Rapid degradation of PrxI and PrxII induced by silica in Rat2 cells

Peroxidases of the peroxiredoxin (Prx) family catalyze the reduction of H2O 2 and lipid peroxides, The effects of H2O2, 12-O-tetradecanoylphorbol 13-ac etate (TPA), and silica on the abundance of two cytosolic isoforms of Prx ( PrxI. and PrxII) were examined in Rata cells. TPA induces the production of reactive oxygen species (ROS) in various mammalian cell types, and silica induces the production of ROS in Rata cells. Whereas H2O2 and TPA did not a ffect the concentration of PrxI or Prx II, silica triggered a rapid degrada tion of both Prx enzymes. Silica also induced degradation of the NF-KB inhi bitor I kappa B-alpha. N-Acetylcysteine and diphenyleneiodonium, both of wh ich inhibit the accumulation of intracellular ROS, each blocked silica-indu ced degradation of I kappa B-alpha but had no effect on that of the Prx enz ymes, suggesting that ROS do not contribute to Prx proteolysis, The silica- induced degradation of Prx enzymes was also insensitive to the proteasome i nhibitors MG132 and lactacystin, whereas I kappa B-alpha proteolysis was co mpletely blocked by these inhibitors. Experiments with the Ca2+ ionophore A 23187 indicated that a Ca2+-dependent protease such as calpain might contri bute substantially to silica-induced degradation of PrxII, but only moderat ely to that of PrxI, These results indicate that silica increases cellular oxidative stress not only by inducing ROS production, but also by triggerin g the degradation of Prx enzymes that are responsible for elimination of ce llular ROS, Such aggravated oxidative stress might be important in the init ial pathogenesis of silica-associated pulmonary diseases. (C) 1999 Academic Press.