REGULATION OF ENDOTHELIAL-CELL XANTHINE DEHYDROGENASE XANTHINE OXIDASE GENE-EXPRESSION BY OXYGEN-TENSION/

Recent studies have demonstrated that xanthine dehydrogenase/xanthine oxidase (XD/XO) activities of bovine endothelial cells (EC) are invers ely regulated by O-2 tensions to which the cells are exposed. We have confirmed these reports and extended the observation to a variety of c ells from other sources. Ah EC that had detectable XD/XO activity demo nstrated the greatest activity at the lowest O-2 level. Bovine pulmona ry artery smooth muscle cells showed XD/XO activity only under hypoxic conditions. The ratio of XO to XO + XD did not change significantly u nder various O-2 concentrations for all cell types tested. Treatment o f bovine pulmonary artery and rat epididymal fat pad EC with actinomyc in D (1 mu g/ml), an inhibitor of transcription, suppressed XO and XO + XD activities in cells exposed both to 20 and 3% O-2. High-dose cycl oheximide (5 mu g/ml), an inhibitor of translation, also reduced XO an d XO + XD activities in these cells, whereas low-dose cycloheximide (0 .5 mu g/ml) enhanced the stimulatory effect of hypoxia on XO + XD acti vity. We developed a digoxigenin-labeled probe that recognizes and hyb ridizes to rat XD cDNA and used it to examine the effect of O-2 concen tration on XD/XO mRNA expression of rat epididymal fat pad EC. XD/XO m RNA concentration was increased in cells exposed to hypoxia and decrea sed in cells exposed to hyperoxia compared with normoxic cells. The in crease in mRNA concentration resulting from exposure to hypoxia was en hanced by cycloheximide. There was no change in XD/XO mRNA stability i n cells exposed to hypoxia compared with normoxia. We conclude that th e regulation of XD/XO by oxygen tension most likely occurs at the tran scriptional level.