Canine mammary growth hormone gene transcription initiates at the pituitary-specific start site in the absence of Pit-1

Growth hormone (GH) gene expression has been reported in the mammary glands of various mammalian' species. The mechanism by which the GH gene becomes activated in extrapituitary tissues is currently unclear. We have character ized the canine mammary and pituitary GH gene transcripts by Northern blot, 5'- and 3'-RACE (rapid amplification of cDNA ends), and DNA sequence analy sis. Northern blot analysis detected GH gene transcripts in mammary glands of dogs which were exposed to high levels of progestins. The mammary and pi tuitary GH cDNAs were shown to be identical in both the coding region and u ntranslated regions. Pituitary GH gene expression is highly dependent upon the transcription factor Pit-1. Analysis of Pit-1 gene expression using RT- PCR followed by Southern hybridization revealed a strong pituitary signal b ut faint, weak or no hybridization signals in mammary gland samples. Among the negative samples were progestin-treated dogs with high mammary GH gene expression. These findings indicate that mammary and pituitary GH gene tran scripts originate from the same transcription start site but are regulated differentially. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.