α1- and β1-Adrenoceptor Signaling Fully Compensates for β3-Adrenoceptor Deficiency in Brown Adipocyte Norepinephrine-Stimulated Glucose Uptake

Abstract

To assess the relative roles and potential contribution of adrenergic receptor subtypes other than the beta3-adrenergic receptor in norepinephrine-mediated glucose uptake in brown adipocytes, we have here analyzed adrenergic activation of glucose uptake in primary cultures of brown adipocytes from wild-type and beta3-adrenergic receptor knockout (KO) mice. In control cells in addition to high levels of beta3-adrenergic receptor mRNA, there were relatively low alpha1A-, alpha1D-, and moderate beta1-adrenergic receptor mRNA levels with no apparent expression of other adrenergic receptors. The levels of alpha1A-, alpha1D-, and beta1-adrenergic receptor mRNA were not changed in the beta3-KO brown adipocytes, indicating that the beta3-adrenergic receptor ablation does not influence adrenergic gene expression in brown adipocytes in culture. As expected, the beta3-adrenergic receptor agonists BRL-37344 and CL-316 243 did not induce 2-deoxy-d-glucose uptake in beta3-KO brown adipocytes. Surprisingly, the endogenous adrenergic neurotransmitter norepinephrine induced the same concentration-dependent 2-deoxy-D-glucose uptake in wild-type and beta3-KO brown adipocytes. This study demonstrates that beta1-adrenergic receptors, and to a smaller degree alpha1-adrenergic receptors, functionally compensate for the lack of beta3-adrenergic receptors in glucose uptake. Beta1-adrenergic receptors activate glucose uptake through a cAMP/protein kinase A/phosphatidylinositol 3-kinase pathway, stimulating conventional and novel protein kinase Cs. The alpha1-adrenergic receptor component (that is not evident in wild-type cells) stimulates glucose uptake through a phosphatidylinositol 3-kinase and protein kinase C pathway in the beta3-KO cells.

β-Adrenoceptors, but not α-adrenoceptors, stimulate AMP-activated protein kinase in brown adipocytes independently of uncoupling protein-1

“…The AMP:ATP ratios were comparable in the two genotypes (AMP:ATP ratio 0.61±0.03 for Ucp1 +/+ vs. 0.74±0.5 for Ucp1 −/− ; n=2 in triplicate), suggesting that the observed decrease in basal AMPK phosphorylation β-Adrenergically mediated glucose uptake requires AMPK β-Adrenoceptors activate glucose uptake in primary brown adipocytes via several intracellular mechanisms, including cAMP, phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) [7,8]. To investigate whether AMPK is involved in adrenergically mediated glucose uptake, glucose uptake was carried out in the presence of Ara-A, an AMPK inhibitor.…”

“…Brown adipose tissue produces all three β-adrenoceptor subtypes, although it is likely that the β 2 -adrenoceptor is limited to the vascular system in the tissue [8,[26][27][28]. Fully differentiated brown adipocytes have low levels of β 1 -adrenoceptors and no β 2 -adrenoceptors [8], and the predominant receptor is the β 3 -adrenoceptor.…”

“…Fully differentiated brown adipocytes have low levels of β 1 -adrenoceptors and no β 2 -adrenoceptors [8], and the predominant receptor is the β 3 -adrenoceptor. The selective β 3 -adrenoceptor agonist CL316243 phosphorylated AMPK (Fig.…”

“…Brown adipose tissue has been shown to play an important role in the regulation of glucose homeostasis and insulin secretion [2]. Glucose uptake is significantly increased in brown adipose tissue in vivo by activation of the sympathetic nervous system independently of insulin [3,4] and by adrenergic agonists in brown adipocytes in vitro [5][6][7][8].…”

β2-Adrenergic activation increases glycogen synthesis in L6 skeletal muscle cells through a signalling pathway independent of cyclic AMP

“…We have previously shown that an increase of glucose uptake occurs through α 1 -and β 2 -adrenoceptors in L6 skeletal muscle cells, and primarily through β 3 -adrenoceptors in brown adipocytes [22][23][24][25][26]. It is not known whether activation of adrenergic receptors leads to glucose uptake for use in glycolysis or whether glucose is taken up to be stored as glycogen.…”