Diabetes is associated with increased generation of cytokines and tissue inflammation, but it is unclear how increased cytokine synthesis is causally related to the development of diabetic complications. Here, we report that exposure to high (25 mm) glucose, but not iso-osmotic concentrations of mannitol or 3-methyl glucose, increased TNF-alpha secretion by rat and human aortic smooth muscle cells in culture. The increase in TNF-alpha production was prevented by actinomycin D and cycloheximide, indicating transcriptional activation of TNF-alpha gene. High glucose (HG)-induced TNF-alpha release was specifically inhibited by protein kinase C (PKC)-delta inhibitor (Rottlerin; EMD Biosciences, San Diego, CA), but not PKC-beta2 inhibitor (CGP53353; Tocris Cookson Inc., Ellisville, MO), indicating the possible involvement of PKC-delta in HG signaling. TNF-alpha secretion was also prevented by pretreating cells with aldose reductase (AR) inhibitors, sorbinil or tolrestat and in cells treated with antisense AR mRNA. Inhibition of AR also prevented the increase in TNF-alpha mRNA. Addition of anti-TNF-alpha antibodies or soluble TNF-alpha receptors 1 and 2 to the medium or RNA interference ablation of TNF-alpha attenuated nuclear factor-kappaB activation and prevented HG-stimulated cell growth. These data indicate that AR is required for HG-induced TNF-alpha synthesis and release. In vivo, the release of TNF-alpha by HG leading to autocrine stimulation of TNF-alpha synthesis may be a critical step in the development of the cardiovascular complications of diabetes. Interruption of the autocrine effects of TNF-alpha may be a useful strategy for treating diabetic vasculopathies.