Background—We recently described autoantibodies (angiotensin-1 receptor autoantibodies, AT₁-AA) directed at the AT₁ receptor in the serum of preeclamptic patients, whose placentas are commonly infarcted and express tissue factor (TF). Mechanisms of how AT₁-AA might contribute to preeclampsia are unknown. We tested the hypothesis that AT₁-AA cause vascular smooth muscle cells (VSMC) to express TF.

Methods and Results—IgG from preeclamptic patients containing AT₁-AA was purified with anti-human IgG columns. AT₁-AA were separated from the IgG by ammonium sulfate precipitation. We transfected Chinese hamster ovary cells overexpressing the AT₁ receptor with TF promoter constructs coupled to a luciferase reporter gene. VSMC were obtained from human coronary arteries. Extracellular signal-related kinase activation was detected by an in-gel kinase assay. AP-1 activation was determined by electromobility shift assay. TF was measured by ELISA and detected by immunohistochemistry. Placentas from preeclamptic women stained strongly for TF, whereas control placentas showed far less staining. We proved AT₁-AA specificity by coimmunoprecipitating the AT₁ receptor with AT₁-AA but not with nonspecific IgG. Angiotensin (Ang) II and AT₁-AA both activated extracellular signal-related kinase, AP-1, and the TF promoter transfected VSMC and Chinese hamster ovary cells, but only when the AP-1 binding site was present. We then demonstrated TF expression in VSMC exposed to either Ang II or AT₁-AA. All these effects were blocked by losartan. Nonspecific IgG or IgG from nonpreeclamptic pregnant women had a negligible effect.

Conclusions—We conclude that AT₁-AA and Ang II both stimulate the AT₁ receptor and initiate a signaling cascade resulting in TF expression. These results show an action of AT₁-AA on human cells that could contribute to the pathogenesis of preeclampsia.