The Pseudomonas aeruginosa autoinducer 3O-C12 homoserine lactone provokes hyperinflammatory responses from cystic fibrosis airway epithelial cells

Pseudomonas aeruginosa biofilm-associated homoserine lactone C12 rapidly activates apoptosis in airway epithelia

Pseudomonas aeruginosa (PA) forms biofilms in lungs of cystic fibrosis (CF) patients, a process regulated by quorum-sensing molecules including N-(3-oxododecanoyl)-l-homoserine lactone (C12). C12 (10-100 µM) rapidly triggered events commonly associated with the intrinsic apoptotic pathway in JME (CF ΔF508CFTR, nasal surface) epithelial cells: depolarization of mitochondrial (mito) membrane potential (Δψ(mito)) and release of cytochrome C (cytoC) from mitos into cytosol and activation of caspases 3/7, 8 and 9. C12 also had novel effects on the endoplasmic reticulum (release of both Ca(2+) and ER-targeted GFP and oxidized contents into the cytosol). Effects began within 5 min and were complete in 1-2 h. C12 caused similar activation of caspases and release of cytoC from mitos in Calu-3 (wtCFTR, bronchial gland) cells, showing that C12-triggered responses occurred similarly in different airway epithelial types. C12 had nearly identical effects on three key aspects of the apoptosis response (caspase 3/7, depolarization of Δψ(mito) and reduction of redox potential in the ER) in JME and CFTR-corrected JME cells (adenoviral expression), showing that CFTR was likely not an important regulator of C12-triggered apoptosis in airway epithelia. Exposure of airway cultures to biofilms from PAO1wt caused depolarization of Δψ(mito) and increases in Ca(cyto) like 10-50 µM C12. In contrast, biofilms from PAO1ΔlasI (C12 deficient) had no effect, suggesting that C12 from P. aeruginosa biofilms may contribute to accumulation of apoptotic cells that cannot be cleared from CF lungs. A model to explain the effects of C12 is proposed.

Exploitation of host signaling pathways by microbial quorum sensing signals

Environmental and commensal microbes that live within, on and around us have an enormous impact on human health. Recent progress in studies of prokaryotic interplay as well as host-bacteria interactions suggests that secreted microbial products, including quorum sensing signals (QSS), are important mediators of these intrakingdom and interkingdom relations. Reports have assigned QSS diverse and sometimes seemingly contradictory effects on mammalian cell physiology ranging from either blunting of the immune response or exerting pro-inflammatory activities to inducing cellular stress pathways and ultimately apoptosis. Thus, it is still unclear whether microbes utilize QSS to establish and maintain infections via modulation of host signaling pathways or if the eukaryotic host uses the conserved microbial QSS structures as molecular danger beacons to detect and fight infections. Along the same lines exactly how and under what circumstances QSS are detected by host cells remains a mystery, especially considering the distinct chemical properties of the QSS classes with some being small enough to passively diffuse across membranes while others most likely require extracellular recognition mechanisms.

CFTR and Ca Signaling in Cystic Fibrosis

Among the diverse physiological functions exerted by calcium signaling in living cells, its role in the regulation of protein biogenesis and trafficking remains incompletely understood. In cystic fibrosis (CF) disease the most common CF transmembrane conductance regulator (CFTR) mutation, F508del-CFTR generates a misprocessed protein that is abnormally retained in the endoplasmic reticulum (ER) compartment, rapidly degraded by the ubiquitin/proteasome pathway and hence absent at the plasma membrane of CF epithelial cells. Recent studies have demonstrated that intracellular calcium signals consequent to activation of apical G-protein-coupled receptors by different agonists are increased in CF airway epithelia. Moreover, the regulation of various intracellular calcium storage compartments, such as ER is also abnormal in CF cells. Although the molecular mechanism at the origin of this increase remains puzzling in epithelial cells, the F508del-CFTR mutation is proposed to be the onset of abnormal Ca²⁺ influx linking the calcium signaling to CFTR pathobiology. This article reviews the relationships between CFTR and calcium signaling in the context of the genetic disease CF.