137-OR: The Selective Human Beta 3 Adrenergic Receptor Mirabegron Potently Activates Lipolysis in Human White Adipocytes

Human obesity is an increasing burden worldwide. Lipids stored as metabolic fuel in white adipose tissue (WAT) can be rapidly mobilized through lipolysis. In rodent adipocytes, activation of the β₃-adrenergic receptors (β₃-ARs) leads to lipolysis. The physiological role of the β₃-AR in human adipocytes remains controversial due to its reported low expression and previous access only to partial β₃-AR agonists. Mirabegron, an FDA-approved drug for overreactive bladder, is a highly-selective human β₃-AR agonist. In clinical trials, mirabegron treatment led to activation of human brown adipose tissue (BAT) thermogenesis and increased plasma NEFA’s. Therefore, we investigated if mirabegron directly mediates lipolysis in human white adipocytes. We found similar gene expression of the three β-ARs in human immortalized and primary white adipocytes, with β₂-AR being the most expressed, followed by β₁-AR and β₃-AR. Next, to assess the functionally of the β₃-AR in human adipocytes, we established dose-response curves and found that mirabegron increased the release of glycerol, a lipolytic product, in the picomolar range. To identify the molecular mechanism controlling mirabegron-induced lipolysis, we assessed the regulation of key downstream lipolytic proteins. Mirabegron mediated lipolysis by rapidly phosphorylating hormone sensitive lipase (HSL), as well as the mitogen activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK). Moreover, acute inhibition of either the PKA or MAPK/ERK pathways decreased lipolysis.

In summary, we identify a major role played by the β₃-AR in the regulation of human WAT lipolysis. Mirabegron acts as a potent pro-lipolytic effector, signaling through the canonical adrenergic receptor pathway as well as the MAPK/ERK pathway. Due to the comparatively minimal effects previously reported of mirabegron on the cardiovascular system, mirabegron may be the optimal choice for targeted lipid mobilization in human WAT.