Inappropriate heat dissipation ignites brown fat thermogenesis in mice with a mutant thyroid hormone receptor α1.

Thyroid hormone is a major regulator of thermogenesis, acting both in peripheral organs and on central autonomic pathways. Mice heterozygous for a point mutation in thyroid hormone receptor α1 display increased thermogenesis as a consequence of high sympathetic brown fat stimulation. Surprisingly, despite the hypermetabolism, their body temperature is not elevated. Here we show, using isolated tail arteries, that defective thyroid hormone receptor α1 signaling impairs acetylcholine-mediated vascular relaxation as well as phenylephrine-induced vasoconstriction. Using infrared thermography on conscious animals, we demonstrate that these defects severely interfere with appropriate peripheral heat conservation and dissipation, which in turn leads to compensatory alterations in brown fat activity. Consequently, when the vasoconstrictive defect in mice heterozygous for a point mutation in thyroid hormone receptor α1 was reversed with the selective α1-adrenergic agonist midodrine, the inappropriate heat loss over their tail surface was reduced, normalizing brown fat activity and energy expenditure. Our analyses demonstrate that thyroid hormone plays a key role in vascular heat conservation and dissipation processes, adding a unique aspect to its well-documented functions in thermoregulation. The data thus facilitate understanding of temperature hypersensitivity in patients with thyroid disorders. Moreover, the previously unrecognized connection between cardiovascular regulation and metabolic activity revealed in this study challenges the interpretation of several experimental paradigms and questions some of the currently derived hypotheses on the role of thyroid hormone in thermogenesis.

Lysosomal lipoprotein processing in endothelial cells stimulates adipose tissue thermogenic adaptation.

In response to cold exposure, thermogenic adipocytes internalize large amounts of fatty acids after lipoprotein lipase-mediated hydrolysis of triglyceride-rich lipoproteins (TRL) in the capillary lumen of brown adipose tissue (BAT) and white adipose tissue (WAT). Here, we show that in cold-exposed mice, vascular endothelial cells in adipose tissues endocytose substantial amounts of entire TRL particles. These lipoproteins subsequently follow the endosomal-lysosomal pathway, where they undergo lysosomal acid lipase (LAL)-mediated processing. Endothelial cell-specific LAL deficiency results in impaired thermogenic capacity as a consequence of reduced recruitment of brown and brite/beige adipocytes. Mechanistically, TRL processing by LAL induces proliferation of endothelial cells and adipocyte precursors via beta-oxidation-dependent production of reactive oxygen species, which in turn stimulates hypoxia-inducible factor-1α-dependent proliferative responses. In conclusion, this study demonstrates a physiological role for TRL particle uptake into BAT and WAT and establishes endothelial lipoprotein processing as an important determinant of adipose tissue remodeling during thermogenic adaptation.

Lipolysis Triggers a Systemic Insulin Response Essential for Efficient Energy Replenishment of Activated Brown Adipose Tissue in Mice.

The coordination of the organ-specific responses regulating systemic energy distribution to replenish lipid stores in acutely activated brown adipose tissue (BAT) remains elusive. Here, we show that short-term cold exposure or acute ß3-adrenergic receptor (ß3AR) stimulation results in secretion of the anabolic hormone insulin. This process is diminished in adipocyte-specific Atgl-/- mice, indicating that lipolysis in white adipose tissue (WAT) promotes insulin secretion. Inhibition of pancreatic ß cells abolished uptake of lipids delivered by triglyceride-rich lipoproteins into activated BAT. Both increased lipid uptake into BAT and whole-body energy expenditure in response to ß3AR stimulation were blunted in mice treated with the insulin receptor antagonist S961 or lacking the insulin receptor in brown adipocytes. In conclusion, we introduce the concept that acute cold and ß3AR stimulation trigger a systemic response involving WAT, ß cells, and BAT, which is essential for insulin-dependent fuel uptake and adaptive thermogenesis.

Combined inhibition of receptor tyrosine and p21-activated kinases as a therapeutic strategy in childhood ALL.

Receptor tyrosine kinase (RTK)-dependent signaling has been implicated in the pathogenesis of acute lymphoblastic leukemia (ALL) of childhood. However, the RTK-dependent signaling state and its interpretation with regard to biological behavior are often elusive. To decipher signaling circuits that link RTK activity with biological output in vivo, we established patient-derived xenograft ALL (PDX-ALL) models with dependencies on fms-like tyrosine kinase 3 (FLT3) and platelet-derived growth factor receptor ß (PDGFRB), which were interrogated by phosphoproteomics using iTRAQ mass spectrometry. Signaling circuits were determined by receptor type and cellular context with few generic features, among which we identified group I p21-activated kinases (PAKs) as potential therapeutic targets. Growth factor stimulation markedly increased catalytic activities of PAK1 and PAK2. RNA interference (RNAi)-mediated or pharmacological inhibition of PAKs using allosteric or adenosine triphosphate (ATP)-competitive compounds attenuated cell growth and increased apoptosis in vitro. Notably, PAK1- or PAK2-directed RNAi enhanced the antiproliferative effects of the type III RTK and protein kinase C inhibitor midostaurin. Treatment of FLT3- or PDGFRB-dependent ALLs with ATP-competitive PAK inhibitors markedly decreased catalytic activities of both PAK isoforms. In FLT3-driven ALL, this effect was augmented by coadministration of midostaurin resulting in synergistic effects on growth inhibition and apoptosis. Finally, combined treatment of FLT3 D835H PDX-ALL with the ATP-competitive group I PAK inhibitor FRAX486 and midostaurin in vivo significantly prolonged leukemia progression-free survival compared with midostaurin monotherapy or control. Our study establishes PAKs as potential downstream targets in RTK-dependent ALL of childhood, the inhibition of which might help prevent the selection or acquisition of resistance mutations toward tyrosine kinase inhibitors.