Formate generated by cellular oxidation of formaldehyde accelerates the glycolytic flux in cultured astrocytes

Abstract

Formaldehyde is a neurotoxic compound that can be endogenously generated in the brain. Because astrocytes play a key role in metabolism and detoxification processes in brain, we have investigated the capacity of these cells to metabolize formaldehyde using primary astrocyte-rich cultures as a model system. Application of formaldehyde to these cultures resulted in the appearance of formate in cells and in a time-, concentration- and temperature-dependent disappearance of formaldehyde from the medium that was accompanied by a matching extracellular accumulation of formate. This formaldehyde-oxidizing capacity of astrocyte cultures is likely to be catalyzed by alcohol dehydrogenase 3 and aldehyde dehydrogenase 2, because the cells of the cultures contain the mRNAs of these formaldehyde-oxidizing enzymes. In addition, exposure to formaldehyde increased both glucose consumption and lactate production by the cells. Both the strong increase in the cellular formate content and the increase in glycolytic flux were only observed after application of formaldehyde to the cells, but not after treatment with exogenous methanol or formate. The accelerated lactate production was not additive to that obtained for azide, a known inhibitor of complex IV of the respiratory chain, and persisted after removal of formaldehyde after a formaldehyde exposure for 1.5 h. These data demonstrate that cultured astrocytes efficiently oxidize formaldehyde to formate, which subsequently enhances glycolytic flux, most likely by inhibition of mitochondrial respiration.

Glutathione-Dependent Detoxification Processes in Astrocytes

“…A similar formaldehyde clearance rate was recently reported for cultured neurons, although these cells release less formate than astrocytes [127]. The formate generated via formaldehyde oxidation may also be harmful for brain cells, as formate-induced inhibition of mitochondrial respiration is the likely reason for the observed accelerated glycolytic flux in formaldehyde-treated astrocytes and neurons [126,127].…”

“…The observed rate of formaldehyde oxidation to formate by astrocytes is with 0.23 lmol/(h 9 mg) in a range similar to that of glucose consumption (0.73 lmol/(h 9 mg) [126], underlining the high capacity of astrocytes to detoxify formaldehyde. A similar formaldehyde clearance rate was recently reported for cultured neurons, although these cells release less formate than astrocytes [127].…”

“…Cultured astrocytes contain the mRNAs of enzymes which generate and oxidize formaldehyde including ADH3 [126], suggesting that astrocytes are able to produce and to detoxify formaldehyde. Indeed, cultured astrocytes remove exogenously applied formaldehyde very efficiently with a rate of around 0.2 lmol/(h 9 mg) [126].…”

“…Indeed, cultured astrocytes remove exogenously applied formaldehyde very efficiently with a rate of around 0.2 lmol/(h 9 mg) [126]. The K M -value for formaldehyde clearance by cultured astrocytes is around 0.19 mM, suggesting that the cytosolic ADH3 which has a low micromolar K M -value for its substrate S-hydroxymethyl-GSH contributes substantially to astrocytic formaldehyde oxidation [126].…”

“…Astrocytes convert the majority ([90 %) of exogenously applied formaldehyde to formate that is subsequently exported from the cells [126], indicating that the S-formyl-GSH generated by the ADH3 reaction is rapidly hydrolyzed in these cells to GSH and formate. The observed rate of formaldehyde oxidation to formate by astrocytes is with 0.23 lmol/(h 9 mg) in a range similar to that of glucose consumption (0.73 lmol/(h 9 mg) [126], underlining the high capacity of astrocytes to detoxify formaldehyde.…”