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British Journal of Pharmacology logoLink to British Journal of Pharmacology
. 2016 May 5;173(12):2016–2029. doi: 10.1111/bph.13494

Combining an epithelial repair factor and anti‐fibrotic with a corticosteroid offers optimal treatment for allergic airways disease

K P Patel 1, A S Giraud 3,4, C S Samuel 1,✉, S G Royce 1,2,✉
PMCID: PMC4882488  PMID: 27060978

Abbreviations

AAD

allergic airways disease

AHR

airway hyperresponsiveness

AI

airway inflammation

AWR

airway remodelling

CO

corn oil

DEX

dexamethasone

cDyn

dynamic compliance

ECM

extracellular matrix

GR

glucocorticoid receptor

H2

human gene‐2

NA

naphthalene

OVA

ovalbumin

PDGF

platelet‐derived growth factor

RLX

recombinant human relaxin drug/serelaxin

RXFP1 receptor

relaxin family peptide receptor 1

TFF2

trefoil factor 2

Tables of Links

TARGETS
GPCRs a Enzymes c
β2‐adrenoceptor Caspase‐3
RXFP1 receptor MMP‐2
Nuclear hormone receptors b MMP‐9
Glucocorticoid receptor (GR)
LIGANDS
Dexamethasone TNF‐α
Fibronectin Trans‐4‐hydroxy‐proline
Methacholine Thymic stromal lymphopoietin (TSLP)
TGF‐β1
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These Tables list key protein targets and ligands in this article which are hyperlinked to corresponding entries in http://www.guidetopharmacology.org, the common portal for data from the IUPHAR/BPS Guide to PHARMACOLOGY (Pawson et al., 2014) and are permanently archived in the Concise Guide to PHARMACOLOGY 2015/16 (a,b,cAlexander et al., 2015b, 2015a, 2015c).

Introduction

Asthma is a chronic inflammatory airways disease and is the most prevalent chronic disease affecting children (Tang et al., 2006). According to the World Health Organization (WHO), ~300 million people worldwide suffer from the disease, attributing to 250 000 deaths annually. The WHO also estimated that a further 100 million people will suffer from asthma by the year 2025 (WHO – The Global Asthma Report 2014). The three central components of the pathogenesis of asthma are airway inflammation (AI), airway remodelling (AWR) and airway hyperresponsiveness (AHR) (Tang et al., 2006). AI is a heterogeneous T helper cell type 2 (Th2)‐driven response, which has long been regarded as the central component of asthma, contributing to AHR. However, it is now known that irreversible structural changes in the airways (AWR) can occur early in the course of asthma pathogenesis and can also lead to AHR, independently of AI. AWR is a collective term used to describe many structural changes including epithelial damage and thickening, goblet cell metaplasia, subepithelial fibrosis, increased airway smooth muscle mass and neovascularization. Epithelial damage has emerged as an important aetiology of asthma, contributing to the development of AWR and subsequently, AHR (Holgate et al., 2003; Tang et al., 2006; Holgate, 2008a; Royce et al., 2014a). The airway epithelium provides physical and immunological protection against specific and non‐specific inhaled stimuli (Holgate et al., 2003; Tang et al., 2006). Genetic susceptibility can impede the ability of the epithelium to protect the airway resulting in denudation, followed by re‐epithelialization, metaplasia and aberrant wound healing (leading to fibrosis) (Trautmann et al., 2005; de Boer et al., 2008; Zhang et al., 2012). Despite this, it is not addressed in commonly used murine models of chronic allergic airways disease (AAD) that mimic several features observed in human asthma.

To address this limitation and the fact that currently used mainstay in asthma therapy, including corticosteroids (which primarily act to suppress AI) and/or short‐ and long‐acting β‐agonists (which suppress AHR independently of regulating the pathogenesis of AI or AWR) do not target epithelial damage or the ensuing AWR, we recently superimposed naphthalene (NA)‐induced epithelial damage onto the well‐established ovalbumin (OVA)‐induced murine model of chronic AAD, which presented with the three central components of asthma pathogenesis (Royce et al., 2014d). Although human asthma is not typically induced by injurious stimuli such as OVA or NA per se, which even in combination would probably not cause the myriad of allergic, chemical and genetic factors associated with disease progression in humans, the strength of combining these agents in mice is that they create a model that allows investigation and therapeutic targeting of the contribution of epithelial damage to several morphological and functional processes that typify the human disease, without any confounding alveolitis (Kumar et al., 2008). The induction of epithelial damage in the OVA + NA model was found to contribute to AWR, fibrosis and related AHR and exacerbate the effects of AI on these parameters (Royce et al., 2014d).

In the current study, we went on to use our newly established model of chronic allergic disease incorporating epithelial damage to assess how therapies targeting AI [with the anti‐inflammatory corticosteroid, dexamethasone (DEX)], epithelial damage [with the epithelial repair factor, trefoil factor 2 (TFF2)] and fibrosis [with the anti‐fibrotic drug, serelaxin (RLX)], in isolation and in combination, affected various endpoints associated with chronic asthma pathogenesis.

TFF2 is a protective molecule of the gut that is produced by intestinal epithelial cells and that promotes epithelial repair and restitution (migration) while inhibiting cell apoptosis (Aamann et al., 2014). TFF2 has also been shown to have a similar protective role in the airways/lungs (Hoffmann, 2007; Royce et al., 2014c). In the setting of chronic AAD, intranasal (i.n.) delivery of recombinant human TFF2 was able to reduce several features of AWR, subepithelial thickening and AHR (Royce et al., 2013a) by inhibiting the actions of TGF‐β1 and PDGF, in the absence of any effects on AI. On the other hand, TFF2 knockout mice underwent exacerbated AWR when exposed to epithelial damage (Royce et al., 2014a).

RLX [a recombinantly produced peptide based on the human gene‐2 (H2) relaxin sequence, which represents the major stored and circulating form of human relaxin] is a potent anti‐fibrotic (Bennett, 2009; Royce et al., 2014c) that primarily acts to inhibit the actions of TGF‐β1 on myofibroblast differentiation and myofibroblast‐mediated aberrant collagen deposition. Additionally, RLX can also promote the expression and activity of various collagen‐degrading MMPs (Unemori et al., 1996) and/or inhibit tissue inhibitor of metalloproteinase activity to facilitate the breakdown of collagen. Both systemic (Royce et al., 2009) and daily i.n. (Royce et al., 2014b) administration of serelaxin significantly reversed several features of established AWR (including epithelial thickening and subepithelial/total lung collagen deposition) and partially reversed AHR, in the absence of any marked effects on AI, when administered over a 2 week treatment period.

We had recently investigated the combined effects of RLX and the corticosteroid, methylprednisolone, in the chronic OVA‐induced model of AAD‐lacking epithelial damage as part of its pathophysiology, and found that the combined effects of both more effectively reduced subepithelial extracellular matrix (ECM) thickness compared with either therapy alone (Royce et al., 2013b). Hence, the current study was designed to further investigate the epithelial repair properties of TFF2, and whether it would potentiate the effects of RLX or RLX and DEX when added in combination, in the OVA + NA model associated with epithelial damage and the central features of human asthma. We hypothesized that combining therapies that target airway epithelial damage and fibrosis (with TFF2 and RLX, respectively) would effectively reverse AWR and AWR‐induced AHR, offering a novel means to treat the structural changes associated with chronic asthma, particularly for patients that are resistant to corticosteroid exposure. We also hypothesized that combining an epithelial repair factor and anti‐fibrotic could act as an adjunct therapy to the anti‐inflammatory effects of DEX, which together would optimally reverse the central components of chronic asthma.

Methods

Animals

Six‐ to eight‐week‐old female Balb/c wild‐type mice (provided by Monash University Animal Services, Clayton, Vic., Australia) were allowed to acclimatize for at least 4–5 days prior to experimentation and were maintained on a 12 h light:12 h dark cycle with free access to standard rodent chow (Barastoc Stockfeeds, Pakenham, Vic., Australia) and water. Female Balb/c wild‐type mice have been shown to be more prone to a Th2 response and undergo higher airway reactivity (in response to allergens) compared with their male counterparts and other commonly used murine strains (Kumar et al., 2008). All experimental procedures were performed according to the regulations approved by Monash University's Animal Ethics Committee, which adheres to the Australian Guidelines for the Care and Use of Laboratory Animal for Scientific Purposes. Animal studies are reported in compliance with the ARRIVE guidelines (Kilkenny et al., 2010; McGrath & Lilley, 2015).

Induction and treatment of chronic AAD incorporating epithelial damage

Mice were subjected to the 9.5 week OVA‐induced model of chronic AAD incorporating epithelial damage, as described previously (Royce et al., 2014d). Briefly, mice (n = 48) were sensitized with two i.p. injections of 10 μg grade V chicken egg OVA (Sigma‐Aldrich, St Louis, MO, USA) and 1 mg aluminium potassium sulfate adjuvant (alum; AJAX Chemicals, Kotara, NSW, Australia) in 0.5 mL of saline on days 0 and 14. They were then subjected to nebulization (inhalation of an aerosol) with 2.5% (w v‐1) OVA for 30 min, three times a week, between days 21 and 63, using an ultrasonic nebulizer (Omron NE‐U07; Omron, Kyoto, Japan). The mice then received a single i.p. injection of the Clara cell‐specific cytotoxin, NA (200 mg·kg−1 body weight; Sigma‐Aldrich) on day 64 (1 day after the last OVA nebulization period; OVA + NA group) and left for a further 3 days. From days 67 to 75, sub‐groups of OVA + NA‐injured mice were treated with daily i.n. administration of either (i) saline (the vehicle for OVA and all drug treatments; injury control group; n = 8), (ii) RLX (0.8 mg·mL−1; kindly provided by Corthera Inc., San Carlos, CA, USA; a subsidiary of Novartis AG, Basel, Switzerland; n = 8), (iii) recombinant human‐glycosylated TFF2 (0.5 mg·mL−1; n = 8), (iv) RLX and TFF2 (n = 8), (v) DEX (0.5 mg·mL−1; n = 8) or (vi) RLX, TFF2 and DEX (n = 8). The doses of RLX (Royce et al., 2014b), TFF2 (Royce et al., 2013a) and DEX (Royce et al., 2014a) used had previously been demonstrated to mediate therapeutic and anti‐fibrotic efficacy. A separate subgroup of mice subjected to (vii) i.p. injections of saline on days 0 and 14, nebulized saline instead of OVA between days 21 and 63, and an i.p. injection of corn oil (CO; the vehicle for NA; n = 8) was included as a control group.

Invasive plethysmography

On day 75, all seven groups of mice (n = 56 in total) had their airway dynamic compliance (cDyn) measured by invasive plethysmography, in response to increasing concentrations of methacholine‐induced airway bronchoconstriction. Mice were briefly anaesthetized with an i.p. injection of ketamine (100 mg·kg−1 body weight) and xylazine (20 mg·kg−1 body weight), tracheostomized and cannulated. Increasing doses of methacholine were nebulized, and AHR was measured (Biosystem XA version 2.7.9, Buxco Electronics, Troy, NY, USA) for 2 min after each dose. Differences in cDyn were analysed for statistical comparisons on the original data obtained. To then better illustrate changes in cDyn between the various treatment groups investigated, the respective baseline cDyns were subtracted from each original data point, and these data were converted to % change for each dose of methacholine used.

Tissue collection

Once airway reactivity measurements were completed, mice were killed by an overdose of anaesthetic containing ketamine and xylazine, before their lung tissue was isolated. The lungs of each animal were then divided along the transverse plane, resulting in four separate lobes. The largest lobe was fixed in 10% neutral buffered formalin overnight, processed routinely and embedded in paraffin wax. The remaining three lobes were snap‐frozen in liquid nitrogen and stored at −80°C for further analyses, as detailed below.

Lung histopathology

Formalin‐fixed, paraffin‐embedded tissues were sectioned (3 μm thickness) and placed on SuperFrost charged microscope slides (Grale Scientific, Melbourne, Vic., Australia). To assess peribronchial inflammation score, one set of serial sections per mouse underwent Mayer's haematoxylin and eosin (H&E) staining. To assess epithelial and subepithelial extracellular matrix thickness, another set of serial sections per mouse underwent Masson's trichrome staining. To assess goblet cell metaplasia, a third set of serial sections per mouse underwent Alcian blue periodic acid Schiff (ABPAS) staining. Stained sections were scanned using the whole‐slide scanning platform Aperio Scanscope CS (Leica Biosystems, Nussloch, Germany). All slides were then scanned at the maximum magnification available (40×) and stored as digital high‐resolution images on a local server associated with the instrument. Digital slides were viewed and morphometrically analysed with the Aperio imagescope v.12.1.0.5029 software (Leica Biosystems).

Histological evaluation of airway inflammation

Histological grading of inflammation severity from 0 to 4 was assigned to every slide, as described previously (Royce et al., 2014d) (0 = no detectable inflammation; 1 = occasional inflammatory cell aggregates, pooled size <0.1 mm2; 2 = some inflammatory cell aggregates, pooled size ~0.2 mm2; 3 = widespread inflammatory cell aggregates, pooled size ~0.3 mm2; and 4 = widespread and massive inflammatory cell aggregates, pooled size ~0.6 mm2), and was performed blinded by the investigator.

Immunohistochemistry

Paraffin‐embedded lung sections were immunohistochemically stained using either a monoclonal antibody to α‐SMA (α‐smooth muscle actin; a marker of myofibroblast differentiation; M0851; 1:200 dilution; DAKO, Carpinteria, CA, USA) or polyclonal antibodies to TGF‐β1 (sc146; 1:1000 dilution; Santa Cruz Biotechnology, Santa Cruz, CA, USA), fibronectin (ab2413; 1:350 dilution; Cambridge, MA, USA) or thymic stromal lymphopoietin (TSLP; a marker of epithelial damage; 1:1000 dilution; EMD Millipore Corp., Temecula, CA, USA). Detection of antibody staining was completed using the DAKO EnVision anti‐mouse or anti‐rabbit kits, respectively, and 3,3‐diaminobenzidine (Sigma‐Aldrich), where sections were counterstained with haematoxylin. Images of five bronchi (measuring 150–350 μm luminal diameter) per section were obtained and quantified by morphometry, as described below.

Morphometric analysis

Representative photomicrographs from H&E‐, Masson's trichrome‐ and ABPAS‐stained slides, as well as immunohistochemically stained slides were captured from scanned images using ScanScope AT Turbo (Aperio, Vista, CA, USA). Stained airways were randomly selected from across the tissue sample. Masson's trichrome‐stained slides were analysed by measuring the thickness of the epithelial and subepithelial layers and expressing values as μm2·mm−1 basement membrane length. ABPAS‐stained slides were analysed by counting the number of stained goblet cells, which were expressed as the number of goblet cells 100 μm‐1 basement membrane length. α‐SMA and TSLP positively stained cells located within the subepithelial and epithelial regions, respectively, of the airway were counted and expressed as number of positively stained cells 100 μm‐1 basement membrane length. Subepithelial fibronectin and epithelial TGF‐β1 staining was quantified by measuring the levels of strong positively stained areas within the subepithelial and epithelial regions, respectively, and expressing the data as percentage staining per field.

Hydroxyproline assay

The second largest lung lobe from each mouse was processed as described previously (Royce et al., 2014b, 2009, 2014d) for the measurement of hydroxyproline content, which was determined from a standard curve of purified trans‐4‐hydroxy‐L‐proline (Sigma‐Aldrich). Hydroxyproline values were multiplied by a factor of 6.94 (based on hydroxyproline representing ~14.4% of the amino acid composition of collagen in most mammalian tissues) (Gallop and Paz, 1975) to extrapolate total collagen content, which in turn, was divided by the dry weight of each corresponding tissue to yield percent collagen concentration.

Gelatin zymography

The third largest lung lobe from each mouse was used for protein isolation and analysis of the gelatinases, MMP2 and MMP9, as described previously (Royce et al., 2009, 2015). Equal amounts of protein extracts (4 μg) were analysed on zymogram gels consisting of 7.5% acrylamide and 1 mg·mL−1 gelatin, and the gels were subsequently treated as described before (Woessner, 1995). Gelatinolytic activity was identified by clear bands at the appropriate molecular weight and quantified by densitometry using a BioRad GS710 Calibrated Imaging Densitometer (BioRad Laboratories, Richmond, CA, USA). The relative OD of the combined latent (L) and active (A) forms of MMP‐2 and MMP‐9 in each group was then expressed as the respective ratio to that of the saline/CO‐treated mouse group, which was expressed as 1 in each case.

Statistical analysis

All data were analysed using GraphPad prism v6.0 (La Jolla, CA, USA) and expressed as the mean ± SEM. Differences in the raw cDyn data were analysed by a two‐way ANOVA with Bonferroni post hoc test, whereas the remaining data were analysed by a one‐way ANOVA with Newman–Keuls post hoc test for multiple comparisons between groups. In each case, the significance was pre‐determined as being P < 0.05. The data and statistical analysis comply with the recommendations on experimental design and analysis in pharmacology (Curtis et al., 2015).

Results

Individual versus combined effects of RLX, TFF2 and DEX on airway inflammation

Confirmation that OVA + NA‐treated mice were sufficiently sensitized/challenged to OVA was demonstrated by the elevated level of peribronchial inflammation score in these mice compared with that in their saline/CO‐treated counterparts (P < 0.05 vs. saline/CO group; Figure 1A and B). Treatment with RLX alone, TFF2 alone or a combination of both resulted in comparable inflammation scores with that in OVA + NA‐treated mice (all P < 0.05 vs. saline/CO group; Figure 1A and B). However, DEX administration, either alone or when added in combination with RLX and TFF2 resulted in a significant and equivalent reduction in inflammation score compared with that in OVA + NA‐treated mice (both P < 0.05 vs. OVA + NA group; Figure 1A and B), although not fully back to that measured in the saline/CO group (both P < 0.05 vs. saline/CO group; Figure 1A and B). These combined findings suggested that the anti‐inflammatory effects measured in the RLX + TFF2 + DEX group were probably the result of the effects of DEX alone.

Figure 1.

Figure 1

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Individual versus combined effects of RLX, TFF2 and DEX on peribronchial inflammation and goblet cell metaplasia. Representative images of H&E‐ (A) and ABPAS‐stained (C) lung sections from each of the groups studied show the extent of bronchial wall inflammatory cell infiltration (A) and goblet cells (indicated by arrows) present within the epithelial layer (C) respectively. Scale bar (A and C) = 100 μm. Also shown is the mean ± SEM inflammation score (B) and goblet cell count (per 100 μm of basement membrane length) (D) from five airways/mouse, where H&E‐stained section were scored for the number and distribution of inflammatory aggregates on a scale of 0 (no apparent inflammation) to 4 (severe inflammation). Numbers in parenthesis represent the number of animals analysed per group. *P < 0.05 versus saline/CO‐treated group; # P < 0.05 versus OVA + NA‐treated group; † P < 0.05 versus OVA + NA + RLX‐treated group; + P < 0.05 vs. OVA + NA + TFF2‐treated group; § P < 0.05 versus OVA + NA + RLX + TFF2‐treated group.

Individual versus combined effects of RLX, TFF2 and DEX on airway remodelling

Goblet cell metaplasia

Goblet cell metaplasia (number of goblet cells 100 μm‐1 of basement length) was assessed by morphometric analysis of ABPAS‐stained lung tissue sections (Figure 1C) and was significantly increased in OVA + NA‐treated mice compared with that in their saline/CO‐treated counterparts (P < 0.05 vs. saline/CO group; Figure 1C and D). Mean goblet cell numbers in RLX‐treated mice were not significantly different from that measured in OVA + NA‐treated animals (P < 0.05 vs. saline/CO group; Figure 1C and D). However, goblet cell numbers were partially but significantly reduced by TFF2 treatment of mice (P < 0.05 vs. OVA + NA group; P < 0.05 vs. saline/CO group; Figure 1C and D) and further reduced to levels that were no longer different from that in saline/CO‐treated mice by RLX + TFF2, DEX alone and the combined effects of RLX + TFF2 + DEX treatment (all P < 0.05 vs. OVA + NA, OVA + NA + RLX and OVA + NA + TFF2 groups; Figure 1C and D). As a result, the combined effects of RLX + TFF2 were significantly greater than either treatment alone (P < 0.05 vs. either RLX alone or TFF2 alone; Figure 1C and D).

Epithelial damage

Epithelial damage (measured as number of TSLP‐positive cells 100 μm‐1 of basement membrane length) was assessed by morphometric analysis of TSLP‐stained lung tissue sections (Figure 2A and B). Expectedly, OVA + NA‐treated mice exhibited significantly exacerbated levels of epithelial damage compared with that measured in saline/CO‐treated mice (P < 0.05 vs. saline/CO group; Figure 2A and B). RLX‐treated mice demonstrated a modest but significant reduction in TSLP‐associated airway epithelial damage compared that in OVA + NA‐treated animals (P < 0.05 vs. OVA + NA group), but which was still greater than that found in saline/CO‐treated mice (P < 0.05 vs. saline/CO; Figure 2A and B). DEX‐treatment of mice also reduced epithelial damage (P < 0.05 vs. OVA + NA group), but not fully back to that measured in saline/CO‐treated controls (P < 0.05 vs. saline/CO; Figure 2A and B). However, the epithelial repair factor TFF2 alone or in combination with RLX or RLX + DEX completely normalized the OVA + NA‐induced increase in TSLP‐associated epithelial damage after 7 days of treatment (all P < 0.05 vs. OVA + NA and OVA + NA + RLX groups; no different to saline/CO‐treated control group; Figure 2A and B).

Figure 2.

Figure 2

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Individual versus combined effects of RLX, TFF2 and DEX on TSLP‐associated epithelial damage and extent of airway epithelial thickness. Representative TSLP‐ (A) and Masson's trichrome‐stained (C) lung sections from each of the groups studied show the extent of airway epithelial damage (A) and associated thickness (C) respectively. Scale bar (A and C) = 100 μm. Also shown is the mean ± SEM TSLP‐stained cell counts (per 100 μm of basement membrane length) (B) and epithelial thickness (μm) relative to basement membrane length (D) from five airways/mouse. Numbers in parenthesis represent the number of animals analysed per group. *P < 0.05 versus saline/CO‐treated group; # P < 0.05 versus OVA + NA‐treated group; † P < 0.05 versus OVA + NA + RLX‐treated group.

Epithelial thickness

Epithelial thickness (relative to basement membrane length) was assessed by morphometric analysis of Masson's trichrome‐stained lung tissue sections (Figure 2C and D) and was significantly increased in OVA + NA‐treated mice compared that in their saline/CO‐treated counterparts (P < 0.05 vs. saline/CO group). All treated groups partially but significantly reduced airway epithelial thickness to a similar extent compared with that measured in OVA + NA‐treated animals (by ~30–40%; all P < 0.05 vs. OVA + NA group), but not fully back to that measured in saline/CO‐treated control mice (all P < 0.05 vs. saline/CO group; Figure 2C and D).

Individual versus combined effects of RLX, TFF2 and DEX on airway fibrosis

Subepithelial ECM thickness

Subepithelial ECM thickness (relative to basement membrane length) was assessed by morphometric analysis of Masson's trichrome‐stained lung tissue sections (Figure 3A) and was significantly increased in OVA + NA‐treated mice compared with that measured from saline/CO‐treated controls (P < 0.05 vs. saline/CO group). All treated groups partially but significantly reduced subepithelial ECM thickness (by ~28–50%; all P < 0.05 vs. OVA + NA group). However, neither of the treatment groups investigated were able to fully restore the OVA + NA‐induced increase in subepithelial collagen thickness back to that measured in saline/CO‐treated mice after 7 days (all P < 0.05 vs. saline/CO group; Figure 3A).

Figure 3.

Figure 3

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Individual versus combined effects of RLX, TFF2 and DEX on subepithelial ECM thickness, lung collagen concentration and subepithelial fibronectin deposition, as measures of fibrosis. Representative fibronectin‐stained (C) lung sections from each of the groups studied show the extent of subepithelial fibronectin deposition within the airways. Scale bar = 100 μm. Also shown is the mean ± SEM subepithelial ECM thickness (μm) relative to basement membrane length (which was morphometrically evaluated from Masson's trichrome‐stained sections) (A), total lung collagen concentration (% collagen content per dry weight lung tissue) (B) and subepithelial fibronectin deposition (D) from five airways/mouse. Numbers in parenthesis represent the number of animals analysed per group. *P < 0.05 versus saline/CO‐treated group; # P < 0.05 versus OVA + NA‐treated group; + P < 0.05 versus OVA + NA + TFF2‐treated group; P < 0.05 versus OVA + NA + DEX‐treated group.

Total lung collagen concentration

Total lung collagen concentration (% collagen content/dry weight lung tissue) was extrapolated from hydroxyproline analysis of lung tissues (Figure 3B) and again was significantly elevated in OVA + NA‐treated mice compared with that in their saline‐CO‐treated counterparts (P < 0.05 vs. saline/CO). Interestingly, neither TFF2 nor DEX treatment alone significantly affected the OVA + NA‐induced increase in lung collagen concentration. However, RLX alone and in combination with TFF2 or TFF2 + DEX significantly ameliorated the aberrant lung collagen concentration (by ~40–60%; all P < 0.05 vs. OVA + NA and OVA + NA + DEX groups; Figure 3B). Again though, total lung collagen concentration in the RLX‐treated groups was not fully reversed back to that measured in saline/CO‐treated mice after 7 days (all P < 0.05 vs. saline/CO group; Figure 3B).

Subepithelial fibronectin expression

As TTF2 and DEX were able to attenuate the OVA + NA‐induced increase in subepithelial ECM thickness but not total lung collagen concentration, the effects of all treatments investigated on subepithelial fibronectin was then evaluated. Fibronectin expression in the subepithelial airway region (relative to basement membrane length) was assessed by morphometric analysis of fibronectin‐stained lung tissue sections (Figure 3C and D) and was significantly greater in OVA + NA‐treated mice compared with that in saline/CO‐treated controls (P < 0.05 vs. saline/CO group). TFF2 or DEX treatment alone significantly ameliorated the OVA + NA‐induced increase in subepithelial fibronectin accumulation by ~30–40% (both P < 0.05 vs. OVA + NA group; Figure 3C and D), suggesting that their ability to attenuate subepithelial ECM thickness was due to effects on fibronectin rather than collagen. RLX alone and in combination with TFF2 further ameliorated the OVA + NA‐induced increase in aberrant subepithelial fibronectin deposition (by ~55–57%) to a greater extent than DEX alone (both P < 0.05 vs. OVA + NA and OVA + NA + DEX groups), while the combined effects of RLX + TFF2 + DEX markedly reduced aberrant subepithelial fibronectin deposition (by ~60%) to a greater extent than either DEX alone or TFF2 alone (P < 0.05 vs. OVA + NA, OVA + NA + DEX and OVA + NA + TFF2 groups; Figure 3D).

TGF‐β1 expression

TGF‐β1 expression and distribution in the airway epithelium (relative to basement membrane length) was assessed by morphometric analysis of TGF‐β1‐stained lung tissue sections (Figure 4A and B), and was markedly increased in OVA + NA‐treated mice compared with that measured from saline/CO‐treated animals (P < 0.05 vs. saline/CO group). RLX alone partially but significantly reduced the aberrant epithelial TGF‐β1 expression levels (by ~50%; P < 0.05 vs. OVA + NA group). In comparison, epithelial TGF‐β1 levels were further reduced to a greater extent (by ~70–80%) by all other treatment groups studied, compared with the effects of RLX alone (all P < 0.05 vs. OVA + NA and OVA + NA + RLX groups; Figure 4A and B). Again though, neither treatment group was able to fully restore the OVA + VA‐induced increase in epithelial TGF‐β1 expression back to that measured in control mice (all P < 0.05 vs. saline/CO‐treated group).

Figure 4.

Figure 4

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Individual versus combined effects of RLX, TFF2 and DEX on epithelial TGF‐β1 expression and subepithelial myofibroblast accumulation. Representative images of immunohistochemically stained lung sections from each of the groups studied show the extent and distribution of epithelial TGF‐β1 expression (A) and subepithelial myofibroblast accumulation (C) respectively. Scale bar (A and C) = 100 μm. Also shown is the mean ± SEM epithelial TGF‐β1 staining (% per field) (B) and α‐SMA‐stained cells (per 100 μm of basement membrane length) within the subepithelial region (D) from 5 airways/mouse. Numbers in parenthesis represent the number of animals analysed per group. *P < 0.05 versus saline/CO‐treated group; # P < 0.05 versus OVA + NA‐treated group; † P < 0.05 versus OVA + NA + RLX‐treated group; P < 0.05 versus OVA + NA + DEX‐treated group.

Myofibroblast differentiation

The number of α‐SMA‐stained positive cells (per 100 μm of basement length) in the subepithelial layer of various airways was assessed by morphometric analysis of α‐SMA‐stained lung tissue sections (Figure 4C and D) and was significantly increased in OVA + NA‐treated mice compared with that measured in their saline/CO‐treated counterparts (P < 0.05 vs. saline/CO group). With the exception of DEX alone, all other treatment groups were able to significantly reduce the OVA + NA‐induced increase in subepithelial myofibroblast accumulation (by ~43–62%; all P < 0.05 vs. OVA + NA and OVA + NA + DEX groups; Figure 4C and D), however, not fully back to that measured in saline/CO‐treated mice (all P < 0.05 vs. saline/CO‐treated group; Figure 4C and D).

Gelatinase expression and activity

Changes in gelatinase expression and activity were assessed by zymography to determine if the treatment‐induced changes in fibrosis observed were associated with their ability to regulate enzymes involved in collagen degradation. Both MMP‐2 (Figure 5A and C) and MMP‐9 (Figure 5A and B) levels were comparable between saline/CO‐treated, OVA + NA‐treated and OVA + NA + DEX‐treated mice. However, MMP‐9 expression and activity was significantly increased by RLX alone (by ~63%), TFF2 alone (by ~65%), RLX + TFF2 (by ~89%) and RLX + TFF2 + DEX (by ~101%) after 7 days of administration (all P < 0.05 vs. saline/CO, OVA + NA and OVA + NA + DEX groups; Figure 5A and B). Similarly, MMP‐2 expression and activity was significantly increased by RLX alone (by ~57%), RLX + TFF2 (by ~68%) and RLX + TFF2 + DEX (by ~95%) (all P < 0.05 vs. saline/CO, OVA + NA and OVA + NA + DEX groups; Figure 5A and C), but not by TFF2 alone over the same time period. The combined effects of all three drugs also promoted both MMP‐9 levels to a greater extent that either RLX alone or TFF2 alone (P < 0.05 vs. OVA + NA + RLX and OVA + NA + TFF2 groups; Figure 5A and B) and MMP‐2 levels to a greater extent than TFF2 alone (P < 0.05 vs. OVA + NA + TFF2 group; Figure 5A and C).

Figure 5.

Figure 5

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Individual versus combined effects of RLX, TFF2 and DEX on lung MMP‐9 and MMP‐2 expression and activity. A representative gelatin zymograph (A) shows latent (L) MMP‐9 (gelatinase B; 92 kDa) and MMP‐2 (gelatinase A; 72 kDa) expression levels and their corresponding active (A) forms from each of the groups studied (2 samples per group). Two separate zymographs, each analysing two additional samples per group, produced similar results. Also shown in the mean ± SEM relative OD (of the combined L‐form and A‐forms of MMP‐9 (B) and MMP‐2 (C)), to that in the saline/CO‐treated control group, which is expressed as 1 in each case; from n = 6 mice per group. *P < 0.05 versus saline/CO‐treated group; # P < 0.05 versus OVA + NA‐treated group; † P < 0.05 versus OVA + NA + RLX‐treated group; + P < 0.05 versus OVA + NA + TFF2‐treated group; P < 0.05 versus OVA + NA + DEX‐treated group.

Individual versus combined effects of RLX, TFF2 and DEX on dynamic airway compliance

Changes in cDyn between treatment groups were measured by invasive plethysmography in response to increasing doses of nebulized methacholine (Figure 6). Consistent with the increased AI, AWR and fibrosis associated with OVA + NA‐treated mice, these mice with chronic AAD demonstrated significantly reduced cDyn (by almost 50%; P < 0.05 vs. saline/CO‐treated group; Figure 6). RLX alone, but not TFF2 alone or DEX alone, was able to normalize the OVA + NA‐induced reduction in cDyn back to that measured in saline/CO‐treated mice (P < 0.05 vs. OVA + NA‐treated group; no different to saline/CO group; Figure 6). The combined effects of RLX + TFF2 or RLX + TFF2 + DEX also completely normalized the OVA + NA‐induced reduction in cDyn after 7 days of treatment (both P < 0.05 vs. OVA + NA‐treated group; Figure 6).

Figure 6.

Figure 6

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Individual versus combined effects of RLX, TFF2 and DEX on cDyn. cDyn was measured in response to increasing concentrations of methacholine‐induced airway bronchoconstriction, as an indicator of the lung's ability to stretch and expand. Shown is the mean ± SEM loss of cDyn to each dose of methacholine tested. Numbers in parentheses represent the number of animals analysed per group. *P < 0.05 versus saline/CO‐treated group; # P < 0.05 versus OVA + NA‐treated group.

Discussion and conclusions

In this study, we evaluated how inhaled therapies targeting AI (with the anti‐inflammatory corticosteroid, DEX), epithelial damage (with the epithelial repair factor, TFF2) or airway/lung fibrosis (with the anti‐fibrotic drug, RLX), in isolation and in combination, affected various endpoints associated with the pathogenesis of asthma (see Table 1 for summary). Given that either RLX (Royce et al., 2009; Royce et al., 2014b) or TFF2 (Royce et al., 2013a) alone demonstrated significant anti‐fibrotic efficacy after 14 days of administration to the OVA‐induced model of chronic AAD, a 7 day treatment period was intentionally chosen so that the potential additive effects of these drugs could be examined in the current investigation.

Table 1.

Summary of the individual versus combined effects of RLX, TFF2 and DEX in the OVA + NA model

Key features of human asthma OVA + NA OVA + NA + RLX OVA + NA + TFF2 OVA + NA + RLX + TFF2 OVA + NA + DEX OVA + NA + RLX + TFF2 + DEX
AI Inflammation score ↑ – – – ↓ ↓
AWR Goblet cell count ↑ – ↓ ↓ ↓ ↓
Epithelial damage ↑ ↓ ↓ ↓ ↓ ↓
Epithelial thickness ↑ ↓ ↓ ↓ ↓ ↓
Fibrosis Subepithelial ECM ↑ ↓ ↓ ↓ ↓ ↓
Total collagen ↑ ↓ – ↓ – ↓
Fibronectin ↑ ↓ ↓ ↓ ↓ ↓
TGF‐β1 ↑ ↓ ↓ ↓ ↓ ↓
α‐SMA ↑ ↓ ↓ ↓ – ↓
MMP‐2 – ↑ – ↑ – ↑
MMP‐9 – ↑ ↑ ↑ – ↑
AHR cDyn ↓ ↑ – ↑ – ↑
Open in a new tab

A summary of the individual versus combined effects of RLX, TFF2 and DEX on chronic AAD‐induced AI, AWR, fibrosis and cDyn. The arrows in the OVA + NA column are reflective of changes from that measured in saline/CO‐treated mice, while the arrows in the various treatment groups are reflective of changes to that in the OVA + NA group. – denotes no changes compared to the effects of OVA + NA alone. AAD, allergic airways disease; AI, airway inflammation; AHR, airway hyperresponsiveness; AWR, airway remodelling; cDyn, dynamic compliance; CO, corn oil; DEX, dexamethasone; NA, naphthalene; OVA, ovalbumin; RLX, serelaxin; TFF2, trefoil factor 2.

DEX alone significantly reversed the AI, goblet cell metaplasia, epithelial damage and thickness as well as the aberrant epithelial TGF‐β1 and subepithelial fibronectin expression induced by chronic AAD, but did not affect the AAD‐induced myofibroblast differentiation, total lung collagen concentration, gelatinase activity nor cDyn over the 7 day treatment period. These findings are consistent with those of others demonstrating that corticosteroids primarily target the AI and, to a lesser extent, epithelial damage, but not AWR and related fibrosis associated with asthma (Knight and Holgate, 2003; Girodet et al., 2011; Royce et al., 2014c), which contributes to their progressive ineffectiveness in treating the airway/lung dysfunction in chronic/severe disease settings. However, while they may seem at odds with the DEX‐induced reduction of MMP‐2 and MMP‐9 that have been reported in mice subjected to OVA‐induced AAD alone (Xu et al., 2012; Gurusamy et al., 2016), this may be explained by the fact that NA administration is likely to change the cellular composition of the airway epithelium, from mainly differentiated secretory cell types (which are the main source of MMPs (Tang et al., 2006)) to cuboidal cells that have a reduced secretory phenotype, which would produce significantly less MMPs. This is consistent with there being no marked changes in MMP‐2 or MMP‐9 level in the OVA + NA model, in which the lack of an up‐regulation of these MMPs may not have provided an aberrant target for DEX to mediate its gelatinase‐inhibitory actions.

On the other hand, TFF2 alone partially reversed the goblet cell metaplasia, epithelial damage and thickness, aberrant TGF‐β1 expression, myofibroblast differentiation and fibronectin deposition associated with chronic AAD and promoted MMP‐9 but not MMP‐2 expression and activity. However, it did not affect the AAD‐induced AI, aberrant lung collagen deposition or loss of cDyn, suggesting that its direct epithelial‐repairing properties were able to indirectly alleviate AWR to some extent, but not to that which affected airway/lung fibrosis and related dysfunction. In comparison, RLX alone significantly reduced the chronic AAD‐induced epithelial damage and thickness, aberrant TGF‐β1 expression, subepithelial myofibroblast differentiation and fibronectin expression as well as total lung collagen concentration in the absence of any direct effects on AI. RLX also significantly increased both MMP‐2 and MMP‐9 expression and activity and normalized the OVA + NA‐induced reduction in cDyn after just 7 days of treatment. These findings are consistent with the anti‐remodelling and anti‐fibrotic effects of both systemic (Royce et al., 2009, 2013b) and i.n. (Royce et al., 2014b) RLX administration that partially restored the chronic AAD‐induced increase in AHR.

Combining RLX and TFF2 was able to markedly reduce the chronic AAD‐induced goblet cell metaplasia to a greater extent than either therapy alone, significantly lower epithelial damage and thickness as well as aberrant epithelial TGF‐β1 expression levels to an equivalent extent as TFF2 alone, reverse the chronic AAD‐induced increase in myofibroblast accumulation, subepithelial ECM/fibronectin deposition and total lung collagen concentration and promote collagen‐degrading MMP‐2 and MMP‐9 levels to an equivalent extent as RLX alone, in the absence of any direct effects on AI, which remarkably resulted in the complete normalization of the AAD‐induced loss of cDyn after as little as 7 days of treatment. Further combining DEX with RLX and TFF2 offered even greater protection against the AI, epithelial damage, AWR and lung dysfunction associated with chronic AAD/asthma, resulting in the complete normalization of chronic AAD‐induced epithelial damage and loss of cDyn, suggesting that these combination therapies may serve as the basis of future personalized therapies for particular asthma endotypes.

This study confirmed a number of important findings: firstly, that airway epithelial damage and the ensuing fibrosis it causes are key contributors to lung dysfunction and exacerbate the effects of AI on AWR and AHR (Royce et al., 2014d) as well as cDyn. Epithelial damage has been shown to result in epithelial cells releasing pro‐inflammatory factors such as IL‐1 and TNF‐α (Elias et al., 1999; Holgate, 2000), which in turn, recruit and stimulate mast cells to release various factors including IL‐10. IL‐10 then actively recruits Th2 cells, which are stimulated to release IL‐13, which in turn, can both promote fibroblast proliferation and differentiation into activated myofibroblasts, while also stimulating the release of pro‐fibrotic TGF‐β1 activity (Elias et al., 1999; Holgate, 2000). TGF‐β1 potently stimulates myofibroblast‐mediated ECM and collagen deposition in the subepithelial basement membrane and, eventually, the interstitial space, resulting in the aberrant increase of subepithelial and interstitial ECM/collagen accumulation. Consistent with this, superimposing epithelial damage onto the pathogenesis of chronic AAD exacerbated airway epithelial TGF‐β1 expression levels, subepithelial myofibroblast differentiation and ECM/fibronectin accumulation, total lung collagen concentration and related AHR (Royce et al., 2014d), while all treatments that targeted both airway epithelial damage and fibrosis were able to somewhat restore, if not fully restore, the OVA + NA‐induced loss of cDyn.

Secondly, with respect to fibrosis, this study confirmed that ameliorating the aberrant collagen deposition (rather than other extracellular matrix proteins such as fibronectin) associated with chronic AAD‐induced AWR is key to improving the lung dysfunction that results from persistent AWR, as only RLX alone or its combined effects with TFF2 or TTF2 and DEX (that significantly ameliorated lung collagen concentration) were able to normalize the OVA + NA‐induced loss of cDyn. On the other hand, TFF2 or DEX administration, which only partially ameliorated the aberrant subepithelial fibronectin accumulation, but not total lung collagen concentration, did not effectively protect against the OVA + NA‐induced loss of lung function. These findings are somewhat consistent with those of others that utilized experimental models of other organ disease (Liao et al., 2010; Qu et al., 2014), suggesting that the amelioration of disease‐induced aberrant collagen deposition is required for protection against organ damage and related dysfunction. Furthermore, they demonstrate the necessity to be cautious when interpreting data from Masson's trichrome‐stained sections, which although stains all ECM proteins blue; most of that staining is thought to represent collagen deposition. The separate analysis of collagen by other means is clearly required to supplement Masson's trichrome staining of ECM deposition.

Thirdly, this study demonstrated that combining an epithelial repair factor with an anti‐fibrotic may provide a novel means to effectively treat several aspects of AWR associated with asthma, including epithelial damage and thickening, goblet cell metaplasia, smooth muscle hyperplasia, epithelial TGF‐β1 expression/activity, subepithelial myofibroblast differentiation and ECM/fibronectin deposition as well as total lung collagen concentration, all of which are associated with human disease pathogenesis (Tang et al., 2006; Holgate, 2008b; Royce et al., 2014c). This combination therapy may therefore offer greater protection against the AWR‐induced AHR and lung dysfunction that is associated with chronic/severe asthma, compared with current standard of care and protection for the 5–10% of asthmatics that are resistant to corticosteroid therapy (Fleming et al., 2007; Hetherington and Heaney, 2015). Furthermore, this combination therapy (of RLX + TFF2) may serve as an effective adjunct therapy to the anti‐inflammatory effects of corticosteroids to treat the three central components of asthma: AI, AWR and AHR. Of further note, corticosteroids can be slow‐acting and cause several side‐effects when chronically administered, particular at high doses. Various inhaled corticosteroids (administered at 0.5–6.4 mg·day−1) (Dahl, 2006) have been shown to induce both local side‐effects such as pharyngitis, dysphonia, cough and bronchospasm, as well as systemic side‐effects primarily involving suppressed hypothalamic–pituitary–adrenal (HPA)‐axis function and growth retardation. Animal mortality or body weight, however, were not affected in this study by DEX alone (0.5 mg·mL−1·day−1; i.n.) or in combination with RLX and TFF2 and in our previous study (Royce et al., 2013b) using methylprednisolone (0.3 mg kg−1/day; i.p) alone or in combination with RLX (0.5 mg·kg−1·day−1; s.c.; equivalent to the 0.8 mg·mL−1·day−1; i.n. used in this study). There is no reason to suggest that RLX or TFF2 treatment would exacerbate the side‐effects of DEX. On the contrary, the muscle‐relaxant properties of RLX (Baccari et al., 2004) may be helpful in protecting from bronchospasm, while the marked epithelial‐repairing and TGF‐β1‐inhibitory properties of TFF2 may aid to a certain extent in protecting from epithelial damaged‐induced AWR. Hence, the wide‐ranging positive effects of RLX and TFF2 might enable optimal dosing of corticosteroids used to be titrated to safer levels when used in combination, and the feasibility of combining these therapies was provided by the current study, albeit at the experimental level.

Even the pleiotropic organ‐protective effects of RLX alone would likely be effective enough to complement and augment the therapeutic effects of corticosteroid‐based therapies against the pathogenesis of asthma, as demonstrated previously in the chronic AAD model (Royce et al., 2013b). Not only has RLX been shown to activate glucocorticoid receptors (GRs) (Dschietzig et al., 2004) and inhibit TNF‐α‐induced endothelial dysfunction via GRs (Dschietzig et al., 2012), but also its ability to reverse AAD‐induced epithelial damage and thickening, TGF‐β1‐mediated myofibroblast contractility (Huang et al., 2011), differentiation and aberrant ECM deposition (Unemori et al., 1996; Royce et al., 2014b), and AWR‐induced AHR (Royce et al., 2009; Royce et al., 2014b) and loss of cDyn are consistent with its reported ability to enhance the therapeutic effects of methylprednisolone against chronic AAD‐induced subepithelial ECM deposition and related AHR (Royce et al., 2013b). In this study, the combined effects of RLX and DEX also markedly enhanced collagen‐degrading gelatinase expression and activity over the effects of DEX alone, suggesting that this combination therapy may be more effective in mediating the gelatinase‐induced breakdown of the mature collagen fibers that contribute to subepithelial fibrosis. Furthermore, RLX's added anti‐inflammatory actions via suppression of mast cell degranulation and leukocyte infiltration (Bani et al., 1997), anti‐apoptotic actions through its ability to increase the Bcl2/Bax ratio (Moore et al., 2007) and inhibit caspase‐3 activity (Masini et al., 2006), vasodilatory actions in blood vessels of several organs including the lung (Bani, 1997), pro‐angiogenic actions via the stimulation of vascular endothelial growth factor expression and new blood vessel growth (Unemori et al., 2000) and muscle‐healing properties (Mu et al., 2010) may also indirectly contribute to its ability to inhibit the AWR associated with chronic AAD and synergistically complement the anti‐inflammatory effects of corticosteroids.

In conclusion, we have evaluated the individual versus combined effects of a clinically used corticosteroid (DEX), an epithelial repair factor (TFF2) and anti‐fibrotic (RLX) in an experimental model of chronic AAD, which incorporates epithelial damage as part of its pathology and demonstrated that (i) RLX alone or in combination with TFF2 offers significant protection against the chronic AAD‐induced AWR, fibrosis and loss of lung function (for which there is currently no effective cure). Both drugs have been shown to be safe, and RLX at least has been well documented to specifically ameliorate pro‐fibrotic cytokine‐induced fibrosis progression without affecting normal fibroblast function and ECM production (Unemori et al., 1996). Furthermore, (ii) combining RLX and TFF2 with the anti‐inflammatory effects of DEX offered even greater protection against the AI, AWR and lung dysfunction associated with chronic AAD, suggesting that this combination therapy may offer a better alternative to current mainstay therapy for asthma sufferers. Not only do our findings demonstrate the feasibility of combining these therapies, but also they represent progress in the field of asthma pharmacology that has been static since the development of corticosteroids and β2‐adrenoceptor agonists some decades ago.

Author contributions

A.S.G., C.S.S. and S.G.R. participated in research design. K.P.P., C.S.S. and S.G.R. conducted experiments. A.S.G., C.S.S. and S.G.R. contributed reagents or tools. K.P.P., C.S.S. and S.G.R. performed data analysis. K.P.P., C.S.S. and S.G.R. wrote or contributed to writing of manuscript.

Conflict of interest

The authors declare no conflicts of interest.

Declaration of transparency and scientific rigour

This Declaration acknowledges that this paper adheres to the principles for transparent reporting and scientific rigour of preclinical research recommended by funding agencies, publishers and other organizations engaged with supporting research.

Acknowledgements

This study was supported in part by a Monash University MBio Postgraduate Discovery Scholarship (MPDS) to K.P.P, and National Health & Medical Research Council (NHMRC) of Australia Senior Research Fellowships to A.S.G. (GNT1002288) and C.S.S. (GNT1041766).

Patel, K. P. , Giraud, A. S. , Samuel, C. S. , and Royce, S. G. (2016) Combining an epithelial repair factor and anti‐fibrotic with a corticosteroid offers optimal treatment for allergic airways disease. British Journal of Pharmacology, 173: 2016–2029. doi: 10.1111/bph.13494.

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