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

Comparison of the effects of insulin and SGLT2 inhibitor on the Renal Renin-Angiotensin system in type 1 diabetes mice

AIMS

SGLT2 inhibitors have been proposed as an adjunct to insulin therapy for glycemic control in type 1 diabetes (T1D) patients. However, concern has been raised due to an increase in renin-angiotensin-system (RAS) activity reported in a clinical trial in which an SGLT2 inhibitor was added while insulin dose was reduced in T1D patients. We previously reported that insulin inhibits intrarenal angiotensinogen (Agt) gene transcription and RAS activation. We hypothesized that insulin, rather than SGLT2 inhibition might regulate the intrarenal RAS.

METHODS

We compared RAS activity in non-diabetic wild type mice, Akita mice (T1D model) and Akita mice treated with insulin or the SGLT2 inhibitor canagliflozin.

RESULTS

Treatment of Akita mice with insulin or canagliflozin produced similar reductions in blood glucose, whereas insulin, but not canagliflozin, reduced elevated systolic blood pressure. Akita mice exhibited increased renal Agt mRNA/protein expression, which was attenuated by insulin, but not by canagliflozin. Furthermore, insulin was more effective than canagliflozin in lowering kidney weight and albuminuria.

CONCLUSIONS

Insulin, but not canagliflozin, lowers intrarenal RAS activity in Akita mice. Our findings can be of potential clinical importance, especially for T1D patients who are not on RAS inhibitors at the time of adding SGLT2 inhibitors.

Insulin Inhibits Nrf2 Gene Expression via Heterogeneous Nuclear Ribonucleoprotein F/K in Diabetic Mice

Oxidative stress induces endogenous antioxidants via nuclear factor erythroid 2-related factor 2 (Nrf2), potentially preventing tissue injury. We investigated whether insulin affects renal Nrf2 expression in type 1 diabetes (T1D) and studied its underlying mechanism. Insulin normalized hyperglycemia, hypertension, oxidative stress, and renal injury; inhibited renal Nrf2 and angiotensinogen (Agt) gene expression; and upregulated heterogeneous nuclear ribonucleoprotein F and K (hnRNP F and hnRNP K) expression in Akita mice with T1D. In immortalized rat renal proximal tubular cells, insulin suppressed Nrf2 and Agt but stimulated hnRNP F and hnRNP K gene transcription in high glucose via p44/42 mitogen-activated protein kinase signaling. Transfection with small interfering RNAs of p44/42 MAPK, hnRNP F, or hnRNP K blocked insulin inhibition of Nrf2 gene transcription. Insulin curbed Nrf2 promoter activity via a specific DNA-responsive element that binds hnRNP F/K, and hnRNP F/K overexpression curtailed Nrf2 promoter activity. In hyperinsulinemic-euglycemic mice, renal Nrf2 and Agt expression was downregulated, whereas hnRNP F/K expression was upregulated. Thus, the beneficial actions of insulin in diabetic nephropathy appear to be mediated, in part, by suppressing renal Nrf2 and Agt gene transcription and preventing Nrf2 stimulation of Agt expression via hnRNP F/K. These findings identify hnRNP F/K and Nrf2 as potential therapeutic targets in diabetes.

Novel mechanism of blood pressure regulation by forkhead box class O1-mediated transcriptional control of hepatic angiotensinogen

The renin-angiotensin system is a major determinant of blood pressure regulation. It consists of a cascade of enzymatic reactions involving 3 components: angiotensinogen, renin, and angiotensin-converting enzyme, which generate angiotensin II as a biologically active product. Angiotensinogen is largely produced in the liver, acting as a major determinant of the circulating renin-angiotensin system, which exerts acute hemodynamic effects on blood pressure regulation. How the expression of angiotensinogen is regulated is not completely understood. Here, we hypothesize that angiotensinogen is regulated by forkhead transcription factor forkhead box class O1 (Foxo1), an insulin-suppressed transcription factor, and thereby controls blood pressure in mice. We generated liver-specific Foxo1 knockout mice, which exhibited a reduction in plasma angiotensinogen and angiotensin II levels and a significant decrease in blood pressure. Using hepatocyte cultures, we demonstrated that overexpression of Foxo1 increased angiotensinogen expression, whereas hepatocytes lacking Foxo1 demonstrated a reduction of angiotensinogen gene expression and partially impaired insulin inhibition on angiotensinogen gene expression. Furthermore, mouse angiotensinogen prompter analysis demonstrated that the angiotensinogen promoter region contains a functional Foxo1-binding site, which is responsible for both Foxo1 stimulation and insulin suppression on the promoter activity. Together, these data demonstrate that Foxo1 regulates hepatic angiotensinogen gene expression and controls plasma angiotensinogen and angiotensin II levels, modulating blood pressure control in mice.

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.

The renin-angiotensin system: a target of and contributor to dyslipidemias, altered glucose homeostasis, and hypertension of the metabolic syndrome

The renin-angiotensin system (RAS) is an important therapeutic target in the treatment of hypertension. Obesity has emerged as a primary contributor to essential hypertension in the United States and clusters with other metabolic disorders (hyperglycemia, hypertension, high triglycerides, low HDL cholesterol) defined within the metabolic syndrome. In addition to hypertension, RAS blockade may also serve as an effective treatment strategy to control impaired glucose and insulin tolerance and dyslipidemias in patients with the metabolic syndrome. Hyperglycemia, insulin resistance, and/or specific cholesterol metabolites have been demonstrated to activate components required for the synthesis [angiotensinogen, renin, angiotensin-converting enzyme (ACE)], degradation (ACE2), or responsiveness (angiotensin II type 1 receptors, Mas receptors) to angiotensin peptides in cell types (e.g., pancreatic islet cells, adipocytes, macrophages) that mediate specific disorders of the metabolic syndrome. An activated local RAS in these cell types may contribute to dysregulated function by promoting oxidative stress, apoptosis, and inflammation. This review will discuss data demonstrating the regulation of components of the RAS by cholesterol and its metabolites, glucose, and/or insulin in cell types implicated in disorders of the metabolic syndrome. In addition, we discuss data supporting a role for an activated local RAS in dyslipidemias and glucose intolerance/insulin resistance and the development of hypertension in the metabolic syndrome. Identification of an activated RAS as a common thread contributing to several disorders of the metabolic syndrome makes the use of angiotensin receptor blockers and ACE inhibitors an intriguing and novel option for multisymptom treatment.

Heterogeneous Nuclear Ribonucleoprotein K Modulates Angiotensinogen Gene Expression in Kidney Cells

The present studies aimed to identify the 70-kDa nuclear protein that binds to an insulin-responsive element in the rat angiotensinogen gene promoter and to define its action on angiotensinogen gene expression. Nuclear proteins were isolated from rat kidney proximal tubular cells and subjected to two-dimensional electrophoresis. The 70-kDa nuclear protein was detected by Southwestern blotting and subsequently identified by mass spectrometry, which revealed that it was identical to 65-kDa heterogeneous nuclear ribonucleoprotein K (hnRNP K). hnRNP K bound to the insulin-responsive element of the rat angiotensinogen gene was revealed by a gel mobility shift assay and chromatin immunoprecipitation assay. hnRNP K inhibited angiotensinogen mRNA expression and promoter activity. In contrast, hnRNP K down-expression by small interference RNA enhanced angiotensinogen mRNA expression. Moreover, hnRNP K interacted with hnRNP F in pulldown and co-immunoprecipitation assays. Co-transfection of hnRNP K and hnRNP F further suppressed angiotensinogen mRNA expression. Finally, in vitro and in vivo studies demonstrated that high glucose increases and insulin inhibits hnRNP K expression in rat kidney proximal tubular cells. In conclusion, our experiments revealed that hnRNP K is a nuclear protein that binds to the insulin-responsive element of the rat angiotensinogen gene promoter and modulates angiotensinogen gene transcription in the kidney.

Insulin stimulates endogenous angiotensin II production via a mitogen-activated protein kinase pathway in vascular smooth muscle cells

OBJECTIVE

The present study was designed to determine the effects of insulin on cytosolic angiotensin II production and proliferation in cultured rat vascular smooth muscle cells.

DESIGN AND METHODS

Vascular smooth muscle cells were incubated with insulin for 48 h. Cytosolic angiotensin I and II were determined by radioimmunoassays of purified cell homogenates. Angiotensin II was also detected by immunohistochemistry of intact cells. Cell proliferation was determined by pulse labeling with radiolabeled thymidine. Angiotensinogen mRNA expression was determined by slot-blot analysis.

RESULTS

Insulin significantly increased cytosolic angiotensin II concentration in vascular smooth muscle cells. Lisinopril, omapatrilat and irbesartan inhibited this increase of angiotensin II, but had no effect on angiotensin I levels. Immunohistochemical staining confirmed the presence of angiotensin II in control and insulin-treated vascular smooth muscle cells. Insulin increased cell proliferation, and addition of lisinopril, omapatrilat or irbesartan inhibited this effect. Insulin also increased expression of angiotensinogen mRNA in cultured vascular smooth muscle cells, but PD98059, a mitogen-activated protein kinase inhibitor, prevented the rise in angiotensinogen expression.

CONCLUSION

These results support the concept that insulin stimulates angiotensin II production in cultured vascular smooth muscle cells through a mitogen-activated, protein kinase-dependent pathway that might be a factor in the progression of atherosclerosis. Agents that block the renin-angiotensin system have direct protective effects, reducing vascular angiotensin II and growth of vascular smooth muscle cells and are thus of cardiovascular benefit.

High glucose stimulates angiotensinogen gene expression and cell hypertrophy via activation of the hexosamine biosynthesis pathway in rat kidney proximal tubular cells

The present study investigated whether activation of the hexosamine biosynthesis pathway might mediate at least in part the high glucose effect on angiotensinogen (ANG) gene expression and immortalized renal proximal tubular cell (IRPTC) hypertrophy. IRPTC were cultured in monolayer. ANG, renin, and beta-actin mRNA expression were determined by specific RT-PCR assays. Phosphorylation of p38 MAPK, activating transcription factor-2 (ATF-2), and cAMP-responsive element-binding protein (CREB) was determined by Western blot analysis. Cell hypertrophy was assessed by flow cytometry, intracellular p27kip1 protein levels, and [3H]leucine incorporation into proteins. Glucosamine stimulated ANG and renin mRNA expression and enhanced p38 MAPK, ATF-2, and CREB phosphorylation in normal glucose (5 mm) medium. Azaserine and 6-diazo-5-oxo-l-norleucine (inhibitors of glutamine: fructose-6-phosphate amino transferase enzyme) blocked the stimulatory effect of high glucose, but not that of glucosamine, on ANG gene expression in IRPTCs. SB 203580 (a specific p38 MAPK inhibitor) attenuated glucosamine action on ANG gene expression as well as p38 MAPK and ATF-2 phosphorylation, but not that of CREB. GF 109203X and calphostin C (inhibitors of protein kinase C) blocked the effect of glucosamine on ANG gene expression and CREB phosphorylation, but had no impact on p38 MAPK and ATF-2 phosphorylation. Finally, both glucosamine and high glucose induced IRPTC hypertrophy. The hypertrophic effect of glucosamine was blocked in the presence of GF 109203X, but not azaserine and SB 203580. In contrast, the hypertrophic effect of high glucose was blocked in the presence of azaserine and GF 109203X, but not SB203580. Our studies demonstrate that the stimulatory effect of high glucose on ANG gene expression and IRPTC hypertrophy may be mediated at least in part via activation of hexosamine biosynthesis pathway signaling.

Dopamine-mediated inhibition of renal Na,K-ATPase is reduced by insulin

Recently we have reported that rosiglitazone treatment of obese Zucker rats reduced plasma insulin and restored the ability of dopamine to inhibit Na,K-ATPase (NKA) in renal proximal tubules. The present study was performed to test the hypothesis that a chronic increase in levels of insulin causes a decrease in expression of the D1 receptor and its uncoupling from G proteins, which may account for the diminished inhibitory effect of dopamine on NKA in obese Zucker rats. We conducted experiments in primary proximal tubule epithelial cells obtained from Sprague-Dawley rat kidneys. These cells at 80% to 90% confluence were pretreated with insulin (100 nmol/L for 24 hours) in growth factor-/serum-free medium. SKF-38393, a D1 receptor agonist, inhibited NKA activity in untreated cells, but the agonist failed to inhibit enzyme activity in insulin-pretreated cells. Basal NKA activity was similar in untreated and insulin-pretreated cells. Measurement of D1 receptors in the plasma membranes revealed that [3H]SCH-23390 binding, a D1 receptor ligand, as well as D1 receptor protein abundance, was significantly reduced in insulin-pretreated cells compared with untreated cells. SKF-38393 (10 micromol/L) elicited significant stimulation of [35S]GTPgammaS binding in the membranes from control cells, suggesting that the D1 receptor-G protein coupling was intact. However, the stimulatory effect of SKF-38393 was absent in membranes from insulin-pretreated cells. We suggest that chronic exposure of cells to insulin causes both the reduction in D1 receptor abundance and its uncoupling from G proteins. These phenomena might account for the diminished inhibitory effect of dopamine on NKA activity in hyperinsulinemic rats.

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.

Protective effect of insulin on ischemic renal injury in diabetes mellitus

BACKGROUND

An exceptional susceptibility to unilateral renal ischemia/reperfusion (I/R) injury resulting in inflammation, fibrosis, atrophy of the kidney, and end-stage renal disease (ESRD) has been demonstrated in the diabetic rat. The aim of this study was to examine whether insulin treatment would reduce I/R injury in diabetic kidneys.

METHODS

Diabetes mellitus (DM) was induced in male Wistar rats by streptozotocin. I/R was achieved by clamping the left renal artery for 30 minutes. Treatment with long acting insulin was started 7 to 14 days before or one day after I/R. Short acting insulin was administrated 2 to 6 hours before the injury. Apoptosis was evaluated six hours after ischemia with the TUNEL-method. Four weeks after the clamping inulin clearance was measured and kidneys were removed for histopathological evaluation.

RESULTS

In DM animals renal I/R caused massive induction of apoptosis in the renal medulla after six hours as well as inflammation, fibrosis, renal atrophy and anuria within four weeks. Treatment with long acting insulin before I/R resulted in decreased cell death and an almost complete protection of both renal function and histomorphology. Treatment with short acting insulin before I/R also decreased the loss of renal function. In contrast, insulin treatment after I/R did not protect the kidney from damage.

CONCLUSIONS

This study shows that insulin treatment with a subsequent improved metabolic control before renal I/R protected kidneys from ESRD.

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.

Early diabetes mellitus stimulates proximal tubule renin mRNA expression in the rat

BACKGROUND

Enhanced intrarenal angiotensin II (Ang II) activity may contribute to diabetic nephropathy. The proximal tubule is a proposed site of significant intrarenal Ang II production. We determined the effect of early diabetes on mRNA expression of components of the proximal tubule renin-angiotensin system.

METHODS

Three groups of male Sprague-Dawley rats were studied after two weeks: (1) control (C), (2) streptozotocin-induced diabetes (STZ), and (3) STZ-induced diabetes, with normoglycemia maintained by insulin implants (STZ-I). Competitive reverse transcription-polymerase chain reaction was used to assay mRNA for renin, angiotensinogen, and angiotensin-converting enzyme in suspensions of proximal tubules; plasma and kidney levels of Ang II were measured by radioimmunoassay, and Western analysis of Ang II subtype 1 (AT1) receptors was performed.

RESULTS

STZ rats tended to have increased plasma and intrarenal levels of Ang II compared with C and STZ-I rats. In proximal tubules, mRNA for renin was significantly increased in STZ rats, with reversal to control values in STZ-I rats (C, 2432 +/- 437 vs. STZ, 5688 +/- 890 fg/0.25 microg RNA, P < 0.05 vs. C, N = 9, vs. STZ-I, 1676 +/- 376 fg/0.25 microg RNA, P = NS vs. C). In STZ rats, the AT1 receptor antagonist losartan caused a further fivefold increase in proximal tubule renin mRNA, associated with proximal tubular renin immunostaining. STZ had no significant effect on mRNA expression for angiotensinogen or angiotensin-converting enzyme in proximal tubules. By Western blot analysis, cortical and proximal tubule AT1 receptor protein expression was significantly decreased in STZ rats.

CONCLUSIONS

These data suggest activation of the proximal tubule renin-angiotensin system in early STZ diabetes, mediated at least partly by enhanced expression of renin mRNA. Increased local production of Ang II could contribute to tubulointerstitial injury in this model.

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.

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.