Bronchopulmonary Dysplasia: Crosstalk Between PPARγ, WNT/β-Catenin and TGF-β Pathways; The Potential Therapeutic Role of PPARγ Agonists

Abstract

Bronchopulmonary dysplasia (BPD) is a serious pulmonary disease which occurs in preterm infants. Mortality remains high due to a lack of effective treatment, despite significant progress in neonatal resuscitation. In BPD, a persistently high level of canonical WNT/β-catenin pathway activity at the canalicular stage disturbs the pulmonary maturation at the saccular and alveolar stages. The excessive thickness of the alveolar wall impairs the normal diffusion of oxygen and carbon dioxide, leading to hypoxia. Transforming growth factor (TGF-β) up-regulates canonical WNT signaling and inhibits the peroxysome proliferator activated receptor gamma (PPARγ). This profile is observed in BPD, especially in animal models. Following a premature birth, hypoxia activates the canonical WNT/TGF-β axis at the expense of PPARγ. This gives rise to the differentiation of fibroblasts into myofibroblasts, which can lead to pulmonary fibrosis that impairs the respiratory function after birth, during childhood and even adulthood. Potential therapeutic treatment could target the inhibition of the canonical WNT/TGF-β pathway and the stimulation of PPARγ activity, in particular by the administration of nebulized PPARγ agonists.

Keywords: PPARγ; TGF-β; bronchopulmonary dysplasia; canonical WNT/β-catenin; fibrosis; myofibroblast; nebulized thiazolidinediones.

Figure 1

WNT/β-catenin pathway on the embryological pulmonary development in humans. Five stages classically follow one another: embryonic, pseudo-glandular, canalicular, saccular, and alveolar. WNT/β-catenin signaling reaches a maximum activation at the 17th week (end of the pseudo glandular stage or beginning of the canalicular stage). In the middle of the canalicular stage (around the 21st week), canonical WNT pathway activity decreases dramatically, a necessary prerequisite for the correct realization of the alveolar stage. In the event of premature birth, the thick alveolar walls do not allow sufficient gas exchanges, which can lead to pulmonary hypoxia. Hypoxia activates the canonical WNT/β-catenin, impairs the alveolar stage, and promotes the synthesis of myofibroblasts and subsequent fibrosis, with abnormalities in the respiratory function.

Figure 2

The canonical β-catenin/WNT pathway: “on” and “off” states. The hallmark of the canonical β-catenin/WNT pathway activation is the elevation of the cytoplasmic β-catenin protein level, the subsequent nuclear translocation of β-catenin and further activation of β-catenin specific gene transcription. The canonical β-catenin/WNT pathway can be either in “on-state” or in “off-state.” The pathway is in “on-state” in the presence of a WNT ligand that binds both Frizzled (FZD) and LRP5/6receptors. This leads to activation of the phosphoprotein Disheveled (DSH). DSH recruits the destruction complex (pGSK-3β + AXIN + APC) to the plasma membrane, where AXIN directly binds the cytoplasmic tail of LRP5/6. APC is the adenomatous polyposis coli and GSK-3β is the glycogen synthase kinase-3β. In “on-state,” pGSK-3β is inactivated which corresponds to the phosphorylated state (pGSK-3β). Activation of DSH leads to the inhibition of both phosphorylation and degradation of beta-catenin. Beta-catenin accumulates into the cytosol and then translocates to the nucleus to bind lymphoid-enhancing/T cell (LEF-TCF) co-transcription factors. This induces the WNT-response gene transcription. In the “off state,” in the absence of WNT ligand or in the presence of the active form of GSK-3β (i.e., the unphosphorylated form of GSK-3β), cytosolic β-catenin is phosphorylated by the active form of GSK-3β. Beta-catenin undergoes the destruction process into the proteasome. (A): at the pseudo-glandular stage of the pulmonary development, the canonical WNT/β-catenin pathway is in “on-state.” (B): at the saccular and alveolar states, and in normal infants, the canonical WNT/β-catenin pathway is in “off-state.” The pulmonary development is normal. (C): at the saccular and alveolar states, in preterm infants with BPD, the canonical WNT/β-catenin pathway is in “on-state” and the pulmonary development is dramatically impaired.

Figure 3

Influence of TGF-β1 on the balance between the canonical WNT/β-catenin signaling and PPARγ. In the presence of the WNT ligands, the WNT receptor binds both LRP5/6 and FZD receptors to initiate LRP phosphorylation and DSH-mediated Frizzled internalization. This leads to the dissociation of the GSK-3 β/AXIN/APC destruction complex. Phosphorylation of β-catenin is inhibited and β-catenin accumulates in the cytosol and then translocates to the nucleus to bind TCF-LEF transcription factors. This leads to the WNT-response gene transcription (PDK, MCT-1, cMyc, and Cyclin D1). PPARγ inhibits the β-catenin/TCF-LEF-induced activation of WNT target genes. TGF-β also enhances WNT signaling through the inhibition of DKK1. DKK1 is activated by PPARγ. TGF-β1 binds type 2 TGF-βR2 receptor (TGF-βR2), which recruits type 1 TGF-βR1 receptor (TGF-βR1). This results in the formation of a heterotetramer that phosphorylates Smad. The Smad complex then translocates to the nucleus and regulates the transcription of target genes (CTGF, COL1A). A non-Smad pathway also occurs through PI3K-AKT. PTEN inhibits PI3K-AKT and PPARγ inhibits AKT. APC, adenomatous polyposis coli; CTGF, Connective tissue growth factor; DKK1, Dickkopf-1; DSH, Disheveled; FZD, Frizzled; GSK-3β, glycogen synthase kinase-3β; LRP5/6, low-density lipoprotein receptor-related protein 5/6; MCT-1, monocarboxylate lactate transporter-1; PPARγ, peroxisome proliferator-activated receptor gamma; PI3K, phosphatidylinositol 3-kinase and AKT, AKT/Protein Kinase B; PTEN, Phosphatase and tensin homolog; PDK, pyruvate; dehydrogenase kinase; TCF/LEF, T-cell factor/lymphoid enhancer factor; TGF, Transforming Growth Factor.