Insulin and adrenergic signaling has been viewed as two opposing anabolic and catabolic pathways that should always oppose each other especially when it comes to glucose homeostasis. The textbook example is that circulating epinephrine from the adrenal gland not only acutely raises the hepatic glucose output but also inhibits insulin-stimulated glucose uptake in skeletal muscle. It has thus been perceived that insulin increase glucose uptake and adrenergic signaling would decrease glucose uptake in peripheral tissues.
We have, however, results that give us good reason to hypothesize that focally released norepinephrine from sympathetic nerve endings (in contrast to circulating epinephrine) have the opposite effect. We have results that show that adrenergic receptors in brown adipocytes and skeletal muscle stimulate glucose uptake by a mechanism independent of insulin.
We have found that stimulation of adrenergic receptors, in addition to increasing typical second messenger levels, also activates signaling molecules normally associated with the insulin-signaling pathway. The activation by adrenergic receptors leads to an increase in glucose uptake of the same magnitude as that mediated by insulin. Interestingly, and in contrast to insulin, the adrenergic receptors activation of these molecules does not lead activation of other classical insulin stimulated molecules such as Akt/PKB.
The anabolic and catabolic pathways of insulin and adrenergic receptors are thus connected at some levels but not on others. It is of high importance to understand the connection between these two pathways due to the impact this can have on how we understand obesity and type II diabetes and how we can find new ways to treat these diseases.
The specific projects are:
How does adrenergic receptors signal to glucose uptake
We examine the role of adrenergic in adrenergic stimulated glucose uptake by using a number of molecular techniques such as mutational analysis of the adrenergic receptoprs by performing (PCR-based) site-directed mutagenesis to induce point mutations along the receptor. Transfections in CHO-cells (expressing tagged GLUT1 and GLUT4), human MADS (brown fat), primary brown fat, L6 skeletal muscle cells, human SKMC (skeletal muscle) and knock-out mice are used as model systems. We also use several constructs with truncated adrenergic receptors (i.e. where phosphorylation and signaling protein binding occurs), as well as kinase-dead mutants and this will with traditional pharmacology tease apart which site(s) of the receptor that are important for the signaling.
What are the mechanisms for adrenergic stimulated glucose uptake
We examine adrenergic signaling effects on glucose transporters. We investigate the localization of proteins involved in the process with cell-biological and microscopically techniques. Furthermore, we investigate a number of in-vivo models (including human skeletal muscle samples) in order to understand the mechanisms involved in animals and humans. In order to detect translocation of GLUTs (native or GLUT-GFPmyc-constructs), confocal immunofluorescence microscopy (in addition to a live cell system) and Fluorescence-Activated Cell Sorting (FACS) is used. In order to make it easier to investigate movement of proteins in myotubes we have developed a technique that we have named the 2DMS-technique (2-dimensional muscle syncytia). This technique makes it possible to get myotubes to fuse in a single direction and to be separated from each other.
Can we treat type II diabetes utilizing adrenergic mechanisms of stimulated glucose uptake?
It is well established that the response to insulin is blunted in diabetes. The adrenergic signaling pathways, if still fully functioning, could potentially be utilized to regulate glucose uptake in type II diabetes. Thus, we will use murine model systems and muscle biopsies from human donors, either healthy or type II diabetic, to study molecular mechanisms of adrenergic signaling and glucose uptake in response to adrenergic agonists.
