Improvement of endothelial nitric oxide synthase activity retards the progression of diabetic nephropathy in db/db mice

Biopterin metabolism and eNOS expression during hypoxic pulmonary hypertension in mice

Tetrahydrobiopterin (BH4), which fosters the formation of and stabilizes endothelial NO synthase (eNOS) as an active dimer, tightly regulates eNOS coupling / uncoupling. Moreover, studies conducted in genetically-modified models demonstrate that BH4 pulmonary deficiency is a key determinant in the pathogenesis of pulmonary hypertension. The present study thus investigates biopterin metabolism and eNOS expression, as well as the effect of sepiapterin (a precursor of BH4) and eNOS gene deletion, in a mice model of hypoxic pulmonary hypertension. In lungs, chronic hypoxia increased BH4 levels and eNOS expression, without modifying dihydrobiopterin (BH2, the oxidation product of BH4) levels, GTP cyclohydrolase-1 or dihydrofolate reductase expression (two key enzymes regulating BH4 availability). In intrapulmonary arteries, chronic hypoxia also increased expression of eNOS, but did not induce destabilisation of eNOS dimers into monomers. In hypoxic mice, sepiapterin prevented increase in right ventricular systolic pressure and right ventricular hypertrophy, whereas it modified neither remodelling nor alteration in vasomotor responses (hyper-responsiveness to phenylephrine, decrease in endothelium-dependent relaxation to acetylcholine) in intrapulmonary arteries. Finally, deletion of eNOS gene partially prevented hypoxia-induced increase in right ventricular systolic pressure, right ventricular hypertrophy and remodelling of intrapulmonary arteries. Collectively, these data demonstrate the absence of BH4/BH2 changes and eNOS dimer destabilisation, which may induce eNOS uncoupling during hypoxia-induced pulmonary hypertension. Thus, even though eNOS gene deletion and sepiapterin treatment exert protective effects on hypoxia-induced pulmonary vascular remodelling, increase on right ventricular pressure and / or right ventricular hypertrophy, these effects appear unrelated to biopterin-dependent eNOS uncoupling within pulmonary vasculature of hypoxic wild-type mice.

Methylglyoxal modulates endothelial nitric oxide synthase-associated functions in EA.hy926 endothelial cells

BACKGROUND

Increased levels of the sugar metabolite methylglyoxal (MG) in vivo were shown to participate in the pathophysiology of vascular complications in diabetes. Alterations of endothelial nitric oxide synthase (eNOS) activity by hypophosphorylation of the enzyme and enhanced monomerization are found in the diabetic milieu, and the regulation of this still remains undefined. Using various pharmacological approaches, we elucidate putative mechanisms by which MG modulates eNOS-associated functions of MG-stimulated superoxide O₂•⁻ production, phosphorylation status and eNOS uncoupling in EA.hy926 human endothelial cells.

METHODS

In cultured EA.hy926 endothelial cells, the effects of MG treatment, tetrahydrobiopterin (BH4; 100 μM) and sepiapterin (20 μM) supplementation, NOS inhibition by N(G)-nitro-L-arginine methyl ester (L-NAME; 50 μM), and inhibition of peroxynitrite (ONOO⁻) formation (300 μM Tempol plus 50 μM L-NAME) on eNOS dimer/monomer ratios, Ser-1177 eNOS phosphorylation and 3-nitrotyrosine (3NT) abundance were quantified using immunoblotting. O₂•⁻-dependent fluorescence was determined using a commercially available kit and tissue biopterin levels were measured by fluorometric HPLC analysis.

RESULTS

In EA.hy926 cells, MG treatment significantly enhanced O₂•⁻ generation and 3NT expression and reduced Ser-1177 eNOS phosphorylation, eNOS dimer/monomer ratio and cellular biopterin levels indicative of eNOS uncoupling. These effects were significantly mitigated by administration of BH4, sepiapterin and suppression of ONOO⁻ formation. L-NAME treatment significantly blunted eNOS-derived O₂•⁻ generation but did not modify eNOS phosphorylation or monomerization.

CONCLUSION

MG triggers eNOS uncoupling and hypophosphorylation in EA.hy926 endothelial cells associated with O₂•⁻ generation and biopterin depletion. The observed effects of the glycolysis metabolite MG presumably account, at least in part, for endothelial dysfunction in diabetes.

Targeted glomerular angiopoietin-1 therapy for early diabetic kidney disease

Vascular growth factors play an important role in maintaining the structure and integrity of the glomerular filtration barrier. In healthy adult glomeruli, the proendothelial survival factors vascular endothelial growth factor-A (VEGF-A) and angiopoietin-1 are constitutively expressed in glomerular podocyte epithelia. We demonstrate that this milieu of vascular growth factors is altered in streptozotocin-induced type 1 diabetic mice, with decreased angiopoietin-1 levels, VEGF-A upregulation, decreased soluble VEGF receptor-1 (VEGFR1), and increased VEGFR2 phosphorylation. This was accompanied by marked albuminuria, nephromegaly, hyperfiltration, glomerular ultrastructural alterations, and aberrant angiogenesis. We subsequently hypothesized that restoration of angiopoietin-1 expression within glomeruli might ameliorate manifestations of early diabetic glomerulopathy. Podocyte-specific inducible repletion of angiopoietin-1 in diabetic mice caused a 70% reduction of albuminuria and prevented diabetes-induced glomerular endothelial cell proliferation; hyperfiltration and renal morphology were unchanged. Furthermore, angiopoietin-1 repletion in diabetic mice increased Tie-2 phosphorylation, elevated soluble VEGFR1, and was paralleled by a decrease in VEGFR2 phosphorylation and increased endothelial nitric oxide synthase Ser(1177) phosphorylation. Diabetes-induced nephrin phosphorylation was also reduced in mice with angiopoietin-1 repletion. In conclusion, targeted angiopoietin-1 therapy shows promise as a renoprotective tool in the early stages of diabetic kidney disease.

Sestrin 2 and AMPK connect hyperglycemia to Nox4-dependent endothelial nitric oxide synthase uncoupling and matrix protein expression

Mesangial matrix accumulation is an early feature of glomerular pathology in diabetes. Oxidative stress plays a critical role in hyperglycemia-induced glomerular injury. Here, we demonstrate that, in glomerular mesangial cells (MCs), endothelial nitric oxide synthase (eNOS) is uncoupled upon exposure to high glucose (HG), with enhanced generation of reactive oxygen species (ROS) and decreased production of nitric oxide. Peroxynitrite mediates the effects of HG on eNOS dysfunction. HG upregulates Nox4 protein, and inhibition of Nox4 abrogates the increase in ROS and peroxynitrite generation, as well as the eNOS uncoupling triggered by HG, demonstrating that Nox4 functions upstream from eNOS. Importantly, this pathway contributes to HG-induced MC fibronectin accumulation. Nox4-mediated eNOS dysfunction was confirmed in glomeruli of a rat model of type 1 diabetes. Sestrin 2-dependent AMP-activated protein kinase (AMPK) activation attenuates HG-induced MC fibronectin synthesis through blockade of Nox4-dependent ROS and peroxynitrite generation, with subsequent eNOS uncoupling. We also find that HG negatively regulates sestrin 2 and AMPK, thereby promoting Nox4-mediated eNOS dysfunction and increased fibronectin. These data identify a protective function for sestrin 2/AMPK and potential targets for intervention to prevent fibrotic injury in diabetes.

New targets for treatment of diabetic nephropathy: what we have learned from animal models

PURPOSE OF REVIEW

There has been an advance in our understanding of the mechanisms of diabetic nephropathy over the past few years and much of that has occurred because of studies in animal models of diabetic nephropathy.

RECENT FINDINGS

Studies in animal models of diabetic nephropathy, especially in mice, have underlined the multifactorial nature of the pathogenesis of the disease process and the recognition that these models only partly replicate the changes found in human disease. Despite these limitations, recent animal model studies have identified a number of new, specific molecular abnormalities that point to pathways and specific molecules as potential targets for preventive or therapeutic intervention. These specific targets include the diabetic nephropathy related decreases in endothelial nitric oxide synthase activity and renal dopamine production and the increases in Nrf-2, JAK/STAT, and mammalian target of rapamycin complex 1 signaling. These and other altered signaling pathways are described in this review. We emphasize the use of a unique investigative resource, Nephromine, to utilize a library of mRNA expression data obtained from the kidney biopsies of humans with diabetic nephropathy, to compare and validate findings in mouse models with human disease.

SUMMARY

Several new pathways have been implicated in the progression of diabetic nephropathy through studies of animal models. Some of these appear to be altered in human diabetic nephropathy and may be targets for therapy.

Decreased levels of proapoptotic factors and increased key regulators of mitochondrial biogenesis constitute new potential beneficial features of long-lived growth hormone receptor gene-disrupted mice

Decreased somatotrophic signaling is among the most important mechanisms associated with extended longevity. Mice homozygous for the targeted disruption of the growth hormone (GH) receptor gene (GH receptor knockout; GHRKO) are obese and dwarf, are characterized by a reduced weight and body size, undetectable levels of GH receptor, high concentration of serum GH, and greatly reduced plasma levels of insulin and insulin-like growth factor-I, and are remarkably long lived. Recent results suggest new features of GHRKO mice that may positively affect longevity-decreased levels of proapoptotic factors and increased levels of key regulators of mitochondrial biogenesis. The alterations in levels of the proapoptotic factors and key regulators of mitochondrial biogenesis were not further improved by two other potential life-extending interventions-calorie restriction and visceral fat removal. This may attribute the primary role to GH resistance in the regulation of apoptosis and mitochondrial biogenesis in GHRKO mice in terms of increased life span.

Maintenance of endothelial guanosine triphosphate cyclohydrolase I ameliorates diabetic nephropathy

In diabetes, endothelial nitric oxide synthase (eNOS) produces superoxide anion rather than nitric oxide, referred to as "eNOS uncoupling," which may contribute to endothelial dysfunction, albuminuria, and diabetic nephropathy. Reduced levels of endothelium-derived tetrahydrobiopterin (BH4), an essential cofactor for eNOS, promote eNOS uncoupling. Accelerated degradation of guanosine triphosphate cyclohydrolase I (GTPCH I), the rate-limiting enzyme in BH4 biosynthesis, also occurs in diabetes, suggesting that GTPCH I may have a role in diabetic microvascular disease. Here, we crossed endothelium-dominant GTPCH I transgenic mice with Ins2(+/Akita) diabetic mice and found that endothelial overexpression of GTPCH I led to higher levels of intrarenal BH4 and lower levels of urinary albumin and reactive oxygen species compared with diabetic control mice. Furthermore, GTPCH I overexpression attenuated the hyperpermeability of macromolecules observed in diabetic control mice. In addition, we treated Ins2(+/Akita) mice with metformin, which activates AMP-activated protein kinase (AMPK) and thereby slows the degradation of GTPCH I; despite blood glucose levels that were similar to untreated mice, those treated with metformin had significantly less albuminuria. Similarly, in vitro, treating human glomerular endothelial cells with AMPK activators attenuated glucose-induced reductions in phospho-AMPK, GTPCH I, and coupled eNOS. Taken together, these data suggest that maintenance of endothelial GTPCH I expression and the resulting improvement in BH4 biosynthesis ameliorate diabetic nephropathy.

Strategies to reverse endothelial dysfunction in diabetic nephropathy

Endothelial dysfunction underlies the basic pathophysiology of microvascular complications of diabetes. Endothelial dysfunction is associated with impaired nitric oxide (NO) availability. Since NO production is tightly regulated by endothelial nitric oxide synthase (eNOS), several therapeutic strategies have been investigated and proposed to improve eNOS bioavailability in the vasculature. The findings of Cheng et al. suggest that increased availability of eNOS may be an effective strategy in restoring endothelial function in patients with diabetic nephropathy.

Growth differentiation factor 15 impairs aortic contractile and relaxing function through altered caveolar signaling of the endothelium

Growth differentiation factor 15 (GDF15) is an independent predictor of cardiovascular disease, and increased GDF15 levels have been associated with endothelial dysfunction in selected patients. We therefore investigated whether GDF15 modulates endothelial function in aortas of wild-type (WT) and GDF15 knockout (KO) mice. Vascular contractions to phenylephrine and relaxation to ACh were assessed in aortas obtained from healthy WT and GDF15 KO mice. The effects of GDF15 pretreatment and the involvement of ROS or caveolae were determined. Phenylephrine-induced contractions and ACh-mediated relaxations were similar in WT and GDF15 KO mice. Pretreatment with GDF15 inhibited contraction and relaxation in both groups. Inhibition of contraction by GDF15 was absent in denuded vessels or after blockade of nitric oxide (NO) synthase. Relaxation in WT mice was mediated mainly through NO and an unidentified endothelium-derived hyperpolarizin factor (EDHF), whereas GDF15 KO mice mainly used prostaglandins and EDHF. Pretreatment with GDF15 impaired relaxation in WT mice by decreasing NO; in GDF15 KO mice, this was mediated by decreased action of prostaglandins. Disruption of caveolae resulted in a similar inhibition of vascular responses as GDF15. ROS inhibition did not affect vascular function. In cultured endothelial cells, GDF15 pretreatment caused a dissociation between caveolin-1 and endothelial NO synthase. In conclusion, GDF15 impairs aortic contractile and relaxing function through an endothelium-dependent mechanism involving altered caveolar endothelial NO synthase signaling.