JAK2 ACTIVATION AND CELL-PROLIFERATION INDUCED BY ANTIBODY-MEDIATED PROLACTIN RECEPTOR DIMERIZATION

Cytokines that interact with receptors of the hematopoietin superfamil y have recently been reported to stimulate receptor-associated JAK tyr osine kinases, including PRL activation of JAK2. Unlike other tyrosine kinases, none of the JAK kinases has thus far been implicated in onco genesis, and their involvement in growth signaling has not been establ ished. Using the PRL-dependent pre-T-cell line Nb2, the present study provided a link between bivalent dimerization of a hematopoietin recep tor and activation of its associated JAK kinase, and demonstrated a st rong positive correlation between the mitogenic potency of a series of bivalent anti-PRL receptor antibodies and the degree of induced tyros ine phosphorylation of JAK2. Antibody bivalency was required for JAK2 phosphorylation. Monovalent anti-PRL receptor Fab fragments alone were inactive, but their activity could be partially restored by cross-lin king with bivalent anti-Fab antibodies. Additional evidence for antibo dy-induced receptor dimerization was provided by a bell-shaped dose-re sponse curve for the most potent receptor agonist, monoclonal antibody T6. This phenomenon is typically seen at pharmacological concentratio ns of bivalent ligands, when bound ligand molecules fail to adjoin a s econd receptor due to occupancy. The present study provided functional support for a model of PRL receptor triggering by ligand-induced rece ptor homodimerization and subsequent activation of the associated tyro sine kinase JAK2.