Diabetic nephropathy: mechanisms of renal disease progression.

Diabetic nephropathy is characterized by excessive amassing of extracellular matrix (ECM) with thickening of glomerular and tubular basement membranes and increased amount of mesangial matrix, which ultimately progress to glomerulosclerosis and tubulo-interstitial fibrosis. In view of this outcome, it would mean that all the kidney cellular elements, i.e., glomerular endothelia, mesangial cells, podocytes, and tubular epithelia, are targets of hyperglycemic injury. Conceivably, high glucose activates various pathways via similar mechanisms in different cell types of the kidney except for minor exceptions that are related to the selective expression of a given molecule in a particular renal compartment. To begin with, there is an obligatory excessive channeling of glucose intermediaries into various metabolic pathways with generation of advanced glycation products (AGEs), activation of protein kinase C (PKC), increased expression of transforming growth factor-beta (TGF-beta), GTP-binding proteins, and generation of reactive oxygen species (ROS). The ROS seem to be the common denominator in various pathways and are central to the pathogenesis of hyperglycemic injury. In addition, there are marked alterations in intraglomerular hemodynamics, i.e., hyperfiltration, and this along with metabolic derangements adversely compounds the hyperglycemia-induced injury. Here, the information compiled under various subtitles of this article is derived from an enormous amount of data summarized in several excellent literature reviews, and thus their further reading is suggested to gain in-depth knowledge of each of the subject matter.

Expression and functional characteristics of tubular transporters: P-glycoprotein, PEPT1, and PEPT2 in renal mass reduction and diabetes.

Renal mass reduction is associated with a compromise in renal excretion, and thus dosages of drugs need to be adjusted to avoid adverse reactions and to ensure their effectiveness. A prototypic example is patients who had undergone transplantation due to a variety of causes, including diabetic nephropathy; the latter appears to be the major cause of renal failure requiring hemodialysis and transplantation. Conceivably, hyperglycemia with reduced renal mass interferes in the delivery of xenobiotics handled by various tubular transporters. In this investigation, effect of renal mass reduction/hyperglycemia on gene and protein expression of P-glycoprotein (Pgp), PEPT1, and PEPT2 was assessed. Also, [H(3)]glycylsarcosine uptake, a prototype of dipeptide, was measured in various groups of rats: sham-operated, uninephrectomized, streptozotocin-induced diabetes, and diabetic + uninephrectomized. An increase in Pgp, PEPT1, and PEPT2 expression was observed in kidneys of uninephrectomy rats, the highest being in the Pgp. Similarly, an increase was observed in diabetic rats who had undergone uninephrectomy, although less than those with nephrectomy alone. No differences were observed between sham-operated and diabetic groups. Increased uptake of [H(3)]glycylsarcosine was also seen in uninephrectomised rats. A modest uptake was observed in diabetic rats who had undergone uninephrectomy. The data suggest that uninephrectomy induces an increase in the expression and activity of transporters localized to renal tubular epithelial brush border. The fact that upregulation and activity of the peptide transporters were less in kidneys of diabetic animals who had undergone uninephrectomy compared with uninephrectomy alone suggests that hyperglycemia interferes in their expression and activity during the compensatory phase.

Renal development in high-glucose ambience and diabetic embryopathy.

Maternal diabetes has an adverse influence on the intrauterine growth of the fetus, which is attributable to the exposure of the mammalian embryo to an abnormal metabolic environment. A sustained exposure of the fetus to such an environment (ie, elevated concentration of glucose), during the first 6 to 8 weeks of gestation in humans may result in diabetic embryopathy, which is characterized by a multitude of congenital birth defects, including those of the nervous, cardiovascular, skeletal, and urogenital systems. The urogenital abnormalities may be associated with caudal regression syndrome or may occur alone in the form of partial or total renal agenesis. Similarly, an increase in the incidence of morphogenetic defects is observed in offsprings of streptozotocin-induced diabetic rats and mice and also in nonobese diabetic mice. In certain instances, failure in the growth of lower part of embryos or newborn mice has been observed in animals with a severe diabetic state. For further delineation of the mechanisms involved in the pathogenesis of diabetic embryopathy, the investigators used whole-embryo culture systems, and found that glucose can induce defects mainly confined to the lower part of the body involving the genitourinary system. Similarly, dysmorphogenesis of the embryonic metanephros is observed when it is subjected to high concentrations of D-glucose and its epimer D-mannose. This article discusses certain aspects of diabetic embryopathy with an emphasis on changes that occur in the fetal metanephros in high-glucose ambience.

Gene regulation of aldose-, aldehyde- and a renal specific oxido reductase (RSOR) in the pathobiology of diabetes mellitus.

Aldose-, aldehyde and renal specific oxido reductase (RSOR) belong to the family of aldo-keto reductases (AKRs). They are monomeric (alpha/beta)8-barrel proteins with a molecular weight ranging from 30 to 40 kDa, and at present include more than 60 members. Except for RSOR, they are expressed in a wide variety of animal and plant species and in various tissues. They catalyze NADPH-dependent reduction of various aliphatic and aromatic aldehyde and ketones. During the past three decades aldehyde reductase (AKR1A) and aldose reductase (AKR1B) have been extensively investigated, and the gene regulation of AKR1B has been noted to be heavily influenced by hyperglycemic state and high glucose ambience in various culture systems. AKR1B catalyzes the conversion of glucose to sorbitol in concert with a coenzyme, NADPH. The newly discovered RSOR has certain structural and functional similarities to AKR1B and seems to be relevant to the renal complications of diabetes mellitus. Like other AKRs, it has a NADPH binding motif, however, it is located at the N-terminus and it probably undergoes N-linked glycosylation in order to achieve functional substrate specificity. Besides the AKR3 motif, it has very little nucleotide or protein sequence homology with other members of the AKR family. Nevertheless, gene regulation of RSOR, like AKR1B, is heavily modulated by carbonyl, oxidative and osmotic stresses, and thus it is anticipated that its discovery would lead to the development of new inhibitors as well as gene therapy targets to alleviate the complications of diabetes mellitus in the future.

High glucose stimulates synthesis of fibronectin via a novel protein kinase C, Rap1b, and B-Raf signaling pathway.

The molecular mechanism(s) by which high glucose induces fibronectin expression via G-protein activation in the kidney are largely unknown. This investigation describes the effect of high glucose (HG) on a small GTP-binding protein, Rap1b, expression and activation, and the relevance of protein kinase C (PKC) and Raf pathways in fibronectin synthesis in cultured renal glomerular mesangial cells (MCs). In vivo experiments revealed a dose-dependent increase in Rap1b expression in glomeruli of diabetic rat kidneys. Similarly, in vitro exposure of MCs to HG led to an up-regulation of Rap1b with concomitant increase in fibronectin (FN) mRNA and protein expression. The up-regulation of Rap1b mRNA was mitigated by the PKC inhibitors, calphostin C, and bisindolymaleimide, while also reducing HG- induced FN expression in non-transfected MCs. Overexpression of Rap1b by transfection with pcDNA 3.1/Rap1b in MCs resulted in the stimulation of FN synthesis; however, the PKC inhibitors had no significant effect in reducing FN expression in Rap1b-transfected MCs. Transfection of Rap1b mutants S17N (Ser --> Asn) or T61R (Thr --> Arg) in MCs inhibited the HG-induced increased FN synthesis. B-Raf and Raf-1 expression was investigated to assess whether Rap1b effects are mediated via the Raf pathway. B-Raf, and not Raf-1, expression was increased in MCs transfected with Rap1b. HG also caused activation of Rap1b, which was largely unaffected by anti-platelet-derived growth factor (PDGF) antibodies. HG-induced activation of Rap1b was specific, since Rap2b activation and expression of Rap2a and Rap2b were unaffected by HG. These findings indicate that hyperglycemia and HG cause an activation and up-regulation of Rap1b in renal glomeruli and in cultured MCs, which then stimulates FN synthesis. This effect appears to be PKC-dependent and PDGF-independent, but involves B-Raf, suggesting a novel PKC-Rap1b-B-Raf pathway responsible for HG-induced increased mesangial matrix synthesis, a hallmark of diabetic nephropathy.

Renal gene expression in embryonic and newborn diabetic mice.

Several novel genes that are upregulated in diabetic kidneys have been identified. Recently, transforming growth factor beta driven secreted proteins, i.e., connective tissue growth factor and gremlin (bone morphogenetic protein 2), have been identified, and their expression has been correlated with the tissue changes seen in diabetic nephropathy in the adult population. However, there are very few studies reported in the literature that describe the gene expression in the diabetic state during embryonic and neonatal life. It is well known that exposure to glucose or its epimer, i.e., mannose, induces marked dysmorphogenesis of the embryonic metanephros in an organ culture system. These changes are associated with ATP depletion and marked apoptosis, suggesting an oxidant stress in the induction of dysmorphogenesis of the embryonic metanephros. In view of the glucose-induced changes in the fetal metanephros, a diabetic state was induced by the administration of streptozotocin during pregnancy, and newborn mouse kidneys were processed for suppression subtractive hybridization-PCR. In addition, a diabetic state was induced in newborn diabetic mice, and after 1 week their kidneys were harvested and subjected to representational difference analysis of cDNA. Four novel genes with upregulated mRNA expression were identified. They included: (1) a translocase inner mitochondrial membrane 44 that is involved in the ATP-dependent import of preproteins from the cytosol into the mitochondrial matrix; (2) a kidney-specific aldo-keto reductase that utilizes NADPH and NADH as cofactors in the reduction of aromatic aldehydes and aldohexoses; (3) Rap1b, a Ras-related small GTP-binding protein that behaves as a GTPase and cycles between GTP-bound (active) and GDP-bound (inactive) states associated with conformational change, and (4) a fusion protein of ubiquitin polypeptide and ribosomal protein L40 (UbA(52) or ubiquitin/60) that is intimately involved in the ubiquitin-dependent proteasome pathway related to the accelerated degradation of proteins under various stress conditions, such as those seen in patients with cancer and diabetes mellitus.

Identification of developmentally regulated mesodermal-specific transcript in mouse embryonic metanephros.

Mesodermal-specific cDNA or transcript (MEST) was identified by suppression subtractive hybridization-PCR of cDNA isolated from embryonic day 13 vs. newborn mice kidneys. At day 13 of mouse gestation, a high expression of MEST, with a single approximately 2.7-kb transcript that was exclusively localized to the metanephric mesenchyme was observed. The MEST mRNA expression gradually decreased during the later stages and then abruptly decreased in the newborn kidneys and subsequent postnatal life, after which a very mild expression persisted in the glomerular mesangium. Regression in mRNA expression during embryonic renal development appears to be related to methylation of the MEST gene. Treatment of metanephroi, harvested at day 13 of gestation with MEST-specific antisense oligodeoxynucleotide resulted in a dose-dependent decrease in the size of the explants and the nephron population. This was associated with a selective decrease in MEST mRNA expression and accelerated apoptosis of the mesenchyme. These findings suggest that MEST, a gene with a putative mesenchymal cell-derived protein, conceivably plays a role in mammalian metanephric development.