The Role of a Key Blood Vessel Protein (SIRT1) in Small Blood Flow and Aging Diseases

NF-κB is a crucial transcriptional regulator of inflammation that is involved in upregulating a wide range of proinflammatory factors, including interleukin (IL)-18, IL-6, and tumor necrosis factor α (TNFα). SIRT1 targets the p65/relA subunit of NF-κB [100]. Deacetylation of this site inhibits nuclear translocation and promotes ubiquitin-proteasome-mediated degradation of NF-κB [101,102]. Microcirculatory inhibition of SIRT1 both directly and indirectly increases the activation of NF-κB, leading to ED, which can be reversed by SIRT1 overexpression or activation [23,24,62,69,103]. Additionally, deletion of toll-like receptor 4, an activator of NF-κB, could attenuate lipid-induced downregulation of SIRT1 expression [24]. In diabetic retinal microvascular endothelial cells (RMECs), NF-κB overactivation upregulates MMP-9, leading to mitochondrial damage, cell death, and subsequent retinopathy [104]. Inflammatory NF-κB overactivation also causes microvascular dysfunction by accelerating the shedding of microvascular endothelial glycocalyx [105], a mechanoresponsive factor in regulating NO-mediated vasodilation [106], which could be prevented by SIRT1 activation. Endothelial SIRT1-deficiency leads to the release of the syndecan-4 ectodomain, producing a profibrogenic signal to nearby endothelial cells, inhibiting eNOS activity and angiogenesis [106,107,108]. Conversely, NF-κB indirectly suppresses SIRT1 activity through its downstream factors. Exposure of intestinal MECs to TNFα, both a target and activator of NF-κB, has been shown to decrease SIRT1 activity associated with reduced NO production and ED [89].

SIRT1 protects cells from ischemia and inflammation by optimizing autophagy, a delicate process involving selective degradation and recycling of intracellular components [8]. In streptozotocin-induced diabetic retinopathy, upregulation of SIRT1 by stachydrine increases autophagy to reduce ROS and inflammation in RMECs [46]. Similarly, sponging of miR-200c-3p by lncRNA MALAT1 upregulated SIRT1 following BMEC Oxygen-Glucose Deprivation/Reperfusion (OGD/R), promoting autophagy and inhibiting apoptosis [70]. On the other hand, SIRT1 also represses autophagy in lung and cerebral Ischemia/Reperfusion (I/R) injury to suppress MEC apoptosis and inflammation [103,109]. Thus, SIRT1 appears to optimize autophagy in MECs under stress to protect against oxidative stress, inflammation, and apoptosis.

Recent evidence suggests a reciprocal regulatory relationship between SIRT1 and autophagy. In senescent cells, SIRT1 interacts more significantly with LC3, upregulating its cytosolic translocation and autophagic degradation [110]. Inhibition of autophagy promotes SIRT1 expression and induces proliferation of endothelial progenitor cells [111]. Similarly, in Parkinson’s disease (PD) mouse models, under oxidative-stress-induced by MPP+, a well-known parkinsonogenic agent, all human sirtuins were identified as autophagy substrates, demonstrating accelerated autophagy-dependent degradation [112]. However, sulfhydration-induced SIRT1 activation upregulated autophagy markers, which attenuated MPP+ induced neuronal damage [113], supporting that autophagic SIRT1 depletion induces senescence, whereas SIRT1-induced autophagy confers cytoprotection. However, SIRT1-induced autophagy may also be detrimental. Zhan et al. found that pulmonary hypoxia/reoxygenation injury leads to miR-141-3p inhibition, which induces SIRT1 activation. Conversely, SIRT1 deacetylates and stabilizes beclin-1 to enhance autophagy, which exacerbates pulmonary microvascular endothelial cell (PMEC) injury [114].

In response to DNA damage, poly (ADP-ribose) polymerase 1 (PARP1) consumes NAD+ to upregulate DNA repair mechanisms. In chronic inflammation, this leads to the depletion of NAD+ and is a major mechanism causing the downregulation of SIRT1 activity [115,116]. Deletion of PARP1 increases NAD+ and SIRT1 activity, thereby promoting mitochondrial function [116]. HG conditions in DM activate PARP, which leads to inhibition of SIRT activity and expression [117]. The loss of SIRT1 due to HG-induced PARP1 activation is a crucial mechanism regulating NF-κB-MMP9-mediated mitochondrial dysfunction in diabetic retinopathy [104]. Conversely, by preserving mitochondrial function and inhibiting mtROS formation, SIRT1 also indirectly suppresses PARP activation in RMECs, accompanied by downregulation of NF-κB activity [118].

Senescent MECs exhibit pro-oxidative phenotypes. Impairment of mitochondrial function is a manifestation of chronic inflammation and oxidative stress, resulting in the production of mtROS and mitochondrial DNA fragments (mtDNA) [39], which induce inflammation and contribute significantly to oxidative stress. SIRT1 is crucial to mitochondrial biogenesis due to its direct interaction with the transcriptional coactivator PGC-1α [119], which stimulates proteins involved in oxidative phosphorylation, mtDNA replication, mitochondrial fusion/fission, and mitophagy. In addition, PGC-1α co-activates transcription factors that are crucial to mitochondrial antioxidative capacity [120]. In BMECs, induction of SIRT1 nuclear translocation by erythropoietin maintains endothelial vascular integrity and prevents apoptosis by regulating mitochondrial function during oxidative stress [121].

Nrf2 is a cytoprotective transcription factor that promotes the expression of major antioxidant proteins SOD, catalase, HO-1, and NQO-1 by binding the antioxidant response element (ARE) in response to oxidative stress [122] and plays a key role in preserving mitochondrial function [123]. SIRT1 directly deacetylates Nrf2 and also promotes its expression [124,125]. In addition, SIRT1 represses the expression of KEAP1, which increases ubiquitination and degradation of Nrf2, and needs to be cleaved for the nuclear translocation of Nrf2 [126]. Additionally, PGC-1α may direct antioxidant gene expression by upregulating Nrf2 [127] and is required for the antioxidant effects of SIRT1. Knockdown of PGC-1α significantly reduces SIRT1-induced activation of the antioxidant proteins MnSOD, catalase, Prx5, TR2, and FOXO3a [128].

In human RMECs, HG-induced miR-221 and miR-34a activation downregulated SIRT1, leading to the loss of mitochondrial biogenesis factors including PGC-1α, Nrf1, Nrf2, and TFAM, mitochondrial antioxidants TrxR2 and SOD2, and premature senescence [42,129], while activation of SIRT1 could prevent HG-induced ROS elevation and apoptosis via activating PGC-1α [43]. SIRT1 also appears to play a significant role in the antioxidative and vasoprotective effects of dulaglutide against HG-induced senescence and ED [91]. Furthermore, activation of SIRT1 by FOXO6 inhibition-mediated CTRP3 upregulation induces Nrf2 signaling to reduce cardiac micro-endothelial barrier permeability and prevent MEC apoptosis [130]. In the cerebral microvascular endothelium, oxidative stress impairs ZO-1, a crucial component of endothelial tight junctions, leading to loss of endothelial integrity [66]. In addition, SIRT1-induced Nrf2 activation upregulated NOX4, HIF-1α, and VEGF expression following intestinal ischemia/reperfusion, leading to increased angiogenesis, cell viability, and migration of human PMECs [131]. A similar relationship between SIRT1 oxidative stress and angiogenesis has been observed in BMECs [77]. This suggests that the mitoprotective and antioxidative interaction between SIRT1 and the PGC-1α/Nrf2 pathway may also contribute to micro-endothelial barrier permeability and angiogenesis/capillarization. In ex vivo human MECs, SIRT1 is also shown to downregulate the expression of pro-oxidant and aging mitochondria proteins p66Shc and arginase II, which further contribute to the maintenance of mitochondrial function, preventing mtROS generation while promoting NO bioavailability and proteins associated with the mitochondrial respiratory chain [10].

Crucially, SIRT1 also regulates the destruction of dysfunctional mitochondria by promoting mitophagy in MECs. Activation of SIRT1 by adiponectin induces stability of PINK1, a molecular marker for abnormal mitochondrial membrane potential, which signals mitophagy of specific dysfunctional mitochondria to suppress PMEC oxidative damage, inflammation, and apoptosis, while knockdown of SIRT1 suppresses PINK1 expression and abolishes the antioxidative effects of mitophagy [44]. Similarly, upregulation of SIRT1/FOXO3a signaling also ameliorated OGD/R-induced cerebral MEC injury by upregulating mitophagy [67].

SIRT1 promotes microvascular angiogenesis primarily via the inhibition of Notch signaling. Upon VEGFR activation, tip-cell Delta-like-ligand 4 (DLL4) binds to stalk-cell Notch, which reduces stalk-cell VEGF sensitivity and inhibits angiogenesis. Meanwhile, tip-cell Jagged 1 (Jag1) competes with DLL4 to promote angiogenesis [132,133]. In endothelial progenitor cells, upregulation of SIRT1 activity by NAMPT following mouse hindlimb ischemia leads to increased SIRT1-dependent deacetylation of the Notch intracellular domain (NICD), inhibiting the DLL4-Notch signaling pathway and upregulating VEGFR-2 and VEGFR-3 to promote angiogenesis. Furthermore, SIRT1 upregulation renders Notch more sensitive to pro-angiogenic Jagged1 as compared to anti-angiogenic DLL4. These changes enhance capillary density and improve post-ischemic blood flow recovery [134]. SIRT1 activation by H2S-induced NAD+ elevation led to interaction with NICD under hypoxic conditions, negatively regulating Notch signaling in micro-endothelial cells to promote sprouting and ameliorate peripheral nerve injury [17,135].

SIRT1 closely interacts with HIF-1α, a master regulator of oxygen homeostasis involved in various pathologies, governing the activity of genes involved in angiogenesis, vascular remodeling, and energy metabolism by targeting VEGF, platelet-derived growth factor (PDGF), and van Willebrand factor (vWF) expression [136,137]. Under normoxia, HIF-1α is inactivated by cytoplasmic ubiquitin-proteasome-mediated degradation. However, under hypoxic conditions, SIRT1 activation stabilizes and accumulates HIF-1α in a low-activity state by blocking p300 recruitment. If SIRT1 is ablated during hypoxia, p300 fully binds to the HIF-1α-HIF-1β complex, leading to a high activity state of HIF-1α target genes [138]. Under hypoxic/HG conditions, linagliptin-induced activation of SIRT1 stabilizes HIF-1α, leading to its accumulation to reduce abnormal proliferation and migration of rat BMECs [92]. Ablation of endothelial SIRT1 under transverse aortic constriction increases protein expression of HIF-1α [55]. Hypoxic SIRT1 activation promotes HIF-1α stability and accumulation in a low-activity state, resulting in the suppression of pathological angiogenesis (Figure 2).

In diabetic microangiopathy, endothelial SIRT1 dysfunction causes capillary loss via enhanced Notch signaling, including upregulated DLL4, Notch target genes Hey1, Hes1, and Notch intracellular domain (NICD) in peritubular and RMECs [32,139], along with diminished NO production [33]. SIRT1 upregulation by NAD+ restored myocardial capillary density and ameliorated myocardial fibrosis [35,56]. HG-exposed MECs overexpress miR-34a, inhibiting SIRT1 and impairing angiogenesis, whereas metformin inhibits miR-34a to activate SIRT1, reversing these effects [140]. In addition, SIRT1 promotes healthy endothelial tube formation and neovascularization by regulating pro-angiogenic chemokine secretion in early outgrowth cells [141]. Simultaneously, a pseudo-hypoxic state induced by diabetic metabolic changes diminishes NAD+ and SIRT1 function, whereas loss of SIRT1 causes HIF-1α activation, resulting in pathological angiogenesis and kidney fibrosis [34].

In diabetic retinopathy, the role of pathological angiogenesis is well characterized. Hypoxic retinal microvascular conditions lead to HIF-1α activation-induced VEGF overproduction, which is associated with abnormal angiogenesis, increased capillary permeability, and retinal dysfunction [36]. Overexpression of SIRT1 suppresses HIF-1α-induced VEGF overproduction [142], resulting in reduced diabetic retinal angiogenesis [27,38], whereas SIRT1 inhibition promotes pathological angiogenesis via HIF-1α activation [37]. This is further supported by the finding that in nondiabetic oxygen-induced-retinopathy, wherein HIF-1α is inactive, deletion of SIRT1 suppresses angiogenesis and retinal revascularization, with a significant downregulation of VEGF-A/VEGFR-2 [143]. As such, SIRT1 seems to promote healthy micro-angiogenesis while inhibiting pathological HIF-1α-induced diabetic angiogenesis.

SIRT1-eNOS also contributes to the maintenance of tissue capillarization. In the kidney, GDNF-modified human adipose mesenchymal stem cell-exosome-induced SIRT1 signaling is associated with increased phosphorylated eNOS, which is positively correlated with peritubular capillary quantity in vivo [33]. In endothelial colony-forming cells, RSV administration increased eNOS expression, NO production, proliferation, and capillary-like outgrowth sprout formation [144].

SIRT1 maintains the endothelial glycocalyx, which plays a crucial role in microcirculatory thrombosis, and prevents its shedding in response to inflammatory stimuli [107]. The endothelial glycocalyx plays a mechanosensory role in the microvascular endothelium to stimulate NO and prostacyclin production [151], and the loss of endothelial glycocalyx is associated with greater platelet adhesion to the endothelial wall [152]. Similarly, upregulation of NO production has been shown to inhibit endothelial adhesion molecule expression, P-selectin, vWF, and platelet activation [151], and endothelial deficiencies in eNOS further exacerbate microvascular thrombosis [153].

SIRT1 interacts with prostacyclin to modulate microvascular thrombosis. Released in response to shear stress, prostacyclin acts similarly to NO to produce a vasodilatory response. Prostacyclin is produced by COX-1 and COX-2. COX-1 also produces pro-thrombotic Thomboxane A2 (TXA2), while COX-2 produces proinflammatory Prostaglandin E2 [154]. It is thought that thromboprotection by low-dose aspirin is derived from shifting the balance between PGI and TXA2 in favor of PGI production [155]. This is supported by the fact that a well-known side effect of selective COX-2 inhibition, the dominant producer of prostacyclin [156], is the increased risk of thrombotic events [157], which have been largely attributed to the inhibition of prostacyclin synthesis [158]. Prostacyclin is positively associated with SIRT1, as antagonism of prostacyclin receptors significantly reduced SIRT1 levels, and SIRT1 inhibition increased the expression of tissue factor, the primary initiator of coagulation, and promoted occlusive thrombi generation in mice [159]; whereas administration of exogenous NAD+ in the form of 1-methylnicotinamide inhibited platelet-mediated thrombosis via prostacyclin activation [160].

SIRT1 is expressed in human platelets, and platelet-specific SIRT1 inhibition causes apoptotic changes and thrombocytopenia due to increased acetylated p53 [161]. However, while SIRT1 is necessary for platelet function, SIRT1 activation/overexpression inhibits the expression of platelet-activating factor receptors and reduces platelet aggregation, attenuating thrombus formation [162]. SIRT1 activation also reduces fibrinogen binding α-granule release and contributes to platelet cytoskeletal reorganization to reduce aggregation [163]. In addition, SIRT1 increases platelet phagocytosis by human endometrial MECs to delay senescence via the deacetylation of Akt [164], showing that SIRT1 also plays a role in regulating platelet lifespan. This is supported by clinical trial evidence, which shows that SIRT1 activation by SRT2104 lowers plasma levels of prothrombin, IL-6, and IL-8 in healthy subjects [165].

RSV, a natural compound found mostly in red grapes, is the most well-known SIRT1 activator and is often treated synonymously with SIRT1 activation. Although RSV seems to provide clear benefits to endothelial function in animals, its effect in human trials has shown inconsistent results, as illustrated in a recent systematic review and meta-analysis [166]. This may be due to its low oral bioavailability [167]. To date, no optimal dose has been established for humans. Many targeted delivery methods have been developed, including lipid nanoparticles [168], cyclodextrin complexes, and nanosponges [169]. Clinical trials have shown that RSV elicits very few adverse reactions with no serious adverse events despite doses of up to 5 g per day. Given this, larger trials of sufficient duration with multi-morbid study populations are needed to clearly elucidate the effects of RSV in humans [166].

Despite its widespread use as a SIRT1 activator, RSV also possesses off-target effects, which may be SIRT1-independent. For instance, carcinogen-induced BMEC inflammation led to the elevation of MMP-9 and COX-2, which was reversible by RSV; however, silencing of SIRT1 did not influence the effects of RSV on MMP-9 and COX-2 [170]. The effects of RSV on SIRT1 activation may also be attenuated in diabetic conditions. In LPS-induced neuroinflammation, micro-endothelial cells subjected to a hyperglycemic state demonstrated reduced SIRT1 expression and attenuated increase in SIRT1 activity following RSV administration compared to normoglycemic and hypoglycemic conditions [171], suggesting that diabetic individuals may require higher doses for therapeutic effect.

Various organic SIRT1 modulators have been shown to protect against streptozotocin-induced diabetic retinopathy models. Coumestrol, an estrogenic phytochemical, increasesSIRT1 expression, suppressing inflammation, oxidative stress, and apoptosis in human RMECs [172]. Wogonoside, a flavonoid, activates SIRT1 and suppresses abnormal angiogenesis, permeability, proliferation, and migration of RMECs [173]. Stachydrine, a plant metabolite, activated AMPK/SIRT1 signaling to upregulate RMEC autophagy alongside suppressing ROS and inflammation [46].

Organic SIRT1 modulators also ameliorated brain injury in middle cerebral artery occlusion/reperfusion (MCAO/R) and OGD/R models of cerebral ischemia. Safranal, the major volatile component of saffron, upregulates SIRT1 expression to promote survival, proliferation, and angiogenesis in rat BMECs following MCAO/R and OGD/R [174]. 14,15-epoxyeicosatrienoic acid, an arachidonic acid metabolite, protected against OGD/R-induced BMEC injury by inducing SIRT1/FOXO3a-mediated mitophagy [67]. Hydroxysafflor Yellow A, a safflower derivative used in Traditional Chinese Medicine (TCM), increased SIRT1 expression, protected BBB integrity, induced angiogenesis, and promoted survival of BMECs following OGD/R [66,175]. Ligustrazine, found in fermented cocoa and natto beans, promotes SIRT1 expression and angiogenesis in BMECs following MCAO/R [176]. In addition, by inhibiting miR-34a-5p to activate SIRT1, ligustrazine is also shown to prevent CMD, platelet activation, and inflammation to alleviate coronary microembolization [54].

Organic SIRT1 modulators also influence physiological regulation by SIRT1 in areas such as lipid metabolism and skeletal muscle function. Boysenberry polyphenols are shown to significantly upregulate endothelial SIRT1, leading to enhanced capillarization and function of BAT, which plays a role in increasing systemic glucose tolerance and thermogenesis [22]. Lycopene, a carotenoid pigment, activated SIRT1, increasing skeletal muscle capillary density and preventing microvascular endothelial damage in aging rats [13]. Through promoting SIRT1 activation, salvianolic acid B, a compound derived from the TCM herb Salvia miltiorrhiza, promoted anti-inflammatory M2 macrophage polarization and angiogenesis, alongside increased muscle capillary density and blood perfusion following mouse limb ischemia [177] (Table 1).

Functionally, the activity of SIRT1 is regulated most significantly by the NAD+/NADH ratio. Thus, boosting cellular NAD+ is a prominent pharmacological strategy for activating SIRT1 [178]. Administration of NAD+ precursors, such as nicotinamide riboside (NR) and NMN, is a popular approach. NMN treatment promotes angiogenesis and suppresses ROS production in CMECs, a process that can be reversed by pharmacological inhibition of SIRT1 via EX-527 [77]. Treatment of mice with NMN also increases exercise-induced skeletal muscle capillary density in elderly mice via SIRT1 activation [17]. Meanwhile, NR has been shown to significantly alleviate intestinal MEC dysfunction by activating the SIRT1-eNOS pathway and reducing ROS production under inflammatory conditions such as TNFα stress and necrotizing enterocolitis [89].

Targeting nicotinamide phosphoribosyl transferase (NAMPT) is another approach to increase biosynthesis of NAD+. FK866 inhibited NAMPT and impaired mobilization of EPCs from the bone marrow upon ischemic stress, reducing angiogenesis and vascular repair, whereas NAMPT overexpression induced opposite effects through SIRT1-dependent enhancement of NICD deacetylation. While silencing SIRT1 reversed the beneficial effects of NAMPT overexpression [134]. Similarly, targeting the nicotinamide mononucleotide adenylyl transferase (NMNAT) pathway for the synthesis and salvage of NAD+ is viable. Intranasal administration of recombinant human NMNAT1 in mice leads to enhanced BBB integrity by upregulating SIRT1 [63].

Besides directly intervening in the NAD+ pathway, studies have also proposed indirect agonism of NAD+-SIRT1 signaling. On its own, H2S induces post-translational sulfhydration of SIRT1, which upregulates its deacetylase activity [179]. The promotion of endogenous H2S production via S-propargyl-cysteine leads to SIRT1 upregulation and microvascular reconstruction following peripheral nerve injury [135]. A combination of sodium hydrosulfide (NaSH), an H2S donor, and NMN supplementation synergistically activates SIRT1 to further promote exercise-induced capillarization of skeletal muscles [17]. Co-administration of H2S boosters and NMN could represent an axis for future research on managing aging-associated frailty (Table 2).

Various hormones and hormone-like substances have also been studied as SIRT1 modulators. Melatonin is a well-known SIRT1 activator [180,181]. In coronary MEC of streptozotocin-induced diabetic mice, melatonin activated AMPK/SIRT1 signaling to reduce oxidative stress and increase antioxidant capacity [41]. Phoenixin 20, a bioactive peptide with hormone-like actions to regulate hypothalamo−pituitary−gonadal hormones and reproduction [182], has been shown to activate SIRT1, leading to inhibition of NLRP3-mediated inflammation and oxidative stress [183]. Desacyl ghrelin, the precursor of ghrelin, protects RMECs from oxidative-stress-induced apoptosis via SIRT1-mediated upregulation of antioxidant enzymes [184].

SRT1720 has been studied as a direct SIRT1 activator in microcirculation, ameliorating obesity-induced ED by promoting antioxidant protein expression while downregulating mitochondrial-aging proteins [10]. SRT1720 also increases PMEC TJ permeability by inhibiting the RhoA/ROCK signaling pathway [148]. In diabetic ulcers, SRT1720 enhanced survival, proliferation, and migration of embryonic artery cluster of differentiation 133+ cells seeded within the wound dressing, which accelerated healing of diabetic ulcers in mice by enhancing wound angiogenesis and capillarization [199].

While most studies have demonstrated that SIRT1 activation exerts microvascular vasoprotection, SIRT1 inhibition may also be beneficial. For instance, SIRT1 inhibition can elicit anti-inflammatory effects. EX-527 is a popular choice for testing the reversibility of the effects of purported SIRT1 activators. However, during the hypo-inflammatory phase of sepsis, SIRT1 inhibition by EX-527 represses adhesion molecule expression in MECs while stabilizing blood pressure and microvascular perfusion [200]. Similarly, inhibition by sirtinol diminished human dermal MEC inflammation by suppressing adhesion molecule expression and chemokine signaling in TNFα, or IL-1β stimulated cells [201]. In addition, SIRT1 inhibition by salermide increased post-ischemic BBB integrity, whereas SIRT1 overexpression had the opposite effect. Suppression of SIRT1 by salermide also attenuated mtROS generation, further preventing post-ischemic BBB permeability and cell damage. In this study, SIRT3, but not SIRT1, protected BBB integrity, and its knockdown significantly increased BBB permeability [60]. Despite this, the mechanisms and circumstances wherein SIRT1 inhibition confers microcirculatory protection are poorly understood and require further investigation.

Several clinically proven drugs have been shown to function as SIRT1 modulators. In diabetic microangiopathy, cilostazol promotes retinal angiogenesis and upregulates SIRT1 via the adiponectin/adipoR signaling pathway, which is necessary for SIRT1 expression [90]. Fenofibrate, a PPARa agonist, increases SIRT1 activity and reduces NF-κB activation to suppress retinal inflammation [202]. Methylene blue, used in the management of methemoglobinemia, protects RMECs against inflammation, apoptosis, and oxidative stress by increasing SIRT1 expression [26].

In the brain, dihydroartemisinin, an antimalarial, ameliorated MCAO/R-induced cerebral injury by upregulating autophagy via SIRT1/FOXO1 signaling [109]. Similarly, donepezil ameliorates BMEC dysfunction following OGD/R by activating SIRT1/FOXO3a/NFkB signaling to promote migration angiogenesis and reduce BBB permeability [69]. Furthermore, SIRT1 activation by cystatin C could shift APP processing toward a non-amyloidogenic pathway in BMECs, providing a potential target for the treatment of Alzheimer’s disease [79].

Evidence suggests that the microcirculatory effects of SIRT1 are heavily involved in the pharmacological mechanism of various antidiabetics, such as gliquidone, metformin, GLP-1 agonists, and DPP-4 inhibitors. Expression of SIRT1 is significantly increased by metformin in MECs under HG conditions, which modulates downstream SIRT1 pathways to alleviate HG-induced senescence, apoptosis, oxidative stress, and inflammation [118,203]. In diabetic retinopathy, gliquidone regulated Notch-mediated angiogenesis to alleviate retinal injury via SIRT1 [139]. Similarly, SIRT1 is involved in the neurological and retinal protective effects of DPP-4 inhibitors and GLP-1 agonists. GLP-1 activation increases SIRT1 to ameliorate HG-induced RMEC senescence, inflammation, and angiogenesis and improves eNOS function. Specifically, SIRT1 activation by dulaglutide contributes to genome stability by restoring telomerase activity [91,204]. On the other hand, research on DPP-4 inhibitors also demonstrated its neuroprotective effects via SIRT1 activation in diabetes, improving angiogenesis, mitochondrial function, and NO bioavailability while suppressing oxidative stress, abnormal proliferation, and migration of BMECs under HG, ischemia, and even psychological stress [80,92,205].

Hydrogen-rich saline has been shown to elicit anti-inflammatory effects by upregulating the SIRT1/NF-κB signaling pathway to prevent microvascular endothelial glycocalyx shedding and maintain micro-endothelial function in sepsis-induced kidney injury [105]. Similarly, Tubastatin A, a histone deacetylase 6 inhibitor, has been shown to increase SIRT1 expression and alleviate oxidative stress in DR [206]. Many compounds and existing drugs are emerging as SIRT1 modulators, and further studies on their mechanisms may reveal great insights into the link between microvascular aging and disease.