Cripto-1 indirectly stimulates the tyrosine phosphorylation of erb B-4 through a novel receptor

Cripto-1 (CR-1) is a recently discovered protein of the epidermal growth fa ctor family that fails to directly bind to any of the four known erb B type 1 receptor tyrosine kinases. The present study demonstrates that CR-1 indi rectly induces tyrosine phosphorylation of erb B-4 but not of the epidermal growth factor-related receptors erb B-2 and erb B-3 in different mouse and human mammary epithelial cell lines. In addition, downregulation of erb B- 4 in NMuMG mouse mammary epithelial cells and in T47D human breast cancer c ells, using an anti-erb B-4 blocking antibody or a hammerhead ribozyme vect or targeted to erb B-4 mRNA, impairs the ability of CR-1 to fully activate mitogen-activated protein kinase. Finally, chemical cross-linking of I-125- CR-1 to mouse and human mammary epithelial cell membranes results in the la beling of two specific bands with a molecular weight of 130 and 60 kDa, sug gesting that the CR-1 receptor represents a novel receptor structurally unr elated to any of the known type I receptor tyrosine kinases. In conclusion, these data demonstrate that CR-1, upon binding to an unknown receptor, can enhance the tyrosine kinase activity of erb B-4 and that a functional erb B-4 receptor is required for CR-1-induced MAPK activation.