Developmental Stage and Sex-Specific Role of IGF/IGF1R Signaling in Hematopoietic Stem and Progenitor Cells

Insulin-like growth factors (IGFs) are critical regulators of cell growth, proliferation and survival. Their activities are mainly mediated through insulin-like growth factor 1 receptor (IGF1R). Both IGFs and IGF1R are commonly involved in human cancers, including leukemia. Thus, a better understanding of the role of IGF/IGF1R signaling in normal hematopoiesis will enhance our understanding of leukemogenesis. We showed previously a developmental stage-specific role of this pathway in regulating fetal (but not adult) megakaryocytic progenitors via interplay with GATA1 (Genes Dev. 24:1659-72). To determine whether IGF/IGF1R signaling also plays differential roles in fetal and adult hematopoiesis in general, we analysed Igf1r conditional knockout mice using Mx1-Cre and Vav-Cre for the adult stage, and Tie2-Cre for the fetal stage. Although both fetal and adult stages of hematopoiesis were not significantly perturbed, loss of Igf1r in fetal (but not in adult) hematopoietic cells led to a significant reduction in the number of CFUs in clonogenic assays. In adult Igf1r-null Lin-Sca-1+c-Kit+ (LSK) hematopoietic stem and progenitor cells (HSPCs), analysis of the long-term hematopoietic stem cell (LT-HSC), short-term HSC (ST-HSC) and multipotent progenitor (MPP) subsets of LSK cells revealed an increase in the percentage of LT-HSCs when compared to that of wild-type (WT) mice, consistent with a recent study (Nature. 500:345-9). These data suggest that IGF/IGF1R signaling may have a developmental stage-restricted role in fetal progenitors; in adults, this pathway may play a role in the transition from LT-HSCs to MPPs.

Recently it was shown that estrogen signaling plays an important role in regulating proliferation of HSCs (Nature. 505:555-8). Since during development, both male and female fetuses are exposed to the same high level of maternal estrogen, whereas in adults, male and female hematopoietic cells are exposed to different levels of circulating estrogen, we hypothesized that adult (but not fetal) HSPCs from male and female mice might exhibit different phenotypes in response to Igf1r-loss. To test this, we analysed the adult LSK population by separating males and females. Interestingly, we found that in females, but not in males, the LSK population is significantly reduced upon Igf1r-loss. Conversely, a preliminary study in fetal liver CFU assay revealed that both Igf1r-null female and male fetuses exhibited a similar reduction in their CFUs compared to matched WT controls. To understand the female-specific role of IGF/IGF1R signaling in adult HSPCs, we analysed the cell cycle status of HSCs. Similar to the reported observation (Nature. 505:555-8), we found that in WT female adults, there were significantly more Ki67+ cycling LT-HSCs than those in males. Intriguingly, Igf1r-loss significantly reduced the percentage of cycling LT-HSCs in females to a level comparable to that of WT males, but had a neglectable effect on cycling LT-HSCs in males. Administration of estradiol (E2) revealed that in females, E2 injection led to an increase in the percentage of Ki67+ LT-HSCs and this increase was partially abolished when under the Igf1r-null background; in males, E2 injection also increased the percentage of Ki67+ LT-HSCs, although we did not observe a notable reduction in this population when under the Igf1r-null background.

Overall, our data suggest that although IGF/IGF1R signaling is not essential for normal hematopoiesis, it may play a more important role under conditions (e.g., fetal development, pregnancy) when there is a higher demand for the output from the hematopoietic system; in particular, this pathway may play an important role in mediating the effect of estrogen on self-renewal and proliferation of HSPCs.