Protein kinase C (PKC)-signaled increases in transforming growth factor beta (TGF beta) have been implicated in the stimulation of matrix protein synthesis induced by high concentrations of glucose, thromboxane, angiotension II (AII), and other stimuli in cultured glomerular mesangial cells. In the present study, the effects of several antioxidants on mesangial cell responses to high glucose, thromboxane, and AII were examined. alpha-Tocopherol blocked increases in PKC, TGF beta bioactivity, collagen, and/or fibronectin synthesis induced in mesangial cells by high glucose, the thromboxane analog U46619, and AII. By contrast, alpha-tocopherol did not alter increases in matrix protein synthesis in mesangial cells in response to exogenous TGF beta, a cytokine that does not activate PKC in mesangial cells and whose actions to stimulate matrix protein synthesis in these cells are not blocked by PKC inhibition or downregulation. Taurine and N-acetylcystein similarly inhibited activation of PKC and increases in TGF beta in response to high glucose, U46619, and AII. alpha-Tocopherol but not taurine or N-acetylcysteine partially blocked increases in PKC activity in mesangial cells in response to the diacylglycerol (DAG) analog, phorbol dibutyrate (PDBu). Thus, alpha-tocopherol may have direct effects on interaction of the PKC system of mesangial cells with DAG that are not shared by N-acetylcysteine or taurine. Increases in TGF beta have been implicated in the pathogenesis of glomerulosclerosis in diabetes and other nephropathies. The capacity of antioxidants to block increases in TGF beta in mesangial cells in response to high glucose, thromboxane, and All suggests their potential therapeutic utility to attenuate glomerulosclerosis.