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.