Mucus hypersecretion, a hallmark of asthma is responsible for the increasing morbidity and mortality associated with this disease. However, the underlying mechanisms of mucus production are still poorly understood. Earlier studies have defined a critical role for IL-13 in the induction of GCM. Periostin, an extracellular matrix protein is induced in epithelial cells by inflammatory cytokines including IL-4 and IL-13 (12
) and in lung fibroblasts by the anti-inflammatory cytokine TGF-β (34
). Moreover, periostin can also co-localize with other extracellular matrix proteins to trigger subepithelial fibrosis (12
). Whether these effects are important in the development of allergic inflammation was not well characterized. Thus, in the present study we determined whether periostin promotes inflammation or has a protective homeostatic effect in regulating airway inflammation. Our study demonstrated that periostin-deficiency led to a marked increase in airway resistance and mucus production upon sensitization and challenge with OVA. Despite the development of eosinophilic airway inflammation to a similar extent in Postn−/−
mice as wild-type littermates, deficiency of periostin specifically augmented mucus production as evident from PAS-staining index and expression of Muc5ac
in allergen-challenged lung tissues.
Recent studies have implicated periostin as a biomarker for IL-13-dependent corticosteroid responsive asthma (35
), expressed in airway epithelial cells of asthma patients. Phenotypic heterogeneity of asthmatics could be classified into “Th2-high” and “Th2-low” asthma based on the epithelial expression of the IL-13-inducible genes POSTN
). Patients with “Th2-high asthma” had significantly increased AHR, eosinophilic inflammation and induction of epithelial mucins MUC5AC
. There are similar associations of increased periostin expression in biopsies of patients with eosinophilic esophagitis (20
). However, these correlations do not determine the biological function of periostin, which could be acting to promote inflammation, or as part of a negative feedback mechanism to control inflammation. Our studies suggest that allergen sensitization and challenge induced periostin expression (), which functioned to attenuate mucus gene expression and GCM. The mechanism of inhibition of GCM by periostin remains to be elucidated. Earlier studies have suggested that periostin acts as a cell adhesion molecule by binding to integrins αvβ3 and αvβ5 in cancer cells (38
). Thus, it is possible that periostin binds to integrins alpha4 and beta 1/2, which are involved in asthma (39
), and may trigger intracellular signaling pathways that repress mucus-inducing transcription factors, including NF-κB, Sp1 and AP-1 (49
). Regardless, our data demonstrate that in the absence of periostin, there was increased differentiation of epithelial cells into mucus-producing goblet cells, suggesting that periostin may contribute to the homeostasis of GCM during allergic inflammation.
How perisotin contributes to airway function is still unclear. There were no differences in cellular infiltration, Penh, or airway resistance and compliance, between wild type and Postn−/− mice in the absence of sensitization and challenge, suggesting that the differences observed in arise from the allergic inflammation and not from a requirement for periostin in lung development. This is consistent with the observation of low perisotin expression in non-challenged lungs (). Analysis of sensitized and challenged mice by whole body plethysmography suggested an increase in airway hyperreactivity. However, use of the more invasive method of testing lung function demonstrated increased airway resistance and decreased compliance in Postn−/− mice compared to control mice, even in the absence of methacholine challenge (). While the airway resistance was significantly increased in Postn−/− mice compared to wild type mice following methacholine challenge, the fold difference between wild type and Postn−/− mice at baseline and after methacholine challenge was similar. These results suggest that periostin-deficiency is not affecting airway hyperreactivity per se, but rather affects baseline lung function following the induction of allergic inflammation. We did not observe obvious changes in epithelial size, smooth muscle content or fibrosis between wild type and Postn−/− lungs, suggesting that the primary effect of perisotin-deficiency is in increased mucus production that results in decreased airflow through the respiratory tract.
The increased Th2 responses in the periphery were somewhat surprising, considering the lack of a direct effect of periostin on Th cell differentiation. As a possible mechanism for this phenomenon, we demonstrated that periostin augments the production of IFNγ, a cytokine that both inhibits Th2 and promotes Th1 responses. We propose a model wherein DCs in the lung would be exposed to periostin induced during allergic inflammation that would alter the amounts of cytokines they secrete, when as mature DC, they migrate to lymph nodes to activate T cells. The ability of periostin to alter Th2 cytokine production might also impact GCM. Thus, the effects of periostin-deficiency on GCM could be through at least two mechanisms; direct effects on airway epithelial cells, and indirect effects of altering APC function and Th2 cytokine production. Although Th2 cytokine production was modestly increased in the lung of Postn
−/− compared to wild type mice, neither inflammation nor Tslp
expression were appreciably increased ( and data not shown). It is possible that the modest increase in Th2 cytokines was not sufficient to alter the amount of inflammation. It is also possible that, as previously reported (20
), periostin plays a role in cell recruitment and this was balanced by increased Th2 cytokines. Further studies can distinguish these possibilities.
In a recent study (20
) of periostin-deficient mice, a decrease in eosinophil recruitment to the lungs and increase in blood eosinophilia was observed with no significant differences in mucus production upon intranasal inhalation with allergens from A. fumigatus
. There may be several reasons for these differences, including the allergen used to generate airway inflammation. It is also possible that genetic background may contribute to the observed differences; the mice used in our study were on a mixed background, the mice in the previous study, while not stated specifically, were likely C57BL/6 mice (50
). Differences in immune pathways activated by these two protocols may result in differential dependence on periostin for regulation of GCM.
In conclusion, our studies have defined a novel previously unrecognized role for periostin in regulation of GCM. Regulation of mucus production is important in minimizing the symptoms and reducing mortality associated with severe asthma attacks (51
). Our data suggest that although periostin is induced by cytokines associated with allergic inflammation, it functions in the lung to limit mucus production and the resulting effects on airway resistance. This negative feedback mechanism may be an important component of maintaining airway function while the immune system combats pathogens, or in the case of allergens, responds to innocuous proteins. Our results examining the function of periostin in vivo
and on primary tracheal epithelial cells suggest that it may be a potential therapeutic in the regulation of mucus production.