Y. Bossé

Leukotriene D4‐induced, epithelial cell‐derived transforming growth factor β1 in human bronchial smooth muscle cell proliferation

Background Cysteinyl‐leukotrienes (cys‐LTs) orchestrate many pathognomonic features of asthma in animal models of allergic airway inflammation, including bronchial smooth muscle cell (BSMC) hyperplasia. However, because cys‐LTs alone do not induce mitogenesis in monocultures of human BSMC, the effect observed in vivo seemingly involves indirect mechanisms, which are still undefined.

Controversy surrounding the increased expression of TGFβ1 in asthma

Asthma is a waxing and waning disease that leads to structural changes in the airways, such as subepithelial fibrosis, increased mass of airway smooth muscle and epithelial metaplasia. Such a remodeling of the airways futher amplifies asthma symptoms, but its etiology is unknown. Transforming growth factor β1 is a pleiotropic cytokine involved in many fibrotic, oncologic and immunologic diseases and is believed to play an essential role in airway remodeling that occurs in asthmatic patients. Since it is secreted in an inactive form, the overall activity of this cytokine is not exclusively determined by its level of expression, but also by extensive and complex post-translational mechanisms, which are all importanin modulating the magnitude of the TGFβ1 response. Even if TGFβ1 upregulation in asthma is considered as a dogma by certain investigators in the field, the overall picture of the published litterature is not that clear and the cellular origin of this cytokine in the airways of asthmatics is still a contemporaneous debate. On the other hand, it is becoming clear that TGFβ1 signaling is increased in the lungs of asthmatics, which testifies the increased activity of this cytokine in asthma pathogenesis. The current work is an impartial and exhaustive compilation of the reported papers regarding the expression of TGFβ1 in human asthmatics. For the sake of comparison, several studies performed in animal models of the disease are also included. Inconsistencies observed in human studies are discussed and conclusions as well as trends from the current state of the litterature on the matter are proposed. Finally, the different points of regulation that can affect the amplitude of the TGFβ1 response are briefly revised and the possibility that TGFβ1 is disregulated at another level in asthma, rather than simply in its expression, is highlighted.

Interleukin-4 and Interleukin-13 Enhance Human Bronchial Smooth Muscle Cell Proliferation

Background: TH2 inflammation and bronchial smooth muscle cell (BSMC) hyperplasia are characteristic features of asthma, but whether these phenomena are linked remains unknown. This study aims to define the effect of the TH2 cytokines IL-4 and IL-13 on human BSMC proliferation when administered alone or in combination with the fibroblast growth factor 2 (FGF2) growth factor. In addition, the effects of the proinflammatory mediators TNFα and IL-1β and the involvement of members of the well-known family of platelet-derived growth factor (PDGF) mitogens were tested. Methods: BSMC proliferation was measured by crystal violet staining and PDGF and PDGF receptor (PDGFR) expression were determined by RT-PCR, immunocytochemistry, ELISA, flow cytometry and dot plot analysis. Results: Neither IL-4 nor IL-13 alone induced BSMC proliferation, despite both being potent inducers of PDGF-CC. However, following a pretreatment with FGF2, which increased PDGFR α chain expression, both IL-4 and IL-13 increased FGF2-induced BSMC proliferation in a time- and concentration-dependent manner. TNFα and IL-1β did not affect basal or FGF2-induced BSMC proliferation, but both proinflammatory mediators enhanced the proliferative synergism between FGF2 and the TH2 cytokines. Conclusions: IL-4 and IL-13 potently induce FGF2-primed BSMC proliferation via an autocrine loop involving PDGFRα and PDGF-CC, and this proliferative synergism is amplified by proinflammatory cytokines.

Leukotriene D4 Up-Regulates Furin Expression through CysLT1 Receptor Signaling

Leukotriene (LT)D(4) is suggested to play a role in airway remodeling, which is characterized by fibrogenesis and airway smooth muscle cell hyperplasia. In this study, we investigated the effects of LTD(4) on the expression of furin, a proprotein convertase involved in the maturation/activation of several substrates implicated in the remodeling processes. HEK293 cells stably transfected with the CysLT1 receptor were used to study the transcriptional regulation of furin by LTD(4). Stimulation of the cells with LTD(4) resulted in a time- and concentration-dependent induction of furin mRNA and protein expression. The study of furin gene (fur) promoters P1, P1A, and P1B revealed a selective transactivation of the P1 promoter by LTD(4). Mutations in the activator protein (AP)-1-binding element of the P1 promoter resulted in the partial loss of transactivation by LTD(4). Binding of AP-1 transcription factor to fur P1 promoter after stimulation with LTD(4) was demonstrated by electrophoretic mobility shift assay, and supershift assays indicated the formation of c-Jun/c-Fos complexes. LTD(4) induced the maturation of the furin substrates membrane-type 1 matrix metalloproteinase and transforming growth factor-beta1, which was inhibited by the furin inhibitor alpha1-PDX. Finally, LTD(4) induced furin gene expression in monocytic THP-1 cells, which was abrogated using a selective CysLT1 receptor antagonist and inhibitors of the mitogen-activated protein kinases MEK-1, p38, and JunK. Our data show for the first time that LTD(4), via the CysLT1 receptor, can transcriptionally activate furin production with consequent maturation of furin substrates relevant to airway remodeling. These findings suggest that CysLT1 is involved in remodeling processes through modulation of furin transcription.