(202f) Modeling for Thermal Injury Induced Acute Inflammation in Rat Liver

Thermal injury is among the most common causes of death from unintentional injury. In the United States, almost 1.2 million burn injuries are reported annually[1] and despite significant advances in burn care, infection induced by thermal injury remains a major cause of morbidity and mortality in burn patients[2; 3]. Response to thermal injury includes cellular protection mechanisms, inflammation, hypermetabolism, prolonged catabolism, organ dysfunction and immune-suppression[4]. Innate immune cell activation leads to production and release of proinflammatory cytokines, which are proximal mediators of the systemic inflammatory response. Following severe trauma, the liver plays a crucial role in mediating a host of physiological responses[5]. Therefore physical stress as a result of burn has a significant impact on the liver, an organ that plays a critical role in modulating immune function, inflammatory processes and the acute phase response in the attempt to restore homeostasis.

Though much is known about the molecular and physiological pathways of the acute inflammatory response induced by thermal injury, effective therapies are still elusive. The complex characteristics of the acute inflammatory response (AIR) and its complications have been thought to be a leading potential reason for the inability to propose effective clinical intervention strategies. Mathematical modeling may provide insights into the global dynamics of the inflammatory process from which therapies may be developed[6]. Thus, combinations of in silico and in vivo approaches are emerging as viable analysis strategy [7]. Previously we proposed a model for endotoxin-induced systemic inflammation using an indirect response model (IDR) in Foteinou et al [8] which not only describes the interaction between the extracellular signal and critical receptors driving downstream signal transduction cascades leading to transcriptional changes, but also can be used after extension to evaluate the effectiveness of corticosteroids under various treatment schedules establishing an in silico zone of therapeutic opportunity thereby facilitating the clinical making decision[9].

Motivated by this successful application, in this study, we explore the development of an in silico thermal injury- induced model in rat liver that aims at coupling extracellular signals with essential transcriptional responses through a receptor mediated indirect response model. Model development needs to be based on relevant experimental measurements with appropriate injury models. In previously published study, male Sprague-Dawley rats were subjected to a cutaneous 3rd degree burn injury consisting of a full skin thickness scald burn of the dorsum, calculated to be ~20% of the rat's total body surface area [10]. Liver samples were obtained at 5 time points (0, 1, 4, 8 and 24 h post burn). RNA extracted from the livers was isolated and subsequently hybridized to an Affymetrix U34A GeneChip that had 8,799 probes represented on each chip. One of the key aspects of this model is the systematic identification and enrichment of an elementary set of temporal responses that describe the trajectory of systemic inflammation in rat liver when exposed to burn stimulus by applying micro-clustering approach[11] and ARRAYTRACK[12] respectively. We first apply a symbolic representation of time series data that allows for clustering probe sets that are highly similar in gene expression profile. Based on the most statistically significant expression motifs we apply an optimization ? based algorithm that gives us five distinct temporal responses that are maximally affected by the stimulus- henceforth termed essential responses. These characterize the critical components the inflammatory response: the early (E), intermediate (M), and late (L) pro-inflammatory response, comprised of earliest, intermediate, and late released pro-inflammatory mediators such as TNF, IL-1α, IL-1β respectively; the anti-inflammatory response (A), comprised of anti-inflammatory mediators such as IL-10, TNFR; and a final response characteristic of hypermetabolism level (D) after thermal injury, comprised of genes associated with the anabolism of the system such as IGF. The relationship such as stimulation or inhibition between one and others have been collected and generated from published literature. These five essential responses and 7 other pivotal elements located in intracellular signaling pathways, along with a standard pharmacodynamic model for simulating the clearance of the extracellular signal are combined in an integrative PK/PD model using the principles of IDR[13]. The resulting model is described by a set of coupled ordinary differential equations containing the key aspects of inflammation such as pro-inflammation, anti-inflammation and organ dysfunction.

Therefore, the thermal injury-induced model is a critical enabler towards understanding the connectivity of the critical components of the immune system, the relationship among various components and offers opportunities for unraveling the control mechanisms of the onset and resolution of systemic inflammation. In addition, the ability of our model to achieve some predictions is also demonstrated: exploring the outcome in a longer time interval; investigating the implication of no insult, decreasing and increasing levels of initial insult; examining possible mechanistic dysregulation which may reflect secondary effects that lead to potential malfunction of the response leading to sustained inflammation; and searching the outcome of administration of receptor antagonist or anti-body.

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