Redox regulation of stem/progenitor cells and bone marrow niche

Abstract

Bone marrow (BM)-derived stem and progenitor cell functions including self-renewal, differentiation, survival, migration, proliferation, and mobilization are regulated by unique cell-intrinsic and -extrinsic signals provided by their microenvironment, also termed the "niche." Reactive oxygen species (ROS), especially hydrogen peroxide (H(2)O(2)), play important roles in regulating stem and progenitor cell functions in various physiologic and pathologic responses. The low level of H(2)O(2) in quiescent hematopoietic stem cells (HSCs) contributes to maintaining their "stemness," whereas a higher level of H(2)O(2) within HSCs or their niche promotes differentiation, proliferation, migration, and survival of HSCs or stem/progenitor cells. Major sources of ROS are NADPH oxidase and mitochondria. In response to ischemic injury, ROS derived from NADPH oxidase are increased in the BM microenvironment, which is required for hypoxia and hypoxia-inducible factor-1α expression and expansion throughout the BM. This, in turn, promotes progenitor cell expansion and mobilization from BM, leading to reparative neovascularization and tissue repair. In pathophysiological states such as aging, atherosclerosis, heart failure, hypertension, and diabetes, excess amounts of ROS create an inflammatory and oxidative microenvironment, which induces cell damage and apoptosis of stem and progenitor cells. Understanding the molecular mechanisms of how ROS regulate the functions of stem and progenitor cells and their niche in physiological and pathological conditions will lead to the development of novel therapeutic strategies.

Figure 1. Brief overview of reactive oxygen species (ROS) reactions and sources

The biological effect of ROS in the cell is dependent on their amount and duration, their source and cellular localization, and type of species. SOD: Superoxide dismutase, GPx: Glutathione peroxidases, Trx-Prx: Thioredoxin-peroxiredoxin, O₂^•−: superoxide anion, H₂O₂: hydrogen peroxide.

Figure 2. Situations of physiologic ROS induction in HSCs and progenitor cells

Growth factor stimulation increases ROS which act as a second messenger in growth factor-mediated redox signaling. Change in oxygen concentration, which is often associated with energy metabolic alteration, actively and passively affects ROS level. Cell status change such as from quiescent to proliferative or migrating (often referred as activated status) involve an increase in ROS and is a physiologically reversible process. By contrast, differentiation, which is normally an irreversible process (such as myeloid commitment of multipotential HSCs) is also concomitant with increased ROS. Although mechanisms are not fully elucidated, these situations which increase ROS are linked to one another. Increase in ROS is achieved by increased their generation and/or decrease in antioxidant(s). Increased ROS may further promote the processes involving the redox alteration as a feed-forward mechanism (orange arrows).

Figure 3. Cell-intrinsic and cell–extrinsic effect of ROS on HSC and progenitor function

Two major sources of ROS in HSCs and progenitor cells are NADPH oxidase (NOX) and mitochondria electron transport chain (ETC) (red arrows). NOX is localized at the plasma membrane and perhaps at the endosome. Mitochondria may release ROS. Each produced ROS can activate specific molecular target(s) to contribute to cell-intrinsic or cell-autonomous regulation of cellular function. As cell-extrinsic or non-cell-autonomous regulation of HSC or progenitor function, ROS released from NOX or passed through the plasma membrane increase ROS in the extracellular space (solid blue arrows) which may instruct HSC or progenitors by targeting membrane or intracellular molecules and may influence extracellular matrix or soluble factors regulating HSC or progenitor function. ROS produced from other cells in the niche may affect an important cell-cell interaction regulating HSC or progenitor function. In addition, ROS in the extracellular space may regulate the cell-cell interaction in the niche support cells (dashed blue arrow).

Figure 4. The relationship between ROS levels and stem and progenitor cell fate and function in the homeostatic state

The bone marrow regulatory niches include hypoxic or normoxic (less hypoxic) niche axis. Given oxygen (O₂) is required for ROS generation, ROS level or redox status of stem or progenitor cells is correlated with O₂ availability. In hematopoietic stem cells (HSCs), especially ones in the quiescent state, oxidative metabolism is suppressed and NADPH oxidase (NOX) enzyme expressions are low, thereby ROS generation from mitochondria and NOX is limited (ROS low). During differentiation or migration of HSCs or in hematopoietic progenitor cells (HPCs), higher ROS (ROS high) are observed with increased mitochondrial ETC (electron transport chain) activities and/or NOX expressions and serve as signaling molecules to promote self-renewal (proliferation), differentiation, migration and survival, which in turn contribute to maintain hematopoiesis and immune function. Antioxidant enzymes play an important role in regulating basal level of ROS or in the cellular adaptation in response to altered ROS generation. These include catalase, Manganese superoxide dismutase, Cu-Zn superoxide dismutase, glutathione peroxidases and peroxiredoxins. On the other hand, further increase in ROS (ROS high) with imbalance between ROS generation and anti-oxidant activity often links to apoptosis, senescence, and oncogenesis or leukemogenesis caused by pathologic HSCs.

Figure 5. Signaling pathways mediated by ROS involving stem cell fate

ROS allow stem cells to shift from the quiescent state to the functional state such as differentiation and migration. ROS can promote the survival pathway, but also lead to senescence. ROS modulate the activities of various kinases and phosphatases, which in turn activate redox-sensitive signaling cascades. Of note, many of these molecules have also been shown to regulate basal ROS levels in stem cells, suggesting that feed-forward or feed-back mechanism by which stem cells respond to redox state and oxidative stress. Please see the main article for the details.

Figure 6. Cellular components of stem and progenitor niche and potential regulation through ROS

Hematopoietic Stem and Progenitor cells (HSPCs) reside in a niche that consists of cellular and non-cellular components. Cellular components include stem or progenitor cells, stromal cells, neurons, immune cells, osteoblastic cells, osteoclast and endothelial cells as well as the progeny of stem or progenitor cells. These cellular niche components regulate stem and progenitor cells directly through cell-cell interactions or indirectly through modifying non-cellular components including secreted neurohormonal factors, growth factors and enzymes, and extracellular matrix and oxygen (O₂) or hypoxia, as well as extracellular ROS.

Figure 7. NADPH Oxidase 2 (NOX2)-derived ROS promote hematopoietic stem/progenitor cell (HSPC) expansion and mobilization in response to ischemic injury

Ischemic injury induces expansion of low oxygen (hypoxic) area, hypoxia inducible factor-1 (HIF-1) expression and Akt activation throughout the BM, in a NOX2-dependent manner. This, in turn, regulates HSPCs expansion and mobilization from BM. Hypoxia might be induced by ROS generation which consumes oxygen, especially at the sites where oxygen supply is limited, such as the bone marrow cavity. Our data also showed matrix metalloproteinases (MMPs) are regulated by NOX2-derived ROS. These ROS-hypoxia-mediated alterations of the BM microenvironment induced by inflammation or tissue injury may play an important role in regulating stem and progenitor function to promote tissue repair and neovascularization. See ref. 24 for the details.