The Role of HIF Hydroxylases in the Regulation of Colonic Epithelial Ion Transport

The intestinal epithelium is a monolayer of cells that lines the entire surface of the gastrointestinal tract from mouth to anus. A key function of the intestinal epithelium is the absorption and secretion of fluids and electrolytes. The intestinal epithelium is primarily an absorptive tissue, but under normal conditions secretion also occurs. In conditions of disease the delicate balance between absorption and secretion can become dysregulated leading to the onset of diarrhoea. Chloride (Cl') secretion, which is the primary driving force for intestinal fluid secretion, is highly dependent on the availability of oxygen (02) for generation of cellular energy. Hypoxia inducible factor (HIF) hydroxylases are the primary intracellular sensors of 0 2 availability. While recent studies have revealed that hydroxylases are important regulators of intestinal epithelial barrier function, there is still little known of their role in regulating epithelial fluid and electrolyte transport. This thesis set out to address this gap in our knowledge. These studies show

that inhibition of hydroxylases exerts profound antisecretory effects in vitro and in vivo. The antisecretory effect of hydroxylase inhibition occurs via specific inhibition of the activity of epithelial sodium potassium adenosine triphosphatase (Na+/K+- ATPase) pumps, the energy-dependent step of the Cl" secretory process. Investigations into potential mechanisms involved in the antisécretory actions of hydroxylase inhibition led to the identification of Modulator of Na+/K+- ATPase (MONaKA), a negative regulator of Na+/K+-ATPase function which, upon hydroxylase inhibition, displays increased expression at the basolateral membrane and forms part of a multimeric complex with Factor Inhibiting HIF (FIH-1) and the Na+/K+-ATPase itself. We propose that this complex acts as a molecular switch that, upon hydroxylase inhibition, downregulates epithelial secretory function. Our data have important implications for our understanding '-v of how epithelial fluid and electrolyte transport can be regulated under physiological and pathophysiological conditions and suggest that by virtue of their ability to alter epithelial transport, hydroxylases may be good targets for the development of new drugs to treat diarrhoeal diseases.