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Follicle-stimulating hormone is important for mammalian reproduction. It acts through specific receptors located on the plasma membrane of granulosa cells in ovaries and Sertoli cells in testes. The binding of follicle-stimulating hormone to its receptor activates intracytoplasmic signaling pathways leading to steroidogenesis. These steroids in turn regulate the follicle-stimulating hormone action from the anterior pituitary through exerting negative feedback effect. In addition to steroids, non-steroidal factors secreted by the ovaries are believed to modulate follicle-stimulating hormone action through autocrine/paracrine mode. One such low molecular weight peptide referred to as follicle-stimulating hormone receptor-binding inhibitor-8 purified from human follicular fluid has been extensively studied. Follicle-stimulating hormone receptor-binding inhibitor-8 has been shown to inhibit binding of follicle-stimulating hormone to its receptor. The present article describes the effect of follicle-stimulating hormone receptor-binding inhibitor-8 on follicle-stimulating hormone-induced signaling in rat granulosa cells. Follicle-stimulating hormone receptor-binding inhibitor-8 inhibited the follicle-stimulating hormone-induced cAMP, and the effect was observed to be mediated through the protein kinase A. Further, an inhibitory effect of follicle-stimulating hormone receptor-binding inhibitor-8 on the granulosa cell proliferation was evaluated using COV434 cell line which is derived from the human granulosa cell tumor. The effect of the peptide on the cell cycle analysis showed an increase in apoptotic population and the arrest of G1 phase. These findings suggest that follicle-stimulating hormone receptor-binding inhibitor-8 acts as a follicle-stimulating hormone antagonist and affects the follicle-stimulating hormone-mediated signaling and proliferation in the granulosa cells.

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.

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The prostate secretory protein of 94 amino acids (PSP94) has been shown to interact with cysteine-rich secretory protein 3 (CRISP-3) in human seminal plasma. Interestingly, PSP94 expression is reduced or lost in the majority of the prostate tumours, whereas CRISP-3 expression is upregulated in prostate cancer compared with normal prostate tissue. To obtain a better understanding of the individual roles these proteins have in prostate tumourigenesis and the functional relevance of their interaction, we ectopically expressed either PSP94 or CRISP-3 alone or PSP94 along with CRISP-3 in three prostate cell lines (PC3, WPE1-NB26 and LNCaP) and performed growth inhibition assays. Reverse transcription-polymerase chain reaction and Western blot analysis were used to screen prostate cell lines for PSP94 and CRISP-3 expression. Mammalian expression constructs for human PSP94 and CRISP-3 were also generated and the expression, localization and secretion of recombinant protein were assayed by transfection followed by Western blot analysis and immunofluorescence assay. The effect that ectopic expression of PSP94 or CRISP-3 had on cell growth was studied by clonogenic survival assay following transfection. To evaluate the effects of co-expression of the two proteins, stable clones of PC3 that expressed PSP94 were generated. They were subsequently transfected with a CRISP-3 expression construct and subjected to clonogenic survival assay. Our results showed that PSP94 and CRISP-3 could each induce growth inhibition in a cell line specific manner. Although the growth of CRISP-3-positive cell lines was inhibited by PSP94, growth inhibition mediated by CRISP-3 was not affected by the presence or absence of PSP94. This suggests that CRISP-3 may participate in PSP94-independent activities during prostate tumourigenesis.

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