Acute increases in O-GlcNAc indirectly impair mitochondrial bioenergetics through dysregulation of LonP1-mediated mitochondrial protein complex turnover

The attachment of O-linked beta-N-acetylglucosamine (O-GlcNAc) to the serine and threonine residues of proteins in distinct cellular compartments is increasingly recognized as an important mechanism regulating cellular function. Importantly, the O-GlcNAc modification of mitochondrial proteins has been identified as a potential mechanism to modulate metabolism under stress with both potentially beneficial and detrimental effects. This suggests that temporal and dose-dependent changes in O-GlcNAcylation may have different effects on mitochondrial function. In the current study, we found that acutely augmenting O-GlcNAc levels by inhibiting O-GlcNAcase with Thiamet-G for up to 6 h resulted in a time-dependent decrease in cellular bioenergetics and decreased mitochondrial complex I, II, and IV activities. Under these conditions, mitochondrial number was unchanged, whereas an increase in the protein levels of the subunits of several electron transport complex proteins was observed. However, the observed bioenergetic changes appeared not to be due to direct increased O-GlcNAc modification of complex subunit proteins. Increases in O-GlcNAc were also associated with an accumulation of mitochondrial ubiquitinated proteins; phosphatase and tensin homolog induced kinase 1 (PINK1) and p62 protein levels were also significantly increased. Interestingly, the increase in O-GlcNAc levels was associated with a decrease in the protein levels of the mitochondrial Lon protease homolog 1 (LonP1), which is known to target complex IV subunits and PINK1, in addition to other mitochondrial proteins. These data suggest that impaired bioenergetics associated with short-term increases in O-GlcNAc levels could be due to impaired, LonP1-dependent, mitochondrial complex protein turnover.