The rhythmic expression of clock genes attenuated in human plaque-derived vascular smooth muscle cells
- Changpo Lin†1,
- Xiao Tang†1,
- Zhu Zhu2,
- Xiaohong Liao2,
- Ran Zhao2,
- Weiguo Fu1,
- Bin Chen1,
- Junhao Jiang1,
- Ruizhe Qian2Email author and
- Daqiao Guo1Email author
© Lin et al.; licensee BioMed Central Ltd. 2014
Received: 8 November 2013
Accepted: 8 January 2014
Published: 13 January 2014
Acute myocardial infarction and stroke are more likely to occur in the early morning. Circadian pacemakers are considered to be involved in the process. Many peripheral tissues and cells also contain clock systems. In this study, we examined whether the primary cultured human plaque-derived vascular smooth muscle cells (VSMCs) process circadian rhythmicity; furthermore, we investigated the expression difference of clock genes between normal human carotid VSMCs and human plaque-derived VSMCs.
Fifty-six human carotid plaques provided the atherosclerotic tissue, and 21 samples yielded viable cultured primary VSMCs. The normal carotid VSMCs were cultured from donors’ normal carotids. The mRNA levels of the target genes were measured by Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR).
After serum shock, both types of cells showed clear circadian expressions of Bmal1, Cry1, Cry2, Per1, Per2, Per3 and Rev-erbα mRNA; meanwhile the Clock mRNA show a rhythmic expression in plaque-derived SMCs but not in normal carotid VSMCs. The expression levels of these main clock genes were significantly attenuated in human plaque-derived VSMCs compared with normal human carotid VSMCs. The rhythm of Bmal1 mRNA in plaque-derived VSMCs was changed.
The present results demonstrate that the human plaque-derived VSMCs possess different circadian rhythmicity from that of normal carotid VSMCs. The rhythm changes of clock genes in plaque-derived VSMCs may be involved in the process of atherosclerosis and finally promote the rupture of plaque.
KeywordsCircadian rhythm Primary cell culture Human vascular smooth muscle cells Atherosclerosis Plaque rupture
In mammals, many behavioral and physiological processes exhibit circadian (approximately 24 h) rhythms that are controlled by a clock system. This system includes the central circadian clock residing in the hypothalamic suprachiasmatic nucleus (SCN) and the peripheral clock located in many peripheral tissues. It is considered that circadian rhythmicity of peripheral tissues is uniquely controlled by SCN via neural and humoral signals. However, recent research demonstrates that peripheral tissues and cells also contain a similar clock system to that in the SCN[2, 3]. The core clock genes include Bmal1, Clock, Cry, Per and Rev-erbα etc., which form a negative feedback loop involving a positive limb (Bmal1 and Clock) and a negative limb (Per and Cry). The heterodimer of BMAL1/CLOCK binds to the E-boxes located within the promoters of Cry and Per genes and activates their transcription. Then, the proteins of PER and CRY form a complex and inhibit the positive limb, resulting in rhythmic oscillation.
Acute myocardial infarction and stroke, severe complications resulted from atherosclerosis, are more likely to occur in the early morning[5, 6]. These fatal complications of atherosclerosis are mainly caused by plaque rupture and subsequent embolism and thrombosis. Epidemiological studies have also indicated that shift workers suffer from a higher risk of atherosclerosis and cardiovascular events. These phenomena cannot simply be explained by the change in blood pressure and platelet function. Although the underlying molecular mechanisms of such diurnal variations were not clarified, the circadian clock could be a potential factor involved in the process. Previous animal research has already illustrated that the disruption of circadian rhythms could impair vessels and enhance atherosclerosis[9, 10].
Vascular smooth muscle cells (VSMCs) are responsible for the structure and function of vessel walls and are involved in the development and progression of a variety of cardiovascular diseases, such as atherosclerosis. However, little is known about the circadian clock system in human VSMCs, especially the VSMCs in human plaques. In the present study, we established a model of primary cultured human plaque-derived VSMCs and normal human carotid VSMCs in vitro (both possess circadian oscillators by the serum shock method) to compare the rhythm changes of clock genes in human plaque-derived VSMCs with that in normal human carotid VSMCs.
Primary cultured VSMCs
Main characteristics of patients succeeded in culturing VSMCs
Number of cases
Age range (mean)
Human plaque derive VSMCs
Normal human carotid VSMCs
Diurnal expression patterns of circadian genes in normal human carotid VSMCs
Expressions of clock genes attenuated in human plaque-derived VSMCs
We then detected that the human plaque-derived VSMCs also possess circadian oscillators (p < 0.05), and the rhythms of their core clock genes expression were similar to that of normal human carotid VSMCs, with consistent peak and trough times (except Bmal1 and Clock) (Figures 4,5 and6). Compared with normal human carotid VSMCs, the peak mRNA level of Bmal1 phase advancing to ZT12 in human plaque-derived VSMCs. Interestingly, the expression of the Clock gene also showed a rhythmic oscillation in plaque-derived SMCs (p < 0.05), which was different to normal human carotid VSMCs (p > 0.05), with a peak at ZT12. However, the amplitude of the above genes was significantly attenuated in human plaque-derived VSMCs compared with normal human carotid VSMCs.
VSMCs are the major cell type in vessel walls and are responsible for the structure and function of vessel walls. They are involved in the pathogenesis of atherogenesis and plaque rupture. Previous research studies indicated that the VSMC phenotype switched in atherosclerosis[12, 13], which is consistent with our findings. VSMCs derived from human plaques can be divided into two distinctly different phenotypes. According to their morphologies, content of lipid and ultrastructure, the fusiform cells would be the contractile VSMCs, and the phenotype of big shape ones was switching to the synthetic type. This phenomenon implied that VSMCs may play different roles at distinct stages of atherogenesis.
Both peripheral tissues and cells in vivo or in vitro possess circadian oscillators, which is similar to that in the SCN. The different cellular elements of vasculature, including vascular endothelial, smooth muscle, and fibroblasts cells, have proved to show a rhythm expression of circadian genes in vitro. The cultured fibroblasts, as well as the hemangioendothelioma cells in culture, present a circadian expression for all clock genes after treatment with serum shock[14, 15]. Moreover, the rhythmic pattern of expression in cultured fibroblasts persists for over 20 days, suggesting the clock system is self-sustained. Several studies have demonstrated that molecular oscillators exist in either mouse or human aortic VSMCs in vitro, and the rhythms are distinct between different species or vessels[17–19].
Main characteristics of patients involved in subgroup analysis (range of age: 60–70 years old)
Number of cases
Age range (mean)
Human plaque derive VSMCs
Normal human carotid VSMCs
Interestingly, we previously found that in apolipoprotein E knockout mice (a widely used atherosclerotic mouse model), the expression levels and the circadian rhythms of clock genes changed compared to C57BL/6 J mice, and these changes were accompanied with a progression of atherosclerosis. Pan, Jiang and Hussain found that the mutation of Clock protein can impair cholesterol metabolism and enhance atherosclerosis in different mouse models. Anea et al. illustrated that in mice with aberrant circadian rhythms, Clock mutant and Bmal1-knockout, pathological remodeling and vascular injury (such as intimal hyperplasia) increased. Moreover, the aged Bmal1-knockout mice exhibit even more severe abnormalities and a significant susceptibility to thrombosis. According to our results, the expressions of clock genes are significant decreased in human plaque-derived VSMCs compared to normal human carotid VSMCs. We hypothesized that the impairment of circadian rhythms in human plaque-derived VSMCs could promote the progress of atherosclerosis. As the plaque VSMCs in our experiment were derived from aged patients, we assumed that these patients with impaired circadian rhythms may also have been more susceptibility to thrombosis and subsequent cardiovascular events. Impressively, besides the reduction of amplitude, we found that the peak time of Bmal1 rhythmic oscillation was also changed. It was a limitation of this study that a certain cause was not found. Whether the change of Bmal1 rhythm is essential to the function of the clock system is also unknown and needs further in-depth studies.
In summary, we found that both the human plaque-derived VSMCs and normal carotid VSMCs possess circadian clock systems. The levels and rhythms of the core clock genes’ expression were changed in plaque-derived VSMCs compared with normal human carotid VSMCs, and these changes together may be involved in the progression of atherosclerosis and its subsequent complications. Of course, further research should be conducted to detect the protein expression of clock genes considered, and to find the downstream genes whose expressions are controlled by clock genes and to reveal how circadian genes regulate these clock-controlled genes and affect the diurnal variations of cardiovascular function.
Materials and methods
Human primary VSMCs culture
We used the established explant culture method to obtain human VSMCs. Human plaque VSMCs were cultured from carotid plaques of patients who had undergone carotid endarterectomy. Donors from the Zhongshan Hospital Transplant Program provided the sections of carotid to culture the normal human carotid VSMCs. Tissues were minced into approximately 1 × 1-mm pieces and then carefully placed into T-25 flasks (NUNC) and cultured in the complete medium involving medium 199 (GIBCO) with 20% fetal bovine serum (GIBCO) and antibiotics (100 IU/ml penicillin, 100 mg/ml streptomycin and 250 ng/ml Amphotericin B). The cultures were incubated at 37°C, with the medium changed twice a week. For the passaging culture, cells were trypsinised with 0.25% trypsin (GIBCO) for approximately 3 min, then centrifuged at 200 × g for 5 min, resuspended, and split 1:2. The second passages of cells were seeded into 35-mm petri dishes for harvesting.
Immunofluorescence microscopic detection of α - smooth muscle actin (α SMA) in primary cultured human VSMCs
Human VSMCs were cultured in collagen-I coated glass bottom dishes. Cells were washed softly and fixed with 4% paraformaldehyde for 10 min at room temperature and then rinsed with PBS (5 min×3 times). Subsequently, cells were permeated with 0.5% Triton-100 for 15 min and washed with PBS (5 min×3 times) before incubating with 1% normal donkey serum in PBS for 30 min. Then, cells were incubated with α - smooth muscle actin (a-SMA) antibody (1:100, BOSTER, CHINA) overnight at 4°C. Cells were washed with PBS (3 min×3 times), and the primary antibody was bound with FITC-labeled donkey anti-rat IgG for 60 min at 37°C. After washing with PBS (3 min×3 times), the cells were viewed through a Zeiss LSM 510 Meta confocal microscope.
Oil Red O staining
Vascular smooth muscle cells were fixed with 4% paraformaldehyde for 20 min and washed with PBS. Fixed cells were stained with freshly prepared Oil Red O working solution (60% Oil Red O stock solution diluted by distilled water) for 30 min. Stained cells were rinsed with PBS until the background became clear, and then observed using an inverted microscope.
Transmission Electron Microscope (TEM)
Vascular smooth muscle cells were centrifuged at 1500×g for 15 min, fixed with 2.5% glutaraldehyde, followed by postfixation for 2 h in 1% osmium tetroxide, dehydrated in graded alcohols and acetones. After that, the samples were embedded in Epon 812, and sectioned with LKB-I ultramicrotome in 50–60 nm. Then the sections were stained with 3% uranyl acetate and lead citrate, and examined by TEM (PHILIPS CM-120).
Serum shock and cells harvesting
The serum shock was performed before cell harvesting, as described. Briefly, human VSMC was grown in the complete medium for 72 h. Then, cells were starved for 24 h in serum-free medium 199 containing antibiotics (the same levels as the above). Subsequently (on the day of serum shock), cells were treated with medium 199 containing 50% horse serum for 2 h (serum shock) and then changed back to a starvation medium until the end of the experiment. The timing of the beginning serum shock was defined as Zeitgeber time 0 (ZT0), and cells were harvested for RNA extraction at ZT0, ZT4, ZT8, ZT12, ZT16 and ZT20.
Total RNA isolation and complementary DNA preparation
The total RNA was extracted from harvested VSMCs using TRIzol reagent (Invitrogen Corporation, USA). First-strand cDNA was synthesized and amplified from 2 μg of total RNA using the ReverTra Ace qPCR RT Kit (TOYOBO, Japan).
Quantitative real-time PCR
The primer sequences used for PCR amplification
Forward primer (5′–3′)
Reverse primer (5′–3′)
SPSS 19.0 software was used to perform the statistical analysis. Results were demonstrated as mean ± SEM. The values for mRNA levels were presented as relative values in all experiments. An unpaired Student’s t test was conducted to examine the differences between the groups, and a two-way analysis of variance (ANOVA) was used to evaluate the oscillation of each gene expression. In all the analyses, p < 0.05 was considered statistically significant.
The study protocol was conducted following the principles outlined in the Declaration of Helsinki and approved by the Ethics Committee of the Zhongshan Hospital, Fudan University. All human tissues were collected from patients who signed the informed consent. And all normal donors signed the consent voluntary standing on altruism. We also obtained the permissions from their immediate family members. No donor organs were obtained from executed prisoners or other institutionalized persons.
Vascular smooth muscle cells
Brain and muscle Arnt-like 1
Circadian locomotor output cycles kaput
NR1D1 (nuclear receptor subfamily 1, group D, member 1) a member of the nuclear receptor family of intracellular transcription factors
Transmission Electron Microscope
Rough endoplasmic reticulum
We thank Professors xiao-bo Li for invaluable suggestions and reagents. We thank Dr. Zhaohua Yang for providing carotid tissues. We thank Dr. xiao Wang, li Huang and jie Hu for their comments on the manuscript.
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