Subcutaneous transplants of embryonic brown adipose tissue (BAT) lead to euglycemia without insulin in mouse models of T1D [Diabetes 2012 61:674]. Euglycemia is accompanied by normalized plasma glucagon levels, restored healthy white adipose tissue, reduced inflammatory cytokines, and increased anti-inflammatory adipokines. Elevated IGF-1 was shown to play a role in successful transplants, putatively by direct stimulation of the insulin receptor [Am. J. Physiol. Endocrinol. Metab. 2015 308:E1043]. However, the mechanism underlying this effect remained unknown. We hypothesized that a secreted product from the transplanted BAT communicates with the recipient allowing a new metabolic equilibrium in the absence of insulin. We tested this using buffer conditioned by cultured immortalized BAT cells. We injected this buffer into diabetic NOD mice once daily for 7 consecutive days. In 5 of the 9 animals studied, blood glucose levels dropped back to the normal range (∼150 mg/dl) for at least 2 months with one animal remaining euglycemic for 5 months with no further intervention. As in embryonic BAT transplants, conditioned buffer injections led to a rapid decrease in plasma glucagon. We found that applying this conditioned buffer to isolated mouse and human islets lowers glucagon secretion directly (without significant changes in insulin or somatostatin). To purify and identify the active BAT-secreted compound, we established a multi-step fractionation protocol using size exclusion, ion exchange, and hydrophobicity columns to concentrate the active compound ∼10 million-fold over the initial BAT-conditioned buffer. Protease, denaturation, and spectroscopy experiments revealed a modified peptide of ∼1600 Da. Initial screening of GPCR targets [Nat Struct Mol Biol. 2015 22:362] revealed binding to the μ-opioid receptor, which is expressed preferentially in α-cells within the islet of Langerhans. Thus, these data suggest a novel path towards clinical antagonism of glucagon secretion.
D.W. Piston: Consultant; Self; Pfizer Inc. A. Ustione: None. S.C. Gunawardana: None. J. Hughes: None.
National Institutes of Health (DK098659); The Leona M. and Harry B. Helmsley Charitable Trust