This article describes pertinent aspects of histochemical and molecular changes of the human pituitary adenomas. The article outlines individual tumor groups with general, speciWc and molecular Wndings. The discussion further extends to the unusual adenomas or carcinomas. The description in this article are pertinent not only for the practicing pathologists who are in the position of making proper diagnosis, but also for the pituitary research scientists who engage in solving basic problems in pituitary neoplasms by histochemistry and molecular biology.

The Wnt signaling pathway has been implicated in the genesis of numerous human cancers. A member of the Wnt family of genes, Wnt4, has been known to regulate proliferation of anterior pituitary cell types in the mouse during embryonic development. In order to elucidate the roles of Wnt signaling in human pituitary adenomas, we examined the expression of Wnt4 and its putative receptor Frizzled6 (Fzd6) by immunohistochemistry in pituitary adenomas and normal pituitaries. Expression of Wnt4 was higher in growth hormone-producing adenomas (GHomas), prolactin-producing adenomas (PRLomas), and thyroid-stimulating hormone-producing adenomas (TSHomas) than in the normal pituitary. Fzd6 was widely expressed in GHomas, PRLomas, TSHomas, and gonadotropin subunit (GnSU)-positive adenomas. In normal pituitary glands, Wnt4 and Fzd6 were colocalized predominantly in follicle-stimulating hormone-, luteinizing hormone-, and alpha-subunits of glycoprotein hormone-positive cells. The canonical Wnt/beta-catenin signaling pathway was analyzed by beta-catenin immunohistochemistry. beta-Catenin was localized at the cell membrane in all pituitary adenomas, but not in the nuclei. On the other hand, Erk1/2 was highly activated in GHomas and TSHomas. These results suggested that activation of Wnt4/Fzd6 signaling through a "beta-catenin-independent" pathway played a role in proliferation and survival of the pituitary adenoma cells. Detailed involvement of transcription factors including Pit-1 remains to be further investigated.

Immunohistochemistry for SSTR2A in pathology specimens from acromegalic patients enabled selection of those experiencing clinical benefit from octreotide. Therefore, performing immunohistochemistry for detection of SSTR2A is recommended for all specimens obtained by surgery.

Wnt signaling is important in many aspects of cell biology and development. In the mouse female reproductive tract, Wnt4, Wnt5a, and Wnt7a show differential expression during the estrus cycle, suggesting that they participate in female reproductive physiology. Although the pituitary is a major gland regulating reproduction, the molecular mechanism of Wnt signaling here is unclear. We elucidated the subcellular distribution of Wnt4 in the pituitary of estrogen-treated ovariectomized female rats. Expression of Wnt4 mRNA increased dramatically, particularly in proestrus compared with estrus and metestrus. Wnt4 protein was observed in the cytoplasm of almost all growth hormone (GH)-producing cells and in only a few thyroid-stimulating hormone β (TSHβ)-producing cells. In rat GH-producing pituitary tumor (MtT/S) cells, estrogen-induced expression of Wnt4 mRNA was completely inhibited by estrogen receptor antagonist ICI 182,780 in vitro. Thus, rat pituitary GH cells synthesize Wnt4 and this is induced by estrogen mediated via an estrogen receptor alpha-dependent pathway.

The functional differentiation of pituitary cells and adenomas follows the combination of transcription factors and co-factors in three cell lineages [growth hormone-prolactin-thyroid-stimulating hormone lineage, adrenocorticotrophic hormone (ACTH)/pro-opiomelanocortin (POMC) lineage, and follicular stimulating hormone (FSH)/luteinizing hormone (LH) lineage], which include Pit-1, GATA-2, SF-1, NeuroD1/beta2, Tpit, ERalpha, and others. Only rarely are hormones from different lineages co-expressed in the same adenoma cells. Most corticotroph cell adenomas belonging to the ACTH/POMC lineage are mono-hormonal. In our study of 89 corticotroph cell adenomas, 5 cases expressed both ACTH and alpha-subunit; these adenomas did not express any other anterior pituitary hormones or subunits. To clarify the mechanism involved, we studied the transcription factors that regulate pituitary cell differentiation. NeuroD1 and T-pit, markers of the ACTH/POMC lineage, and SF-1 and DAX-1, related to the LH/FSH cell lineage were expressed in all cases. GATA2, a synergistic factor in the gonadotroph cell lineage with SF-1, was also expressed in three of five cases. As ACTH and alpha-subunit are the earliest hormones to appear during development, we speculate that these particular adenomas are derived from committed ACTH progenitor cells. The molecular process governing functional differentiation of these adenomas requires further investigation.