Introduction

Bone remodelling is a tightly coupled lifelong process, whereby old bone is removed by osteoclasts (bone resorption) and new bone is formed by osteoblasts (bone formation).1,2 Osteocytes, which act as mechanosensors/endocrine cells, and bone lining cells3 are also involved in bone remodelling.4 Myriad pathophysiological factors affecting bone remodelling have been observed in skeletal diseases such as osteoporosis, arthritis and periodontal disease.5 Oxidative stress is one of the pathophysiological factors affecting bone remodelling. Oxidative stress stimulates osteoclast differentiation, thereby enhancing bone resorption.6,7 Reactive oxygen species (ROS) stimulate the apoptosis of osteoblasts and osteocytes, thus affecting bone formation. ROS also activate mitogen-activated protein kinases (MAPKs), such as extracellular signal-regulated kinases (ERK1/2), c-Jun-N terminal kinase (JNK) and p38, and enhance osteoclastogenesis and bone resorption.811 These phenomena skew the bone remodelling process in favour of bone loss.

Antioxidants are compounds which reduce free radicals and oxidative stress.12 Antioxidants have been reported to promote differentiation of osteoblasts, bone formation and survival of osteocytes, as well as suppressing osteoclast differentiation and activity.8,1315 Some studies associate the age-related reduction in circulating antioxidants to osteoporosis in rats and women.1618 A decline in antioxidant levels has been reported to promote bone loss by triggering the tumour necrosis factor-alpha (TNF)-dependent signalling pathway,6 while administration of antioxidants, such as vitamin C, E, N-acetylcysteine and lipoic acid, have been reported to exert favourable effects in animal models of osteoporosis1921 and individuals with osteoporosis.2225

Caffeic acid (CA) is a metabolite of hydroxycinnamate and phenylpropanoid commonly synthesized by all plant species. It is a polyphenol present in many food sources like coffee, tea, wine, blueberries, apples, cider, honey and propolis.26 CA and its major derivatives including caffeic acid phenethyl ester (CAPE) and caffeic acid 3,4-dihydroxy-phenethyl ester (CADPE) are reported to possess potential antibacterial, antidiabetic, antioxidant, anti-inflammatory, antineoplastic and cardioprotective activities (reviewed in2729). As a potent antioxidant, CA has been demonstrated to decrease lipoperoxyl radicals (ROO) by donating a hydrogen atom to its corresponding hydroperoxide, which terminates the lipid peroxidation chain reaction. It also inhibits human low-density lipoprotein (LDL) oxidation induced by cupric ions.30 Furthermore, it interacts with other compounds, such as -tocopherol, chlorogenic and caftaric acids, to exert more potent antioxidant activity in a variety of different systems.3133 Therefore, the antioxidant activities of CA might protect against the negative effects of oxidative stress on bone cells and the skeletal system. This systematic review aims to summarise the effects of CA and its derivatives on bone cells and bone in literature.

A systematic literature search was conducted from July until November 2020 using PubMed, Scopus, Cochrane Library and Web of Science databases to identify studies on the effects of caffeic acid on bone and bone cells including osteoblasts, osteoclasts and osteocytes. The search string used was (1) caffeic acid AND (2) (bone OR osteoporosis OR osteoblasts OR osteoclasts OR osteocytes).

Studies with the following characteristics were included: (1) original research article with the primary objective of determining the effects of caffeic acid on bone and bone cells; (2) studies using cellular or animal models, or humans; (3) studies administering caffeic acid as a single compound but not in a mixture or food. Articles were excluded if they (1) do not contain original data; (2) use food rich in caffeic acid or mixtures containing caffeic acid. The bibliography of relevant review articles was traced for potential articles missed during database search. The search results were organised using EndNoteTM software (Clarivate Analytics, Philadelphia, USA). Duplicates were identified using EndNoteTM and confirmed by manual checking.

Two authors (S.O.E. and K.L.P.) searched the same databases using the search string mentioned and screened the search results. All the articles that did not match the selection criteria were excluded. Next, the articles which used caffeic acid in treating models other than bone-related diseases were removed. Finally, articles which used caffeic acid in combination with other compounds were also excluded. Any disagreement on the inclusion or exclusion of articles was resolved through discussion among the two authors. The corresponding author (K.Y.C.) had the final decision on articles included if a consensus could not be reached between authors responsible for screening. This systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and checklist.34 Steps in the selection process, from identification, screening, eligibility to the inclusion of articles, are shown in Figure 1.

Figure 1 Flowchart of the article selection process.

From the literature search, 381 articles were identified, of which 87 were obtained from PubMed, 182 were from Scopus, 3 from Cochrane Library and 109 from Web of Science. A total of 155 duplicate articles were identified and removed. Of the 226 articles screened, 202 articles were excluded based on the selection criteria, whereby 51 articles did not contain primary data (3 book chapters, 2 commentary and 46 review articles), 147 articles and 2 conference abstracts presented topics irrelevant to the current review, a conference abstract had been published as a full-length research article and another conference abstract did not contain sufficient experiment details (Supplementary Material). Finally, 24 articles fulfilling all criteria mentioned were included in the review.

The included studies were published between 2006 and 2020. Seven studies were in vitro experiments using mouse bone marrow macrophages (BMMs), RAW264.7, RAW D and MG63 osteoblast cell lines3541 while 19 studies were in vivo studies using Sprague Dawley/Sprague Dawley albino rats, Wistar/Wistar albino rats, Balb/c mice, lipopolysaccharide (LPS)-resistant C3H/HEJ mice, C57BL/6J mice and ICR mice.35,38,4258 No human studies on this topic were reported.

Six in vitro studies focused on the effects of CA on osteoclast differentiation from haematopoietic cells using macrophage colony-stimulating factor (M-CSF), receptor activator of NF-B (RANK) ligand (RANKL) or TNF-,3539,41 while one in vitro study focused on the effect of CA on osteoblasts using MG63 osteoblast cell line.40 Four in vitro studies used CA doses between 0.15 M.35,37,38,40 Ang et al.36 used doses between 00.3 M and Sandra et al.41 and Sandra and Ketherin39 used a dose of 10 g/mL (55.5 M). The treatment period was 57 days for the differentiation of osteoclasts.

For animal studies, Duan et al.,55 Zawawi et al.,58 William et al.,51 Wu et al.,38 Zych et al.49 and Folwarczna et al.48,52 used CA or its derivatives at doses between 0.550 mg/kg via oral or intraperitoneal (i.p.) administration. Ucan et al.,57 Erdem et al.,53 Cicek et al.,54 Yigit et al.,45 Yildiz et al.50 and Tolba et al.56 used doses between 1020 mol/kg/day (2.845.69 mg/kg/day) via i.p. administration. Kizilda et al.4244 and Kazanciolu et al.46,47 used the dose of 10 mmol/kg/day (2.843 g/kg/day) for an i.p. administration, Kazanciolu et al.47 employed 50100 mmol/kg/day (14.2228.43 g/kg/day) for a localised administration, while Ha et al.35 used a collagen sponge soaked with CAPE with the final dose of 250 g/mouse. For oral administration, first-pass effect might affect the enteric absorption of CA or its derivatives.59 For i.p. administration, the injection is commonly performed at the lower left or right quadrant of the abdomen. The peritoneum can absorb the compounds fast and reach systemic circulation with greater bioavailability with fewer handling errors.60

The bone-related disease models used included ovariectomy (OVX)- or glucocorticoids (dexamethasone)-induced osteoporosis, polyethylene particle-induced bone defect and osteolysis, electromagnetic force (EMF)-stimulated bone loss, osteotomy- or anti-collagen antibody-induced arthritis (CAIA) and rapid maxillary expansion (RME) and LPS-induced periodontitis. The endpoints studied included bone microstructure, histomorphometry, bone remodelling and oxidative status. The effects of CA and its derivatives on bone remodelling have been summarized in Table 1.

Melguizo-Rodrguez et al. reported that 24-hour CA (1 M) incubation increased the number of MG63 osteoblast cells compared with control.40 Gene expression studies revealed that CA increased the expression of osteoblast-related genes such as bone morphogenetic protein-2 and -7 (BMP-2 and BMP-7), transforming growth factor-beta 1 (TGF-1), transforming growth factor-beta receptor 1, 2 and 3 (TGF-R1, TGF-R2 and TGF-R3) and osteoblastogenesis genes including Runt-related transcription (RUNX-2), alkaline phosphatase (ALP), collagen type 1 (COL-I), osterix (OSX) and osteocalcin (OSC).40 Additionally, pretreatment of CA (10 g/mL or 55.5 M) on RAW D cells for 2 h also significantly inhibited the RANKL and TNF-induced osteoclastogenesis with the suppression of p38 MAPK phosphorylation and tartrate-resistant acid phosphatase (TRAP)-positive osteoclast-like cells (OCLs) formation.39 Similarly, pretreatment of CA (0.1, 1 and 10 g/mL or 0.555, 5.55 and 55.5 M) on RAW D cells and BMMs for 3 days significantly inhibited the RANKL and TNF-induced osteoclastogenesis and NF-B activity in RAW-D cells and RANKL, TNF and M-CSF-induced osteoclastogenesis in BMMs.41

On the other hand, CAPE treatment (00.3 M; 57 days) suppressed the formation of TRAP-positive OCLs on RANKL-treated RAW264.7 cells and BMMs.36 Apoptosis occurred in CAPE-treated RAW264.7 cells with the disruption of the microtubule network in OCLs.36 Similarly, Kwon et al. reported that CAPE treatment (0.15 M) for 5 days suppressed OCLs formation from RANKL-stimulated RAW264.7 cells.37 Another study by Ha et al. treating M-CSF and RANKL-stimulated BMMs with CAPE (05 M for 57 days) also showed decreased OCLs formation in a concentration-dependent manner.35 The amount of TRAP-positive OCLs was decreased upon 0.1 and 0.5 M CAPE treatment by 30% and 95% respectively.35 No OCL formation was observed upon 1 M CAPE treatment.35 The anti-osteoclastogenic activities of CAPE are mainly contributed by its anti-inflammatory and antioxidant properties. Mechanistically, CAPE reduces superoxide anion generation by downregulating the nicotinamide adenine dinucleotide phosphate oxidase 1 (Nox1) expression through the interruption of nuclear factor-kappa B (NF-B) and c-Jun N-terminal kinase (JNK) signalling pathways.37 CAPE suppresses RANKL-mediated activation of the NF-B pathway by downregulating NF-B p65 subunit expression and its nuclear translocation,37 suppressing nuclear factor of activated T cells (NFAT) activities36 and degradation of NF-B inhibitor (IB),36,37 as well as inducing the degradation of IB kinase (IKK).37 CAPE also suppresses the expression and activation of JNK and its downstream transcription factors, such as c-Fos and c-Jun, which subsequently interrupt the protein activator-1 (AP-1) complex formation.37 Additionally, CAPE suppressed RANKL-induced activation of the Nox1 by inhibiting the Nox p47PHOX subunit translocation to the cell membrane and downregulation of Ras-related C3 botulinum toxin substrate 1 (Rac1) expression.37

On the other hand, Wu et al. reported that CADPE (0.15 M for 7 days) also concentration-dependently reduced OCL formation in the M-CSF and RANKL-stimulated BMMs and RAW264.7 cells.38 Mechanistic and characterisation examination revealed that CADPE suppressed RANKL-induced tumour necrosis factor receptor-associated factor 6 (TRAF6) activation and protein kinase B (PKB or also known as Akt) and activation of major MAPKs including ERK, JNK and p38.38 Subsequently, CADPE suppressed downstream expression of nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), nuclear translocation of c-Fos protein and expression of osteoclastic markers, such as TRAP and cathepsin K, possibly through the non-receptor tyrosine kinase c-Src signalling.38 Interestingly, CADPE did not significantly affect the NF-kB signalling pathway and M-CSF-induced proliferation and differentiation of BMMs.

Supplementation of CA in animal models of bone loss yielded heterogeneous findings.48,49,52 This observation might be attributable to oral administration. Folwarczna et al. reported that CA (5 and 50 mg/kg, by stomach tube for 4 weeks) improved the bone mechanical properties by increasing the width of the trabecular metaphysis of the femur and decreasing the transverse growth in endosteal of the femur in OVX rats.48 Folwarczna et al. then demonstrated that CA (10 mg/kg/day; oral administration for 4 weeks) could reduce the width of tibial periosteal and endosteal osteoid compared with untreated OVX rats.52 However, CA did not promote or reduce the resorption of compact bone in the tibia of OVX-induced osteoporotic rats as evidenced by negligible changes of bone mass, bone mineral mass, bone mass/body mass ratio and bone mineral mass/body mass ratio.52 On the other hand, Zych et al. reported that CA at a similar dose (10 mg/kg/day; by stomach tube for 4 weeks) worsened the bone mechanical properties of healthy female Wistar Cmd:(WI)WU rats by decreasing the load of fracture at the femoral neck, decreasing the width of periosteal osteoid in the tibia and decreasing the width of the epiphysis and metaphysis trabecular in the femur compared with the negative control group.49

CAPE is the most extensively studied caffeic acid derivative in animal studies. The beneficial effects on new bone formation and healing upon systemic administration of CAPE had been reported.46,47,53,57 Erdem et al. reported that a low dose of CAPE (10 mol/kg; i.p. injection for 22 days) increased new bone formation and bone strength by increasing maximum torsional fracture momentum and degree of rigidity compared with negative control in rats that underwent unilateral femoral lengthening (osteotomy).53 Similarly, a 30-day i.p. injection of CAPE (10 mol/kg/day) also increased bone healing level in Sprague Dawley rats with cranial critical size bone defect.57 A higher dose of CAPE (10 mmol/kg/day, i.p. for 20 days) also further promoted the RME procedure-induced new bone formation in midpalatal suture of male Sprague Dawley rats.47 Similarly, a longer treatment period of CAPE (10 mmol/kg/day; i.p. injection for 28 days) also significantly promoted bone healing by increasing the total new bone areas in surgical-induced calvarial defects of male Wistar rats compared with the negative control.46 However, localised administration of CAPE (28 days) on surgical-induced calvarial defects by pre-mixing 50 and 100 mmol/kg CAPE solutions with gelatin sponges did not significantly improve the new bone formation.46

Localised and systemic administration of CAPE was reported to be beneficial in reducing osteolysis and bone loss.35,4245,50,5456,58 Ha et al. reported that collagen sponge implant impregnated with 250 g CAPE and RANKL could reduce osteoclastogenesis with significantly lesser TRAP-stained area in mouse calvariae compared with implants with RANKL only.35 Subcutaneous injection of CAPE (1 mg/kg/day for 10 days) reduced the polyethylene particle-induced calvarial osteolysis, surface bone resorption and TRAP-positive cells formation with an increase of bone volume (BV) on LPS-resistant C3H/HEJ female mice.58 However, no significant changes were observed in carboxy-terminal cross-linked type 1 collagen (CTX-1) and osteoclast-associated receptor levels among untreated and CAPE-treated rats with calvarial osteolysis.58

Similarly, Duan et al. reported that lower dose and frequency of CAPE injection (0.5 mg/kg twice a week; i.p. injection for 4 weeks) also increased the BV and trabecular number (Tb.N) due to the decrease of bone osteoclast formation (evidenced by decreased osteoclast number/bone perimeter) in OVX mice.55 Tolba et al. also reported that i.p. injection of CAPE (10 and 20 mol/kg) for 3 weeks increased femur weight and length in rats with dexamethasone-induced bone loss.56 The preservation of skeletal health in their study was associated with an improved antioxidant defence, such as higher levels of glutathione (GSH) and superoxide dismutase (SOD), and the reduction of malondialdehyde (MDA, lipid peroxidation product).56 This event led to an increase of osteoblastogenesis indicated by upregulation of RUNX-2 and ALP (osteoblast marker) levels56 On the other hand, decreased RANKL/osteoprotegerin (OPG) ratio was observed with CAPE treatment, indicating the suppression of osteoclastogenesis, which was further confirmed by lower acid phosphatase level and TRAP activity.56 In another study by Yildiz et al., CAPE (10 mol/kg/day; i.p. injection for 22 days) also increased the spine and femur BMD in rats with EMF-induced bone loss.50 Similarly, Cicek et al. reported a longer treatment of CAPE (10 mol/kg/day; i.p. injection for 28 days) also significantly improved the mechanical strength of cortical bone by increasing the breaking force, bending strength and total fracture energy in rats with EMF-induced bone loss compared with negative control.54

Additionally, a study by Wu et al. treated mice with an OVX-induced bone loss with a moderately high dose of CADPE (10 mg/kg; i.p. injection) every 2 days for 3 months.38 Results showed that CADPE could increase the BV fraction (BV/TV) and Tb.N, as well as decreased trabecular spacing (Tb.Sp) compared with the negative control.38 The improvement in the bone structure was contributed by reduced osteoclast number and eroded surface on the bone.38 Assessment of bone remodelling markers also revealed that serum TRAP5b and CTX-1 levels were reduced in CADPE-treated group compared with the negative control.38

On the other hand, CAPE was effective in reducing periodontitis-related bone loss and osteolysis.4245 CAPE (10 mol/kg/day, i.p. for 14 days) significantly reduced the subgingival ligature placement-induced periodontitis-mediated articular bone loss, histopathological features and severity of periodontal inflammation with lesser polymorphonuclear cells (PMNLs) infiltration in the junctional epithelium and connective tissues among Wistar albino rats.45 CAPE also suppressed the periodontitis-upregulated interleukin (IL)-1, IL-6, IL-10, TNF, MDA levels and the percentage of gingival apoptosis with the parallel restoration of periodontitis-downregulated GSH and glutathione peroxidase (GPx).45 Administration of high-dose CAPE (10 mmol/kg/day; i.p. for 15 days) in streptozotocin (STZ)-induced diabetic male Sprague Dawley rats reduced RANKL-positive osteoclast number, IL-1 levels, oxidative stress index (OSI), alveolar bone loss and histological analysis score in LPS-induced periodontitis. The treated rats also suffered lesser inflammatory reactions, ulcers and hyperemia.42 Similar changes of osteoclast number, IL-1 and OSI were observed in male Sprague Dawley rats with chronic stress and LPS-induced periodontitis treated with CAPE (10 mmol/kg/day, i.p. for 14 days).44 In addition, CAPE also increased the mesial and distal periodontal bone supports (MPBS and DPBS) in these rats.44 The effects of CAPE were sustained with a longer treatment period of CAPE (10 mmol/kg/day, i.p. for 28 days) on male Sprague Dawley rats with LPS-induced periodontitis.43

In contrast to the above findings, Williams et al. reported that subcutaneous injection of CAPE (1 mg/kg; at day 3, 7 and 10) did not reduce paw inflammation or bone loss in CAIA mice.51 Cartilage and bone degradation, as well as TRAP-positive cells on the bone surface and soft tissues, were still apparent in the supplemented CAIA group compared with the normal control.51

This systematic review found that although CA and its derivatives is a potential anti-osteoporosis agent by suppressing the formation of osteoclasts and their bone resorption activity, it worsened bone mechanical properties in some cases. The anti-osteoclastogenesis action of CA and its derivatives was mediated by the antioxidant activities, which blocked RANKL-induced TRAF6/Akt and MAPK signalling, as well as M-CSF/c-Src signalling. In animals, CA and its derivatives (mainly CAPE) prevented bone resorption in rodent calvariae when implanted in situ, facilitated the healing of bone defects, preserved bone structure and improved mechanical strength in osteoporosis models induced by OVX, dexamethasone, osteotomy, LPS-mediated periodontitis and EMF. However, CA did not alter bone resorption in OVX-induced osteoporotic rats and worsened the mechanical properties in normal rats. Additionally, CAPE did not suppress bone loss in rats with CAIA-induced bone loss.

Osteoblasts are bone-forming cells derived from bone marrow mesenchymal stem cells and are responsible for the synthesis, secretion and mineralisation of bone matrix.61 The expression of osteoblast markers was increased following CA or CAPE supplementation, an indication that CA and CAPE stimulated osteoblast proliferation, differentiation and maturation.40,56 Osteoblasts and osteocytes regulate the formation of osteoclasts through RANKL/OPG axis. Osteoblasts and osteocytes synthesise RANKL, which binds to RANK to activate the canonical pathway for osteoclastogenesis. They also secrete OPG, which is a decoy receptor for RANKL to suppress osteoclastogenesis. The production of RANKL is stimulated under conditions such as oestrogen deficiency62 and oxidative stress.63 Osteoclastogenesis can also be stimulated via a non-canonical pathway, for instance, through the binding of TNF with TNF receptor I or II.64 Glucocorticoids are potential modulators of RANKL/OPG axis, whereby dexamethasone is shown to downregulate OPG levels in osteoblasts.65 Tolba et al. showed that the RANKL/OPG level reduced in rats induced with dexamethasone with CAPE treatment.56 Other cellular studies showed that CA and its derivatives suppressed RANKL- and TNF-induced formation of OCLs from haematopoietic cells,3539 indicating that CA and its derivatives suppressed both canonical and non-canonical osteoclastogenesis.

The complex formed by the binding of RANKL to RANK causes the recruitment of the adaptor molecules tumour necrosis factor receptor-associated factors (TRAFs), including TRAF6.66 This event leads to the activation of several downstream signalling pathways, including c-Src/Akt/phosphatidylinositol 3-kinase and MAPKs (ERK/p38/JNK). CADPE was shown to suppress RANKL-induced activation of TRAF6 activation and the subsequent signalling pathways in multiple osteoclast progenitors, such as BMMs,38 RAW264.738 and RAW D cells.39 Sandra and Ketherin suggested that the downregulation of p38 is the key step of CA-mediated osteoclastogenesis.39 Upon activation, p38 initiates osteoclastogenesis by inducing NF-B and NFATc1 expression.67,68 Inhibition of p38 MAPK reduces RANKL (canonical) and TNF-induced (non-canonical) osteoclast formation.69

The NF-B pathway is another signalling pathway downstream of TRAFs critical for osteoclast differentiation and bone reabsorption activity. Upon activation, IKK (consisting of IKK, IKK and IKK) phosphorylates and degrades IB, which enables translocation of NF-B p65/p50 heterodimers into the nucleus to allow transcription of osteoclast-related genes.70 Kwon et al. demonstrated that the anti-osteoclastogenesis effects of CAPE were mediated via the degradation of total IKK, thereby preventing the phosphorylation and degradation of IB and subsequently suppresses the nuclear translation of p65.37 On the other hand, Wu et al. reported that CADPE did not affect phosphorylation or degradation of IB, as well as nuclear translocation, and DNA-binding activity of p65.38 This observation suggests that compared with CAPE, CADPE does not influence the NF-B signalling pathway.

ROS are one of the important secondary signals in the early stages of osteoclast differentiation.71,72 These ROS are mainly produced as superoxide anions by Nox1.73 Blocking of Nox1 ameliorates ROS production and the downstream MAPKs (JNK, p38 and ERK) and NF-B activation74 and subsequently suppresses the osteoclast formation.71 The reduction of Nox 1 and Rac1 expression by CAPE is accompanied by RANKL-downstream signalling, denoting that anti-osteoclastogenesis effects of CAPE are dependent on suppression of Nox1-mediated superoxide anion production. Besides, dexamethasone has been reported to increase the expression of oxidative stress-related genes in human osteoblasts.75 Tolba et al. showed that CAPE increased GSH and SOD but reduced MDA in the bone of the rats exposed to dexamethasone, indicating an improvement of redox status in the skeletal environment.56 Additionally, CAPE also reduced the OSI and bone loss with an improvement of bone support in rats with LPS-induced periodontitis.

NFATc1 is the master regulator of osteoclast-related gene expression, and it is activated by c-Fos and NF-B.76 Ha et al. observed that CAPE inhibited the recruitment of NF-B to NFATc1 promoter, and the combined effect of NF-B inhibition on c-Fos and NFATc1 may have caused CAPE to suppress osteoclastogenesis effectively.35 Holland et al. demonstrated a new fluorinated derivative of CAPE possesses potent anti-osteoclastogenic properties on RAW 264.7 cells by downregulating NFATc1 via suppression of c-Fos and NF-B signalling pathways.77 Besides, this new fluorinated CAPE also exhibits improved stability with a 2-fold higher potency than CAPE.77 On the other hand, although CADPE did not alter NF-B signalling, it still could suppress NFATc1 and other osteoclast-related markers, indicating other mechanisms of suppression could be involved, for instance, c-Src and MAPKs signalling pathways.38

Matrix metalloproteinases (MMPs), including gelatinases (MMP-2 and MMP-9) are examples of zinc-dependent extracellular matrix-degrading enzymes, which actively participate in bone resorption.78 MMPs are expressed as inactive proenzymes or zymogens that can be activated by several mediators including AP-1, NF-B, TNF and TGF.78 Currently, there is no study conducted to investigate the inhibitory effects of CA and CAPE on osteoclastic MMPs activity and its subsequent linkage in bone resorption; interestingly, CA and CAPE were reported to inhibit MMP-9 activity in human hepatocellular carcinoma HEP3B cells.79,80 This observation renders an interesting research gap in osteoclastic MMP inhibition upon CA and its derivatives treatment.

Suppression of osteoclastogenesis by CA or its derivatives have significant therapeutic potential against bone disorders induced by excessive bone resorption. Bone loss after osteotomy is a rapid process that affects both fractured and unfractured bone and may be incompletely reversible.81 CAPE was reported to improve bone formation and mechanical strength of bone in osteotomy.53 Exposure to EMF radiation caused by high-voltage transmission lines and transformers could affect bone health through decreased BMD, serum calcium and ALP level leading to the increase of bone resorption.82 CAPE increased the spine and femur BMD levels50 and increased mechanical strength of bones54 in rats exposed to EMF radiation. Total hip arthroplasty without cement often caused osteolysis induced by polyethylene particles.83 CAPE was shown by Zawawi et al. to prevent calvarial bone resorption in a murine polyethylene particle-induced osteolysis model.58 Therefore, biomaterials impregnated with CA or its derivatives could be adopted to prevent osteolysis in the arthroplasty procedure. CA has been incorporated in chitosan/(3-chloropropyl) trimethoxysilane scaffold for hard-tissue engineering applications and this adopted material exhibits antibacterial and anticancer effects.84 Ucan et al. observed that CAPE increased cranial bone healing in rats with critical size bone defect, suggesting that it could be administered systematically or locally to treat bone fracture/defect healing.57

Similarly, CAPE also effectively reduced the articular bone loss, inflammatory cytokines production and oxidative stress in rats with LPS-mediated periodontitis. Additionally, Wu et al.38 and Duan et al.55 demonstrated that CADPE prevented the ovariectomy-induced bone loss by suppressing osteoclast activity in a mouse model, while Folwarczna et al. showed increased width of trabecular metaphysis in the femur of OVX rats.48 Similarly, Tolba et al. showed improved bone formation and skeletal health in rats with dexamethasone-induced bone loss upon receiving CAPE.56 Additionally, CA and its derivatives may be involved in oestrogen production and signalling. Zych et al. reported that an oral administration of CA (10 mg/kg/day for 4 weeks) significantly restored the serum oestradiol levels in OVX rats.85 Interestingly, CA at 10 and 100 M did not cause any alteration in calcium content in the femoral-diaphyseal and metaphyseal ex vivo culture, suggesting its bone-protecting effect may not involve calcium metabolism and regulation.86 Additionally, CAPE was reported as a selective human oestrogen receptor agonist with the EC50 value of 3.72 M in oestrogen-responsive element transcription.87 A recent in silico study by Zhao et al. suggested potential osteoimmunological effects of CAPE, which may explain its biological activities on both immune and skeletal systems.88 However, the findings from this modelling study requires further validation through in vitro and in vivo models. As oestrogen deficiency due to menopause and glucocorticoids present the most significant cause of primary and secondary osteoporosis globally, CA and its derivatives have the potential to be used as an adjuvant therapy to existing osteoporosis management strategies. The mechanisms of action of CA and its derivatives in osteoclastogenesis have been summarized in Figure 2.

Figure 2 Mechanism of action of caffeic acid and its derivatives.

Abbreviations: , decrease or downregulate; ?, unknown mechanism; Akt, protein kinase B; AP-1, activator protein 1; CA, caffeic acid; CADPE; caffeic acid 3,4-dihydroxy-phenethyl ester; CAPE, caffeic acid phenethyl ester; c-Src, cellular sarcoma tyrosine kinase; ERK1/2, extracellular signal-regulated kinases 1/2; GM-CSF, granulocyte-macrophage colony-stimulating factor; Grb2, growth factor receptor-bound protein 2; IFN-, interferon-gamma; IL, interleukin; IL1R, interleukin-1 receptor; IB, NF-B inhibitor protein; IKK, IB kinase; LPS, lipopolysaccharide; M-CSF, macrophage colony-stimulating factor; M-CSF-R, M-CSF receptor; MAPKs, mitogen-activated protein kinases; NFAT, nuclear factor of activated T cells; NF-B, nuclear factor kappa B; NIK, MAPK kinase kinase 14; Nox1, nicotinamide adenine dinucleotide phosphate oxidase 1; OPG, osteoprotegerin; PI3k, phosphoinositide 3-kinase; Rac1, Ras-related C3 botulinum toxin substrate 1; RANK, receptor activator of NF-B; RANKL, receptor activator of NF-B ligand; ROS, reactive oxygen species; TAK, MAPK kinase kinase 7; TLR4, Toll-like receptor 4; TNF, tumour necrosis factor-alpha; TNFR1/2, TNF receptor 1/2; TRAF2, tumour necrosis factor receptor-associated factor 2; TRAF6; tumour necrosis factor receptor-associated factor 6.

Regardless of the positive effects of CA on bone status, some studies have reported negative effects associated with supplementation of CA and its derivatives. CA supplementation did not affect the bone resorption52 and reduced transverse growth of endosteal in femur48 of rats with OVX-induced osteoporosis. In normal rats, CA supplementation even negatively affected their bone mechanical properties.49 Moreover, CAPE supplementation has been reported to stimulate the synthesis of PGE2,89 which mediates osteoclastogenesis through RANKL stimulation and activation of the NF-B pathway.90 This event will eventually increase TRAP-positive OCLs. Similarly, Williams et al. showed that CAPE did not suppress osteoclastogenesis in rats with CAIA.51

In term of safety, the International Agency for Cancer Research classifies CA as Class 2B (possibly carcinogenic to humans),91 and it was reported to induce renal tubular cell hyperplasia, forestomach hyperplasia, renal cell adenoma and forestomach cancer in rodents.9294 CA has been reported to be non-mutagenic and non-clastogenic.91 Therefore, its carcinogenicity may involve epigenetic modification. Human toxicity and carcinogenicity of CA and its derivatives remain unknown. CA also showed anti-implantation activity in pregnant mice at a median effective dose of 4.26 mg/kg/day.95 Similarly, 5 mg/kg/day and 150 mg/kg of CA in mice demonstrated anti-implantation activity in early pregnancy.96 On the other hand, 0.15 mg/kg/day, 5 mg/kg/day and 150 mg/kg/day of CA for 21 days in mice showed no maternal toxicity, foetal teratogenesis or post-natal effects on pup development and mortality.96 The same experiment stated that the no-observed-adverse-effect level of CA for pregnant female mice was 0.15 mg/kg/day.96 Therefore, high-dose CA should be cautioned in humans, especially pregnant women.

Several common limitations can be identified from the studies reviewed. Most studies did not adopt a positive control to compare against the anti-osteoclastogenesis or anti-osteoporosis effect of CA. Therefore, the therapeutic effects of CA and currently available anti-resorptive therapy cannot be compared. Although osteoblastogenesis and bone formation are also important in bone remodelling, evidence of CA on these processes is limited in the literature. The actions of CA in humans cannot be confirmed due to the lack of human clinical trials. These aspects can be improved in future studies.

The current review also has several limitations. We only considered articles indexed by PubMed, Scopus, Cochrane Library and Web of Science; therefore, non-indexed articles could be overlooked. We only selected articles studying CA or its derivatives as a single compound to understand its mechanism of action properly without other interference, but not a mixture of compounds or natural products rich in CA. CA are present in foods, and interaction with other compounds in the food matrix might alter its absorption, bioavailability and action on the target tissue. Moreover, the heterogeneous findings of CA in bone loss reduction upon oral administration further emphasise these possibilities.

The current preclinical evidence agrees that CA and its derivatives exert promising skeletal protective effects by inhibiting osteoclastogenesis and bone resorption, but literature on bone formation is limited. Notwithstanding that, the skeletal effects of CA and its derivatives in models of normal bone health should be investigated because the limited studies available show undesirable effects. Human clinical trials to validate the skeletal effects of CA are lacking. Therefore, a well-planned clinical trial should be conducted to confirm the potential of CA as an antiresorptive agent. This information is critical for CA and its derivatives to be incorporated as part of the strategies to prevent bone loss.

The researchers are funded by Universiti Kebangsaan Malaysia through Research University Grant (GUP-2020-021). S.O.E. and K.L.P. are post-doctoral researchers funded by Universiti Kebangsaan Malaysia through FPR-1 and RGA-1 grants.

The authors report no conflicts of interest in this work.

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