IntroductionHematopoietic stem and progenitor cells (HSPCs) are located in the bone marrow (BM) in close association with a highly organized 3-dimensional structure formed by stroma cells, referred to as the niche. 1,2 It has been demonstrated that these cell-cell interactions are vital for the biology of HSPCs. Systemic administration of cytokines and chemokines or cytotoxic agents mobilize HSPCs from the BM into peripheral blood (PB), where they are collected in clinically useful quantities for stem cell therapies. 3,4 Mobilization of HSPCs in response to these factors requires the de-adhesion of HSPCs from the niche. 5,6 Evidence accumulating over the past decade has proven that there is a measurable and successive functional decline in hematopoietic, intestinal, and muscle stem cell function. 7,8 Given that stem cell activity is necessary to replenish lost differentiated cells, it has been hypothesized that aging of hematopoietic stem cells (HSCs) leads to reduced stem cell renewal and thus reduced tissue homeostasis in aged animals, [7][8][9][10][11] which is emphasized by ageassociated anemia and a decline in function of immune cells in elderly persons. 12-19 HSC aging is intrinsic to the aged cell and cannot be reverted by exposing HSCs from aged animals to a young microenvironment. [18][19][20] The ability of healthy, older patients to undergo stem cell mobilization in response to granulocyte colony-stimulating factor (G-CSF), the standard regimen used for clinical HSPC mobilization, has thus far not been investigated in detail because autologous hematopoietic stem cell transplantation is most often administered to adults younger than 50 years and rarely to patients older than 60 years. 21,22 The limited available information is not conclusive in terms of efficiency of mobilization because it is based primarily on the analysis of mobilization of elderly patients after severe chemotherapy 23,24 and possibly also because 2 different methods of determining mobilization efficiency are used (colony-forming cell [CFC] frequency vs CD34 ϩ cell frequency in PB). Combining general clinical wisdom and these published reports, it is anticipated that the ability to mobilize HSPCs in response to G-CSF may be reduced in elderly patients, [23][24][25][26] hampering the efficient collection of HSPCs for subsequent hematopoietic stem cell therapies. Whether a negative correlation exists between age and mobilization is still under debate. 24,25 The mouse has been used in a wide variety of studies to model human G-CSF-induced mobilization of HSCs, with an overall good correlation between results obtained from murine studies and subsequent results obtained in humans. By investigating mobilization proficiency in aged mice, we report here that aged mice show not only decreased but increased efficiency in mobilizing hematopoietic progenitor cells (HPCs) and HSCs to PB. Materials and methods AnimalsYoung C57BL/6J mice were obtained from The Jackson Laboratory (Bar Harbor, ME) and were subsequently housed in the animal barrier ...

The platelet-derived growth factor (PDGF)-A promoter is regulated by a number of GC-rich regulatory elements that possess non-B-form DNA structures. Screening of a HeLa cDNA expression library with the C-rich strand of a PDGF-A silencer sequence (5-S1 nuclease-hypersensitive site (SHS)) yielded three cDNA clones encoding NM23-H1, a protein implicated as a suppressor of metastasis in melanoma and breast carcinoma. Recombinant human NM23-H1 cleaved within the 3-portions of both 5-SHS strands in either singlestranded or duplex forms. In contrast, NM23-H2, known as a transcriptional activator with a DNA cleavage function, cleaved within the 5-portions of both strands, revealing that NM23-H1 and NM23-H2 cleave at distinct sites of the 5-SHS and by different mechanisms. NM23-H1 and NM23-H2 also cleaved within the PDGF-A basal promoter region, again exhibiting preferences for cleavage within the 5-and 3-portions of the element, respectively. Transient transfection analyses in HepG2 cells revealed that both NM23-H1 and -H2 repressed transcriptional activity driven by the PDGF-A basal promoter (؊82 to ؉8). Activity of the negative regulatory region (؊1853 to ؊883), which contains the 5-SHS, was also inhibited modestly by NM23-H1 and NM23-H2. These studies demonstrate for the first time that NM23-H1 interacts both structurally and functionally with DNA. They also indicate a role for NM23 proteins in repressing transcription of a growth factor oncogene, providing a possible molecular mechanism to explain their metastasis-suppressing effects.The platelet-derived growth factor (PDGF) 1 family consists of three structurally similar glycoproteins (M r 30,000) that induce proliferation and other growth-related effects in cells of mesenchymal origin. These proteins arise from covalent dimerization of two PDGF subunits, designated the A-chain and B-chain, yielding the heterodimer PDGF-AB and two homodimers, PDGF-AA and PDGF-BB (1, 2). PDGF was implicated in tumorigenesis following the discovery of high sequence homology between the PDGF B-chain (PDGF-B) and the viral oncogene, v-sis (for a review, see Ref.3). Other studies suggest that both PDGF-A and PDGF-B may also mediate tumor progression to the metastatic phenotype (4, 5). Transcription of the PDGF-A gene is regulated by several enhancer and silencer elements that are poly-purine/pyrimidine-rich and possess a high degree of single-stranded, non-B DNA structure. Other laboratories (6, 7) as well as our own (8) have demonstrated that a highly GC-rich and nuclease-hypersensitive element (PDGF-A NHE) in the proximal 5Ј-flanking sequence of the PDGF-A promoter (Ϫ82 to Ϫ40) contributes most of the basal transcriptional activity of the gene. This activity is mediated by the binding of members of the Sp1 family of transcription factors and can be induced in vascular endothelial cells by phorbol ester treatment through displacement of Sp1 and Sp3 by the early growth response factor Egr-1 (7, 9) or repressed by binding of the Wilms' tumor gene product WT1 (10). More recently, we local...