β₁ to β₃ Switch in Control of Cyclic Adenosine Monophosphate during Brown Adipocyte Development Explains Distinct β-Adrenoceptor Subtype Mediation of Proliferation and Differentiation

Abstract

To explain the distinctive pharmacological profiles observed for adrenergic stimulation of cell proliferation (β₁) and cell differentiation (β₃), the adrenergic control of cAMP accumulation was investigated during brown adipocyte development. In preadipocytes, norepinephrine (NE) increased cAMP levels but theβ₃-agonists BRL-37344 and CGP-12177 did not; in contrast, when the cells had differentiated into mature brown adipocytes, a large cAMP response to the β₃-agonists had emerged and was now double that to NE (although the affinity of NE had increased 10-fold).β₁-messenger RNA (mRNA) levels were high in both pre- and mature brown adipocytes; β₃-mRNA did not appear until maturation but then abruptly. Although β₁-receptors remained detectable by [³H]CGP-12177 binding in the mature brown adipocytes, the cAMP response to NE (based on propranolol inhibitory potency) switched from β₁ to β₃. Even the established β₁-agonist dobutamine acted throughβ₃-receptors in the mature brown adipocytes. The increases in cAMP levels could adequately explain the increased cell proliferation in NE-stimulated preadipocytes and the NE-induced UCP1 gene expression in mature brown adipocytes. The distinctive adrenergic profiles for stimulation of proliferation and of differentiation were thus not due to the existence of additional pathways but to a switch in the type of β-receptor mediating the NE response, coordinated with an alteration in the nuclear response to increased cAMP levels. The study implies that full recruitment of brown adipose tissue cannot be induced by exclusive β₃-stimulation.