Identification of a Functional Leukocyte-Type NADPH Oxidase in Human Endothelial Cells : A Potential Atherogenic Source of Reactive Oxygen Species

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Abstract
Cultured human endothelial cells (EC) exposed to atherogenic low-density lipoprotein levels have increased reactive oxygen species (ROS) generation. The enzyme responsible for this ROS production elevation is unknown. We have examined for the presence of a functional leukocyte-type NADPH oxidase in EC to elucidate whether this enzyme could be the ROS source. The plasma membrane fraction of disrupted EC showed a reduced-minus-oxidized difference spectra with absorption peaks identical to those observed in the spectra of the leukocyte NADPH oxidase component, cytochrome b558. Western-blot analysis, using anti-gp91-phox. anti-p22-phox. anti-p41-phox. and anti p67-phox antibodies, demonstrated the protein expression of NADPH oxidase subunits in EC. Reverse transcriptase-polymerase chain reaction (RT-PCR) showed the mRNA expression p22-phox, p47-phox, and p67-phox in EC. Sonicates from unstimulated EC produced no measurable superoxide; whereas, exogenously applied arachidonic acid activated superoxide generation in a manner that was dependent upon the presence of NADPH and both membrane and cytosolic fractions combined. Apocynin, a specific leukocyte NADPH oxidase inhibitor, was shown by Western-blot analysis of membrane and cytoplasmic fractions to inhibit the translocation of p41-phox to the membrane of stimulated EC. These findings support the presence of a functionally active leukocyte-type NADPH oxidase in EC. NADPH oxidase could be the major cellular ROS source in EC perturbation, which has been hypothesized to be a major contributing factor in the pathogenesis of atherosclerosis.

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