Cell-based functional studies
Several studies investigated miRNA-based molecular mechanisms ex vivo /in vitro in different cell types involved in asthma pathogenesis138-143. miR-155 is one of the most frequently investigated miRNAs in regards to asthma and AD. miR-155 was shown to be induced by hyper-stretch in human bronchial epithelial cells144 and targets Src homology 2 domain–containing inositol 5-phosphatase 1 (SHIP1) production and activates Janus Kinase (JNK) signaling leading to KC (the functional IL-8 paralog) secretion in mouse models. miR-181b was observed to be decreased in bronchial brushings and plasma from patients with asthma and inversely correlated with eosinophil counts in sputum145. Overexpressing this miRNA in a bronchial epithelial cell line (BEAS-2B) confirmed the regulation of the target Secreted Phospho Protein 1 (SPP1) and reduced IL-13 induced secretion of IL-1β and CC-Motif Chemokine Ligand 11 (CCL11). In this line, miR-181b was induced following addition of dexamethasone. Further, miR-27b has been described to be decreased in HDM induced experimental asthma, with a proposed function in the regulation of the PI3K-AKT pathway via targeting Spleen Associated Tyrosine Kinase (SYK) and Epidermal Growth Factor Receptor (EGFR) in a bronchial epithelial cell line (16-HBE)55.The let-7 miRNA family is also very abundant in the lung and their inhibitionin vivo ameliorated murine experimental asthma69; in particular, let-7a was shown to regulate IL-13 expression135 in vitro . In concordance to its effect in skin keratinocytes,35,36miR-146a was shown to have anti-inflammatory function in human lung alveolar epithelial cell line A549146,147and in HBECs.148
Besides epithelial cells, several studies have investigated miRNA-regulated mechanisms in airway smooth muscle cells. In vitro stimulation of hASMCs with a cytokine cocktail (IL-1β, TNF-α, IFN-γ) caused an increase in miR-146a with the observed effect being stronger in cells from asthmatic donors compared to healthy controls149. As inhibition of miR-146a increases cyclooxygenase-2 (COX-2) levels and IL-1β secretion by hASMCs, the authors suggested that miR-146a may be an interesting anti-inflammatory factor in asthma. In line with this study, upregulation of miR-145 in hASMCs was demonstrated upon cytokine stimulation and was associated with enhanced migration and proliferationin vitro 150. Inhibition of miR-145 reversed this effect through the reduced expression of collagen type I and contractile protein MHC via targeting of Krüppel-like factor 4 (KLF-4). Finally, miR-21 was shown to modulate hASMCs proliferation in vitro , via targeting PTEN, as identified by lentiviral overexpression experiments151. miR-21 has been previously associated with asthma development, mainly due its targeting of i.e. IL-12p35125,152, highlighting the multi-functional roles of miRNAs in several cell types contributing synergistically to asthma pathology.