Early growth response factor 1 is essential for cigarette smoke-induced MUC5AC expression in human bronchial epithelial cells.

Early growth response factor 1 (Egr-1) is a zinc finger transcription factor which responses rapidly to a variety of extracellular stimuli. Previous studies have suggested that Egr-1 exerts pathological functions in chronic obstructive pulmonary disease (COPD) by regulation of cigarette smoking-induced autophagy, cell death, and inflammation. However, little is known about the role of Egr-1 in regulation of mucus production in airway epithelium. In this study, we observed that cigarette smoke extract (CSE) induced a successive expression of Egr-1 and MUC5AC in human bronchial epithelial (HBE) cells. Knockdown of Egr-1 markedly attenuated CSE-induced MUC5AC production, and chromatin immunoprecipitation revealed that Egr-1 transcriptionally bound to MUC5AC promoter upon CSE stimulation. Concurrently, CSE increased the expression of c-Jun and c-Fos, two subunits of activator protein 1 (AP-1) which also critically regulates CSE-induced MUC5AC in HBE cells. CSE also induced a physical interaction of Egr-1 and AP-1, and knockdown of Egr-1 significantly decreased CSE-induced expression of c-Fos and c-Jun. Furthermore, knockdown of c-Fos remarkably attenuated the CSE-induced Egr-1 binding to MUC5AC promoter. These data taken together demonstrate that Egr-1 is essential for CSE-induced MUC5AC production in HBE cells likely through interaction with and modulation of AP-1, and re-emphasize targeting Egr-1 as a novel therapeutic strategy for COPD.

Interaction of caveolin-1 with ATG12-ATG5 system suppresses autophagy in lung epithelial cells.

Autophagy plays a pivotal role in cellular homeostasis and adaptation to adverse environments, although the regulation of this process remains incompletely understood. We have recently observed that caveolin-1 (Cav-1), a major constituent of lipid rafts on plasma membrane, can regulate autophagy in cigarette smoking-induced injury of lung epithelium, although the underlying molecular mechanisms remain incompletely understood. In the present study we found that Cav-1 interacted with and regulated the expression of ATG12-ATG5, an ubiquitin-like conjugation system crucial for autophagosome formation, in lung epithelial Beas-2B cells. Deletion of Cav-1 increased basal and starvation-induced levels of ATG12-ATG5 and autophagy. Biochemical analyses revealed that Cav-1 interacted with ATG5, ATG12, and their active complex ATG12-ATG5. Overexpression of ATG5 or ATG12 increased their interactions with Cav-1, the formation of ATG12-ATG5 conjugate, and the subsequent basal levels of autophagy but resulted in decreased interactions between Cav-1 and another molecule. Knockdown of ATG12 enhanced the ATG5-Cav-1 interaction. Mutation of the Cav-1 binding motif on ATG12 disrupted their interaction and further augmented autophagy. Cav-1 also regulated the expression of ATG16L, another autophagy protein associating with the ATG12-ATG5 conjugate during autophagosome formation. Altogether these studies clearly demonstrate that Cav-1 competitively interacts with the ATG12-ATG5 system to suppress the formation and function of the latter in lung epithelial cells, thereby providing new insights into the molecular mechanisms by which Cav-1 regulates autophagy and suggesting the important function of Cav-1 in certain lung diseases via regulation of autophagy homeostasis.

Caveolin-1 inhibits expression of antioxidant enzymes through direct interaction with nuclear erythroid 2 p45-related factor-2 (Nrf2).

The Nrf2 (nuclear erythroid 2 p45-related factor-2) signaling pathway is known to play a pivotal role in a variety of oxidative stress-related human disorders. It has been reported recently that the plasma membrane resident protein caveolin-1 (Cav-1) can regulate expression of certain antioxidant enzymes and involves in the pathogenesis of oxidative lung injury, but the detailed molecular mechanisms remain incompletely understood. Here, we demonstrated that Cav-1 inhibited the expression of antioxidant enzymes through direct interaction with Nrf2 and subsequent suppression of its transcriptional activity in lung epithelial Beas-2B cells. Cav-1 deficiency cells exhibited higher levels of antioxidant enzymes and were more resistant to oxidative stress induced cytotoxicity, whereas overexpression of Cav-1 suppressed the induction of these enzymes and further augmented the oxidative cell death. Cav-1 constitutively interacted with Nrf2 in both cytosol and nucleus. Stimulation of 4-hydroxynonenol increased the Cav-1-Nrf2 interaction in cytosol but disrupted their association in the nucleus. Knockdown of Cav-1 also disassociated the interaction between Nrf2 and its cytoplasmic inhibitor Keap1 (Kelch-like ECH-associated protein 1) and increased the Nrf2 transcription activity. Mutation of the resembling Cav-1 binding motif on Nrf2 effectively attenuated their interaction, which exhibited higher transcription activity and induced higher levels of antioxidant enzymes relative to the wild-type control. Altogether, these studies clearly demonstrate that Cav-1 inhibits cellular antioxidant capacity through direct interaction with Nrf2 and subsequent suppression of its activity, thereby implicating in certain oxidative stress-related human pathologies.