Angiotensinogen gene expression is stimulated by the cAMP-responsive element binding protein in opossum kidney cells

A kidney-brain neural circuit drives progressive kidney damage and heart failure

Chronic kidney disease (CKD) and heart failure (HF) are highly prevalent, aggravate each other, and account for substantial mortality. However, the mechanisms underlying cardiorenal interaction and the role of kidney afferent nerves and their precise central pathway remain limited. Here, we combined virus tracing techniques with optogenetic techniques to map a polysynaptic central pathway linking kidney afferent nerves to subfornical organ (SFO) and thereby to paraventricular nucleus (PVN) and rostral ventrolateral medulla that modulates sympathetic outflow. This kidney-brain neural circuit was overactivated in mouse models of CKD or HF and subsequently enhanced the sympathetic discharge to both the kidney and the heart in each model. Interruption of the pathway by kidney deafferentation, selective deletion of angiotensin II type 1a receptor (AT1a) in SFO, or optogenetic silence of the kidney-SFO or SFO-PVN projection decreased the sympathetic discharge and lessened structural damage and dysfunction of both kidney and heart in models of CKD and HF. Thus, kidney afferent nerves activate a kidney-brain neural circuit in CKD and HF that drives the sympathetic nervous system to accelerate disease progression in both organs. These results demonstrate the crucial role of kidney afferent nerves and their central connections in engaging cardiorenal interactions under both physiological and disease conditions. This suggests novel therapies for CKD or HF targeting this kidney-brain neural circuit.

Heterogeneous nuclear ribonucleoproteins F and K mediate insulin inhibition of renal angiotensinogen gene expression and prevention of hypertension and kidney injury in diabetic mice

AIMS/HYPOTHESIS

We investigated whether heterogeneous nuclear ribonucleoproteins F and K (hnRNP F, hnRNP K) mediate insulin inhibition of renal Agt expression and prevention of hypertension and kidney injury in an Akita mouse model of type 1 diabetes.

METHODS

Adult male Akita mice (12 weeks old) were treated with insulin implants and killed at week 16. Untreated non-Akita littermates served as controls. The effects of insulin on blood glucose, systolic BP (SBP), renal proximal tubular cell (RPTC) gene expression and interstitial fibrosis were studied. We also examined immortalised rat RPTCs stably transfected with control plasmid or with plasmid containing rat Agt promoter in vitro.

RESULTS

Insulin treatment normalised blood glucose levels and SBP, inhibited renal AGT expression but enhanced hnRNP F, hnRNP K and angiotensin-converting enzyme-2 expression, attenuated renal hypertrophy and glomerular hyperfiltration and decreased urinary albumin/creatinine ratio, as well as AGT and angiotensin II levels, in Akita mice. In vitro, insulin inhibited Agt but stimulated Hnrnpf and Hnrnpk expression in high-glucose media via p44/42 mitogen-activated protein kinase signalling in RPTCs. Transfection with Hnrnpf or Hnrnpk small interfering RNAs prevented insulin inhibition of Agt expression in RPTCs.

CONCLUSIONS/INTERPRETATION

These data indicate that insulin prevents hypertension and attenuates kidney injury, at least in part, through suppressing renal Agt transcription via upregulation of hnRNP F and hnRNP K expression in diabetic Akita mice. HnRNP F and hnRNP K may be potential targets in the treatment of hypertension and kidney injury in diabetes.

CREB, NF-κB, and NADPH oxidase coordinately upregulate indoxyl sulfate-induced angiotensinogen expression in proximal tubular cells

In chronic kidney disease (CKD), indoxyl sulfate, a uremic toxin, accumulates in serum, and the expression of angiotensinogen (AGT) is upregulated in renal proximal tubular cells. The present study aimed to determine the relationship between indoxyl sulfate and the upregulation of AGT expression in proximal tubular cells. Indoxyl sulfate induced expression of AGT in rat renal cortex and in cultured human proximal tubular cells (HK-2). In proximal tubular cells, indoxyl sulfate induced phosphorylation of cAMP response element-binding protein (CREB) on Ser-133, and small interfering RNA (siRNA) specific to CREB inhibited indoxyl sulfate-induced AGT expression. Our previous study demonstrated that indoxyl sulfate activated nuclear factor-κB (NF-κB) through reactive oxygen species (ROS) production. NF-κB inhibitors (pyrrolidine dithiocarbamate and isohelenin), NF-κB p65 siRNA, an antioxidant [ N-acetylcysteine (NAC)], and a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor [diphenyleneiodonium (DPI)] suppressed indoxyl sulfate-induced AGT expression. Both NAC and DPI suppressed indoxyl sulfate-induced expression of NF-κB p65 and CREB. CREB siRNA suppressed indoxyl sulfate-induced NF-κB p65 expression, whereas both NF-κB inhibitors and NF-κB p65 siRNA prevented indoxyl sulfate-induced CREB expression. Furthermore, we focused on the expression of NADPH oxidase 4 (NOX4), because indoxyl sulfate induced NOX4 expression in vascular smooth muscle cells and vascular endothelial cells. Indoxyl sulfate induced the expression of NOX4 in proximal tubular cells, which was suppressed by NAC, DPI, NF-κB inhibitors, NF-κB p65 siRNA, and CREB siRNA. Taken together, CREB, NF-κB, and NOX4 coordinately upregulate indoxyl sulfate-induced AGT expression in proximal tubular cells.

Heterogeneous nuclear ribonucleoprotein F suppresses angiotensinogen gene expression and attenuates hypertension and kidney injury in diabetic mice

We investigated the impact of heterogeneous nuclear ribonucleoprotein F (hnRNP F) overexpression on angiotensinogen (Agt) gene expression, hypertension, and renal proximal tubular cell (RPTC) injury in high-glucose milieu both in vivo and in vitro. Diabetic Akita transgenic (Tg) mice specifically overexpressing hnRNP F in their RPTCs were created, and the effects on systemic hypertension, Agt gene expression, renal hypertrophy, and interstitial fibrosis were studied. We also examined immortalized rat RPTCs stably transfected with control plasmid or plasmid containing hnRNP F cDNA in vitro. The results showed that hnRNP F overexpression attenuated systemic hypertension, suppressed Agt and transforming growth factor-β1 (TGF-β1) gene expression, and reduced urinary Agt and angiotensin II levels, renal hypertrophy, and glomerulotubular fibrosis in Akita hnRNP F-Tg mice. In vitro, hnRNP F overexpression prevented the high-glucose stimulation of Agt and TGF-β1 mRNA expression and cellular hypertrophy in RPTCs. These data suggest that hnRNP F plays a modulatory role and can ameliorate hypertension, renal hypertrophy, and interstitial fibrosis in diabetes. The underlying mechanism is mediated, at least in part, via the suppression of intrarenal Agt gene expression in vivo. hnRNP F may be a potential target in the treatment of hypertension and kidney injury in diabetes.

cAMP-positive regulation of angiotensinogen gene expression and protein secretion in rat adipose tissue

The adipose renin-angiotensin system (RAS) has been assigned to participate in the control of adipose tissue development and in the pathogenesis of obesity-related hypertension. In adipose cells, the biological responses to beta-adrenergic stimulation are mediated by an increase in intracellular cAMP. Because cAMP is known to promote adipogenesis and because an association exists between body fat mass, hypertension, and increased sympathetic stimulation, we examined the influence of cAMP on angiotensinogen (ATG) expression and secretion in rat adipose tissue. Exposure of primary cultured differentiated preadipocytes to the cAMP analog 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) or cAMP-stimulating agents (forskolin and IBMX) results in a significant increase in ATG mRNA levels. In adipose tissue fragments, 8-BrcAMP also increases ATG mRNA levels and protein secretion, but not in the presence of the protein kinase A inhibitor H89. The addition of isoproterenol, known to stimulate the synthesis of intracellular cAMP via beta-adrenoreceptors, had the same stimulatory effect on ATG expression and secretion. These results indicate that cAMP in vitro upregulates ATG expression and secretion in rat adipose tissue via the protein kinase A-dependent pathway. Further studies are required to determine whether this regulatory pathway is activated in human obesity, where increased sympathetic tone is frequently observed, and to elucidate the importance of adipose ATG to the elevated blood pressure observed in this pathological state.

Polycystin-1 interacts with intermediate filaments

Polycystin-1, the protein defective in a majority of patients with autosomal dominant polycystic kidney disease, is a ubiquitously expressed multi-span transmembrane protein of unknown function. Subcellular localization studies found this protein to be a component of various cell junctional complexes and to be associated with the cytoskeleton, but the specificity and nature of such associations are not known. To identify proteins that interact with the polycystin-1 C-tail (P1CT), this segment was used as bait in a yeast two-hybrid screening of a kidney epithelial cell library. The intermediate filament (IF) protein vimentin was identified as a strong polycystin-1-interacting partner. Cytokeratins K8 and K18 and desmin were also found to interact with P1CT. These interactions were mediated by coiled-coil motifs in polycystin-1 and IF proteins. Vimentin, cytokeratins K8 and K18, and desmin also bound directly to P1CT in GST pull-down and in in vitro filament assembly assays. Two observations confirmed these interactions in vivo: (i) a cell membrane-anchored form of recombinant P1CT decorated the IF network and was found to associate with the cytoskeleton in detergent-solubilized cells and (ii) endogenous polycystin-1 distributed with IF at desmosomal junctions. Polycystin-1 may utilize this association for structural, storage, or signaling functions.

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.

Polycystin-1, the PKD1 gene product, is in a complex containing E-cadherin and the catenins

Autosomal dominant polycystic kidney disease (ADPKD) is a common human genetic disease characterized by cyst formation in kidney tubules and other ductular epithelia. Cells lining the cysts have abnormalities in cell proliferation and cell polarity. The majority of ADPKD cases are caused by mutations in the PKD1 gene, which codes for polycystin-1, a large integral membrane protein of unknown function that is expressed on the plasma membrane of renal tubular epithelial cells in fetal kidneys. Because signaling from cell-cell and cell-matrix adhesion complexes regulates cell proliferation and polarity, we speculated that polycystin-1 might interact with these complexes. We show here that polycystin-1 colocalized with the cell adhesion molecules E-cadherin and alpha-, beta-, and gamma-catenin. Polycystin-1 coprecipitated with these proteins and comigrated with them on sucrose density gradients, but it did not colocalize, coprecipitate, or comigrate with focal adhesion kinase, a component of the focal adhesion. We conclude that polycystin-1 is in a complex containing E-cadherin and alpha-, beta-, and gamma-catenin. These observations raise the question of whether the defects in cell proliferation and cell polarity observed in ADPKD are mediated by E-cadherin or the catenins.

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.

Identification of the major site of in vitro PKA phosphorylation in the polycystin-1 C-terminal cytosolic domain

Sequence analysis of the C-terminal cytosolic domain of human and mouse polycystin-1 has identified three RxS consensus protein kinase A (PKA) phosphorylation motifs. GST-fusion proteins containing the full-length and truncated C-terminal cytosolic domain of murine polycystin-1 were phosphorylated in vitro by the purified catalytic subunit of PKA. This identified a sequence of 25 amino acids, immediately downstream of a previously identified heterotrimeric G-protein activation sequence, as the major site of PKA phosphorylation. Phosphorylation of wild-type and alanine substituted synthetic peptides containing this motif demonstrated that alanine substitution of serine 4159 largely eliminated phosphorylation. Mutation of this residue in the fusion protein reduced phosphorylation by about 70%, whereas mutation of the other two conserved phosphorylation motifs had little effect. We conclude that serine 4159 is the major site of PKA phosphorylation in the C-terminal cytosolic domain of murine polycystin-1.

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.

The polycystic kidney disease-1 protein, polycystin-1, binds and activates heterotrimeric G-proteins in vitro

Analysis of the C-terminal cytosolic domain of human and mouse polycystin-1 has identified a number of conserved protein motifs, including a 20-amino-acid heterotrimeric G-protein activation sequence. A peptide specific for this sequence was synthesized and shown to activate purified bovine brain heterotrimeric Gi/Go in vitro. To test whether the C-terminal cytosolic domain of polycystin-1 stably binds G-proteins, GST-fusion constructs were used in pull-down and co-immunoprecipitation assays with purified bovine brain Gi/Go and rat brain lysates. This identified a 74-amino-acid minimal binding domain that includes the G-protein activation sequence. This region of polycystin-1, including the G-protein activation peptide and flanking amino acid sequences, is highly conserved in mouse, human, and puffer fish, lending further support to the functional importance of the minimal binding domain. These results suggest that polycystin-1 may function as a heterotrimeric G-protein coupled receptor.

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

BACKGROUND

Norepinephrine (NE) is the major endogenous neurotransmitter of the renal sympathetic nerves interacting with both the alpha- and beta-adrenoceptors in the renal proximal tubules. We have previously reported that isoproterenol and iodoclonidine stimulate the expression of the angiotensinogen (ANG) gene in opossum kidney (OK) proximal tubular cells via the beta1-adrenoceptor and alpha2-adrenoceptor, respectively. We hypothesized that NE may interact with the beta- and/or alpha2-adrenoceptors to stimulate the expression of the ANG gene in OK cells.

METHODS

The fusion genes containing the various lengths of the 5'-flanking regulatory sequence of the rat ANG gene fused with a human growth hormone (hGH) gene as a reporter were stably transfected into the OK cells. The stimulatory effect of NE on the expression of the fusion genes was evaluated by the amount of immunoreactive hGH (IR-hGH) secreted into the culture medium.

RESULTS

The addition of NE stimulated the expression of the fusion gene, pOGH (ANG N-1498/+18) in a dose-dependent manner. The stimulatory effect of NE was inhibited in the presence of propranolol, atenolol, Rp-cAMP, yohimbine, staurosporine, H-7 and U73122 but not in the presence of ICI 118,551 and prazosin. The addition of a combination of isoproterenol and iodoclonidine synergistically stimulated the expression of pOGH (ANG N-1498/+18) as compared to the addition of isoproterenol and iodoclonidine alone. Furthermore, the addition of NE, forskolin, 8-Br-cAMP or phorbol 12-myristate (PMA) stimulated the expression of pOGH (ANG N-806/-779/-53/+18), a fusion gene containing the putative cAMP responsive element (CRE, ANG N-806/-779) upstream of the ANG promoter (ANG N-53/+ 18) in OK 95 cells, but had no effect on the expression of fusion genes containing the mutant of the CRE.

CONCLUSION

These studies demonstrate that the stimulatory effect of NE on the expression of the ANG gene in OK cells may be mediated via both the beta1- and alpha2-adrenoceptors and via the CRE (ANG N-806/-779) in the 5'flanking region of rat ANG gene.

The polycystic kidney disease 1 gene product mediates protein kinase C alpha-dependent and c-Jun N-terminal kinase-dependent activation of the transcription factor AP-1

Autosomal dominant polycystic kidney disease (ADPKD) is a common hereditary disorder that accounts for 8-10% of end stage renal disease. PKD1, one of two recently isolated ADPKD gene products, has been implicated in cell-cell and cell-matrix interactions. However, the signaling pathway of PKD1 remains undefined. We found that the C-terminal 226 amino acids of PKD1 transactivate an AP-1 promoter construct in human embryonic kidney cells (293T). PKD1-induced transcription is specific for AP-1; promoter constructs containing cAMP response element-binding protein, c-Fos, c-Myc, or NFkappaB-binding sites are unaffected by PKD1. In vitro kinase assays revealed that PKD1 triggers the activation of c-Jun N-terminal kinase (JNK), but not of mitogen-activated protein kinases p38 or p44. Dominant-negative Rac-1 and Cdc42 mutations abrogated PKD1-mediated JNK and AP-1 activation, suggesting a critical role for small GTP-binding proteins in PKD1-mediated signaling. Several protein kinase C (PKC) inhibitors decreased PKD1-mediated AP-1 activation. Conversely, expression of the C-terminal domain of PKD1 increased PKC activity in 293T cells. A dominant-negative PKC alpha, but not a dominant-negative PKC beta or delta, abrogated PKD1-mediated AP-1 activation. These findings indicate that small GTP-binding proteins and PKC alpha mediate PKD1-induced JNK/AP-1 activation, together comprising a signaling cascade that may regulate renal tubulogenesis.

New insights into polycystic kidney disease and its treatment

Major advances in the understanding of the genetics and pathogenesis of autosomal dominant polycystic kidney disease have occurred within the past year. The proteins encoded by the PKD1 and PKD2 genes, polycystin 1 and polycystin 2, are membrane proteins, capable of interacting physically in vitro, and are likely components of a complex signalling pathway. The majority of PKD1 and PKD2 mutations so far identified are unique inactivating mutations dispersed over the entire genes. Immunohistochemical studies have shown that polycystin 1 and polycystin 2 are developmentally regulated and are overexpressed in polycystic kidneys. The cysts probably result from clonal expansions of single cells. The demonstration of loss of heterozygosity for PKD1 and the absence of immunoreactive polycystin 1 in approximately 20% of the cysts supports a two-hit tumor suppressor gene model of cystogenesis. Regardless of the nature of the initial pathogenic mechanism, the cysts in autosomal dominant polycystic kidney disease are accompanied by partial dedifferentiation of the epithelial cells, disregulation of epithelial cell proliferation, expression of a secretory phenotype, and disarray of cell matrix interactions which leads to interstitial inflammation and matrix accumulation. Recent observations in animal models of inherited polycystic kidney disease have implicated oxidative stress in its pathogenesis. These downstream pathogenetic events have been targeted for intervention, and an increasing number of studies have demonstrated that the course of polycystic kidney disease in rodents can be altered by environmental and pharmacological interventions. Nevertheless, these experimental observations cannot be extrapolated to human autosomal dominant polycystic kidney disease. The recent generation of mice with PKD1 or PKD2 targeted mutations will help to bridge this gap.

Identification of a novel mouse hepatic 52 kDa protein that interacts with the cAMP response element of the rat angiotensinogen gene

To identify the nuclear protein(s) that interact with the putative cAMP response element (CRE) of the rat angiotensinogen (ANG) gene (i.e. nt 806-779 upstream of the transcriptional start site), mouse liver nuclear proteins were prepared for the present studies. The DNase 1 footprinting protection analysis revealed that nt -799/-788 in the 5'-flanking region of the rat ANG gene are protected by the mouse liver nuclear protein. Gel mobility-shift assays revealed that the addition of the unlabelled DNA fragment, ANG nt -806/-779 competed effectively with the binding of the labelled ANG nt -806/-779 to the mouse liver nuclear proteins but the addition of unlabelled mutants of ANG nt -806/-779 were only weakly effective in competing with the labelled ANG nt -806/-779. The addition of unlabelled CRE of the somatostatin (SOM) gene and the CRE of the tyrosine aminotransferase (TAT) gene was also ineffective in competing with the labelled ANG nt -806/-779. Southwestern blot analysis revealed that the labelled ANG nt -806/-779 interacted with two mouse liver nuclear proteins with apparent molecular masses of 52 and 43 kDa, whereas the labelled SOM-CRE, TAT-CRE and the CRE of the phosphoenolpyruvate carboxykinase (PEPCK) gene interacted with one molecular species of 43 kDa. The binding of the labelled ANG nt -806/-779 to the 52 kDa protein was effectively competed for by the addition of unlabelled ANG nt -806/-779 but not by unlabelled SOM-CRE, TAT-CRE and PEPCK-CRE. Finally, Western blot analysis revealed that polyclonal antibodies against the CRE-binding protein (CREB) interacted with the mouse liver nuclear 43 kDa protein but not with the 52 kDa protein. These studies demonstrate that the CRE of the rat ANG gene (ANG nt -806/-779) interacts with the 43 kDa CREB and a novel 52 kDa protein from mouse liver. The novel 52 kDa protein is immunologically distinct from the 43 kDa CREB. These studies suggest that the 52 kDa protein might have a role in the expression of the hepatic ANG gene.

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

To investigate whether D(+)-glucose has a stimulatory effect on the expression of the angiotensinogen (Ang) gene in opossum kidney (OK) cells, we used OK cells with a fusion gene containing various lengths of the 5'-flanking regulatory sequence of the rat Ang gene fused with the human growth hormone (hGH) gene as a reporter, stably integrated into their genomes. The level of expression of the fusion gene was quantified by the amount of immunoreactive-human growth hormone (IR-hGH) secreted into the medium. The addition of D(+)-glucose stimulated the expression of pOGH (Ang N-1498/+18) in OK 27 cells in a dose-dependent manner (5 to 25 mM), whereas the addition of D-mannitol, L-glucose and 2-deoxy-D-glucose (25 mM) had no effect. The stimulatory effect of D(+)-glucose (25 mM) was blocked by the presence of staurosporine or H7 (an inhibitor of protein kinase C) or U73122 (an inhibitor of phospholipase C and A2) but not blocked by the presence of Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A). The addition of D(+)-glucose (25 mM) also stimulated the expression of pOGH (Ang N-960/+18) and pOGH (Ang N-688/+18) in OK 960 and OK 688 cells, respectively. It had no stimulatory effect, however, on the expression of pOGH (Ang N-280/+18) and pOGH (Ang N-35/+18) in OK 280 and OK 35 cells, respectively. The addition of D(+)-glucose also had no effect on the expression of pTKGH in OK 13 cells, an OK cell line, into which had been stably integrated a fusion gene, pTKGH containing the promoter/enhancer DNA sequence of the viral thymidine-kinase (TK) gene fused with a human growth hormone gene as a reporter. These studies demonstrate that the stimulatory effect of high D(+)-glucose concentration (25 mM) on the expression of the angiotensinogen-growth hormone fusion genes in OK cells is mediated via the 5'-flanking region of the angiotensinogen gene and the protein kinase C signal transduction pathway. Our data indicate that a high glucose concentration may activate the renin-angiotensin system in the renal proximal tubular cells.