Yoked complexes of human choriogonadotropin and the lutropin receptor: Evidence that monomeric individual subunits are inactive

Abstract

Human choriogonadotropin (hCG) contains an α-subunit, common to other members of the glycoprotein hormone family, and a unique β-subunit that determines hormone specificity. It is generally thought that heterodimer formation is obligatory for full hormonal activity, although other studies have indicated that individual subunits and homodimeric hCGβ were capable of low affinity binding to the LH receptor (LHR) and subsequent activation. Previously, we constructed two yoked hormone (hCG)-LHR complexes, where the two hormone subunits and the heptahelical receptor were engineered to form single polypeptide chains, i.e. N-β-α-LHR-C and N-α-β-LHR-C. Expression of both complexes led to constitutive stimulation of cAMP production. In the present study, we investigated whether the human α-subunit and hCGβ can act as functional agonists when covalently attached to or coexpressed with the LH receptor. Our initial results showed that hCGβ, but not α, was able to activate LHR with an increase in intra-cellular cAMP in human embryonic kidney 293 cells but not in Chinese hamster ovary or COS-7 cells. Further examination of this apparent cell-specific agonist activity of hCGβ revealed that low levels of endogenous α-subunit were expressed in human embryonic kidney 293 cells, thus enabling sufficient amounts of active heterodimer to form with the transfected hCGβ to activate LHR. The studies in Chinese hamster ovary and COS-7 cells clearly demonstrate that, even under experimental conditions where hormone-receptor interactions are maximized, individual subunits of hCG can not act as functional agonists, at least in their monomeric form.