We are identifying genes and proteins involved in development of insulin resistance and stress responses in adipose cells. Our focus is on spleen tyrosine kinase (Syk) as a critical mediator of the development and function of both white and brown adipose cells, and on the role of mitochondrial ROS signaling in activation of Syk.
A Respiratory Chain Controlled Signal Transduction Cascade in the Mitochondrial Intermembrane Space Mediates H2O2 Signaling
Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) govern cellular homeostasis by inducing signaling. H2O2 modulates the activity of phosphatases and many other signaling molecules through oxidation of critical cysteine residues, which led to the notion that initiation of ROS signaling is broad and non-specific and thus fundamentally distinct from other signaling pathways. Heide Christine Patterson, a post-doc and pathologist at Brigham and Women's Hospital, and colleagues recently showed that H2O2 signaling bears hallmarks of a typical hierarchical signal transduction cascade. The mitochondrial respiratory chain is upstream of Lyn, and Lyn upstream of spleen tyrosine kinase (Syk), which induces phosphorylation of Bruton’s tyrosine kinase (Btk), Akt and other targets that critically regulate signaling, transcription, translation, metabolism and the cell cycle. The active mediators of H2O2 signaling co-localize as H2O2 induces mitochondria associated Lyn and Syk phosphorylation and Lyn and Syk reside in the mitochondrial intermembrane space near the respiratory chain. Finally, the same intermediaries control the signaling response in many tissues and species responsive to H2O2 as the respiratory chain, Lyn and Syk were similarly required for H2O2 signaling in mouse B cells, fibroblasts, and chicken DT40 B cells. Consistent with a broad role, the Syk pathway is co-expressed across tissues, is of early metazoan origin, and displays evidence of evolutionary constraint in the human. These results suggest that H2O2 signaling is under control of a signal transduction pathway linking the respiratory chain to the mitochondrial intermembrane space localized ubiquitous and ancient Syk pathway in hematopoietic and non-hematopoietic cells.
Syk Signaling Mediates Brown Adipocyte Activation and Development
Marko Knoll and Sally Winther, a visiting graduate student from the University of Copenhagen, together with Heide Christine Patterson, showed that Syk is a critical mediator of beta-adrenergically-induced thermogenesis and is required for brown adipocyte function and development. β-adrenergic stimulation activates Syk, and Syk is required for beta-adrenergic mediated protein kinase A (PKA) and mechanistic target of rapamycin (mTOR) signaling in brown adipocytes. Syk promotes β-adrenergically - induced mitochondrial respiration in brown adipocytes in vitro, and mice with an inducible deletion of Syk fail to elevate BAT oxygen consumption following adrenergic stimulation.
Syk is required for β-adrenergic mediated expression of numerous genes among them mitochondrial brown fat uncoupling protein 1 (Ucp1), as well as genes involved in mitochondrial biogenesis and function, brown adipogenesis and the ROS response in brown adipocytes. Syk is also required for brown preadipocyte proliferation and brown adipocyte differentiation in vitro. Syk deletion by Cre recombinase expressed constitutively in brown adipocyte precursors resulted in smaller BAT depots, and the BAT that developed consisted of Syk expressing brown adipocytes that had escaped complete Syk deletion. As BAT is of great benefit to metabolic health, pharmacological modulation of Syk has the potential to treat obesity and associated metabolic disorders.