The long term goal of this project is to understand, at the cellular level, the mechanisms by which gastric mucosa protects itself against damage by trauma and systemic sepsis or stress. This proposal focuses on zinc as a signal of oxidative stress that occurs in gastric mucosa in response to injury followed by acute inflammation. Our preliminary studies suggest that the intracellular concentration of Zn ([Zn2+]i) is maintained at extraordinarily low levels by the actions of various zinc transporters, vesicular storage sites, and metal binding proteins in the cytoplasm, of which metallothionein (MT) is a major reservoir. Our studies also indicate that exposure to some oxidants leads to substantial increases in [Zn2+]i in epithelial cells of the acid-secreting gastric glands and mucus/HCO3-secreting surface epithelium. Little is known of the mechanisms regulating Zn2+ homeostasis in epithelial cells of the gastric mucosa- or in the gastrointestinal tract generally. We hypothesize that hypoxic injury and the ensuing inflammatory response lead to accumulation of Zn2+, in cells of the glands and of the surface epithelium. The downstream consequences of increases in [Zn2+]i include: suppression of acid secretion and enhancement of mucosal protective functions, alterations in second messenger pathways (Ca2+, cAMP/PKA, PKC), restraint of glycolysis and mitochondrial respiration, and containment of the intrinsic pathway of apoptosis. In general, oxidant-induced increases in [Zn2+]i would be viewed as a protective and anti-apoptotic. However, we also hypothesize that uncontrolled accumulation of [Zn2+]i may contribute to non-apoptotic, oxidantinduced epithelial cell injury and necrosis.
The Specific Aims of this proposal are: 1) to identify alterations in the mechanisms of uptake, release and disposal of labile Zn2+ gastric glands and surface epithelium, using in vitro models of oxidative stress; to evaluate Zn2+ as an intra-cellular messenger of oxidative stress, using in vitro and in vivo models of gastric gland and surface epithelial function to monitor responses in signal transduction and apoptosis pathways in response to oxidant-induced alterations in intracellular [Zn2+] signals; and 3) to explore the role of Zn2+ as an extra-cellular messenger of oxidative stress, using both in vitro and in vivo models to characterize oxidant-induced disturbances in [Zn2+] in the the lumen and subepithelial spaces of gastric mucosa and their effects on mucosal function and integrity. The proposed studies promise novel insights into the role of Zn2+ as an intracellular signal that regulates epithelial function in the gastric mucosa. In addition, these studies may identify therapeutic targets that are effective through control of Zn2+ homeostasis during oxidative stress. Such therapeutic strategies would be applicable not only to injury in the stomach, but to other regions of the GI tract affected by systemic stress and acute inflammation.
