Stimulation of the beta-2 adrenergic receptor for activating human brown adipose tissue

The prevalence of obesity and associated metabolic diseases is increasing at a
disturbing rate. Type 2 diabetes mellitus (T2DM) and cardiovascular diseases
(CVD) are currently the leading cause of global death. Obesity is the result of
energy intake exceeding energy expenditure. An important contributor to energy
metabolism and a promising target to counterbalance the positive energy balance
in obesity is brown adipose tissue (BAT). BAT is a thermogenic organ that is
able to combust triglyceride-derived fatty acids and glucose into heat.
Naturally, the most well-acknowledged activator of BAT is cold exposure, which
provokes an increased sympathetic outflow towards beta-adrenergic receptors on
BAT. In rodents, the beta-3-adrenergic receptor (ADRB3) is predominantly found
on brown and white adipocytes and activation of the ADRB3 has been shown to
effectively activate BAT and improve (cardio)metabolic outcomes.
Although in humans the ADRB3 agonist mirabegron increases
18F-fluorodeoxyglucose (18F-FDG) uptake by BAT, increases whole body lipolysis
and increases resting energy expenditure, this only occurs after administration
of a very high dosage of 200 mg, which highly exceeds the therapeutic dose to
treat hyperactive bladder via ADRB3 activation (i.e. 50 mg). Since at 200 mg
also cardiovascular side effects occur, mirabegron probably cross-reacts with
ADRB1 and ADRB2. Furthermore, RNA sequencing analyses on human BAT showed, in
contrast to mice, a negligible ADRB3 expression with a high ADRB2 expression.
In vitro studies indeed indicate that ADRB2 is responsible activation of human
BAT: 1) mirabegron increases oxygen consumption by human brown adipocytes that
is inhibited by a specific ADRB2 antagonist, 2) the ADRB2 agonist formoterol
activates oxygen consumption by human brown adipocytes, and 3) specific
knock-down of ADRB2, and not ADRB1 or ADRB3, reduces oxygen consumption by
human brown adipocytes. Whether an ADRB2 agonist is able to activate human BAT
in vivo has not been investigated yet, but the fact that several studies
unequivocally show an increase in resting energy expenditure, lipolysis and
lipid oxidation after intravenous (IV) administration of the ADRB2 agonist
salbutamol is promising.
Everything considered, we hypothesize that sympathetic activation of human BAT
is mainly mediated by the ADRB2 rather than the ADRB3.

1. To investigate the acute effect of ADRB2 activation, via intravenous
administration of salbutamol (250 µg), on 18F-FDG uptake by BAT.
2. To assess the acute effect of ADRB2 activation via intravenous
administration of salbutamol (250 µg) on resting energy expenditure, serum
markers for lipid- and glucose metabolism and plasma BAT markers.
3. To confirm that the stimulatory effect of salbutamol on 18F-FDG uptake by
BAT is not mediated via the ADRB3, by showing that the acute effect of i.v.
salbutamol (250 µg) on BAT is blunted by co-administration of the
ADRB1/2-blocker propranolol.

The study is a randomized double-blinded cross-over trial, that will be carried
out at the Leiden University Medical Center (LUMC). This trial encompasses one
screening and two study days.

For all subjects on both study days the intervention consists of intravenous
(IV) injection of salbutamol. This will be combined with either placebo (day 1
or 2) or propranolol (day 1 or 2) oral capsules. In addition, to visualize
supraclavicular BAT all subjects will undergo a dynamic 18F-FDG PET/CT scan on
both study days.

This study consists of three study visits: one screening visit (~1 hour) and
two study days (~3.5 hours per day). A total amount of 87.5 mL blood will be
drawn, divided over the three visits. In addition, on both study days
participants will undergo an 18F-FDG PET/CT scan, with a radiation burden of
8.4 mSv in total. Prior to the PET/CT scan, on both study days participants
will receive 250 µg salbutamol via IV injection, either in combination with
placebo or with 80 mg propranolol in capsules. Several pre-cautions regarding
the IV administration of salbutamol will be taken into account to prevent from
severe side-effects.
Subjects will not directly benefit from participation in this study. However,
the results from this study are indispensable for unravelling the working
mechanism of human BAT. In addition, the results from this study could reveal
new therapeutic targets to activate human BAT and could therefore contribute to
the fight against the worldwide obesity epidemic. Therefore, the risks of this
study are considered defensible.