Physiological Relevance of Renin/Prorenin Binding and Uptake

Collecting Duct Renin: A major player in Angiotensin II-dependent Hypertension

Recently, the focus of interest on the role of the renin angiotensin system in the pathophysiology of hypertension has shifted towards greater emphasis on new developments in local renin angiotensin systems in specific tissues. We have focused our recent investigations on the role of the intrarenal-intratubular RAS in hypertension. All of the components needed for angiotensin II generation are present within the various compartments in the kidney. This brief review is focused on recent evidence that inappropriate activation of renin in distal nephron segments, by acting on angiotensinogen generated in the proximal tubule cells and delivered to the distal nephron may contribute to increased distal intrarenal angiotensin II formation, sodium retention and development and progression of hypertension.

Evolving concepts on regulation and function of renin in distal nephron

Sustained stimulation of the intrarenal/intratubular renin-angiotensin system in a setting of elevated arterial pressure elicits renal vasoconstriction, increased sodium reabsorption, proliferation, fibrosis, and eventual renal injury. Activation of luminal AT(1) receptors in proximal and distal nephron segments by local Ang II formation stimulates various transport systems. Augmented angiotensinogen (AGT) production by proximal tubule cells increases AGT secretion contributing to increased proximal Ang II levels and leading to spillover of AGT into the distal nephron segments, as reflected by increased urinary AGT excretion. The increased distal delivery of AGT provides substrate for renin, which is expressed in principal cells of the collecting tubule and collecting ducts, and is also stimulated by AT(1) receptor activation. Renin and prorenin are secreted into the tubular lumen and act on the AGT delivered from the proximal tubule to form more Ang I. The catalytic actions of renin and or prorenin may be enhanced by binding to prorenin receptors on the intercalated cells or soluble prorenin receptor secreted into the tubular fluid. There is also increased luminal angiotensin converting enzyme in collecting ducts facilitating Ang II formation leading to stimulation of sodium reabsorption via sodium channel and sodium/chloride co-transporter. Thus, increased collecting duct renin contributes to Ang II-dependent hypertension by augmenting distal nephron intratubular Ang II formation leading to sustained stimulation of sodium reabsorption and progression of hypertension.

Prorenin induces vascular smooth muscle cell proliferation and hypertrophy via epidermal growth factor receptor-mediated extracellular signal-regulated kinase and Akt activation pathway

BACKGROUND

It is widely acknowledged that the (pro)renin receptor mediates angiotensin (Ang) II-dependent and Ang II-independent effects of prorenin.

METHOD

We examined the effect of prorenin on vascular smooth muscle cell (VSMC) signal transduction, proliferation, and hypertrophy.

RESULTS

Recombinant rat prorenin dose-dependently increased extracellular signal-regulated kinase (ERK) 1/2 and Akt phosphorylation in rat VSMCs. Prorenin also significantly increased cell number, and [H]-thymidine and [H]-leucine incorporation, which were attenuated by pretreatment with inhibitors for ERK kinase and phosphatidylinositol 3 kinase. Prorenin was also found to stimulate epidermal growth factor (EGF) receptor and Src phosphorylation. Pretreatment of VSMCs with an EGF receptor tyrosine kinase inhibitor and a Src inhibitor significantly attenuated the prorenin-induced increase in ERK 1/2 and Akt phosphorylation, as well as DNA and protein synthesis. Prorenin-induced phosphorylation of the EGF receptor, ERK 1/2, and Akt, as well as DNA and protein synthesis were all blocked by (pro)renin receptor siRNA, but not by an Ang II type 1 receptor blocker, candesartan, nor an Ang-converting enzyme inhibitor, captopril.

CONCLUSION

These results reveal that prorenin directly stimulates VSMC proliferative and hypertrophic changes, dependent on the (pro)renin receptor, independent of Ang II. Furthermore, EGF receptor-mediated ERK 1/2 and Akt activation contributes to prorenin-dependent proliferative and hypertrophic effects in VSMCs.

Plasma and renal renin concentrations in adult sheep after prenatal betamethasone exposure

This study examined whether renin expression and secretion and plasma angiotensin II (Ang II) levels were altered in adult sheep exposed to antenatal betamethasone. Pregnant sheep received injections of 0.17 mg/kg betamethasone or vehicle, at 80 and 81 days of gestation, and offspring were studied at 6 and 18 months of age. At 6 months, plasma prorenin concentrations were significantly lower in betamethasone animals (4.63 +/- 0.64 vs 7.09 +/- 0.83 ng angiotensin I/mL/h, P < .01). The percentage of plasma active renin was significantly higher in the betamethasone group (31.93 +/- 4.09% vs 18.57 +/- 2.79%, P < .01). Plasma and renocortical renin levels were similar in both groups at 18 months, but plasma renin activity was lower than at 6 months. Ang II levels were suppressed by betamethasone. The data indicate that prenatal exposure to betamethasone alters processing and secretion of renin in offspring at 6 months, but that this difference is not apparent at 18 months.

Collecting duct renin is upregulated in both kidneys of 2-kidney, 1-clip goldblatt hypertensive rats

Renin in collecting duct cells is upregulated in chronic angiotensin II-infused rats via angiotensin II type 1 receptors. To determine whether stimulation of collecting duct renin is a blood pressure-dependent effect; changes in collecting duct renin and associated parameters were assessed in both kidneys of 2-kidney, 1-clip Goldblatt hypertensive (2K1C) rats. Renal medullary tissues were used to avoid the contribution of renin from juxtaglomerular cells. Systolic blood pressure increased to 184+/-9 mm Hg in 2K1C rats (n=19) compared with sham rats (121+/-6 mm Hg; n=12). Although renin immunoreactivity markedly decreased in juxtaglomerular cells of nonclipped kidneys (NCK: 0.2+/-0.0 versus 1.0+/-0.0 relative ratio) and was augmented in clipped kidneys (CK: 1.7+/-1.0 versus 1.0+/-0.0 relative ratio), its immunoreactivity increased in cortical and medullary collecting ducts of both kidneys of 2K1C rats (CK: 2.8+/-1.0 cortex; 2.1+/-1.0 medulla; NCK: 4.6+/-2.0 cortex, 3.2+/-1.0 medulla versus 1.0+/-0.0 in sham kidneys). Renal medullary tissues of 2K1C rats showed greater levels of renin protein (CK: 1.4+/-0.2; NCK: 1.5+/-0.3), renin mRNA (CK: 5.8+/-2.0; NCK: 4.9+/-2.0), angiotensin I (CK: 120+/-18 pg/g; NCK: 129+/-13 pg/g versus sham: 67+/-6 pg/g), angiotensin II (CK: 150+/-32 pg/g; NCK: 123+/-21 pg/g versus sham: 91+/-12 pg/g; P<0.05), and renin activity (CK: 8.6 microg of angiotensin I per microgram of protein; NCK: 8.3 microg of angiotensin I per microgram of protein; sham: 3.4 microg of angiotensin I per microgram of protein) than sham rats. These data indicate that enhanced collecting duct renin in 2K1C rats occurs independently of blood pressure. Upregulation of distal tubular renin helps to explain how sustained intrarenal angiotensin II formation occurs even during juxtaglomerular renin suppression, thus allowing maintained effects on tubular sodium reabsorption that contribute to the hypertension.

(Pro)renin receptor-mediated activation of mitogen-activated protein kinases in human vascular smooth muscle cells

Blockade of (pro)renin receptor has benefits in diabetic angiotensin II type-1a-receptor-deficient mice, suggesting the importance of (pro)renin receptor-mediated intracellular signals. To determine the mechanism whereby the human (pro)renin receptor activates mitogen-activated protein kinases in human vascular smooth muscle cells (hVSMC), we treated the cells with recombinant human prorenin. Prorenin enhanced hVSMC proliferation and activated extracellular-signal-related protein kinase (ERK) in a dose- and time-dependent manner but did not influence activation of p38 or c-Jun NH(2)-terminal kinase. The activated ERK level was reduced to the control level by the tyrosine kinase inhibitor genistein, and the MEK inhibitor U0126 markedly reduced the activated ERK level to the control level, whereas the level of activated ERK was unaffected by the angiotensin-converting enzyme inhibitor imidaprilat or the angiotensin II receptor blocker candesartan. A human (pro)renin receptor was present in hVSMCs, and its knockdown with small interfering RNA (siRNA) significantly inhibited the prorenin-induced ERK activation. These results suggest that prorenin stimulates ERK phosphorylation in hVSMCs through the receptor-mediated activation of tyrosine kinase and subsequently MEK, independently of the generation of angiotensin II or the activation of its receptor. The (pro)renin receptor-mediated ERK signal transduction is thus a possible new therapeutic target for preventing vascular complications.

Immediate hypersensitivity elicits renin release from cardiac mast cells

BACKGROUND

We recently reported that murine and cavian heart mast cells are a unique extrarenal source of renin. Ischemia/reperfusion releases this renin leading to local angiotensin formation and norepinephrine release. As mast cells are a primary target of hypersensitivity, we assessed whether anaphylactic mast cell degranulation also results in renin and norepinephrine release.

METHODS

Hearts isolated from presensitized guinea pigs were challenged with antigen.

RESULTS

Cardiac anaphylaxis was characterized by mast cell degranulation, evidenced by beta-hexosaminidase release and associated with renin and norepinephrine release. Mast cell stabilization with cromolyn or lodoxamide markedly attenuated the release of beta-hexosaminidase, renin and norepinephrine. Renin inhibition with BILA2157 did not affect mast cell degranulation, but attenuated norepinephrine release.

CONCLUSIONS

Our findings disclose that immediate-type hypersensitivity elicits renin release from mast cells, activating a local renin-angiotensin system, thereby promoting norepinephrine release. As renin is stored in human heart mast cells, allergic reactions could initiate renin release, leading to local angiotensin formation and hyperadrenergic dysfunction.

Comparison of Active Renin Concentration and Plasma Renin Activity as a Prognostic Predictor in Patients With Heart Failure

BACKGROUND

Plasma renin activity (PRA) may be limited to angiotensinogen levels, which decrease in patients with heart failure (HF) because of liver congestion.

METHODS AND RESULTS

To evaluate whether the plasma active renin concentration (ARC) is a more useful prognostic predictor than PRA, the plasma levels of ARC, PRA, angiotensin II, aldosterone, brain natriuretic peptide (BNP), norepinephrine, and hemodynamic parameters were measured in 214 consecutive HF patients who were already taking angiotensin-converting enzyme inhibitors (ACEI) or angiotensin-receptor blockers (ARB). Median follow-up period was 1,197 days. Of the clinical variables, including pulmonary capillary wedge pressure, right atrial pressure, left ventricular ejection fraction, and neurohumoral factors, only high plasma levels of log ARC (p<0.0001) and log BNP (p=0.0009), but not log PRA, were significant independent prognostic predictors. Log ARC/PRA ratio was significantly higher in nonsurvivors than in survivors. Log ARC/PRA significantly correlated with pulmonary capillary wedge pressure (r=0.305, p<0.0001), right atrial pressure (r=0.222, p=0.0011), and log BNP (r=0.242, p=0.0004).

CONCLUSIONS

Plasma ARC is superior to PRA and a high plasma ARC is an independent prognostic predictor in HF patients who are already receiving ACEI or ARB.

Prorenin induces intracellular signaling in cardiomyocytes independently of angiotensin II

Tissue accumulation of circulating prorenin results in angiotensin generation, but could also, through binding to the recently cloned (pro)renin receptor, lead to angiotensin-independent effects, like p42/p44 mitogen-activated protein kinase (MAPK) activation and plasminogen-activator inhibitor (PAI)-1 release. Here we investigated whether prorenin exerts angiotensin-independent effects in neonatal rat cardiomyocytes. Polyclonal antibodies detected the (pro)renin receptor in these cells. Prorenin affected neither p42/p44 MAPK nor PAI-1. PAI-1 release did occur during coincubation with angiotensinogen, suggesting that this effect is angiotensin mediated. Prorenin concentration-dependently activated p38 MAPK and simultaneously phosphorylated HSP27. The latter phosphorylation was blocked by the p38 MAPK inhibitor SB203580. Rat microarray gene (n=4800) transcription profiling of myocytes stimulated with prorenin detected 260 regulated genes (P<0.001 versus control), among which genes downstream of p38 MAPK and HSP27 involved in actin filament dynamics and (cis-)regulated genes confined in blood pressure and diabetes QTL regions, like Syntaxin-7, were overrepresented. Quantitative real-time RT-PCR of 7 selected genes (Opg, Timp1, Best5, Hsp27, pro-Anp, Col3a1, and Hk2) revealed temporal regulation, with peak levels occurring after 4 hours of prorenin exposure. This regulation was not altered in the presence of the renin inhibitor aliskiren or the angiotensin II type 1 receptor antagonist eprosartan. Finally, pilot 2D proteomic differential display experiments revealed actin cytoskeleton changes in cardiomyocytes after 48 hours of prorenin stimulation. In conclusion, prorenin exerts angiotensin-independent effects in cardiomyocytes. Prorenin-induced stimulation of the p38 MAPK/HSP27 pathway, resulting in alterations in actin filament dynamics, may underlie the severe cardiac hypertrophy that has been described previously in rats with hepatic prorenin overexpression.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.