Prorenin accumulation and activation in human endothelial cells: importance of mannose 6-phosphate receptors

Megalin: A Novel Endocytic Receptor for Prorenin and Renin

Megalin is an endocytic receptor contributing to protein reabsorption. Impaired expression or trafficking of megalin increases urinary renin and allowed the detection of prorenin, which normally is absent in urine. Here, we investigated (pro)renin uptake by megalin, using both conditionally immortalized proximal tubule epithelial cells and Brown Norway Rat yolk sac cells (BN16). To distinguish binding and internalization, cells were incubated with recombinant human (pro)renin at 4°C and 37°C, respectively. (Pro)renin levels were assessed by immunoradiometric assay. At 4°C, BN16 cells bound 3× more prorenin than renin, suggestive for a higher affinity of prorenin. Similarly, at 37°C, prorenin accumulated at 3- to 4-fold higher levels than renin in BN16 cells. Consequently, depletion of medium prorenin (but not renin) content occurred after 24 hours. No such differences were observed in conditionally immortalized proximal tubule epithelial cells, and M6P (mannose-6-phosphate) greatly reduced conditionally immortalized proximal tubule epithelial cells (pro)renin uptake, suggesting that these cells accumulate (pro)renin largely via M6P receptors. M6P did not affect (pro)renin uptake in BN16 cells. Yet, inhibiting megalin expression with siRNA greatly reduced (pro)renin binding and internalization by BN16 cells. Furthermore, treating BN16 cells with albumin, an endogenous ligand of megalin, also decreased binding and internalization of (pro)renin, while deleting the (pro)renin receptor affected the latter only. Exposing prorenin's prosegment with the renin inhibitor aliskiren dramatically increased prorenin binding, while after prosegment cleavage with trypsin prorenin binding was identical to that of renin. In conclusion, megalin might function as an endocytic receptor for (pro)renin and displays a preference for prorenin. Megalin-mediated endocytosis requires the (pro)renin receptor.

Renin Activity in Heart Failure with Reduced Systolic Function-New Insights

Regardless of the cause, symptomatic heart failure (HF) with reduced ejection fraction (rEF) is characterized by pathological activation of the renin–angiotensin–aldosterone system (RAAS) with sodium retention and extracellular fluid expansion (edema). Here, we review the role of active renin, a crucial, upstream enzymatic regulator of the RAAS, as a prognostic and diagnostic plasma biomarker of heart failure with reduced ejection fraction (HFrEF) progression; we also discuss its potential as a pharmacological bio-target in HF therapy. Clinical and experimental studies indicate that plasma renin activity is elevated with symptomatic HFrEF with edema in patients, as well as in companion animals and experimental models of HF. Plasma renin activity levels are also reported to be elevated in patients and animals with rEF before the development of symptomatic HF. Modulation of renin activity in experimental HF significantly reduces edema formation and the progression of systolic dysfunction and improves survival. Thus, specific assessment and targeting of elevated renin activity may enhance diagnostic and therapeutic precision to improve outcomes in appropriate patients with HFrEF.

Unravelling novel functions of the endosomal transporter mannose 6-phosphate/insulin-like growth factor receptor (CD222) in health and disease: An emerging regulator of the immune system

Properly balanced cellular responses require both the mutual interactions of soluble factors with cell surface receptors and the crosstalk of intracellular molecules. In particular, immune cells exposed unceasingly to an array of positive and negative stimuli must distinguish between what has to be tolerated and attacked. Protein trafficking is one of crucial pathways involved in this labour. The approximately >270-kDa protein transporter called mannose 6- phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222) is a type I transmembrane glycoprotein present largely intracellularly in the Golgi apparatus and endosomal compartments, but also at the cell surface. It is expressed ubiquitously in a vast majority of higher eukaryotic cell types. Through binding and trafficking multiple unrelated extracellular and intracellular ligands, CD222 is involved in the regulation of a plethora of functions, and thus implicated in many physiological but also pathophysiological conditions. This review describes, first, general features of CD222, such as its evolution, genomic structure and regulation, protein structure and ligands; and second, its specific functions with a special focus on the immune system.

Lectin chromatography of extrarenal renin protein in human plasma and tissues: Potential endocrine function via the renin receptor

Secretion of prorenin from extrarenal tissues comprises approximately half of the renin protein in plasma; its origin is unknown. The discovery of a prorenin/renin receptor that activates vascular tissue kinases raises interest in this otherwise inactive component. We have studied its glycosylation as this may distinguish it from renal renin. The binding of renin protein (active and prorenin) in human plasma and tissues to concanavalin (Con A) and wheat germ lectins was deployed. Immunoradiometric and enzyme kinetic assays were applied to column fractions. Mannosylated renin protein binds to Con A and has been shown to be taken up by human vascular and hepatic cells on mannose-6-phosphate receptors, possibly as a clearance mechanism. But the other binding sites of prorenin/renin that elicit a cellular phosphorylation response are apparently independent of glycosylation. The tissues examined (kidney, adrenal, ovary) each contain high proportions of Con A binding renin, but the plasma of normal resting males and females contain mainly non-binding renin, the proportion increasing as renal renin secretion decreases. The extreme of this relationship is seen in anephric patients and in some normal women on oral contraceptives with suppressed renal renin, in whom plasma renin is entirely non-binding to Con A. Conversely, when renal renin secretion was stimulated, the increased plasma active renin bound to Con A. However, extrarenal tissues containing exclusively Con A non-binding renin protein, and hence potential sources of anephric plasma renin protein, were not identified, but are unlikely to include adrenal or female reproductive tract. The findings are consistent with the view that normal human plasma contains a considerable amount of amannosylated prorenin of extrarenal origin that escapes hepatic clearance and has a longer half-life than renal renin. This plasma renin form would be expected to be activated in association with the recently described renin/prorenin aspartyl protease receptor and to participate in local pathophysiological processes.

A local renal renin-angiotensin system activation via renal uptake of prorenin and angiotensinogen in diabetic rats

The mechanism of activation of local renal renin-angiotensin system (RAS) has not been clarified in diabetes mellitus (DM). We hypothesized that the local renal RAS will be activated via increased glomerular filtration and tubular uptake of prorenin and angiotensinogen in diabetic kidney with microalbuminuria. Streptozotocin (STZ)-induced DM and control rats were injected with human prorenin and subsequently with human angiotensinogen. Human prorenin uptake was increased in podocytes, proximal tubules, macula densa, and cortical collecting ducts of DM rats where prorenin receptor (PRR) was expressed. Co-immunoprecipitation of kidney homogenates in DM rats revealed binding of human prorenin to the PRR and to megalin. The renal uptake of human angiotensinogen was increased in DM rats at the same nephron sites as prorenin. Angiotensin-converting enzyme was increased in podocytes, but decreased in the proximal tubules in DM rats, which may have contributed to unchanged renal levels of angiotensin despite increased angiotensinogen. The systolic blood pressure increased more after the injection of 20 μg of angiotensinogen in DM rats than in controls, accompanied by an increased uptake of human angiotensinogen in the vascular endothelium. In conclusion, endocytic uptake of prorenin and angiotensinogen in the kidney and vasculature in DM rats was contributed to increased tissue RAS and their pressor response to angiotensinogen.

The Role of the (Pro)renin Receptor in Hypertensive Disease

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin (together denoted as (pro)renin). Since tissue renin levels are usually higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. The (pro)renin receptor ((P)RR), discovered in 2002, appeared a promising candidate, although its nanomolar affinity for renin/prorenin is many orders of magnitude above their levels in blood. This review discusses (P)RR-related research since its discovery. First, encouraging in vitro findings supported detrimental effects of (pro)renin-(P)RR interaction, even resulting in angiotensin-independent signaling. Moreover, the putative (P)RR blocker "handle region peptide" (HRP) yielded beneficial effects in various cardiovascular animal models. Then doubt arose whether such interaction truly occurs in vivo, and (P)RR deletion unexpectedly turned out to be lethal. Moreover, HRP results could not be confirmed. Finally, it was discovered that the (P)RR actually is a component of vacuolar-type H(+)-ATPase, a multisubunit protein found in virtually every cell type which is essential for vesicle trafficking, protein degradation, and coupled transport. Nevertheless, selective (P)RR blockade in the brain with the putative antagonist PRO20 (corresponding with the first 20 amino acids of prorenin's prosegment) reduced blood pressure in the deoxycorticosteroneacetate (DOCA)-salt model, and (P)RR gene single nucleotide polymorphisms associate with hypertension. To what degree this relates to (pro)renin remains uncertain. The concept of (P)RR blockade in hypertension, if pursued, requires rigorous testing of any newly designed antagonist, and may not hold promise given the early death of tissue-specific (P)RR knockout animals.

Maternal insulin-like growth factor 1 and 2 differentially affect the renin-angiotensin system during pregnancy in the guinea pig

OBJECTIVE

Insulin-like growth factors (IGFs) are known to interact with the renin-angiotensin system (RAS). We previously demonstrated that administration of IGF1 to guinea pigs in early to mid pregnancy promotes placental function and fetal growth in mid to late gestation. Early administration of IGF2 had sustained, but not acute, effects on these parameters and also on placental structural differentiation. Here, we aimed to determine whether the IGFs interact with the placental RAS in early to mid gestation to modulate placental development and increase fetal growth and survival, and if IGF2 binding the IGF2R is implicated in the sustained effects of IGF2 treatment.

DESIGN

At day 20 of pregnancy, guinea pigs were infused with 1m g/kg/day of IGF1, IGF2, (Leu27)IGF2 or vehicle for 18days and sacrificed on either day 62 (late pregnancy) or during the infusion period on day 35 (early-mid pregnancy). Placental structure at day 35 was analyzed using morphometric technique and expression of RAS genes in the placenta and placental and plasma renin activity were measured at both time points.

RESULTS

Compared with vehicle at day 35 of gestation, IGF1 infusion reduced the total midsagittal cross-sectional area of the placenta (-17%, p = 0.02) and the labyrinth area (-22%, p = 0.014) but did not alter the labyrinth volume nor labyrinth:interlobium ratios. IGF2 treatment did not affect placental structure. IGF1 did not alter placental mRNA for any of the RAS genes quantified at day 35 (AGTR1, ACE, AGT, TGFB1) but increased TGFB1 expression by more than 16-fold (p = 0.005) at day 62. IGF2 increased placental expression of AGTR1 (+88%, p = 0.03) and decreased AGT (-73%, p = 0.01) compared with the vehicle-treated group at day 35, and both IGF2 and (Leu27)IGF2 increased expression of TGFB1 at day 62 by 9-fold (p = 0.016) and 6-fold (p = 0.019) respectively. Both IGFs increased the ratio of active:total placental renin protein (+22% p = 0.026 p = 0.038) compared to vehicle compared to vehicle at day 35 but not 62. At day 62, IGF2-treated mothers showed a marked increase in total plasma renin (+495%) and active renin (+359%) compared to vehicle but decreased the ratio of active to total renin by 41% (p = 0.042). (Leu27)IGF2-treated animals had higher levels of placental active renin (+73%, p = 0.001) and total renin (+71%, p = 0.001) compared with the vehicle control.

CONCLUSIONS

The data obtained in the current study suggest the potential for alternate roles for the induction of the RAS after IGF treatment. IGF1 and 2 treatments increase the activation of prorenin to renin in the placenta, possibly due to increased protease activity. In addition, IGF2 treatment in early pregnancy may enhance the maternal adaptation to pregnancy through stimulation of renin in the kidney. The sustained effects on placental differentiation and function after IGF2 treatment suggest therapeutic potential for exogenous administration of IGFs in improving pregnancy outcomes.

Renin and the IGFII/M6P receptor system in cardiac biology

Nonenzymatic cardiac activities of renin are well described during the last years and contribute either to cardiac-specific effects of the renin-angiotensin-aldosterone-system (RAAS) or to the pharmacological effects of RAAS inhibition. The interaction of renin with insulin-like growth factor II/mannose-6-phosphate (IGFII/M6P) receptors participates in nonclassical renin effects and contributes to cardiac remodelling caused by RAAS activation. The current findings suggest an important role for renin IGFII/M6P receptor interaction in cardiac adaptation to stress and support the idea that excessive accumulation of renin during inhibition of RAAS directly contributes to blood pressure-independent effects of these pharmacological interventions. It becomes a challenge for future studies focussing on chronic hypertension or myocardial infarction to comprise regulatory adaptations of the kidney, the main source of plasma renin and prorenin, because they directly contribute to key steps in regulation of cardiac (mal)adaptation via IGFII/M6P receptors. This receptor system is part of peptide/receptor interactions that modifies and possibly limits adverse remodelling effects caused by angiotensin II. Evaluation of interactions of renin with other pro-hypertrophic agonists is required to decide whether this receptor may become a target of pharmacological intervention.

The (pro)renin receptor. A decade of research: what have we learned?

The discovery of a (pro)renin receptor ((P)RR) in 2002 provided a long-sought explanation for tissue renin–angiotensin system (RAS) activity and a function for circulating prorenin, the inactive precursor of renin, in end-organ damage. Binding of renin and prorenin (referred to as (pro)renin) to the (P)RR increases angiotensin I formation and induces intracellular signalling, resulting in the production of profibrotic factors. However, the (pro)renin concentrations required for intracellular signalling in vitro are several orders of magnitude above (patho)physiological plasma levels. Moreover, the phenotype of prorenin-overexpressing animals could be completely attributed to angiotensin generation, possibly even without the need for a receptor. The efficacy of the only available putative (pro)renin receptor blocker handle region peptide remains doubtful, leading to inconclusive results. The fact that, in contrast to other RAS components, (P)RR knock-outs, even tissue-specific, are lethal, points to an important, (pro)renin-independent, function of the (P)RR. Indeed, recent research has highlighted ancillary functions of the (P)RR as an essential accessory protein of the vacuolar-type H⁺-ATPase (V-ATPase), and in this role, it acts as an intermediate in Wnt signalling independent of (pro)renin. In conclusion, (pro)renin-dependent signalling is unlikely in non-(pro)renin synthesizing organs, and the (P)RR role in V-ATPase integrity and Wnt signalling may explain some, if not all of the phenotypes previously associated with (pro)renin-(P)RR interaction.

(Pro)renin and its receptors: pathophysiological implications

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin [together denoted as (pro)renin]. Since tissue renin levels are usually somewhat higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. Several candidates have been evaluated in the past three decades, including a renin-binding protein, the mannose 6-phosphate/insulin-like growth factor II receptor and the (pro)renin receptor. Although the latter seemed the most promising, its nanomolar affinity for renin and prorenin is several orders of magnitude above their actual (picomolar) levels in blood, raising doubt on whether (pro)renin–(pro)renin receptor interaction will ever occur in vivo. A wide range of in vitro studies have now demonstrated (pro)renin-receptor-induced effects at nanomolar renin and prorenin concentrations, resulting in a profibrotic phenotype. In addition, beneficial in vivo effects of the putative (pro)renin receptor blocker HRP (handle region peptide) have been observed, particularly in diabetic animal models. Despite these encouraging results, many other studies have reported either no or even contrasting effects of HRP, and (pro)renin-receptor-knockout studies revealed lethal consequences that are (pro)renin-independent, most probably due to the fact that the (pro)renin receptor co-localizes with vacuolar H+-ATPase and possibly determines the stability of this vital enzyme. The present review summarizes all of the recent findings on the (pro)renin receptor and its blockade, and critically compares it with the other candidates that have been proposed to mediate (pro)renin uptake from blood. It ends with the conclusion that the (pro)renin–(pro)renin receptor interaction, if it occurs in vivo, is limited to (pro)renin-synthesizing organs such as the kidney.

Renin and prorenin have no direct effect on aldosterone synthesis in the human adrenocortical cell lines H295R and HAC15

INTRODUCTION

Transgenic rats expressing the human (pro)renin receptor (h(P)RR) have elevated plasma aldosterone levels despite unaltered levels, in plasma and adrenal, of renin and angiotensin II.

MATERIALS AND METHODS

To investigate whether renin/prorenin-(P)RR interaction underlies these elevated aldosterone levels, the effect of (pro)renin on steroidogenesis was compared with that of angiotensin II in two (P)RR-expressing human adrenocortical cell lines, H295R and HAC15. Angiotensin II rapidly induced extracellular signal-regulated kinase (ERK) phosphorylation and increased the expression of STAR, CYP21A2, CYP11B2, and CYP17A1 at 6 and 24 hours, whereas the expression of CYP11A1 and HSD3B2 remained unaltered. Incubation with renin or prorenin at nanomolar concentrations had no effect on the expression of any of the steroidogenic enzymes tested, nor resulted in ERK phosphorylation. Angiotensin II, but not renin or prorenin, induced aldosterone production.

CONCLUSION

Although the (P)RR is present in adrenocortical cells, renin and prorenin do not elicit ERK phosphorylation nor directly affect steroid production via this receptor at nanomolar concentrations. Thus, direct (pro)renin-(P)RR interaction is unlikely to contribute to the elevated aldosterone levels in human (P)RR transgenic rats. This conclusion also implies that the aldosterone rise that often occurs during prolonged renin-angiotensin system blockade is rather due to the angiotensin II 'escape' during such blockade.

Angiotensin II-aldosterone interaction in human coronary microarteries involves GPR30, EGFR, and endothelial NO synthase

AIMS

The aim of this study was to investigate the aldosterone-angiotensin (Ang) II interaction in human coronary microarteries (HCMAs).

METHODS AND RESULTS

HCMAs, obtained from 75 heart-beating organ donors, were mounted in myographs and exposed to Ang II, either directly or following a 30-min pre-incubation with aldosterone, 17β-oestradiol, hydrocortisone, the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580, the extracellular regulated kinase 1/2 (ERK1/2) inhibitor PD98059, the GPR30 antagonist G15, or the epidermal growth factor receptor (EGFR) antagonist AG1478. Ang II constricted HCMAs in a concentration-dependent manner. All steroids, at nanomolar levels, potentiated Ang II and G15 prevented this effect. The potentiation disappeared or was reversed into Ang II antagonism at micromolar steroid levels. NO synthase (NOS) inhibition prevented the latter antagonism in the case of 17β-oestradiol, whereas both aldosterone and 17β-oestradiol at micro- (but not nano-) molar levels induced endothelial NOS phosphorylation in human umbilical vein endothelial cells. AG1478, but not SB203580 or PD98059, abolished the Ang II-induced contraction in the presence of aldosterone or 17β-oestradiol, and none of these drugs affected Ang II alone.

CONCLUSION

Steroids including aldosterone affect Ang II-induced vasoconstriction in a biphasic manner. Potentiation occurs at nanomolar steroid levels and depends on GPR30 and EGFR transactivation. At micromolar steroid levels, this potentiation either disappears (aldosterone and hydrocortisone) or is reversed into an inhibition (17β-oestradiol), and this is due to the endothelial NOS activation that occurs at such concentrations.

Aliskiren, a renin inhibitor, downregulates TNF-α-induced tissue factor expression in HUVECS

Angiotensin (Ang)II, the effector arm of the locally active renin—angiotensin system (RAS), modulates Tissue Factor (TF), the principal initiator of blood coagulation and a key promoter of atherothrombotic events. Consistent with that knowledge, previous data showed inhibitory properties of angiotensin-converting enzyme inhibitor (ACEI)s and angiotensin II type-1 receptor blocker (ARB)s, but no data are available about the effect of renin inhibition. We aimed to evaluate whether aliskiren, a direct renin inhibitor (DRI), modulates TNF-α-stimulated TF expression in cultured human umbilical vein endothelial cells (HUVECs). Zofenopril, an ACEI, and olmesartan, an ARB, were the controls. HUVECs were incubated with experimental drugs (1 nM) 30 min prior to TNF-α stimulation (0.1 ng/ml × 4 h). Main evaluation variables were procoagulant activity (single-stage clotting assay), TF antigen (ELISA) and mRNA expression (real-time polymerase chain reaction) in cell lysates. TNF-α stimulated procoagulant activity and increased TF antigen and mRNA expression. Aliskiren inhibited TNF-α-mediated TF stimulation; zofenopril and olmesartan exerted a comparable effect. We conclude that aliskiren, a DRI, downregulates TNF-α-stimulated TF expression in HUVECs, possibly as a reflection of endothelial renin activation by the cytokine.

The increase in renin during renin inhibition: does it result in harmful effects by the (pro)renin receptor?

Renin inhibitors, similar to all renin-angiotensin system (RAS) blockers, increase the plasma concentration of renin because they attenuate the negative feedback effect of angiotensin (Ang) II on renin release. The increase in renin has been suggested to be higher than that during other types of RAS blockade. This could potentially limit the effectiveness of renin inhibition, either because Ang II generation might occur again ('Ang II escape'), possibly even at the levels above baseline, as has been described before for angiotensin-converting enzyme inhibitors, or because high levels of renin will stimulate the recently discovered (pro)renin receptor, and thus induce effects in an Ang-independent manner. This review shows first that the cause(s) of the renin increase during treatment with the renin inhibitor aliskiren is the consequence of a combination of factors, including an assay artifact, allowing the detection of prorenin as renin, and a change in renin half-life. When correcting for these phenomena the increase is unlikely to be as excessive as originally thought. The review then critically describes the consequence(s) of such a increase, concluding (i) that an Ang II escape is highly unlikely, given the [aliskiren]/[renin] stoichiometry, and (ii) that renin and prorenin downregulate their receptor (similar to many agonists). On the basis of the latter, one could even speculate that this will be more substantial when the renin and prorenin levels are higher. Thus, from this point of view the larger increase in renin during renin inhibition will cause a stronger reduction in (pro)renin receptor expression, and a greater suppression of (pro)renin receptor-mediated effects than other renin-Ang blockers.

Diabetic complications: a role for the prorenin-(pro)renin receptor-TGF-beta1 axis?

Morbidity and mortality of diabetes mellitus are strongly associated with cardiovascular disease including nephropathy. A discordant tissue renin-angiotensin system (RAS) might be a mediator of the endothelial dysfunction leading to both micro- and macrovascular complications of diabetes. The elevated plasma levels of prorenin in diabetic subjects with microvascular complications might be part of this discordant RAS, especially since the plasma renin levels in diabetes are low. Prorenin, previously thought of as an inactive precursor of renin, is now known to bind to a (pro)renin receptor, thus activating locally angiotensin-dependent and -independent pathways. In particular, the stimulation of the transforming growth factor-beta (TGF-beta) system by prorenin could be an important contributor to diabetic disease complications. This review discusses the concept of the prorenin-(pro)renin receptor-TGF-beta(1) axis, concluding that interference with this pathway might be a next logical step in the search for new therapeutic regimens to reduce diabetes-related morbidity and mortality.

Prolonged exposure of cardiac cells to renin plus angiotensinogen reduces intracellular renin in the failing heart. On the role of angiotensin II-AT1 complex internalization

To investigate the influence of prolonged exposure of cardiac cells to renin plus angiotensinogen (Ao) on intracellular renin levels, myocytes were isolated from the ventricle of cardiomyopathic hamsters(TO-2) and incubated in Krebs solution containing renin(128 pmol Ang ml/min) plus Ao (110 pmol Ang I generated by renin to exhaustion) for a period of 24 h. Membrane-bound and intracellular AT1 receptors levels as well as intracellular renin were studied using immunological methods and quantified by flow cytometry. The results indicated: a) intracellular renin levels were higher in the failing heart at an advanced stage of the disease (8 months) than in age-matched controls; b) the intracellular renin levels were significantly reduced in cells exposed to renin (128 pmol Ang I.ml/min) plus angiotensinogen (Ao)(110 pmol Ang I generated by renin to exhaustion) for a period of 24 h; c) incubation of the cardiomyocytes with renin (128 pmol Ang I.ml/min) alone did not reduced the intracellular renin levels; d) the fall of the intracellular renin level was related to the formation of angiotensin II (Ang II) at the surface cell membrane and internalization of the Ang II-AT1 complex because losartan (10(-7) M) added to the incubation medium containing renin plus Ao, blocked the internalization of AT1 and suppressed the decline of the intracellular renin levels; e) no internalization of renin or renin secretion was found in these experiments.

IN CONCLUSION

prolonged exposure of cardiac cells to renin plus Ao (24 h) reduced intracellular renin levels through the internalization of Ang II-AT1 complex and inhibition of renin expression.

The (pro)renin receptor: a new addition to the renin-angiotensin system?

The renin-angiotensin system is still incompletely understood. In particular, the function of prorenin, the inactive precursor of renin, is unknown. Yet, prorenin levels are >10-fold higher than renin levels, and prorenin increases even further in subjects with diabetes mellitus displaying microvascular complications. The recent discovery of a (pro)renin binding receptor may shed light on the role of prorenin. This review discusses the possibility that prorenin binding to this receptor results in prorenin activation, thereby allowing angiotensin generation, and that prorenin simultaneously acts as an agonist of this receptor, inducing angiotensin-independent effects. Transgenic animals overexpressing the receptor, as well as a receptor antagonist are now available, and future studies should reveal to what degree this concept is applicable to humans as well.

Prorenin anno 2008

For many years, prorenin has been considered to be nothing more than the inactive precursor of renin. Yet, its elevated levels in diabetic subjects with microvascular complications and its extrarenal production at various sites in the body suggest otherwise. This review discusses the origin, regulation, and enzymatic activity of prorenin, its role during renin inhibition, and the angiotensin-dependent and angiotensin-independent consequences of its binding to the recently discovered (pro)renin receptor. The review ends with the concept that prorenin rather than renin determines tissue angiotensin generation.

Mechanisms of Disease: intracrine physiology in the cardiovascular system

The field of intracrine physiology attempts to codify the biological actions of intracrines--extracellular signaling proteins or peptides that also operate in the intracellular space, either because they are retained in their cells of synthesis or because they have been internalized by a target cell. Intracrines are structurally diverse; hormones, growth factors, DNA-binding proteins and enzymes can all display intracrine functionality. Here, we review the role of intracrines in the heart and vasculature, including the intracrine actions of renin-angiotensin-system components in cardiac pathology, dynorphin B in cardiac development, and a variety of factors in pathologic and therapeutic angiogenesis. We argue that principles of intracrine physiology can inform our understanding of important pathologic processes such as left ventricular hypertrophy, diabetic cardiomyopathy and arrythmogenesis, and can aid the development of more-effective therapeutic interventions in cardiovascular disease.

Functional renin receptors in renal mesangial cells

Activation of the renin-angiotensin system (RAS) and generation of angiotensin II (Ang II) play a crucial role in fibrotic renal disease beyond this system's hemodynamic actions. Ang II blockade was a great therapeutic breakthrough for renal and cardiovascular diseases; however, this slows, but does not stop, disease progression. These limitations leave other molecules unopposed to sustain disease progression. One is renin, which is markedly elevated by Ang II blockade. Recently, a new renin receptor was cloned in renal mesangial cells. This receptor acts as a renin/prorenin cofactor on the cell surface, enhancing efficiency of angiotensinogen cleavage by renin and unmasking prorenin catalytic activity. Unexpectedly, the receptor induces angiotensin-independent cellular effects in renal mesangial cells, suggesting that renin has novel receptor-mediated actions that could play a role in renal fibrosis. Proof of this could lead to a pharmacological compound blocking renin/prorenin binding and activity as an alternative or adjunct to classical inhibitors of the RAS.

The (pro)renin receptor: pathophysiological roles in cardiovascular and renal pathology

PURPOSE OF REVIEW

The pathophysiological role of the (pro)renin receptor is yet to be established. The present review summarizes the findings, suggesting that it may play pathological role in cardiac and renal fibrosis, and in hypertensive and diabetic nephropathy.

RECENT FINDINGS

In-vitro and animal studies have shown that increased receptor expression could be linked to high blood pressure and to cardiac and glomerular fibrosis by activating mitogen-activated protein kinases and by upregulating gene expression of profibrotic molecules. Studies also suggest that the receptor is involved in diabetic nephropathy by activating receptor-bound prorenin, thereby increasing angiotensin II tissue generation. Moreover, in diabetic mice, a peptide able to block prorenin binding to the receptor was claimed to be more effective for renal protection than angiotensin-converting enzyme inhibitor.

SUMMARY

The experimental data confirmed the pivotal role of the receptor in cell surface generation of angiotensin and suggested its potential role in tissue fibrosis via receptor activation and intracellular signaling. The data also questioned the ability of soon available renin inhibitors to inhibit the activity of receptor-bound renin and prorenin, and the benefit of a new class of drug--(pro)renin receptor blockers--to prevent tissue damage.

Mannose-6-phosphate/insulin-Like growth factor-II receptors may represent a target for the selective delivery of mycophenolic acid to fibrogenic cells

PURPOSE

The insulin-like growth factor axis plays an important role in fibrogenesis. However, little is known about mannose-6-phosphate/Insulin-like growth factor-II receptor (M6P/IGF-IIR) expression during fibrosis. When expressed preferentially on fibrogenic cells, this receptor may be used to selectively deliver drugs to these cells.

METHODS

We investigated M6P/IGF-IIR expression in livers of bile duct-ligated (BDL) rats and in renal vascular walls of renin transgenic TGR(mRen2)27 rats. Both models are characterized by fibrogenic processes. Furthermore, we studied whether drug delivery via M6P/IGF-II-receptor-mediated uptake is possible in fibroblasts.

RESULTS

M6P/IGF-IIR mRNA expression was investigated 3, 7 and 10 days after BDL. At all time-points hepatic M6P/IGF-IIR expression was significantly increased compared to healthy controls. Moreover, immunohistochemical staining revealed that alpha-sma-positive cells were M6P/IGF-IIR-positive. In kidneys of TGR(mRen2)27 rats, the number of M6P/IGF-IIR-positive arteries per microscopic field was increased 5.5 fold over healthy controls. To examine whether M6P/IGF-IIRs could be used as a port of entry for drugs, we coupled mycophenolic acid (MPA) to mannose-6-phosphate-modified human serum albumin (M6PHSA). M6PHSA-MPA inhibited 3T3-fibroblast proliferation dose-dependently, which was reversed by co-incubation with excess M6PHSA, but not by HSA.

CONCLUSIONS

M6P/IGF-IIRs are expressed by fibrogenic cells and may be used for receptor-mediated intracellular delivery of the antifibrogenic drug MPA.

Prorenin and Angiotensin-Dependent Renal Vasoconstriction in Type 1 and Type 2 Diabetes

Prorenin is a powerful marker for risk of nephropathy and retinopathy in diabetes, but the responsible mechanism remains unclear. Studied were 35 patients with diabetes (18 with type 1 and 17 with type 2) and 69 age-matched healthy subjects with para-aminohippurate and inulin clearances and their response to captopril. All patients with diabetes had normal renal function and no microalbuminuria. Prorenin was calculated as the difference between total renin and active renin. Active renin level in patients with diabetes (11.6 +/- 0.9 microU/ml) was significantly lower than in normal subjects (14.5 +/- 1.3 microU/ml; P < 0.05); despite this, the renal vascular response to captopril was much larger (82.9 +/- 11.5 versus 13.6 +/- 5.8 ml/min per 1.73 m(2); P < 0.01). Prorenin in both patients with type 1 and type 2 diabetes (175.7 +/- 15.1 microU/ml) also was significantly higher than in normal subjects (128 +/- 5.8 microU/ml; P < 0.01). Active renin correlated with prorenin in normal subjects (r = 0.44, P = 0.0002), and this correlation was much more striking in patients with diabetes (r = 0.72, P = 0.0001). The active renin and prorenin correlation was identical in type 1 and type 2 diabetes. There was a clear correlation between plasma prorenin and the renovascular response to captopril in patients with diabetes (P < 0.01) but not in normal subjects (P > 0.13). The strong correlation between plasma prorenin concentration and the renovascular response to captopril in diabetes supports the hypothesis of a direct effect of prorenin, but the unanticipated high degree of correlation between plasma prorenin and active renin limits the conclusions that can be drawn.

The (pro)renin receptor: therapeutic consequences

It is generally assumed that the beneficial effects of renin-angiotensin system blockers in cardiovascular disease are due to blockade of the generation or action of angiotensin at tissue sites. Such generation depends on the uptake of renin and/or its inactive precursor prorenin from the circulation. Recently, a (pro)renin receptor has been cloned that might perform this task. Unexpectedly, this receptor also induced angiotensin-independent effects, suggesting that renin and/or prorenin may act as agonists for this receptor. Ultimately, this could lead to the development of (pro)renin receptor blockers (i.e., drugs that not only prevent tissue angiotensin generation but also inhibit renin- or prorenin-induced effects).

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.

Is angiotensin II made inside or outside of the cell?

Angiotensin synthesis at tissue sites is well-established, and depends largely, if not completely, on kidney-derived renin. The exact tissue site of angiotensin generation (extracellular fluid, cell surface, intracellular compartment) is still being debated. In this review, we discuss the various possibilities, taking into consideration the intracellular occurrence/absence of prorenin, renin, angiotensinogen, angiotensin-converting enzyme, and angiotensin receptors; the local activation of prorenin to renin; the differences between in vivo and in vitro studies; and the methodologic difficulties related to angiotensin measurements. It is eventually concluded that angiotensin generation at tissue sites occurs extracellularly, most likely on the cell surface.

Physiological Relevance of Renin/Prorenin Binding and Uptake

There is compelling physiological evidence of binding and uptake of renin and prorenin in tissues. A number of molecules with the ability to bind renin and prorenin have been identified and have been characterized to varying degrees. It remains unclear, however, just how many renin/prorenin binding proteins and receptors exist and what their physiological functions may be. The possible functions of renin/prorenin binding and uptake are manifold, and include clearance of renin and prorenin from the circulation, local generation of angiotensins, activation of prorenin on the cell surface, trafficking of prorenin between cellular and extracellular compartments as part of a complex processing machinery, and signal transduction both via direct receptor mediated signaling, and via modulation of O-linkage of N-acetyl-glucosamine to cellular proteins. Some of these functions may involve single renin/prorenin binding sites or receptors, while others may require multiple binding sites and receptors. This review describes the physiological studies that have provided evidence of renin/prorenin uptake from the circulation, summarizes our knowledge of renin/prorenin binding proteins and receptors, and postulates new roles for renin/prorenin binding and uptake in tissues.

The potentials of MS-based subproteomic approaches in medical science: the case of lysosomes and breast cancer

Because of the great number of women who are diagnosed with breast cancer each year, and though this disease presents the lowest mortality rate among cancers, breast cancer remains a major public health problem. As for any cancer, the tumorigenic and metastatic processes are still hardly understood, and the biochemical markers that allow either a precise monitoring of the disease or the classification of the numerous forms of breast cancer remain too scarce. Therefore, great hopes are put on the development of high-throughput genomic and proteomic technologies. Such comprehensive techniques should help in understanding the processes and in defining steps of the disease by depicting specific genes or protein profiles. Because techniques dedicated to the current proteomic challenges are continuously improving, the probability of the discovery of new potential protein biomarkers is rapidly increasing. In addition, the identification of such markers should be eased by lowering the sample complexity; e.g., by sample fractionation, either according to specific physico-chemical properties of the proteins, or by focusing on definite subcellular compartments. In particular, proteins of the lysosomal compartment have been shown to be prone to alterations in their localization, expression, or post-translational modifications (PTMs) during the cancer process. Some of them, such as the aspartic protease cathepsin D (CatD), have even been proven as participating actively in the disease progression. The present review aims at giving an overview of the implication of the lysosome in breast cancer, and at showing how subproteomics and the constantly refining MS-based proteomic techniques may help in making breast cancer research progress, and thus, hopefully, in improving disease treatment.

Food vacuole plasmepsins are processed at a conserved site by an acidic convertase activity in Plasmodium falciparum

Intraerythrocytic Plasmodium falciparum digests vast amounts of hemoglobin within an acidic food vacuole (FV). Four homologous aspartic proteases participate in hemoglobin degradation within the FV. Plasmepsin (PM) I and II are thought to initiate degradation of the native hemoglobin molecule. PM IV and histo-aspartic protease (HAP) act on denatured globin further downstream in the pathway. PM I and II have been shown to be synthesized as zymogens and activated by proteolytic removal of a propiece. In this study, we have determined that the proteolytic processing of FV plasmepsins occurs immediately after a conserved Leu-Gly dipeptidyl motif with uniform kinetics and pH and inhibitor sensitivities. We have developed a cell-free in vitro processing assay that generates correctly processed plasmepsins. Our data suggest that proplasmepsin processing is not autocatalytic, but rather is mediated by a separate processing enzyme. This convertase requires acidic conditions and is blocked only by the calpain inhibitors, suggesting that it may be an atypical calpain-like protease.

Local renin-angiotensin systems: the unanswered questions

The concept of local renin-angiotensin systems has been introduced almost 20 years ago to explain the beneficial blood pressure-independent effects of ACE inhibitors and AT(1) receptor antagonists in cardiovascular diseases. In the past decade, research has focussed on the local effects of angiotensin II rather than on the mechanism(s) of its local generation. This review addresses several of the unanswered questions with regard to tissue angiotensin II generation, focussing in particular on the heart and vascular wall: (1) what is the origin of the renin that is required to generate angiotensin II locally, (2) where does tissue angiotensin generation occur (intra- versus extracellular), (3) what is the importance of alternative (non-renin, non-ACE) angiotensin-generating enzymes, (4) do ACE inhibitors and AT(1) receptor antagonists exert local effects that are renin-angiotensin system independent (thereby incorrectly leading to the conclusion that they interfere with the local generation or effects of angiotensin II), and (5) to what degree do differences in tissue angiotensin generation underlie the association between cardiovascular diseases and renin-angiotensin system gene polymorphisms?

The renin receptor: the facts, the promise and the hope

PURPOSE OF REVIEW

The renin-angiotensin system plays a major role in the control of blood pressure and of salt balance, but it is also involved in physiological and pathological processes, development, inflammation and cardiac hypertrophy. A concept has emerged suggesting that these effects are due to a local activation of the renin-angiotensin system. The search for a receptor of renin was based on the idea that tissue (pro)renin is taken up from the circulation and on data suggesting that renin has cellular effects independent of angiotensin II.

RECENT FINDINGS

Endothelial cells and cardiac myocytes bind (pro)renin via the mannose-6-phosphate receptor, mainly a clearance receptor as no cellular effect has been specifically attributed to prorenin binding. A functional receptor was cloned recently. It mediates intracellular signalling by activating the mitogen activated protein kinases, extracellular signal regulated kinases 1 and 2, and acts as a co-factor by increasing the efficiency of angiotensinogen cleavage by receptor-bound (pro)renin. The receptor is abundantly expressed in heart, brain, placenta and eye, compared with a lower expression in liver and kidney. In normal human kidney and heart, it is localized in the mesangium and in the coronary and kidney artery, associated with smooth-muscle cells and co-localized with renin.

SUMMARY

This receptor provides a functional role for prorenin and may help to understand the physiological and pathological role of elevated levels of prorenin and of local activation of the renin-angiotensin system. From a practical point of view, it questions the need for a pharmacological compound blocking (pro)renin binding and activity as an alternative to the classical inhibitors of the renin-angiotensin system.

Prorenin uptake in the heart: a prerequisite for local angiotensin generation?

Interference with locally generated angiotensin II most likely underlies the beneficial effects of renin-angiotensin system blockers in cardiac disorders. Since renin is not synthesized in the heart, this enzyme must be sequestered from the circulation in order to allow angiotensin generation at cardiac tissue sites. This review addresses the various ways through which circulating (i.e., kidney-derived) renin may reach cardiac tissue sites, considering in particular the possibility that prorenin, the inactive precursor of renin, is involved in cardiac angiotensin generation, as the plasma concentrations of prorenin are tenfold higher than those of renin. Renin and prorenin diffuse into the cardiac interstitial space and bind to cardiac (pro)renin receptors/renin-binding proteins. One of these receptors is the mannose 6-phosphate/insulin-like growth factor II receptor. This receptor not only binds mannose 6-phosphate-containing ligands like renin and prorenin, it also internalizes these enzymes, and activates prorenin intracellularly. This process possibly represents (pro)renin clearance, since intracellular angiotensin generation could not be demonstrated following (pro)renin uptake by cardiomyocytes. Angiotensin II-mediated myocyte proliferation did occur when incubating cardiomyocytes with prorenin plus angiotensionogen. The effects of prorenin plus angiotensinogen were comparable to those of 100nmol/l angiotensin II, although the angiotensin II levels in the medium during exposure of the cells to prorenin plus angiotensinogen were <1nmol/l. This suggests that cardiac angiotensin II generation by circulating renin occurs predominantly on the cell surface. The presence of ACE and/or renin on the cell membrane, in the microenvironment of angiotensin receptors, would allow maximal efficiency of local angiotensin II generation, i.e., immediate binding of angiotensin II to its receptors with minimal loss into the extracellular space.

Transendothelial transport of renin-angiotensin system components

BACKGROUND

Vascular (interstitial) angiotensin (ANG) II production depends on circulating renin-angiotensin system (RAS) components. Mannose 6-phosphate (man-6-P) receptors and angiotensin II type 1 (AT(1)) receptors, via binding and internalization of (pro)renin and ANG II, respectively, could contribute to the transportation of these components across the endothelium.

OBJECTIVE

To investigate the mechanism(s) contributing to transendothelial RAS component transport.

METHODS

Human umbilical vein endothelial cells were cultured on transwell polycarbonate filters, and incubated with RAS components in the absence or presence of man-6-P, eprosartan or PD123319, to block man-6-P, AT(1) and angiotensin II type 2 (AT(2)) receptors, respectively.

RESULTS

Apically applied (pro)renin and angiotensinogen slowly entered the basolateral compartment, in a similar manner as horseradish peroxidase, a molecule of comparable size that reaches the interstitium via diffusion only. Prorenin transport was unaffected by man-6-P. Apical ANG I and ANG II rapidly reached the basolateral fluid independent of AT(1) and AT(2) receptors. Basolateral ANG II during apical ANG I application was as high as apical ANG II, whereas during apical ANG II application it was lower. During basolateral ANG I application, ANG II generation occurred basolaterally only, in an angiotensin-converting enzyme (ACE)-dependent manner.

CONCLUSIONS

Circulating (pro)renin, angiotensinogen, ANG I and ANG II enter the interstitium via diffusion, and interstitial ANG II generation is mediated, at least in part, by basolaterally located endothelial ACE.