Thyrotropin (
TSH) beta is a subunit of TSH, the expression of which is limited to the thyrotrope cells of the anterior pituitary gland. Tissue-specific expression of the mouse
TSH beta gene is conferred by sequences between -270 and -80 of the 5'-flanking region. We have investigated tissue-specific expression of the
TSH beta promoter in two thyrotrope-derived cell types: 1) TtT-97 thyrotropic
tumors, which express the endogenous
TSH beta gene, and 2) an alpha-TSH cell line, which was generated from a thyrotropic
tumor that has lost the ability to express the
TSH beta gene. The
pituitary-specific transcription factor Pit-1 is present in thyrotropes and interacts with three cis-acting elements in the functionally important region of the
TSH beta promoter. Pit-1
protein is present in TtT-97
tumor cells but is absent from alpha-TSH cells. Reintroduction of Pit-1 into alpha-TSH cells by transient transfection does not restore
TSH beta promoter activity. We have identified an alternately spliced variant of Pit-1, called Pit-1T, the
mRNA and
protein expression of which is limited to thyrotrope-derived cells. Pit-1T contains a 14-
amino acid insert in the transactivation domain due to an alternate 3'
splice acceptor site. Transiently transfected Pit-1T increases
TSH beta promoter activity in TtT-97 thyrotropic
tumor cells, whereas additional Pit-1 has no effect. The alpha-TSH cell line, which lacks all Pit-1
proteins, requires both
isoforms in order to stimulate
TSH beta promoter activity. These data suggest that Pit-1T is a thyrotrope-specific splice variant of Pit-1 that is required for
TSH beta promoter stimulation; furthermore, both Pit-1 and Pit-1T are required for
TSH beta promoter activity in thyrotrope cells.