Although the precise mechanisms for their activities are still a matter of debate, most studies suggest that the NRPs function as obligate coreceptors by cooperatively enhancing the activity of the VEGF kinase receptors in nonneuronal tissues ( 6 , 11–15 ).
Although NRP1 and NRP2 are not normally expressed in adult tissues, their expression is detected on some human tumor cells ( 6 , 7 ).
Tumor angiogenesis is a complex process that requires interactions among endothelial cells, tumor cells, and other components of the microenvironment.
Among numerous secreted factors that promote angiogenesis, perhaps the most important is vascular endothelial growth factor (VEGF) ( 1 , 2 ).
Preliminary reports suggest that carcinomas primarily express NRP1, whereas neural tumors and melanomas largely express NRP2 [reviewed in ( 7 )].