Introduction
Radiological accidents or belligerent use of a nuclear device is an
ever-present global concern. Thus, the development of radiation
countermeasures that mitigate the acute radiation syndrome (ARS) is a
priority for every nation to protect first responders, the military, and
civilians at large. At this time, no FDA approved radiomitigator exists.
The gastrointestinal (GI) tract has been considered as a highly
vulnerable target of irradiation. The radiation-induced GI dysfunction
is commonly referred to as GI-ARS. GI-ARS is characterized by
rapid-onset diarrhea during the early stage of radiation injury,
followed by endotoxemia and bacteremia, leading to septicemia in the
later stage. The fact that endotoxemia and bacteremia are essential
events in the pathogenesis of ARS and the colonic bacteria are the
primary source of endotoxins highlights the critical importance of
colonic tissue injury in the pathogenesis of ARS. Endotoxemia and
bacteremia are significant factors involved in multiple organ
dysfunction in ARS.
Typically, the colonic epithelial tight junction and mucosal barrier
function prevent diffusion of endotoxins and bacteria from the lumen
into the colonic tissue (Anderson & Van Itallie, 2009; Rao, 2008; Van
Itallie & Anderson, 2006). The tight junction, a multiprotein complex
that seals the intercellular space of the epithelium, is tightly
regulated by cell signaling networks (Basuroy, Seth, Elias, Naren &
Rao, 2006; Basuroy, Sheth, Kuppuswamy, Balasubramanian, Ray & Rao,
2003; Rao, 2008; Rao, Basuroy, Rao, Karnaky Jr & Gupta, 2002; Seth,
Sheth, Elias & Rao, 2007; Sheth, Samak, Shull, Seth & Rao, 2009b;
Sheth et al., 2007). Occludin, one of the transmembrane protein
component of the tight junction, interacts with a scaffold protein ZO-1,
which is an essential interaction for the assembly and maintenance of
tight junction. Adherens junction is located beneath the tight junction
and is known to indirectly regulate the integrity of tight junction
(Rao, Seth & Sheth, 2004). The tight junction and adherens junction
integrated to the underlying actin cytoskeleton define the apical
junctional complex (AJC) of the epithelium. Our recent study indicated
that the tight junction and adherens junction of the mouse colon are
highly sensitive to radiation (Shukla et al., 2016). These junctions are
disrupted as early as two hours post-irradiation with sustained damage
lasting at least for five days. These findings emphasize the importance
of early disruption of colonic epithelial tight junction and barrier
dysfunction in leading to endotoxemia, bacteremia, and septicemia in
GI-ARS. Understanding the cellular and molecular mechanisms of this ARS
that we propose to designate as “Colonic Radiation Sub-syndrome” is
crucially important for the development of effective radiomitigators and
treatment strategies.
Lysophosphatidic acid (LPA) is a growth factor-like lipid mediator that
rescues cells from genotoxic stress, including radiation-induced
apoptosis and cellular injury in vitro and in vivo when
administered post-irradiation (Deng, Balazs, Wang, Van Middlesworth,
Tigyi & Johnson, 2002). Exogenously delivered LPA, due to its short
half-life in plasma (~9 min), is not suitable for use as
a therapeutic. However, stabilized analogs of LPA such as
Radioprotectin-1 [RP1, (Patil et al., 2014; Patil et al., 2015)]
have radioprotective (Deng, Balazs, Wang, Van Middlesworth, Tigyi &
Johnson, 2002; Deng et al., 2015; Deng et al., 2007) as well as
radio-mitigative (Deng et al., 2015; Kiss et al., 2013) efficacy in
animal models of GI-ARS. In vitro receptor add-back studies and
experiments conducted with Lpar2 knockout
(Lpar2-/- ) mice indicate that this receptor
subtype is necessary and sufficient to protect cells and mice from
radiation-induced apoptotic cell death and tissue injury to the gut
(Deng et al., 2007; Lin, Lai, Makarova, Tigyi & Lin, 2007). Thus,
improved non-lipid analogs specific to the LPA2 receptor such as
RP1could be suitable for the treatment of colonic ARS without activation
of the other LPA receptor subtypes.
In this study, we evaluated the protective and mitigating effects of RP1
on the radiation-induced disruption of intestinal epithelial AJC, an
increase in mucosal permeability, and endotoxemia.