HYPOXIA REGULATES XANTHINE DEHYDROGENASE-ACTIVITY AT PRETRANSLATIONALAND POSTTRANSLATIONAL LEVELS

Hypoxia increases the activity of xanthine oxidase (XO) and its precur sor, xanthine dehydrogenase (XDH), but the mechanism of regulation is unclear. In hypoxic Swiss 3T3 cells, an early (0-24 h) cycloheximide-i nsensitive increase in XO-XDH activity, coupled with a lack of increas e in de novo XO-XDH synthesis (immunoprecipitation) or mRNA levels (qu antitative RT-PCR), demonstrated a posttranslational effect of hypoxia . Similarly, hyperoxia decreased XO-XDH activity faster than could be accounted for by cessation of XO-XDH protein synthesis, In further sup port of a posttranslational effect, cells transfected with a constitut ively driven XDH construct displayed an exaggerated increase in activi ty in hypoxia but no increase in activity in hyperoxia. However, more prolonged exposure to hypoxia (24-48 h) induced an increase in XO-XDH mRNA levels and de novo XO-XDH protein synthesis, suggesting an additi onal pretranslational effect, Finally, hypoxic induction of XO-XDH act ivity was found to be cell-type-restricted. We conclude that control o f XO-XDH levels by oxygen tension is a complex process which involves several points of regulation. (C) 1997 Academic Press.