Prolactin (PRL) is required not only for maintenance of gestation in pigs but also for mammary gland development and subsequent lactogenesis. The actions of PRL are modulated by both long and short isoforms of the PRL receptor (PRLR), where short isoforms can interfere with the essential signaling function of the long isoform. Using 3′ RACE we have isolated a unique splice variant of the porcine PRLR (pPRLR) that contains a short intracellular domain of 38 aa that is encoded by splicing from exon 9 to a novel exon 11 located 17.5 kb downstream of exon 10 on chromosome 16. The short pPRLR was detected as a 42 kDa protein in membranes from porcine mammary glands. Functional analyses indicated that the short pPRLR functions as a dominant negative against the differentiative function of the long pPRLR and does not transduce a signal to the rat β-casein promoter. Differential abundance of long pPRLR and short pPRLR mRNA was established in a range of porcine tissues. The binding affinity of the short pPRLR for pPRL was lower (K d=3.7 nM) than the affinity of the long pPRLR (K d=1.6 nM) despite a fourfold higher level of binding sites for the short pPRLR. Our data raise the possibility that the short pPRLR in pigs may function independently from the long pPRLR, where the splicing strategy used to generate the short pPRLR likely evolved under different selection pressures to those acting on the long pPRLR.
Josephine F Trott, Anke Schennink and Russell C Hovey
Anke Schennink, Josephine F Trott, Bradley A Freking and Russell C Hovey
Endocrine, paracrine, and autocrine prolactin (PRL) acts through its receptor (PRLR) to confer a wide range of biological functions, including its established role during lactation. We have identified a novel first exon of the porcine PRLR that gives rise to three different mRNA transcripts. Transcription of this first exon is tissue specific, where it increases during gestation in the adrenal glands and uterus. Within the mammary glands, its transcription is induced by estrogen and PRL, while in the uterus, its expression is downregulated by progestin. The promoter region has an enhancer element located between −453 and −424 bp and a putative repressor element between −648 and −596 bp. Estrogen, acting through the estrogen receptor, activates transcription from this promoter through both E-box and transcription factor AP-2 α binding sites. These findings support the concept that the multilevel hormonal regulation of PRLR transcription contributes to the various biological functions of PRL.
Anke Schennink, Josephine F Trott, Rodrigo Manjarin, Danielle G Lemay, Bradley A Freking and Russell C Hovey
Prolactin (PRL), acting via the PRL receptor (PRLR), controls hundreds of biological processes across a range of species. Endocrine PRL elicits well-documented effects on target tissues such as the mammary glands and reproductive organs in addition to coordinating whole-body homeostasis during states such as lactation or adaptive responses to the environment. While changes in PRLR expression likely facilitates these tissue-specific responses to circulating PRL, the mechanisms regulating this regulation in non-rodent species has received limited attention. We performed a wide-scale analysis of PRLR 5′ transcriptional regulation in pig tissues. Apart from the abundantly expressed and widely conserved exon 1, we identified alternative splicing of transcripts from an additional nine first exons of the porcine PRLR (pPRLR) gene. Notably, exon 1.5 transcripts were expressed most abundantly in the heart, while expression of exon 1.3-containing transcripts was greatest in the kidneys and small intestine. Expression of exon 1.3 mRNAs within the kidneys was most abundant in the renal cortex, and increased during gestation. A comparative analysis revealed a human homologue to exon 1.3, hE1N2, which was also principally transcribed in the kidneys and small intestines, and an exon hE1N3 was only expressed in the kidneys of humans. Promoter alignment revealed conserved motifs within the proximal promoter upstream of exon 1.3, including putative binding sites for hepatocyte nuclear factor-1 and Sp1. Together, these results highlight the diverse, conserved and tissue-specific regulation of PRLR expression in the targets for PRL, which may function to coordinate complex physiological states such as lactation and osmoregulation.
Joseph E Morabito, Josephine F Trott, Dorian M Korz, Heather E Fairfield, Sarah H Buck and Russell C Hovey
Progesterone (P) and prolactin (PRL) fulfill crucial roles during growth and differentiation of the mammary epithelium, and each has been implicated in the pathogenesis of mammary cancer. We previously identified that these hormones synergistically stimulate the proliferation of mouse mammary epithelial cells in vivo, although the mechanism(s) underlying their cooperative effect are unknown. We now report a novel pathway by which P and PRL synergize to activate transcription from the long terminal repeat (LTR) of the mouse mammary tumor virus-LTR (MMTV-LTR) in T47D breast cancer cells. Using serial 5′ and 3′ deletions of the MMTV-LTR, in addition to selective mutations, we identified that a previously uncharacterized inverted palindrome on the distal enhancer (−941/−930), in addition to a signal transducer and activator of transcription 5 site, was essential for the synergistic activation of transcription by P and PRL. Notably, hormone synergy occurred via a mechanism that was independent of the P receptor DNA-binding elements found in the proximal MMTV-LTR hormone-response element. The palindrome specifically recruited a protein complex (herein termed mammary gland-specific complex) that was almost exclusive to normal and cancerous mammary cells. The synergy between P and PRL occurred via a Janus kinase 2 and c-Src/Fyn-dependent signaling cascade downstream of P and PRL receptors. Combined, our data outline a novel pathway in T47D cells that may facilitate the action(s) of P and PRL during mammary development and breast cancer.