Imbalanced follicle-stimulating hormone beta-subunit hormone biosynthesis in human pituitary adenomas.

Abstract

Clinically nonfunctioning pituitary adenomas represent approximately 25% of all pituitary tumors. Recent studies using a number of in vitro techniques show that the majority of such tumors produce gonadotropins. Hypersecretion of uncombined gonadotropin subunits by these tumors has also been identified raising the possibility that gonadotropin biosynthetic alterations may occur in neoplastic pituitary tissue. To determine whether underlying intracellular biosynthetic alterations lead to imbalanced secretion of the gonadotropin subunits by such tumors, we investigated 1) steady state gonadotropin-subunit messenger ribonucleic acid (mRNA) levels in tumor tissue from 49 patients with clinically nonfunctioning adenomas, 2) secretion of gonadotropins in dispersed pituitary tumor cultures, and 3) serum concentrations of gonadotropins and free subunits. Northern blots of RNA extracted from surgically obtained pituitary tumor tissue were hybridized with complementary DNA probes for FSH beta, LH beta, and alpha-subunit, and quantitative analysis was done to compare alpha- and beta-subunit biosynthesis in individual tumors. Of these tumors, 47 contained sufficient RNA for Northern analysis and 77% of these tumors contained one or more of the gonadotropin-subunit mRNAs. Steady state alpha-subunit mRNA was detected in 57% of tumors, FSH beta mRNA in 49%, and LH beta in 1 (2%). We found FSH beta mRNA in excess of alpha-subunit mRNA in one-third of tumors, including 9 tumors where alpha-subunit mRNA was undetectable. In cultured cells, FSH beta was secreted in excess of alpha-subunit in 41% of tumors. For those tumors in which both mRNA and culture data were available, FSH beta mRNA and secreted subunit levels were in excess of alpha-subunit in 64% of tumors. We conclude that clinically nonfunctioning pituitary adenomas frequently synthesize excess FSH beta subunit relative to alpha-subunit. This finding is in contrast to previous data in normal pituitary or placental tissue where alpha-subunit is present in excess of beta-subunits at both the mRNA and protein levels. The free-beta-subunit hypersecretion identified in pituitary adenomas may be due to biosynthetic abnormalities intrinsic to neoplastic gonadotrophs.