Redox homeostasis: the linchpin in stem cell self-renewal and differentiation

Abstract

Stem cells are characterized by their unique ability of self-renewal to maintain the so-called stem cell pool. Over the past decades, reactive oxygen species (ROS) have been recognized as toxic aerobic metabolism byproducts that are harmful to stem cells, leading to DNA damage, senescence or cell death. Recently, a growing body of literature has shown that stem cells reside in redox niches with low ROS levels. The balance of Redox homeostasis facilitates stem cell self-renewal by an intricate network. Thus, to fully decipher the underlying molecular mechanisms involved in the maintenance of stem cell self-renewal, it is critical to address the important role of redox homeostasis in the regulation of self-renewal and differentiation of stem cells. In this regard, we will discuss the regulatory mechanisms involved in the subtly orchestrated balance of redox status in stem cells by scavenger antioxidant enzyme systems that are well monitored by the hypoxia niches and crucial redox regulators including forkhead homeobox type O family (FoxOs), apurinic/apyrimidinic (AP) endonuclease1/redox factor-1 (APE1/Ref-1), nuclear factor erythroid-2-related factor 2 (Nrf2) and ataxia telangiectasia mutated (ATM). We will also introduce several pivotal ROS-sensitive molecules, such as hypoxia-inducible factors, p38 mitogen-activated protein kinase (p38) and p53, involved in the redox-regulated stem cell self-renewal. Specifically, all the aforementioned molecules can act as 'redox sensors' by virtue of redox modifications of their cysteine residues, which are critically important in the control of protein function. Given the importance of redox homeostasis in the regulation of stem cell self-renewal, understanding the underlying molecular mechanisms involved will provide important new insights into stem cell biology.

Figure 1

Schematic illustration of cellular maintenance of redox homeostasis. Mitochondria electron-transport chain (ETC), membrane-bound NADPH oxidase (NOX) complex and endoplasmic reticulum (ER) are the three major intracellular sources of reactive oxygen species (ROS). Anion superoxide (O₂⁻) is the principal form of ROS and can be rapidly converted into hydrogen peroxide (H₂O₂) by superoxide dismutases (SODs) or can alternatively, form peroxynitrite (ONOO⁻) through reacting with the nitric oxide (NO^·). H₂O₂ can be catalyzed to HO^· in the presence of Fe²⁺ or Cu²⁺ ions or be converted to H₂O and O₂ catalyzed by catalase, glutathione peroxidase (Gpx) or peroxiredoxins (Prx). To maintain the redox homeostasis, the living cells engage powerful scavenger antioxidant enzyme systems to eliminate the intracellular ROS, major ROS-scavenging enzymes are shown in green. NOS, nitric oxide synthase; GR, glutathione reductase; GSH, reduced glutathione; GSSG, oxidized glutathione

Figure 2

Key regulators upstream of ROS in the self-renewal of stem cells. Key regulators, including forkhead homeobox type O family (FoxOs), nuclear factor erythroid-2-related factor 2 (Nrf2), apurinic/apyrimidinic (AP) endonuclease1/redox factor-1 (APE1/Ref-1) and ataxia telangiectasia mutated (ATM), play pivotal roles in the regulation of self-renewal of stem cells by modulating redox homeostasis. FoxOs, including FoxO1, FoxO3, FoxO4 and FoxO6, are negatively regulated by phosphoinositide 3-kinase (PI3K)/Akt kinase signaling cascades and positively regulated by c-Jun N-terminal kinase (JNK) phosphorylation and Sirtuin 1 (SIRT1) deacetylation. Activated FoxOs translocate into the nucleus and initiate the transcription of target antioxidant genes, such as catalase and superoxide dismutase 2 (SOD2). Nrf2 is sequestered in the cytoplasm by binding with Kelch-like ECH-associated protein 1 (Keap1), which causes proteasomal degradation of Nrf2. Nrf2 is also a transcription factor that combats intracellular detrimental reactive oxygen species (ROS) by transcribing genes encoding key antioxidant molecules, such as glutamate cysteine ligase (GCL), heme oxygenase-1 (HO-1), glutathione transferase (GST), NAD(P)H quinine oxidoreductase-1 (NQO1) and peroxiredoxins (Prx). APE1/Ref-1 inhibits Rac1-regulated membrane-bound NADPH oxidase (NOX) to decrease ROS production or by binding with oxidized transcription factors (such as hypoxia-inducible factor 1α (HIF-1α), Nrf2 and p53) that transcribing genes encoding antioxidant enzymes, to maintain the cysteine residues of these transcription factors in the reduced state. ATM controls the intracellular levels of ROS by regulating the expression of antioxidant enzymes. FoxOs, Nrf2, APE1/Ref1 and ATM modulate the self-renewal of stem cells by monitoring the redox homeostasis. PTEN, phosphatase and tensin homolog; Ub, ubiquitin; ARE, antioxidant responsive element

Figure 3

Redox homeostasis in stem cell self-renewal and differentiation. Low reactive oxygen species (ROS) levels maintain the self-renewal of stem cells by monitoring the redox homeostasis, which is well regulated by the antioxidant enzymatic defense systems and hypoxia niches as well as several key redox regulators such as forkhead homeobox type O family (FoxOs), nuclear factor erythroid-2-related factor 2 (Nrf2), apurinic/apyrimidinic (AP) endonuclease1/redox factor-1 (APE1/Ref-1) and ataxia telangiectasia mutated (ATM); high ROS levels cause abnormal differentiation, apoptosis or senescence of stem cells by the ROS-sensitive molecules, including hypoxia-inducible factors (HIFs), p38 and p53

Figure 4

Oxidative thiol modifications in redox-sensitive cysteine residues. Active thiol groups are easily oxidized to sulfenic acids (RSOH), the initial oxidation product. These transient sulfenic acids can also result from the hydrolysis of S-nitrosothiols (RSNO), which are the oxidative products of thiol groups in response to RNS. Sulfenic acids often condense with nearby thiols to form intermolecular or intramolecular disulfide bonds (RS-SR' or RS-SR), or with GSH resulting in S-glutathionylation (RS-SG). These oxoforms are reversible and can be restored to free thiols through the action of cellular reductants. Alternatively, sulfenic acids can be further oxidized to sulfinic acids (RSO₂H) and, under more severe oxidizing conditions, to sulfonic acids (RSO₃H), both of which are irreversible modifications. Cys, cysteine; ROS, reactive oxygen species; RNS, reactive nitrogen species; GSH, reduced glutathione; Reduc, Reductants

Figure 5

Schematic illustration of key ‘redox sensors' that might be involved in the self-renewal of stem cells. All key molecules shown are involved in the regulation of stem cell self-renewal by modulating redox homeostasis, including forkhead homeobox type O family (FoxOs), nuclear factor erythroid-2-related factor 2 (Nrf2), apurinic/apyrimidinic (AP) endonuclease1/redox factor-1 (APE1/Ref-1) and ataxia telangiectasia mutated (ATM), hypoxia-inducible factor 1α (HIF-1α), p38 and p53, as well as several upstream signaling molecules including phosphatase and tensin homolog (PTEN), Sirtuin 1 (SIRT1), c-Jun N-terminal kinase (JNK) and Kelch-like ECH-associated protein 1 (Keap1), are subject to redox modifications, or named ‘redox sensors.' These redox modifications might play an important role in the regulation of redox homeostasis thereby modulating the self-renewal of stem cells. SH, thiol; SNO, S-nitrosothiols; PI3K, phosphoinositide 3-kinase; Grx, glutaredoxin; Trx, thioredoxin; DTT, Dithiothreitol; SOD2, superoxide dismutase 2; Ub, ubiquitin; ARE, antioxidant responsive element; GCL, glutamate cysteine ligase; HO-1, heme oxygenase-1; GST glutathione transferase; NQO1, NAD(P)H quinine oxidoreductase-1; Prx, peroxiredoxins; ROS, reactive oxygen species. The blue asterisk denotes activated state of ‘redox sensors', while the black asterisk denotes inactivated state. The black solid dot indicates that the redox modification has no effect on the turnover of activated/inactivated state of ‘redox sensors', or this effect is not affected or has not been clarified; ‘S?' indicates that the types of oxidative modification are not clear. ‘?' denotes that the active cysteine site is undefined