Nonsense-mediated decay of mRNA for the selenoprotein phospholipid hydroperoxide glutathione peroxidase is detectable in cultured cells but masked orinhibited in rat tissues

Previous studies of mRNA for classical glutathione peroxidase I (GPx1) demo nstrated that hepatocytes of rats fed a selenium-deficient diet have less c ytoplasmic GPx1 mRNA than hepatocytes of rats fed a selenium-adequate diet. This is because GPx1 mRNA is degraded by the surveillance pathway called n onsense-mediated mRNA decay (NMD) when the selenocysteine codon is recogniz ed as nonsense. Here, we examine the mechanism by which the abundance of ph ospholipid hydroperoxide glutathione peroxidase (PHGPx) mRNA, another selen ocysteine-encoding mRNA, fails to decrease in the hepatocytes and testicula r cells of rats fed a selenium-deficient diet. We demonstrate with cultured NIH3T3 fibroblasts or H35 hepatocytes transiently transfected with PHGPx g ene variants under selenium-supplemented or selenium-deficient conditions t hat PHGPx mRNA is, in fact, a substrate for NMD when the selenocysteine cod on is recognized as nonsense. We also demonstrate that the endogenous PHGPx mRNA of untransfected H35 cells is subject to NMD. The failure of previous reports to detect the NMD of PHGPx mRNA in cultured cells is likely attrib utable to the expression of PHGPx cDNA rather than the PHGPx gene. We concl ude that 1) the sequence of the PHGPx gene is adequate to support the NMD o f product mRNA, and 2) there is a mechanism in liver and testis but not cul tured fibroblasts and hepatocytes that precludes or masks the NMD of PHGPx mRNA.