Aldo–keto reductases mediate constitutive and inducible protection against aldehyde toxicity in human neuroblastoma SH-SY5Y cells

Reactive aldehydes including methyl glyoxal, acrolein and 4-hydroxy-2-nonenal (4-HNE) have been implicated in the progression of neurodegenerative diseases. The reduction of aldehydes to alcohols by the aldo–keto reductase (AKR) family of enzymes may represent an important detoxication route within neuronal cells. In this study, the ability of AKR enzymes to protect human neuroblastoma SH-SY5Y cells against reactive aldehydes was assessed. Using gene-specific RNA interference (RNAi), we report that AKR7A2 makes a significant contribution to the reduction of methyl glyoxal in SH-SY5Y cells, with its knockdown altering the IC₅₀ from 410 to 25.8 μM, and that AKR1C3 contributes to 4-HNE reduction, with its knockdown lowering the IC₅₀ from 1.25 to 0.58 μM. In addition, we have shown that pretreatment of cells with sub-lethal concentrations of 4-HNE or methyl glyoxal leads to a significant increase in IC₅₀ when cells are exposed to higher concentrations of the toxic aldehyde. The IC₅₀ for methyl glyoxal increased from 410 μM to 1.9 mM, and the IC₅₀ for 4-HNE increased from 120 to 690 nM. To investigate this protection, we show that pretreatment of cells with the AKR inhibitor sorbinil lead to decreased resistance to aldehydes. We show that AKR1C can be induced 8-fold in SH-SY5Y cells by treatment with sub-lethal concentrations of methyl glyoxal, and 5-fold by 4-HNE treatment. AKR1B is not induced by methyl glyoxal but is induced 10-fold by 4-HNE treatment. Furthermore, we have shown that this adaptive response can also be induced using the chemoprotective agent tert-butyl hydroquinone (t-BHQ), and that this also evokes an increase in the expression and activity of AKR1B and AKR1C. These findings highlight the potential for the interventional upregulation of AKR via non-toxic derivatives or natural compounds as a novel therapeutic approach towards the detoxication of aldehydes, with the aim of halting the progression of aldehyde-dependent neurodegenerative diseases.