The role of insulin and AMP-activated protein kinase in the regulation of metabolism and vascular health
Type 2 diabetes, a condition in which patients have reduced sensitivity to the hormone insulin, is closely linked with cardiovascular disease. My laboratory is interested in identifying the mechanisms that underlie this link between diabetes and cardiovascular disease. We have shown that insulin itself acts on the endothelial cells that line blood vessels to produce nitric oxide, a substance that relaxes blood vessels and prevents the clots that lead to heart attacks and strokes. In addition, we have demonstrated that a protein called “AMP-activated protein kinase (AMPK)” has similar beneficial effects on endothelial cells. We are currently investigating how insulin and AMPK regulate the cardiovascular system under conditions that mimic diabetes and cardiovascular disease. Our hope is that the results of our work will further expand our knowledge of how insulin and AMPK affect arteries, which will aid the development of future drugs and new therapies for the treatment of diabetes and cardiovascular disease.
Diabetes & Cardiovascular Disease
Cardiovascular disease is the principal cause of morbidity and mortality in patients with diabetes and is associated with endothelial dysfunction, characterised by reduced nitric oxide (NO) bioavailability, a key early mechanism in the progression of vascular disease. The molecular mechanisms that underlie the association between diabetes, obesity, vascular disease and endothelial dysfunction, however, are not fully characterised. For several years my laboratory has been focussed on determining the signalling pathways that regulate adipose and vascular cell function in response to insulin and stimulation of AMP-activated protein kinase (AMPK).
Insulin is the key hormonal regulator of carbohydrate and lipid metabolism, and has also been demonstrated to be a direct-acting vasodilator in intact vessels. We have previously demonstrated that insulin stimulates NO synthesis in cultured endothelial cells, the result of protein kinase B (PKB/Akt)-mediated phosphorylation and activation of endothelial NO synthase (eNOS) at Ser1177 and Ser615 [Salt et al., 2003; Ritchie et al., 2010]. Furthermore, incubation of endothelial cells in high glucose concentrations to mimic the hyperglycaemia observed in diabetes abrogated insulin-stimulated NO synthesis, without altering insulin-stimulated PKB/Akt activity; phosphorylation of eNOS at Ser1177 or Ser615 [Salt et al., 2003; Ritchie et al., 2010]. We are currently further investigating the mechanism underlying impaired insulin-stimulated NO synthesis in response to high culture glucose concentrations and proinflammatory cytokines.
AMPK is a serine/threonine protein kinase involved in the regulation of cellular and organismal metabolism. As a result, AMPK has been proposed to be a candidate target for therapeutic intervention in the treatment of type 2 diabetes and insulin resistance. Indeed, we have recently demonstrated that the commonly-utilised hypoglycaemic drug metformin stimulates AMPK in adipose tissue from patients with diabetes [Boyle et al., 2011]. In adipocytes, we have demonstrated that activation of AMPK is associated with reduced insulin-stimulated GLUT4-mediated glucose transport [Salt et al., 2000] and that insulin-stimulated glucose uptake is impaired in hypertensive rats [Collison et al., 2005]. Furthermore, in cultured human endothelial cells we have previously demonstrated that AMPK activation stimulates NO synthesis via phosphorylation and activation of eNOS at Ser1177 after incubation with VEGF and the thiazolidinedione hypoglycaemic drugs [Morrow et al., 2003; Reihill et al., 2007; Boyle et al., 2008]. Furthermore, AMPK activation attenuated TNFα-stimulated adhesion of monocytic cells to endothelial cells [Ewart et al., 2008] and was required for VEGF-mediated endothelial cell migration and proliferation [Reihill et al., 2011]. These published data demonstrate that AMPK activation promotes an anti-inflammatory, anti-atherogenic phenotype in human endothelial cells independent of its effects on whole body carbohydrate and lipid metabolism. We are currently further characterising the molecular mechanisms by which AMPK elicits this anti-inflammatory effect, in collaboration with Dr. Tim Palmer, Dr. Simon Kennedy and Dr. Christian Delles. We are also examining further the role of AMPK in adipocytes/adipose tissue.
We propose that the further characterisation of the molecular mechanisms by which insulin and AMPK regulate endothelial and vascular function may identify novel mechanisms to serve as therapeutic targets for the treatment of the vascular complications of diabetes. As a consequence of this focus in metabolic disease, my laboratory is part of the Metabolic Disease & Diabetes Research and Vascular Research themes within the Institute of Cardovascular & Medical Sciences (ICAMS).
- Boyle JG, Logan PJ, Ewart MA, Reihill JR, Ritchie SA, Connell JMC, Cleland SJ, Salt IP. Rosiglitazone stimulates nitric oxide synthesis in human aortic endothelial cells via AMP-activated protein kinase. J Biol Chem 2008;283:11210-11217.
- Boyle JG, Logan PJ, Jones GC, Small M, Sattar N, Connell JMC, Cleland SJ, Salt IP. AMP-activated protein kinase is activated in adipose tissue of individuals with type 2 diabetes treated with metformin: a randomised glycaemia-controlled crossover study. Diabetologia 2011;54:1799-1809.
- Collison M, James DJ, Graham D, Holman GD, Connell JMC, Dominiczak AF, Gould GW, Salt IP. Reduced insulin-stimulated GLUT4 bioavailability in stroke-prone spontaneously hypertensive rats. Diabetologia 2005;48:539-546.
- Ewart MA, Kohlhaas CF, Salt IP. Inhibition of TNFα-stimulated monocyte adhesion to human aortic endothelial cells by AMP-activated protein kinase. Arterioscler Thromb Vasc Biol 2010;28:2255-2257.
- Morrow VA, Foufelle F, Connell JMC, Petrie JR, Gould GW, Salt IP. Direct activation of AMP-activated protein kinase stimulates nitric oxide synthesis in human aortic endothelial cells. J Biol Chem 2003;278:31629-31639.
- Reihill JR, Ewart MA, Hardie DG, Salt IP. AMP-activated protein kinase mediates VEGF-stimulated endothelial NO production. Biochem Biophys Res Commun 2007;354:1084-1088.
- Reihill JA, Ewart MA, Salt IP. The role of AMP-activated protein kinase in the functional effects of vascular endothelial growth factor-A and -B in human aortic endothelial cells. Vascular Cell 2011;3:9.
- Ritchie SA, Kohlhaas CF, Boyd AR, Yalla KC, Walsh K, Connell JMC, Salt IP. Insulin-stimulated phosphorylation of endothelial nitric oxide synthase at Ser615 contributes to nitric oxide synthesis. Biochem J 2010;426:85-90.
- Salt IP, Connell JMC, Gould GW. 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes. Diabetes 2000;49:1649-1656.
- Salt IP, Morrow VA, Brandie FM, Connell JMC, Petrie JR. High glucose inhibits insulin-stimulated nitric oxide production without reducing eNOS Ser1177 phosphorylation in human aortic endothelial cells. J Biol Chem 2003;278:18791-18797.