Endothelial-Specific Reduction in Arf6 Impairs Insulin-Stimulated Vasodilation and Skeletal Muscle Blood Flow Resulting in Systemic Insulin Resistance in Mice

BACKGROUND

Much of what we know about insulin resistance is based on studies from metabolically active tissues such as the liver, adipose tissue, and skeletal muscle. Emerging evidence suggests that the vascular endothelium plays a crucial role in systemic insulin resistance; however, the underlying mechanisms remain incompletely understood. Arf6 (ADP ribosylation factor 6) is a small GTPase that plays a critical role in endothelial cell function. Here, we tested the hypothesis that the deletion of endothelial Arf6 will result in systemic insulin resistance.

METHODS

We used mouse models of constitutive endothelial cell-specific Arf6 deletion (Arf6f/- Tie2Cre+) and tamoxifen-inducible Arf6 knockout (Arf6f/f Cdh5CreER+). Endothelium-dependent vasodilation was assessed using pressure myography. Metabolic function was assessed using a battery of metabolic assessments including glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps. We used a fluorescence microsphere-based technique to measure tissue blood flow. Skeletal muscle capillary density was assessed using intravital microscopy.

RESULTS

Endothelial Arf6 deletion impaired insulin-stimulated vasodilation in white adipose tissue and skeletal muscle feed arteries. The impairment in vasodilation was primarily due to attenuated insulin-stimulated nitric oxide bioavailability but independent of altered acetylcholine-mediated or sodium nitroprusside-mediated vasodilation. Endothelial cell-specific deletion of Arf6 also resulted in systematic insulin resistance in normal chow-fed mice and glucose intolerance in high-fat diet-fed obese mice. The underlying mechanisms of glucose intolerance were reductions in insulin-stimulated blood flow and glucose uptake in the skeletal muscle and were independent of changes in capillary density or vascular permeability.

CONCLUSIONS

Results from this study support the conclusion that endothelial Arf6 signaling is essential for maintaining insulin sensitivity. Reduced expression of endothelial Arf6 impairs insulin-mediated vasodilation and results in systemic insulin resistance. These results have therapeutic implications for diseases that are associated with endothelial cell dysfunction and insulin resistance such as diabetes.

Endothelial cell-specific reduction in mTOR ameliorates age-related arterial and metabolic dysfunction

Systemic inhibition of the mammalian target of rapamycin (mTOR) delays aging and many age-related conditions including arterial and metabolic dysfunction. However, the mechanisms and tissues involved in these beneficial effects remain largely unknown. Here, we demonstrate that activation of S6K, a downstream target of mTOR, is increased in arteries with advancing age, and that this occurs preferentially in the endothelium compared with the vascular smooth muscle. Induced endothelial cell-specific deletion of mTOR reduced protein expression by 60-70%. Although this did not significantly alter arterial and metabolic function in young mice, endothelial mTOR reduction reversed arterial stiffening and improved endothelium-dependent dilation (EDD) in old mice, indicating an improvement in age-related arterial dysfunction. Improvement in arterial function in old mice was concomitant with reductions in arterial cellular senescence, inflammation, and oxidative stress. The reduction in endothelial mTOR also improved glucose tolerance in old mice, and this was associated with attenuated hepatic gluconeogenesis and improved lipid tolerance, but was independent of alterations in peripheral insulin sensitivity, pancreatic beta cell function, or fasted plasma lipids in old mice. Lastly, we found that endothelial mTOR reduction suppressed gene expression of senescence and inflammatory markers in endothelial-rich (i.e., lung) and metabolically active organs (i.e., liver and adipose tissue), which may have contributed to the improvement in metabolic function in old mice. This is the first evidence demonstrating that reducing endothelial mTOR in old age improves arterial and metabolic function. These findings have implications for future drug development.

Reduction of double-strand DNA break repair exacerbates vascular aging

Advanced age is the greatest risk factor for cardiovascular disease (CVD), the leading cause of death. Arterial function is impaired in advanced age which contributes to the development of CVD. One underexplored hypothesis is that DNA damage within arteries leads to this dysfunction, yet evidence demonstrating the incidence and physiological consequences of DNA damage in arteries, and in particular, in the microvasculature, in advanced age is limited. In the present study, we began by assessing the abundance of DNA damage in human and mouse lung microvascular endothelial cells and found that aging increases the percentage of cells with DNA damage. To explore the physiological consequences of increases in arterial DNA damage, we evaluated measures of endothelial function, microvascular and glycocalyx properties, and arterial stiffness in mice that were lacking or heterozygous for the double-strand DNA break repair protein ATM kinase. Surprisingly, in young mice, vascular function remained unchanged which led us to rationalize that perhaps aging is required to accumulate DNA damage. Indeed, in comparison to wild type littermate controls, mice heterozygous for ATM that were aged to ~18 mo (Old ATM +/-) displayed an accelerated vascular aging phenotype characterized by increases in arterial DNA damage, senescence signaling, and impairments in endothelium-dependent dilation due to elevated oxidative stress. Furthermore, old ATM +/- mice had reduced microvascular density and glycocalyx thickness as well as increased arterial stiffness. Collectively, these data demonstrate that DNA damage that accumulates in arteries in advanced age contributes to arterial dysfunction that is known to drive CVD.

Endothelial cell telomere dysfunction induces senescence and results in vascular and metabolic impairments

In advanced age, increases in oxidative stress and inflammation impair endothelial function, which contributes to the development of cardiovascular disease (CVD). One plausible source of this oxidative stress and inflammation is an increase in the abundance of senescent endothelial cells. Cellular senescence is a cell cycle arrest that occurs in response to various damaging stimuli. In the present study, we tested the hypothesis that advanced age results in endothelial cell telomere dysfunction that induces senescence. In both human and mouse endothelial cells, advanced age resulted in an increased abundance of dysfunctional telomeres, characterized by activation of DNA damage signaling at telomeric DNA. To test whether this results in senescence, we selectively reduced the telomere shelterin protein telomere repeat binding factor 2 (Trf2) from endothelial cells of young mice. Trf2 reduction increased endothelial cell telomere dysfunction and resulted in cellular senescence. Furthermore, induction of endothelial cell telomere dysfunction increased inflammatory signaling and oxidative stress, resulting in impairments in endothelial function. Finally, we demonstrate that endothelial cell telomere dysfunction-induced senescence impairs glucose tolerance. This likely occurs through increases in inflammatory signaling in the liver and adipose tissue, as well as reductions in microvascular density and vasodilation to metabolic stimuli. Cumulatively, the findings of the present study identify age-related telomere dysfunction as a mechanism that leads to endothelial cell senescence. Furthermore, these data provide compelling evidence that senescent endothelial cells contribute to age-related increases in oxidative stress and inflammation that impair arterial and metabolic function.

Endothelial specific reduction in Arf6 impairs insulin-stimulated vasodilation and skeletal muscle blood flow resulting in systemic insulin resistance

Background

Much of what we know about insulin resistance is based on studies from metabolically active tissues such as liver, adipose tissue, and skeletal muscle. Emerging evidence suggests that the vascular endothelium plays a crucial role in systemic insulin resistance, however, the underlying mechanisms remain incompletely understood. ADP ribosylation factor 6 (Arf6) is a small GTPase that plays a critical role in endothelial cell (EC) function. Here, we tested the hypothesis that the deletion of endothelial Arf6 will result in systemic insulin resistance.

Methods

We used mouse models of constitutive EC-specific Arf6 deletion (Arf6 f/- Tie2Cre) and tamoxifen inducible Arf6 knockout (Arf6 f/f Cdh5Cre). Endothelium-dependent vasodilation was assessed using pressure myography. Metabolic function was assessed using a battery of metabolic assessments including glucose- and insulin-tolerance tests and hyperinsulinemic-euglycemic clamps. A fluorescence microsphere-based technique was used to measure tissue blood flow. Intravital microscopy was used to assess skeletal muscle capillary density.

Results

Endothelial Arf6 deletion impaired insulin-stimulated vasodilation in white adipose tissue (WAT) and skeletal muscle feed arteries. The impairment in vasodilation was primarily due to attenuated insulin-stimulated nitric oxide (NO) bioavailability but independent of altered acetylcholine- or sodium nitroprusside-mediated vasodilation. In vitro Arf6 inhibition resulted in suppressed insulin stimulated phosphorylation of Akt and endothelial NO synthase. Endothelial cell-specific deletion of Arf6 also resulted in systematic insulin resistance in normal chow fed mice and glucose intolerance in high fat diet fed obese mice. The underlying mechanisms of glucose intolerance were reductions in insulin-stimulated blood flow and glucose uptake in the skeletal muscle and were independent of changes in capillary density or vascular permeability.

Conclusion

Results from this study support the conclusion that endothelial Arf6 signaling is essential for maintaining insulin sensitivity. Reduced expression of endothelial Arf6 impairs insulin-mediated vasodilation and results in systemic insulin resistance. These results have therapeutic implications for diseases that are associated with endothelial cell dysfunction and insulin resistance such as diabetes.

Pyridoxamine treatment ameliorates large artery stiffening and cerebral artery endothelial dysfunction in old mice

Age-related increases in large artery stiffness are associated with cerebrovascular dysfunction and cognitive impairment. Pyridoxamine treatment prevents large artery stiffening with advancing age, but the effects of pyridoxamine treatment on the cerebral vasculature or cognition is unknown. The purpose of this study was to investigate the effects of pyridoxamine on blood pressure, large artery stiffness, cerebral artery function, and cognitive function in old mice. Old male C57BL/6 mice consumed either pyridoxamine (2 g/L) or vehicle control in drinking water for ∼7.5 months and were compared with young male C57BL/6 mice. From pre- to post-treatment, systolic blood pressure increased in old control mice, but was maintained in pyridoxamine treated mice. Large artery stiffness decreased in pyridoxamine-treated mice but was unaffected in control mice. Pyridoxamine-treated mice had greater cerebral artery endothelium-dependent dilation compared with old control mice, and not different from young mice. Old control mice had impaired cognitive function; however, pyridoxamine only partially preserved cognitive function in old mice. In summary, pyridoxamine treatment in old mice prevented age-related increases in blood pressure, reduced large artery stiffness, preserved cerebral artery endothelial function, and partially preserved cognitive function. Taken together, these results suggest that pyridoxamine treatment may limit vascular aging.

T cells mediate cell non-autonomous arterial ageing in mice

KEY POINTS

Increased large artery stiffness and impaired endothelium-dependent dilatation occur with advanced age. We sought to determine whether T cells mechanistically contribute to age-related arterial dysfunction. We found that old mice exhibited greater proinflammatory T cell accumulation around both the aorta and mesenteric arteries. Pharmacologic depletion or genetic deletion of T cells in old mice resulted in ameliorated large artery stiffness and greater endothelium-dependent dilatation compared with mice with T cells intact.

ABSTRACT

Ageing of the arteries is characterized by increased large artery stiffness and impaired endothelium-dependent dilatation. T cells contribute to hypertension in acute rodent models but whether they contribute to chronic age-related arterial dysfunction is unknown. To determine whether T cells directly mediate age-related arterial dysfunction, we examined large elastic artery and resistance artery function in young (4-6 months) and old (22-24 months) wild-type mice treated with anti-CD3 F(ab'2) fragments to deplete T cells (150 μg, i.p. every 7 days for 28 days) or isotype control fragments. Old mice exhibited greater numbers of T cells in both aorta and mesenteric vasculature when compared with young mice. Old mice treated with anti-CD3 fragments exhibited depletion of T cells in blood, spleen, aorta and mesenteric vasculature. Old mice also exhibited greater numbers of aortic and mesenteric IFN-γ and TNF-α-producing T cells when compared with young mice. Old control mice exhibited greater large artery stiffness and impaired resistance artery endothelium-dependent dilatation in comparison with young mice. In old mice, large artery stiffness was ameliorated with anti-CD3 treatment. Anti-CD3-treated old mice also exhibited greater endothelium-dependent dilatation than age-matched controls. We also examined arterial function in young and old Rag-1^-/- mice, which lack lymphocytes. Rag-1^-/- mice exhibited blunted increases in large artery stiffness with age compared with wild-type mice. Old Rag-1^-/- mice also exhibited greater endothelium-dependent dilatation compared with old wild-type mice. Collectively, these results demonstrate that T cells play an important role in age-related arterial dysfunction.

Aging differentially impacts vasodilation and angiogenesis in arteries from the white and brown adipose tissues

Aging adipose tissues (ATs) manifest reduced vascularity and increased hypoxia and inflammation that contribute to local and systemic metabolic dysfunction. However, the mechanisms that underlie these age-related changes are incompletely understood. In this study, we sought to examine insulin-stimulated vasodilation and angiogenesis in the arterial vasculature from three major AT depots, perigonadal white (pgWAT), subcutaneous white (scWAT) and brown (BAT) from young and old mice. Here, we demonstrate that in young mice, insulin-stimulated vasodilation is lower in feed arteries from pgWAT compared to scWAT (p < 0.05), but no differences were found between feed arteries in other AT depots (p > 0.05). Insulin-stimulated vasodilation was lower in old compared to young feed arteries from all three AT depots (p < 0.05 for all). In the presence of endothelial nitric oxide synthase inhibitor, L-NAME, insulin-stimulated vasodilation was decreased in young (p < 0.05), but was unaffected in old (p > 0.05) from all AT depots. We also observed no age-related differences in endothelium-independent dilation, as assessed by sodium nitroprusside (p > 0.05). We next investigated angiogenic capacity of the vasculature in these AT depots. In young mice, BAT vasculature demonstrated the highest angiogenic potential, followed by pgWAT and scWAT. We found that aging decreased angiogenic sprout formation in pgWAT and BAT (both p < 0.05), but increased angiogenic potential in scWAT (p < 0.05), indicating dissimilar impact of aging on angiogenesis in different AT depots. Collectively, these data suggest that aging leads to a consistent impairment in insulin-stimulated vasodilation and reduction in NO bioavailability in all three AT, although aging differentially impacts angiogenic capacity across different AT depots.

Impact of high-fat diet on vasoconstrictor reactivity of white and brown adipose tissue resistance arteries

Blood flow regulation is a critical factor for tissue oxygenation and substrate supply. Increased reactivity of arteries to vasoconstrictors may increase vascular resistance, resulting in reduced blood flow. We aimed to investigate the effect of a high-fat (HF) diet on stiffness and vasoconstrictor reactivity of white adipose tissue (WAT) and brown adipose tissue (BAT) resistance arteries and also investigated the interconversion of both adipose depots in the setting of a HF diet. Vasoconstrictor reactivity and passive morphology and mechanical properties of arteries from B6D2F1 mice (5 mo old) fed normal chow (NC) or a HF diet (8 wk) were measured using pressure myography. Receptor gene expression in WAT and BAT arteries and markers of WAT and BAT were assessed in whole tissue lysates by real-time RT-PCR. Despite greater receptor-independent vasoconstriction (in response to KCl, P < 0.01), vasoconstriction in response to angiotensin II ( P < 0.01) was lower in NC-BAT than NC-WAT arteries and similar in response to endothelin-1 ( P = 0.07) and norepinephrine ( P = 0.11) in NC-BAT and NC-WAT arteries. With the exception of BAT artery reactivity to endothelin-1 and angiotensin II, the HF diet tended to attenuate reactivity in arteries from both adipose depots and increased expression of adipose markers in BAT. No significant differences in morphology or passive mechanical properties were found between adipose types or diet conditions. Alterations in gene expression of adipose markers after the HF diet suggest beiging of BAT. An increase in brown adipocytes in the absence of increased BAT mass may be a compensatory mechanism to dissipate excess energy from a HF diet. NEW & NOTEWORTHY Despite no differences in passive mechanical properties and greater receptor-independent vasoconstriction, receptor-mediated vasoconstriction was either lower in brown than white adipose tissue arteries or similar in brown and white adipose tissue arteries. A high-fat diet has a greater impact on vasoconstrictor responses in white adipose tissue but leads to altered adipose tissue gene expression consistent with beiging of the brown adipose tissue.

The impact of ageing on adipose structure, function and vasculature in the B6D2F1 mouse: evidence of significant multisystem dysfunction

The critical influence of the white adipose tissue (WAT) on metabolism is well-appreciated in obesity, but adipose tissue dysfunction as a mechanism underlying age-associated metabolic dysfunction requires elucidation. To explore this possibility, we assessed metabolism and measures of epididymal (e)WAT mitochondria and artery function in young (6.1 ± 0.4 months) and old (29.6 ± 0.2 months) B6D2F1 mice. There were no group differences in average daily oxygen consumption, fasted blood glucose or plasma free fatty acids, but fasted plasma insulin and the homeostatic model assessment of insulin resistance (HOMA-IR%) were higher in the old (∼50-85%, P < 0.05). Tissue mass (P < 0.05) and adipocyte area were lower (∼60%) (P < 0.01) and fibrosis was greater (sevenfold, P < 0.01) in eWAT with older age. The old also exhibited greater liver triglycerides (∼60%, P < 0.05). The mitochondrial respiratory oxygen flux after the addition of glutamate and malate (GM), adenosine diphosphate (d), succinate (S) and octanoyl carnitine (O) were one- to twofold higher in eWAT of old mice (P < 0.05). Despite no change in the respiratory control ratio, substrate control ratios of GMOd/GMd and GMOSd/GMd were ∼30-40% lower in old mice (P < 0.05) and were concomitant with increased nitrotyrosine (P < 0.05) and reduced expression of brown adipose markers (P < 0.05). Ageing reduced vascularity (∼50%, P < 0.01), angiogenic capacity (twofold, P < 0.05) and expression of vascular endothelial growth factor (∼50%, P < 0.05) in eWAT. Finally, endothelium-dependent dilation was lower (P < 0.01) in isolated arteries from eWAT arteries of the old mice. Thus, metabolic dysfunction with advancing age occurs in concert with dysfunction in the adipose tissue characterized by both mitochondrial and arterial dysfunction.

Life-long caloric restriction reduces oxidative stress and preserves nitric oxide bioavailability and function in arteries of old mice

Aging impairs arterial function through oxidative stress and diminished nitric oxide (NO) bioavailability. Life-long caloric restriction (CR) reduces oxidative stress, but its impact on arterial aging is incompletely understood. We tested the hypothesis that life-long CR attenuates key features of arterial aging. Blood pressure, pulse wave velocity (PWV, arterial stiffness), carotid artery wall thickness and endothelium-dependent dilation (EDD; endothelial function) were assessed in young (Y: 5-7 month), old ad libitum (Old AL: 30-31 month) and life-long 40% CR old (30-31 month) B6D2F1 mice. Blood pressure was elevated with aging (P < 0.05) and was blunted by CR (P < 0.05 vs. Old AL). PWV was 27% greater in old vs. young AL-fed mice (P < 0.05), and CR prevented this increase (P < 0.05 vs. Old AL). Carotid wall thickness was greater with age (P < 0.05), and CR reduced this by 30%. CR effects were associated with amelioration of age-related changes in aortic collagen and elastin. Nitrotyrosine, a marker of cellular oxidative stress, and superoxide production were greater in old AL vs. young (P < 0.05) and CR attenuated these increase. Carotid artery EDD was impaired with age (P < 0.05); CR prevented this by enhancing NO and reducing superoxide-dependent suppression of EDD (Both P < 0.05 vs. Old AL). This was associated with a blunted age-related increase in NADPH oxidase activity and p67 expression, with increases in superoxide dismutase (SOD), total SOD, and catalase activities (All P < 0.05 Old CR vs. Old AL). Lastly, CR normalized age-related changes in the critical nutrient-sensing pathways SIRT-1 and mTOR (P < 0.05 vs. Old AL). Our findings demonstrate that CR is an effective strategy for attenuation of arterial aging.