Methylglyoxal (MG), a reactive dicarbonyl produced during glucose metabolism, is known as a preferred substrate of aldose reductase (AR; AKR1B1) that concomitantly catalyzes the reduction of glucose in the polyol pathway. MG modifies cellular proteins to form cross-links of amino groups, generating so-called advanced glycation end products. Increased rates of MG formation under hyperglycemic conditions and ensuing high serum levels of MG are reported in diabetic patients. As involvement of AR in the pathogenesis of diabetic complications has been suggested, we investigated the effects of MG on AR activity using cultured vascular smooth muscle cells (SMC) isolated from rat aorta. MG-induced a dose- and time-dependent increase in AR mRNA levels to a maximum of 4.5-fold. This increase in mRNA was accompanied by elevated enzyme activity and protein levels. Pretreatment of SMC with N-acetyl-L-cysteine significantly suppressed the MG-induced AR expression, while DL-buthionine-(S,R)-sulfoximine further augmented the MG-induced increase in AR mRNA level. Intracellular levels of reactive oxygen species determined using 2',7'-dichlorofluorescein diacetate were significantly elevated in SMC treated with MG, suggesting the involvement of oxidative stress in this process. Under oxidative stress generated by hydrogen peroxide, the major signaling pathway mediating the up-regulation of AR expression was demonstrated to be the epidermal growth factor receptor-ERK pathway. In contrast, the p38 kinase pathway appears to mediate MG-induced AR expression. The cytotoxic effect of MG was significantly enhanced in the presence of the AR inhibitor ponalrestat, indicating a protective role of AR against MG-induced cell damage.