Leukopenia is a frequent side effect of many forms of cancer therapy, including chemotherapy and radiotherapy. In this study, we extended our previous findings on in vivo
administration of a Chinese herbal medicine, Cordyceps sinensis,
to show that it not only promotes the recovery from radiation-induced leukopenia (18
) but also rescues mice from Taxol-induced leukopenia. This study also provides evidence that CS influences the survival and differentiation of HSCs both directly and indirectly by affecting the MSC niche.
We believe that this is the first report to show that CS affects the differentiation of BM-HSCs and BM-MSCs. GM-CFUs and preosteoblast colony numbers were increased by the addition of CS to nonadherent and adherent BM cells, respectively. Furthermore, an osteoblast differentiation assay demonstrated that CS could promote the differentiation of MSCs into bone tissue. Although the precise mechanism by which CS affects osteoblast differentiation is unclear, the enhanced expression of Cbfa1 and BMP-2 mRNA indicates that CS acts at early stage on osteoblastic progenitors. In contrast, we also found that CS retards the development of osteoclasts. This is probably due to its inhibitory effect on the expression of ODF (RANKL) in osteoblasts. RANKL produced by marrow osteoblasts causes osteoclast precursors within the monocytic/macrophage precursor pool to express TRAT and form multi-nucleated osteoclasts (29
). The delicate balance between osteoblast and osteoclast differentiation is crucial during bone development, homeostasis, and repair (see a review in [30
]) but may also determine the size of the niche in which HSCs develop. By suppressing HSC differentiation to osteoclast lineages, CS may also switch them to make other blood cell lineages by enhancing Cbfa1 and BMP-2 expression by MSCs.
The hematopoietic system and bone have an intimate relationship that is manifested in multiple ways. Osteoblasts produce hematopoietic growth factors such as GM-CSF and hepatocyte growth factor (33
) that cooperatively permit the survival of hematopoietic progenitors. Taichman and Emerson (33
) first proposed that the osteoblast may have a role in hematopoietic recovery. This has been further supported by Zhang et al.
) and Calvi et al.
), who used genetic strategies to increase the size of the osteoblast population in specific regions of bone. They found that this caused parallel increases in the HSC population and that bone not only provides cavities in which HSC can differentiate into specialized blood cells but also that osteoblasts lining the trabecular bones provide “niches” for HSC development (3
). Calvi et al.
) also demonstrated that osteoblastic cells were a regulatory component of the niche in vivo
that influences HSC function through Notch activation. The ability of CS water extracts to promote the recovery of leukopenia after cancer therapy may therefore partially be through its effects on osteoblast clonogenicity, which includes inhibiting their differentiation into osteoclasts, as well as a direct effect in increasing the colony-forming ability of HSCs. However, this study does not exclude the possibility that CS directly protects blood leukocytes or BM stem cells from Taxol cytotoxicity.
A critical issue is which component of CS is responsible for its multiple effects and is the same component active in all cases. Polysaccharides seem to be responsible for many CS effects (37
). The β-glucan polysaccharide extract from the fruit body of the Maitake mushroom (Grifola frondosa
) also enhanced hematopoietic BM cell growth and differentiation into colony-forming cells (19
), similar to what we found for CS in this study. We have been working to identify the CS ingredient(s) responsible for stimulating BM, but our preliminary data indicate that multiple components express activity (data not shown) and that these individual components are less effective than the whole extract and have a higher risk of side effects in vivo.
This has been a major issue for many other traditional Chinese medicines. Multiple ingredients may be necessary, perhaps because they balance the side effects associated with a single compound (40) and stimulate multiple pathways and multiple cell types, as was seen in this study.
Nevertheless, this study demonstrates the potential of CS, a traditional Chinese medicine, as a remedy for leukopenia after cancer therapy and shows that it can promote the differentiation of HSCs both directly and indirectly through its action on osteoblast differentiation.