PARENT SESSION
SLIDE SESSION 18: HORMONE RECEPTORS, STEROID HORMONE ACTION
Chairs: Terry Nett, Leslie Heckert, Dustin Vale-Cruz (Trainee)
Univ Ottawa-Arts Hall 257
1:30 PM-3:30 PM

THE -SUBUNIT OF HUMAN CHORIONIC GONADOTROPIN CAN ACTIVATE THE LUTEINIZING HORMONE RECEPTOR.

Narayan, Prema¹, Gray, Judy¹, Puett, David¹, ¹

ABSTRACT- Human chorionic gonadotropin (hCG) is a member of the glycoprotein hormone family that includes luteinizing hormone, follicle stimulating hormone and thyroid stimulating hormone. All members share a common -subunit and a hormone-specific -subunit. Structure-function studies on hCG have identified amino acid residues on both subunits as being important in receptor binding and activation. We have previously shown that single chain hCG, when covalently linked to the luteinizing hormone receptor (LHR) to produce a yoked hormone-receptor complex, causes receptor activation in transfected cells and results in an elevated basal level of intracellular cAMP. Individual subunits yoked to the receptor are also active; yoked hCG-LHR is a significantly more potent agonist than yoked h-LHR, the latter of which exhibits minimal if any activity. These results suggested that the covalently attached hCG and hCG are functionally bound to LHR. In this study, we have examined if free hCG can activate LHR when it is not covalently attached to it. To address this question, hCG was co-transfected with myc-tagged LHR into HEK 293 cells. Basal levels of cAMP were elevated greater than 10-fold when compared to levels in cells transfected with myc-LHR alone. Addition of exogenous hCG did not increase the cAMP levels further, and cell surface binding of ¹²⁵I-hCG was not detected, implying that hCG was occupying the hormone binding site. Furthermore, co-transfection of myc-LHR with both h and hCG subunits did not increase the basal levels of cAMP over that measured in myc-LHR/hCG co-transfected cells. Cell surface expression of LHR, examined by antibody binding against the myc epitope showed that LHR was expressed at similar levels in myc-LHR-, myc-LHR/hCG- and myc-LHR/hCG/h-transfected cells. These results suggest that hCG by itself is a functional agonist and that a heterodimer is not absolutely required for receptor activation. Supported by NIH DK33973.

KEY WORDS: human chorionic gonadotropin , luteinizing hormone receptor, G-protein coupled receptor