Because of the central role of adrenergic mechanisms in the expression of crucial genes during brown adipocyte differentiation, we examined the activation (phosphorylation) of CREB (cAMP-response-element-binding protein) in mouse brown adipocytes in primary culture. We found that noradrenaline (‘norepinephrine’) stimulated CREB phosphorylation rapidly (maximum effect in ≤ 5min with slow decay) and efficiently (EC50, 6nM). The increase in CREB phosphorylation coincided with increased expression of an artificial cAMP-response-element-containing reporter construct. CREB phosphorylation was partly inhibitable, both by the β-adrenergic antagonist propranolol and by the α1-adrenergic antagonist prazosin. Adenylate cyclase hyperactivation (by forskolin) could stimulate CREB phosphorylation to the same extent as noradrenaline. The α1-adrenergic agonist cirazoline also increased CREB phosphorylation. An increase in intracellular [Ca2+] had, however, no effect, but protein kinase C activation by PMA was a potent stimulator. The cirazoline-stimulated (α1-adrenergic) CREB phosphorylation was inhibited by a desensitizing pretreatment with PMA, demonstrating that the α1-stimulation was mediated via protein kinase C activation; neither Src nor extracellular-signal-regulated kinases 1 and 2 activation was involved in the signalling process. We conclude that CREB phosphorylation in brown adipocytes is mediated not only through the classical β-adrenergic/cAMP pathway but also through a novel α1-adrenergic/protein kinase C/CREB pathway, which has not been described previously in any tissue.
Abbreviations used: CaM kinase, Ca2+/calmodulin-dependent protein kinase; CRE, cAMP-response element; CREB, CRE-binding protein; Erk1/2, extracellular-signal-regulated kinases 1 and 2; MAPK, mitogen-activated protein kinase; pCREB, CREB phosphorylated at serine-133; DTT, dithiothreitol.