Cigarette smoking is a secondary cause of folliculin loss

Background: 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 (FLCN) gene.
Chronic obstructive pulmonary disease (COPD), characterised by an
irreversible airflow limitation, is primarily caused by cigarette smoking.
Objective: Given that COPD often shares structural features with
BHD, we investigated the link between COPD, cigarette smoke (CS) exposure
and FLCN expression.MethodsWe 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. Results: 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. Conclusions: Our study
demonstrates that CS exposure is a secondary cause of FLCN deficiency due
to the enhanced proteolysis, which promoted airway epithelial cell death.