Mechanism for nicotine-induced up-regulation of Wnt signaling in human alveolar interstitial fibroblasts

Nicotine exposure alters normal homeostatic pulmonary epithelial-mesenchymal paracrine signaling pathways, resulting in alveolar interstitial fibroblast (AIF)-to-myofibroblast (MYF) transdifferentiation. Since the AIF versus MYF phenotype is determined by the expression of peroxisome proliferator-activated receptor gamma (PPAR gamma) and Wingless/Int (Wnt) signaling, respectively, the authors hypothesized that nicotine-induced AIF-to-MYF transdifferentiation is characterized by the down-regulation of PPAR gamma, and the up-regulation of the Wnt signaling pathway. As nicotine is known to activate protein kinase C (PKC) signaling, the authors also hypothesized that in AIFs, nicotine-induced up-regulation of Wnt signaling might be due to PKC activation. Embryonic human lung fibroblasts (WI38 cells) were treated with nicotine (1 x 10(-6) M) for either 30 minutes or 24 hours, with or without 30-minute pretreatment with calphostin C (1 x 10(-7) M), a pan-PKC inhibitor. Then the authors examined the activation of PKC (p-PKC) and Wnt signaling (p-GSK-3 beta, beta-catenin, LEF-1, and fibronectin). Furthermore, activation of nicotinic acetylcholine receptor (nAChR)-alpha 3 and -alpha 7 and whether a PPAR gamma agonist, rosiglitazone (RGZ), blocks nicotine-mediated Wnt activation were examined. Following nicotine stimulation, there was clear evidence for nAChR-alpha 3 and -alpha 7 up-regulation, accompanied by the activation of PKC and Wnt signaling, which was further accompanied by significant changes in the expression of the downstream targets of Wnt signaling at 24 hours. Nicotine-mediated Wnt activation was almost completely blocked by pretreatment with either calphostin C or RGZ, indicating the central involvement of PKC activation and Wnt/PPAR gamma interaction in nicotine-induced upregulation of Wnt signaling, and hence AIF-to-MYF transdifferentiation, providing novel preventive/therapeutic targets for nicotine-induced lung injury.