Pharmaco-nutritional approaches to combat heat stress-induced intestinal barrier dysfunction

Abstract

Climate changes have increased the prevalence and intensity of environmental and exertional heat stress (HS) conditions. Under HS conditions the thermoregulatory mechanism of the body shifts the splanchnic blood flow towards the peripheral circulation in order to facilitate heat dissipation. Subsequently splanchnic ischemia followed by hypoxia in the intestines can lead to a disruption of intestinal integrity and a “leaky gut syndrome”. The aim of the research project described in this thesis was I) to broaden the current knowledge about the vulnerability of the intestinal epithelium to HS-induced injury and to evaluate the endogenous defence mechanisms, such as the heat shock response (HSR) and antioxidant defence system and II) to characterize the gut-health promoting effects of functional nutritional supplements. By selecting compounds of different classes, the molecular mechanisms, by which these supplements may regulate the HSR, cellular redox status, intestinal integrity and inflammatory reactions under HS conditions, were elucidated. Using an in vitro model with Caco-2 cell monolayers (human colon cancer cells) grown on transwell inserts, it could be demonstrated that exposure to elevated temperatures (40-42°C) indeed resulted in a loss of barrier integrity, a derangement of E-cadherin adherens junction proteins and an increase in the paracellular transport across the intestinal epithelial cell monolayers. In addition, in vivo investigations using broiler chicken as the most heat susceptible animal model, confirmed these findings and clarified the differences of individual intestinal segments to HS. In the first part of this thesis in vitro and in vivo HS models were used to investigate the direct and indirect gut health-promoting effects of dietary galacto-oligosaccharides (GOS). Results showed that GOS prevented the heat-induced adverse effects on intestinal epithelial integrity by mitigating the alterations in the expression/distribution of tight junctions and adherens junctions and modulated the stress and inflammatory markers, such as heat shock protein (HSP) 70, HSP90, haem-oxygenase-1, hypoxia inducible factor-1, Toll-like receptor-4, IL-6 and IL-8. In the second part of this thesis, the beneficial effect of the essential amino acidL-Arginine, serving as a precursor of nitric oxideproduction, was investigated in the in vitro model with Caco-2 cell monolayers exposed to HS. Results showed that L-Arginine pre-treatment could prevent the loss of intestinal epithelial integrity and mitigate the adverse effect of HS on E-cadherin expression and cellular distribution by preserving the base-line NO production in Caco-2 cells. In the third part of this thesis, the effect of α-lipoic acid (ALA), as one of the most potent cellular antioxidants, was measured. Results showed that ALA could increase the resilience of Caco-2 cells to HS by inducing a protective HSR, preserving the intestinal integrity, enhancing epithelial cell proliferation, and reducing the inflammatory response in the cells exposed to HS. In summary, the experimental work described in this thesis identified a number of different promising candidates for a pharmaco-nutritional intervention to prevent HS-induced intestinal injuries.