Overexpression of UCP3 in cultured human muscle lowers mitochondrial membrane potential, raises ATP/ADP ratio, and favors fatty acid versus glucose oxidation

The skeletal muscle mitochondrial uncoupling protein-3 (UCP3) promotes subs trate oxidation, but direct evidence for its metabolic role is lacking. Her e, we show that UCP3 overexpression in cultured human muscle cells decrease d mitochondrial membrane potential (Delta Psim). Despite this, the ATP cont ent was not significantly decreased compared with control cells, whereas AD P content was reduced and thus the ATP/ADP ratio raised. This finding was i n contrast with the effect caused by the chemical protonophoric uncoupler, CCCP, which lowered Delta Psim, ATP, and the ATP/ADP ratio. UCP3-overexpres sion enhanced oxidation of oleate, regardless of the presence of glucose, w hereas etomoxir, which blocks fatty acid entry to mitochondria, suppressed the UCP3 effect. Glucose oxidation was stimulated in UCP3-overexpressing ce lls, but this effect was inhibited by oleate. UCP3 caused weak increase of both 2-Deoxyglucose uptake and glycolytic rate, which differed from the mar ked stimulation by CCCP. We concluded that UCP3 promoted nutrient oxidation by lowering Delta Psim and enhanced fatty acid-dependent inhibition of glu cose oxidation. Unlike the uncoupler CCCP, however, UCP3 raised the ATP/ADP ratio and modestly increased glucose uptake and glycolysis. We propose tha t this differential effect provides a biological significance to UCP3, whic h is up-regulated in metabolic stress situations where it could be involved in nutrient partitioning.