Hematopoietic stem cells (HSCs) are multipotent precursors with the unique ability to self-renew into all cell types and self-regenerate in order to resume proliferation in the blood-forming system. They are crucial for the life maintenance of bio-organisms. Investigation into the functioning of HSCs remains a prominent and dynamic area of exploration by researchers. Studies have shown that various factors can shape the profile of HSCs. This review systematically summarizes the intrinsic factors (i.e., RNA-binding protein, modulators in epigenetics and enhancer–promotor-mediated transcription) that are reported to play a pivotal role in the function of HSCs, therapies for bone marrow transplantation, and the relationship between HSCs and autoimmune diseases. It also demonstrates the current studies on the effects of high-fat diets and nutrients (i.e., vitamins, amino acids, probiotics and prebiotics) on regulating HSCs, providing a deep insight into the future HSC research.
The homing and engraftment of hematopoietic stem cells are crucial for the efficiency of bone marrow transplantation, and when donor-cell numbers are low, clinical engraftment can be severely compromised. Christopherson et al. reported that endogenous CD26 expression on donor cells negatively regulates homing and engraftment. The inhibition or deletion of CD26 greatly increases the efficiency of transplantation [55]. A group of researchers showed that the administration of ACK2, an antibody that inhibits c-kit function, resulted in the temporary depletion of more than 98% of endogenous hematopoietic stem cells (HSCs) in immunodeficient mice. Subsequent transplantation of donor HSCs into these mice led to high levels of chimerism, which was as high as 90%. Extrapolating these findings to humans may potentially facilitate the development of mild yet effective conditioning regimens for transplantation [56]. One finding revealed that in competitive repopulation experiments, hemizygous (Cxcr4+/o) HSCs presented a stable proliferation without depleting long-term hematopoietic stem cells (LT-HSCs), indicating that partial inactivation of CXCR4 may be a strategy to promote HSC engraftment in patients who have WHIM syndrome (Cxcr4+/S338X) after transplantation [57]. UM171 functions by augmenting the self-renewal capacity of human LT-HSCs regardless of whether AhR is suppressed. In contrast, the activity of AhR inhibitors seems to be limited to cells with transient self-renewal capacity. In the in vitro expansion of LT-HSCs and cells derived from them, using UM171 may be an alternative approach for prioritizing small, well-HLA-matched cord blood units as potential cell sources for transplantation involving donor selection algorithms in the future [58]. Moreover, viral therapy has recently been used to cure specific types of HSC-related diseases.
Since matched donors are not always available, the infusion of autologous HSPCs modified ex vivo via gene therapy is considered to be an alternative approach to hematopoietic stem/progenitor cell (HSPC) transplantation in the treatment of many diseases. Researchers used a lentiviral vector encoding functional WASP, a protein regulating the cytoskeleton, to genetically correct HSPCs obtained from three Wiskott–Aldrich syndrome (WAS) patients and reinfused the edited cells after a reduced-intensity conditioning regimen was administered [59]. Cartier et al. initiated the lentiviral-mediated gene therapy of hematopoietic stem cells in X-linked adrenoleukodystrophy (ALD) patients and found that this therapy led to successful allogeneic hematopoietic cell transplantation, indicating that this approach offered therapeutic benefits to ALD patients [60]. Metachromatic leukodystrophy (MLD), usually caused by arylsulfatase A (ARSA) deficiency, is an inherited lysosomal storage disease. Researchers used a lentiviral vector to transfer a functional ARSA gene into hematopoietic stem cells (HSCs). After the reinfusion of the gene-corrected HSCs, extensive and stable ARSA gene replacement was detected in MLD patients, suggesting a new method to cure MLD [61].
ADs are characterized by the immune system attacking the host’s own tissues due to the loss of self-tolerance [62], and HSCs have been implicated in the pathogenesis of autoimmune diseases [63][64][65]. They are involved in the production of lymphocytes, specifically T cells and B cells, which play a central role in autoimmune responses. Abnormalities in the differentiation or regulation of lymphocytes derived from HSCs can lead to the production of self-reactive cells, contributing to the development of autoimmune diseases. It has been reported that CD8 T cells are detrimental to BM failure and the function of HSCs, causing anemia [63]. In one study, it was found that HSCs of patients with type 1 diabetes were possibly genetically programmed to facilitate the proliferation of autoreactive memory B cells [64]. Another study showed that the hyperactivation of mTOR impaired the hematopoiesis of HSCs in mice [65].
Hematopoietic stem cell transplantation (HSCT) holds its potential as a therapeutic approach to autoimmune diseases (ADs). Studies demonstrated that HSCT could serve as an effective treatment in some severe ADs such as multiple sclerosis, systemic sclerosis and systemic lupus erythematosus [66][67]. Manipulating HSCs to enhance immune tolerance or modulate the inflammatory microenvironment may offer promising avenues for future treatment strategies.
The mTORC1 pathway tends to be elevated in overnutrition conditions such as obesity, which can subsequently induce diabetes [68]. It has been reported that during nutrient deprivation, two repressors of mTORC1, seizure threshold 2 (SZT2) and tuberous sclerosis complex-1 (TSC1), are crucial for HSC homeostasis. The loss of SZT2 in HSCs decreased the HSC pool and reduced the repopulating capacity of HSCs. Moreover, the ablation of both SZT2 and TSC1 markedly depleted the HSC pool and led to a marked synergistic effect that increased mTORC1 activity and ROS production by 10-fold [4].
Spred1, which is a negative regulator of RAS-MAPK signaling, has been reported to participate in maintaining HSC homeostasis, particularly in animals fed on HFDs. Specifically, under normal conditions, Spred1 negatively modulates HSC self-renewal and fitness, partially through the regulation of Rho kinase activity. Spred1 deficiency mitigates HSC failure induced by pathogen mimetics, extending the HSC lifespan, but it does not initiate leukemia cells due to the compensatory upregulation of Spred2. However, after animals were fed on an HFD, Spred1-deficient HSCs exhibited a hyperactivation of the ERK pathway and abnormal self-renewal, resulting in functional HSC failure, severe anemia and the development of myeloproliferative neoplasm-like disease. These HFD-induced hematopoietic abnormalities were, at least in part, mediated by alterations in the gut microbiota [5].
Hermetet et al. investigated the impact of HFDs on TGF-β signaling in HSCs [6]. They utilized C57BL/6J model mice fed on an HFD and a standard diet in a short-term study and observed a reduction in the number of primitive HSCs among the BM cells of the HFD-fed mice. These HFD-fed mice exhibited a diminished hematopoietic reconstitution potential after transplantation. The disrupted maintenance of HSCs was attributed to a reduction in the number of dormant HSCs following an HFD intake. The researchers also found that mice fed on an HFD showed disrupted TGF-β receptor localization within lipid rafts, leading to impaired Smad2/3-dependent TGF-β signaling, which was the primary molecular mechanism underlying these effects. However, the administration of recombinant TGF-β1 to HFD-fed mice attenuated the loss of HSCs and the compromised recovery ability of the BM cells. Another study revealed that metabolic stress induced by HFDs affected the bone marrow niche by altering the gut microbiota and the balance between the numbers of osteoblasts and adipocytes. Specifically, HFDs resulted in a decrease in the number of long-term Lin- Sca-1+ c-Kit+ (LSK) stem cells and a shift in the number of cells differentiating from lymphoid to myeloid lineages. The function of the bone marrow niche was impaired after HFD intake, as shown by the poor reconstitution of hematopoietic stem cells. HFD feeding led to the robust activation of PPARγ2, which hindered osteoblastogenesis while promoting adipogenesis in the bone marrow. Additionally, the expression of key genes, such as Jag-1, SDF-1 and IL-7, involved in forming the bone marrow niche was significantly suppressed after HFD intake. Moreover, HFD-feeding-induced changes in the structure of the gut microbiota were associated with alterations in the bone marrow. The partial rescue of HFD-mediated effects on the bone marrow niche was achieved through antibiotic treatment, and the transplantation of stools from HFD-fed mice into normal mice led to the normal mice presenting with the same symptoms as the HFD-fed mice [7].
Interestingly, the HSC pattern was passed to the next generation. Maternal obesity, especially when combined with an HFD intake during pregnancy, has been found to impede the normal expansion of fetal HSPCs while promoting the differentiation of these cells into other cell types. Notably, these effects were only partially attenuated when dietary adjustments were made for mothers with obesity during pregnancy. Through competitive transplantation experiments, HSPCs were shown to be programmed by HFDs and exhibited a reduced ability to repopulate the hematopoietic system and showed a bias toward myeloid cell differentiation. Importantly, these impairments were found to depend on the microenvironment in HFD-conditioned male recipients. Deficiencies in fetal HSPCs coincided with disruptions in the expression of genes involved in metabolism, immune and inflammatory processes and the stress response. Additionally, genes that are crucial for hematopoietic stem cell self-renewal were downregulated, and the activation of pathways that regulate cell migration was inhibited [8]. Another study demonstrated that in type 2 diabetic HFD-fed mice, elevated blood sugar levels, the criteria for a hyperglycemia diagnosis, led to the simultaneous expression of insulin and TNF-α in Lin- Sca-1+ c-Kit+ (LSK) progenitor cells. This coexpression pattern was maintained in the progeny of these cells. Interestingly, when these progenitor cells were transferred to animals with normal blood sugar levels, they were found fuse with neurons [69].
As an indispensable group of substances to support life, nutrients can affect HSCs in many aspects. Here we discuss how vitamins, amino acids, prebiotics and probiotics exert their influences on the behavior of HSCs and HSC-related diseases (Table 1).
Targeting vitamin A receptors may be a therapy for chronic graft-versus-host disease (GVHD). Heat shock protein 47 (HSP47) contributes to multiorgan fibrosis during allogeneic HSCT and reduces the life expectancy of patients after SCT. Through treatment with HSP47 small interfering RNA (siRNA) delivered via vitamin-A-coupled liposomes, HSP47 expression was downregulated in cells expressing vitamin A receptors, which effectively attenuated fibrosis [70]. The serum concentrations of retinol-binding protein and transferrin are considered to be biochemical indices for the assessment of individual nutritional status during autologous and allogeneic HSCT [9]. The stimulation of retinoic acid (RA) signaling in HE (aorta-gonad-mesonephros-derived hematopoietic endothelial) cells in an ex vivo setting significantly enhanced their potential to become HSCs. In contrast, when the enzyme critical for RA metabolism, retinal dehydrogenase 2, was conditionally inactivated in VE-cadherin-expressing endothelial cells in vivo, HSC development was completely abolished. Wnt signaling completely inhibited the HSC-inducing effects of RA modulators. In contrast, inhibiting the Wnt pathway promoted HSC development even in the absence of RA signaling [10]. Moreover, treatment with all-trans retinoic acid hindered the activation of HSCs induced via stress through the mechanism involved in protein translation suppression and reactive oxygen species (ROS) and Myc level reduction. Mice that were maintained on a vitamin-A-free diet experienced a loss of HSCs and exhibited disrupted HSC reentry into a dormant state following exposure to inflammatory stress stimuli [11].
In a mouse model, dietary supplementation with vitamin B3, an NAD+ precursor nicotinamide riboside, increased HSC function by stimulating hematopoiesis. NAD+ is a vital coenzyme in many fundamental reactions, such as those in the TCA cycle, that generates energy, and it has also been found to increase mitophagy and reduce mitochondrial metabolism rates [71]. A recent study revealed that dietary supplementation with NAD+ precursors contributed to HSC function maintenance by attenuating mitochondrial stress and shrinking the mitochondrial network size, reducing the acquisition of the age-associated phenotype of HSCs [12].
Vitamin C, also known as ascorbate, is a common reductant that reduces ROS levels. Patients with low levels of vitamin C in plasma, which are possibly associated with ASXL1 mutations, are at greater risk for developing acute myeloid leukemia than other myeloid malignancies. One study reported that oral vitamin C supplementation temporarily restores and maintains adequate vitamin C concentrations (1 g twice daily) in patients undergoing myeloablative chemotherapy and stem cell transplantation. However, intolerance issues were reported in patients one week after chemotherapy treatment [72]. Thus, intravenous vitamin C may be a better treatment choice. Agathocleous et al. showed that systemic ascorbate depletion in mice increased HSC frequency and function because the effect was enhanced by Flt3 internal tandem duplication (Flt3ITD) leukemic mutations, while dietary ascorbate reversed the acceleration of leukemogenesis [13].
Rats were fed for one month without vitamin D or vitamin D supplementation, after which rat serum was collected, and bone marrow stem cells were cultured. The results showed that the stem cells divided at a slower rate and produced fewer stem cells than those in the group that received supplemental vitamin D. This study showed that targeted nutritional supplementation can restore the function of aging stem cells involved in the hematopoietic system [73]. One study demonstrated the effect of vitamin D on brain stem cells in a murine model of multiple sclerosis (MS). The investigators used an MS mouse model to determine whether vitamin D treatment increased neural function by protecting neuronal stem cells and promoting their function. Surprisingly, they found that vitamin D supplementation reversed the nerve cell damage caused by MS. This study provides additional insights into how vitamin D directly contributes to the mitigation of diseases caused by impaired stem cell function in adults [74]. In one study, subjects were provided supplements containing ellagic acid and vitamin D and fermented lactic acid from probiotic bacteria twice per day for 2 weeks. The researchers found a significant increase in the number of circulating bone marrow stem cells. This finding suggests that nutrients can enhance the function of stem cells and further delay the aging associated with stem cells [75].
6.2. Amino Acids
Restricting dietary valine in mice resulted in the depletion of the bone marrow niche and facilitated the engraftment of donor HSCs without the need for chemotherapy or irradiation. This suggests that valine plays a crucial role in maintaining HSC function, and dietary valine restriction may potentially reduce complications associated with HSC transplantation [76]. By using HSCs obtained from mice in ex vivo experiments, Wilkinson et al. [77] found that a branched-chain amino acid (BCAA) imbalance reduced HSC proliferation and survival, whereas low valine levels resulted in poor HSC maintenance. Recently, Li et al. discovered direct links between metabolic alterations and translation regulation in HSCs during homeostasis and proliferation. Specifically, they found that to maintain homeostasis, HSCs actively engaged in high-amino-acid (AA) catabolism to decrease cellular AA levels, and this effect was facilitated by activation of the GCN2-eIF2α axis. This process was a protein synthesis inhibitory checkpoint, limiting protein synthesis to support HSC maintenance. In this study, under proliferation-promoting conditions, HSCs exhibited enhanced mitochondrial oxidative phosphorylation (OXPHOS) and generated higher energy levels. This metabolic adaptation resulted in decreased AA catabolism and the accumulation of cellular AAs. As a consequence, the GCN2-eIF2α axis, which is critical for inhibiting protein synthesis, was inactivated. This inactivation led to increased protein synthesis and, in combination with proteotoxic stress, affected cellular proteostasis. GCN2 plays a pivotal role in the maintenance of proteostasis and the inhibition of Src-mediated AKT activation via this process to repress mitochondrial OXPHOS in HSCs. Therefore, GCN2 deletion reduces HSC repopulation and regeneration. Furthermore, the glycolytic metabolite nicotinamide riboside (NR), which is a precursor to NAD+, promotes the catabolism of AA to activate GCN2 and thus supports the long-term functionality of HSCs [78].
6.3. Probiotics and Prebiotics
Several reviews have noted that the administration of probiotics and prebiotics alleviates gut microbial dysbiosis in the contexts of acute leukemia, multiple myeloma and post-transplantation [79][80][81]. We summarize the recent studies on probiotics and prebiotics that indicated that they attenuated other disease symptoms in this section. Probiotics and prebiotics can alleviate the gastrointestinal symptoms caused by chemotherapy and autologous hematopoietic stem cell transplantation (auto-HSCT). Graft-versus-host disease (GVHD) occurs when alloreactive donor T cells attack the mucous layer, and it causes gut inflammation and bacterial translocation after allogeneic hematopoietic stem cell transplantation (allo-HSCT) treatment [82]. One finding revealed that prebiotic intake may be an alternative strategy for preventing acute graft-versus-host disease (aGVHD) after allo-HSCT. Specifically, prebiotic treatment attenuated mucosal injury and decreased the cumulative incidence of skin aGVHD. Concomitantly, the gut environment in the patients was effectively maintained, and the butyrate-producing bacterial population was preserved. Thus, prebiotic intake can be utilized to increase treatment outcomes after stem cell transplantation [83]. Synbiotics, a mixture of live microorganisms and substrate (s) selectively utilized by host microorganisms confers a health benefit on the host [84], and it reportedly ameliorates chemotherapy-induced mucosal damage. One RCT experiment demonstrated that the intake of Bifidobacterium longum (BB536) and guar gum significantly attenuated diarrhea and anorexia after engraftment, indicating that synbiotics attenuated gastrointestinal toxicity effects [85]. Although many studies have reported the beneficial effects of prebiotics and probiotics on bone marrow transplantation, the direct influence of prebiotics and probiotics on HSCs needs to be elucidated.
Table 1. Effects of different nutrients on the influence of HSC.
Nutrients |
Functional Changes |
Regulatory Mechanism |
Ref. |
Vitamin A |
Improves fibrosis, facilitates HSC development |
Downregulates HSP47 expression, suppresses level of ROS |
[55–58] |
Vitamin B3 |
Stimulates hematopoiesis, attenuates age-associated metabolic and functional changes in HSC |
A precursor to NAD+; increases mitophagy and reduces mitochondrial metabolism |
[59,60] |
Vitamin C |
Improves acute myeloid leukemia condition, slackens leukemogenesis |
Removes ROS, combines with Flt3 internal tandem duplication (Flt3ITD) |
[61,62] |
Vitamin D |
Rescues aging stem cells, improves neural function |
- |
[63–65] |
Amino acids |
Maintains HSCs, reduces iatrogenic complications in HSC transplantation |
Activates the GCN2-eIF2α axis, inhibits Src-mediated AKT activation |
[66–68] |
Probiotics, prebiotics and synbiotics |
Prevents acute graft-versus-host disease (aGVHD), improves mucosal injury, ameliorates chemotherapy-induced mucosal damage improve diarrhea and anorexia after engraftment |
Maintains butyrate-producing bacterial population, reduces gastrointestinal toxicity |
[69–73,75] |
The homeostatic regulation of HSCs has been intensively studied in the past few decades. Most studies have focused on how transcription factors bind promoter or cofactors to regulate transcription and RNA-binding protein or noncoding RNA regulation via posttranscriptional or translational modifications, and affecting the homeostasis of hematopoietic stem cells is relatively preliminarily. How epigenetic regulation precisely controls hematopoietic stem cell function is unclear. Moreover, enhancer–promoter interactions and their regulatory networks may be important mechanisms contributing to HSC homeostasis. The chaperone properties of prolyl isomerase in HSCs suggest that condensates formed by biomacromolecules may play important roles in the renewal and self-differentiation of HSCs. Thus, an in-depth understanding of how RNA-binding proteins as well as epigenetic modifications regulate stem cell function will undoubtedly generate new strategies for the maintenance of healthy HSCs in vitro and in vivo. Understanding the intrinsic factors affecting HSCs will help us develop targeted nutritional interventions that will ultimately lead to dietary improvements in human health.
In this review, we introduced the definition of HSCs and summarized the factors that regulate HSC bioactivity. We also discussed various nutrients that can affect HSC activity, either directly or indirectly. However, there is limited research that can clearly demonstrate the interaction between specific nutrients and HSC activity. Current studies have mainly focused on identifying factors that directly regulate HSCs without exploring the links between these factors and nutrients. Clinical studies should be carried out more broadly and precisely. Most studies were based on mouse models, which ought to be applied further into human patients in a safe way. More methods have been developed to study HSCs. Adam and his colleagues created a new method to achieve the albumin-free and long-term ex vivo expansion of functional HSCs. Since serum albumin was one poorly defined albumin supplement in HSC cultures, this study made a fully defined culture by replacing serum albumin with polyvinyl alcohol [86].
In one study, a metabolomics method was utilized for the analysis of rare cell populations isolated directly from tissues and to compare mouse hematopoietic stem cells (HSCs) to restricted hematopoietic progenitors [13]. An article adopted single-cell RNA sequencing coupled with iterative clustering and guide-gene selection and clonogenic assays to identify different states of cells, which can be utilized to manifest genes of HSPCs from mixed-lineage intermediates [87]. Based on the role that nutrients play in HSCs, efficient therapies can be developed for patients possessing HSC-related disease, especially for those treated by bone marrow transplantation. One study showed that pretreatment with NAC (N-acetyl-L-cysteine) enhanced the engraftment of transplanted hematopoietic stem cells (HSCs) in the bone marrow [88]. This pretreatment facilitates the successful homing of HSCs to the bone marrow and may create a supportive microenvironment with low levels of reactive oxygen species (ROS) that helps in the retention of stem cells. These findings suggest that NAC pretreatment holds promise in improving HSC transplantation outcomes and maintaining a favorable microenvironment for HSC function.
Due to the complex digestion and absorption process of nutrients, it is necessary to track their transforming metabolism in the body and to identify the main products using biochemical techniques. Moreover, identifying the regulation of HSC homeostasis by these products could help us develop new foods that are healthier and safer. The molecular mechanisms of how these in vivo transformation products affect the genome, transcriptome or proteome of HSCs often involve multiple steps and therefore need to be explored through molecular networks using more advanced frontier technologies. As people around the world become increasingly concerned about the steady growth of their daily diet and life expectancy, it is still worthwhile to explore the link between daily nutrient intake and HSC function.
This entry is adapted from the peer-reviewed paper 10.3390/nu15112605