Hexosamine-induced fibronectin protein synthesis in mesangial cells is associated with increases in cAMP responsive element binding (CREB) phosphorylation and nuclear CREB: the involvement of protein kinases A and C.

Abstract

Hyperglycemia-induced alterations in mesangial (MES) cell function and extracellular matrix protein accumulation are seen in diabetic glomerulopathy. Recent studies have demonstrated that some of the effects of high glucose (HG) on cellular metabolism are mediated by the hexosamine biosynthesis pathway (HBP), in which fructose-6-phosphate is converted to glucosamine 6-phosphate by the rate-liming enzyme glutamine:fructose-6-phosphate amidotransferase (GFA). In this study, we investigated the role of HBP on HG-stimulated fibronectin protein synthesis, a matrix component, in SV-40-transformed rat kidney MES cells. Treatment of MES cells with 25 mmol/l glucose (HG) for 48 h increases cellular fibronectin levels by two- to threefold on Western blots when compared with low glucose (5 mmol/l). Glucosamine (GlcN; 1.5 mmol/l), which enters the hexosamine pathway distal to GFA action, also increases fibronectin synthesis. Azaserine (AZA; 0.5 micromol/l), an inhibitor of GFA, blocks the HG- but not the GlcN-induced fibronectin synthesis. Fibronectin contains cAMP responsive element (CRE) consensus sequences in its promoter and the phosphorylation of CRE-binding protein (CREB) may regulate its expression. On Western blots, HG and GlcN stimulate two- to threefold the phosphorylation of CREB at Ser 133, whereas CREB protein content was unaltered by either HG or GlcN. In addition, nuclear CREB activity was increased by HG and GlcN on gel-shift assays using (32)P-CRE oligonucleotides. AZA impeded the HG-enhanced CREB phosphorylation and CRE binding but had no effect on GlcN-mediated CREB phosphorylation and CRE binding. Pharmacologic inhibition of protein kinase C (PKC) and protein kinase A (PKA), which are involved in hexosamine-mediated matrix production, blocked the CREB phosphorylation and fibronectin synthesis seen in HG and GlcN conditions. We conclude that the effects of HG on fibronectin synthesis in the mesangium are mediated by the HBP possibly via hexosamine regulation of CREB and PKC/PKA signaling pathways. These results support the hypothesis that the HBP is a sensor and regulator of the actions of glucose in the kidney.