No abstract

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...

The universal cyclin-Cdk inhibitor p27Kip1 functions as a tumor suppressor and reduced levels of p27Kip1 connote poor prognosis in several human malignancies. p27Kip1 levels are predominately regulated by ubiquitin-mediated turnover of the protein, which is marked for destruction by the E3 ubiquitin ligase SCFSkp2 complex following its phosphorylation by the cyclin E-Cdk2 complex. Binding of phospho-p27Kip1 is directed by the Skp2 F-box protein, and this is greatly augmented by its allosteric regulator Cks1. We have established that programmed expression of c-Myc in the B cells of Eμ-Myc transgenic mice triggers p27Kip1 destruction by inducing Cks1, that this response controls Myc-driven proliferation, and that loss of Cks1 markedly delays Myc-induced lymphomagenesis and cancels the dissemination of these tumors. Here, we report that elevated levels of Skp2 are a characteristic of Eμ-Myc lymphomas and of human Burkitt lymphoma that bear MYC/immunoglobulin chromosomal translocations. As expected, Myc-mediated suppression of p27Kip1 was abolished in Skp2-null Eμ-Myc B cells. However, the impact of Skp2 loss on Myc-driven proliferation and lymphomagenesis was surprisingly modest compared to the effects of Cks1 loss. Collectively these findings suggest that Cks1 targets in addition to p27Kip1 are critical for Myc-driven proliferation and tumorigenesis.