ABSTRACT
Follicle-stimulating hormone (FSH) receptor binding inhibitor (FRBI-8) is a novel octapeptide purified from human ovarian follicular fluid. In vitro, it inhibits the binding of FSH to granulosa cells and in vivo, it induces atresia in developing follicles in rodents. This peptide, when administered to marmosets and bonnet monkeys, altered the circulating progesterone levels. This study was carried out to elucidate structure of the FRBI-8 and understand its mechanism of inhibiting interaction of FSH to its receptors. Homology modeling predicted that the FRBI-8 adopts a turn and random coil. This is further confirmed by circular dichroism and NMR. Docking studies of the FRBI-8 with reported FSH-FSHR hormone binding (FSHR(HB)) domain complex using ZDOCK algorithm revealed that the FRBI-8 binds to FSHbetaL2-FSHR(HB) binding interface which is otherwise known to be crucial for activation of signal transduction cascade. FRBI-8 analogs were designed by replacing the acidic amino acid residues at positions 2, 5 and 6 with Ala, individually. Docking studies revealed that D6A mutant (FRBI-8(D6A)) had a higher binding affinity than the native FRBI-8. In vitro radioreceptor assay with FRBI-8(D6A) showed 50% lower IC(50) compared with the FRBI-8, confirming the in silico observations. Thus, the study reveals that both FRBI-8 and FRBI-8(D6A) interfered with the binding of FSH to its receptor.
Subject(s)
Carrier Proteins/chemistry , Follicle Stimulating Hormone/metabolism , Peptide Fragments/chemistry , Peptides/chemistry , Receptors, FSH/metabolism , Amino Acid Sequence , Amino Acid Substitution , Animals , Callithrix , Carrier Proteins/pharmacology , Computer Simulation , Databases, Protein , Follicle Stimulating Hormone/antagonists & inhibitors , Humans , Macaca radiata , Peptide Fragments/pharmacology , Peptides/pharmacology , Protein Binding , Receptors, FSH/antagonists & inhibitors , SoftwareABSTRACT
Pituitary gonadotropins, follicle-stimulating hormone and luteinizing hormone, are the key regulators of ovarian folliculogenesis; these are known to be directly or indirectly modulated by many intraovarian factors. Our group has identified and studied one such novel peptide from human ovarian follicular fluid. Its partial N-terminal eight amino acid sequence has been deduced, referred to as octapeptide (OP). OP induces follicular atresia in mice and interferes with normal ovarian function in non-human primates, this action being similar to the native peptide. Thus, in this study, an attempt has been made to elucidate the mechanism of action of the synthetic OP by studying the pathway of follicular atresia in mouse ovary. Changes in granulosa cells were studied using various apoptotic markers by flow cytometry and immunohistochemistry. An increase in apoptotic cell population in atretic- and peptide-treated groups was observed compared with normal controls. Interestingly, both these groups exhibited differences in the apoptotic pathway. Results showed that the mitochondrial pathway was predominant in the atretic group, whereas the Fas-FasL pathway was predominant in the peptide-treated groups. The ultrastructural study also showed apoptotic changes in the OP-treated and atretic groups; the pattern of apoptosis differed at the subcellular level.
