Uncoupling protein-1 is not leaky

Abstract

The activity of uncoupling protein-1 (UCP1) is rate-limiting for nonshivering thermogenesis and diet-induced thermogenesis. Characteristically, this activity is inhibited by GDP experimentally and presumably mainly by cytosolic ATP within brown-fat cells. The issue as to whether UCP1 has a residual proton conductance even when fully saturated with GDP/ATP (as has recently been suggested) has not only scientific but also applied interest, since a residual proton conductance would make overexpressed UCP1 weight-reducing even without physiological/pharmacological activation. To examine this question, we have here established optimal conditions for studying the bioenergetics of wild-type and UCP1-/- brown-fat mitochondria, analysing UCP1-mediated differences in parallel preparations of brown-fat mitochondria from both genotypes. Comparing different substrates, we find that pyruvate (or palmitoyl-L-carnitine) shows the largest relative coupling by GDP. Comparing albumin concentrations, we find the range 0.1-0.6% optimal; higher concentrations are inhibitory. Comparing basic medium composition, we find 125 mM sucrose optimal; an ionic medium (50-100 mM KCl) functions for wild-type but is detrimental for UCP1-/- mitochondria. Using optimal conditions, we find no evidence for a residual proton conductance (not a higher post-GDP respiration, a lower membrane potential or an altered proton leak at highest common potential) with either pyruvate or glycerol-3-phosphate as substrates, nor by a 3-4-fold alteration of the amount of UCP1. We could demonstrate that certain experimental conditions, due to respiratoty inhibition, could lead to the suggestion that UCP1 possesses a residual proton conductance but find that under optimal conditions our experiments concur with implications from physiological observations that in the presence of inhibitory nucleotides, UCP1 is not leaky.

Neural control of white, beige and brown adipocytes

“…In the unstimulated state, UCP-1 mRNA and protein are found in mesenteric WAT, EWAT, retroperitoneal WAT, IWAT and especially periovarian WAT, but they do not produce heat because in the unstimulated state UCP-1-bearing mitochondria do not show a natural leaky mitochondrial membrane to protons necessary for heat production. 59,60 This may seem in conflict to the data of Bi 49 who found temperature increases in the browned IWAT of the DMH NPY knockdown rats, but the latter animals were not housed at thermoneutrality and thus the animals were moderately thermogenically challenged. The study by Bi 49 is the only one we are aware of where the real physiology of brite/beige cells has been assessed in vitro in terms of their heat production.…”

Yes, even human brown fat is on fire!

“…There is, however, at least one important caveat in this line of thought. Studies of isolated brown fat mitochondria (22), isolated brown fat cells, and intact mice all converge on the same conclusion: when not acutely activated, brown adipose tissue does not do anything. This is also the outcome of the study of Ouellet et al (2).…”

Nonshivering thermogenesis and its adequate measurement in metabolic studies

“…However, the basal respiration of the cells is identical regardless of whether they possess UCP1. This demonstrates that UCP1 does not show any proton transport activity when non-stimulated, that is to say, it is not 'leaky' (Shabalina et al, 2010b) (i.e. it does not allow for proton flux over the mitochondrial membrane when it is not directly stimulated).…”