['Arachidonic acid-derived 12-lipoxygenase (12-LO) products have potent growth and chemotactic properties. The present studies examined whether 12-LO and fibronectin are induced in cultured rat mesangial cells (MCs) exposed to high glucose and whether they are expressed in experimental diabetic nephropathy.', 'To determine the effect of high glucose on MC 12-LO mRNA and protein expression, rat MCs were incubated with RPMI medium containing 100 (NG) or 450 mg/dL glucose (HG). For animal studies, rats were injected with diluent (control) or streptozotocin. The latter were left untreated (DM) or treated with insulin (DM + I). At sacrifice after four months, GAPDH, 12-LO, and fibronectin mRNA were measured by competitive reverse transcription-polymerase chain reaction (RT-PCR) in microdissected glomeruli (G). Renal sections were semiquantitatively scored (0 to 4+) for diabetic changes and for 12-LO and fibronectin by immunohistochemistry.', '12-LO mRNA expression in MC exposed to HG (12.71 +/- 1.17 attm/microL) and DM G (1.78 +/- 0.65 x 10-3 attm/glomerulus) was significantly higher than those of MCs in NG media (6.71 +/- 0.78 attm/microL) and control G (0.34 +/- 0.12 x 10-3 attm/glomerulus, P < 0.005), respectively. Western blot revealed a 1.7- and a 2.8-fold increase in MC and G 12-LO protein expression, respectively (P < 0.05). The immunohistochemistry score for G 12-LO and diabetic nephropathy score was significantly greater in DM and DM + I than controls. MC and G GAPDH mRNA remained unchanged.'] Is 12-lipoxygenase increased in glucose-stimulated mesangial cells and in experimental diabetic nephropathy?