A Review of the Anti-Inflammatory Effects of Rosmarinic Acid on Inflammatory Diseases

1Inflammation is an integral part of innate immunity. It includes the body’s removal of harmful signals and the initiation of protective responses and tissue healing processes. The inflammatory response is an essential physiological process that maintains the homeostasis of the immune system. Inflammation is divided into acute inflammation and chronic inflammation, both of them have a significant impact on human health. The acute inflammatory process is characterized by the rapid recruitment of granulocytes (i.e., neutrophils, eosinophils, and basophils) into the body. Disorders of sustained inflammatory stimuli or ablation stages can lead to chronic inflammation, including rheumatoid arthritis, systemic lupus erythematosus, silicosis, atherosclerosis, and inflammatory bowel disease. Macrophages and T lymphocytes are the primary immune cells involved in chronic inflammation and generate cytokines and enzymes that result in tissue destruction, manifested as tissue fibrosis. Inadequate inflammation can cause continuous infection of pathogens, while excessive inflammation can lead to chronic or systemic inflammatory diseases.

2The pattern-recognition receptors (PRRs) on immune cells sense “danger” from protein-associated molecular patterns (PAMPs) linked to a pathogen or from danger-associated molecular patterns (DAMPs) triggered by a large number of endogenous stress signals from the host. The interleukin 1 (IL-1) cytokine family (IL-1α, IL-1β, IL-18, IL-33, IL-36α, IL-36β, and IL-36γ) acts as DAMPs and stimulates sterile inflammation caused by necrosis and increases the inflammation with infection-related tissue damage. The cyclooxygenase (COX) and 5-lipoxygenase (5-LOX) pathways, which are metabolized by arachidonic acid (AA), produce highly pro-inflammatory lipid mediators that are involved in the classic signs of inflammation, including redness, fever, pain, swelling, and loss of function, which are designed to eliminate harmful and harmful stimuli. A family of important receptors that stimulate inflammation includes Toll-like receptors (TLRs). TLR4 signaling mediated by the linker MyD88 activates the transcription factor nuclear factor kappa B (NF-κB), thereby inducing gene expression of pro-inflammatory factors like tumor necrosis factor (TNF), IL-6, and IL-1β. These proteins play an important role in inflammatory diseases.

3Rosmarinic acid (RosA, Figure 1 ) is an ester of caffeic acid and 3, 4-dihydroxyphenyl lactic acid. It is usually discovered in species of the Boraginaceae family and the subfamily Nepetoideae of the Lamiaceae family. RosA’s presence in medicinal plants, herbs, and spices is linked to beneficial and health-promoting effects. In plants, RosA is considered to be a cumulative defense compound, while in human, RosA has many biological activities, including antiviral, antibacterial, antioxidant, antimutagenic, and anti-inflammatory activities. Many in vitro and in vivo studies have reported the anti-inflammatory effects of RosA in inflammatory diseases. This review systematically describes the therapeutic potential of RosA for inflammatory diseases and discusses its possible mechanisms.

4Arthritis is an inflammatory disease that involves damage to one or more joints. It has more than one hundred types, the most common of which are osteoarthritis and rheumatoid arthritis. Osteoarthritis (OA) is a progressive degenerative disease characterized by inflammation of the synovial, abrasion of the cartilage surface, subchondral sclerosis, and osteophyte generation, leading to loss of pain and movement. OA has long been considered as a degenerative disease of cartilage and is the only result of any process that causes increased pressure on a particular joint or the fragility of the cartilage matrix. The pathogenesis of OA is complicated and not fully understood, but an increasing number of researches have indicated that inflammation exerts a key role in the pathogenesis of OA. Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease described as extensive infiltration and activation of inflammatory and mesenchymal cells, synovial cell proliferation, neovascularization, and occasional cartilage and bone destruction. RA is usually treated with nonsteroidal anti-inflammatory drugs (NSAIDs) and disease-resistant antirheumatic drugs (DMARDs), but they have adverse reactions, potential toxicity, and high cost, thus limiting their use. Presently, the field of arthritis study is rapidly developing in the direction of herbal research to find safe and effective drugs.

5OA is a multifactorial disease described primarily as the destruction of articular cartilage. Collagen 2 (COL2) and aggrecan (ACAN) are the main components of cartilage extracellular matrix (ECM). The depletion of ACAN and COL2 results in the degradation of cartilage in OA. A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4 and ADAMTS-5 are responsible for ACAN depletion in osteoarthritic cartilage. The inflammatory cytokine interleukin-1-beta (IL-1β) also exerts an important role in ECM degradation. An effect of RosA on OA has been reported in rat chondrocytes. In this experiment, chondrocytes were isolated from rat cartilage and incubated with RosA in the presence of IL-1β. RosA was found to inhibit IL-6 secretion and inhibit the gene and protein levels of ADAMTS-4 and ADAMTS-5. Moreover, RosA also inhibited the ACAN and COL2 gene expression induced by IL-1β. The results indicate that RosA can degrade ECM in OA and may have a therapeutic effect on OA. Another study found that drinking high-RosA spearmint tea can be a potential complementary treatment for OA pain relief. The study indicated that taking high RosA tea for 16 weeks per day could significantly improve stiffness and physical disability scores in adults with knee OA and could significantly reduce pain.

6T cells exert a crucial role in the development and progression of RA, and the apoptosis of potentially pathogenic T cells is considered to be an important therapeutic option. The study found that RosA can induce apoptosis in activated T cell subsets in RA patients by the mitochondrial pathway. The results showed that RosA induced CD3+CD25+ activated T cell apoptosis in 57.1% of RA patients in a dose-dependent manner, and RosA showed stronger apoptotic activity against the CD4+CD45RO+ effector T cell subset than the CD4+CD45RA+ naive T cell subset. In addition, RosA inhibited MMP destruction, reduced Bcl-2 expression, and induced Cyt c release from mitochondria to the cytoplasm. These results supported the view that RosA induced the apoptosis of activated T cells from RA patients through the mitochondrial pathway. Another experiment found that RosA can improve arthritis symptoms in the mouse model of collagen-induced arthritis (CIA). RosA could significantly reduce the arthritis index and the number of affected paws. Histopathological images indicated that RosA inhibits synovitis, and synovial tissue of RosA-treated mice indicated a great reduce in the frequency of COX-2-expressing cells. Therefore, the administration of RosA in a clinical setting provided therapeutic effects in the treatment of RA. Furthermore, it has been reported that RosA extracted from pomegranate peel showed significant anti-arthritic potential in arthritis induced by Freund’s complete adjuvant (FCA).

7Inflammatory bowel disease (IBD) is a chronic recurrent intestinal inflammation that includes Crohn’s disease (CD) and ulcerative colitis (UC). IBD is thought to be caused by an abnormal and sustained immune response to microorganisms in the intestine caused by the genetic susceptibility of the individual. UC is a chronic disease described as diffuse inflammation of the colon and rectal mucosa. The typical clinical symptom of UC is bloody diarrhea. In contrast to CD, the inflammation of UC is limited to the colonic mucosa and the affected colon. In recent decades, the incidence of IBD has elevated in some areas of the world, especially in developing countries. Consequently, the need for effective and safe natural compounds is increasing for IBD patients.

8Studies have found that RosA can improve dextran sulfate sodium (DSS)-induced colitis. In this study, RosA markedly decreased the disease activity index (DAI) and inhibited DSS-stimulated colon length shortening and splenomegaly in mice. RosA significantly improved inflammatory cell infiltration in the DSS-induced colitis model. RosA inhibited the induction of COX-2 and iNOS expression and the generation of IL-1β, IL-6, and IL-22. Immunohistochemistry analysis indicated that RosA significantly inhibited the expression of NF-κBp65 and pSTAT3 and their transport to the nucleus in the inflamed mucosa. Western blot analysis indicated that RosA inhibited the increase in NF-κB and STAT3-associated proteins in the colon of colitis mice. Another article showed the effect of RosA and black rice anthocyanin-rich extract (BRAE) alone or in combination on colitis induced by DSS. At the appropriate dose, the combination of BRAE and RosA significantly reduced the DAI and inhibited the NO content, serum IL-1β and TNF-α expressions as well as IL-1β and TNF-α mRNA expression. Furthermore, the combination of BRAE and RosA exerted its anti-inflammatory activity through decreasing myeloperoxidase (MPO) and NO expressions as well as the mRNA levels of IL-6, IL-1β, and iNOS.

9Some researchers have modified RosA to reduce its hydrophilicity, effectively inhibiting hypoxia-inducible factor-prolyl hydroxylase-2 (HPH) and causing the activation of hypoxia-inducible factor (HIF)-1 to exert anti-TNBS-induced colitis effects in rats.

10Atopic dermatitis (AD), also known as atopic eczema, is a chronic recurrent inflammatory skin disease. The clinical features of the disease are the exacerbation and relief of eczema skin, accompanied by inflammation, itching and flaking, desquamation, dry skin and susceptibility to skin bacteria, and mold infections. The pathophysiology of AD is intricate and multifactorial, including barrier dysfunction, cell-mediated immune response changes, IgE-mediated hypersensitivity, and environmental factors. Currently, a lot of researchers are energetically trying to develop therapeutic drugs with great anti-inflammatory effects and few side reactions.

11T cells play an important role in AD’s pathogenesis. AD is a bipolar inflammatory skin disease that can be thought of as having two distinct stages. In the acute phase, AD skin lesions are infiltrated by CD4+ T cells, which chiefly secrete the Th2 cytokines IL-4, IL-5, and IL-13. However, in the chronic phase, Th1 cells secrete interferon-γ (IFN-γ). Some researchers have reported that RosA can alleviate 2, 4-dinitrofluorobenzene (DNFB)-induced AD in NC/Nga mice and have revealed the mechanism of its involvement. The authors showed in this report that RosA could significantly inhibit the generation of IFN-γ and IL-4 by activated CD4+ T cells. RosA significantly inhibited the development of skin lesions and the thickness of the ear and reduced serum total IgE levels. RosA suppressed the infiltration of CD4+ T cells, CD8+ T cells, and mast cells into DNFB-induced skin lesions in NC/Nga mice. The above results indicated that RosA inhibited the development of AD-like dermatitis in DNFB-stimulated NC/Nga mice by decreasing the production of IFN-γ and IL-4 via activated T cells and the level of total serum IgE. Furthermore, RosA had the effect of improving AD in clinical studies. This effect was observed by the topical application of a RosA (0.3%) emulsion twice daily to the elbow flexion of 21 patients with mild AD. It was found that RosA could significantly reduce the statistically significant Severity Scoring of Atopic Dermatitis (SCORAD) score and reduce pruritus and transepidermal water loss (TEWL). In the patch test, no patient responded, indicating that RosA could be safely applied to human skin. These consequences indicate that RosA was able to be used as a therapeutic agent for AD. The author also suggested at the end of the article that RosA could improve AD symptoms by inhibiting IKK-β.

12Asthma is a common chronic airway disease described as a complicated interaction between airway obstruction, bronchial hyperresponsiveness (BHR), and airway inflammation. Lots of cells and cellular components exert a role in this process, especially mast cells, eosinophils, T lymphocytes, macrophages, neutrophils, and epithelial cells.

13RosA has been found to inhibit ovalbumin (Ova)-stimulated airway inflammation in a mouse model of asthma. The mitogen-activated protein kinases (MAPKs) regulate the synthesis and secretion of pro-inflammatory mediators during the inflammatory process, and their family contains three different stress-activated protein kinase pathways: p38, JNK, and ERK. Constant activation of NF-κB has been found in allergic asthma, and the suppression of the NF-κB pathway attenuates asthma induced by OVA. In this experiment, RosA significantly inhibited the increase in inflammatory cells and Th2 cytokines in bronchoalveolar lavage fluid (BALF), decreased total IgE and Ova-specific IgE concentrations, and significantly improved airway hyperresponsiveness (AHR). Histological analysis showed that RosA significantly reduced the number of inflammatory cells and excessive mucus secretion in the airways. Pretreatment with RosA led to a significant decrease in AMCase, CCL11, CCR3, Ym2, and E-selectin mRNA levels in lung tissue and a significant regulation of NF-κB and MAPK activation. Therefore, this study suggested that RosA may be a promising candidate for asthma treatment. The protective effect of RosA may be through the inhibition of ERK, JNK, and p38 phosphorylation and the activation of NF-κB.

14Another experiment evaluated the immunomodulatory effects of Ocimum gratissimum (Og) and RosA in a mouse respiratory allergy model caused by Blomia tropicalis (Bt) mite. This experiment found that RosA significantly inhibited the number of total inflammatory cells and eosinophils. Lung histopathology images showed that treatment with RosA reduced inflammatory cell infiltration around the bronchi and perivascular areas and inhibited mucus secretion in lung tissue. In addition, RosA could reduce the level of IL-4. The results of this experiment strongly supported the potential use of RosA as an anti-inflammatory drug for the treatment of allergic asthma. In addition, an article reported the effect of rosmarinus officinalis extracts on asthmatic subjected resistant to routine treatments. It was found that the Asthma Control Test (ACT) score displayed marked improvement after treatment with rosmarinus officinalis. Clinical evaluation revealed that cough, sputum generation, and wheezing were markedly improved in the rosmarinus officinalis group. At the same time, Exhaled Nitric Oxide (FENO) was significantly reduced after treatment with rosmarinus officinalis. The author finally concluded that rosmarinus officinalis and RosA had the potential to treat asthma.

15Allergic rhinitis (AR) is a symptomatic nose inflammation caused by immunoglobulin E (IgE)-mediated endocardial inflammation. AR is one of the most common chronic diseases in the world, influencing 10 to 20% of the population. Pollen is the biggest reported reason of seasonal allergic rhinoconjunctivitis (SAR). Prospective researches indicate that SAR may be a predisposing factor for the development of asthma.

16RosA improved inflammation in the OVA-induced AR animal model. Administrate RosA could decrease elevated IgE levels in the serum, spleen, and nasal mucosa of OVA-sensitized mice. After treatment with RosA, the level of histamine in the serum was also significantly reduced. RosA suppressed the protein expressions and mRNA expressions of IL-1β, IL-6, and TNF-α in nasal mucosa or spleen as well. The article also found that the increase in mast cell and eosinophil infiltration caused by OVA sensitization was reduced in the drug-administered group. Furthermore, COX-2 expression and caspase-1 activity can be prevented by administering RosA in nasal mucosa tissue. In activated human mast cells, RosA suppressed the activation of NF-κB and caspase-1. The above consequences demonstrated the therapeutic potential of RosA for allergic rhinitis and allergic rhinoconjunctivitis.

17Some researchers have studied whether oral RosA is effective in patients with SAR through clinical trials, and animal experiments have estimated the anti-inflammatory mechanism of RosA in the ear edema model. In clinical trials, the addition of RosA significantly reduced the rate of remission of each symptom compared with placebo. RosA markedly reduced the number of neutrophils and eosinophils in nasal lavage fluid as well. In the 12-tetradecanoylphorbol 13-acetate (TPA)-stimulated mouse ear edema model, the up-regulation of ICAM-1, VCAM-1 cyclooxygenase-2 (COX-2), keratinocyte chemoattractant (KC), and Macrophage inflammatory protein-2 (MIP-2) through TPA was significantly decreased by pretreatment with RosA. RosA can reduce neutrophil infiltration, as demonstrated by histological examination with hematoxylin-eosin staining. Another article also reported that RosA had a therapeutic effect on patients with SAR. Compared to placebo supplementation, supplementation with RosA-enriched Perilla frutescens extract led to a marked augment in responder rates for itchy nose, watery eyes, itchy eyes, and total symptoms. RosA could effectively treat mild SAR at least in part by inhibiting the infiltration of polymorphonuclear leukocytes (PMNLs) into the nostrils.

18Periodontal disease (PD) is thought to be a multifactorial disease caused by pathogenic infections, promoted by inflammation, and immune responses to bacteria, and altered by different environmental and genetic factors. The complex composition and organization of the periodontal ligament may be affected by damage to the steady-state equilibrium between the oral microbiome and the host, which may cause two main illnesses: gingivitis and periodontitis. Gingivitis is caused by microbial plaques that accumulate at or near the gingival sulcus. It was determined that gingivitis was mainly caused by B lymphocytes and plasma cells. Gingivitis is thought to be an early periodontal disease that may or may not develop into periodontitis. Periodontitis is a chronic inflammatory disease of dental support tissue with high incidence and alternating relief and acute exacerbation. Periodontitis is described as persistent leukocyte infiltration, possibly regulated by the production of topical chemokine, but its pathogenesis has not been fully elucidated.

19An article has reported the effects of the topical anti-inflammatory drugs ebselen and RosA on plaque-induced gingivitis progression in a rhesus monkey model. Nonparametric indicators (G.I.) and plaque accumulation index (P.I.) were used to ascertain the extent of gingival inflammation and plaque accumulation. G.I. ranges from 0 (without erythema or edema) to 4 (serious erythema or edema, spontaneous bleeding, and ulceration). P.I. ranges from 0 (no plaque) to 4 (tooth completely covered by plaque). In this study, animals treated with RosA and ebselen showed lower G.I. and P.I. levels than controls. Therefore, the article drew a conclusion that, at least in the short term, RosA and ebselen were effective in decreasing gingival inflammation and plaque buildup when using a skin graft macaque model.

20In addition, researchers have detected the effects of Prunella vulgaris L. (PVE) and its composition RosA on oxidative damage and inflammation in human gingival fibroblasts induced by LPS. The pathogenesis of periodontitis is related to the imbalance of homeostasis between reactive oxygen species (ROS) and antioxidant defense systems. The authors pointed out that PVE and RosA reduced the production of ROS, the consumption of intracellular glutathione (GSH) and lipid peroxidation in LPS-treated cells. PVE and RosA not only inhibited the upregulation of IL-1β, IL-6, and TNF-α induced by LPS, but also inhibited the expression of iNOS. These results indicated that PVE and RosA may slow the progression of periodontitis by reducing the inflammatory response and the production of oxidative mediators in gingival fibroblasts.

21Acute pancreatitis (AP) is an inflammatory disease described as acute inflammation and necrosis of the pancreatic parenchyma. AP is considered to be a topical inflammatory response including premature intracellular activation of digestive enzymes in acinar cells, resulting in tissue self-digestion and possibly involving distant organs. Secretory acinar cells are considered to release chemokines and cytokines as well, which recruit white blood cells and trigger an inflammatory response that causes pancreatic edema and neutrophil accumulation. The prognosis of AP patients depends largely on the incidence of organ failure and infected pancreatic necrosis. Despite the increasing incidence, there are currently no drugs to alleviate the symptoms of the disease and its course.

22RosA may have a protective effect on sodium taurocholate (NaTC)-induced AD. The pathology of NaTC-induced rat AP is very similar to that of severe acute pancreatitis (SAP) in humans, characterized by rapid onset of necrosis and inflammation of the pancreas and/or peripancreatic tissue. Inflammatory responses, pro-inflammatory cytokines like IL-1β, IL-6, and TNF-α, and the activation of NF-κB exert a crucial role in AP. The consequences indicated that RosA pretreatment markedly ameliorated pathological change in the pancreas; decreased serum amylase and lipase activity; decreased pancreatic myeloperoxidase activity; decreased systemic and pancreatic leukocyte IL-1β, IL-6, and TNF-α expression; and suppressed NF-κB translocation in the pancreas. RosA appeared to reduce the damage to AD caused by NaTC and reduced the release of inflammatory cytokines via suppressing the activation of NF-κB.

23Mastitis is a breast inflammation which is usually caused by a bacterial infection and is able to influence any mammal that is lactating. Human epidemiological studies have found that more than one-third of all lactating women develop mastitis, and the clinical manifestations of this illness are the main reason for mothers to stop breastfeeding. The development of mastitis is related to the extent to which the breast is exposed to bacterial pathogens. Multiple Gram-positive and Gram-negative pathogens lead to mastitis. Gram-negative bacterial lipopolysaccharide (LPS) is considered to be an important factor in establishing animal models of inflammation.

24Recently, an article reported the anti-inflammatory effect of RosA on LPS-induced mouse mastitis. This experiment found that RosA treatment significantly improved mammary gland structural damage and decreased myeloperoxidase activity. ELISA and qPCR consequences showed that RosA reduced the level of TNF-α, IL-1β, and IL-6 in tissues and mMECs in a dose-dependent manner. TLRs are a vital class of pathogen recognition receptors. TLR4 is the most characteristic member of the TLR family and exerts a key role in the innate immune response to LPS infection. A developing number of studies have shown that TLR4 can bring the production of inflammatory cytokines and regulate the activation of the NF-κB signaling pathway. RosA dose-dependently decreased TLR4 level in HEK293-mTLR4/mMD-2 cells, suggesting that RosA can interdict the inflammatory response by straight targeting TLR4. The levels of the downstream signaling factors MyD88, IRAK1, TRAF6, and IKKβ of the TLR4 pathway were also significantly reduced. Furthermore, the administration of RosA significantly suppressed the phosphorylation of IκB and the activation of p65. The DNA binding activity assay further confirmed the similar suppression of the nuclear translocation of NF-κB via RosA. Therefore, RosA can attenuate LPS-induced mastitis through suppressing the TLR4/MyD88/NF-κB signaling pathway.

25In addition, studies have found that RosA has been reported in Japanese encephalitis and neuritis. RosA reduced the mortality of murine infected with Japanese encephalitis virus (JEV). The viral load and pro-inflammatory cytokine levels of JEV-infected animals receiving RosA were significantly reduced 8 to 9 days after infection compared to the levels of untreated infected mice. RosA greatly decreased the level of TLR4 and CD14 and the activation of NF-κB and NLRP3 inflammatory bodies, which is associated with anti-neuroinflammation.

26In addition to its therapeutic effects in inflammatory diseases, RosA also exerts an anti-inflammatory effect on other diseases. The anti-inflammatory effects of RosA on different disease models are presented in Table 1.

27Inflammation is an acute reaction to infection and tissue lesion to prevent damage to the body. Excessive inflammation can lead to chronic or systemic inflammatory diseases. In the past few decades, the prevalence of inflammatory diseases has been on the rise, especially in developed countries. Rosmarinic acid is a class of aqueous phenolic compounds. Many reports have demonstrated that RosA has an important role in treating inflammatory diseases through multiple mechanisms, and RosA exerts anti-inflammatory effects to treat various diseases. Of all retrieved articles, we found that there was less clinical data of RosA in inflammatory diseases. Therefore, we mainly outlined the therapeutic potential of RosA in inflammatory diseases and its possible mechanisms in preclinical research. We hope that this review can obtain a reference for the future treatment of inflammatory disorders.

28All the authors have written, reviewed the manuscript and agreed the ultimate version.

29This work was supported by Liaoning provincial department of education project (LZ2019013) and Special Program of Talents Development for Excellent Youth Scholars in Tianjin (TJTZJH-QNBJRC-2-7).

30SS is employed by Tianjin Institute of Pharmaceutical Research New Drug Evaluation Co Ltd.

31The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.