific aortic valves

We first examined the levels of histones 3 and 4 acetylation in human AV leaflets. The immunostaining results showed that acetylated histone 3 and histone 4 were present in both CAVs and noncalcific control AVs (NCAVs) from patients receiving AV replacement. The levels of AcH3 or AcH4 were significantly higher in CAVs than in NCAVs (Fig. 1a, b).

Fig. 1

Levels of histone acetylation in human calcific and noncalcific aortic valves.

a Representative images of immunofluorescence staining of human aortic valves for AcH3. The expression of AcH3 was slightly higher in CAVs than in NCAVs. b Representative images of immunofluorescence staining of human aortic valves for AcH4. The expression of AcH4 was slightly higher in CAVs than in NCAVs. Scale bar = 50 μm. n = 3 for each group. ##p < 0.01 compared with NCAVs

P300 inhibition attenuates high calcium/high phosphate–induced pAVIC calcification

To address whether inhibition of histone acetylation affects AV calcification, an inhibitor of histone acetyltransferase p300 named C646 was used to treat pAVICs cultured with calcification medium. The calcium content of pAVICs was significantly increased following a 3-day treatment with calcification medium. However, calcium deposition was inhibited by C646 at doses of 5 and 10 μM (Fig. 2a). In addition, 10 μM C646 treatment significantly decreased ALP activity (Fig. 2b). We chose 10 μM for subsequent experiments because 10 μM C646 exhibited a stronger effect on pAVIC calcification than 5 µM C646.

Fig. 2

The effect of C646 on pAVIC calcification induced by high-calcium/high-phosphate treatment.

a Calcium deposition was measured with a QuantiChromTM calcium assay kit. b ALP activity was measured to assess the degree of osteoblastic differentiation and calcification. c Calcium deposition was assessed by alizarin red staining. d–f The expression of Runx2, OPN, and OCN was assessed by qPCR. (G-H) The expression of Runx2 and OPN was assessed by western blotting. The relative protein levels were normalized to GAPDH levels. The data are shown as the means ± standard errors of the means of triplicates and are representative of three independent experiments performed. #p < 0.05, ##p < 0.01 compared with control. **p < 0.01, ***p < 0.001 compared with high Ca+P

As shown in Fig. 2c, alizarin red S staining indicated that 10 μM C646 decreased calcium deposition in pAVICs treated with calcification medium. To assess the effect of C646 on osteogenic transdifferentiation, the expression of some osteogenic markers was examined. The results showed that C646 decreased the mRNA levels of osteogenic markers, including Runx2, OCN, and OPN (Fig. 2d–f), and the protein levels of Runx2 and OPN after calcification medium treatment (Fig. 2g, h).

The effect of p300 inhibition on apoptosis of pAVICs after calcification

In view of the proapoptotic effect of C646 in tumor cells[23,24], we examined the effect of C646 on pAVICs after calcification medium treatment. In contrast to tumor cells, pAVICs treated with 10 μM C646 exhibited an inhibition of apoptosis induced by high-calcium/high-phosphate conditions (Fig. 3a, b). In addition, the effects of high calcium/high phosphate on the expression of cleaved Caspase 3 and Bcl-2 were significantly attenuated in the presence of C646 (Fig. 3c, d).

Fig. 3

The effect of C646 on apoptosis and proliferation of pAVICs after calcification.

a, b Flow cytometry analysis of apoptosis revealed that C646 inhibited apoptosis induced by high-calcium/high-phosphate treatment. The right bar graph shows the results of statistical analysis of the apoptosis ratios. c, d The expression of cleaved caspase 3 and Bcl-2 was assessed by western blotting. The relative protein levels were normalized to GAPDH levels. The data are shown as the means ± standard errors of the means of triplicates and are representative of three independent experiments performed (#p < 0.05, ###p < 0.001 compared with control; *p < 0.05, **p < 0.01 compared with high Ca+P)

P300 inhibition changes the levels of histone acetylation and histone deacetylase in pAVICs treated with high calcium/high phosphate

The levels of total AcH3, H3 acetylated on lysine 9 (AcH3 K9), H3 acetylated on lysine 14 (AcH3 K14), H3 acetylated on lysine 27 (AcH3 K27), and H3 acetylated on lysine 56 (AcH3 K56) were all significantly increased after calcification medium treatment. Similar to AcH3 levels, AcH4 levels were also significantly higher in the calcification medium group than in the control group. The expression levels of H4 acetylated on lysine 5 (AcH4 K5) and H4 acetylated on lysine 8 (AcH4 K8) were consistent with those of total AcH4. C646 treatment significantly decreased the levels of AcH3, AcH3 K14, AcH3 K27 and AcH3 K56 (Fig. 4a, b). The elevations in AcH4, AcH4 K5 and AcH4 K8 levels were also attenuated by C646 treatment (Fig. 4c, d).

Fig. 4

HAT activity in pAVICs treated with high calcium/high phosphate.

a, b The expression of AcH3, AcH3 K9, AcH3 K14, AcH3 K27, and AcH3 K56 was assessed by western blotting. The relative protein levels were normalized to H3. c, d The expression of AcH4, AcH4 K5, and AcH4 K8 was assessed by western blotting. The relative protein levels were normalized to H3 levels. e, f The expression of HDAC1, HDAC2, HDAC3, and HDAC4 was assessed by western blotting. The relative protein levels were normalized to H3 levels. The data are shown as the means ± standard errors of the means of triplicates and are representative of three independent experiments performed. (#p < 0.05, ##p < 0.01, ###p < 0.001 compared with control; *p < 0.1, **p < 0.01, ***p < 0.001 compared with high Ca+P)

To examine the effect of C646 on HDAC levels with or without high-calcium/high-phosphate treatment, we analyzed the protein levels of HDACs and assessed the associated epigenetic changes in H3 and H4. We found that high-calcium/high-phosphate treatment led to significant decreases in the levels of HDACs, including HDAC1, HDAC2, and HDAC3, but did not influence the class II histone deacetylase HDAC4. C646 treatment increased the levels of class I HDACs independently of high-calcium/high-phosphate treatment (Fig. 4e, f).

HDAC inhibition promotes pAVIC calcification induced by high calcium/high phosphate

To determine whether inhibition of HDACs affects AV calcification, we used the specific chemical inhibitor SAHA to inhibit histone deacetylation activity. Calcium deposition was enhanced by SAHA at a dose of 5 μM (Fig. 5a). In addition, SAHA treatment significantly increased ALP activity (Fig. 5b). As shown in Fig. 5c, alizarin red S staining indicated that SAHA enhanced calcium deposition in pAVICs treated with calcification medium. Figure 5d and E illustrate that SAHA increased the protein levels of Runx2 and OPN after calcification medium treatment.

Fig. 5

The effect of HDAC inhibition on pAVIC calcification induced by high-calcium/high-phosphate treatment.

a Calcium deposition was measured with a QuantiChromTM calcium assay kit. b ALP activity was measured to assess the degree of osteoblastic differentiation and calcification. c Calcium deposition was assessed by alizarin red staining. d, e The expression of Runx2 and OPN was assessed by western blotting. The relative protein levels were normalized to GAPDH levels. The data are shown as the means ± standard errors of the means of triplicates and are representative of three independent experiments performed (#p < 0.05, ##p < 0.01, ###p < 0.001 compared with control; *p < 0.05, **p < 0.01, ***p < 0.001 compared with high Ca+P)

P300 inhibition attenuates AV calcification induced by adenine and vitamin D treatment via H3 and H4 acetylation

To gain insights into the therapeutic potential of C646 inhibition in CKD-associated AV calcification, we established a mouse CAV model by inducing calcification with adenine-containing chow and VitD injections. Blood biochemical analysis demonstrated that adenine treatment led to marked increases in blood urine nitrogen (BUN) and creatinine. Furthermore, adenine plus VitD treatment significantly increased serum calcium concentrations; however, C646 treatment showed no effects on BUN, creatinine, or serum calcium concentrations (Fig. 6a–c). As expected, von Kossa staining indicated marked calcification in AVs from adenine- and VitD-treated mice compared to control mice. When the treated mice received C646 supplementation, the calcium deposition in the AVs induced by adenine and VitD treatment was almost completely eliminated (Fig. 6d, e).

Fig. 6

The effect of C646 on calcification induced by adenine-containing chow and VitD injection.

a, b The effect of adenine treatment on BUN and serum creatinine concentrations. c Adenine and vitamin D treatment on serum calcium levels. d Histological von Kossa staining. e Quantification of calcification in the aortic valves. (Scale bar = 100 μm; n = 3 for each group; #p < 0.05, ###p < 0.001 compared with control)

We further examined the effect of C646 on osteogenic marker gene expression in our CAV models. C646 treatment reduced the adenine plus VitD-induced increase in Runx2 expression (Fig. 7a). In addition, C646 counteracted the decline in HDAC1 expression in response to adenine and VitD treatment (Fig. 7b). Similar to the in vitro results, the levels of AcH3 and AcH4 were significantly upregulated in the adenine plus VitD treatment group compared with the control group, while C646 supplementation inhibited the increases in AcH3 and AcH4 expression (Fig. 8).

Fig. 7

The effects of HAT inhibition on osteogenesis and HDAC1.

a Representative images of immunofluorescence staining of mouse aortic valves for Runx2. b Representative images of immunofluorescence staining of mouse aortic valves for HDAC1 (scale bar = 50 μm; n = 3 for each group; #p < 0.05, ##p < 0.01 compared with control; *p < 0.05, **p < 0.01 compared with adenine+vitamin D)

Fig. 8

The effect of HAT inhibition on histone acetylation.

a Representative images of immunofluorescence staining of mouse aortic valves for AcH3. b Representative images of immunofluorescence staining of mouse aortic valves for AcH4 (scale bar = 50 μm; n = 3 for each group; ##p < 0.01 compared with control; *p < 0.05 compared with adenine+vitamin D)

Discussion

The following are the four major findings of this study: (1) the levels of AcH3 and AcH4 are increased in human CAVs; (2) inhibition of AcH3 and AcH4 with the inhibitor of histone acetyltransferase p300 attenuates pAVIC apoptosis and calcification induced by high-calcium/high-phosphate treatment in vitro; (3) inhibition of AcH3 and AcH4 reverses the decreased expression of class I HDACs induced by high-calcium/high-phosphate treatment; and (4) inhibition of AcH3 and AcH4 attenuates AV calcification induced by adenine and VitD treatment in vivo.

Our data demonstrated that AcH3 and AcH4 are increased in human calcific AVs, which may be related to the development of aortic valve calcification. Previous studies have reported that increases in AcH3 and AcH4 due to inhibition of HDACs promote vascular calcification[25] and bone formation[26], indicating that increased histone acetylation contributes to osteogenic differentiation and ectopic calcification. Our study suggests that increased AcH3 and AcH4 levels play critical roles in high calcium/high phosphate–induced AV calcification and that reduction of histone 3/4 acetylation via HAT inhibition may be a novel strategy for controlling calcium metabolic disorders associated with AV calcification in patients with chronic renal insufficiency.

To achieve satisfactory inhibition of AcH3/AcH4, we used C646, the most potent competitive inhibitor available for p300/CBP in live cells[27]. C646 succeeded in inducing G1 phase cell cycle arrest and apoptosis in various cancer cells[28,29]. These results suggest that p300 can participate in AML1-ETO fusion protein–mediated transcriptional activation of Bcl-2 and that downregulation of c-kit and Bcl-2 may be involved in C646-mediated growth inhibition, cell cycle arrest, and apoptosis in AML1-ETO-positive acute myeloid leukemia cells[30]. However, the role of C646 in diseases other than cancer has been studied less often. To explore the effect of C646 on apoptosis in the context of high calcium/high phosphate–induced calcification, we evaluated apoptosis using flow cytometry and assessed the expression levels of apoptosis markers, including cleaved caspase 3 and Bcl-2, in pAVICs. In contrast to the role of C646 in cancer cells, our results showed that C646 plays a protective role in high calcium/high phosphate–induced apoptosis and upregulates the level of Bcl-2 in pAVICs. In our previous study, we found higher baseline levels of the tumor suppressor p53 in AVICs than in other cell types[22]. We speculate that C646 may have opposing effect in different cell and chromatin status contexts or that some AVIC-specific expressed proteins may hinder the proapoptotic effect of C646. Further studies are needed to reveal the antiapoptotic mechanisms of C646 in AVICs.

C646 is a promising anticancer agent due to its ability to promote terminal differentiation and induce growth arrest in numerous tumor cells[31,32]. However, p300 modulates profibrotic responses and plays a critical role in the osteogenesis of osteoblastic cells[21,33,34]. The effects of C646 on many normal cell types, including AVICs, remain poorly understood. In AVIC osteogenesis induced by TNF-α and p300 inhibition, Li et al. found that HATs can attenuate the osteogenic effects of TNF-α through modulation of induced Klotho deficiency[7]. Our data showed that C646 reversed the decreases in HDAC1, HDAC2, and HDAC3 induced by high-calcium/high-phosphate medium in AVICs. It is widely accepted that HDAC inhibitors have beneficial effects as therapeutics for epilepsy, angiogenesis, cardiac hypertrophy, heart failure, fibrosis, and myocardial infarction[35-37]. In contrast, however, loss of HDAC1 activity via either chemical inhibitor treatment or genetic ablation enhances vascular calcification[25,38]. This finding is quite interesting because it suggests new possible adverse effects of the application of HDAC inhibitors in other diseases as well as possible novel pathways associated with HDACs in vascular calcification. It seems that there is major involvement of class I HDACs in the anticalcification of AVICs, so we need to further demonstrate that class I HDACs are directly involved in the regulation of calcification by overexpressing HDACs, especially HDAC1 and HDAC2, in vitro and in vivo. In addition, further work is necessary to elucidate the direct effects of HDACs by inhibiting HDACs with MS-275 (an inhibitor of HDAC1, HDAC2, and HDAC3). Notably, histone deacetylase inhibition promotes osteoblast maturation by altering the histone H4 epigenome, and enhanced H4 acetylation is characterized by altered expression of osteoblast gene regulatory programs[39]. In the current study, we found that high-calcium/high-phosphate treatment significantly increased histones 3 and 4 acetylation and promoted AV calcification both in vivo and in vitro. Our results suggest that certain levels of HDACs are important in preventing AV calcification and that the clinical use of HAT inhibition may be better than that of HDAC inhibition based on the potential side effects of HDAC inhibitors on AV calcification.

Osteogenic differentiation of AVICs is essential for AV calcification[40]. Runx2 is a master transcription factor for osteoblast differentiation and bone formation[41]. Runx2 also acts as a potent regulator for the commitment of mesenchymal stem cells to the osteoblastic lineage and regulates the expression levels of osteogenic marker genes, including ALP, osteopontin, type I collagen, and OCN[41-43]. Increased p300 protein levels and HAT activity can increase the acetylation and stability of Runx2[43]. A previous report clearly demonstrated that p300-mediated acetylation of Runx2 is essential not only for Runx2 stabilization but also for its transcriptional activity[44]. In our study, suppression of p300 activity with C646 inhibited Runx2 protein expression. Our data indicate that the attenuation of calcification by C646 may be partially mediated by decreases in the posttranslational modification and stability of the Runx2 protein.

In addition to revealing the role of histone acetylation during AVIC and AV calcification induced by high-calcium/high-phosphate treatment, we developed a short-term CAV mouse model via 3-week administration of an adenine-supplemented diet with 10-day intraperitoneal VitD injection. Our data demonstrated that this short-term modeling method is a very efficient strategy for inducing severe AV calcification, which simulates clinical AV calcification in CKD patients. Our model will provide a useful tool for further research on AV calcification induced by calcium and phosphate disorders.

Our model mice exhibited significantly elevated serum calcium concentrations after VitD treatment compared to control mice. A high concentration of serum calcium has been identified as an independent risk factor for cardiovascular mortality in CKD patients[45]. Our previous study indicated that serum calcium is not positively correlated with heart valve calcification[46]. To minimize the risk of mortality, high serum calcium concentrations should be lowered and maintained within the target range in patients with CKD[45]. In this study, mice treated with VitD exhibited higher serum calcium concentrations than control mice, and administration of C646 did not reduce these elevations. However, despite the high serum calcium concentrations, C646 inhibited AV calcification according to the histological von Kossa staining results. These findings suggest that the inhibition of AcH3/AcH4 by C646 is not associated with serum calcium homeostasis and that C646-mediated inhibition of AV calcification may be due to a localized inhibitory effect on osteogenic differentiation in AVICs.

In summary, we have demonstrated that increased histone acetylation is involved in AV calcification. In addition, we found that inhibition of this acetylation by C646 significantly attenuates AV calcification induced by high calcium/high phosphate both in vitro and in vivo via prevention of decreased expression of class I HDACs. Our study indicates that targeting HAT inhibition may be a potential therapeutic strategy for clinical AV calcification in CKD patients.