Background: Inhibins are multipotent dimeric glycoproteins, composed of an alpha-subunit and one of two possible beta-subunits (betaA or betaB). Aims of this study were (a): the immunohistochemical characterisation of normal human endometrium for the inhibin-alpha subunit; (b) the assessment of the secretion and metabolism of inhibin, E2 and cortisol; (c) the evaluation of any relationship between these three substances in cell culture medium of isolated and cultivated normal human endometrial glandular cells.
Materials and methods: Samples of human endometrium were obtained from 34 premenopausal patients. Nineteen endometrial specimen (proliferative [PP] n=8; early secretory [ES] n=7; late secretory phase [LS] n=4) were brought into cell culture. Fifteen endometrial specimen (PP n=5; ES n=5; LS n=5) were paraffin-fixed and used for the immunohistochemical analysis for inhibin-alpha. Stromal and epithelial cells were separated by collagenase digestions, filtrations, sedimentations and Ficoll-gradient centrifugation. E2 and cortisol were measured with radioimmunoassay (RIA) and inhibin with enzyme-immunoassay (EIA). Statistical analysis was performed with the non-parametric Mann-Whitney rank-sum test and linear regression analysis.
Results: Inhibin-alpha showed a weak (positive) expression during proliferative phase, which increased significantly as the menstrual cycle continued. In secretory glands the mean inhibin concentration was higher than that from proliferative samples. A significant correlation was observed between inhibin and E2 (p<0.001) as well as cortisol and inhibin (p<0.0001) in glands from proliferative phase. Between inhibin and E2 (p<0.05) as well as inhibin and cortisol (p<0.002) a significant correlation in early secretory glands was also noted. In late secretory phase inhibin and E2 (r2=0.78650; p<0.0001), inhibin and cortisol (r2=0.58326; p<0.001) and E2 and cortisol (r2=0.52880; p<0.001) showed a significant correlation.
Discussion: In conclusion, we found a cyclical expression of inhibin-alpha subunit in the endometrium demonstrated by immunohistochemical means. A higher in vitro secretion of inhibin from secretory glands was also observed. In addition, a significant correlation between inhibin with E2 and cortisol in PP and ES glands and a significant correlation between inhibin, E2 and cortisol in LS glands could also be demonstrated. We conclude that inhibin can be associated with E2 and cortisol metabolism, playing an important role in paracrine/autocrine mechanisms in the endometrium and possibly exerting its function through cortisol and E2. The cortisol concentration also correlates with E2, suggesting a link between these steroids in the endometrial function. The correlation of inhibin, E2 and cortisol suggest complex autocrine/ paracrine mechanisms in human endometrial glands, modulated and controlled by all these three substances.