Medicine, VA Pittsburgh Healthcare System University Drive Division, Pittsburgh, Pennsylvania, USA.
Division of Pulmonary, Allergy, and Critical Care Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
Thorax. 2023 Apr;78(4):402-408. doi: 10.1136/thoraxjnl-2021-217197. Epub 2022 Mar 17.
Birt-Hogg-Dubé syndrome (BHD) is a clinical syndrome manifesting with cystic lung disease and pneumothorax. Features of BHD result from the loss-of-function mutations of the folliculin () gene. Chronic obstructive pulmonary disease (COPD), characterised by an irreversible airflow limitation, is primarily caused by cigarette smoking.
Given that COPD often shares structural features with BHD, we investigated the link between COPD, cigarette smoke (CS) exposure and FLCN expression.
We measured the expression of FLCN in human COPD lungs and CS-exposed mouse lungs, as well as in CS extract (CSE)-exposed immortalised human airway epithelial cells by immunoblotting.
We found that the lung FLCN protein levels in smokers with COPD and CS exposure mice exhibit a marked decrease compared with smokers without COPD and room air exposure mice, respectively. We confirmed CS induced degradation of FLCN in immortalised human bronchial epithelial Beas-2B cells via ubiquitin proteasome system. Further, siRNA targeting FLCN enhanced CSE-induced cytotoxicity. By contrast, FLCN overexpression protected cells from CSE-induced cytotoxicity. We found that FBXO23, the ubiquitin E3 ligase subunit, specifically binds to and targets FLCN for degradation. Inhibition of ATM (ataxia-telangiectasia mutated) attenuated CSE induced FLCN degradation, suggesting a role of ATM in FLCN proteolysis. We further confirmed that the mutant of major FLCN phosphorylation site serine 62A is resistant to CSE-induced degradation and cytotoxicity.
Our study demonstrates that CS exposure is a secondary cause of FLCN deficiency due to the enhanced proteolysis, which promoted airway epithelial cell death.
Birt-Hogg-Dubé 综合征（BHD）是一种表现为囊性肺病和气胸的临床综合征。BHD 的特征是由于滤泡素（）基因的功能丧失突变所致。慢性阻塞性肺疾病（COPD）以不可逆的气流受限为特征，主要由吸烟引起。
鉴于 COPD 通常与 BHD 具有结构特征，我们研究了 COPD、香烟烟雾（CS）暴露和 FLCN 表达之间的联系。
我们通过免疫印迹法测量了人类 COPD 肺和 CS 暴露小鼠肺以及 CS 提取物（CSE）暴露的永生化人气道上皮细胞中的 FLCN 表达。
我们发现，与没有 COPD 和在室内空气暴露的吸烟者相比，患有 COPD 和 CS 暴露的吸烟者的肺 FLCN 蛋白水平明显降低。我们通过泛素蛋白酶体系统证实 CS 诱导了永生化人支气管上皮细胞 Beas-2B 中的 FLCN 降解。此外，靶向 FLCN 的 siRNA 增强了 CSE 诱导的细胞毒性。相比之下，FLCN 过表达可保护细胞免受 CSE 诱导的细胞毒性。我们发现，FBXO23，泛素 E3 连接酶亚基，特异性结合并靶向 FLCN 进行降解。ATM（共济失调毛细血管扩张突变）的抑制减弱了 CSE 诱导的 FLCN 降解，表明 ATM 在 FLCN 蛋白水解中的作用。我们进一步证实，主要 FLCN 磷酸化位点丝氨酸 62A 的突变体对 CSE 诱导的降解和细胞毒性具有抗性。
我们的研究表明，CS 暴露是由于增强的蛋白水解导致 FLCN 缺乏的次要原因，这促进了气道上皮细胞的死亡。
