Metabolic signalling to somatotrophs: Transcriptional and post‐transcriptional mediators

Abstract

In normal individuals, pituitary somatotrophs optimise body composition by responding to metabolic signals from leptin. To identify mechanisms behind the regulation of somatotrophs by leptin, we used Cre-LoxP technology to delete leptin receptors (LEPR) selectively in somatotrophs and developed populations purified by fluorescence-activated cell sorting (FACS) that contained 99% somatotrophs. FACSpurified, Lepr-null somatotrophs showed reduced levels of growth hormone (GH), growth hormone-releasing hormone receptor (GHRHR), and Pou1f1 proteins and Gh (females) and Ghrhr (both sexes) mRNAs. Pure somatotrophs also expressed thyroidstimulating hormone (TSH) and prolactin (PRL), both of which were reduced in pure somatotrophs lacking LEPR. This introduced five gene products that were targets of leptin. In the present study, we tested the hypothesis that leptin is both a transcriptional and a post-transcriptional regulator of these gene products. Our tests showed that Pou1f1 and/or the Janus kinase/signal transducer and activator of transcription 3 transcriptional regulatory pathways are implicated in the leptin regulation of Gh or Ghrhr mRNAs. We then focused on potential actions by candidate microRNAs (miR-NAs) with consensus binding sites on the 3' UTR of Gh or Ghrhr mRNAs. Somatotroph Lepr-null deletion mutants expressed elevated levels of miRNAs including miR1197-3p (in females), miR103-3p and miR590-3p (both sexes), which bind Gh mRNA, or miRNA-325-3p (elevated in both sexes), which binds Ghrhr mRNA. This elevation indicates repression of translation in the absence of LEPR. In addition, after detecting binding sites for Musashi on Tshb and Prl 3' UTR, we determined that Musashi1 repressed translation of both mRNAs in in vitro fluc assays and that Prl mRNA was enriched in Musashi immunoprecipitation assays. Finally, we tested ghrelin actions to determine whether its nitric oxide-mediated signalling pathways would restore somatotroph functions in deletion mutants. Ghrelin did not restore either GHRH binding or GH secretion in vitro. These studies show an unexpectedly broad role for leptin

Post-Transcriptional Regulation of Gnrhr: A Checkpoint for Metabolic Control of Female Reproduction

“…Metabolic disorders are associated with disordered levels of miRNA production and secretion (reviewed in [ 83 ]). Specifically, leptin has been shown to modulate miRNA levels (see [ 84 , 85 , 86 , 87 ] for a few recent examples), and we have demonstrated this in somatotropes [ 88 ]. Since our in silico analyses predicted binding sites for miRNAs within the 3′UTR of Gnrhr ( Table 1 ), we wished to determine whether or not leptin may regulate Gnrhr post-transcriptionally via regulation of gonadotrope miRNA levels.…”

Leptin: A Metabolic Signal for the Differentiation of Pituitary Cells

“…We continued the investigation of leptin signaling pathways in somatotropes and reported that they included both transcriptional and posttranscriptional regulators [3]. Our tests of pathway inhibitors showed that full GH expression may be maintained by leptin through the JAK/STAT3 pathway but not nitric oxide.…”

“…This contrasts with leptin pathways that regulate gonadotropins, which include NOS. Leptin regulation is likely to be transcriptional as loss of LEPR in somatotropes reduced Gh and Ghrhr mRNA and proteins [3]. In addition, leptin regulation of the Pou1f1 transcription factor may also serve as a pathway for the transcriptional regulation of Gh and Ghrhr [2,3,60].…”

“…Leptin is one of the most important of these extrinsic signals. However, recent studies show that leptin does more than simply signal levels of fat stores [1][2][3][4][5][6][7][8][9][10][11]. Leptin plays a trophic role that optimizes and maintains differentiation of at least two of these cell types, somatotropes and gonadotropes.…”

Growth hormone and prolactin: So much still to learn

“…In addition to hypothalamic regulation, peripheral signals also regulate GH and prolactin production and release, although the mechanism of many of these is poorly understood. Cell‐specific manipulation of receptor expression can provide insights into this, as is demonstrated in the study by Allensworth‐James et al, 9 who show that the decreased GH secretion following loss of leptin receptors specifically in GH‐secreting somatotrophs is mediated by alterations in both gene transcription and translation. Key mediators in the regulation of prolactin have also been identified in fish, where prolactin has an osmoregulatory role 10 .…”