| Glutathione (GSH) is the most abundant low molecular weight thiol in cells, and has been found to be essential for normal intestinal function. Considerable evidence has demonstrated the involvement of GSH in cell proliferation both in intestinal and other cell systems. GSH/Glutathione disulfide (GSSG) redox potential has been implicated in cellular processes such as proliferation, differentiation and apoptosis. In response to chemical inducers of differentiation, the GSH/GSSG redox of intestinal cells in culture becomes more oxidized. Also, numerous studies demonstrate that depletion of cellular GSH inhibits proliferation. Extracellularly, cysteine (Cys) is the most abundant low molecular weight thiol. The extracellular redox environment maintained by the cysteine/cystine (CySS) couple has also been implicated in intestinal cell proliferation, with reducing conditions stimulating proliferation. However, specific mechanisms for extracellular redox-induced cell growth are not known.; The first aim of this dissertation was to evaluate the response of the GSH/GSSG and Cys/CySS redox in response to spontaneous differentiation of intestinal cells. The findings demonstrate that the GSH/GSSG redox of proliferative cells is more reduced than cells undergoing differentiation. Evaluation of the extracellular Cys/CySS redox also showed a similar growth-dependent change in redox potential.; The second aim of this dissertation determined the mechanism(s) by which extracellular Cys/CySS redox affects proliferation independent of increases in intracellular GSH levels. The data show that more reducing extracellular Cys/CySS redox cause a marked increase in p44/42 MAPK activation when compared to the oxidizing conditions. Reducing Cys/CySS redox conditions increased EGFR (epidermal growth factor receptor) phosphorylation, upstream of p44/42 MAPK (mitogen-activated protein kinase). Inhibitors of EGFR phosphorylation, metalloproteinases and TGFalpha attenuated Cys/CySS redox-dependent activation of MAPK and cell proliferation, indicating that reducing conditions stimulate cell proliferation by activating signaling via the EGFR, in a metalloproteinase-dependent manner. The results provide a mechanistic basis for extracellular redox control of proliferation.; Finally, the third aim of this dissertation was (i) to determine if modification of dietary sulfur amino acid (SAA) content could affect the thiol/disulfide redox in an in vivo model and (ii) to determine if dietary supplementation with SAA could lead to reducing conditions and improve adaptation in a rat model of SBS. Rats consumed an isocaloric and isonitrogenous diet containing either 25%, 100% or 300% SAA for 7 days. Results show that reduction of SAA intake leads to oxidation of plasma Cys/CySS and GSH/GSSG redox states, as well as oxidation of gut mucosal GSH/GSSG redox state. SAA supplementation improved plasma redox for the Cys/CySS couple. The data therefore show that a reduction in SAA intake can oxidize the Cys/CySS and GSH/GSSG redox in plasma and tissues and that this can be countered by increased dietary SAA. (Abstract shortened by UMI.)... |
