Induction of MnSOD gene expression in a hepatic model of TNF-α toxicity does not result in increased protein

The model of toxic liver injury was used to examine the role of manganese superoxide dismutase (MnSOD) expression in cellular resistance to tumor necrosis factor (TNF)-α toxicity. The effects of the hepatotoxin D- galactosamine (GalN) and lipopolysaccharide (LPS) on hepatic and splenic TNF- α and MnSOD expression were studied. Treatment with GalN and LPS alone or in combination led to equivalent increases in hepatic and splenic TNF-α gene expression. Hepatic MnSOD mRNA levels were not affected by GalN or GalN with LPS but were increased 13-fold by LPS alone. Splenic MnSOD mRNA levels were increased twofold by GalN and 12-fold by either LPS alone or GalN plus LPS. The determination of MnSOD protein content, however, revealed no changes in hepatic or splenic steady-state levels of the protein with any of the treatments, despite the marked increases in MnSOD gene expression. Hepatic MnSOD enzyme activity was also unchanged by LPS or GalN plus LPS administration. Biosynthesis of MnSOD protein in rat hepatocytes isolated from an in vivo LPS-treated rat was unchanged compared with control. MnSOD mRNA levels were increased when GalN treatment was combined with uridine rescue, but again no change in protein was seen. The lack of any increase in MnSOD protein after GalN or LPS administration indicates that MnSOD upregulation is not involved in cellular resistance against TNF-α cytotoxicity in the liver in vivo.