Development of a specific and sensitive two-site enzyme-linked immunosorbent assay for measurement of inhibin-A in serum

Inhibin at 90: from discovery to clinical application, a historical review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

Inhibins in female and male reproductive physiology: role in gametogenesis, conception, implantation and early pregnancy

A great deal of new information has arisen in the recent years concerning inhibin physiology and clinical relevance in reproductive medicine. It is now recognized that the two inhibin isoforms, named inhibin A and inhibin B, are produced by the gonads in the course of gamete maturation and in women have a different pattern of secretion throughout the menstrual cycle. Since inhibins are also produced by placenta and fetal membranes, it has been suggested that there is an involvement in physiological adaptation of pregnancy. Evidence from several sources has underlined the clinical usefulness of the measurement of inhibin-related proteins in the diagnosis and follow-up of different fertility disturbances and early pregnancy viability. In the male, inhibin B is produced in the testis, principally by the Sertoli cells. Inhibin B expression and secretion are positively correlated with Sertoli cell function, sperm number, and spermatogenic status and are negatively correlated with FSH. This review covers the most recent advances on the role of inhibins in human reproductive function. Considerable progress in the understanding of inhibin physiology has resulted from selective measurement of the two inhibin molecular forms, named inhibin A and B. Newly recognized alterations of inhibin levels in gynaecological diseases as well as in normal and pathological pregnancy are discussed, with particular emphasis on the potential clinical usefulness of assessing inhibin levels in serum and other biological fluids.

Effect of intracellular interactions on the processing and secretion of bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9. Implication of the aberrant ovarian phenotype of BMP-15 mutant sheep

Bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9 (GDF-9) are members of the transforming growth factor-beta superfamily. Both molecules are closely related in their primary structures and share a nearly identical spatiotemporal expression pattern in the oocyte during folliculogenesis in mammals. Here we have established a series of cell lines, which express recombinant BMP-15, GDF-9, or both, and investigated whether they form homodimers and/or heterodimers. We demonstrate the first evidence that both BMP-15 and GDF-9 can form non-covalent homodimers when expressed individually, while when both are co-expressed BMP-15/GDF-9 heterodimers are produced. Interestingly, when GDF-9 and BMP-15 are co-expressed the processing of both proproteins are significantly impaired as compared with that of the singly expressed proproteins, suggesting that the proprotein heterodimer is less susceptible to proteolytic cleavage than the individual homodimers. Since BMP-15 mutant sheep, called Inverdale, exhibit severe defects in ovarian function we have also established stable transformants expressing the mutant BMP-15 (InvBMP-15) alone or together with GDF-9. Although InvBMP-15 was previously predicted to be unable to form homodimers, we show here that it does form non-covalent dimers; however, the processing efficiency of InvBMP-15 proprotein is significantly lower than wild-type BMP-15. Surprisingly, when GDF-9 is co-expressed, the processing and secretion of InvBMP-15 is abolished, and the processing of GDF-9 is also severely impaired, suggesting that the heterodimers of InvBMP-15/GDF-9 proproteins are not susceptible to proteolytic cleavage and thus degrade in the cells. Based on these findings we propose a novel hypothesis that a decrease in GDF-9 secretion may be involved in causing infertility in homozygous Inverdale ewes.

Inhibin gene expression in a large cell calcifying Sertoli cell tumour and serum inhibin and activin levels

Inhibin is a potential tumour suppressor gene product in the gonads. While inhibin gene products may have a role in tumourigenesis, serum inhibin levels can be used as a marker for ovarian tumours derived from granulosa cells. Tumours derived from Sertoli cells, testicular counterparts of granulosa cells, are rare. To assess whether inhibin could be used as a human Sertoli cell tumour marker, serum inhibin and activin levels and inhibin subunit mRNA expression in the testis were studied. Northern blot and in situ hybridization revealed abundant expression of inhibin alpha, beta A, and beta B subunit mRNAs in large cell calcifying Sertoli cell tumours found in a 12-year old boy with Carney complex. The tumours were multifocal and bilateral. Serum inhibin levels were clearly elevated at the time of the diagnosis, decreased by 50% after one of the testes was removed, and were low or undetectable after the second orchidectomy six weeks later. Activin was undetectable before the orchidectomies, while a low concentration of activin-A was measured after them. Follicle stimulating hormone (FSH) concentration increased from normal pubertal value to castration level as expected. Normal seminiferous tubules also showed inhibin subunit alpha and beta B mRNA expression, whereas inhibin beta A mRNA was expressed in normal Leydig cells. These data suggest that serum inhibin reflects Sertoli cell activity and can be used as a human tumour marker.

Alterations in plasma and ovarian immunoreactive inhibin during reproductive aging in the female rat

Previously, we showed that ovarian inhibin α- and β;(A)-subunit mRNAs are elevated in middle-aged and old persistent-estrous (PE) female rats. To determine whether higher inhibin subunit mRNA expressions result in increased circulating inhibins during reproductive aging, plasma immunoreactive inhibin α (ir-inh α) and gonadotropins were measured in young, middle-aged and PE rats. Plasma LH profiles were distinctly divergent in the middle-aged rats with some showing LH surges indistinguishable from young rats and others showing significantly attenuated LH surges. Plasma ir-inh α in middle-aged rats with LH surges were similar to those of young rats. However, animals with attenuated LH surges had higher peak ir-inh α levels than young and middle-aged animals with LH surges. Immunohistochemistry revealed increased levels of ovarian inhibin α staining in those animals with attenuated LH surges. Overall, the highest plasma and ovarian inhibin α were found in PE rats which lack LH surges. However, significant decreases of plasma and ovarian inhibin α were seen after reinstatement of estrous cyclicity with progesterone implant treatment. Thus, increases in both plasma and ovarian inhibin α appear to be closely associated with attenuation or loss of the preovulatory gonadotropin surge that occurs during aging.

Inhibin as a marker for ovarian cancer

Inhibin is a polypeptide hormone produced by the granulosa cells of the ovary, and is present in body fluids as dimers of various sizes each comprising an alpha- and beta-subunit. Free forms of the alpha-subunit also circulate, and the presently available radioimmunoassay (Monash assay) cannot distinguish these from biologically active dimeric inhibin. Recently we described a new two-site enzyme immunoassay able for the first time to measure the levels of dimeric inhibin throughout the human menstrual cycle. The sensitivity limit of this assay is 2 pg ml-1 in human serum with cross-reactivity against activin of 0.05%. The normal range of inhibin in post-menopausal women is < 5 pg ml-1, in pre-menopausal women 2-80 pg ml-1 (2-10 pg ml-1 in the follicular phase, 40-80 pg ml-1 in the luteal phase). This assay was used to determine inhibin levels in sera from 15 (five pre-menopausal and ten post-menopausal) patients with granulosa cell tumours of the ovary. It was raised in a pre-menopausal patient preoperatively (261 pg ml-1), in six post-menopausal patients (32, 43, 54, 66, 24 and 58 pg ml-1) and one pre-menopausal patient with recurrent tumour, (237 pg ml-1), all confirmed clinically. Inhibin was normal in six patients in remission. Oestradiol levels were normal in all patients. Serial levels of inhibin predicted recurrence before overt clinical relapse in two patients. In 29 patients with malignant epithelial ovarian tumours inhibin levels were modestly elevated in nine and normal in the rest. Three patients with endometrioid histology, two with undifferentiated tumours, three with mucinous adenocarcinoma and one with clear cell carcinoma had elevated inhibin levels. Functional inhibin is secreted by all granulosa cell tumours of the ovary studied and can be used as a tumour marker to determine response to therapy and predict recurrence and is superior to oestradiol. A more detailed analysis of the levels of inhibin, and its subunits in epithelial ovarian cancer is needed to identify the molecular forms of the immunoreactive material before optimised assays can be applied to this more common tumour.