INTRODUCTION
Bronchial asthma is a chronic respiratory disease characterized by reversible airflow limitation, including airway hyperresponsiveness, airway inflammation and airway remodeling. At present, bronchial asthma can be divided into T helper 2 cell (Th2)-high asthma and Th2-low asthma according to the status of Th2 inflammation. In Th2-high asthma, allergens stimulate Th2 cells to secrete IL-5, IL-4 and IL-13, and activate B cells. B cells produce IgE and bind to high affinity immunoglobulin epsilon receptor (FcεRI) of mast cells, which induces mast cells to release mediators such as leukotrienes (LT), histamine, and interleukin (ILs). In low Th2 asthma, poor response to glucocorticoid is a common feature, and IL-17 is considered to play an important role in this process [1].
Fatty acids are usually classified according to the length of their fat chains. Short chain fatty acids (SCFAs) are considered to be fatty acids with 6 or less carbon chains. Medium chain fatty acids (MCFAs) have 7–12 carbon chains. Long chain fatty acids have 13–21 carbon chains. Fatty acids have been shown to regulate gene expression in epigenetics by enhancing histone acetylation [2]. Fatty acids can also function through an olfactory receptor [3]. Fatty acids are ligands of FFARs. GPR40, GPR41, GPR43, GPR120 and GPR84 have been identified as FFARs. FFARs can affect the occurrence and progression of asthma through β-arrestins, ras homolog family member A (RhoA)/Rho associated coiled-coil containing protein kinase (ROCK1), phospholipase C (PLC)/inositol-1,4,5-trisphosphate (IP3), mitogen activated kinase-like protein (MAPK), AMP-activated protein kinase (AMPK) and other pathways.
GPR40
G protein-coupled receptor 40 (GPR40) is also known as free fatty acid receptor 1 (FFAR1). It can be activated by medium- and long-chain saturated and unsaturated fatty acids and is expressed in the pancreas, brain, heart, liver, kidney and airway [4]. Mizuta et al. found that under the stimulation of agonists, FFAR1 couples with Gq and releases calcium ions from the sarcoplasmic reticulum into the cytoplasm through the PLC/IP3 pathway, which leads to airway smooth muscle contraction. They also observed other effects of FFAR1 activation, including increasing the ratio of F/G actin, enhancing the contraction effect of acetylcholine (ACh), and weakening the relaxation effect of isoproterenol on airway smooth muscle [5]. Lin et al. found that after treatment with the GPR40 antagonist DC260126, the levels of IL-4, IL-5, IL-13, IL-1β and tumor necrosis factor-α (TNF-α) in the serum of obese asthmatic mice were decreased, the proliferation of goblet cells in the airway epithelium was reduced, and smooth muscle cell proliferation was inhibited. After treatment with DC260126, the airway resistance stimulated by acetylcholine was decreased and the expression of RhoA and ROCK1 in mouse lung was decreased. It has been suggested that GPR40 may promote the contraction of airway smooth muscle through the RhoA/ROCK1 signaling pathway [6, 7]. These studies suggest that activating GPR40 may exacerbate airway hyperresponsiveness in asthma. In other studies, activation of FFAR1 can inhibit the contraction of human airway smooth muscle (HASM). The agonist TAK875 of FFAR1 could attenuate histamine-induced HASM contraction and myosin light-chain kinase (MLC) phosphorylation. This may be related to the biased excitation of FFAR1 [8]. Mancini et al. found that the FFAR1 agonist TAK-875 had a significant effect on the recruitment of β-arrestins, which was more effective than palmitate or oleate. Compared with palmitate or oleate, TAK-875 has a weaker activation effect on Gq. TAK-875 recruits β-arrestin through Gq-dependent and Gq-independent mechanisms [9].
Airway smooth muscle hyperplasia is one of the important characteristics of airway remodeling. A study found that FFAR1 promotes the proliferation of airway smooth muscle cells through the MEK/extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3K)/Akt kinase (Akt) signaling pathways. FFAR1 can also inhibit cyclic AMP (cAMP)/protein kinase A (PKA) through Gi coupling to reduce the inhibition of Raf-1 proto-oncogene and serine/threonine kinase (c-Raf)/ERK to promote airway smooth muscle cell proliferation [10].
GPR40 may promote airway repair. Gras et al. found that activation of GPR40 can stimulate the proliferation of human bronchial epithelial cells [11]. Another study found that in Calu-3 cells, activating PLC after activating GPR40 increased the intracellular calcium signal, and then activated calcium/calmodulin dependent protein kinase kinase 2 (CAMKK2) and AMPK. Activating AMPK promotes tight junction assembly. Similar results were observed in human bronchial epithelial cells (HBE) [12]. These results suggest that activation of GPR40 may reduce the damage of the epithelial barrier in asthma. Zhao et al. found that pulmonary GPR40 deficiency in bleomycin-induced pulmonary fibrosis (PF) model mice aggravated pulmonary fibrosis and pulmonary inflammation. Vincamine, a GPR40 agonist, inhibits the deposition of extracellular matrix in the lung via the GPR40/β arrestin2/ SMAD family member 3 (Smad3) pathway. Vincamine can also inhibit pulmonary inflammation in PF mice through the GPR40/nuclear factor of κ light polypeptide gene enhancer in B cells 1 (NF-κB)/NLR family pyrin domain containing 3 (NLRP3) pathway. These observations suggest that activation of GPR40 receptor may inhibit pulmonary fibrosis and inflammation caused by asthma [13].
GPR120
G protein-coupled receptor 120 (GPR120), also known as free fatty acid receptor 4 (FFAR4), can be activated by long-chain fatty acids and is expressed in adipose tissue, intestines, lungs, pituitary gland, macrophages, eosinophils, etc. Prihandoko et al. found that stimulation of FFAR4 can be coupled with Gq, inducing intracellular calcium influx through the phospholipase C/phosphoinositol pathway but not causing HASM contraction. The treatment of FFAR4 agonists can alleviate the airway resistance caused by carbachol. In the smoke model, long-term treatment of the agonist TUG-891 can alleviate airway inflammation and nebulization of TUG-891 before stimulation with bronchoconstrictors can alleviate airway hyperresponsiveness. In mice sensitized to house dust mites (HDMs), treatment with TUG-891 can also alleviate airway hyperresponsiveness [14].
In the ovalbumin (OVA)-induced allergic asthma mouse, stimulation of FFAR4 could inhibit mast cell degranulation and reduce IL-4, IL-5, IL-13, interferon-γ (IFN-γ), IL-17A and mucin production in lung tissue [15]. Another study also found that activating FFAR4 can inhibit the release of inflammatory mediators by mast cells. After treatment with the FFAR4 antagonist AH7614, the inhibitory effect of eicosapentaenoic acid (EPA) on the degranulation of human mast cells was weakened [16]. In normal human bronchial epithelial (NHBE) cells, 17,18-EpETE (a GPR120 agonist) inhibits TNF-α induced production of IL-6 and IL-8 [17]. FFAR4 can exert anti-inflammatory effects through the β-arrestin pathway. In lipopolysaccharide (LPS) stimulated RAW 264.7 cells, after internalization of docosahexaenoic acid (DHA) stimulated GPR120/β-arrestin2 complex, β-arrestin2 can bind to TGF-β activated kinase 1 (TAB1), blocking the binding of TAB1 to TGF-β activated kinase 1 (TAK1), inhibiting TAK1 activation and downstream signal transduction to I-kappaB kinase beta (IKKβ)/NF-κB and c-Jun N-terminal kinase (JNK)/activator protein-1 (AP1) systems, thereby inhibiting Toll-like receptors (TLR) and TNF-α pro-inflammatory signaling pathways [18].
FFAR4 also plays an important role in airway repair. Sveiven et al. found significant epithelial dysplasia in lung tissue of FFAR4 knockout mice exposed to extracts of dust (DE). The yes1 associated transcriptional regulator (YAP) in the lungs of FFAR4 knockout mice was significantly reduced, suggesting that FFAR4 may regulate YAP in some way to maintain lung epithelial homeostasis [19]. Moonwiriyakit et al. found that FFAR4 can reduce mucin production, airway fibrosis, and epithelial barrier disruption. In human bronchial epithelial cells treated with IL-4 and IL-13, the GPR120 agonist GSK137647 can reduce the production of fibronectin, actin α2 (ACTA2), and airway mucin. GPR120 agonists also inhibit the downregulation of zonula occludens-1 (ZO-1) protein by IL-4 and IL-13. Stimulating GPR120 can achieve these effects by weakening the phosphorylation of signal transducer and activator of transcription 6 (STAT6) and Akt [20]. In a naphthalene-induced acute airway injury mouse after treatment with Omacor, it was found that activating FFAR4 could accelerate the repair of airway epithelium, while Omacor treatment in FFAR4 knockout mice could not accelerate the repair of airway injury. FFAR4 can promote club cell proliferation in in vitro experiments [21]. The lack of secretion from club cells in club cell secretory protein-16 (CC16) exacerbates airway hyperresponsiveness and airway remodeling [22]. These observations suggest that activating GPR120 may contribute to the repair of airway injury.
GPR41
G protein-coupled receptors 41 (GPR41), also known as free fatty acid receptor 3 (FFAR3), can be activated by short chain fatty acids. GPR41 is expressed in cells such as the ileum, colon, adipose tissue, monocytes, neutrophils and other tissues. In the airway, FFAR3 expression is found in bronchial epithelial cell lines, human lung fibroblasts, and airway smooth muscle cells. In human airway smooth muscle cells, stimulation of FFAR3 receptors with propionic acid via Gi coupling can activate PLC-β and lead to the release of Ca2+ in sarcoplasmic reticulum (SR), which does not alter the tension of human airway smooth muscle but enhances ACh-induced airway smooth muscle contraction. The FFAR3 receptor also inhibits the synthesis of cyclic AMP [23]. Cyclic AMP can inhibit airway smooth muscle contraction. The production of cyclic AMP can induce activation of PKA, which can inhibit Gq and PLC activation to inhibit intracellular calcium mobilization [24]. Cyclic AMP can also inhibit MLC phosphorylation and RhoA activity through exchange of protein directly activated by cAMP (Epac), thereby inhibiting smooth muscle contraction [25]. The FFAR3 receptor may inhibit the relaxation of airway smooth muscle cells by inhibiting the synthesis of cyclic AMP.
It has been shown that activation of FFAR3 with propionate in human airway fibroblasts and smooth muscle promotes TNF-α-induced IL-6 and C-X-C motif chemokine ligand 8 (CXCL8) expression through the p38 MAPK pathway [26]. FFAR3 has also been observed to have anti-inflammatory effects. The expression of IL-4, IL-5, and IL-17A decreased in the lung tissue of mice treated with propionate. Wild-type, GPR41-deficient, and GPR43-deficient mice were exposed to indoor dust mite extracts and treatment with propionic acid. It was observed that both wild-type and GPR43-deficient mice showed reduced airway inflammation, but not GPR41-deficient mice. It is considered that propionate exerts anti-inflammatory effects through GPR41 [27]. Another study also found that the FFAR3 agonist AR420626 can inhibit the expression of IL-4, IL-13, IFN, and IL-17A in OVA group mice [28].
In pulmonary fibroblasts, GPR41 couples with Gi and promotes the activation of Smad2/3 and ERK1/2. Knocking out GPR41 can alleviate pulmonary fibrosis in mice [29]. This suggests that GPR41 may promote airway remodeling in asthma.
GPR43
G protein-coupled receptors43 (GPR43), also known as free fatty acid receptor 2 (FFAR2), can be activated by short chain fatty acids. GPR43 is expressed in the appendix, small intestine, spleen, bone marrow and other tissues. In the lung, FFAR2 expression is found in bronchial epithelial cells and human lung fibroblasts. In acute allergic airway inflammation model mice, GPR43 knockout mice showed more severe inflammation compared to wild-type mice [30]. Another study found that, in OVA asthmatic mice treated with FFAR2 agonist 4-CMTB, the expression of IL-4, IL-5 and IL-13 in bronchoalveolar lavage fluid (BALF) was inhibited, the production of mucin was reduced, and the degranulation of mast cells was inhibited [31]. In other inflammation models, activating GPR43 exhibits inhibitory effects on the inflammatory reaction. Xu et al. found that overexpression of GPR43 significantly alleviated the infiltration of inflammatory cells in the lungs of acute lung injury (ALI) model mice. Also the expression levels of inflammatory factors such as TNF-α, IL-1β and IL-6 decreased. In alveolar type II epithelial cells with GPR43 overexpression stimulated by LPS, the apoptosis rate and the expression of inflammatory factors were reduced. It alleviates the inflammatory response and reduces apoptosis, which is related to GPR43 through reducing the phosphorylation levels of JNK and the ETS transcription factor ELK1 (ELK1) [32]. GPR43 can bind with β-arrestins, which can inhibit NF-κB activity, thereby inhibiting production of the inflammatory factors IL-1β and IL-6 [33]. These results suggest that GPR43 may alleviate the inflammatory response in asthma. Acetate binding to GPR43 induces activation of PLCβ/IP3 via activating Gq and then reducing calcium icon signaling mobilization. Soluble adenylate cyclase (sAC) can be activated by calcium to convert ATP into cyclic AMP, with subsequent activation of protein kinase A. Activation of PKA promotes ubiquitination of NLRP3 inflammasomes, ultimately inducing NLRP3 degradation through autophagy [34]. This suggests that FFAR2 may reduce IL-1 and Th2 cytokine levels by inhibiting NLRP3, which can reduce airway hyperresponsiveness and steroid resistance [35].
GPR84
G protein-coupled receptor 84 (GPR84) can be activated by MCFAs. There are few studies about GPR84 in asthma. There is some indirect evidence that inhibition of GPR84 may alleviate asthma. In an ALI mouse model, GPR84 deficiency can reduce lung inflammation. GPR84 stimulates reactive oxygen species (ROS) production in neutrophils via the Akt/ERK pathway [36]. In a bleomycin-induced mouse model of pulmonary fibrosis, PBI-4050 treatment can reduce pulmonary fibrosis [37]. PBI-4050 can also reduce pulmonary fibrosis and airway remodeling in heart failure in reduced ejection fraction model mice [38].
With the in-depth study of bronchial asthma, the role of free fatty acids and their receptors in bronchial asthma has attracted much attention. FFARs play an important role in airway hyperresponsiveness, airway inflammation and airway remodeling through various factors and pathways. There are many studies on fatty acids and asthma. However, whether this represents the role of FFARs needs further study. At present, the agonists of FFAR1 have been used in clinical trials for the treatment of diabetes, but there are few studies on the agonists or antagonists of FFARs for the treatment of asthma. The role of FFARs in asthma needs to be further studied, and more therapeutic drugs targeting FFARs will be developed to effectively treat asthma and improve prognosis.
