Adaptation of rats to a high-protein, carbohydrate-free (HP) diet induced a marked reduction of brown adipose tissue (BAT) fatty acid (FA) synthesis from both3H2O and [14C]glucose in vivo, with pronounced decreases in the activities of four enzymes associated with lipogenesis: glucose-6-phosphate dehydrogenase, malic enzyme, citrate lyase, and acetyl-CoA carboxylase. In both HP-adapted and control rats, in vivo incorporation of3H2O and [14C]glucose into BAT glyceride-glycerol was much higher than into FA. It could be estimated that most of the glycerol synthetized was used to esterify preformed FA. Glycerol synthesis from nonglucose sources (glyceroneogenesis) was increased in BAT from HP rats, as evidenced by an increased capacity of tissue fragments to incorporate [1-14C]pyruvate into glycerol and by a fourfold increase in the activity of phospho enolpyruvate carboxykinase activity, a key glyceroneogenic enzyme. The data suggest that high rates of glyceroneogenesis and of esterification of preformed FA in BAT from HP-adapted rats are essential for preservation of tissue lipid stores, necessary for heat generation when BAT is recruited in nonshivering thermogenesis.

No abstract

The effect of brown adipose tissue (BAT) sympathetic hemidenervation on the activity of glycerokinase (GyK) was investigated in different physiological conditions. In rats fed a balanced diet, the activity of the enzyme was ∼50% lower in BAT-denervated pads than in intact, innervated pads. In rats adapted to a high-protein, carbohydrate-free diet, norepinephrine turnover rates and BAT GyK activity were already reduced, and BAT denervation resulted in a further decrease in the activity of the enzyme. Cold acclimation of normally fed rats at 4°C for 10 days markedly increased the activity of the enzyme. Cold exposure (4°C) for 6 h was insufficient to stimulate BAT GyK, but the activity of the enzyme was already increased after 12 h of cold exposure. The cold-induced BAT GyK stimulation was completely blocked in BAT-denervated pads. The data indicate that an adequate sympathetic flow to BAT is required for the maintenance of normal levels of GyK activity and for the enzyme response to situations, such as cold exposure, which markedly increase BAT sympathetic flow.

We have previously found that glyceroneogenesis is very active in brown adipose tissue (BAT) and increases in fasted, diabetic and high-protein-diet-fed rats, situations of reduced thermogenic activity. To understand better the role of glyceroneogenesis in BAT glycerol-3-phosphate (G3P) generation, we investigated its activity during cold exposure (10 days at 4 degrees C), a condition in which, in contrast to the above situations, BAT thermogenesis is markedly activated. Rates of total (from all sources) BAT fatty acid (FA) synthesis and rates of incorporation of glucose carbon into BAT glyceride-FA and -glycerol in vivo were markedly increased by cold exposure. Cold exposure induced a marked increase in BAT glyceroneogenic activity, evidenced by (1) increased rates of non-glucose carbon incorporation into glyceride-glycerol in vivo and of [1-14C]-pyruvate incorporation into glyceride-glycerol in vitro, and (2) a threefold increase in phosphoenolpyruvate carboxykinase activity. Most of the glyceride-glycerol synthesized by BAT via glyceroneogenesis or from glucose was used to esterify preformed FA. This use was markedly increased by cold exposure, in parallel with a pronounced activation of BAT lipoprotein lipase activity. In conclusion, during cold exposure BAT glyceroneogenesis is markedly activated, contributing to increase the generation of G3P, which is mostly used to esterify preformed FA.

Brown adipose tissue (BAT) glyceroneogenesis was evaluated in rats either fasted for 48 h or with streptozotocin-diabetes induced 3 days previously or adapted for 20 days to a high-protein, carbohydrate-free (HP) diet, conditions in which BAT glucose utilization is reduced. The three treatments induced an increase in BAT glyceroneogenic activity, evidenced by increased rates of incorporation of [1-14C]pyruvate into triacylglycerol (TAG)-glycerol in vitro and a marked, threefold increase in the activity of BAT phosphoenolpyruvate carboxykinase (PEPCK). BAT glycerokinase activity was not significantly affected by fasting or diabetes. After unilateral BAT denervation of rats fed either the HP or a balanced diet, glyceroneogenesis activity increased in denervated pads, evidenced by increased rates of nonglucose carbon incorporation into TAG-glycerol in vivo (difference between 3H2O and [14C]glucose incorporations) and of [1-14C]pyruvate in vitro. PEPCK activity was not significantly affected by denervation. The data suggest that BAT glyceroneogenesis is not under sympathetic control but is sensitive to hormonal/metabolic factors. In situations of reduced glucose use there is an increase in BAT glyceroneogenesis that may compensate the decreased generation of glycerol-3-phosphate from the hexose.