Hexosamine regulation of glucose-mediated laminin synthesis in mesangial cells involves protein kinases A and C

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Abstract
Hyperglycemia leads to alterations in mesangial cell function and extracellular matrix (ECM) protein accumulation. These adverse effects of glucose may be mediated by glucose metabolism through the hexosamine biosynthesis pathway (HBP). The HBP converts fructose-6-phosphate to glucosamine-6-phosphate via the rate-limiting enzyme, glutamine:fructose-6-phosphate amidotransferase (GFA). We have investigated the effects of high glucose (HG, 25 mM) and glucosamine (GlcN, 1.5 mM) on the synthesis of the ECM protein laminin in a SV-40-transformed rat kidney mesangial (MES) cell line. The roles of protein kinases C (PKC) and A (PKA) in mediating laminin accumulation were also investigated. Treatment of MES cells with HG or GlcN for 48 h increased laminin levels in cellular extracts more than twofold compared with 5 mM glucose (low glucose; LG). The presence of the GFA inhibitor diazo-oxo-norleucine (DON, 10 μM) blocked HG but not GlcN-induced laminin synthesis. HG resulted in a time-dependent increase in total PKC and PKA activities, 57±11.3 (P< 0.01 vs. LG) and 85±17.4% (P < 0.01 vs. LG), respectively. GlcN had no effect on the total PKC activity; however, both glucose and glucosamine increased membrane-associated PKC activity by twofold compared with LG. GlcN stimulated total PKA activity by 47±8.4% (P < 0.01 vs. LG). Similarly, membrane- associated PKA activity was also increased by HG and GlcN ∼1.8 and 1.5-fold, respectively. HG and GlcN increased cellular cAMP levels 2.2- and 3.4-fold, respectively. Pharmacological downregulation of PKC by long-term incubation of MES cells with 0.5 μM phorbol 12-myristate 13-acetate (PMA) or inhibition of PKA activity by 2 μM H-8 blocked the effects of HG and GlcN on laminin synthesis. These results demonstrate that glucose-induced laminin synthesis in MES cells is mediated by flux through the HBP and that this stimulation involves PKC and PKA signaling pathways.

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