Given that earlier studies have shown that HRV-infection of human airway epithelial cells releases products that are known to contribute to airway remodeling [16–19], and that HRV-infections synergizes with TGFβ1 in triggering epithelial to mesenchymal transition , we initially hypothesized that conditioned medium from HRV-infected epithelial cells would enhance TGFβ1-induced FMT, as monitored by increased α-SMA expression. Our data consistently demonstrated, however, that medium from epithelial cells infected with HRV significantly inhibited the ability of TGFβ1 to induce α-SMA expression in airway fibroblasts, using both western blotting and immunohistochemistry for α-SMA.
Studies using the non-selective cyclooxygenase inhibitor, diclofenac, demonstrated that this drug significantly, but not completely, reversed the ability of conditioned medium from HRV-infected HBE to inhibit TGFβ1 to induce α-SMA expression in airway fibroblasts. This implied that, while other mediators likely contribute to the inhibitory response seen, one or more prostaglandins play a significant role in inhibiting the ability of TGFβ1 to induce α-SMA expression in airway fibroblasts. It has previously been reported that HTV-infection of HBE can induce the generation of several prostaglandins, including PGE2 . Because PGE2 has previously been reported to inhibit TGFβ-induced α-SMA expression [24, 26], we tested the hypothesis that HRV infection induced production of from HBE and that this contributed to inhibition of TGFβ1induced α-SMA expression in human airway fibroblasts. Our data clearly demonstrated that HRV infection of HBE induced release of PGE2 at levels similar to those reported previously . We also confirmed that PGE2 inhibited TGFβ1induced α-SMA expression in a concentration dependent manner. Although mRNA analysis showed that fibroblasts significant expressed mRNA for both EP2 and EP4 receptors for PGE2, data using both selective antagonists and agonists demonstrated that effects of PGE2 were mediated entirely by interactions at the EP2 receptor, a G-protein coupled receptor with a Gsa subunit that activated adenylyl cyclase to increase intracellular levels of cAMP .
It is apparent that the interaction of HRV-infected airway epithelial cells with airway fibroblasts is complex. Although our current data show that supernatants from infected epithelial cells can reduce FMT, HRV-infected cells also release chemokines that are chemotactic for fibroblasts and could contribute to increased fibroblast numbers in the region of the lamina reticularis . Such fibroblasts also secrete matrix proteins that could lead to enhanced matrix protein deposition. Moreover, infected epithelial cells secrete a number of growth factors that can stimulate matrix protein deposition from fibroblasts and myofibroblasts .
A limitation of our study is that we used only epithelial cells and fibroblasts from healthy normal controls and, as such, our data may not fully represent data that may be obtained with cells derived from asthmatic airways. It is known that airway epithelial cells phenotype is altered in both children and adults with asthma [28, 29]. Moreover, several phenotypic changes in epithelial cells from asthmatic subjects, including increased expression of cytokeratin 5, altered production of cytokines and chemokines, and enhanced production of several growth factors linked to airway remodeling are maintained in culture [28, 30–33]. Thus, given that airway remodeling occurs in patients with asthma but not in healthy normal subjects, it would seem likely that epithelial cells from asthmatic subjects may exert properties that would be more supportive of aspects of remodeling. In support of this concept, it has been reported that co-culture of epithelial cells from asthmatic subjects with fibroblasts in the presence of TGFβ enhances expression compared to cells from normal subjects . Interestingly, it has been reported that, while expression of TGFβ is increased upon co-culture of fibroblasts with asthmatic epithelial cells, expression of prostaglandin E2 synthase is downregulated compared to co-cultures with normal epithelial cells . Thus, one may speculate that enhanced PGE2 expression in normal cells, along with reduced expression may help to prevent airway remodeling in normal subjects. Further studies will be required to address this. Finally, it must be noted that we are unaware of any studies thus far in which the interaction of asthmatic airway epithelial cells with fibroblasts also derived from asthmatic patients have been examined. It is feasible that this such studies may further exaggerate differences between normal and asthmatic cell models.
In summary, our data demonstrate that HRV-induced production of PGE2 from airway epithelial cells derived from normal subjects inhibits TGFβ-induced FMT. We speculate that this may contribute to preventing airway remodeling in normal subjects.