- The regulation of insulin signaling and the pathophysiology of insulin resistance as it relates to diabetes mellitus.
- The pathogenesis of the complications of diabetes, particularly diabetic nephropathy.
- The mechanisms of high glucose-induced alterations in cell signalling.
Detailed Description: Our research program is focused on the regulation of insulin action in particular, in understanding the cellular and molecular mechanisms which lead to insulin resistance, a metabolic abnormality associated with obesity and Type 2 diabetes mellitus.
Our previous studies of the role of Tyr phosphorylation in insulin biological effects utilizing the protein Tyr phosphatase inhibitor vanadate as a probe, has revealed the following principles:
- Inhibition of protein Tyr phosphatases (PTPs) mimics several acute (glucose transport, lipogenesis) and more chronic (receptor downregulation) actions of insulin.
- Inhibition of PTPs leads to enhanced sensitivity to insulin.
- Inhibition of PTPs leads to a prolonged duration of insulin biological responses. We demonstrated that the amplitude of the insulin receptor Tyr kinase signal is a determinant of response duration.
- Tyr phosphorylation is important in promoting IR internalization but Tyr dephosphorylation appears to be required for degradation of multiple receptors and the movement of receptors from a late endosomal to lysosomal compartment.
- Tyr dephosphorylation is important not only in terminating the insulin signal but also for propagation of several insulin biological effects, namely amino acid uptake and mitogenesis. This action results in PTP inhibitors, such as vanadate, enhancing the ratio of metabolic versus mitogenic insulin bioeffects.
- The insulin-mimetic effects of vanadium compounds on glucose transport are tyrosine-kinase dependent but in contrast to insulin, these agents can stimulate glucose uptake independent of the enzyme phosphatidylinositol-3-kinase and Akt/PKB (protein kinase B).
- The sensitivity to vanadate of tissues depends on the redox state of the cell. The presence of reactive oxygen species (ROS) will promote the oxidized (+5) state (vanadate) rather than the less active (+4) (vanadyl) oxidation state. A second determinant of tissue sensitivity to vanadate is the duration of exposure to the compound.
Our overall goals are:
- To understand the role of altered cell signaling in the pathogenesis of diabetic nephropathy.
- To determine the various mechanisms by which insulin action is inhibited in disease states and identify potential therapeutic targets to increase insulin sensitivity.
The pathogenesis of diabetic nephropathy - We are investigating the effects of high glucose on signaling by the mitogen activated protein kinase (MAPK) family of enzymes, ERKs and p38 and the EGF receptor, the role of the tyrosine kinase Src, the mechanism of action of Thioredoxin-interacting protein (TxNIP) which is highly induced by hyperglycemia. TxNIP knockout mice are protected from diabetic nephropathy.
Insulin Resistance: We are investigating the model of insulin resistance caused by exposure to high levels of glucose in combination with insulin. We are exploring a) the signaling defects responsible for the insulin resistance in adipocytes b) the possible cellular mechanisms of the defects including oxidative stress, activation of protein kinase C and degradation of the glucose transporter, GLUT4, and c) examining agents, eg bradykinin, to increase insulin sensitivity.
Cardiomyocytes, Adiocytes, Mesangial Cells, Podocytes, Muscle Cells
Procedures: Adenovirus, Elisa, Gene Expression Analysis, Glucose Clamp, Immunohistochemistry, Mass Spectrometry, Microarrays, Protemics, qRT-PCR, RIA, RT-PCR, Signal Transduction Characterization, siRNA, Western Blot
Analytical Balances, Benchtop Centrifuge, Blotting Apparatus, Confocal Microscope, Culture hood, Culture Incubators, Cryostat, Departmental beta and gamma counters, Digital Microscope, Fluorescence Microscope, Fresh Tissue sectioning systems, Gel Apparatus, Low- and High Speed Centrifuge, Low and Ultralow Freezers, Microwave Oven, Mini Vortexer, Plate Reader, ProBlot Hybridization Oven, Real-time/ Thermocycler, Setups for electropherosis, Stirrer/Hot Plate, Water Baths
Outside the Department of Physiology:
Tianru Jin LMP/University of Toronto, Canada
Susan Quaggin Medicine/Northwestern University, USA
Marc Grynpas LMP/University of Toronto, Canada
Rohan John LMP/University of Toronto, Canada
Committee member/officer of national/international scientific organizations
PRESENT GRANT COMMITTEES SERVED ON
Agency: Canadian Diabetes Association
Canadian Institutes of Health Research
Committee: Operating (Invitation)