Methylglyoxal downregulates Raf-1 protein through a ubiquitination-mediated mechanism

Abnormal accumulation of methylglyoxal, a physiological glucose metabolite, is considered a potential link between hyperglycemia and diabetes complications. Evidence has shown that methylglyoxal modifies cellular proteins by glycation and oxidation, resulting in dysfunction or loss of cellular proteins. Raf-1 protein-serine/threonine kinase serves as a central switch board in the transmission of many growth and developmental signals. It was reported that Raf-1 levels appear to decrease in some diabetic subjects. But the potential mechanisms have not yet been clarified. Here, we tested the hypothesis that methylglyoxal-mediated proteolysis might contribute to the downregulation of Raf-1 levels. We observed that a rapid and detectable decrease in Raf-1 protein levels was induced by methylglyoxal, which was accelerated by treating with a Raf-1 activator, phorbol-12-myristate-13-acetate, and by expressing active forms of Raf-1 and Ras. Moreover, immunoprecipitation and immunoblotting assays showed that co-treatment of cells with methylglyoxal and phorbol-12-myristate-13-acetate caused dramatic ubiquitination in both total intracellular proteins and Raf-1. Blocking phosphorylation with the protein kinase C inhibitor bisindolylmaleimide, or inhibiting intracellular oxidation by addition of the antioxidant N-acetyl-l-cysteine could reverse the ubiquitination and downregulation of Raf-1 induced by methylglyoxal and phorbol-12-myristate-13-acetate. These results suggest that methylglyoxal-mediated intracellular oxidation and ubiquitin/proteasome-dependent proteolysis are involved in the downregulation of Raf-1, which may be closely related to the development complications in diabetes.