Influence of Vanin-1 and Catalytic Products in Liver During Normal and Oxidative Stress Conditions.

In liver, cysteamine in all probability represents a "low-capacity, high-affinity" scavenger of ROS. The available body of evidence suggests that reduced cysteamine and oxidized cystamine exist in equilibrium and that this ratio acts as an active redox sensor within the cell much like GSH. During normal liver homeostasis cysteamine's antioxidant properties are evident. Highly metabolic and/or pro-oxidative conditions, such as in mice treated with peroxisome proliferators, shift this equilibrium to favor the oxidized form. Under these conditions, cystamine is likely able to inactivate proteins involved in energy biogenesis through cysteaminylation of critical Cys residues as has been shown in vitro. This would allow cystamine to function as a "metabolic brake" to prevent the formation of additional ROS. In vivo, subcellular localization, pH, reducing capacity, FMO status and metabolic rate are all probable factors in determining the cysteamine:cystamine ratio. The availability of free cysteamine is also regulated by hydrolysis of pantetheine by pantetheinase. This cleavage results in the formation of pantothenic acid, a precursor to Coenzyme A which is prominently involved with lipid metabolism and energy production by the β -oxidation pathway and TCA cycle, respectively. Expression of pantetheinase is controlled by the Vnn1 gene and is upregulated in response to free fatty acids, PPAR activation or oxidative stress. The use of Vnn1 knockout mice has provided clear evidence that Vnn1 modulates redox and immune pathways In vivo, both of which appear at least partially due to a loss of cysteamine/cystamine. Immunologically, Vnn1 expression may influence cell signaling indirectly through maintenance of disulfide bonds or directly by interaction of pantetheinase on the cell surface. Cysteamine treatment has been used clinically as an antidote to APAP poisoning and in animal models against hepatotoxicants including APAP, galactosamine and CCl4. Protection in animal models occurs even when administered up to 12 hours following intoxication, suggesting that protection is the result of effects that occur downstream of bioactivation and covalent binding of reactive metabolites to target cellular macromolecules. Currently, the downstream influences of Vnn1 expression and cysteamine at endogenous concentrations remain largely unknown. Vnn1 knockout mice represent a valuable tool available to researchers investigating these events. Future studies in the field are needed to elucidate the precise mechanisms by which pantetheinase and/or cysteamine impact immune cell recruitment, cell signaling and survival, though it is clear that these factors have far reaching implications in the fields of immunology and toxicology.