Brownlee and colleagues now show that methylglyoxal ... can post-translationally modify a coregulator protein to alter gene expression.

Brownlee and colleagues now show that methylglyoxal — a highly reactive α-oxoaldehyde of previously unknown function that is mainly formed in cells from the triose phosphate intermediates of glycolysis — can post-translationally modify a coregulator protein to alter gene expression. Increased methylglyoxal levels have been implicated in various diseases, for example, diabetic vascular disease. Hyperglycaemia results in increased retinal levels of methylglyoxal-modified proteins, as well as in an increased retinal expression of angiopoietin-2, which causes vascular disease. So, in Cell, the authors proposed that methylglyoxal might directly regulate the angiopoietin-2 gene by covalently modifying proteins that bind to its promoter.

Working in retinal Müller cells, Brownlee and co-workers first showed that overexpressing glyoxalase I — which reduces methylglyoxal to D-lactate — prevented the hyperglycaemia-induced increased transcription of the angiopoietin-2 gene. By generating promoter-deletion constructs, they identified a GC-box sequence in the angiopoietin-2 promoter that is required for its glucose responsiveness. Searching for nuclear proteins that bind to this sequence, they showed that Sp1 binding increased under hyperglycaemic conditions, whereas Sp3 binding decreased. Both effects were prevented by overexpressing glyoxalase I, although neither Sp1 nor Sp3 were modified by methylglyoxal.

Instead, the authors found that mSin3A — a corepressor that interacts with Sp3 — is modified by methylglyoxal: hyperglycaemia increased the modification of mSin3A, and the overexpression of glyoxalase I blocked this increase. mSin3A has been reported to bind to O-linked N-acetylglucosamine transferase (OGT), and Brownlee and colleagues showed that hyperglycaemia resulted in an increased association between mSin3A and OGT — an effect that was blocked by overexpressing glyoxalase I.

The increased association of mSin3A and OGT led to an increased interaction between OGT and Sp3, and to Sp3 glycosylation. Overexpressing glyoxalase I abolished this effect. Hyperglycaemia led to a reduced binding of mSin3A–Sp3 to the angiopoietin-2 promoter, but to increased Sp1 binding — effects that were again blocked by the overexpression of glyoxalase I. The glycosylation of Sp3 was shown to be the cause of the reduced binding, because binding could be restored by inhibiting glycosylation.

To summarize, Brownlee and colleagues have described a novel mechanism for regulating gene expression. In retinal Müller cells, hyperglycaemia causes an increased methylglyoxal modification of mSin3A, which results in the recruitment of OGT to the mSin3A–Sp3 complex. Sp3 is subsequently glycosylated by OGT, which reduces the binding of this repressor complex to a glucose-responsive GC-box sequence in the angiopoietin-2 promoter. This allows more Sp1 to bind and results in increased angiopoietin-2 transcription. It will be interesting to learn whether the methylglyoxal modification of further coregulator proteins has a role in the pathobiology of other diseases that are associated with altered methylglyoxal levels, such as cancer, malaria and renal failure.