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

Reoxygenation of hypoxic human umbilical vein endothelial cells (HUVECs) in creases protein expression of the complement regulators CD46 and CD55. As t he receptor for C3b is known to be present on injured bovine endothelial ce lls, we investigated whether hypoxia or inflammatory mediators induce compl ement receptor type 1 (CR1; CD35) expression on HUVECs. CR1 protein express ion increased 3.7 +/- 0.6-fold as measured by ELISA on HUVECs following hyp oxia (48 h, 1% O-2) Colocalization of CD35 and von Willebrand factor by con focal microscopy confirmed that CD35 was predominantly intracellular. Lipop olysaccharide or tumor necrosis factor-alpha also significantly increased H UVEC CR1 protein expression. Western blot analysis of neutrophil or hypoxic HUVEC lysates revealed a 221-kDa CR1 band under nonreducing conditions. RT -PCR of hypoxic HUVEC mRNA revealed a single band that, after sequencing, w as identified as CD35. In situ hybridization of hypoxic HUVECs, but not nor moxic HUVECs or fibroblasts, demonstrated increased CD35 mRNA. Hypoxic HUVE Cs bound immune complexes and acted as a cofactor for factor I-mediated cle avage of C3b. Thus hypoxia induces functional HUVEC CR1 expression.