Targeting mitochondria to restore failed adaptation to exercise in diabetes

Abstract

Our translational research group focuses on addressing the problem of exercise defects in diabetes with basic research efforts in cell and rodent models and clinical research efforts in subjects with diabetes mellitus. CREB (cAMP-response-element-binding protein) regulates cellular differentiation of neurons, β-cells, adipocytes and smooth muscle cells; it is also a potent survival factor and an upstream regulator of mitochondrial biogenesis. In diabetes and cardiovascular disease, CREB protein content is decreased in the vascular media, and its regulation in aberrant in β-cells, neurons and cardiomyocytes. Loss of CREB content and function leads to decreased vascular target tissue resilience when exposed to stressors such as metabolic, oxidative or sheer stress. This basic research programme set the stage for our central hypothesis that diabetes-mediated CREB dysfunction predisposes the diabetes disease progression and cardiovascular complications. Our clinical research programme revealed that diabetes mellitus leads to defects in functional exercise capacity. Our group has determined that the defects in exercise correlate with insulin resistance, endothelial dysfunction, decreased cardiac perfusion and diastolic dysfunction, slowed muscle perfusion kinetics, decreased muscle perfusion and slowed oxidative phosphorylation. Combined basic and clinical research has defined the relationship between exercise and vascular function with particular emphasis on how the signalling to CREB and eNOS [endothelial NOS (nitric oxide synthase)] regulates tissue perfusion, mitochondrial dynamics, vascular function and exercise capacity. The present review summarizes our current working hypothesis that restoration of eNOS/NOS dysfunction will restore cellular homoeostasis and permit an optimal tissue response to an exercise training intervention.

Figure 1. Targets of CREB regulation

CREB is a transcriptional hub integrating signals from G-protein-coupled receptors, tyrosine kinase receptors, Ca²⁺ and oxidant injury to regulate genes essential for survival, differentiation and metabolic adaptation. BDNF, brain-derived neurotrophic factor; CBP, CREB-binding protein; CRE, cAMP-response element; IAP-2, inhibitor of apoptosis 2; IRS-2, insulin receptor substrate 2; NRF-2, nuclear factor-erythroid 2-related factor 2; POLII, RNA polymerase II; TFAM, transcription factor A, mitochondrial; TFIID, transcription factor IID.

Figure 2. Aortic mitochondrial protein content with exercise intervention

Mitochondrial protein expression profile in SD (Sprague–Dawley) and SHHF lean and obese rats with and without exercise training. Aortic lysates were generated from SD (n = 8 and 9 for sedentary and exercise respectively), SHHF lean (n = 8 and 10 for sedentary and exercise respectively) and SHHF obese (n = 9 for both sedentary and exercise) rats as labelled: 30 µg of protein was run on SDS/PAGE (10% gels), transferred on to nitrocellulose membranes and Western blot analysis was carried out. Results are means±S.E.M. *P < 0.05; †P=0.05–0.10 (Student’s t test). Reproduced with permission from [36]: Knaub, L.A., McCune, S., Chicco, A.J., Miller, M., Moore, R.L., Birdsey, N., Lloyd, M.I., Villarreal, J., Keller, A.C., Watson, P.A. and Reusch, J.E. (2013) Impaired response to exercise intervention in the vasculature in metabolic syndrome. Diab. Vasc. Dis. Res. 10, 222–238. © 2013 SAGE Publications.

Figure 3. Impact of NOS/eNOS on aortic mitochondrial protein content and dynamics baseline and with exercise intervention

eNOS deletion and NOS disruption mimic diabetes and disrupt adaptive mitochondrial dynamics. (A) eNOS deletion or pharmacological NOS inhibition decrease aortic mitochondrial content. (B) NOS inhibition decreases mitochondrial regulator PGC1α without changing SIRT1 (sirtuin 1) protein content. (C) NOS inhibition decreases mitochondrial fusion proteins (Mfn1, Mfn2 and OPA1) and increases mitochondrial fission proteins (Fis1 and Drp1). (D) Exercise intervention increases mitochondrial protein and Mfn2 and decreases Fis1 in control rat aorta, whereas no change is observed in rats exercising with the NOS inhibitor treatment. Adapted from [37]: Miller, M.W., Knaub, L.A., Olivera-Fragoso, L.F., Keller, A.C., Balasubramaniam, V., Watson, P.A. and Reusch, J.E. (2013) Nitric oxide regulates vascular adaptive mitochondrial dynamics. Am. J. Physiol. Heart Circ. Physiol. 304, H1624–H1633. © 2013 The American Physiological Society.