Effects of Hypoxia and Inflammation on Citrulline Synthesis by Ornithine Transcarbamylase in Human Enterocytes

Overview

Chronic Obstructive Pulmonary Disease (COPD) is characterized by chronic systemic hypoxia and low-grade inflammation as well as by an alteration of arginine (ARG) metabolism. As ARG is synthetized from circulating citrulline (CIT), an alteration of CIT homeostasis, particularly its production by ornithine transcarbamylase (OCT) in small intestine could be involved. We hypothesized that hypoxia +/- inflammation, classically associated to COPD, has effects on OCT regulation in enterocytes. This study aims at exploring the effects of hypoxia and inflammation on the production of citrulline by ornithine transcarbamylase (OTC) activity in enterocytes from explant cultures of duodenal tissue.

Citrulline is an amino-acid almost exclusively released by the small intestine after its synthesis from glutamine by the OTC in enterocytes. Citrulline from the small intestine is released into the portal vena and, because it does not enter hepatocytes, it reaches the systemic circulation to be metabolized in arginine by kidneys. By this way, is an important source of endogenous ARG. Moreover, evidence suggests that circulating citrulline could have a direct action on the regulation of muscle protein synthesis. Therefore, the administration of citrulline might be an interesting nutritional strategy to preserve or restore muscle mass and function. Muscle mass is a determinant of respiratory function and muscle weakness explains for a large part morbidity and mortality in patients suffering from Chronic Obstructive Pulmonary Disease (COPD). Evidence suggests that citrulline plasmatic levels would be lower in several states involving systemic inflammation and hypoxia. Indeed, it has been observed in rats that hypoxia leads to a sharp decline in plasma CIT concentration and also in human (hypocitrullinemia has been observed in Intensive Care Unit patients and in subjects suffering from sepsis and trauma) but the cause of relationship is not yet established. Therefore, it may be supposed that a decreased plasma CIT level could be responsible for a decrease in de novo ARG synthesis leading to an impairment of NO production (endothelial dysfunction) in these pathological situations. Because chronic hypoxia and systemic inflammation are both systemic traits of patients suffering from COPD, the fact that hypoxia and/or systemic inflammation might directly affect OCT, decreasing intestinal citrulline production which, in turn, could contribute to endothelial dysfunction and muscle weakness is considered. In order to explore this hypothesis, the potential consequences of hypoxia and inflammation (alone or in association) on citrulline synthesis by the OCT in human enterocytes will be determined, thanks to an "explant" culture model of duodenal tissue. Duodenal biopsies will be removed during oesophago-gastro-duodenoscopies performed in the Hepato-gastro-enterology unit of Grenoble University Hospital, among patients expected to undergo a gastroscopy for any diagnostic purpose. 30 patients will be selected during a period of 6 months. After complete information and written agreement, 8 biopsy specimens will be removed from the duodenum of each patient and processed for organ culture. Then, duodenal biopsies will be incubated in the presence or not of cytokines and exposed or not to hypoxia. Indeed, 4 groups will be constituted: a control group (no stimulus), a group exposed to hypoxic conditions, a group exposed to inflammatory conditions (cytokines) and a group exposed to both hypoxic an inflammatory conditions. As primary endpoint, for each group, the OTC activity and the citrulline production in the culture medium will be studied.