the transcriptional regulators LasR and RhlR, respectively [1]; in
turn, these regulators can alter gene expression of as much as 10%
of the entireP. aeruginosagenome [2, 3]. Such extensive control of
gene expression via quorum sensing givesP. aeruginosathe means
to coordinate multi-faceted and aggressive infections once the cell
population has achieved a significant density.
The secretion of these autoinducers into the environment
affords the opportunity for microbes to coordinate an orchestrated
attack; however, these chemical signals are also sensed by host cells.
Mammalian cells detect and respond to molecules produced by
microorganisms, such asP. aeruginosa3O-C 12 -HSL, in a process
known as inter-kingdom signaling. There are numerous microbe–-
host interactions in the human gastrointestinal tract, including
those between the gut microbiota and intestinal epithelial cells
that can impact host health and the development of disease [1, 4].
However, the downstream consequences on host cells that incur as a
result of inter-kingdom signaling are still poorly understood.
Given the plethora of host–microbe activity in the gut, it comes
as no surprise that intestinal epithelial cells can be disrupted by a
variety of exogenous agents, including those of bacterial origin.
3O-C 12 -HSL has been shown to affect intestinal epithelial cells by
disrupting junctional proteins, which regulate epithelial barrier
integrity and the permeability of small molecules [5–10]. Human
Caco-2 cells, which are derived from a human epithelial colorectal
adenocarcinoma, are routinely employed to evaluate aspects of
epithelial barrier integrity such as absorption and permeability
[11]. These cells functionally and morphologically mimic epithelial
cells of the small intestine when cultured under specific conditions,
making them an excellent model to study various aspects of epithe-
lial barrier activities [12, 13]. 3O-C 12 -HSL can disrupt not only the
barrier integrity of Caco-2 cells by altering tight junction and
adherens junction proteins but can also perturb cell migration
and Rac1- and Cdc42-dependent reorganization of the actin cyto-
skeleton [7], which can have significant implications on bacterial
translocation.
Tight junctions compose a complex of proteins on the apical
membrane of epithelial cells and function to maintain cell–cell
adhesion and barrier permeability. Similarly, at the basolateral
membrane of epithelial cells, adherens junctions also play a role in
barrier integrity in addition to maintaining the structure of tight
junctions. TheP. aeruginosaquorum sensing molecule 3O-C 12 -
HSL interacts with and targets the IQ-motif-containing GTPase-
activating protein (IQGAP1) in human epithelial cells. Interaction
with scaffold IQGAP1 triggers changes in the cytoskeleton net-
work and signaling cascades [7]. Exposure to 3O-C 12 -HSL
increases intracellular free calcium in Caco-2 cells and alters the
phosphorylation status of tight and adherens junction protein com-
plexes [8]. As a result, 3O-C 12 -HSL downregulates expression of214 Jake Everett et al.