Stimulation of brown adipose tissue activity in tumor-bearing rats.

Despite anorexia, cancer development is frequently accompanied by an increase of energy expenditure. Considering the pivotal role played by brown adipose tissue (BAT) in the energy metabolism of small mammals, we investigated the functional and compositional modification in BAT of anorexic tumor-bearing (Yoshida sarcoma) and pair-fed control rats. BAT thermogenic activity (assessed by maximal mitochondrial GDP binding) was 1.8-fold greater in tumor-bearing rats than in controls, while the thermogenic capacity (assessed by measurement of uncoupling protein) was unchanged. This suggests that tumor bearing had induced an unmasking of uncoupling protein sites. BAT hypertrophy and hyperplasia, characteristic of full-fledged BAT activation, did not occur. The mitochondrial oxidative capacity of BAT (assessed by cytochrome c oxidase activity) was 1.6-fold lower in tumor-bearing than in control rats. The main compositional modification observed in BAT of tumor-bearing rats was an increase in the saturation of cardiolipin fatty acids. These results suggest that the BAT stimulation induced by tumor bearing after 10 days is almost exclusively functional and that the tissue development is limited, probably by anorexia. However, a suppressive effect of anorexia inhibition by tumor bearing cannot be excluded.