Endocrine Abstracts

There are many effects on adipose tissue metabolism of housing mice at different temperatures. Considering the situation from a thermoregulatory perspective, it may – in retrospect – be evident that one cannot expect to observe any effect of e.g. UCP1 ablation under “normal housing” conditions. This is because at normal animal house conditions (≈20 °C), the mice experience significant cold, and they must increase their metabolism by 50-100 % (as compared to mice living at thermoneutrality) to compensate for the heat loss. This clearly means that any heat production deriving from diet-induced thermogenesis will be used in the struggle to keep up the body temperature: the mice have to produce extra heat or they will become hypothermic and die. Thus, any heat from any diet-induced thermogenesis will merely be experienced as extra heat - but if this is absent, it has to be substituted for with some other “heat”. Thus, UCP1 KO mice have to produce the same amount of heat as have wildtype mice, to survive under normal housing conditions. How this is accomplished is presently not really clarified. Under normal housing conditions, UCP1 KO do not show overt shivering (as they do when exposed to the much greater “standard” cold (=4°C). As these mice are devoid of adrenergically induced nonshivering thermogenesis, the heat must be produced in other ways, perhaps through increased muscular tone. In any case, these mice cannot allow themselves to reduce their metabolism due to the absence of UCP1: thermogenesis must be maintained, and thus there will be no “unused energy” to be stored, resulting in no obesity development under these conditions. The situation is principally very different with mice living at thermoneutrality that resembles the human situation. Here, any differences in thermogenesis (energy expenditure) should be directly observable since they are not used for thermoregulation. Mice at thermoneutrality therefore yield a fully different result. When given a high-fat diet, UCP1 KO mice become clearly more obese than the wildtype mice – although they do not develop the massive obesity seen in ob/ob mice.