AN ALTERNATIVELY SPLICED FORM OF PIT-1 REPRESSES PROLACTIN GENE-EXPRESSION

The pituitary-specific transcription factor Pit-1 is required for expression of the PRL gene. Transcription of the PRL gene in the anterior pituitary is both activated and repressed in response to neuroendocrine signals. The molecular events that mediate repression are unknown. Transplantation of GH3 pituitary tumor cells from culture to female Wistar-Furth rats resulted in repression of PRL gene expression. When the transplanted cells were returned to culture, PRL gene expression was rapidly activated. We used this model to study potential mechanisms by which PRL gene expression was silenced. In addition to the appropriate size Pit-1 proteins of 33 and 31 kilodaltons, smaller forms of the transcription factor, migrating at approximately 27 and 24 kilodaltons, were found in transplanted cells in which PRL gene expression was repressed. These smaller forms of Pit-1 protein were observed to disappear when transplanted cells were returned to culture, coincident with the activation of PRL gene expression. A transcript approximately 170 base pairs shorter than expected for that encoding full-length Pit-1 was detected in transplanted GH3 cell RNA by polymerase chain reaction. The shorter Pit-1 transcript was abundant only in GH3 cells after in vivo passage and was not readily detected in transplanted cells after as little as 12 h in culture. This shorter transcript was found to result from excision of sequence corresponding to exon iv and encodes a Pit-1 protein lacking 54 amino acids of the POU-specific domain. Gene transfer studies demonstrated this alternative form of Pit-1 inhibited PRL promoter activity. The results implicate an alternatively spliced form of Pit-1 as a potential mediator of repression of PRL gene expression.