Distinct Effects of SCF and Hes1 On Normal Hematopoietic Stem and Progenitor Cells During Leukemic Cell Expansion in a T-ALL Mouse Model

Abstract 1216

During leukemia development, emerging leukemic cells out-compete normal hematopoietic cells and become predominant in the body. How hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) respond to the growth of leukemic cell population is an important, yet less investigated area. Our previous study demonstrated differential effects of a leukemic environment on normal HSCs and HPCs in the Notch1-induced T-ALL mouse model (Hu X, et al. Blood 2009). We found that normal HSCs were better preserved in the leukemic bone marrow in part due to increased quiescence of the HSCs and in contrast, HPCs were exhausted during the expansion of leukemic cells. Our current work is aimed to further explore the molecular mechanisms concerning the distinct impacts of leukemic environment on normal HSCs and HPCs in the T-ALL mouse model.

Given the previous report by others showing that increased secretion of stem cell factor (SCF) by myeloid leukemia cells played an important role in inducing normal HSCs/HPCs out of their niche and thus allowing leukemic cells to occupy the niche in the human-NOD/SCID xeno-graft model (Sipkins DA et al, Science 2008), we first examined the expression of SCF by ELISA, Western blot and real-time RT PCR in both normal hematopoietic and leukemic cell fractions in the Notch1-induced T-ALL mouse model as previously reported. We found that while expression of SCF in peripheral blood (PB) or bone marrow (BM) was increased in the leukemic mice, both mRNA and protein levels of SCF in normal hematopoietic cells were higher than that in leukemic cells, thereby suggesting that elevated SCF might be mainly secreted by non-leukemic cells in the leukemic hosts of our model. Further assessments on the role of SCF in leukemogenesis with the mice specifically deficient in SCF in different niche cell types are currently under investigation in our laboratory.

In order to define potential mediators in HSCs in response to leukemic cell growth, a microarray study on normal HSCs isolated from T-ALL leukemic mice and the control mice was conducted. Gene expression profiling showed significantly differed expression of 169 genes (127 up and 42 down). Especially, real-time RT PCR confirmed an increase of Hes1, p21, Fbxw11, IL-18R1 and Itgb3, and a decrease of CXCR4 and Mmp2. Interestingly, the expression of Hes1 and its target gene, p21 were elevated in normal HSCs but not in HPCs, letting us to hypothesize that Hes1 might be in part mediate the different responses of HSCs and HPCs to the T-ALL leukemic environment. To test this hypothesis, we ectopically expressed Hes1 in normal hematopoietic cells and then examined their functions under the leukemic condition. BM cells from B6.SJL mice were transduced with either MSCV-Hes1-IRES-GFP or control MSCV-GFP vector. After transduction, Hes1-GFP⁺or control-GFP⁺cells were co-transplanted with the Notch1-induced T-ALL cells into lethally irradiated C57BL/6J recipients. The engrafted cells from the leukemic BM were analyzed and Hes1-GFP⁺or control-GFP⁺cells were sorted for functional assessments. Interestingly, although over-expression of Hes1 inhibited the growth of colony forming cell (CFC) in vitro, it could potentiate the long-term repopulating cells by maintaining more cells in the quiescent (G0) state in vivo. Taken together, our current study supports a role of Hes1 in mediating the distinct responses of normal HSCs and HPCs to the T-ALL leukemic environment.