Endocrine Abstracts

Prolactin is produced by pituitary lactotrophic cells and by several extrapituitary tissues, including immune cells and the endometrium in man. It has a wide range of actions at different sites, and hyperprolactinaemia and prolactinomas give rise to common clinical problems. The human prolactin gene contains six exons, with two distinct promoters that direct pituitary or extra-pituitary transcription respectively. The pituitary-specific promoter extends over 5000 base-pairs and comprises multiple binding sites for the Pit-1 transcription factor that are essential for normal intracellular regulation of gene expression as well as for pituitary-specific transcription. The alternative promoter located -6kbp upstream is differently regulated, and binds a different array of transcription factors. A frequent single-nucleotide polymorphism in this upstream promoter has opposite effects on transcriptional activity in lymphocytes and endometrium, and may have important effects on prolactin's local immuno-modulating function.
Recently we have investigated the dynamics of regulation of the pituitary-specific promoter by real time imaging of the luciferase reporter gene in living pituitary cells. Using both stably transfected cell lines and adenovirus-mediated transfection of normal pituitary cells, we found that prolactin promoter activity is highly unstable from hour to hour, and heterogeneous amongst different individual cells even within a clonal population. The growth hormone promoter is similar in structure and displays similar behaviour, with irregular pulses of activity in resting cells suggesting stochastic regulation. The nature of hormonal regulation of this unstable system can now be addressed in new ways. Cameleon fluorescent proteins now allow simultaneous imaging of signals such as calcium or cAMP in parallel with transcription; using models of transcriptionally synchronised cells, the dynamics of transcription factor and co-activator recruitment to chromatin may also be related to promoter activity and hence to help understand hormone production.