Journal of Clinical Endocrinology & Metabolism, Vol 77, 1275-1280, Copyright © 1993 by Endocrine Society

ARTICLES

Cell type-specific expression of the pituitary transcription activator pit-1 in the human pituitary and pituitary adenomas

SL Asa, LA Puy, AM Lew, VC Sundmark and HP Elsholtz
Department of Pathology, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario, Canada.

Pit-1 is a transcription factor that has been shown to be critical for pituitary-specific activation of the GH and PRL genes. In rodents and humans, differentiation and/or maintenance of somatotroph, lactotroph, and thyrotroph phenotypes are dependent on expression of a functional pit-1 gene. In rodents, Pit-1 protein is detectable in only these three cell types; however, pit-1 mRNA transcripts appear to be present at comparable levels in all adenohypophysial cell types, suggesting that translational controls may dictate the pattern of Pit-1 expression. We examined the distribution of pit-1 transcripts in the human pituitary and pituitary adenomas. All tumors were characterized by immunocytochemistry, electron microscopy, and tissue culture for accurate classification. Northern blot analysis demonstrated abundant levels of pit-1 mRNA in somatotroph, mammosomatotroph, and lactotroph adenomas. Two clinically silent adenomas that expressed TSH as well as gonadotropins contained detectable levels of pit-1 mRNA. No pit-1 expression was otherwise detected in corticotroph, gonadotroph, null cell, or oncocytic adenomas. In situ hybridization localized pit-1 mRNA transcripts in adenomas that contained GH, PRL, or TSH, but not in adenomas composed of other cell types. Pit-1 mRNA was also localized to selected subpopulations of the human nontumorous adenohypophysis that contained immunoreactivity for GH, PRL, and/or TSH. Pit-1 protein immunoreactivity was detected in the nuclei of adenomas that expressed pit-1 mRNA, but not in those that were negative for pit-1 mRNA; it was also localized only in cells containing GH, PRL, or TSH beta in the nontumorous adenohypophysis. These data demonstrate selective expression of the human pit-1 gene in adenohypophysial cell types responsible for GH, PRL, and/or TSH synthesis and are consistent with a predominantly pretranslational regulatory mechanism for Pit-1 expression in the human.

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