Inhibin alpha and beta A subunit messenger ribonucleic acids expression in human pituitary adenomas: studies by quantitative reverse transcription-polymerase chain reaction

In vitro secretion of FSH by cultured clinically nonfunctioning and gonadotroph pituitary adenomas is directly correlated with locally produced levels of activin A

OBJECTIVE

Expression of mRNAs encoding activin and its antagonists inhibin and follistatin has been described in human pituitary adenomas, including clinically nonfunctioning adenomas (NFAs) and gonadotroph adenomas (Gn-omas). Since many of the NFAs and Gn-omas secrete FSH in vitro, we hypothesized that locally produced activin may stimulate secretion of FSH in these pituitary adenomas.

PATIENTS AND METHODS

Pituitary adenoma tissue was obtained from 38 patients diagnosed preoperatively as having NFAs (n = 17), Gn-omas (n = 5), prolactinomas (n = 6) or growth hormone (GH)-producing adenomas (n = 10). Actual protein levels of activin, inhibin, follistatin, FSH and LH were measured in media of these 38 cultured pituitary adenomas. In addition, we investigated correlations between concentrations of these growth factors and hormones in NFAs and Gn-omas.

RESULTS

Gn-omas were found to secrete significantly more activin A in their culture medium than PRL- and GH-producing adenomas (P < 0.05). Inhibin A and inhibin B protein levels in culture media were very low. A positive correlation between levels of activin A and FSH (r = 0.56, P < 0.005) was found, while no correlation between activin A and LH could be detected. Furthermore, levels of follistatin were positively correlated with activin A levels (r = 0.73, P < 0.0005). Comparison of the activin A:follistatin ratio with the measured FSH protein levels showed an even stronger relationship (r = 0.79, P < 0.0005).

CONCLUSIONS

It is concluded that levels of activin A, follistatin and FSH in media of cultured nonfunctioning adenomas and gonadotroph adenomas are positively correlated. This suggests that these adenomas secrete FSH in response to the relatively high locally produced levels of activin A.

Localization of inhibin/activin subunits in normal pituitary and in pituitary adenomas

The localization of inhibin/activin (I/A) subunits was investigated in human normal adenohypophysial cells and in 87 pituitary adenomas of different types, using immunohistochemistry. Monoclonal antibodies directed against alpha, beta A and beta B subunits of I/A were employed. In normal pituitary, alpha subunit of inhibin was detected only in FSH-positive gonadotrophs, while beta A subunit of I/A was expressed in FSH-positive gonadotrophs, GH-cells and in a few PRL-cells. beta B subunit was found in FSH-positive gonadotrophs, TSH-cells and a few LH-positive gonadotrophs. The three subunits of I/A were detected in the majority of nonfunctioning tumors, while functioning adenomas showed a significantly lower expression. This study shows that alpha, beta A and beta B subunits of I/A are expressed by specific adenohypophysial cell types and that they are characteristically present in nonfunctioning adenomas. These results suggest that inhibins and activins may play a role in the local regulation of pituitary hormonal secretion both in normal adenohypophysial cells and in pituitary adenomas.