Increasing evidences have revealed that microenvironment plays a critical role in protecting HSPCs from cytotoxic agents and metabolic by-products, and in maintaining dormancy. Perturbation of niche or stromal cells may cause malfunctions in HSPCs, hence lead to hematological diseases. Most recently, it was found that hematopoietic reconstitution was compromised in aging BM due to ROS invasion of stromal cells, which, however, was partly reversed by a polyphenolic antioxidant curcumin [33]. In the current study, we have shown that ASP, polysaccharides extractive taken from traditional Chinese medicine, attenuated ROS load in stromal cells, which consequently prevented HSPCs from SIPS due to oxidative DNA damage.
There has been growing proof that increased levels of ROS with age renders oxidative stress and lead to dysregulation of tissues, especially BM. Interestingly, steadily intracellular ROS accumulation is significantly more evident in the stromal compartment. The reason that ROS level varies between HSCs and stromal cells is that HSCs are metabolically less active with various of robust ROS-detoxifying systems, whereas high proliferative stromal cells consume more O2 hence increase ROS content and are self-exposed to oxidative stress [34]. Herein, the current study showed that in vivo, ASP administration promoted CFU-GEMM formation and protected BM hematopoietic cells from D-gal induced senescence via down-regulation of stress-activated signals including p38 MAPK, p21 and p16 proteins. Meanwhile, ASP was found to exert the role of ameliorating the aging niche: rejuvenated BMSCs; prevented BMSCs from ROS accumulation and p53, p21, and p16 activation. Also, ASP functioned as a potent quencher of cellular ROS in BM niche cells as improving ROS scavenger SOD level whereas reducing lipid peroxidation. However, it is not known that whether aging BM niche cells induce aging-related changes in HSPCs. We hypothesized that under the condition of oxidative stress, ASP rejuvenated BMSCs to provide enough support to HSPCs thus prevented HSPCs from aging. Therefore, BMSCs and hematopoietic cells co-culture system was established. Sca-1+ HSPCs were co-cultivated with D-gal-treated or -untreated stromal cells for 48h. D-gal-induced aging feeder layer provoked DDR machinery in Sca-1+ cells; whereas ASP-treated feeder layer attenuates ROS-mediated cellular DNA damage in co-cultivated Sca-1+ HSPCs. How the interplay with the stromal niche controls HSPC function including the aging process remains to be elucidated. Hence, we further focused on the differences between the aging niche and ROS load-attenuated niche.
BMSCs share the hypoxic niche with the hematopoietic progenitor cells that they support. Stromal cells participate in the regulation of intracellular ROS of primitive hematopoietic cells, to adjust and balance ROS content in primitive cells via intercellular communication, adhesion, and cytokine milieu. Cx43, postulated to be a self-renewal gene, maintains hematopoietic precursors in hematopoiesis. In stress circumstance, it can protect HSPCs via transferring the intracellular ROS of HSPCs to HM. CXCL12 facilitates HSPC maintenance and low ROS level in HM elicits CXCL12 presentation on the membrane of BMSCs [18]. Hematopoietic growth factors SCF can also regulate intracellular ROS level in HSPCs and GM-CSF is vital for CFU-GEMM formation. However, the BMSCs and niche themselves might be altered by ROS accumulation in hematopoietic microenvironment. Oxidative stress will drive stromal progenitor cells differentiation, and different levels of ROS dictate osteogenisis versus adipogenisis differentiation. As known, osteoblastic differentiation facilitates hematopoiesis. Increases in adipocytes in BM related to repression of growth and differentiation of HSPCs partly due to the reduced production of GM-CSF and G-CSF. Mice which were genetically incapable of adipocyte formation demonstrated an increase in HSC engraftment after irradiation [35]. Adipogenisis differentiation is regulated by transcription factors such as CREB and C/EBPb, which act together with PPARγ, a downstream signal of ROS. Additionally, hemeoxygenase-1 (HO-1) activity involves reduction of ROS levels in stromal progenitor cells by promoting osteoblast differentiation [36, 37]. In the current study, after ectogenic oxidant D-gal treatment, high level of ROS caused senescence of stromal cells. Also, the expression of the osteolineage related transcription factor Runx2 in stromal cells decreased, however, the adipogenic lineage related transcription factor PPARγ was up-regulated, suggesting gradual potential shift from osteogenesis to adipogenesis differentiation in stromal progenitor cells. The result was in accordance with the documents that aging BM is accompanied with a prominent increase in adipocytes, and aging alters cellular composition of stroma with a decrease in the frequency of stromal cells, osteoblasts, endothelial cells, MSCs, and CAR+ cells [38, 39].
Moreover, it was reported that the niche microenvironment was altered under the condition of oxidative stress. First, ROS may damage perivascular niche and osteoblastic niche from which most SCF and CXCL12 originate, therefore the fundamental hematopoietic factors are reduced [38, 40]. In this study, after D-gal treatment, high level of ROS inhibited SCF secretion and CXCL12 presentation on the stromal cell membrane. It was speculated that decreased SCF and CXCL12 may subsequently translocate HSCs from the BM endosteal area to areas around the sinuses which promote ROS accumulation in HSCs [41, 42]. Second, D-gal induced oxidative load altered intercellular gap junction Cx43, which is a major ROS scavenger transferring from HSCs to stromal cells during genotoxic stress, and also declined the ability of GJIC. Third, during injury process of aging, BMSCs produced various pro-inflammatory cytokines including IL-1, IL-6 and TNF-α. The over-production of pro-inflammatory cytokines is one mechanism of activation of the oxidative DNA damage checkpoint in HSPCs [21]. Among the pro-inflammatory cytokines, RANTES promotes myeloid-biased lineage differentiation via up-regulation of pro-myeloid transcription factors like Gata2 and inhibits early lymphopoiesis and T-cell development via down-regulation of lymphoid-specification genes including Ikaros and Gata3 [32, 43]. Functional redundancy among stromal cells to maintain hypoxia in HSPCs remains not clear, anyway, the excessive oxidative load in the niche provoked oxidative stress in the co-cultivated HSPCs.
It is encouraging that with hematopoietic stimulating property and ROS scavenging property, ASP exerted protective effects on HSPCs from DDR induced premature senescence. The underlying mechanism is that ASP reduced ROS load in stromal cells, minimized stress-responsive signaling, attenuated ROS-mediated cellular aging; ameliorate hematopoietic milieu thus rescued HSPCs from aging and dysfunction due to a damaged bone marrow niche. Generally, as major constituents of root extraction of Chinese Angelica Sinensis, ASP underlines the niche-mediated hematopoietic protective function to some extent, and it might lead to new strategies for screening of hematopoietic protective agents.