Expression of MSX1 in human normal pituitaries and pituitary adenomas.

Transcription factors play specific roles in the development and differentiation of normal pituitary tissues and pituitary adenoma. The transcription factor, muscle segment homeobox 1 (MSX1), which belongs to the homeobox gene family, binds the promoter region of the glycoprotein hormone alpha-subunit (SU) in TSH-producing cells in the mouse pituitary and regulates alpha-SU expression. The present study investigated MSX1 expression in the normal human pituitary. In addition, 50 pituitary adenomas were examined using immunohistochemistry and reverse transcription polymerase chain reaction (RT-PCR) to clarify the role of MSX1 in the development and functional differentiation of pituitary adenoma cells. In the normal pituitary, MSX1 was predominantly expressed in the cytoplasm of GH-producing cells. Furthermore, MSX1 immunoreactivity was observed in the cytoplasm of some alpha-SU-producing cells. It is interesting to note that, in the pituitary adenoma, MSX1 was expressed in the nucleus of GH- and TSH-producing adenomas. RT-PCR using RNA extracted and purified from formalin-fixed paraffin-embedded pituitary adenoma specimens revealed MSX1 mRNA expressed in GH- and TSH-producing adenomas. Immunoelectron microscopy demonstrated MSX1 localized at intranuclear heterochromatin and euchromatin, which suggests transcriptional activity. These results suggest that MSX1 plays a specific role in human pituitary adenoma.