Molecular targets and signalling pathways in asthma explored by network pharmacology

Network pharmacology studies consistently identify key molecular targets involved in asthma pathogenesis. Prominent targets include Interleukins (ILs) like IL-17, IL-4, IL-13 and IL-6, pro-inflammatory factors like tumour necrosis factor (TNF), Signal Transducers and Activators of Transcription (STAT3), AKT1, Vascular Endothelial Growth Factor A (VEGFA) and Epidermal Growth Factor Receptor (EGFR). These molecules participate in the modulation of inflammatory and immune responses, which are critical for asthma development [7,24,38,40].

Signalling pathways commonly enriched in asthma network pharmacology analyses include the IL-17 signalling pathway, which is involved in neutrophilic inflammation and airway remodelling [20]. The TNF signalling pathway is central to inflammatory cytokine production and cellular recruitment [20,38,43,44]. HSP90AB1 protein is associated with the immunological response, as it can stimulate macrophages to produce inflammatory mediators (IL-6 and TNF-alpha). The processes of airway remodelling, excessive mucus secretion and the immune responses elicited by airway inflammation are all influenced by EGFR and VEGFA [7].

Pathways in asthma

Akt kinase signalling axis

Phosphoinositide 3-kinase (PI3K) signalling cascade is profoundly linked to type 2 immune response and Th2-driven pulmonary response, which subsequently facilitates collagen accumulation, ultimately resulting in airway remodelling. The allergic asthma phenotype is elicited by sensitization to environmental allergens and is primarily mediated by Th2 cells, type 2 innate lymphoid cells (ILC2s), eosinophils, mast cells and immunoglobulin E (IgE). Conversely, intrinsic asthma arises independently of allergic stimuli and is associated with dysregulated inborn immunity driven by a multitude of factors, including pathogen invasion, adiposity, nicotine consumption and ecological toxins. This specific variant of asthma is distinguished by the participation of Th17 cells, ILC3s and neutrophils. The fundamental pathophysiological mechanisms underlying asthma involve the stimulation of immune cells, which subsequently elicit responses from non-immune cells, such as airway smooth muscle (ASM) and airway epithelial cells, culminating in AHR, inflammation and remodelling. The phosphoinositide 3-kinase (PI3K)/Akt signalling cascade plays a critical role in regulating a myriad of biological processes, including cellular proliferation, differentiation and migration. The activation of PI3K may lead to a decline in histone deacetylase 2 (HDAC2), which facilitates the recruitment of inflammatory cells, particularly neutrophils and promotes the release of pro-inflammatory factors and ROS overload. This signalling cascade is essential for orchestrating the responses of airway immune cells and structural cells implicated in the pathophysiological processes associated with asthma [45].

Janus kinases - signal transducers/activators of transcription

Th2 cells are responsible for the production of interleukins IL-4, IL-5, IL-6, IL-10 and IL-13, whereas Th1 cells release IL-2, IFN-γ and TNF-β. Th2 cytokines, primarily IL-4, IL-5 and IL-13, orchestrate the core elements of asthmatic inflammation, including IgE class switching, mucin production and eosinophil recruitment and activation. Stat6 is stimulated by interleukin 4 (IL-4) through the stimulation of Janus kinase 1 (Jak1) and Janus kinase 3 (Jak3). Conversely, engagement of interleukin 12 (IL-12) with its receptor activates Janus kinase 2 (Jak2) and tyrosine kinase 2 (Tyk2), thereby leading to the phosphorylation of signal transducer and activator of transcription 4 (Stat4). The function of Stat4 is primarily associated with the differentiation of T helper 1 (Th1) cells, whereas Stat6 is integral to the development of T helper 2 (Th2) cells. STAT6 also promotes B cells to switch antibody production to IgE. It also regulates chemokines that recruit inflammatory cells to the airways and promotes mucus production by airway epithelial cells. The pathway also controls the polarization and expansion of Th2 cells: STAT6 is required for Th2 development and memory, while STAT4 (activated by IL-12) promotes Th1 differentiation. STAT6 acts not only in immune cells but also in lung parenchymal cells (e.g. epithelial cells), where it drives mucus cell changes and the recruitment of Th2 cells by regulating chemokines such as eotaxin [46].

Mitogen-activated protein kinase signalling cascade

Within the mitogen-activated protein kinase (MAPK) class, the p38 subtype is the most significantly associated with pneumonitis. Various environmental factors, including aeroallergens, tobacco smoke, atmospheric pollutants and respiratory microbes, activate the p38α isoform, which subsequently upregulates the expression of an array of proinflammatory cytokines and chemokines, as well as the synthesis of certain fibrogenic factors. Consequently, p38 MAPK-mediated bronchial inflammation and remodelling are instrumental in the initiation, maintenance and aggravation of airflow obstruction, a hallmark of asthma. Eosinophilic asthma emerges from both atopic and non-atopic mechanisms, predominantly influenced by Th2 lymphocytes and group 2 innate lymphoid cells (ILC2). Th2 and ILC2 cells secrete substantial quantities of IL-4, IL-5 and IL-13, which are instrumental in IgE production, eosinophilic inflammation and airway hyperreactivity, respectively. Th2 lymphocytes are activated by IL-4, whereas ILC2 are stimulated by cytokines from the innate immune response, notably thymic stromal lymphopoietin (TSLP), IL-25 and IL-33. The p38 mitogen-activated protein kinase (MAPK) pathway is instrumental in mediating both the differentiation and activation processes of Th2 cells, consequently promoting the secretion of Th2 cytokines, specifically IL-4, IL-5 and IL-13. The upregulation of gene networks associated with the p38 MAPK signalling cascade is significantly correlated with neutrophilic inflammation in bronchial tissues. Furthermore, p38 MAPK enhances the expression of intercellular adhesion molecule-1 (ICAM-1) on the endothelial cells of pulmonary vasculature and increases the secretion of tumour necrosis factor-alpha (TNF-α) from neutrophils, thereby facilitating the recruitment of these immune cells into the airway passages. Transforming growth factor beta (TGF-β) can induce apoptosis in human airway epithelial cells via p38 MAPK activation. Additionally, p38 MAPK seems to play a pivotal role in the structural modifications that underlie airway remodelling in asthma, exemplified by the hypertrophy of the sub-epithelial basement membrane. Intercellular interactions involving mast cells and lung fibroblasts undergo proliferation and secrete substantial quantities of collagen via the p38 MAPK-dependent IL-6 secretion [47].

Hypoxia-inducible factor-1 alpha

Key transcription factor induced by low oxygen is hypoxia-i factor-1 alpha (HIF-1α), which is upregulated under hypoxic conditions. Tumour necrosis factor-alpha (TNF-α) exerts regulatory influence on the expression levels of both the protein and mRNA of hypoxia-inducible factor 1-alpha (HIF-1α), which serves as an essential component in the functional efficacy of airway smooth muscle cells (ASMCs) and in the pathogenesis of airway inflammatory disorders. The upregulation of HIF-1α, induced by hypoxic conditions, facilitates the ubiquitination of P53 by modulating MDM2, thereby intensifying airway swelling in asthmatic conditions and promoting airway restructuring. HIF-1α is known to augment the concentrations of pro-inflammatory cytokines such as IL-4, IL-5 and IL-13. Furthermore, HIF-1α may enhance the capacity of ASMCs to endure, proliferate, migrate and elicit an inflammatory response within a hypoxic milieu, while concurrently impeding their apoptotic processes [48].

Notch signalling pathway

The notch signalling pathway participates in airway inflammation, airway hyperresponsiveness (AHR) and structural remodelling. It is integral to the regulation of an array of cellular processes, including proliferation, development and differentiation. Generally, the Notch signalling pathway is activated by ligands that associate with their specific receptors. The Notch signalling pathway is instrumental in the differentiation of T helper 1 (Th1) and T helper 2 (Th2) cells and has the potential to influence asthma by modulating the Th1/Th2 cell equilibrium: Notch ligand delta is predominantly linked with Th1 cells, whereas Jagged Notch ligands exhibit a stronger association with Th2 cells. The manifestation of allergic asthma is mitigated by downregulating Notch-1 and upregulating Jagged 1 and 2, resulting in reduced Th2 and Th17 cytokine concentrations while increasing interferon-gamma (IFN-γ) levels. A balance between Th1 and Th2 subsets is also present, wherein Th1 cells express the transcription factor T-bet and secrete cytokines such as IFN-γ and tumour necrosis factor-alpha (TNF-α), which are involved in the cellular immune response against viral and bacterial pathogens. Conversely, Th2 cells express the transcription factor GATA-3 and secrete cytokines, including interleukin-4 (IL-4), IL-5 and IL-13, that enhance humoral immunity following pathogen invasion. The cytokine IFN-γ inhibits Th2 differentiation and function, while the cytokines IL-4 and IL-10 inhibit Th1 differentiation and function. The Notch signalling pathway has been identified as playing a significant role in CD8+ T cell-mediated AHR and inflammatory responses, with Delta1 emerging as a pivotal regulator of allergic airway inflammation. Furthermore, IL-17A, produced by Th17 cells, can directly induce Airway Hyper Responsiveness (AHR) by acting on smooth muscle within the airways. It has been elucidated that the interaction between the Notch ligand on CD4+ T cells and Jagged1 on antigen-presenting cells (APCs) is critical for Th2 cell differentiation and the initiation of IL-4 production, which further promotes AHR and airway inflammation. NOTCH3 modulates the expression of MUC5AC, a mucin protein that is expressed at elevated levels in airway epithelial cells and constitutes a primary component of airway mucus [49].

Interactions and crosstalk among these pathways enhance airway inflammation and hyperresponsiveness and network pharmacology explains how multi-component herbal compounds can simultaneously modulate these pathways, providing mechanistic insight into the therapeutic advantages over single-target drugs.