Abstract
Inhibin is a heterodimeric peptide hormone produced in the ovary that antagonizes activin signaling and FSH synthesis in the pituitary. The inhibin β-subunit interacts with the activin type II receptor (ActRII) to functionally antagonize activin. The inhibin α-subunit mature domain (N terminus) arose relatively early during the evolution of the hormone, and inhibin function is decreased by an antibody directed against the α-subunit N-terminal extension region or by deletion of the N-terminal region. We hypothesized that the α-subunit N-terminal extension region interacts with the activin type I receptor (ALK4) to antagonize activin signaling in the pituitary. Human or chicken free α-subunit inhibited activin signaling in a pituitary gonadotrope-derived cell line (LβT2) in a dose-dependent manner, whereas an N-terminal extension deletion mutant did not. An α-subunit N-terminal peptide, but not a control peptide, was able to inhibit activin A signaling and decrease activin-stimulated FSH synthesis. Biotinylated inhibin A, but not activin A, bound ALK4. Soluble ALK4-ECD bioneutralized human free α-subunit in LβT2 cells, but did not affect activin A function. Competitive binding ELISAs with N-terminal mutants and an N-terminal region peptide confirmed that this region is critical for direct interaction of the α-subunit with ALK4. These data expand our understanding of how endocrine inhibin achieves potent antagonism of local, constitutive activin action in the pituitary, through a combined mechanism of competitive binding of both ActRII and ALK4 by each subunit of the inhibin heterodimer, in conjunction with the co-receptor betaglycan, to block activin receptor-ligand binding, complex assembly, and downstream signaling.
Highlights
The inhibin -subunit interacts with activin type II receptor to antagonize activin signaling
Competition for ActRII binding alone cannot account for the powerful antagonism of constitutive activin signaling in the pituitary by ovarian inhibin
Other clues suggested that the ␣-subunit, and its N-terminal region, may be involved in inhibin antagonism of activin: antibodies targeting the inhibin ␣-subunit N-terminal region and the N-terminal point mutant A257T associated with POF can decrease the antagonistic function of inhibin in the pituitary
Summary
The inhibin -subunit interacts with activin type II receptor to antagonize activin signaling. Human or chicken free ␣-subunit inhibited activin signaling in a pituitary gonadotropederived cell line (LT2) in a dose-dependent manner, whereas an N-terminal extension deletion mutant did not. Competitive binding ELISAs with N-terminal mutants and an N-terminal region peptide confirmed that this region is critical for direct interaction of the ␣-subunit with ALK4. These data expand our understanding of how endocrine inhibin achieves potent antagonism of local, constitutive activin action in the pituitary, through a combined mechanism of competitive binding of both ActRII and ALK4 by each subunit of the inhibin heterodimer, in conjunction with the co-receptor betaglycan, to block activin receptor-ligand binding, complex assembly, and downstream signaling
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