Protein X has been shown to enhance protection against oxidative stress and studies have shown that it can decrease cardiac hypertrophy in cardiomyocytes. Although Protein X was discovered more than 20 years ago, its specific function or functions are unclear. Here we have studied the function of Protein X in mouse embryonic fibroblast (MEFs) from a knock-in model overexpressing (OE) Protein X, and its resistance to oxidative stress. Cells were exposed to 1 mM tert-butyl hydroperoxide for 1 hour and cell survival and recovery were evaluated after 24 and 48 hours of incubation. Surprisingly, Protein X OE did not result in an increased resistance to oxidative stress in MEFs under the conditions used herein. We also evaluated the previously suggested roles and mechanisms of Protein X. The results from treating cells with 400 nM of Compound A for 24 hours indicate that Protein X OE protects against Enzyme Y inhibition, suggesting that protein X can have a supplementary function to Enzyme Y. Wild type cells were less harmed by Enzyme Y inhibition when it was combined with mild Protein Z inhibition (5 nM of Compound B). This implicates that Protein X OE can have the same effect as partly inhibiting Protein Z during enzyme Y inhibition, opening up for

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