Research Article: X-Box Binding Protein 1 (XBP1s) Is a Critical Determinant of Pseudomonas aeruginosa Homoserine Lactone-Mediated Apoptosis

Date Published: August 22, 2013

Publisher: Public Library of Science

Author(s): Cathleen D. Valentine, Marc O. Anderson, Feroz R. Papa, Peter M. Haggie, Matthew R. Parsek.


Pseudomonas aeruginosa infections are associated with high mortality rates and occur in diverse conditions including pneumonias, cystic fibrosis and neutropenia. Quorum sensing, mediated by small molecules including N-(3-oxo-dodecanoyl) homoserine lactone (C12), regulates P. aeruginosa growth and virulence. In addition, host cell recognition of C12 initiates multiple signalling responses including cell death. To gain insight into mechanisms of C12-mediated cytotoxicity, we studied the role of endoplasmic reticulum stress in host cell responses to C12. Dramatic protection against C12-mediated cell death was observed in cells that do not produce the X-box binding protein 1 transcription factor (XBP1s). The leucine zipper and transcriptional activation motifs of XBP1s were sufficient to restore C12-induced caspase activation in XBP1s-deficient cells, although this polypeptide was not transcriptionally active. The XBP1s polypeptide also regulated caspase activation in cells stimulated with N-(3-oxo-tetradecanoyl) homoserine lactone (C14), produced by Yersinia enterolitica and Burkholderia pseudomallei, and enhanced homoserine lactone-mediated caspase activation in the presence of endogenous XBP1s. In C12-tolerant cells, responses to C12 including phosphorylation of p38 MAPK and eukaryotic initiation factor 2α were conserved, suggesting that C12 cytotoxicity is not heavily dependent on these pathways. In summary, this study reveals a novel and unconventional role for XBP1s in regulating host cell cytotoxic responses to bacterial acyl homoserine lactones.

Partial Text

Pseudomonas aeruginosa is an opportunistic bacterium and infections caused by this pathogen constitute a significant heath-care burden. P. aeruginosa is the fourth most-commonly isolated nosocomial pathogen and infections can be fatal, particularly in immuno-compromised subjects such as those with burns or undergoing chemotherapy [1], [2], [3], [4]. Chronic conditions including cystic fibrosis (CF), chronic obstructive pulmonary disease, acquired immunodeficiency syndrome, and non-CF bronchiectasis are also associated with P. aeruginosa infection [5], [6], [7]. Antimicrobials are currently used against P. aeruginosa; however, infections are typically refractory to therapeutic interventions [8]. Furthermore, drug resistant P. aeruginosa strains have been isolated and biofilm formation in chronic conditions enhances antimicrobial resistance and pathogenicity [9], [10]. As such, there is urgent need to understand mechanisms of P. aeruginosa virulence and for new strategies to combat infections [10], [11].

Infections associated with P. aeruginosa result in significant mortality and constitute a major worldwide healthcare burden. Our understanding of P. aeruginosa virulence mechanisms is limited and there is an urgent need to identify new approaches that can be employed to reduce P. aeruginosa infectivity. P. aeruginosa employ QS to regulate growth, virulence factor expression and biofilm formation. In addition to quorum signalling, P. aeruginosa-derived C12 homoserine lactone activates several host cell responses including cytotoxicity. The major conclusion of this study is that C12-mediated host cell cytotoxic responses largely require expression of the XBP1s transcription factor (Figure 7). Cell death and caspase activation were dramatically reduced (∼95%) in cells that cannot generate XBP1s (Ire1α−/− MEFs) or lack the XBP1 gene (Xbp1−/− MEFs). To the best of our knowledge, this study represents the first description of cells that are C12-tolerant due to genetic ablation of a protein. Restoring XBP1s expression in either Ire1α−/− or Xbp1−/− MEFs was sufficient to re-establish caspase 3 cleavage upon C12 stimulation. Although the present study does not provide full mechanistic detail, we demonstrate that a fragment of XBP1s encompassing the leucine zipper and transcriptional activation domains (XBP1Δ2) that is not transcriptionally active is sufficient to restore C12-mediated caspase 3 cleavage. Knockout cell lines were used to incisively demonstrate the role of XBP1s in homoserine lactone-mediated cell death; however, expression of XBP1Δ2 in rat and human epithelial cells resulted in enhanced caspase activation in response to C12- and C14-stimulation, indicating that XBP1s can mediate apoptotic responses in distinct cell types. Further experiments will be required to establish that XBP1s regulates C12-mediated apoptosis in cells that are exposed to P. aeruginosa, including macrophages, neutrophils and airway epithelial cells; however, XBP1s is expressed at high levels in these cells [45].




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