Secreted frizzled-related protein 1 (Sfrp1) is highly expressed by stromal cells maintaining hematopoietic stem cells (HSCs). Sfrp1 loss in stromal cells increases production of hematopoietic progenitors, and in knockout mice, dysregulates hemostasis and increases Flk2- Cd34- Lin- Sca1+ Kit+ (LSK) cell numbers in bone marrow. Also, LSK and multipotent progenitors (MPPs) resided mainly in the G0/G1 phase of cell cycle, with an accompanying decrease in intracellular beta-catenin levels. Gene-expression studies showed a concomitant decrease Ccnd1 and Dkk1 in Cd34- LSK cells and increased expression of Pparg, Hes1, and Runx1 in MPP. Transplantation experiments showed no intrinsic effect of Sfrp1 loss on the number of HSCs or their ability to engraft irradiated recipients. In contrast, serial transplantations of wild-type HSCs into Sfrp1(-/-) mice show a progressive decrease of wild-type LSK and MPP numbers. Our results demonstrate that Sfrp1 is required to maintain HSC homeostasis through extrinsic regulation of beta-catenin.

IntroductionAll mature blood cells derive from HSCs. These HSCs have been shown to reside mainly in specialized microenvironments, referred to as niches. It is thought that the niche regulates HSC quiescence (dormancy), self-renewal, and differentiation by expression of surface molecules and secretion of soluble factors. Which signals are provided by the niche and how exactly these signals affect HSCs still remains uncertain. 1 We have established a number of stromal cell clones from mid gestation embryonic sources, of which we identified 2 cell lines (EL08-1D2 and UG26-1B6) which maintain fetal as well as adult HSCs, even though they had no direct contact with the hematopoietic cells (noncontact cocultures). 2,3 In gene expression studies, we observed that, in comparison to a number of nonsupporting stromal cell lines, those 2 cell lines both expressed larger amounts of mRNA corresponding to a number of secreted molecules. We recently described that one of these factors, secreted frizzledrelated protein 1, is required for sustained self-renewal of HSCs in vivo and that this was because of extrinsic regulation of HSCs by the microenvironment, most probably, through regulating ␤-catenin (Ctnnb1) and peroxisome proliferator-activated receptor ␥ (Pparg), both mediators of the Wnt signaling pathways. 4 One other overrepresented factor was the pleiotrophic cytokine pleiotrophin (Ptn). 3 Ptn was also found to be overexpressed by other HSC supportive cells, such as human brain endothelial cells 5,6 or the stromal cell line AFT024, 7 suggesting that high expression of Ptn may be a common feature among HSC-supportive stromal cells.Ptn, because of its pleiotrophic activities, is known under many alternative names, including heparin-binding growth-associated molecule and osteoblast-stimulating factor. Ptn is a highly conserved 17-kDa cytokine, 8 which, together with Midkine, forms a small family of low molecular weight factors. 9 Several receptors are known to bind Ptn as a ligand: receptor protein tyrosine phosphatase ␤/ (Rptpz1), 10 nucleolin, 11 and N-syndecan. 12 It was recently shown that Rptpz1 is expressed on BM-derived lineage Ϫ Ly6a ϩ Kit ϩ (LSK) cells. 6 Binding of Ptn to RPTP␤/ inactivates the phosphatase domain through dimerization of the receptor. This leads to an increasing phosphorylation status of the numerous targets of RPTP␤/, including ␤-catenin, ALK, ␤-adducin, CD81, c-Fyn, and others. 10,13 The effects of Ptn on proliferation and differentiation appear to converge in ␤-catenin and its downstream factor Dlk1. 10,14 Interestingly, Dlk1 has previously been identified to be overrepresented in the Ptn-overexpressing HSC-maintaining cell line AFT024 and to promote cobblestone area formation by HSC-derived progeny. 15 Because it was shown that Rptpz1 is expressed on BM-derived LSK cells, 6 these pathways may well be relevant in HSC regulation.Ptn is known to play important roles in proliferation and differentiation in various cell types. It was shown that Ptn is mitogenic for fibroblasts, epithelial, and endo...

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Cks1 is an activator of the SCF Skp2 ubiquitin ligase complex that targets the cell cycle inhibitor p27 Kip1 for degradation. The loss of Cks1 results in p27 Kip1 accumulation and decreased proliferation and inhibits tumorigenesis. We identify here a function of Cks1 in mammalian cell cycle regulation that is independent of p27 Kip1 . Specifically, Cks1 −/− ; p27 Kip1−/− mouse embryonic fibroblasts retain defects in the G 1 -S phase transition that are coupled with decreased Cdk2-associated kinase activity and defects in proliferation that are associated with Cks1 loss. Furthermore, concomitant loss of Cks1 does not rescue the tumor suppressor function of p27 Kip1 that is manifest in various organs of p27 Kip1−/− mice. In contrast, defects in mitotic entry and premature senescence manifest in Cks1 −/− cells are p27 Kip1 dependent. Collectively, these findings establish p27 Kip1 -independent functions of Cks1 in regulating the G 1 -S transition.