Atherosclerotic specimens and histological staining
Human carotid plaques were collected from patients who received endarterectomy. Japanese white rabbits that were treated by a cholesterol-rich diet containing 0.3% cholesterol and 3% corn oil for 6 or 16 weeks to produce atherosclerosis (n = 3, respectively), and then rabbits were euthanised by using pentobarbital sodium at each time point. The aortic arch of each rabbit was cut into 10 cross sections (4 µm) (9). Mice were euthanised by cervical dislocation. Segments of heart tissue crossing the ascending aorta and aortic sinus from male ApoE−/− mice (n = 4) were embedded within OCT, and serial sections (8 µm thick) were made as previously described (10). Hematoxylin and eosin (H&E), oil red O and Masson’s trichrome stains were performed according to the protocols as previously described (11, 12). Moreover, sections were performed with immunohistochemically staining against DDR2 in mouse (1:200; Abcam, Cambridge, UK; CST, Beverly, MA, USA) human and rabbit (1:200; Santa Cruz Biotechnology, Inc, Dallas, TX, USA), RAM11 of macrophages (1:200; Dako, CA, USA) and α-actin of SMC (1:200; Thermo Fisher Scientific, CA, USA) as previously described (11).
Japanese white rabbits and apolipoprotein E (ApoE)-deficient mice were purchased from the laboratory animal center at Xi’an Jiaotong University (Xi’an, China). All animal experiments were performed in the animal facility of Institute of Basic and Translational Medicine at Xi'an Medical University and were implemented in accordance with the guidelines and regulations set by the Ethics Committee of Xi’an Medical University (Permit No. XYJZS‑201609027-1). Informed consents were obtained from all patients enrolled in the study. The animal experiments were strictly following the guidelines of animal experiment in Xi’an Medical University, which was adapted from the Guide for the Care and Use of Laboratory Animals (NIH; Bethesda, MD, USA; NIH Publication No. 85‑23, revised 2011). The Laboratory Animal Administration Committee of Xi'an Medical University approved all animal experiments (Institutional Animal Care and Use Committee; Permit No. XYJZS‑201608012-2).
Vsmc Culture
VSMCs were obtained from the aortas of male SD rats (200–300 g) as previously described (13). Cells were used in the experiments from passages 3–6. Before the initiation of each experiment, an additional incubation of serum-free DMEM for 24 h renders cells to quiescent. Then, cells were exposed to ox-LDL (0, 25, 50 and 100 mg/L; Yiyuan Biotechnologies, Guangzhou, China) for 24 h to simulate the pro-atherosclerotic condition. To activate DDR2, cells were incubated with collagen I (Sigma-Aldrich) for 48 h. To study the inhibition of signaling pathways, cells were treated with inhibitors for 30 min. SP600125 (20 µmol/l; Calbiochem) is an inhibitor for JNK (c-Jun N-terminal kinase) and SB203580 (10 µmol/l; Calbiochem) is an inhibitor for p38 MAPK (mitogen-activated protein kinase) and PD98059 (20 µmol/l; Calbiochem) is an inhibitor for MEK [MAPK/ERK (extracellular-signal-regulated kinase) kinase]. The doses of the inhibitors were referenced by the previously studies (14, 15). Then, cells exposed to ox-LDL (100 mg/L).
Sirna Interference
Small interfering RNA (siRNA) was used to knockdown DDR2 expression in VSMCs as previously described (16). Referencing a previous study, siRNA sequences were synthesized by a commercial company (sense strand, 5′- GAUGAUAGCAACACUCGGAUU-3′; antisense strand, 5′- UCCGAGUGUUGCUAUCAUCUU-3′; Ribo Bio, Guangzhou, China) (17). siN05815122147 (Ribo Bio, Guangzhou, China) was used as a universal negative control. Transfection was performed with X-tremeGENE siRNA Transfection Reagent (Roche) and accorded to the manufacturer’s instructions. Briefly, the cells were rinsed twice with phosphate-buffered saline (PBS) to reduce background interference. DDR2 siRNA with various doses (400 and 800 ng; approximately 40 and 80 pmol) were transfected into VSMCs for 6 h, and were then treated with ox-LDL (100 mg/L). The cells were collected to examine the DDR2 expression or to be performed by migration and proliferation assay.
Migration And Proliferation Assay
The migration of VSMCs was assessed by using the Transwell permeable support insert (Corning, Lowell, MA, USA) as previously described (18). After incubation of siRNA for 24 h without FBS, VSMCs were seeded on Matrigel (5 mg/mL; BD Biosciences, San Diego, CA, USA) of the upper compartment, and DMEM supplemented with 10% FBS was added into the lower compartment. Cells were cultured for another 24 h and then detected by crystal violet staining. Five different high-power fields per well were photographed. The positively stained VSMCs were counted by an observer blinded to the treatment protocol.
The proliferation of VSMCs was assessed by the wound-healing assay as previously described (19). Briefly, after 24 h of siRNA treatment, a 10-l pipette tip was used to scrap an artificial wound in the monolayer across the bottom of the dish. After extensively washed, medium containing 10% FBS was removed, and cells started to migrate for the appropriate time in a 37 °C incubation chamber with 5% CO2. At various time-points, images were obtained with a Nikon TE2000 Inverted Microscope. Meanwhile, some representative dishes were performed by immunofluorescence against α-actin of SMC (1:200; Thermo Fisher Scientific, CA, USA) with Alexa Fluor 488 (1:200; Thermo Fisher Scientific, CA, USA). The remaining open area of the wound was quantified by using ImageJ as previously described, with some modifications (20).
Rna Extraction And Real-time Pcr
Total RNA was extracted from the aorta and VSMCs. Real-time PCR was performed as previously described (21, 22). The sequences of the primers are listed in Table 1.
Table 1
Primers were used for Real-time PCR
Gene | Forward (5′-3′) | Reverse (5′–3′) |
DDR2 | GATCATGTTTGAATTTGACCGA | GCACTGGGGTTCACATC |
MMP-2 | TTGACCAGAACACCATCG | GGTCCAGGTCAGGTGTGT |
MMP-3 | GCTGTGTGCTCATCCTACC | TGACAACAGGGCTACTGTC |
MMP-8 | AGGAATGCCACTATGATTG | CAAGAAATCACCAGAGTCG |
MMP-9 | ACAGCGAGACACTAAAGGC | GGCAAGTCTTCGGTGTAGC |
MMP-12 | GCTGGTTCGGTTGTTAGG | GTAGTTACACCCTGAGCATAC |
MMP-13 | ACTCAAATGGTCCCAAAC | TATCAGCAGTGCCATCAT |
MMP-14 | GTACCCACACACAACGCT | TTATCTGGAACACCACAGC |
GAPDH | TACCCACGGCAAGTTCAACG | CACCAGCATCACCCCATTTG |
Protein Extraction And Western Blotting Analysis
Total protein was extracted from the aorta of rabbits and VSMCs as previously described (22). The primary antibodies were against rabbit’s DDR2 (1:500; Santa Cruz Biotechnology, Santa Cruz, CA), rat’s DDR2 (1:500; Santa Cruz Biotechnology, Santa Cruz, CA; CST, Beverly, MA, USA). MMP-2 (1:500; Abcam, Cambridge, MA), TIMP-1 (1:500; Abcam, Cambridge, MA), TIMP-2 (1:500; Abcam, Cambridge, MA), p-ERK1/2 (1:1000; CST, Beverly, MA, USA) and GAPDH (1:1000; Santa Cruz Biotechnology, Santa Cruz, CA). Western blotting analysis was applied as previously described, and relative protein expression was measured by ImageJ with gel analysis (22).
Zymography
The supernatant along with the total protein were extracted from cultured VSMCs. Under nonreducing conditions, the equal amounts of sample protein were analyzed by SDS-PAGE in gelatin-containing acrylamide gels (2 mg/mL gelatin and 7.5% polyacrylamide) as previously described (23).
Statistical Analysis
All data are expressed as the mean ± SE. Two groups of comparisons were used by Student's t‑test. Multiple groups of comparisons were performed by using one‑way ANOVA with the Bonferroni test. P < 0.05 was considered statistically significan. The statistical calculations were performed by using SPSS 19.0 software (IBM Corp., Armonk, NY, USA).