Differential modulation of mitogenic and metabolic actions of insulin-like growth factor I in rat glomerular mesangial cells in high glucose culture

High glucose concentration decreases insulin-like growth factor type 1-mediated mitogen-activated protein kinase activation in bovine retinal endothelial cells

Clinical trials have incontrovertibly demonstrated that the onset and progression of diabetic retinopathy (DR) is influenced by the control of glucose levels in patients. In the present study, we examined the effect of glucose concentration on the responsiveness of bovine retinal endothelial cells (BREC) to insulin-like growth factor type 1 (IGF-1). Retinal endothelial cells were isolated from bovine retina and cultured in 5 or 20 mmol/L glucose with or without 100 ng/mL IGF-1. The level of cell growth and p42/44 and p38 mitogen-activated protein kinase (MAPK) activation was determined using the alamarBlue (Serotech) assay and Western blotting, respectively. IGF-1 significantly enhanced cell growth in BREC exposed to 5 mmol/L glucose but not in cells exposed to high glucose concentrations (20 mmol/L). IGF-1 induced a transient activation of p42/44 MAPK, with peak activation at 15 minutes in cells exposed to 5 mmol/L glucose; however, no increase in p42/44 MAPK was evident at the higher glucose concentration of 20 mmol/L. There was no significant change in the level of p38 MAPK during the time period examined when IGF-1 was also present. However, high glucose concentrations alone increased the level of p38 MAPK after 60 minutes and the level of p42/44 MAPK after only 15 minutes exposure in 20 mmol/L glucose. Thus, BREC exposed to high glucose concentrations are not sensitive to IGF-1 and this is due, at least in part, to a reduced activation of the p42/44 MAPK pathway. Furthermore, the presence of IGF-1 appears to exert a protective effect on the cells in high glucose concentration by preventing progression through the cell cycle.

Leptin induces mitogenic effect on human choriocarcinoma cell line (JAr) via MAP kinase activation in a glucose-dependent fashion

Leptin and glucose effect on cell growth has been investigated in the JAr human choriocarcinoma cells. When JAr cells were cultured in the presence of 6m M glucose (LG), proliferation and thymidine incorporation were induced by serum but not by leptin. At variance, at 25m M glucose (HG), proliferation and thymidine incorporation were stimulated by leptin and serum to a comparable extent. HG culturing also enhanced leptin-stimulated insulin receptor substrate 1 (IRS1) and MAPK phosphorylation. Blockage of MAPK activity with PD98059 caused an inhibition of glucose- and leptin-dependent thymidine incorporation. At variance with HG conditions no effects were observed in cells cultured in 6m M glucose upon treatment with PD98059. Neither glucose nor leptin determined a modification in leptin receptors total content. In this study, we provide evidence that in placental cells, leptin, similarly to that observed with insulin, stimulates cell proliferation by inducing the IRS1/MAPK pathway in a glucose-dependent fashion.

Glucose regulates insulin mitogenic effect by modulating SHP-2 activation and localization in JAr cells

The glucose effect on cell growth has been investigated in the JAr human choriocarcinoma cells. When JAr cells were cultured in the presence of 6 mm glucose (LG), proliferation and thymidine incorporation were induced by serum, epidermal growth factor, and insulin-like growth factor 1 but not by insulin. In contrast, at 25 mm glucose (HG), proliferation and thymidine incorporation were stimulated by insulin, serum, epidermal growth factor, and insulin-like growth factor 1 to a comparable extent, whereas basal levels were 25% lower than those in LG. HG culturing also enhanced insulin-stimulated insulin receptor and insulin receptor substrate 1 (IRS1) tyrosine phosphorylations while decreasing basal phosphorylations. These actions of glucose were accompanied by an increase in cellular tyrosine phosphatase activity. The activity of SHP-2 in HG-treated JAr cells was 400% of that measured in LG-treated cells. SHP-2 co-precipitation with IRS1 was also increased in HG-treated cells. SHP-2 was mainly cytosolic in LG-treated cells. However, HG culturing largely redistributed SHP-2 to the internal membrane compartment, where tyrosine-phosphorylated IRS1 predominantly localizes. Further exposure to insulin rescued SHP-2 cytosolic localization, thereby preventing its interaction with IRS1. Antisense inhibition of SHP-2 reverted the effect of HG on basal and insulin-stimulated insulin receptor and IRS1 phosphorylation as well as that on thymidine incorporation. Thus, in JAr cells, glucose modulates insulin mitogenic action by modulating SHP-2 activity and intracellular localization.

Advanced glycation end product-induced peroxisome proliferator-activated receptor gamma gene expression in the cultured mesangial cells

We identified the AGEs-induced expression of peroxisome proliferator-activated gamma (PPAR gamma) in the cultured mesangial cells using reverse transcription-polymerase chain reaction, electrophoretic mobility shift assay (EMSA), and Western immunoblotting. Administration of AGEs-BSA into the cultured mesangial cells resulted in an increase in the levels of mRNA and proteins for PPAR gamma in a dose-dependent manner. Specific bands which indicate the protein binding to PPAR gamma responsive element (PPRE) in the nuclear extracts were also detected in AGEs-BSA-treated mesangial cells, but not found in BSA-treated cells by EMSA. Antioxidants, NAC, PDTC, and aminoguanidine, attenuated the gene expression and activity of PPAR gamma induced by AGEs. These results indicate that PPAR gamma was induced and activated by the oxidative signal(s) evoked by AGEs-ligand-receptor interactions. AGEs-induced gene expression of PPAR gamma and the signal intensity of PPAR gamma and PPRE complex were attenuated furthermore by protein kinase C inhibitors, calphostin C and staurospolin, but not abolished completely, indicating that both signal transduction pathways through the induction of PKC activation and independent of PKC activation were involved in the AGEs-mediated expression and activation process of PPAR gamma. AGEs also increased the gene expression of smooth muscle alpha-actin, which is a marker for phenotypic change in mesangial cells. It is suggested therefore that AGEs-induced transcription factor as the oxidative stress may have a role in the differentiation of mesangial cells.

Growth factors and the kidney in diabetes mellitus

Nephromegaly and mesangial matrix expansion observed in the diabetic kidney are all clues of a role of growth factors in the pathogenesis of these lesions. A growing body of evidence shows that changes in (1) insulin-like growth factor I regulation, and (2) the transforming growth factor beta loop exist in the kidney in the diabetic hypertrophic kidney and in diabetic glomerulosclerosis. However, other growth factors may be involved in some diabetic renal changes. The abnormalities in growth factor content and regulation, the role of growth factors in the diabetic kidney, and the effect of hyperglycemia and advanced glycosylation end products on growth factors in the kidney are reviewed.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Posttranscriptional effects of glucose on proteoglycan expression in mesangial cells

Hyperglycemic conditions are known to increase mRNA and protein levels of several extracellular matrix molecules in cultured mesangial cells, but accompanying increases in proteoglycan mRNA have not been found, and there are discrepant reports of normal or decreased proteoglycan synthesis with or without undersulfation in diabetic kidneys and hyperglycemic cultures. We examined the effects in proliferating cells of glucose on [35S]Sulfate incorporation into heparan and dermatan sulfates and on mRNA levels of decorin, biglycan, and basement membrane perlecan. In both mesangial cells and vascular smooth muscle cells, 30 mmol/L glucose caused a decrease of 15% to 25% in the amount of sulfate incorporated into each proteoglycan in cultures confluent for 1 to 4 days, compared with 10 mmol/L glucose. The effect showed no specificity for the class of proteoglycan and was not a consequence of changes in total protein synthesis, which increased, or cell proliferation, which was unaffected. No decrease in charge density of any of the proteoglycan fractions was observed by ion-exchange chromatography. Therefore, the decrease in labeling was due to a decrease in synthesis and not undersulfation. mRNA levels for biglycan and perlecan increased slightly and transiently, and these changes cannot account for the decreased synthesis. Decorin mRNA was detected only in smooth muscle cells, where it and biglycan were differentially affected by glucose, apparently at the transcriptional level; stabilities of the two messages were unaffected by glucose. Although transforming growth factor-beta 1 (TGF-beta 1) mRNA levels increased in response to glucose, the cytokine did not appear to regulate proteoglycan synthesis, because structural changes in proteoglycans elicited by addition of TGF-beta 1 to the culture medium did not occur in the hyperglycemic cultures. On the other hand, inhibition and downregulation of protein kinase C (PKC), while decreasing net sulfate incorporation into mesangial cell proteoglycans, prevented the effect of high glucose. We conclude that a high glucose concentration causes a general decrease in the synthesis of all classes of proteoglycans at a posttranscriptional level, and can do so without affecting the charge density of individual proteoglycan molecules.

Increased activity of the insulin-like growth factor system in mesangial cells cultured in high glucose conditions. Relation to glucose-enhanced extracellular matrix production

Recent evidence suggests that several growth factors participate in diabetic glomerular disease by mediating increased extracellular matrix accumulation and altered cell growth and turnover leading to mesangial expansion. Transforming growth factor (TGF)-beta has been demonstrated to be upregulated both in vivo and in vitro, whereas studies on the activity of the renal insulin-like growth factor (IGF) system in experimental diabetes have provided conflicting results. We investigated the effects of prolonged exposure (4 weeks) of cultured human and rat mesangial cells to high (30 mmol/l) glucose vs iso-osmolar mannitol or normal (5.5 mmol/l) glucose levels on: 1) the autocrine/paracrine activity of the IGF system (as assessed by measuring IGF-I and II, IGF-I and II receptors, and IGF binding proteins); and, in parallel, on 2) TGF-beta 1 gene expression; 3) matrix production; and 4) cell proliferation. High glucose levels progressively increased the medium content of IGF-I and the mRNA levels for IGF-I and IGF-II, increased IGF-I and IGF-II binding and IGF-I receptor gene expression, and reduced IGF binding protein production. TGF-beta 1 transcripts and matrix accumulation and gene expression were increased in parallel, whereas cell proliferation was reduced. Iso-osmolar mannitol did not affect any of the above parameters. These experiments demonstrated that high glucose levels induce enhanced mesangial IGF activity, together with enhanced TGF-beta 1 gene expression, increased matrix production, and reduced cell proliferation. It is possible that IGFs participate in mediating diabetes-induced changes in matrix turnover leading to mesangial expansion, by acting in a paracrine/autocrine fashion within the glomerulus.

Enhancement and inhibition of mitogenic action of insulin-like growth factor I by high glucose in cultured bovine retinal pericytes

To explore the possible relationship between insulin-like growth factor I(IGF-I) and diabetic retinopathy, we examined the effects of glucose on IGF-I stimulated thymidine incorporation into DNA and IGF-I binding in cultured bovine retinal pericytes. IGF-I significantly increased thymidine incorporation, and its effect was completely inhibited by IGF-I receptor antibody. The exposure to high glucose for 8 h significantly enhanced the IGF-I induced increase in thymidine incorporation as compared with those to normal glucose and to high osmolarity. In contrast, the incubation with high glucose for 24 h decreased thymidine incorporation in response to IGF-I. Specific binding of IGF-I to pericytes was increased by the incubation with high glucose for both 8 and 24 h. These results suggest that glucose may play an important role in the regulation of mitogenic action of IGF-I in retinal pericytes.

Translocation of protein kinase C alpha and zeta in rat glomerular mesangial cells cultured under high glucose conditions

The activities and expression of protein kinase C isoenzymes were examined in glomerular mesangial cells cultured under high glucose conditions. Exposure of cells to high glucose concentrations (27.8 mmol/l) for more than 3 days resulted in a significant elevation of protein kinase C activities in the membrane fraction. Of the protein kinase C isoenzymes, the levels of protein kinase C alpha significantly increased in the membrane fraction after 3 days of exposure to glucose, and protein kinase C zeta increased after 5 days of exposure. Levels of protein kinase C delta and epsilon remained unchanged and protein kinase, C beta and gamma were not detected. These results indicate that protein kinase C alpha and zeta are translocated under high glucose conditions possibly through different mechanisms.