Cold tolerance of UCP1-ablated mice: A skeletal muscle mitochondria switch toward lipid oxidation with marked UCP3 up-regulation not associated with increased basal, fatty acid- or ROS-induced uncoupling or enhanced GDP effects

Abstract

Mice lacking the thermogenic mitochondrial membrane protein UCP1 (uncoupling protein 1)--and thus all heat production from brown adipose tissue--can still adapt to a cold environment (4 degrees C) if successively transferred to the cold. The mechanism behind this adaptation has not been clarified. To examine possible adaptive processes in the skeletal muscle, we isolated mitochondria from the hind limb muscles of cold-acclimated wild-type and UCP1(-/-) mice and examined their bioenergetic chracteristics. We observed a switch in metabolism, from carbohydrate towards lipid catabolism, and an increased total mitochondrial complement, with an increased total ATP production capacity. The UCP1(-/-) muscle mitochondria did not display a changed state-4 respiration rate (no uncoupling) and were less sensitive to the uncoupling effect of fatty acids than the wild-type mitochondria. The content of UCP3 was increased 3-4 fold, but despite this, endogenous superoxide could not invoke a higher proton leak, and the small inhibitory effect of GDP was unaltered, indicating that it was not mediated by UCP3. Double mutant mice (UCP1(-/-) plus superoxide dismutase 2-overexpression) were not more cold sensitive than UCP1(-/-), bringing into question an involvement of reactive oxygen species (ROS) in activation of any alternative thermogenic mechanism. We conclude that there is no evidence for an involvement of UCP3 in basal, fatty-acid- or superoxide-stimulated oxygen consumption or in GDP sensitivity. The adaptations observed did not imply any direct alternative process for nonshivering thermogenesis but the adaptations observed would be congruent with adaptation to chronically enhanced muscle activity caused by incessant shivering in these mice.

Cold tolerance, cold-induced hyperphagia, and nonshivering thermogenesis are normal in α1-AMPK−/−mice

“…2). Since body weight remained nearly identical in the two genotypes, it appears that the ␣ 1 -AMPK Ϫ/Ϫ mice 1) preferentially used nonepididymal fat or 2) catabolized nonadipose tissue sources such as skeletal muscles to generate the heat needed to maintain body temperature (28). Our data indicate that failure of ␣ 1 -AMPK Ϫ/Ϫ mice to use epididymal fat was not due to lower-than-normal levels of P-HSL.…”

“…For example, ␣ 2 -AMPK activity may be sufficient to accommodate the increased metabolic demands of chronic cold exposure. Interestingly, chronic cold exposure caused a further upregulation of ␣ 2 -AMPK beyond that caused by loss of ␣ 1 -AMPK in WAT and skeletal muscle, both of which are major contributors to body temperature maintenance during cold exposure (11,28).…”

Targeting of mitochondrial reactive oxygen species production does not avert lipid-induced insulin resistance in muscle tissue from mice

“…Mitochondrial isolation, respiration and ROS measurements In a second set of mice (n07 per group) on the same diet and intervention regimen as that described above, skeletal muscle mitochondria were isolated and respiration was measured as previously described [25,26]. Respiration rates in isolated mitochondria (0.1 mg/ml) were determined at 37°C by polarographic oxygen sensors in a two-chamber Oxygraph (Oroboros Instruments, Innsbruck, Austria) using pyruvate (5 mmol/l) plus malate (3 mmol/l), or palmitoyl-coenzyme A (CoA) (50 μmol/l) plus carnitine (2 mmol/l) as substrates.…”

The preservation of in vivo phosphorylated and activated uncoupling protein 3 (UCP3) in isolated skeletal muscle mitochondria following administration of 3,4-methylenedioxymethamphetamine (MDMA aka ecstasy) to rats/mice

“…For instance Wijers et al (2008) have demonstrated increased state 4 oxygen consumption in skeletal muscle mitochondria isolated from cold-acclimated humans. However, a recent study using cold acclimated wild-type and UCP1 knock-out mice states that "UCP3 is not innately uncoupling, that it cannot be induced to act as an uncoupler by either fatty acids or by endogenous reactive oxygen species, and that effects of GDP on respiration are not mediated by UCP3 study" (Shabalina et al, 2010). Interestingly, Sun et al (2003) have demonstrated that increased UCP3 protein levels in rat skeletal muscle following lipopolysaccharide/dexamethosone injection, possibly indicating a role for UCP3 involvement in thermogenesis in sepsis.…”