Asthma is a common disease of the airway, causing expiratory airflow limitation that is partially reversible with inhaled bronchodilators. Bronchodilators are the first-line treatment for asthma, and they work by acting on beta-2-adrenergic receptors on airway smooth muscle (ASM) cells in the lower respiratory tract, allowing muscle relaxation and bronchodilation (46) as well as inhibits liquid accumulation and mucus section into the lumen(47–49). In this study, we tried to identify serum miRNAs as indicator of BDR in a cohort of children with asthma (GACRS) followed by replication study in an adult asthma (LOCCS) cohort. We found differences in ICS use, baseline FEV1, and PD20 among participants with high vs. low BDR in the GACRS cohort. Sometimes, BDR is greater in patients with lower starting, since they have more to gain from a bronchodilator FEV1 (regression beta = -0.41, p-value < 10− 15). We have attempted to correct for this effect by including baseline FEV1 as a covariate in our analysis, so that miRNAs associated with BDR should be more indicative of the airway’s plasticity rather than the magnitude of lung function deficit. ICS use increases pre-BDR FEV1 (50, 51) and would then decrease BDR (50), however, we noticed that ICS use was higher in patients with high BDR group. This may be due to confounding by indication, with patients with more serious disease and lower lung function requiring ICS therapy. Although PD20 differed by BDR response group, we did not include this as a covariate since it was anti-correlated with BDR, and inclusion in our model would result in decreased power.
We performed differential expression analysis using DeSeq2 to identify miRNAs associated with high vs low BDR and adjusting model for age, sex, use of inhaled corticosteroids (ICS) in the previous year and, baseline (pre-BDR) percent predicted FEV1. We found 11 miRNAs significantly associated with high vs low BDR in a study of Costa Rican children with asthma. In subjects with a high bronchodilator response, 10 of these 11 miRNAs were down-regulated, while one was up-regulated. Two of these miRNAs (miR-1246 and miR-200b-3p) were validated as being significant in the LOCCS cohort and regulated in the same direction, i.e., miR-1246 was down-regulated and miR-200b-3p was up-regulated in subjects with high BDR. These 11 miRNAs, and particularly miRs 200b-3p and 1246, may be indicative of general biological state that leads to difference in BDR.
The two replicated miRNAs were previously reported as potential biomarkers for respiratory diseases. miR-1246 has been reported to predict response to benralizumab in severe eosinophilic asthma (52), to distinguish healthy subjects from those with asthma (53), and to differentiate asthma from COPD or asthma–COPD overlap (ACO) along with two other miRNAs in a logistic regression model (54). Over-expression of miR-200-3p has been shown to reduce airway inflammation, mucus hypersecretion, and remodeling in asthma (55). Another study also suggested adenosine to inosine (A-to-I) edited sites in miR-200-3p in lower airway cells is associated with moderate-to-severe asthma (56). The putative target identification of these miRNAs revealed that miR-200b-3p regulates the expression of SPATS2L, a gene that was previously reported as a BDR gene (20) and miR-1246 regulates ADCY9, another BDR gene (17, 57). DIANA-miTED: a microRNA tissue expression database (58) also shows that these replicated miRNAs, namely miR-200b-3p (42986.7 RPM) and miR-1246 (147.7 RPM), are expressed in the bronchus.
Both gene targets of all 11 DE miRNAs in aggregate and gene targets of the two replicated miRNAs separately were enriched in regulation of cholesterol biosynthesis by SREBPs, ESR-mediated signaling, G1/S transition, RHO GTPase cycle, and signaling by TGFB family pathways (Figs. 3 & 4).
Regulation of cholesterol biosynthesis by the SREBPs pathway promotes cholesterol accumulation through uptake (low-density lipoprotein receptor) and synthesis (e.g., hydroxymethylglutaryl coenzyme A reductase) in macrophages and other cells (59). Recent findings indicate that cholesterol trafficking and inflammation are associated in the lung (60–63). In the present study we found that the target genes of DE miRNAs were enriched in this pathway, which may indicate the role of BDR-responsive miRNAs in cholesterol trafficking and inflammatory response in asthma. This is of interest as there are studies that link BDR to the presence of inflammation.
RHO GTPase pathway is known to regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion (64). We found that miR-200b-3p regulates the expression of ROCK2 gene that encodes Rho-kinase, known to play a role in regulating mucus overproduction (65), airway smooth muscle (ASM) tone (66) and ASM cytoskeletal stiffness (67). Further, ROCK2 expression is increased in ASM and pulmonary blood vessels in human asthma (68). This indicates a possible role of miR-200b-3p in regulating bronchoconstriction. Further exploration of the mechanisms by which miR-200b-3p and its target gene ROCK2 affect BDR may be worth pursuing.
TGFB family pathway is known to play a role in epithelial shedding, mucus hyper-secretion, angiogenesis, airway hyperresponsiveness, ASMC hypertrophy and hyperplasia in an asthmatic mouse model (69–71). Previously, it has been reported that eosinophils constitute a major source of TGF-β in asthmatic airways (72, 73). In this study, participants with a high bronchodilator response had a higher eosinophil count than those with a low bronchodilator response (Table 1). Previous investigations suggest that TGF-β1 may play a role in the development of resistance to bronchodilators in asthma by reducing the efficacy of β2-agonists and by inducing PDE4D gene expression in a Smad2/3-dependent pathway manner (74–77). The DE miRNAs (miR-26b-5p, miR-378a-3p, miR-378i, miR-200b-3p, and miR-885-5p) were found to regulates the expression of TGFB1, TGFB2, TGFBR1, TGFBR2, and TGFBR3 genes encoding TGF-β and TGF-β receptor (Supplementary Table S1). Additionally, the target genes of DE miRNAs were found to be enriched in TGFB pathway and pathway associated with downregulation of SMAD2/3: SMAD4 transcriptional activity (Fig. 3), showing possible role of DE miRNAs in regulation of TGF-β associated pathway and thus involved in smooth muscle remodeling (Fig. 5). TGF-β works upstream of the RHO GTPase pathway, TGF-β activates RhoA/rho-associated protein kinase (ROCK), and the cross-talk between these two pathways promotes airway remodeling (78) and mucus formation (69).
Strengths of our study include leveraging a large cohort of childhood asthmatics with circulating miRNA sequencing data and careful spirometric evaluation. That two of the identified miRNAs were able to replicate in an adult asthma population, despite etiological differences between childhood and adult asthma, gives weight to their importance in determining efficacy of SABAs as rescue inhalers. Weaknesses of our study include the retrospective study design and inability to assess miRNA differences in airway smooth muscle cells. We anticipate that future work into ASM miRNAs would provide additional biological insight into differences in BDR.