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Overexpression of the Complementary DNA for Human Glutamine:Fructose-6-Phosphate Amidotransferase in Mesangial Cells Enhances Glucose-Induced Fibronectin Synthesis and Transcription Factor Cyclic Adenosine Monophosphate-Responsive Element Binding Phosphorylation
  1. Lalit P. Singh,
  2. Michelle Alexander,
  3. Kenneith Greene,
  4. Errol D. Crook
  1. From the Department of Internal Medicine, Division of Nephrology, Wayne State University School of Medicine, and John D. Dingell VA Medical Center, Detroit, MI.
  1. This work was supported by grants from the Veterans Administration, Washington, DC, and The Kidney Care, Inc. Foundation, Jackson, MS.
  2. Portions of this work were presented at the American Diabetes Association annual meeting, June 2002, San Francisco,CA.
  3. Address correspondence to: Dr. Errol D. Crook, Harper University Hospital, 2 Hudson, 3990 John R., Detroit, MI 48201. E-mail: ecrook{at}dmc.org

Abstract

Hyperglycemia-induced alterations in mesangial cell function and extracellular matrix protein (ECM) accumulation are seen in diabetic glomerulopathy. The hexosamine biosynthesis pathway (HBP) is implicated in mediating several metabolic effects of high glucose (HG) in cells. This pathway converts fructose-6-phosphate to glucosamine (GlcN)-6-phosphate by the rate-limiting enzyme glutamine:fructose-6-phosphate amidotransferase (GFA). We have previously shown that metabolism of glucose through the HBP regulates the effects of glucose on ECM (fibronectin) synthesis and transcription factor (cyclic adenosine monophosphate-responsive element binding [CREB]) phosphorylation in SV-40-transformed rat kidney mesangial cells. UDP-N-acetyl-GlcN is the end product of the HBP and serves as a precursor for O-linked serine/threonine glycosylation of cytoplasmic and nuclear proteins. Here we show that culturing mesangial cells in HG and GlcN increases the level of O-N-acetylglucosamine in several cytoplasmic and nuclear proteins. Inhibition of O-glycosylation by benzyl-2-acetamido-2-deoxy-α-D-galactopyranoside blocks both HG and GlcN-induced fibronectin synthesis and CREB phosphorylation. To further support the hypothesis that the HBP mediates HG-induced ECM synthesis, a complementary deoxyribonucleic acid (DNA) for human GFA was stably expressed in mesangial cells. Mesangial and GFA-overexpressing cells were cultured in 5 to 25 mM glucose for 48 hours. GFA-overexpressing cells were more sensitive to glucose as they demonstrated increases in fibronectin and CREB phosphorylation at lower glucose concentrations than seen in control cells. In addition, the response to 25 mM glucose for both proteins was increased in GFA when compared with controls. There is no difference in DNA synthesis and cellular adenosine triphosphate levels between the two cell lines. These results suggest that the HBP is a glucose sensor and mediator of the effects of hyperglycemia in the diabetic mesangium.

Key Words
  • diabetic nephropathy
  • hexosamine pathway
  • transforming growth factor-β
  • fibronectin
  • cyclic adenosine monophosphate-responsive element binding (CREB)

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