Beta-1 and Not Beta-3 Adrenergic Receptors May Be the Primary Regulator of Human Brown Adipocyte Metabolism

Abstract

Purpose Brown adipose tissue (BAT) activation in humans has gained interest as a potential target for treatment of obesity and insulin resistance. In rodents, BAT is primarily induced through beta-3 adrenergic receptor (ADRB3) stimulation, whereas the primary beta adrenergic receptors (ADRBs) involved in human BAT activation are debated. We evaluated the importance of different ADRB subtypes for uncoupling protein 1 (UCP1) induction in human brown adipocytes. Methods A human BAT cell model (TERT-hBA) was investigated for subtype-specific ADRB agonists and receptor knockdown on UCP1 mRNA levels and lipolysis (glycerol release). In addition, fresh human BAT biopsies and TERT-hBA were evaluated for expression of ADRB1, ADRB2, and ADRB3 using RT-qPCR. Results The predominant ADRB subtype in TERT-hBA adipocytes and BAT biopsies was ADRB1. In TERT-hBA, UCP1 mRNA expression was stimulated 11.0-fold by dibutyryl cAMP (dbcAMP), 8.0-fold to 8.4-fold by isoproterenol (ISO; a pan-ADRB agonist), and 6.1-fold to 12.7-fold by dobutamine (ADRB1 agonist), whereas neither procaterol (ADRB2 agonist), CL314.432, or Mirabegron (ADRB3 agonists) affected UCP1. Similarly, dbcAMP, ISO, and dobutamine stimulated glycerol release, whereas lipolysis was unaffected by ADRB2 and ADRB3 agonists. Selective knockdown of ADRB1 significantly attenuated ISO-induced UCP1 expression. Conclusion The adrenergic stimulation of UCP1 and lipolysis may mainly be mediated through ADRB1. Moreover, ADRB1 is the predominant ADRB in both TERT-hBA and human BAT biopsies. Thus, UCP1 expression in human BAT may, unlike in rodents, primarily be regulated by ADRB1. These findings may have implications for ADRB agonists as future therapeutic compounds for human BAT activation.

The metabolic effects of mirabegron are mediated primarily by β3‐adrenoceptors

“…In human brown adipocytes, the effects of isoprenaline on lipolysis and gene expression were due to β 1 ‐adrenoceptors, that we showed previously compensate for the lack of β 3 ‐adrenoceptors even in brown adipocytes derived from β 3 ‐adrenoceptor knockout mice 23 . However, care should be used when interpreting data derived from human brown adipocyte cell cultures, 33 as we have previously shown 34 that in general they express much lower levels of β 3 ‐adrenoceptor mRNA than native BAT. This may pose challenges when using immortalized human brown adipocytes to investigate the role of β 3 ‐adrenoceptors in human physiology.…”

“…Since cultured mouse brown adipocytes express β 1 ‐adrenoceptor mRNA, 10 it is likely that the residual responses to mirabegron in β 3 ‐adrenoceptor knockout mice are mediated by the β 1 ‐adrenoceptor. Interestingly, a recent paper 33 suggested that the β 1 ‐ and not the β 3 ‐adrenoceptor is the primary adrenergic regulator of human brown adipocyte metabolism. Using a human immortalized brown adipocyte cell model, they showed that mirabegron had no effect on lipolysis or on the expression of several genes including UCP1 33 .…”

“…Interestingly, a recent paper 33 suggested that the β 1 ‐ and not the β 3 ‐adrenoceptor is the primary adrenergic regulator of human brown adipocyte metabolism. Using a human immortalized brown adipocyte cell model, they showed that mirabegron had no effect on lipolysis or on the expression of several genes including UCP1 33 . This correlated with little expression of β 3 ‐adrenoceptors at the mRNA level, and supports our finding that mirabegron is a β 3 ‐selective adrenoceptor agonist (Figure 1).…”

Studying Brown Adipose Tissue in a Human in vitro Context

“…Second, whereas treatment with β 3 -adrenergic receptor (β 3 AR) agonists in mice is able to selectively stimulate BAT and protect against diet-induced obesity, these effects are not seen in humans at doses low enough to prevent cardiovascular side effects (26,27). Interestingly, a recent study found that the β 1 AR may be the predominant βAR in human BAT (28), highlighting another confounding issue when translating murine BAT studies to human biology. Other attempts to selectively activate thermogenesis through sympathomimetic drugs or mitochondrial uncouplers have not been successful (26).…”

Substrate Utilization by Brown Adipose Tissue: What’s Hot and What’s Not?

“…While well characterised in rodents, until recently BAT was thought only to be found in human infants, however BAT has been identified in the supraclavicular, axillary, and paraspinal regions of adult humans and retains its thermogenic function ( 5 ). BAT is stimulated by cold exposure which induces sympathetic activation of β3-adrenergic receptors [β3AR, and possibly β1AR and β2AR ( 6 , 7 )] which activates the inner mitochondrial membrane (IMM) transporter uncoupling protein 1 (UCP1) ( 8 , 9 ). UCP1 transports protons generated from the electron transport chain across the IMM in a process uncoupled from ATP synthesis, generating heat for non-shivering thermogenesis ( 10 ).…”