Hui Medicine Moxibustion Promotes the Absorption of Lumbar Disc Herniation and the Recovery of Motor Function in Rats through Fas/FasL Signaling Pathway.

Objectives: To study the resorption of the herniated lumbar disc (RHLD) and its mechanism in the SD rats of lumbar intervertebral disc herniation treated with Hui medicine moxibustion (HMM).
Methods: Forty SD rats were randomly divided into four groups, normal group, lumbar disc herniation (LDH) group, HMM group, and antagonist (HMM+Met12) group, with 10 rats in each group. The rat model of LDH was prepared with the method of lumbar epidural emplacement of the caudal intervertebral disc. In the HMM group and HMM+Met12 groups, 4 weeks after modeling, HMM therapy was performed in the lumbar spine for 3 months with 1 time per day and 20 min each time, the samples were collected 8 weeks after the treatment. The histological degeneration was observed through HE staining, and the neovascularization of intervertebral disc tissues was detected by the expression of CD34 and vascular endothelial growth factor (VEGF). The apoptosis of nucleus pulpous cells was detected by TUNEL assay, and the activity of caspase-3, -8, and -9 and extracellular matrix enzymes was detected by western blotting.
Results: HMM treatment significantly improved the behavioral ability of rats with LDH surgery. The morphological structure was obviously destroyed in the LDH group, but disc structure was significantly repaired in the HMM group, and mild structure alterations were observed in the HMM+Met12 group. Higher levels of CD34 and VEGF were detected in the HMM group indicating that neovascularization is formed. The expression level of FasL was significantly increased in the HMM group. The protein expression levels of cleaved-caspase-3, cleaved-caspase-8, and cleaved-caspase-9 in nucleus pulposus (NP) tissues were also elevated when treated with HMM, and the TUNEL staining showed the same results. The protein expression levels of matrix metalloproteinases- (MMP-) 1, MMP-2, MMP-3, MMP-13, and ADAMTS-4 were markedly promoted in the HMM group. Met12, a small peptide antagonist of FasL, significantly reduced the effects of HMM.
Conclusion: HMM can promote the formation of neovascularization of lumbar intervertebral disc, support the apoptosis of NP cells through Fas/FasL signaling, and regulate the degradation of extracellular matrix enzyme, which then accelerates the absorption of lumbar intervertebral disc herniation and the recovery of motor function in rats.

1. Introduction

The resorption of the herniated lumbar disc (RHLD) refers to the phenomenon of natural absorption or even complete disappearance after conservative treatment without surgical intervention or chemical drug injection [1]. Since the first report of computed tomography- (CT-) confirmed RHLD [2], this phenomenon has been broadly reported in many pieces of research [3, 4]. RHLD theory brings new enlightenment to traditional Chinese medicine and Hui medicine and other conservative therapy in the prevention and treatment of lumbar disc herniation. In the treatment of lumbar disc herniation, Hui medicine moxibustion (HMM) therapy has the effect of promoting blood circulation, detumescence, and strengthening muscle and bone. It can not only prevent the degeneration of the lumbar disc but also promote the resorption of partially ruptured lumbar disc herniation, reduce the recurrence of lumbar disc herniation, and improve the quality of life of patients with lumbar disc herniation [5].

There are many theories about the mechanism of RHLD after lumbar disc herniation, such as autoimmunity, inflammatory reaction, neovascularization, imbalance of matrix metabolism, intervertebral disc degeneration, and NP cell apoptosis [6]. More experiments have confirmed that the apoptosis of NP cells mediated by signal pathways may be an important mechanism of reabsorption [7]. Prominent NP cells express a variety of inflammatory factors and matrix-degrading enzymes after contacting with blood supply, which worsens the living environment of NP cells, affects the growth and function of NP cells, and increases the apoptosis of the nucleus pulposus cells. Promoting the apoptosis of NP cells can promote the resorption of NP tissues [8].

Macrophage infiltration after disc herniation can be accompanied by the release of a variety of cytokines, such as IL-1, HIF-1, and monocyte chemoattractant protein-1. These cytokines can participate in the regulation of physiological activities through multiple signaling pathways (such as Fas/FasL, MAPKs, and NF kappa B) [9], among which, the Fas/FasL signaling pathway may be one of the most important pathways regulating the NP cell apoptosis, and moxibustion can regulate this signaling pathway [9].

Fas is a kind of type I transmembrane protein, belonging to the superfamily of tumor necrosis factor (TNF) and nerve growth receptor protein [10]. Fas can induce apoptosis by binding with its ligand FasL. FasL is a type II transmembrane protein belonging to the TNF family [11]. Fas recruits the adaptor protein FADD (Fas-associating protein with death domain) in the cytoplasm through the interaction between DD (death domain) in the intracellular domain and DD in the carboxyl end of FADD. Then, FADD binds with caspase-8 through the death effect domain to form a death-inducing signal complex and activates caspase-8 and downstream caspase-3 family members, which eventually leads to apoptosis. Regulation of the Fas/FasL signaling pathway can lead to a variety of complex biological effects on nucleus pulposus cells: regulating the apoptosis of NP cells and controlling the balance of extracellular matrix anabolism [12]. The Fas/FasL signaling pathway plays an important role in the resorption after lumbar disc herniation, so regulating this signaling pathway becomes an ideal choice to promote the resorption after lumbar disc herniation.

With the deepening of the basic research on the reabsorption of NP, the mechanism of reabsorption is gradually clear. People gradually realize that surgery is not the final treatment for lumbar disc herniation, and it is not the only treatment. When there is no serious nerve injury, the discovery of reabsorption provides more possibilities for the conservative treatment of lumbar disc herniation with traditional Chinese medicine. It will be the focus of future research to give full play to the advantages of traditional Chinese medicine and develop a more targeted and targeted conservative treatment plan for lumbar disc herniation.

HMM therapy is a treatment method of burning moxa or in the moxibustion tank directly on the body surface of the affected place, making the local skin red and foaming, and promoting the recovery of the body. The treatment needs to achieve the unique moxibustion effect that the skin must be red and foaming. It is the development of the Chinese medical moxibustion with purulent moxibustion therapy to promote the balance of body fluids and to achieve Yin and Yang balance.

HMM therapy has the advantages of fast heat transfer and simple operation, which is deeply loved by patients clinically. However, the mechanism of HMM remains unclear. Herein, we will study the specific mechanism of HMM in traditional Chinese medicine.

2. Methods

2.1. Animals

Forty healthy male Sprague Dawley rats (2-3 months old and 200-250 g in weight), obtained from Ningxia Medical University Animal Centre, were used in the present study. Rats were cultured in separated cages and fed with distilled water and standard rat chow under a pathogen-free environment of a 12 h light/12 h dark cycle. The rats were fed for 5 days to adapt to the environment before the experiments. Then, the rats were randomly divided into 4 groups: Hui medicine moxibustion (HMM) group, antagonist (Met12) group, LDH model group, and normal group, with 10 rats in each group. Except for 10 rats as a normal group, the remaining 30 rats were made into the model of nucleus pulposus resorption after lumbar disc herniation. Useless laboratory rats were euthanized through carbon dioxide (CO2) asphyxiation. Compressed CO2 gas in cylinders flowed to the rats' chamber at a rate of 10-30% displacing air in the chamber volume per minute. All experiments involving rats in our study were approved by the Ethics Committee of the General Hospital of Ningxia Medical University (approval no.: 2017-052).

2.2. Establishment of LDH Model

The method of lumbar epidural emplacement of the caudal intervertebral disc was used to establish the LDH model. Thirty Sprague Dawley rats in the HMM group, Met12 group, and LDH group were anesthetized by intraperitoneal injection of pentobarbital sodium. After successful anesthesia, the back was depilated and the tail was tied with a rubber band. Under the condition of routine disinfection and asepsis, the caudal vertebrae including two complete disc lengths, including the upper and lower cartilage endplates, were cut from each animal. The upper and lower endplates were punctured with needles to expose the NP. The caudal vertebrae were cut with a no. 7 surgical line and put into the normal saline vessel for standby. The skin of the lumbar vertebrae of rats was depilated and sterilized with 75% alcohol. Under an aseptic condition, the skin and muscle of the back were cut along the posterior midline to expose the spine. The L2-5 spine lamina was removed to expose the dura mater of the lumbar vertebrae. The caudal disc was carefully placed in the lumbar epidural and sutured layer by layer. The rats in the HMM group and Met12 group were intervened for 1 month after the LDH model establishment. According to our previous experimental results, the materials were obtained after 3 months of intervention.

2.3. Hui Medicine Moxibustion Treatment

The rats in the HMM group were treated one month after being modeled. LDH rats were fixed on the fixator and shaved along the longitudinal axis of the rat lumbar spine, and then, the special 2 cm × 0.5 cm × 0.5 cm rectangular iron moxibustion device was placed along the longitudinal axis of lumbar vertebrae, covering the surface of L1-L6 vertebrae. The center of the iron moxibustion device was placed in the L3-L4 vertebrae gap of rats, and then, 2 g of moxa was ignited in the moxibustion groove. The hot iron moxibustion device could make the local skin flush, once a day, 10 minutes each time, and the samples were collected 3 months later.

LDH rats in the HMM+Met12 group were intervened one month after modeling, and the method of moxibustion intervention was the same as that of the HMM group. After HMM treatment, the rats were intraperitoneally injected with Met12 (10 μg/kg/week), and the samples were collected 3 months after the intervention.

Rats in the LDH group were fixed on the fixator for 10 min, once a day after 1 month of modeling, without other intervention, and the samples were taken after 3 months of fixation.

Rats in the normal group were LDH rats that were not given modeling and intervention, and the samples were taken after 3 months.

2.4. Locomotor Activity Test

The locomotor activity tests were proceeded according to the previous study [13]. Briefly, the gait recovery of rats was observed and recorded every day after the surgery. Rats were placed in the open field to observe their motor function. Rats with a normal gait and no deformity of toes were recorded as having 0 score. Rats with a slight limp in gait were recorded as 1 score. Rats with weakness of hind limbs or moderate claudication were recorded as 2 scores. Rats with paraplegia of hind limbs, obvious claudication, or an abnormal contralateral toe were recorded as 3 scores.

2.5. Mechanical Allodynia Test

The pain threshold was measured by a paw tenderness instrument when the rats were waking [14]. Rats were placed in plexiglass boxes with a wire mesh floor. After 15 min, the needle was pointed at the palmar surface of the rat's hind foot through the reflector, and the needle vertically stimulated the middle of the rat's hind foot continuously from the lower part of the metal screen. When the rat's foot retracted or licked, the needle fell, which was regarded as a positive reaction. The stimulation intensity was automatically displayed on the instrument screen, which was the threshold of mechanical claw retraction. Each test was repeated 3 times, with an interval of 5 min. The change percentage of tactile threshold (%) = (immediate pain threshold − basic pain threshold)/basic pain threshold × 100%.

2.6. Histopathological Analysis

Nucleus pulposus tissues were fixed in 4% paraformaldehyde for 24 h and embedded in paraffin and then sliced into 5-6 μm paraffin sections. Sections were stained with hematoxylin and eosin (HE, Beyotime, China) stain. Five fields were randomly selected and observed by light microscopy (Olympus, Tokyo, Japan).

2.7. Immunohistochemistry (IHC)

IHC experiments were performed as described in a previous report [15]. Briefly, nucleus pulposus tissues were harvested into PBS-buffered formaldehyde, embedded in paraffin, and then cut into 4 μm thick sections. A monoclonal antibody against FasL (Abcam, Cambridge, UK) was used for this analysis. A horseradish peroxidase diaminobenzidine kit was used to detect immunoreactivity, and samples were counterstained with hematoxylin. A microscope (Olympus, Tokyo, Japan) was used to capture a representative area containing FasL-positive tissue (magnification, 200x).

2.8. Western Blotting

Based on a previous report, proteins were measured through western blotting [15]. For the isolation of proteins in the tissues, nucleus pulposus tissues were first ground in chilled mortar in the presence of liquid nitrogen. Immediately after grinding, the tissue powder was lysed with radioimmunoprecipitation assay (RIPA) buffer. Total proteins were then separated on polyacrylamide gels and transferred to a polyvinylidene difluoride membrane before being probed overnight at 4°C with antibodies (anticleaved-caspase-3, anticleaved-caspase-8, anti-cleaved-caspase-9, anti-MMP-1, anti-MMP-2, anti-MMP-3, anti-MMP-13, and anti-ADAMTS-4; Abcam, Cambridge, UK), then washed three times with PBST. Then, membranes were treated with the corresponding secondary antibodies. Finally, the membrane was processed using an ECL kit for color reaction. Western blotting results were normalized to those of GAPDH for quantification.

2.9. TUNEL

The TUNEL method using an in situ cell death detection kit (Roche, USA) was performed to detect the apoptosis of NP cells. Paraffin sections were deparaffinized and rehydrated with xylene and gradient ethanol. Then, sections were treated with proteinase K at room temperature for 30 min. After washing twice with PBS, specimens were added with 50 μL TUNEL reaction mixture and incubated in the dark for 60 min at room temperature, and then, 50 μL converter POD was added and reacted at 37°C in the dark for 30 min. After rinsing with PBS 3 times, the sections were colored with 50 μL DAB for 10 min at room temperature and counterstained with hematoxylin for a few seconds. A microscope (Olympus, Tokyo, Japan) was used to capture the representative stained pictures.

2.10. Statistical Analysis

The SPSS version 26.0 and GraphPad Prism 8 statistical software packages were used for data analysis. Each result is presented as means ± standard deviation. The significance of differences among samples was teased by one-way ANOVA followed by Turkey's post hoc test. P < 0.05 was considered significant. ImageJ software was used to carry out the semiquantitative analysis.

3. Results

3.1. Hui Medicine Moxibustion (HMM) Promotes Recovery of Motor and Tactile Function in Rats with LDH

We established the LDH rat model by nucleus pulposus transplantation and assessed the behavior of LDH rats. There was no obvious motor dysfunction and no difference in mechanical withdrawal thresholds to stimulations before the LDH surgical procedures in all rats. As shown in Figures 1(a) and 1(b), the locomotor activity of rats with LDH was significantly decreased compared with normal rats, and the mechanical withdrawal thresholds of LDH rats were markedly reduced compared with withdrawal latencies before surgery. HMM treatment significantly improved the behavioral ability of rats with LDH surgery, and the tactile function was also recovered to a large extent by the end of our experiments. The behavior of LDH rats was improved with HMM treatment but suppressed with the inhibitor of Fas, MET12 (Figures 1(c) and 1(d)).

Figure 1

Behavioral assessments in lumbar disc herniation (LDH) rats with moxibustion treatment of Hui medicine (HMM). Locomotor activity tests (a) and tactile function tests (b) were performed to assess the success of LDH rat models. Locomotor activity tests (c) and tactile function tests (d) were performed with or without the moxibustion treatment of Hui medicine. A negative value of paw withdrawal latency indicates mechanical hyperalgesia. ∗P < 0.05, ∗∗P < 0.01, and ∗∗∗P < 0.001 vs. the normal group; #P < 0.05 and ##P < 0.01 vs. the LDH group; and < 0.05 vs. the HMM group.

3.2. HMM Promotes the Formation of Neovascularization of the Lumbar Intervertebral Disc

Vascularization plays an important role in the reabsorption process of intervertebral disc herniation. HE staining was used to observe the histological degeneration of the lumbar intervertebral disc. We observed clearly morphological alterations and degenerative nucleus pulposus in the LDH group, which were repaired in the HMM group and mild reparation in the HMM+Met12 group (Figure 2(a)). CD34 immunohistochemical staining further proved the formation of new blood vessels (Figure 2(b)). The expression of VEGF was also detected to further assess the formation of neovascularization of the lumbar intervertebral disc (Figure 2(c)).

Figure 2

HMM promotes the formation of neovascularization of lumbar intervertebral disc tissue. (a) Representative HE staining (10x) of nucleus pulposus tissues, which was used to observe the histological degeneration of the intervertebral disc. Scale bar = 200 μm. (b) Representative of CD34 expression, determined by immunohistochemical staining (40x). Scale bar = 50 μm. (c) Western blotting was used to detect the protein expression of VEGF. ∗P < 0.05, ∗∗P < 0.01, and ∗∗∗P < 0.001 vs. the normal group; #P < 0.05 and ##P < 0.01 vs. the LDH group; and < 0.05 vs. the HMM group.

3.3. HMM Induces Apoptosis in Rat NP Tissues through FasL

To examine whether HMM facilitates the reabsorption of herniated disc tissue by inducing apoptosis, the expression of FasL in NP tissues was assessed by immunohistochemistry. As shown in Figure 3(a), the number of FasL-positive cells was significantly increased in the LDH group compared with the normal group, with an even higher number in the HMM group, which were then decreased in the HMM+Met12 group. The TUNEL staining in the NP tissues showed a similar trend among the four groups (Figure 3(b)). The protein expression levels of C-caspase-3, C-caspase-8, and C-caspase-9 in NP tissues were also detected to assess the apoptosis of NP cells at the molecular level. The protein expression levels of C-caspase-3, C-caspase-8, and C-caspase-9 were significantly increased in the LDH group compared with the normal group, which were even higher in the HMM group and restored in the HMM+Met12 group (Figure 3(c)).

Figure 3

HMM promotes the apoptosis of lumbar intervertebral disc tissue through FasL. (a) Representative images of the results of the expression of FasL in nucleus pulposus cells, assessed by immunohistochemistry (40x). Black arrows indicate positive staining, and blue arrows indicate negative staining. Scale bar = 50 μm. (b) Representative images of TUNEL staining (40x) in the nucleus pulposus cells. Scale bar = 50 μm. (c) The expression levels of C-caspase-3, C-caspase-8, and C-caspase-9 in nucleus pulposus cells were detected by western blotting. ∗P < 0.05, ∗∗P < 0.01, and ∗∗∗P < 0.001 vs. the normal group; ##P < 0.01 and ###P < 0.001 vs. the LDH group; and < 0.05 and < 0.01 vs. the HMM group.

3.4. HMM Promotes the Reabsorption of Lumbar Disc Herniation through Extracellular Matrix Degradation

The reabsorption of herniated disc tissue is also related to the imbalance of matrix synthesis and degradation. The protein expression levels of MMP-1, MMP-2, MMP-3, MMP-13, and ADAMTS-4 were detected. The protein expression levels of MMP-1, MMP-2, MMP-3, MMP-13, and ADAMTS-4 were markedly increased in the HMM group and rescued with the FasL inhibitor (Figure 4).

Figure 4

HMM promotes the reabsorption of lumbar disc herniation by promoting extracellular matrix degradation. (a) The protein expression levels of MMP-1, MMP-2, MMP-3, MMP-13, and ADAMTS-4 were detected using western blotting. (b–f) Semiquantitative analysis of western blotting. ∗P < 0.05, ∗∗P < 0.01, and ∗∗∗P < 0.001 vs. the normal group; ##P < 0.01 and ###P < 0.001 vs. the LDH group; and < 0.01 and < 0.001 vs. the HMM group.

4. Discussion

In recent decades, the incidence rate of LDH has increased worldwide, especially among adolescent and middle-aged people [16]. Not all LDH patients can be treated with surgery. According to statistics, only 10% to 20% of patients are suitable for surgical treatment [17], the incidence of postoperative complications is 15-30%, and the recurrence rate of LDH after the operation can reach 25% [18]. Therefore, nonsurgical treatment is still the best choice for most LDH patients. HMM is one of the most distinctive treatments of Chinese Hui medicine [19]. This therapy not only has good effects in the treatment of lumbar disc herniation but also can prevent the degeneration of the lumbar disc [20], but the mechanism of HMM remains unclear. In this study, we established the LDH rat model and confirmed that the simulation of self-nucleus pulposus implantation led to persistent motor dysfunction and mechanical hyperalgesia and revealed that HMM promoted the absorption of the nucleus pulposus through the Fas/FasL pathway and alleviated disc herniation.

Vascularization plays an important role in the resorption of herniated disc tissue, and neovascularization is closely related to the degree of absorption and prognosis of LDH [21]. Previous studies have shown that the higher the degree of vascularization of the herniated disc tissue is, the more obvious the spontaneous absorption is. It has been reported that blood vessels have appeared in five types of disc hernia—extruded, bulging, prolapsed, protrusion, and sequestrated—and mainly in the extruded type [22]. Therefore, the growth of neovascularization is a potential factor for the resorption of prominent nucleus pulposus. VEGF and CD34 have been proven to be implicated in the neovascularization of herniated disc tissues [23, 24]. In our study, HMM therapy effectively promoted the neovascularization of LDH rats, with high expression of VEGF and CD34 in the rats treated with HMM.

Apoptosis of the nucleus pulposus plays an important role in the pathological process of RHLD. FasL has many effects on maintaining immunity, inducing apoptosis, and mediating inflammation under different conditions. FasL is expressed in the lumbar disc and nucleus pulposus, and the abnormal expression of FasL induces apoptosis of the nucleus pulposus cells, which may play an important role in the pathogenesis of lumbar disc herniation [25]. Apoptosis was observed when the nucleus pulposus cells were cultured with FasL for 24 h, and caspase-3 and caspase-8 expression levels were increased with the increase of the FasL concentration, which indicated that this membrane pathway was involved in the apoptosis of Fas-induced nucleus pulposus cells; caspase-9 was activated when FasL reached a certain concentration, which activated the mitochondrial apoptosis pathway [26]. The effect of the Fas/FasL signaling pathway on nucleus pulposus cells has been widely accepted. Besides, Fas/FasL has also been validated to have a connection with vascularization [27]. Our experiments have also detected that HMM therapy can activate caspase-3, caspase-8, and caspase-9 and observed abnormal expression of FasL.

The resorption of herniated disc tissue is also related to the imbalance of matrix synthesis and degradation. The apoptosis of nucleus pulposus cells will gradually cause the imbalance of extracellular matrix components and metabolic disorder [28]. MMPs are important enzymes in the extracellular matrix. When the activity of MMPs increases, the levels of TIMPs decrease, and matrix synthesis and degradation are unbalanced, which promotes the degradation of protrusions [29]. It is suggested that the reabsorption of the herniated disc is accomplished by the coordination of various protein enzymes. It has been found that the expression of ADAMTS-4 and MMP-3, MMP-8, and MMP-9 in the herniated lumbar disc is highly consistent [30]. Studies have shown that macrophage infiltration and MMP-l and MMP-3 expression were observed by implanting the disc tissue into the back muscles of rats, and the resorption was promoted by activating the MMPs [31]. As a kind of proteoglycan hydrolase, ADAMTS also participates in the decomposition of the disc matrix and the process of reabsorption [32]. Therefore, it is of great significance to study the effects of the Fas/FasL signaling pathway on the extracellular matrix environment of the nucleus pulposus. We detected the activity of MMP-1, MMP-2, MMP-3, MMP-13, and ADAMTS-4 in the rat nucleus pulposus treated with HMM, which indicated that the treatment of HMM effectively promoted the resorption of the herniated disc.

5. Conclusions

Our study found that the Hui medicine moxibustion promoted the apoptosis of the nucleus pulposus cells through the Fas/FasL pathway and then promoted the absorption of the nucleus pulposus. We also clarified the mechanism of the therapeutic effect of the moxibustion on lumbar disc herniation and provided a theoretical and experimental basis for the prevention and treatment of disc herniation by Hui medicine moxibustion.