We previously identified a critical pathogenic role for mineralocorticoid receptor (MR) activation in cardiomyocytes that included a potential interaction between the MR and the molecular circadian clock. While glucocorticoid regulation of the circadian clock is undisputed, studies on MR interactions with circadian clock signalling are limited. We hypothesised that the MR influences cardiac circadian clock signalling, and vice versa. Aldosterone or corticosterone (10 nM) regulated Cry1, Per1, Per2 and ReverbA (Nr1d1) gene expression patterns in H9c2 cells over 24 h. MR-dependent regulation of circadian gene promoters containing GREs and E-box sequences was established for CLOCK, Bmal, CRY1 and CRY2, PER1 and PER2 and transcriptional activators CLOCK and Bmal modulated MR-dependent transcription of a subset of these promoters. We also demonstrated differential regulation of MR target gene expression in hearts of mice 4 h after administration of aldosterone at 08:00 h vs 20:00 h. Our data support MR regulation of a subset of circadian genes, with endogenous circadian transcription factors CLOCK and BMAL modulating the response. This unsuspected relationship links MR in the heart to circadian rhythmicity at the molecular level and has important implications for the biology of MR signalling in response to aldosterone as well as cortisol. These data are consistent with MR signalling in the brain where, like the heart, it preferentially responds to cortisol. Given the undisputed requirement for diurnal cortisol release in the entrainment of peripheral clocks, the present study highlights the MR as an important mechanism for transducing the circadian actions of cortisol in addition to glucocorticoid receptor (GR) in the heart.
Elizabeth K Fletcher, Monica Kanki, James Morgan, David W Ray, Lea M Delbridge, Peter J Fuller, Colin D Clyne and Morag J Young
R J Lacey, S L F Chan, H C Cable, R F L James, C W Perret, J H B Scarpello and N G Morgan
Sequences from cDNA molecules encoding α2-adrenoceptor subtype genes were subcloned into prokaryotic vectors and riboprobes generated to hybridise selectively with each of the human α2C2-, α2C4- and α2C10-adrenoceptor subtype mRNA species. The riboprobes were labelled with either 32P or digoxigenin and used to study the expression of α2-adrenoceptor subtypes in sections of human pancreas, in isolated human islets of Langerhans and in clonal HIT-T15 pancreatic β-cells. Using a ribonuclease protection assay protocol, expression of mRNA species encoding both α2C2 and α2C10 was demonstrated in preparations of isolated human islets of Langerhans. mRNA encoding α2C4 was also detected in human islet RNA, using reverse transcription coupled with the polymerase chain reaction. In situ hybridisation was then employed to examine the distribution of each α2-adrenoceptor subtype in sections of human pancreas. All three subtypes of α2-adrenoceptor mRNA were identified in sections of formalin-fixed, paraffinembedded human pancreas using riboprobes labelled with digoxigenin. Although some labelling of the three α2-adrenoceptor mRNA subtypes was seen in the islets, the labelling was most intense in the exocrine tissue of the pancreas for each receptor subtype. The specificity of the digoxigenin-labelled RNA probes was confirmed in several control tissues and by in situ hybridisation studies using sense probes in the pancreas. The integrity of the pancreas sections was confirmed by in situ hybridisation with an antisense riboprobe derived from human insulin cDNA. The results demonstrate that multiple α2-adrenoceptor subtypes are expressed in human pancreas. Both the exocrine and endocrine cells express more than one receptor subtype, although the islets stain less intensely than the bulk of the tissue suggesting that the islet cells may have lower levels of expression than the acinar tissue. The presence of α2-adrenoceptor subtype mRNA species in pancreatic β-cells was confirmed by Northern blotting of RNA extracted from the clonal β-cell line, HIT-T15. Transcripts encoding each of the three cloned α2-adrenoceptor subtypes were detected in HIT-T15 cells.
Hybridisation of sections of human pancreas with oligodeoxynucleotide probes designed to hybridise with β2-adrenoceptor mRNA revealed expression of this species in islet β-cells but not in the exocrine tissue of the pancreas.
Journal of Endocrinology (1996) 148, 531–543
R. J. Lacey, H. C. Cable, R. F. L. James, N. J. M. London, J. H. B. Scarpello and N. G. Morgan
The effects of the mixed α/β-agonist adrenaline on insulin secretion from isolated human islets of Langerhans were studied. In static incubation experiments, adrenaline (0·1 nmol/l to 10 μmol/l) caused a concentration-dependent inhibition of glucose-induced insulin secretion from isolated human islets. However, perifusion experiments revealed that the time-course of the secretory changes induced by adrenaline was complex. When employed at a high concentration (1 μmol/l), adrenaline caused a sustained inhibition of glucose-induced insulin secretion, which could be relieved by the addition of the α2-antagonist yohimbine (10 μmol/l). By contrast, infusion of adrenaline at a lower concentration (10 nmol/l), produced a large initial potentiation of glucose-induced insulin secretion. This response was, however, short-lived and followed by sustained inhibition of secretion, which could be relieved by yohimbine (10 μmol/l). The initial stimulation of insulin secretion provoked by 10 nmol adrenaline/l was abolished when islets were incubated in the presence of the β-antagonist, propranolol (1 μmol/l), consistent with activation of β-adrenoceptors. In support of this, treatment of human islets with the selective β2-agonist clenbuterol, was also associated with marked stimulation of insulin secretion. By contrast, each of two selective β3-agonists tested failed to alter insulin secretion from human islets. The results indicate that human pancreatic B-cells are equipped with both α2-and β2-adrenoceptors which can affect insulin secretion. Adrenaline interacts with both of these but the α2-response is predominant and can overcome the tendency of β2-adrenoceptors to potentiate insulin release.
Journal of Endocrinology (1993) 138, 555–563