Metabolic diseases like type II diabetes (T2D) and obesity largely stems from an unbalanced energy homeostasis with the fails of the insulin pathway to the point in which the glucose homeostasis is severely disturbed leading to hyperglycemia. We have investigated if the β-adrenergic signaling pathways, in both brown adipose tissue (BAT) and skeletal muscle, could be used as a strategy to alleviate metabolic disease.

As an important protein regulating energy metabolism, Akt has been an interesting target for study also in BAT, but its role in glucose uptake downstream the β-adrenergic receptors (β-ARs) have had conflicting outcomes. We have therefore made efforts to separate Akt and insulin from the β-adrenergic pathway and shown that Akt is not involved in β-adrenergic glucose uptake or thermogenesis in the brown adipocyte and norepinephrine (NE)-driven glucose clearance in vivo (Paper I). We have also shown that a β2-adrenergic agonist, clenbuterol, at low dose, can be used to induce glucose uptake to skeletal muscle, glucose clearance and increase insulin sensitivity in diet-induced obese mice, independently of insulin (Paper II). Administrating an agonist that binds to either the β-ARs on BAT or on skeletal muscle, at low dose to minimize cardiovascular adverse effects, would initiate processes independent of Akt and insulin, and could therefore be administered to patients with T2D to target idle assets.

Further, we have identified Myosin 1c (Myo1c) as a major regulator of basal protein kinase A (PKA) activity and basal glucose uptakein the brown adipocyte (Paper III) and also as a cofactor in chromatin remodeling for uncoupling protein 1 (UCP1) and peroxisome proliferator-activated receptor gamma coactivator 1α (Pgc1α) transcription in the thermogenic adipocyte (Paper IV). These novel functions of Myo1c will open up the possibilities for future explorations concerning motor proteins in thermogenic adipocytes.

This thesis has expanded current understanding about the independence of Akt and insulin in β-adrenergic metabolism as well as identifying Myo1c as a key component of the β-adrenergic pathway in the thermogenic adipocyte. The work presented herein will hopefully contribute to further exploration into adrenergic signaling in BAT and skeletal muscle.

The primary aim of this thesis was to investigate ways to safely leverage the adrenergic signalling pathway to utilize thermogenic fat and skeletal muscle for treating metabolic disease. To this end, our research first provided key evidence that adrenergically stimulated glucose uptake in brown adipocytes operates independently of the canonical insulin/AKT pathway, highlighting that this pathway provides a mechanism to bypass the core signalling defects present in insulin resistant states (Paper I). We subsequently identified Myo1c as a novel, specific regulator of this process in a BAT specific manner, providing a new molecular target within this pathway (Paper II).

To enable the direct identification of novel modulators of thermogenesis, we established isothermal microcalorimetry as a high-throughput platform capable of quantifying both UCP1 dependent and independent heat production in murine and human adipocytes (Paper III). Additionally, we complimented this work through a detailed pharmacological characterization of the β₃ AR agonist Mirabegron, clarifying that its beneficial effects in our rodent models were indeed β₃ AR and UCP1 dependent (Paper IV). Concurrently, to address the critical issue of translatability, we developed a physiologically humanized mouse model, which demonstrated that rodent classical BAT recapitulates the molecular and morphological signatures of human thermogenic tissue (Paper V).

This mechanistic work provided the framework for the design of next generation therapeutics that could activate adrenergic signalling in a functionally selective manner to avoid cardiovascular side effects and desensitization associated with conventional agonism. We first developed ATR-127, a dual β_2/3 AR agonist that served as an essential proof of concept to show that the separation of metabolic efficacy from cardiovascular effects was indeed possible (Paper VI). This led to the further development of the refined, clinically validated candidate, ATR-258, a GRK2 biased β₂ agonist that demonstrated broad preclinical efficacy, inducing healthy weight loss characterized by a significant reduction of fat mass with the preservation of lean mass. This potent muscle sparing effect was also observed in models of late-stage diabetes and sarcopenia. Furthermore, ATR-258 showed significant utility in combination regimens, providing complementary benefits such as preventing the lean mass loss associated with incretin analogues and producing additive glycemic effects with SGLT2 inhibitors. This robust preclinical profile was ultimately confirmed for its safety and tolerability in a first-in-human clinical trial (Paper VII)