NAD metabolism in oncogene-induced senescence and mitochondrial dysfunction-associated senescence

The uppermost part of the pathway includes part of the general NAM salvage pathway in the cytosol as it is relevant to senescence-induced changes to NAD metabolism. In this pathway, NAD levels are maintained through recycling back to NAD from nicotinamide (NAM) and nicotinamide mononucleotide (NMN) (Braidy et al., 2019). The conversion from NAM to NMN is catalyzed by nicotinamide phosphoribosyltransferase (NAMPT), while the conversion from NMN to NAD is catalyzed by nicotinamide mononucleotide adenylyl transferases (NMNATs). Other sources, such as nicotinic acid (NA) and nicotinamid riboside (NR), are not shown here as they are not affected by senescence, at least from current research. OIS-specific interactions are highlighted in orange, while MiDAS-specific interactions are highlighted in purple. General interactions for both (or other senescent types) remain a black color. The OIS pathway, induced by Ras singalling in this case, results in the upregulation of HMGA1, and stimulation of the NAMPT enzyme (Nacarelli et al., 2019). Resulting increased levels of NMN (the direct metabolite of NAMPT) translate to increased NAD levels, and a high NAD-NADH ratio. This leads to decreased ADP-ATP levels, which causes a decreased phosphorylated AMPK expression (Nacarelli et al., 2019). This interaction causes increased p38 and p65 activation, and increased NF-κB activity. The NF-κB signalling pathway has been known to play a key role in the promotion of the proinflammatory SASP (Freund et al., 2011). Furthermore, this is correlated with increased expression of interleukins IL1B, IL6 and IL8, all key factors in the proinflammatory wave of the SASP. In addition, Nacarelli et al. (2019) found that the proinflammatory environment created as a result of the increased NAD-NADH ratio leads to acceleration of cancer progression. NAMPT upregulation through HMGA1 also resulted in the expression of senescence markers SA-ß-gal, p16 and p21. The resulting phenotype from this high NAD-NADH ratio is a high proinflammatory SASP. Malate is another important metabolite in redox reactions and in many senescence types, including OIS and MiDAS. Of interest to NAD metabolism is the malate-aspartate shuttle, where NADH is transferred from the cytosol to the mitochondrial matrix through malate dehydrogenase 1 (MDH1) (Lee et al., 2012). In senescence, levels of MDH1 decrease. On the other hand, decreased activity of MDH1 can induce a senescence response. This reduction in MDH1 activity results in a decreased cytosolic NAD-NADH. Lastly, this inhibition may result in loss of cell proliferation due to the requirement of aspartate synthesis in response to inhibition of the electron transport chain (Birsoy et al., 2015). Mitochondrial dysfunction-associated senescence (MiDAS), on the other hand, causes a decrease in the NAD-NADH ratio, which induces three main responses: (1) the inhibition of sirtuins, (2) the activation of AMPK and (3) the inhibition of PARP which blocks the NF-kB pathway. First, low levels of NAD+ decrease sirtuin activity. A decrease in the activity of SIRT3 and SIRT5, located in the mitochondria, is associated with the activation of cell senescence (Wiley et al., 2016)). Second, a decreased NAD+/NADH ratio activates AMPK and p53, which inhibits the RNA binding protein Hu antigen R (HuR) from degrading the mRNAs encoding the cyclin-dependent kinase inhibitors, p21 and p16INK4a. This increases the activity of the pRB tumor suppressor, resulting in cell proliferation and growth arrest ((Wiley et al., 2016)). Additionally, p53 activation leads to the release of SASPs that lack IL-1-dependent factors but include the secretion of anti-inflammatory cytokine IL-10 and high levels of the pro-inflammatory cytokines CCL27 and TNF-α (Wiley et al., 2016). The activation of p53 also reduces glycolysis and promotes mitochondrial respiration, by inhibiting phosphoglycercate mutase (PGM) and inducing the expression of synthesis of cytochrome c oxidase 2 (SCO2). Furthermore, p53 activation inhibits the pentose phosphate pathway (PPP) by binding to glucose-6-phosphate dehydrogenase (G6PDH). Lastly, the low NAD+,NADH ratio inhibits ADP-ribose donor for poly-ADP ribose polymerase (PARP), which consecutively inhibits the NF-kB pathway. A downregulated NF-κB pathway then contributes to the pathogenic processes of various inflammatory diseases as well as the expression of various proinflammatory SASPs (Liu et al., 2017). As visible in this pathway, when senescence is induced by either OIS or MiDAS distinguishable effects on NAD metabolism are evident. Not only do these stimuli release distinct SASPs, but they exhibit distinct responses on the NAD-NADH ratio and subsequent related pathways. While MiDAS leads to a decrease in the NAD-NADH ratio, OIS causes an increase in this ratio and the NAD+ levels.

该途径的最高部分包括细胞质中与衰老诱导的NAD代谢变化相关的部分NAM修复途径。在此途径中，NAD水平通过将其从烟酰胺（NAM）和烟酰胺单核苷酸（NMN）（Braidy等，2019年）循环回收至NAD而得以维持。从NAM到NMN的转化由烟酰胺磷酸核糖转移酶（NAMPT）催化，而从NMN到NAD的转化则由烟酰胺单核苷酸腺苷转移酶（NMNATs）催化。其他来源，如烟酸（NA）和烟酰胺核糖苷（NR），在此未展示，因为它们不受衰老的影响，至少根据目前的研究。OIS特异性相互作用以橙色突出显示，而MiDAS特异性相互作用则以紫色突出。两者（或其他衰老类型）的通用相互作用保持黑色。由Ras信号诱导的OIS途径导致HMGA1的上调，以及NAMPT酶（Nacarelli等，2019年）的刺激。NAMPT的直接代谢产物NMN水平的增加导致NAD水平的增加，以及NAD-NADH比率的升高。这导致ADP-ATP水平降低，从而引起磷酸化AMPK表达降低（Nacarelli等，2019年）。这种相互作用导致p38和p65的激活增加，以及NF-κB活性的增加。NF-κB信号通路已被证明在促进促炎SASP（Freund等，2011年）的生成中起着关键作用。此外，这与IL1B、IL6和IL8等促炎SASP炎症波的关键因子的表达增加相关。此外，Nacarelli等（2019年）发现，由于NAD-NADH比率的增加而形成的促炎环境导致癌症进展的加速。HMGA1通过NAMPT的上调还导致衰老标志物SA-β-gal、p16和p21的表达。这种高NAD-NADH比率导致的高促炎SASP是这一现象的结果。苹果酸是红氧反应和许多衰老类型（包括OIS和MiDAS）中的另一种重要代谢物。值得注意的是，苹果酸-天冬氨酸穿梭，其中NADH通过苹果酸脱氢酶1（MDH1）（Lee等，2012年）从细胞质转移到线粒体基质。在衰老过程中，MDH1水平降低。另一方面，MDH1活性的降低可以诱导衰老反应。MDH1活性的降低导致细胞质中NAD-NADH的降低。最后，这种抑制可能导致细胞增殖的丧失，因为这是对电子传递链抑制反应中天冬氨酸合成的需求（Birsoy等，2015年）。另一方面，与线粒体功能障碍相关的衰老（MiDAS）导致NAD-NADH比率的降低，这诱导三种主要反应：（1）sirtuin的抑制，（2）AMPK的激活，（3）PARP的抑制，从而阻断NF-kB通路。首先，低水平的NAD+降低sirtuin活性。线粒体中定位的SIRT3和SIRT5活性的降低与细胞衰老的激活相关（Wiley等，2016年）。其次，NAD+/NADH比率的降低激活AMPK和p53，抑制RNA结合蛋白Hu抗原R（HuR）降解编码周期依赖性激酶抑制剂的mRNA，即p21和p16INK4a。这增加了pRB肿瘤抑制因子的活性，导致细胞增殖和生长停滞（Wiley等，2016年）。此外，p53的激活导致缺乏IL-1依赖因子的SASP的释放，包括抗炎细胞因子IL-10和促炎细胞因子CCL27和TNF-α的高水平分泌（Wiley等，2016年）。p53的激活还通过抑制磷酸甘油酸变位酶（PGM）和诱导合成细胞色素c氧化酶2（SCO2）的表达，降低糖酵解并促进线粒体呼吸。此外，p53的激活通过结合葡萄糖-6-磷酸脱氢酶（G6PDH）抑制戊糖磷酸途径（PPP）。最后，低NAD+、NADH比率抑制多ADP核糖聚合酶（PARP）的ADP-核糖供体，从而依次抑制NF-kB通路。随后，下调的NF-kB通路参与各种炎症疾病的致病过程以及各种促炎SASP的表达（Liu等，2017年）。在此途径中可见，当由OIS或MiDAS诱导衰老时，对NAD代谢的区分性影响是显而易见的。这些刺激不仅释放不同的SASP，而且在NAD-NADH比率和后续相关途径上表现出不同的反应。MiDAS导致NAD-NADH比率的降低，而OIS则导致该比率和NAD+水平的升高。