Regulation of Prolactin Receptor (PRLR) Gene Expression in Insulin-producing Cells: PROLACTIN AND GROWTH HORMONE ACTIVATE ONE OF THE RAT PRLR GENE PROMOTERS VIA STAT5a AND STAT5b

Abstract

Expression of the prolactin receptor (PRLR) gene is increased in pancreatic islets during pregnancy and in vitro in insulin-producing cells by growth hormone (GH) and prolactin (PRL). The 5'-region of the rat PRLR gene contains at least three alternative first exons that are expressed tissue-specifically because of differential promoter usage. We show by reverse transcription-polymerase chain reaction analysis that both exon 1A- and exon 1C-containing PRLR transcripts are expressed in rat islets and that human (h)GH, ovine (o)PRL, and bovine (b)GH increase exon 1A expression 6.5 +/- 0. 8-fold, 6.8 +/- 0.7-fold, and 3.9 +/- 0.7-fold and exon 1C expression 4.8 +/- 0.4-fold, 4.4 +/- 0.6-fold, and 2.5 +/- 0.7-fold, respectively. Expression of exon 1B was not detectable. The transcriptional activities of reporter constructs containing the 1A, 1B, or 1C promoter were found to be 22.8-fold, 2.7-fold, and 8. 0-fold, respectively, above that of a promoterless reporter construct when transfected into the insulin-producing INS-1 cells. The transcriptional activity of the 1A promoter construct was increased 8.9 +/- 1.9-fold by 0.5 microgram/ml hGH. Responsiveness to hGH of the 1A promoter was localized to the region from -225 to +81 with respect to the transcription start site. This region contains the sequence TTCTAGGAA that by gel retardation experiments was shown to bind the transcription factors STAT5a and STAT5b in response to stimulation by hGH, oPRL, or bGH. Mutation of this gamma-activated sequence-like element completely abolished transcriptional induction of the 1A promoter by hGH. Our results suggest that GH and PRL increase the levels of exon 1A- and 1C-containing PRLR mRNA species and furthermore that the transcriptional activity of the 1A promoter is increased via activation of STAT5a and STAT5b.