Brown adipose tissue hyperplasia is a fundamental response to low ambient temperature. We show here that cold exposure of an animal markedly increased the phosphorylation of mitogen-activated protein kinase (p42/p44) Erk1 and Erk2 in brown adipose tissue, and protected cells in the tissue from apoptosis. We also show that cessation of the sympathetic stimulus, by transferring cold-adapted animals to 28°C, caused an increased rate of apoptosis in the tissue. In primary cultures of brown adipose

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... issue, norepinephrine (NE) stimulated both the phosphorylation and the activity of Erk1/2 via the Erk kinase MEK, and protected the cells form apoptosis. Similarly, agonist stimulation of ␣ 1 -and ␤-adrenergic receptors and increases in the intracellular level of Ca 2؉ and cAMP stimulated the phosphorylation of Erk1/2. Agonist stimulation of ␣ 1 -and ␤-adrenergic receptors, and increased intracellular cAMP level also promoted the cell survival. Furthermore, NE stimulated the expression and secretion of basic fibroblast growth factor (bFGF), which further promoted the cell survival via MEK-dependent activation of Erk1/2. In essence, we show that Erk1/2 has a critical role in promoting NE-and bFGF-dependent survival of brown adipocytes, and propose that NE-and bFGF-dependent regulation of the cell survival is involved in the coldinduced hyperplasia of brown adipose tissue.