Trying to define the precise role played by insulin regulating the survival of brown adipocytes, we have used rat fetal brown adipocytes maintained in primary culture. The effect of insulin on apoptosis and the mechanisms involved were assesed.

Different from the known effects of insulin as a survival factor, we have found that long-term treatment (72 h) with insulin induces apoptosis in rat fetal brown adipocytes. This process is dependent on the PI3K/mTOR/p70S6K pathway. Short-term treatment with the conditioned medium from brown adipocytes treated with insulin for 72 h, mimicked the apoptotic effect of insulin. During the process, caspase 8 activation, Bid cleavage, cytochrome c release and activation of caspases 9 and 3 are sequentially produced. Treatment with the caspase inhibitor, Z-VAD, prevents activation of this apoptotic cascade. The antioxidants, ascorbic acid and superoxide dismutase, also impair this process of apoptosis. Moreover, generation of reactive oxygen species (ROS), probably through NADPH oxidases, and a late decrease in GSH content are produced. According to this, antioxidants prevent caspase 8 activation and Bid cleavage, suggesting that ROS production is an important event mediating this process of apoptosis. However, the participation of UCP-1, 2 and 3 regulating ROS is unclear, since their levels remain unchanged upon insulin treatment for 72 h.

Our data suggest that the prolonged hyperinsulinemia might cause insulin resistance through the loss of brown adipose tissue.