Thioredoxin reductase is the major selenoprotein expressed in human umbilical-vein endothelial cells and is regulated by protein kinase C

Damage to the endothelium by reactive oxygen species favours atherogenesis. Such damage can be prevented by selenium, which is thought to exert its ac tions through the expression of selenoproteins. The family of glutathione p eroxidases (GPXs) may have antioxidant roles in the endothelium but other i ntracellular and extracellular selenoproteins with antioxidant actions may also be important. The selenoproteins expressed by cultured human umbilical -vein endothelial cells (HUVECs) were labelled with [Se-75]selenite and sep arated using SDS/PAGE. HUVECs secreted no extracellular selenoproteins. The re were distinct differences between the intracellular selenoprotein profil e of Se-75-labelled HUVECs and those of other tissues. A single selenoprote in with a molecular mass of 58 kDa accounted for approx. 43% of the intrace llular Se-75-labelled proteins in HUVECs, This protein was identified by We stern blotting as the redox-active lipid-hydroperoxide-detoxifying selenopr otein, thioredoxin reductase (TR). TR expression in HUVECs was down-regulat ed by transiently exposing cells to the phorbol ester PMA for periods as sh ort as 1 min. However, there was a delay of 48 h after PMA exposure before maximal down-regulation of TR was observed. The protein kinase C (PKC) inhi bitor bisindolylmaleimide I hydrochloride had no effect on TR expression wh en added alone, but the agent prevented the down-regulation of TR expressio n seen with PMA. The calcium ionophore A23157 increased TR expression in HU VECs after a 12-h exposure, but the maximal effect was only observed after a 35-h exposure. These findings suggest that TR may be an important factor in the known ability of Se to protect HUVECs from peroxidative damage. Furt hermore, the results also suggest that TR expression can be negatively regu lated through PKC. It is possible that TR expression may be positively regu lated by the calcium-signalling cascade, although TR induction by A23187 ma y be due to toxicity.