Renin

Succinate receptor GPR91 provides a direct link between high glucose levels and renin release in murine and rabbit kidney

Diabetes mellitus is the most common and rapidly growing cause of end-stage renal disease in developed countries. A classic hallmark of early diabetes mellitus includes activation of the renin-angiotensin system (RAS), which may lead to hypertension and renal tissue injury, but the mechanism of RAS activation is elusive. Here we identified a paracrine signaling pathway in the kidney in which high levels of glucose directly triggered the release of the prohypertensive hormone renin. The signaling cascade involved the local accumulation of succinate and activation of the kidney-specific G protein-coupled metabolic receptor, GPR91, in the glomerular endothelium as observed in rat, mouse, and rabbit kidney sections. Elements of signal transduction included endothelial Ca2+, the production of NO and prostaglandin (PGE2), and their paracrine actions on adjacent renin-producing cells. This GPR91 signaling cascade may serve to modulate kidney function and help remove metabolic waste products through renal hyperfiltration, and it could also link metabolic diseases, such as diabetes, or metabolic syndrome with RAS overactivation, systemic hypertension, and organ injury.

Insulin Induces Renal Vasodilation, Increases Plasma Renin Activity, and Sensitizes the Renal Vasculature to Angiotensin Receptor Blockade in Healthy Subjects

Insulin stimulates the renin-angiotensin system and induces renal vasodilation. The relationship between these opposing influences of insulin on renal vascular tone has not been explored. A hyperinsulinemic euglycemic clamp and sham insulin clamp each of 270 min duration were performed in 15 healthy individuals during high sodium balance. An angiotensin receptor blocker was administered at time 180 min. Renal plasma flow and plasma renin activity were measured serially. The response to insulin or sham insulin infusion was defined as the change from time 0 to 180 min; the response to angiotensin receptor blockade (ARB) was defined as the change from time 180 to 270 min. Insulin infusion increased plasma renin activity (P < 0.01) and renal plasma flow (P < 0.01); the latter effect plateaued by time 150 min. ARB caused a greater vasodilator response during insulin infusion compared with during sham insulin infusion (P = 0.02). Increasing renin response to insulin predicted blunting of the renal vasodilator response to insulin infusion (R(2) = 0.36, P = 0.02) and sensitizing of the renal vasodilator response to ARB during insulin infusion (R(2) = 0.59, P < 0.01). Insulin-induced activation of the renin-angiotensin system modulates insulin-induced renal vasodilation in healthy individuals. Further studies are warranted to address this balance in states of insulin resistance and the possible implications for the association of insulin resistance with risk for chronic kidney disease.

Erythropoietin and renin as biological markers in critically ill patients

Introduction

During sepsis the endocrine, immune and nervous systems elaborate a multitude of biological responses. Little is known regarding the mechanisms responsible for the final circulating erythropoietin (EPO) and renin levels in septic shock. The aim of the present study was to assess the role of EPO and renin as biological markers in patients with septic shock.

Methods

A total of 44 critically ill patients with septic shock were evaluated.

Results

Nonsurvivors had significantly higher serum EPO levels than did survivors on admission (median [minimum–maximum]; 61 [10–602] versus 20 [5–369]). A negative relationship between serum EPO and blood haemoglobin concentrations was observed in the survivor group (r = -0.61; P < 0.001). In contrast, in the nonsurvivors the serum EPO concentration was independent of the blood haemoglobin concentration. Furthermore, we observed significant relationships between EPO concentration and lactate (r = 0.5; P < 0.001), arterial oxygen tension/fractional inspired oxygen ratio (r = -0.41; P < 0.005), arterial pH (r = -0.58; P < 0.001) and renin concentration (r = 0.42; P < 0.005). With regard to renin concentration, significant correlations with lactate (r = 0.52; P < 0.001) and arterial pH (r = -0.33; P < 0.05) were observed.

Conclusion

Our findings show that EPO and renin concentrations increased in patients admitted to the intensive care unit with septic shock. Renin may be a significant mediator of EPO upregulation in patients with septic shock. Further studies regarding the regulation of EPO expression are clearly warranted.

Controlling the release and production of renin

The renin-angiotensin system (RAS) plays a pivotal role for a variety of cardiovascular functions. The diversity of renin actions is reflected by its complex control. The major stimulus for the release of renin from the vesicles in juxtaglomerular cells is determined by stretch, as induced by changes in arterial pressure. The production of renin underlies a vastly complex control network, which takes place at different levels, such as transcription and translation. With regard to transcription, important regions for binding transcription factors have been identified several years ago, but the conservation of nucleotide sequences throughout different species suggests that there might be additional binding regions of importance. At the post-transcriptional level, the modulation of renin mRNA stability is seems pivotal. The half-life of renin mRNA appears to be controlled by the interaction between several regulatory proteins, most of which are well known in other systems. Moreover, in addition to the modulation of mRNA stability, the translation efficiency seems to play a key role in determining the amount of renin to be produced.

Renin: origin, secretion and synthesis

Renin is a central hormone in the control of blood pressure and various other physiological functions. In spite of the very early discovery of renin over 100 years ago, we have only recently gained a deeper understanding of the origin of renin-producing cells and of the mechanisms responsible for renin synthesis and secretion. The main source of renin is the juxtaglomerular cells (JGCs), which release renin from storage granules. Besides the renin-angiotensin system (RAS) in the JGCs, there exist local RASs in various tissues. JGCs originate in situ within the metanephric kidney from mesenchymal cells that are not related to smooth muscle lineages, as hitherto assumed. The previous notion that JGCs stem from vascular smooth muscle cells may be explained by JGC differentiation: they acquire smooth muscle markers that are maintained throughout adulthood. It has become clear that increasing intracellular free [Ca2+] inhibits renin secretion in JGCs. In contrast, cAMP stimulates renin release. Over the last decade, numerous studies on isolated JGCs and intact animals have provided contradictory results as to whether cGMP has a stimulatory or inhibitory action on renin release. More recent results strongly suggest that the effects of cGMP on renin release from JGCs involve the degradation of cAMP, which is modulated by cGMP. Finally, it has been found that not only is the production of renin modulated by enhancing or attenuating renin transcription, but renin mRNA stability is controlled by various proteins present in renin-producing cells.

Control of renin synthesis

Studies published recently have considerably enhanced our understanding of the mechanisms controlling renin production. With regard to the control of renin transcription, two enhancer regions have been identified that markedly augment renin synthesis in cell lines. In the absence of this enhancer activity, the basic promoter of the renin gene increases transcription only two- to threefold. The location of one (Jones CA, Sigmund CD, McGowan RA, Kane-Haas CM, and Gross KW. Mol Endocrinol 4: 375-383, 1990) transcription enhancer in the mouse gene is at about -2.7 kb and in humans at roughly -11 kb. A second important region has been identified in a chorionic cell line to be located approximately 5 kb upstream of the transcription start site in humans. Another potentially important regulatory region may lie within approximately 3.9 kb upstream of the -11 kb enhancer, as suggested by several conserved sequences among species in this region. In addition to the control of renin transcription, it seems that renin translation and the stability of renin mRNA are also effectively regulated. This occurs via the 3'-untranslated region, to which several proteins can bind. The binding proteins were identified as hnRNP K and E1, dynamin, nucleolin, MINT homologous protein, and Y-Box 1.

An evolutionary approach for identifying potential transcription factor binding sites: the renin gene as an example

Evolutionary pressure has resulted in the conservation of certain nucleotide sequences. These conserved regions are potentially important for certain functions. Here we give an example of a comparison between noncoding sequences combined with other independent database information to shed light onto the regulation of the renin gene, a gene that has great importance for cardiovascular and renal homeostasis. To combine the information regarding conservation and weight matrices of transcription factor (TF) binding sites, an algorithm was developed (TFprofile). Notably, a local peak in the resulting binding profile coincides with a previously experimentally identified regulatory region for the renin gene. The existence of further peaks in the binding profile in the conserved 3.9-kb-long hRENc DNA block upstream of the renin gene suggests additional regions of potential importance for gene regulation. The algorithm TFprofile may be used to integrate information on cross-species evolutionary conservation and aspects of TF binding characteristics to provide putative regulatory DNA regions for experimental verification.

Tumor necrosis factor-alpha inhibits renin gene expression

Renin, produced in renal juxtaglomerular (JG) cells, is a fundamental regulator of blood pressure. Accumulating evidence suggests that cytokines may directly influence renin production in the JG cells. TNF-alpha, which is one of the key mediators in immunity and inflammation, is known to participate in the control of vascular proliferation and contraction and hence in the pathogenesis of cardiovascular diseases. Thus TNF-alpha may exert its effects on the cardiovascular system through modulation of renal renin synthesis. Therefore we have tested the effect of TNF-alpha on renin transcription in As4.1 cells, which represent transformed mouse JG cells, and in native mouse JG cells in culture. Renin gene expression was also determined in mice lacking the gene for TNF-alpha (TNF-alpha knockout mice). TNF-alpha inhibited renin gene expression via an inhibition of the transcriptional activity, targeting the proximal 4.1 kb of the renin promoter in As4.1 cells. TNF-alpha also attenuated forskolin-stimulated renin gene expression in primary cultures of mouse JG cells. Mice lacking the TNF-alpha gene had almost threefold higher basal renal renin mRNA abundance relative to the control strain. The general physiological regulation of renin expression by salt was not disturbed in TNF-alpha knockout mice. Our data suggest that TNF-alpha inhibits renin gene transcription at the cellular level and thus may act as a modulator of renin synthesis in (physio)pathological situations.