Hypoxia-induced expression of complement receptor type 1(CR1, CD35) in human vascular endothelial cells

Reoxygenation of hypoxic human umbilical vein endothelial cells(HUVECs) increases protein expression of the complement regulators CD46and CD55. As the receptor for C3b is known to be present on injuredbovine endothelial cells, we investigated whether hypoxia or inflammatory mediators induce complement receptor type 1 (CR1; CD35) expression on HUVECs. CR1 protein expressionincreased 3.7 ± 0.6-fold as measured by ELISA on HUVECsfollowing hypoxia (48 h, 1%O₂). Colocalization of CD35 andvon Willebrand factor by confocal microscopy confirmed that CD35 waspredominantly intracellular. Lipopolysaccharide or tumor necrosisfactor- also significantly increased HUVEC CR1 proteinexpression. Western blot analysis of neutrophil or hypoxicHUVEC lysates revealed a 221-kDa CR1 band under nonreducingconditions. RT-PCR of hypoxic HUVEC mRNA revealed a singleband that, after sequencing, was identified as CD35. In situhybridization of hypoxic HUVECs, but not normoxic HUVECs or fibroblasts, demonstrated increased CD35 mRNA.Hypoxic HUVECs bound immune complexes and acted as a cofactorfor factor I-mediated cleavage of C3b. Thus hypoxia induces functionalHUVEC CR1 expression.