Since the frequency of HSCs with low levels of ROS decreases with age, ROS generation/accumulation can be considered a distinctive characteristic of aging
[22]. Proper levels of ROS are important mediators of various signal transduction pathways. However, increased levels of ROS affect HSCs’s lifespan
[20], self-renewal
[23[23][24],
24], and differentiation
[25,26][25][26]. ROS contribute to HSC aging and senescence, and excessive ROS generation induces apoptotic cell death in HSCs
[20,23,27,28][20][23][27][28]. Increase in ROS levels in adult HSC have similarities with the aging phenotypes, such as myeloid lineage skewing and defective long-term repopulation activity
[29,30][29][30]. On the other hand, very low levels of intracellular ROS in HSCs are essential to maintaining HSCs quiescence
[22]. Mitochondria produce around 90% of cellular ROS, and the impairment of mitochondrial function, for example, by the loss of the Polycomb repressor
BMI1, causes a major increase in intracellular ROS
[31]. Aging of mitochondria leads to an overload of ROS, which further damage the mitochondria, resulting in perpetual cell cycling
[32]. Evidence for this has come from studies of the effects of FOXO transcription factors, key players in the oxidative stress response, on HSC fitness. Genetic variation within the
FOXO3 gene is associated with human longevity and aging phenotypes
[33]. Mice carrying triple conditional deletions of
FOXO1,
FOXO3a, and
FOXO4 genes in the adult hematopoietic system exhibited myeloid lineage expansion, lymphoid developmental abnormalities, and a decreased long-term repopulation ability in vivo while increased ROS levels
[30]. Another study from Dr. Tosho Suda’s group also demonstrated that
FOXO3a-deleted HSCs can neither maintain quiescence nor support long-term reconstitution of hematopoiesis in vivo
[34].
FOXO3a deficiency increased levels of ROS and downregulated several cyclin-dependent kinases inhibitors, resulting in the exit of HSCs from quiescence. This boosted sensitivity to cell-cycle-specific myelotoxic injury, and loss of self-renewal capacity during aging
[34].
FOXO3a knockout HSCs also showed lower expression of mitochondrial
Superoxide dismutase 2 (
SOD2) and
Catalase, two FOXO targets involved in ROS detoxification
[35,36][35][36]. Further, FOXO3 has been shown to be crucial for the regulation of mitochondrial respiration in HSCs, which, under disrupted conditions, generate more ROS
[29]. This strengthens the hypothesis that
FOXO3a deficiency causes HSCs cell cycle abnormalities via mitochondrial ROS dysregulation.