Smoke from cigarettes blocks self-healing processes in the lungs and consequently can lead to chronic obstructive pulmonary disease (COPD), according to researchers at the Helmholtz Zentrum München, partner in the German Centre for Lung Research (DZL), and their international colleagues.
Cough, bronchitis and breathing difficulties – these are the typical manifestations of COPD. Exact figures are not available, but estimates assume that 10% to 12% of the adults over 40 years of age in Germany suffer from the disease. Experts estimate the national economic costs of the disease at almost €6bn annually. Scientists around the world are attempting to discover how the disease develops and what biological adjustments can be made to stop it.
One approach involves the lung’s natural self-healing, which no longer takes place in COPD. “In healthy patients, the so-called WNT/beta-catenin signaling pathway is responsible for the lung’s homeostasis. Until now, it was not clear why it was silenced in patients with COPD,” explains Dr Melanie Königshoff, head of the Lung Repair and Regeneration (LRR) Research Unit of the Comprehensive Pneumology Centre (CPC) at Helmholtz Zentrum München. She and her team spent the last few years tackling this question in the framework of an ERC Starting Grant and discovered that one of the Frizzled molecules – Frizzled 4 – plays an important role.
“Frizzled 4 is a receptor molecule that sits on the surface of lung cells, where it regulates their self-renewal via WNT/beta-catenin,” explains first author Wioletta Skronska-Wasek, doctoral candidate at the LRR. “However, if the cells are exposed to cigarette smoke, Frizzled 4 disappears from the surface and cell growth comes to a halt.”
The starting point for the current study was the team’s observation that in the lung tissue of COPD patients, and especially that of smokers, there were significantly fewer Frizzled 4 receptors than in non-smokers. “In the next step, we were able to prove in cell culture and model systems that inhibition of Frizzled 4 signaling on the cells led to decreased WNT/beta-catenin activity and consequently to reduced wound healing and repair capacity,” described Dr Ali Önder Yildirim, an expert on the effects of cigarette smoke in the lung. He is a group leader at the Institute of Lung Biology of the CPC at the Helmholtz Zentrum München and also participated in the study. The authors additionally recognized that without the receptor, there was a loss of certain proteins that are important for the structure of lung tissue (including elastin, fibulin and IGF1) and the lung’s elasticity, allowing patients to breathe.
To confirm their results, the scientists artificially increased Frizzled 4 levels in a cell culture test to stimulate its production. The increase in Frizzled 4 reactivated the blocked repair process and led to the production of many of the previously reduced proteins.
“The activation of the Frizzled 4 receptor can restore the WNT/beta-catenin signaling pathway and consequently lead to repair in the lung,” explains Königshoff. This is an exciting starting point for further research which might develop new therapies for COPD patients.
Rationale: Chronic obstructive pulmonary disease (COPD), in particular emphysema, is characterized by loss of parenchymal alveolar tissue and impaired tissue repair. WNT/β-catenin signaling is reduced in COPD, however, the mechanisms thereof, specifically the role of WNT receptors Frizzled (FZD), remains unexplored.
Objective: To identify and functionally characterize specific FZD receptors that control downstream WNT signaling in impaired lung repair in COPD.
Methods: FZD receptor expression was analyzed in lung homogenates and primary alveolar epithelial type II (ATII) cells of never-smokers, smokers, and COPD patients, as well as two experimental emphysema models by qRT-PCR, immunoblotting, and immunofluorescence. The functional effects of cigarette smoke on WNT/β-catenin signaling and FZD4 function were investigated in primary ATII cells and cell lines. Gain- and loss-of-function approaches were applied to determine the effects of increased/decreased FZD4 expression on alveolar epithelial cell viability, wound repair, and elastogenesis.
Measurements and Main Results: FZD4 receptor expression was reduced in human and experimental COPD lung tissues as well as primary human ATII cells from COPD patients. Cigarette smoke exposure downregulated FZD4 expression in vitro and in vivo, along with reduced WNT target gene expression. Inhibition of FZD4 decreased WNT/β-catenin activity and epithelial cell proliferation, and interfered with ATII to ATI cell trans-differentiation, whereas FZD4 overexpression augmented WNT/β-catenin signaling and epithelial cell proliferation. Moreover, FZD4 overexpression rescued the cigarette smoke-induced decrease in elastin expression.
Conclusions: Reduced FZD4 expression in COPD contributes to impaired alveolar repair capacity. Thus, FZD4 represents a potential therapeutic target for COPD.
Wioletta Skronska-Wasek, Kathrin Mutze, Hoeke A Baarsma, Ken R Bracke, Hani N Alsafadi, Mareike Lehmann, Rita Costa, Mariano Stornaiuolo, Ettore Novellino, Guy G Brusselle, Darcy E Wagner, Ali Ö Yildirim, Melanie Königshoff