Vascular actions of insulin in cardiometabolic disease : effects of metformin, physical activity, and intrinsic aerobic fitness

Insulin has important vascular and metabolic actions. The vascular effects of insulin typically account for as much as 40-50% of insulin-stimulated glucose disposal. However, the vascular reactivity to insulin is impaired in obesity and type 2 diabetes, in part due to an imbalance in endothelium-derived nitric oxide (NO) and endothelin-1 (ET-1), limiting the perfusion and delivery of glucose and insulin to target tissues. Consequently, aberrations in the vascular actions of insulin contribute to reduced glycemic control and insulin sensitivity. The studies described in this dissertation were designed to test hypotheses that focus on mechanisms and treatments of impaired vascular reactivity to insulin using rodent models of insulin resistance, obesity, and type 2 diabetes. We investigated the efficacy of metformin (Chapter 2), or daily physical activity (Chapter 3) treatments to improve the vasomotor response to insulin in conduit and skeletal muscle resistance arteries of obese, insulin resistant Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Additionally we evaluated the influence of inherited aerobic fitness (Chapter 4), independent of physical activity levels, on the vascular reactivity to insulin in a large conduit artery of rats selectively bred for low or high aerobic running capacity. We present evidence that obesity-related impairments in micro- and macro-vasomotor reactivity to insulin is largely independent of changes in body composition and adiposity. Specifically, metformin does not enhance the microvascular reactivity to insulin, despite reductions in body weight, food consumption, and improvements in glycemic control in OLETF rats. In contrast, daily physical activity in OLETF rats, significantly improved insulin-stimulated vasorelaxation, an effect that was in part mediated by reduced insulin-stimulated ET-1 aortic vasocontraction. Our data also indicate that low intrinsic aerobic fitness is paradoxically associated with greater insulin-mediated vasorelaxation and an exclusive dependence on nitric oxide in aortic vasomotor function. Collectively, our results shed new light on mechanisms underlying the influence of intrinsic aerobic fitness on the vasoreactivity to insulin, and provided strong evidence of physical activity's powerful insulin-sensitizing effects on the vasculature with obesity.Includes bibliographical references

An Alternant Method to the Traditional NASA Hindlimb Unloading Model in Mice

The Morey-Holton hindlimb unloading (HU) method is a widely accepted National Aeronautics and Space Administration (NASA) ground-based model for studying disuse-atrophy in rodents 4-6. Our study evaluated an alternant method to the gold-standard Morey-Holton HU tail-traction technique in mice. Fifty-four female mice (4-8 mo.) were HU for 14 days (n=34) or 28 days (n=20). Recovery from HU was assessed after 3 days of normal cage ambulation following HU (n=22). Aged matched mice (n=76) served as weight-bearing controls

Endurance or sprint interval exercise, & metformin treatment differently modify insulin-induced vasodilation in skeletal muscle arterioles of obese insulin resistant rats

A key contributor to insulin-mediated glucose uptake is insulin-induced vasodilation of skeletal muscle arterioles, which is impared with obesity and Type 2 diabetes (T2D). Abnormalities in the vascular reactivity to insulin can limit perfusion, and delivery of glucose and insulin to muscle tissue. In human patients with T2D, exercise improves insulin sensitivity and glucose uptake T2D. Furthermore, we have previously shown that daily exercise prevents impairments in insulin-induced vasodilation in OLETF rats. However, the efficacy of exercise interventions which utilize different muscle recruitment patterns (i.e. aerobic vs. sprint training) to ameliorate or reverse impairments in microvascular insulin reactivity has not been elucidated. The current ADA standard of care for T2D is treatment with metformin in combination with a diet and exercise program. Therefore, we studied the effects of endurance exercise and interval sprint training with and without metformin on the vasoreactivity to insulin in skeletal muscle arterioles from red and white muscles

Divergent role of nitric oxide in insulin‐stimulated aortic vasorelaxation between low‐ and high‐intrinsic aerobic capacity rats

Low‐intrinsic aerobic capacity is associated with increased risk for cardiovascular and metabolic diseases and is a strong predictor of early mortality. The effects of intrinsic aerobic capacity on the vascular response to insulin are largely unknown. We tested the hypothesis that rats selectively bred for a low capacity to run (LCR) exhibit vascular dysfunction and impaired vascular reactivity to insulin compared to high capacity running (HCR) rats. Mature female LCR (n = 21) and HCR (n = 17) rats were maintained under sedentary conditions, and in vitro thoracic aortic vascular function was assessed. LCR exhibited greater body mass (13%), body fat (35%), and subcutaneous, perigonadal, and retroperitoneal adipose tissue mass, than HCR. During an intraperitoneal glucose tolerance test, glucose area under the curve (AUC) was not different but insulin AUC was 2‐fold greater in LCR than HCR. Acetylcholine and insulin‐stimulated aortic vasorelaxation was significantly greater in LCR (65.2 ± 3.8%, and 32.7 ± 4.1%) than HCR (55.0 ± 3.3%, and 16.7 ± 2.8%). Inhibition of nitric oxide synthase (NOS) with L‐NAME entirely abolished insulin‐mediated vasorelaxation in the aorta of LCR, with no effect in HCR. LCR rats exhibited greater expression of Insulin Receptor protein, lower Endothelin Receptor‐A protein, a down‐regulation of transcripts for markers of immune cell infiltration (CD11C, CD4, and F4/80) and up‐regulation of pro‐atherogenic inflammatory genes (VCAM‐1 and MCP‐1) in the aorta wall. Contrary to our hypothesis, low‐aerobic capacity was associated with enhanced aortic endothelial function and NO‐mediated reactivity to insulin, despite increased adiposity and evidence of whole body insulin resistance.Rats selectively bred for low‐aerobic capacity displayed enhanced aortic endothelial function and nitric oxide‐mediated insulin‐stimulated vasorelaxation, despite increased adiposity and evidence of whole body insulin resistance. The vascular reactivity to insulin in high‐intrinsic aerobic capacity rats was independent of nitric oxide. Our findings demonstrate that endothelial and nitric oxide insulin‐mediated vasomotor function in the rat aorta is not always associated with aerobic capacity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112223/1/phy212459.pd

Fine-tuning and autoregulation of the intestinal determinant and tumor suppressor homeobox gene CDX2 by alternative splicing

On the basis of phylogenetic analyses, we uncovered a variant of the CDX2 homeobox gene, a major regulator of the development and homeostasis of the gut epithelium, also involved in cancer. This variant, miniCDX2, is generated by alternative splicing coupled to alternative translation initiation, and contains the DNA-binding homeodomain but is devoid of transactivation domain. It is predominantly expressed in crypt cells, whereas the CDX2 protein is present in crypt cells but also in differentiated villous cells. Functional studies revealed a dominant-negative effect exerted by miniCDX2 on the transcriptional activity of CDX2, and conversely similar effects regarding several transcription-independent functions of CDX2. In addition, a regulatory role played by the CDX2 and miniCDX2 homeoproteins on their pre-mRNA splicing is displayed, through interactions with splicing factors. Overexpression of miniCDX2 in the duodenal Brunner glands leads to the expansion of the territory of these glands and ultimately to brunneroma. As a whole, this study characterized a new and original variant of the CDX2 homeobox gene. The production of this variant represents not only a novel level of regulation of this gene, but also a novel way to fine-tune its biological activity through the versatile functions exerted by the truncated variant compared to the full-length homeoprotein. This study highlights the relevance of generating protein diversity through alternative splicing in the gut and its diseases