Sirtuins and NAD+ in the Development and Treatment of Metabolic and Cardiovascular Diseases

Abstract

The sirtuin family of nicotinamide adenine dinucleotide-dependent deacylases (SIRT1-7) are thought to be responsible, in large part, for the cardiometabolic benefits of lean diets and exercise and when upregulated can delay key aspects of aging. SIRT1, for example, protects against a decline in vascular endothelial function, metabolic syndrome, ischemia-reperfusion injury, obesity, and cardiomyopathy, and SIRT3 is protective against dyslipidemia and ischemia-reperfusion injury. With increasing age, however, nicotinamide adenine dinucleotide levels and sirtuin activity steadily decrease, and the decline is further exacerbated by obesity and sedentary lifestyles. Activation of sirtuins or nicotinamide adenine dinucleotide repletion induces angiogenesis, insulin sensitivity, and other health benefits in a wide range of age-related cardiovascular and metabolic disease models. Human clinical trials testing agents that activate SIRT1 or boost nicotinamide adenine dinucleotide levels are in progress and show promise in their ability to improve the health of cardiovascular and metabolic disease patients.

Keywords: aging; atherosclerosis; cardiomyopathies; dyslipidemias; insulin resistance; metabolic syndrome; obesity.

Figure 1.. Synthesis, salvage and metabolism of NAD⁺.

NAD⁺ supplies in the body come from either de novo synthesis, or via salvage pathways. NAD⁺ is synthesized de novo from dietary tryptophan via a series of enzymatic reactions ultimately resulting in the production of nicotinic acid adenine dinucleotide (NAAD), which is converted to NAD⁺ by NAD-synthase. However, in the heart the vast majority of NAD⁺ is synthesized via salvage routes from nicotinamide (NAM), nicotinamide riboside (NR) and nicotinic acid (NA). NAM and NR are converted to nicotinamide mononucleotide (NMN) by nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide riboside kinases (NRKs) respectively. NMN is then converted to NAD⁺ by nicotinamide mononucleotide adenylyltransferease (NMNAT)2 in the cytoplasm, NMNAT3 in the mitochondria, and NMNAT1 in the nucleus. NA is converted, via a series of reactions into NAAD and finally to NAD⁺ by NAD-synthase. NAD⁺ is a necessary substrate of sirtuin deacetylation reactions, by which it is converted into NAM. NAD⁺ is broken down by sterile α and TIR motif–containing protein 1 (SARM1), cluster of differentiation 38 (CD38), Poly ADP-ribose polymerase 1 (PARP1) and SIRTS1–7. SIRTs 3, 4 and 5 are located in the mitochondria, SIRT 2 is located in the cytoplasm, SIRT6 and 7 are found in the nucleus and SIRT1 is found in both the cytoplasm and nucleus. Sirtuins and NAD⁺ have a variety of roles in cardiac and vascular cells that can effect cardiovascular function. For example, SIRT3 acts to block opening of the mitochondrial permeability transition pore (mPTP), to reduce cell swelling and hypertrophy. Figure adapted from Rajman et al 2018 and Bonkowksi & Sinclair 2016, using adapted images from http://smart.servier.com.

Figure 2.. The role of Sirtuins and NAD⁺ in the aging vasculature.

(A) A young blood vessel shows healthy vascular smooth muscle cells (VSMCs), lined with health endothelial cells. Stimulation with nitric oxide (NO) results in vessel contraction. Angiogenesis, the growth of new blood vessels, is induced when vascular endothelial growth factor (VEGF) acts on SIRT1 to relieve notch intracellular domain (NICD) inhibition of Notch. (B) Aging results in changes to the structure and function of the vasculature. With increasing age there are increased atherosclerotic plaques, increased low density lipoprotein (LDL) cholesterol in the blood stream, decreased sprouting angiogenesis, increased thrombosis, increased oxidative stress, decreased nitric oxide, increased inflammation, senescence of endothelial and VSMCs, arterial thickening and fibrosis. These changes contribute to reduced blood flow, and reduced reactivity of the blood vessels in aging. Sirtuins and NAD⁺ can improve or reverse some aspects of vascular aging (marked in blue on the figure) through modulation of targets (marked in brown). In particular SIRT1 decreases LDL cholesterol levels, reduces vascular cell senescence, increases NO through endothelial nitric oxide synthase (eNOS), reduces inflammation through nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) modulation, decreases oxidative stress by modulating FOXOs and thrombosis via tissue factor (TF) and peroxisome proliferator-activated receptor delta (PPARδ), and increases angiogenesis through modulation of FOXO1 and NICD-Notch. SIRT3 also has a protective role in angiogenesis and NAD⁺ reduces vascular inflammation. Figure made using adapted images from http://smart.servier.com.

Figure 3.. The role of SIRT1 and NAD⁺ in age-related endothelial dysfunction.

In young healthy capillaries angiogenesis occurs after stimulation of endothelial cells with VEGF released from myocytes. Characteristic changes of the vasculature with old age include dysfunctional endothelial cells, and reduced VEGF-induced angiogenesis. These age-related changes result in reduced capillary density, reduced blood flow and ultimately reduced mobility and endurance. A mechanism of this reduced angiogenesis in old age is reduced endothelial SIRT1 and NAD⁺, which ostensibly reduces the sensitivity of endothelial cells to VEGF. Increasing the hydrogen sulfide (H₂S)-NAD⁺-SIRT1 axis in endothelial cells, either with NaHS to boost H₂S, NMN to boost NAD+ or with exercise, causes SIRT1-dependent inhibition of Notch Intracellular Domain (NICD) which results in increased sprouting angiogenesis. These interventions can reverse endothelial dysfunction and increase angiogenesis in old age, to return capillary density and endurance to the levels of that seen in young mice. Figure adapted from Das et al 2018, using adapted images from http://smart.servier.com.

Figure 4.. The role of Sirtuins and NAD⁺ in age-related metabolic and heart diseases.

In old age there is increased risk of cardiovascular and metabolic diseases including fatty liver, dyslipidemia, obesity, type 2 diabetes, hypertension, arrhythmias, fibrosis, hypertrophy and ischemia/reperfusion injury. Sirtuins and NAD⁺ have roles in protecting against or preventing the development of these diseases (shown in blue on figure), through modulation of a variety of proteins (shown in brown on figure). SIRT1, 3, 6 and NAD⁺ have roles in modulating dyslipidemia, with the liver X receptor (LXR) and malonyl-CoA decarboxylase (MCD) identified as targets, and SIRT1, 3, 4 and 6 have roles in preventing obesity. SIRT1, 3, 4, 6, 7 and NAD⁺ may have protective roles in Type 2 Diabetes and identified targets include mitochondrial uncoupling protein 2 (UCP2), PGC1α and FOXO1. SIRT1 and 3 have protective roles in hypertension, and SIRT1, 2, 3, 5, 7 and NAD⁺ have been associated with reduced cardiac fibrosis. SIRT1, 2, 3, 4, 6, 7 and NAD⁺ protect against cardiac hypertrophy with identified targets including the mitochondrial permeability transition pore (mPTP), insulin-like growth factor (IGF)-1, PPARα, eukaryotic initiation Factor 2 (elF2a), FOXO3a and AMP-activated protein kinase (AMPK). SIRT1, 3, 5 and NAD⁺ protect against ischemia/reperfusion injury via FOXO1, manganese superoxide dismutase (MnSOD), Bcl-2-associated X protein (Bax) and succinate dehydrogenase (SDH). SIRT1, 2 and NAD⁺ improve arrhythmias via sodium channel Nav1.5 and BubR1. Figure made using adapted images from http://smart.servier.com.