Effect of glucose on the expression of the angiotensinogen gene in opossum kidney cells

Insulin signalling and glucose transport in the ovary and ovarian function during the ovarian cycle

Data derived principally from peripheral tissues (fat, muscle and liver) show that insulin signals via diverse interconnecting intracellular pathways and that some of the major intersecting points (known as critical nodes) are the IRSs (insulin receptor substrates), PI3K (phosphoinositide kinase)/Akt and MAPK (mitogen-activated protein kinase). Most of these insulin pathways are probably also active in the ovary and their ability to interact with each other and also with follicle-stimulating hormone (FSH) and luteinizing hormone (LH) signalling pathways enables insulin to exert direct modulating influences on ovarian function. The present paper reviews the intracellular actions of insulin and the uptake of glucose by ovarian tissues (granulosa, theca and oocyte) during the oestrous/menstrual cycle of some rodent, primate and ruminant species. Insulin signals through diverse pathways and these are discussed with specific reference to follicular cell types (granulosa, theca and oocyte). The signalling pathways for FSH in granulosa cells and LH in granulosa and theca cells are summarized. The roles of glucose and of insulin-mediated uptake of glucose in folliculogenesis are discussed. It is suggested that glucose in addition to its well-established role of providing energy for cellular function may also have insulin-mediated signalling functions in ovarian cells, involving AMPK (AMP-dependent protein kinase) and/or hexosamine. Potential interactions of insulin signalling with FSH or LH signalling at critical nodes are identified and the available evidence for such interactions in ovarian cells is discussed. Finally the action of the insulin-sensitizing drugs metformin and the thiazolidinedione rosiglitazone on follicular cells is reviewed.

The relationship between urinary renin-angiotensin system markers, renal function, and blood pressure in adolescents with type 1 diabetes

The relationship between the renal renin-angiotensin aldosterone system (RAAS) and cardiorenal pathophysiology is unclear. Our aims were to assess 1) levels of urinary RAAS components and 2) the association between RAAS components and HbA1c, the urine albumin/creatinine ratio (ACR), estimated glomerular filtration rate (eGFR), and blood pressure (BP) in otherwise healthy adolescents with type 1 diabetes mellitus (TID) vs. healthy controls (HC). Urinary angiotensinogen and angtionsin-converting enzyme (ACE) 2 levels, activity of ACE and ACE2, BP, HbA1c, ACR, and eGFR were measured in 65 HC and 194 T1D from the Adolescent Type 1 Diabetes Cardio-Renal Intervention Trial (AdDIT). Urinary levels of all RAAS components were higher in T1D vs. HC ( P < 0.0001). Higher HbA1c was associated with higher urinary angiotensinogen, ACE2, and higher activity of ACE and ACE2 ( P < 0.0001, P = 0.0003, P = 0.003, and P = 0.007 respectively) in T1D. Higher ACR (within the normal range) was associated with higher urinary angiotensinogen ( P < 0.0001) and ACE activity ( P = 0.007), but not with urinary ACE2 activity or ACE2 levels. These observations were absent in HC. Urinary RAAS components were not associated with BP or eGFR in T1D or HC. Otherwise healthy adolescents with T1D exhibit higher levels of urinary RAAS components compared with HC. While levels of all urinary RAAS components correlate with HbA1c in T1D, only urinary angiotensinogen and ACE activity correlate with ACR, suggesting that these factors reflect an intermediary pathogenic link between hyperglycemia and albuminuria within the normal range.

Detailed localization of augmented angiotensinogen mRNA and protein in proximal tubule segments of diabetic kidneys in rats and humans

In the intrarenal renin-angiotensin system, angiotensinogen levels are well known to be increased in diabetes, and these enhanced intrarenal angiotensinogen levels may initiate the development and accelerate the progression of diabetic nephropathy. However, the specific localization of the augmented angiotensinogen in proximal tubule segments in diabetes is still unknown. We investigated the detailed localization of angiotensinogen in 3 proximal tubule segments in the diabetic Otsuka Long-Evans Tokushima fatty (OLETF) rats and the control Long-Evans Tokushima Otsuka (LETO) rats. We also prepared OLETF rats treated with angiotensin II type 1 receptor blocker, olmesartan or with a combination of vasodilator agents. Moreover, biopsied samples of human kidney cortex were used to confirm the results of animal studies. We examined the co-localization of angiotensinogen with segment-specific markers by double staining using fluorescence in situ hybridization and/or immunofluorescence. Angiotensinogen mRNA expression was barely detectable in segment 1. In segment 3, the area of angiotensinogen mRNA expression was augmented in the OLETF rats compared with the LETO rats. Angiotensinogen protein expression areas in segments 1 and 3 were also increased in the OLETF rats compared with the LETO rats. Chronic treatment with olmesartan ameliorated these areas of augmented angiotensinogen expression. Biopsied human kidney samples showed similar results. These data suggest that the augmented angiotensinogen mRNA levels in segment 3 and angiotensinogen protein levels in segments 1 and 3 may contribute to the progression of diabetic nephropathy.

Enhancive effects of D-glucose and its analogs on expression of d-glucose-unrelated transgenes in mammalian cells

We studied the effects of d-glucose on transgene expression in mammalian cells by a reporter gene assay using CV-1 cells and a CMV promoter-controlled EGFP gene. Treatment of CV-1 cells with 5% D-glucose unchanged the number of fluorescent cells in fluorescence microscopic observation but significantly intensified fluorescence in the fluorometric assay. Furthermore, EGFP itself and mRNA became more abundant in Western blot and quantitative RT-PCR analyses of 5% D-glucose-treated cells, respectively. These results indicate that elevated D-glucose can activate transgene expression via transcriptional stimulation, at least in part. The same concentrations of L-glucose led to only negligible increases in transgene expression, indicating that D-glucose's effect is different from its osmotic effect. The D-glucose-induced augmentation of fluorescence was observed not only in the experiment using the CMV promoter-controlled EGFP gene but also in experiments using the SV40 and RSV promoter-controlled ones, suggesting that elevated D-glucose can enhance transgene expression regulated by various promoters commonly used in transgene expression. The assessment of D-glucose analogs for their enhancive effects on transgene expression revealed that 1,6-anhydro-D-glucose and β-methyl-D-glucoside had stronger effects than D-glucose. From this result, we can expect to find more effective carbohydrates to enhance transgene expression. The α- and β-M-D-glucosides, which are slightly different from each other in three-dimensional structure, exerted largely distinct stimulative effects on transgene expression, suggesting that fundamental rules determine the enhancive effects of saccharides and that the modification of the saccharide by applying such rules will enable us to develop more powerful substances for transgene expression.

Sequential activation of the reactive oxygen species/angiotensinogen/renin-angiotensin system axis in renal injury of type 2 diabetic rats

1. The present study was performed to test the hypothesis that the reactive oxygen species (ROS)-angiotensinogen (AGT)-renin angiotensin system (RAS) axis is sequentially activated in the development of diabetic nephropathy in Zucker diabetic fatty (ZDF) obese rats. 2. Genetic pairs of male ZDF obese and control ZDF lean rats (n = 12 of each species) were killed every 3 weeks from 12 to 21 weeks of age (n = 6 at each time point). 3. The ZDF obese rats developed diabetes mellitus at 12 weeks. At that time, urinary excretion rates of 8-isoprostane were similar between the groups; however, urinary 8-isoprostane levels were significantly increased at 15 weeks in ZDF obese rats compared with controls (36 +/- 6 vs 15 +/- 2 ng/day, respectively). At 15 weeks, protein levels of cortical angiotensinogen were similar between groups; however, cortical angiotensinogen levels were significantly increased at 18 weeks in ZDF obese rats compared with controls (relative ratio of 2.32 +/- 0.21 vs 1.00 +/- 0.20, respectively). At 12 weeks, angiotensin (Ang) II-like immunoreactivity was similar between groups in both the glomeruli and tubules; however, AngII-like immunoreactivity was increased significantly at 21 weeks in ZDF obese rats compared with controls (relative ratios of 1.98 +/- 0.55 vs 1.00 +/- 0.03, respectively, for glomeruli and 1.58 +/- 0.16 vs 1.00 +/- 0.13, respectively, for tubules). Moreover, at 21 weeks, the desmin-positive area in the glomeruli (0.63 +/- 0.08 vs 0.22 +/- 0.05%) and Masson's trichrome stain-positive area in the interstitium (4.97 +/- 0.05 vs 3.18 +/- 0.41%) were significantly increased in ZDF obese rats compared with controls, even though these differences had not been observed earlier. 4. These data suggest that the sequential activation of the ROS-AGT-RAS axis plays an important role in the development of diabetic nephropathy in ZDF obese rats.

Beneficial effects of thiazolidinediones on diabetic nephropathy in OLETF rats

PURPOSE

Diabetic nephropathy is the most serious of complications in diabetes mellitus. Thiazolidinedione (TZD) is thought to ameliorate diabetic nephropathy; however, the mechanism underlying this effect has not been elucidated. We hypothesized that the vascular endothelial growth factor (VEGF) participates in the pathogenesis of diabetic nephropathy and that TZD may be beneficial for the treatment of diabetic nephropathy because of the effect it has on VEGF.

MATERIALS AND METHODS

23 Otsuka- Long-Evans-Tokushima-Fatty (OLETF) rats and eight control Long-Evans-Tokushima-Otsuka (LETO) rats were divided into the following four groups: LETO group, control OLETF group, pioglitazone treated group (10mg/ kg/day), and rosiglitazone treated group (3mg/kg/day).

RESULTS

A progressive increase in urinary protein excretion was observed in the diabetic rats. Glomerular VEGF expression in the control OLETF rats was significantly higher than in the control LETO rats. However, there was a significant reduction in both the glomerular VEGF expression and the VEGF mRNA levels after treatment with pioglitazone and rosiglitazone. The twenty-four hour urine protein levels were significantly decreased in both groups of the treated OLETF rats.

CONCLUSION

These results suggest that TZD may have beneficial effects on diabetic nephropathy by reducing the VEGF expression.

Activation of the renin-angiotensin system within podocytes in diabetes

The autocrine and paracrine activation of the renin-angiotensin system (RAS) within cells of the kidney plays a role in the overall pathophysiology of the renal disease due to diabetes. In this study, we focus on components of the RAS in the podocyte as these cells are important in the pathogenesis of glomerulosclerosis and proteinuria. Immortalized mouse podocytes were exposed to media containing normal glucose (NG) or high glucose (HG) for in vitro studies. In vivo studies utilized kidney tissue obtained from rats treated for 3 months with streptozotocin to induce diabetes. Angiotensinogen (AGT) and the angiotensin II (AII) type 1 receptor mRNA and protein were significantly increased in the podocytes cultured under the high glucose conditions. Both angiotensins I and II levels were significantly higher in cell lysates and the conditioned media of cells grown in high glucose. There were no differences in renin activity, angiotensin-converting enzyme level, or AII type 2 receptor level. Glomerular AGT and AII type 1 receptor assessed by means of immunohistochemistry were increased in diabetic rats compared with the control rats. Other measured components of the RAS within the glomeruli were not different. We suggest that increased AGT, an attendant increase in AII and increased AII type 1 receptor in podocytes experiencing diabetic conditions play an important role in the pathogenesis of diabetic nephropathy.

Intrarenal oxidative stress and augmented angiotensinogen are precedent to renal injury in Zucker diabetic fatty rats

The Zucker diabetic fatty (ZDF) rat is a model of type II diabetes and metabolic syndrome based on impaired glucose tolerance caused by the inherited insulin-resistance gene. The ZDF rat exhibits progressive nephropathy; however, the detailed mechanisms have remained unclear. This study was performed to examine the possible involvement of enhanced intrarenal angiotensinogen in the development of renal injury in ZDF rats. Genetic pairs of male ZDF rats and control lean rats (N=6 each) were maintained from 12 to 17 weeks of age. At 17 weeks of age, fasting blood glucose and urinary 8-isoprostane levels were significantly higher in ZDF rats compared with the controls. Systolic blood pressure progressively increased in ZDF rats from 120+/-1 to 137+/-1 mmHg during this period. In contrast, systolic blood pressure did not increase in the controls. Kidney angiotensinogen protein levels were significantly increased in ZDF rats compared with the controls (1.83+/-0.34 vs. 1.00+/-0.17, relative ratio). Expression of angiotensin II type 1a receptor mRNA was similar between these groups. The measured indices of renal damage in the present study (glomerular sclerosis, interstitial expansion, glomerular macrophage infiltration, and renal arterial proliferation) were not significantly increased at this stage in ZDF rats. However, we previously showed that the increased reactive oxygen species-related angiotensinogen enhancement plays an important role in the development of renal injury in a genetic salt-sensitive hypertension. Thus, the present data suggest that elevated reactive oxygen species and reactive oxygen species-associated augmentation of intrarenal angiotensinogen may initiate the development of renal injury in ZDF rats.

Nuclear factor-kappaB activation in diabetic rat kidney: evidence for involvement of P-selectin in diabetic nephropathy

Activation of a transcription factor, nuclear factor-kappaB (NF-kappaB), is a key step in the pathogenesis of diabetic nephropathy. In this study, we investigated the role of P-selectin, a platelet-derived adhesion molecule, in diabetic nephropathy by examining the activation status of NF-kappaB in the renal cortex of streptozotocin (STZ)-treated rats. The STZ treatment induced pathogenetic parameters such as increased creatinine clearance, increased blood glucose and massive albuminuria in a time-dependent manner. Electrophoretic mobility shift assays (EMSAs) with a specific probe, representing the P-selectin gene promoter, revealed the activation status of NF-kappaB in the STZ-treated rats, as judged by the time-dependent increase in the formation of the specific protein-DNA complexes. This increase was associated with the increased pathogenetic parameters. Supershift assays with specific antibodies revealed that p50, but not p52, p65, Rel B, or c-Rel, may be involved in the activation of NF-kappaB, though the component primarily responsible for the increase could not be determined. Western blot analysis confirmed an increase in P-selectin in STZ-treated rats. Notably, treatment with ammonium pyrrolidinedithiocarbamate, an antioxidant and inhibitor of NF-kappaB, inhibited the activation of NF-kappaB in STZ-treated rats and decreased P-selectin in the renal cortical tissue. Our results indicate that expression of the P-selectin gene is induced through the activation of NF-kappaB and that P-selectin may be involved in the pathogenesis of diabetic nephropathy.

Angiotensin II and the glomerulus: focus on diabetic kidney disease

The renin-angiotensin system plays a key role in the progression of kidney disease, in addition to its well-described role in the maintenance of extracellular fluid volume and blood pressure. Recent studies have shown that blockade of the renin-angiotensin system at the level of the angiotensin II type 1 receptor can have important effects on proteinuria and the rate of progression of kidney disease in patients with type 2 diabetes mellitus. This review first discusses recent experimental studies relating angiotensin II to kidney function in diabetes mellitus and changes in glomerular permselectivity, and then focuses on recent clinical trials with angiotensin II receptor blockers in patients with type 2 diabetes mellitus.

Insulin inhibits dexamethasone effect on angiotensinogen gene expression and induction of hypertrophy in rat kidney proximal tubular cells in high glucose

The present studies investigated whether insulin inhibits the stimulatory effect of dexamethasone (DEX) on angiotensinogen (ANG) gene expression and induction of hypertrophy in rat immortalized renal proximal tubular cells (IRPTCs) in a high-glucose milieu. Rat IRPTCs were cultured in monolayer. ANG and ANG mRNA expression in IRPTCs were quantified by a specific RIA for rat ANG and by RT-PCR assay, respectively. A fusion gene containing the full length of the 5'-flanking region of the rat ANG gene linked to a chloramphenicol acetyl transferase reporter gene was introduced into IRPTCs. The level of fusion gene expression was determined by cellular chloramphenicol acetyl transferase enzymatic activity. Cellular hypertrophy was assessed by flow cytometry, cellular p27(Kip1) protein expression, and protein assay. Our results showed that high glucose (i.e. 25 mM) and DEX (10(-7) M) additively stimulated ANG gene expression and induced IRPTC hypertrophy. Insulin inhibited the effect of high glucose and DEX on these parameters. The inhibitory effect of insulin was reversed by PD 98059 (a MAPK inhibitor) but not by wortmannin (a phosphatidylinositol-3-kinase inhibitor). These results demonstrate that insulin is effective in blocking the stimulatory action of high glucose and DEX on ANG gene expression and induction of IRPTC hypertrophy, suggesting its important role in preventing local intrarenal renin-angiotensin system activation and renal proximal tubular cell hypertrophy induced by hyperglycemia and glucocorticoids in vivo.

High glucose levels alter angiotensin II-induced Ca(2+) uptake via PKC and cAMP pathways in renal proximal tubular cells

Although a dysfunction of the calcium metabolism occurs in diabetes mellitus, alterations of Ca(2+) uptake induced by angiotensin II (ANG II) in renal proximal tubular cells (PTCs) grown in high-glucose medium are not fully elucidated. Thus, we examined whether high glucose concentrations can induce an alteration of the ANG II effect on the Ca(2+) uptake and its action mechanism in primary cultured renal PTCs. PTCs were exposed to different glucose concentrations (5-100 mM) and time intervals (0-48 h). There was a sustained increase of Ca(2+) uptake at glucose concentrations >15 mM. Thus, we selected 25 mM glucose and incubation for 48 h to maintain a hyperglycemic condition in vitro, unlike short-time regulatin. ANG II significantly inhibited the Ca(2+) uptake in a dose-dependent manner in a 5-mM glucose medium. In addition, downregulation of ANG II receptors appeared in a glucose dose dependent manner. However, PTCs treated with 25 mM glucose for 48 h, not 12 h, did not exhibit the inhibitory effect of ANG II (10(-7) M) on Ca(2+) uptake, although the inhibitory effect of ANG II on Ca(2+) uptake occurred in the presence of 25 mM mannitol or L-glucose. Staurosporine, bisindolylmaleimide I (protein kinase C, PKC, inhibitors), 12-o-tetradecanoylphorbol 13-acetate pretreatment, SQ 22536 (an adenylate cyclase inhibitor), and myristoylated protein kinase A inhibitor amide 14-22 (a protein kinase A inhibitor) blocked the 25-mM-glucose-induced alteration of ANG II effect on Ca(2+) uptake. These results suggest that both PKC and cyclic adenosine monophosphate (cAMP) pathways are involved in the high-glucose-induced alteration of ANG II effect on Ca(2+) uptake. Indeed, 25 mM glucose increased PKC activity and cAMP contents. In conclusion, a high glucose concentration altered ANG II induced inhibition of Ca(2+) uptake via PKC and cAMP pathways in the PTCs.

The relationship between active renin concentration and plasma renin activity in Type 1 diabetes

AIMS

Circulating activity of the renin-angiotensin-aldosterone system (RAAS) can be assessed by measuring plasma active renin concentration (ARE), as well as by measuring plasma renin activity (PRA). We aimed to assess the relationships between ARE and PRA in Type 1 diabetic compared with non-diabetic control subjects. We also assessed concentrations of the active renin precursor, prorenin.

PATIENTS AND METHODS

Thirty-five Type 1 diabetic subjects and 34 non-diabetic control subjects were assessed. Groups had similar ages, sex distributions, body mass indices, systolic and diastolic blood pressures. PRA was measured by radioimmunoassay of angiotensin I generation from endogenous substrate. ARE and total renin concentration (TRE) were measured by immunoradiometric assay (Nichols Institute Diagnostics, USA). Prorenin concentration was calculated as the difference between ARE and TRE.

RESULTS

PRA was significantly lower in Type 1 diabetic than in control subjects (0.8 (0.4-1.1) vs. 1.1 (0.9-1.9) pmol/ml per h; P < 0.005), while ARE was similar (17 (9-33) vs. 18 (15-25) mU/l; P = 0.548). PRA (loge transformed) correlated strongly with ARE in diabetic (r = 0.49; P = 0.003) and control subjects (r = 0.59; P = 0.0002), but there was significant vertical separation of the regression lines for the two groups (P < 0.0001). Prorenin concentrations were significantly higher in Type 1 diabetic subjects (249 (170-339) vs. 171 (153-219) mU/l; P = 0.005). Diabetic subjects with high prorenin concentrations (> 400 mU/l (control mean + 3 SD)) were more likely to have microalbuminuria (P = 0.027) and peripheral neuropathy (P = 0.049).

CONCLUSIONS

Type 1 diabetes is associated with an altered relationship between ARE and PRA, such that ARE is higher for a given PRA compared with non-diabetic control subjects. Both ARE and PRA are used to assess circulating RAAS activity. The altered relationship between the two in Type 1 diabetic subjects suggests that neither parameter alone is necessarily an adequate and reliable index of such activity. Higher prorenin concentrations, particularly in association with microvascular complications, were confirmed in the Type 1 diabetic subjects. Diabet. Med. 18, 451-458 (2001)

Characterization of a putative insulin-responsive element and its binding protein(s) in rat angiotensinogen gene promoter: regulation by glucose and insulin

We previously demonstrated that high glucose activates angiotensinogen (ANG) expression and that insulin inhibits this activation. The present studies aim to investigate whether insulin regulates ANG gene expression in kidney proximal tubular cells at the transcription level via interaction of the putative insulin-response element (IRE) with its binding protein(s) in the 5'-flanking region of the ANG gene. Fusion genes containing various lengths of the 5'-flanking region of the rat ANG gene fused to a human GH (hGH) gene as reporter were constructed and transiently introduced into rat immortalized renal proximal tubular cells (IRPTCs). The expression of the fusion genes was monitored by the amount of immunoreactive hGH secreted into the medium as assayed by a specific RIA for hGH. Insulin inhibited the expression of pOGH (rANG N-1498/+18), pOGH (rANG N-1120/+18) and pOGH (rANG N-882/+18) but not pOGH (rANG N-854/+18), pOGH (rANG N-820/+18), pOGH (rANG N-688/+18) and pOGH (rANG N-53/+18) in high-glucose (i.e. 25 mM) medium. Site-directed mutagenesis of nucleotides N-874 to N-867 (5' CCC GCC CT 3') in the 5'-flanking region of the rat ANG gene abolished the response to insulin. Insulin also inhibited the expression of the fusion gene containing the DNA fragment ANG N-882 to N-855 inserted upstream of the ANG gene promoter (N-53/+18), but had no effect on a mutant of N-882 to N-855. Gel mobility shift assays revealed that the labeled putative rat ANG-IRE motif (N-878 to N-864, 5' CCT TCC CGC CCT TCA 3') was bound to the nuclear proteins of IRPTCS: This binding was displaced by unlabeled ANG-IRE and IRE of human glyceraldehyde phosphate dehydrogenase but not by mutants of ANG-IRE and IRE of the rat glucagon gene. Southwestern blotting analysis revealed that the labeled putative ANG-IRE motif bound to a major nuclear protein with an apparent molecular mass of 48 kDA: Finally, high glucose levels enhanced 48-kDa nuclear protein expression and induced an additional 70-kDa nuclear protein expression in IRPTCs, as revealed by Southwestern blotting. Insulin inhibited both 48- and 70-kDa nuclear proteins expression induced by high glucose levels. Its inhibitory effect was reversed by the presence of PD98059 (an inhibitor of mitogen-activated protein kinase, MAPK) but not by wortmannin (an inhibitor of phosphatidylinositol 3- kinase). These studies demonstrate that insulin action on ANG gene expression is at the transcriptional level. The molecular mechanism (s) of insulin action is mediated, at least in part, via interaction of the functional IRE with unidentified 48- and 70- kDa nuclear proteins in the rat ANG gene and is MAPK dependent.

Blood pressure in diabetic nephropathy--current controversies

Hypertension has been recognized as an early and constant feature of diabetic nephropathy, but recent studies also suggest that a genetic predisposition to hypertension is an important risk factor for diabetic nephropathy. Antihypertensive treatment attenuates progression in diabetic nephropathy, but there is increasing evidence that very early treatment and very low target blood pressures should be implemented. There is also evidence for local activation of the renin system in the kidney as a result of hyperglycaemia. Apart from blood pressure, proteinuria should be monitored and dosing of ACE inhibitors should be guided, also by reduction of protein excretion.

Early streptozotocin-diabetes mellitus downregulates rat kidney AT2 receptors

The interaction of ANG II with intrarenal AT1 receptors has been implicated in the progression of diabetic nephropathy, but the role of intrarenal AT2 receptors is unknown. The present studies determined the effect of early diabetes on components of the glomerular renin-angiotensin system and on expression of kidney AT2 receptors. Three groups of rats were studied after 2 wk: 1) control (C), 2) streptozotocin (STZ)-induced diabetic (D), and 3) STZ-induced diabetic with insulin implant (D+I), to maintain normoglycemia. By competitive RT-PCR, early diabetes had no significant effect on glomerular mRNA expression for renin, angiotensinogen, or angiotensin-converting enzyme (ACE). In isolated glomeruli, nonglycosylated (41-kDa) AT1 receptor protein expression (AT1A and AT1B) was increased in D rats, with no change in glycosylated (53-kDa) AT1 receptor protein or in AT1 receptor mRNA. By contrast, STZ diabetes caused a significant decrease in glomerular AT2 receptor protein expression (47.0 +/- 6.5% of C; P < 0.001; n = 6), with partial reversal in D+I rats. In normal rat kidney, AT2 receptor immunostaining was localized to glomerular endothelial cells and tubular epithelial cells in the cortex, interstitial, and tubular cells in the outer medulla, and inner medullary collecting duct cells. STZ diabetes caused a significant decrease in AT2 receptor immunostaining in all kidney regions, an effect partially reversed in D+I rats. In summary, early diabetes has no effect on glomerular mRNA expression for renin, angiotensinogen, or ACE. AT2 receptors are present in glomeruli and are downregulated in early diabetes, as are all kidney AT2 receptors. Our data suggest that alterations in the balance of kidney AT1 and AT2 receptor expression may contribute to ANG II-mediated glomerular injury in progressive diabetic nephropathy.

High Levels of Glucose Stimulate Angiotensinogen Gene Expression Via the P38 Mitogen-Activated Protein Kinase Pathway in Rat Kidney Proximal Tubular Cells* *This work was supported by grants from the Medical Research Council of Canada (MT-13420 to J.S.D.C. and J.G.F., and MT-12573 to J.G.F.) and from the NIH (HL-48455 to J.R.I., and D.K-50836 to S.S.T.)

The present studies investigated whether the effect of high levels of glucose on angiotensinogen (ANG) secretion and gene expression in kidney proximal tubular cells is mediated at least in part via the activation of p38 mitogen-activated protein kinase (p38 MAPK). Rat immortalized renal proximal tubular cells (IRPTCs) were cultured in monolayer. The levels of immunoreactive rat ANG (IR-rANG) secreted into the medium and the levels of cellular ANG messenger RNA were determined by a specific RIA for rat ANG and a RT-PCR assay, respectively. Phosphorylation of cellular p38 MAPK was determined by Western blot analysis using the Phospho Plus p38 MAPK antibody kit. High levels of glucose (i.e. 25 mM) and phorbol 12-myristate 13-acetate (PMA; 10(-7) M) increased the secretion of IR-rANG and cellular ANG messenger RNA as well as phosphorylation of p38 MAPK in IRPTCs. This stimulatory effect of high levels of glucose and PMA was blocked by SB 203580 (a specific inhibitor of p38 MAPK), but not by SB 202474 (a negative control of SB 203580). High levels of D-sorbitol or 2-deoxy-D-glucose (i.e. > or = 35 mM) also stimulated the phosphorylation of p38 MAPK, but did not stimulate ANG secretion or gene expression. GF 109203X (an inhibitor of protein kinase C) blocked the stimulatory effect of high levels of glucose and PMA on ANG gene expression, whereas it did not block the effect of high levels of glucose, sorbitol, or 2-deoxy-D-glucose on p38 MAPK phosphorylation in IRPTCs. These studies demonstrate that the stimulatory effect of a high level of glucose (25 mM) on ANG gene expression in IRPTCS may be mediated at least in part via activation of p38 MAPK signal transduction pathway and is protein kinase C independent.

Angiotensin II and its receptors in the diabetic kidney

Studies using either angiotensin-converting enzyme inhibitors or type 1 (AT(1)) angiotensin II (ANG II)-receptor blockers indicate that ANG II is a mediator of progressive injury in diabetic nephropathy. However, suppression of the systemic renin-angiotensin system (RAS) generally has been shown in diabetes mellitus. Evidence suggests that intrarenal RASs within glomeruli and proximal tubules may be activated with hyperglycemia, leading to stimulation of local ANG II production, which may exert feedback inhibition of systemic renin release. Once formed, intrarenal ANG II exerts most of its well-described effects through binding to AT(1) receptors that are abundantly present in cells of the glomeruli, tubules, vasculature, and interstitium. Thus, AT(1)-receptor activation increases vascular resistance, reduces renal blood flow, and stimulates production of extracellular matrix in the mesangium and tubulointerstitium. Recent studies suggest that the adult kidney also expresses type 2 (AT(2)) ANG II receptors in glomeruli, tubular segments, and vasculature. AT(2)-receptor activation is associated with increased intrarenal nitric oxide production, stimulation of natriuresis, and inhibition of cell growth and matrix synthesis, effects that oppose those of kidney AT(1) receptors. A number of studies have shown a reduction in kidney AT(1)-receptor expression in diabetic nephropathy, suggesting that the balance between AT(1)- and AT(2)-receptor-mediated cell-signaling events may be a determinant of progression rate in diabetic nephropathy and that unopposed stimulation of AT(2) receptors by ANG II with use of AT(1)-receptor blockers may contribute to the beneficial properties of these agents. Determination of the expression pattern of AT(2) receptors in diabetes and further definition of the role of AT(2) receptors in opposing the detrimental effects of AT(1) receptors may lead to more selective targeting of the RAS in diabetic nephropathy.

Molecular mechanism(s) of insulin action on the expression of the angiotensinogen gene in kidney proximal tubular cells

To investigate the molecular mechanism(s) of insulin action on the expression of the angiotensinogen (ANG) gene in kidney proximal tubular cells, we constructed a fusion gene, pOGH (hANG N-1064/+27), containing the 5'-flanking regulatory sequence of the human ANG gene fused with the human growth hormone (hGH) gene as a reporter and stably integrated the fusion gene into the opossum kidney (OK) cell genomes. The level of expression of pOGH (hANG N-1064/+27) was quantified by the amount of immunoreactive hGH secreted into the medium. The addition of a high level of D(+)-glucose (25 mM) or phorbol 12-myristate 13-acetate (PMA, 10(-7) M) stimulated the expression of the fusion gene in OK cells. The stimulatory effect of glucose (25 mM) was blocked by insulin and tolrestat (an inhibitor of aldose reductase). Tolrestat also inhibited the increase of cellular DAG and PKC activity stimulated by 25 mM glucose. While insulin did not affect the cellular DAG and PKC activity, it did block the stimulatory effect of high glucose (25 mM) and PMA on the expression of the fusion gene. Finally, PD98059 (an inhibitor of mitogen-activated protein kinase kinase (MEK)) enhanced the stimulatory effect of high levels of glucose and blocked the inhibitory effect of insulin on the expression of the fusion gene as well as on the phosphorylation of MEK and mitogen-activated protein kinase (MAPK). In contrast, Wortmannin (an inhibitor of phosphatidylinositol-3-kinase) did not block the inhibitory effect of insulin on the ANG gene expression. These studies demonstrate that the action of insulin, blocking the stimulatory effect of a high level of D(+)-glucose (25 mM) on the ANG gene expression is mediated, at least in part, via the 5'-flanking region of the ANG gene and MAPK signal transduction pathway.

Insulin inhibits angiotensinogen gene expression via the mitogen-activated protein kinase pathway in rat kidney proximal tubular cells

The present study aimed to investigate the molecular mechanism(s) of insulin action on angiotensinogen (ANG) secretion and gene expression in kidney proximal tubular cells exposed to high levels of glucose. Immortalized rat proximal tubular cells (IRPTC) were cultured in monolayer. The levels of rat ANG and ANG messenger RNA in the IRPTC were quantified by a specific RIA for rat ANG (RIA-rANG) and by an RT-PCR assay. Insulin inhibited the stimulatory effect of a high level of glucose (25 mM) and phorbol 12-myristate 13-acetate, an activator of protein kinase C) on the secretion of ANG and the expression of the ANG messenger RNA in IRPTC. This inhibitory action of insulin on the ANG secretion and gene expression was blocked by PD98059 (an inhibitor of mitogen-activated protein kinase kinase) but not by Wortmannin (an inhibitor of phosphatidylinositol-3-kinase). PD98059 was effective in inhibiting the phosphorylation of MEK 1/2 and p44/42 MAP kinase in IRPTC stimulated by insulin. These studies demonstrate that insulin prevents the stimulatory effect of high levels of glucose on the expression of the renal ANG gene in IRPTC, at least in part, via the MAPK kinase signal transduction pathway, subsequently inhibiting the activation of the local renal renin-angiotensin system.

Impact of hyperglycemia on the renin angiotensin system in early human type 1 diabetes mellitus

It has been demonstrated previously that moderate hyperglycemia without glucosuria can increase plasma renin activity and mean arterial pressure in young healthy males with early uncomplicated type 1 diabetes mellitus. This study was conducted to extend these observations by testing the hypothesis that mild to moderate hyperglycemia can affect renal function by increasing renin angiotensin system (RAS) activity in diabetic humans. The study included 10 men and women with early, uncomplicated type 1 diabetes (duration <5 yr), all ingesting a controlled sodium and protein diet. They were studied on four separate occasions, during a subdepressor dose of the angiotensin II (AngII) receptor blocker losartan, and during graded AngII infusion, 1.5 and 2.5 ng/kg per min, while euglycemic (blood glucose 4 to 6 mmol/L) and again while hyperglycemic without glucosuria (blood glucose 9 to 11 mmol/L), according to a randomized crossover design. Outcome measures included mean arterial pressure (MAP), GFR, effective renal plasma flow (ERPF), renal vascular resistance (RVR), filtration fraction (FF), and urine sodium excretion (UNaV) at baseline and in response to the above maneuvers. During hyperglycemic conditions, MAP was significantly higher compared with euglycemia, as were RVR and FF. After the administration of losartan, a significant renal and peripheral depressor effect was noted, with decreases in MAP, RVR, and FF, whereas during euglycemia the responses to losartan were minimal. AngII infusion resulted in elevations in MAP, RVR, and FF and a decline in UNaV during both glycemic phases, but the responses during hyperglycemia, most significantly at the 1.5 ng/kg per min infusion rate, were blunted. These data support the hypothesis that hyperglycemia affects renal function by activating the RAS. The mechanism remains obscure, but these contrasting responses may provide a link between the observations that maintenance of euglycemia and blockade of the RAS prevent or delay diabetic kidney disease, and furthermore, may clarify the mechanism whereby high glucose promotes renal disease progression in diabetes.

Angiotensin IV stimulates plasminogen activator inhibitor-1 expression in proximal tubular epithelial cells

BACKGROUND

Angiotensin II (Ang II) has been shown to be implicated in the development of renal fibrosis in several forms of chronic glomerulonephritides, but the precise mechanisms of its effects remain unclear. It has recently been reported that Ang II stimulates the expression of plasminogen activator inhibitor-1 (PAI-1) in several cell lines. PAI-1 is a major physiological inhibitor of the plasminogen activator/plasmin system, a key regulator of fibrinolysis and extracellular matrix (ECM) turnover. PAI-1 induction by Ang II in endothelial cells seems to be mediated by Ang IV via a receptor that is different from Ang II type 1 and 2 receptors (AT1 and AT2).

METHODS

In this study, we sought to evaluate the effects of Ang IV on PAI-1 gene and protein expression in a well-characterized and immortalized human proximal tubular cell line (HK2) by Northern blot and enzyme-linked immunosorbent assay.

RESULTS

Ang IV stimulated PAI-1 mRNA expression, whereas it did not induce a significant increase in tritiated thymidine uptake after 24 hours of incubation. This effect was dose and time dependent. Ang IV (10 nM) induced a 7.8 +/- 3.3-fold increase in PAI-1 mRNA expression. The PAI-1 antigen level was significantly higher in conditioned media and the ECM of cells treated with Ang II and Ang IV than in control cells (both P < 0.02). Although Ang II induced a 4.2 +/- 2. 1-fold increase in PAI-1 mRNA expression, its effect underwent a dose-dependent reduction when amastatin, a potent inhibitor of the endopeptidases that catalyzes the conversion of Ang II to Ang IV, was added. In contrast, amastatin was not able to prevent the expression of PAI-1 mRNA induced by Ang IV. Finally, pretreatment of HK2 cells with losartan and N-Nicotinoyl-Tyr-N3-(Nalpha-CBZ-Arg)-Lys-His-Pro-Ile, the specific antagonists of AT1 and AT2 receptors, failed to modify PAI-1 mRNA expression as induced by Ang II.

CONCLUSIONS

Our results demonstrate that Ang II stimulates PAI-1 mRNA expression and the production of its protein in human proximal tubular cells. This is mainly-if not exclusively-due to Ang IV, which acts on a receptor that is different than AT1 or AT2. Therefore, it can be hypothesized that the induction of PAI-1 by Ang IV may be implicated in the pathogenesis of renal interstitial fibrosis in several forms of chronic glomerulonephritides.

Molecular mechanism(s) of action of isoproterenol on the expression of the angiotensinogen gene in opossum kidney proximal tubular cells

BACKGROUND

beta-adrenoceptors are present in the renal proximal tubules. We have previously reported that isoproterenol stimulates the accumulation of intracellular cAMP and the expression of the angiotensinogen (ANG) gene in opossum kidney (OK) proximal tubular cells via the beta 1-adrenoceptor. We hypothesized that the molecular mechanism(s) of action of isoproterenol on the expression of the ANG gene is mediated via the interaction of the phosphorylated cAMP-responsive element binding protein (CREB) and the cAMP-responsive element (CRE; that is, ANG N-806/-779) in the 5'-flanking region of the rat ANG gene.

METHODS

The fusion genes containing the putative ANG-CRE of the rat ANG gene inserted upstream of the rat ANG basal promoter (ANG N-53/+18) fused to a human growth hormone (hGH) gene as reporter were stably cotransfected, with or without the plasmid containing the cDNA for 43 kDa CREB, into the OK cells. The effect of various agonists and antagonists of adrenoceptors on the expression of the fusion genes was evaluated by the amount of immunoreactive hGH secreted into the culture medium. The interactions of OK cellular nuclear protein(s) with the ANG N-806/779 were determined by gel mobility shift assays and by Southwestern and Western blot analysis.

RESULTS

The addition of isoproterenol, forskolin, or 8-Bromo-cAMP (8-Br-cAMP) stimulated the expression of pOGH (ANG N-806/-779/-53/+18) by 135, 150, and 160%, respectively, but not mutants of the ANG N-806/-779. The stimulatory effect of isoproterenol was blocked in the presence of propranolol, Rp-cAMP, and atenolol, but not by the presence of stauro-sporine, U73122, and ICI 118,551. Transient transfection of the plasmid containing the cDNA for the catalytic subunit of protein kinase A further enhanced the stimulatory effect of 43 kDa CREB on the expression of the fusion gene. The gel mobility shift assays revealed the the nuclear protein(s) of OK cells binds to the radioactive-labeled ANG N-806/-779. The binding of the labeled ANG N-806/-779 to the OK cell nuclear protein(s) was displaced by unlabeled ANG N-806/-779, but not by the CRE of the somatostatin gene, the CRE of the tyrosine amino-transferase gene, or the mutants of the ANG N-806/-779. Southwestern blot analysis revealed that the labeled ANG N-806/-779 binds to two nuclear species of 43 and 35 kDa proteins. Western blot analysis, however, revealed that rabbit polyclonal antibodies against the 43 kDa CREB interacted with only the 43 kDa molecular species but not with the 35 kDa species.

CONCLUSION

These studies demonstrate that the stimulatory effect of isoproterenol on the expression of the ANG gene may be mediated, at least in part, via the interaction of the phosphorylated CREB and the CRE in the 5'-flanking region of the rat ANG gene. The novel 35 kDa nuclear protein that is immunologically different from the 43 kDa CREB may also play a role in the expression of the ANG gene.

Molecular mechanisms of glucose action on angiotensinogen gene expression in rat proximal tubular cells

BACKGROUND

Clinical studies have shown that the angiotensin-converting enzyme (ACE) inhibitors or angiotensin II (Ang II) receptor antagonists decrease proteinuria and slow the progression of nephropathy in diabetes, indicating that Ang II plays an important role in the development of nephropathy. We have previously reported that high levels of glucose stimulate the expression of rat angiotensinogen (ANG) gene in opossum kidney (OK) proximal tubular cells. We hypothesized that the stimulatory effect of D(+)-glucose on the expression of the ANG gene in kidney proximal tubular cells is mediated via de novo synthesis of diacylglycerol (DAG) and the protein kinase C (PKC) signal transduction pathway.

METHODS

Immortalized rat proximal tubular cells (IRPTCs) were cultured in monolayer. The stimulatory effect of glucose on the activation of polyol pathway and PKC signal transduction pathway in IRPTCs was determined. The immunoreactive rat ANG (IR-rANG) in the culture medium and the cellular ANG mRNA were measured with a specific radioimmunoassay and a reverse transcription-polymerase chain reaction assay, respectively.

RESULTS

D(+)-glucose (25 mM) markedly increased the intracellular levels of sorbitol, fructose, DAG, and PKC activity as well as the expression of IR-rANG and ANG mRNA in IRPTCs. These stimulatory effects of D(+)-glucose (25 mM) were blocked by an inhibitor of aldose reductase, Tolrestat. PKC inhibitors also inhibited the stimulatory effect of D(+)-glucose (25 mM) on the expression of the IR-rANG in IRPTCs. The addition of phorbol 12-myristate 13-acetate further enhanced the stimulatory effect of D(+)-glucose (25 mM) on the expression of the IR-rANG in IRPTCs and blocked the inhibitory effect of Tolrestat.

CONCLUSION

These studies suggest that the stimulatory effect of a high level of D(+)-glucose (25 mM) on the expression of the ANG gene in IRPTCs is mediated, at least in part, via the de novo synthesis of DAG, an activator of PKC signal transduction pathway.