Identification of Adiponectin as a Novel Hemopoietic Stem Cell Growth Factor 1 Leah DiMascio,* Carlijn Voermans,* Mweia Uqoezwa,* Andrew Duncan,* Danhong Lu, † Judy Wu,* Uma Sankar,* and Tannishtha Reya 2 * The hemopoietic microenvironment consists of a diverse repertoire of cells capable of providing signals that influence hemopoietic stem cell function. Although the role of osteoblasts and vascular endothelial cells has recently been characterized, the function of the most abundant cell type in the bone marrow, the adipocyte, is less defined. Given the emergence of a growing number of adipokines, it is possible that these factors may also play a role in regulating hematopoiesis. Here, we investigated the role of adiponectin, a secreted molecule derived from adipocytes, in hemopoietic stem cell (HSC) function. We show that adiponectin is expressed by components of the HSC niche and its’ receptors AdipoR1 and AdipoR2 are expressed by HSCs. At a functional level, adiponectin influences HSCs by increasing their proliferation, while retaining the cells in a functionally immature state as determined by in vitro and in vivo assays. We also demonstrate that adiponectin signaling is required for optimal HSC proliferation both in vitro and in long term hemopoietic reconstitution in vivo. Finally we show that adiponectin stimulation activates p38 MAPK, and that inhibition of this pathway abrogates adiponectin’s proliferative effect on HSCs. These studies collectively identify adiponectin as a novel regulator of HSC function and suggest that it acts through a p38 dependent pathway. The Journal of Immunology, 2007, 178: 3511–3520. A ll stem cells have the remarkable ability to develop into multiple cell types while maintaining a constant reserve of undifferentiated cells for the lifetime of an organism. Hemopoietic stem cells (HSCs) 3 in particular, are capable of gen- erating all of the blood forming and immune cells found in the body both during homeostatic tissue maintenance and during re- generation in response to injury (1). HSCs exist primarily in a quiescent state, infrequently entering cycle and choosing between a fate of self-renewal or commitment. The outcome of this choice is dictated by signals present in the bone marrow microenviron- ment. Elucidation of the signals present in the stem cell niche and their specific roles will allow a clearer view of how HSC function is regulated and is thus of critical importance to both basic stem cell biology and transplantation based therapy. The cells of the hemopoietic microenvironment include osteoblasts, endothelial cells, adipocytes and fibroblasts. These cells are likely to provide support for hemopoietic cells through secretion of soluble factors as well as cell-cell contact. Indeed, several studies have iden- tified osteoblasts as a primary component of the HSC niche (2– 4). These bone producing cells have been shown to produce multiple factors involved in HSC quiescence and maintenance, including an- giopoietin (4), N-cadherin (5), and the Notch ligand, Jagged1(2). Re- cently, HSCs have also been shown to associate with the sinusoidal endothelium in the bone marrow (6 – 8), indicating that endothelial cells may also form an important supportive niche for HSCs. The adipocyte is the most abundant stromal cell type found in the marrow (9); however, the role of this cell in hematopoiesis has not been clearly defined. Mature adipocytes are present in long- term bone marrow cultures (10) and are capable of supporting both lymphopoiesis (11) and granulopoiesis (12). Additionally, follow- ing irradiation injury, adipocytes first appear after 7 days, a time point corresponding to the initiation of hemopoietic proliferation (13). These fat-containing cells are known to secrete a number of proteins, or adipokines, that play a role in hematopoiesis. IL-6 (14 –16) and IL-8 (16), two growth factors derived from adipocytes, have well established roles in the proliferation and differentiation of hemopoietic cells. Prostaglandin, another adipokine, has been demonstrated to have an inhibitory influence on HSCs through induction of apoptosis (17, 18). Additionally, leptin has been shown to be required for nor- mal lymphopoiesis (19) by differentially regulating the proliferation of naive and memory T cells (20) and is capable of stimulating the pro- liferation of myelocytic progenitors (21). The role of another adipokine, adiponectin, in hemopoietic stem cell function is less well defined. Adiponectin was originally iden- tified as being produced exclusively by differentiated adipocytes (22–24). It has since been shown to be expressed by multiple other cell types including fibroblasts and osteoblasts (25, 26). It has three known receptors, AdipoR1, AdipoR2 (27), and T-cadherin (28). At the cellular level, stimulation with adiponectin increases insulin sen- sitivity (29), glucose uptake (30), and fatty acid oxidation (31). Fur- thermore, mice deficient for adiponectin exhibit delayed clearance of free fatty acids from the plasma, severe diet-induced insulin resistance (32), and impaired angiogenic (33) and myocardial (34) repair fol- lowing ischemic injury. Previous studies have examined the effect of adiponectin on myeloid (35) and lymphoid (18) cells, showing that adiponectin can suppress the growth of these mature cell types in *Department of Pharmacology and Cancer Biology; and † Sarah Stedman Center for Nutrition and Diabetes Research, Duke University Medical Center; Durham, NC 27710 Received for publication May 12, 2006. Accepted for publication January 10, 2007. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 L.D. and A.D. are recipients of American Heart Association predoctoral awards. C.V. is the recipient of a Netherlands Organization for Scientific Research Talent postdoctoral fellowship. U.S. is a recipient of an American Cancer Society postdoc- toral fellowship. T.R. is the recipient of a Cancer Research Institute Investigator Award and an Ellison Medical Foundation New Scholar Award. This work was also supported by the Lisa Stafford Memorial Prize, Research Discovery Group Award (Duke University), and National Institutes of Health Grants DK63031 and DK072234 (to T.R.). 2 Address correspondence and reprint requests to Dr. Tannishtha Reya, Duke Uni- versity, Pharmacology and Cancer Biology, C333 LSRC, Research Drive, Durham, NC 27710. E-mail address: t.reya@duke.edu 3 Abbreviations used in this paper: HSC, hemopoietic stem cell; AMPK, AMP-acti- vated protein kinase; DAPI, 4',6'-diamidino-2-phenylindole. Copyright © 2007 by The American Association of Immunologists, Inc. 0022-1767/07/$2.00 The Journal of Immunology www.jimmunol.org