Immunohistochemical expression of inhibin-alpha in human endometrium and the in vitro secretion of inhibin, estradiol and cortisol in cultured human endometrial glandular cells

Inhibins are multipotent dimeric glycoproteins, composed of an α-subunit and one of two possible β-subunits (β_A or β_B). Aims of this study were (a): the immunohistochemical characterisation of normal human endometrium for the inhibin-α 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.

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-α. 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.

Inhibin-α 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.

In conclusion, we found a cyclical expression of inhibin-α 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.