Membrane glycoprotein PC-1 inhibition of insulin receptor function occurs via direct interaction with the receptor alpha-subunit

Plasma cell membrane glycoprotein-1 (PC-1) inhibits insulin receptor (IR) t yrosine kinase activity and subsequent cellular signaling. PC-1 content is elevated in fibroblasts, muscle, and adipose tissue from insulin-resistant subjects, and its elevation correlates with in vivo insulin resistance. In vitro, when PC-1 is transfected and overexpressed in cultured cells, it inh ibits IR tyrosine kinase activity. To determine the mechanism whereby PC-I regulates the IR, we studied how PC-1 interacts with this protein. Overexpr ession of PC-1 in MCF-7 cells inhibited tyrosine kinase activity of the IR, but not of the IGF-I receptor. When the IR was immunocaptured by specific IR monoclonal antibodies, PC-1 was associated with this receptor. In contra st, after specific immunocapture, PC-1 was not associated with the IGF-I re ceptor. We next studied HTC cells that were overexpressing an IR ar-subunit mutant. This LR mutant binds insulin but has a deletion in the tyrosine ki nase regulatory domain located in amino acids 485-599. In contrast to norma l IRs, PC-1 did not associate with this mutant and did not affect tyrosine kinase activity. To determine whether decreasing PC-1 expression would reve rse the inhibition of tyrosine kinase activity, we treated MCF-7 cells over expressing PC-1 with a monoclonal antibody to PC-1. This treatment decrease d PC-1 levels; concomitantly, IR tyrosine kinase activity increased. In con trast, IGF-I receptor tyrosine kinase activity was not increased. These stu dies indicate, therefore, that PC-1 may inhibit the IR by interacting direc tly with a specific region in the IR alpha-subunit. These studies also rais e the possibility that monoclonal antibodies to PC-1 could be a new treatme nt for insulin resistance.