Sticking It to the Niche: CD98 Mediates Critical Adhesive Signals in AML
Authors: Andreas Reinisch, Ravindra Majeti
Journal: Cancer Cell (2016)

In this issue of Cancer Cell, Bajaj and colleagues report that CD98, a heterodimeric protein highly expressed in acute myeloid leukemia (AML) and largely dispensable for basal hematopoiesis, plays an important role in facilitating leukemia stem cell adhesion to bone marrow vasculature and is a potential therapeutic target in AML. In this issue of Cancer Cell, Bajaj and colleagues report that CD98, a heterodimeric protein highly expressed in acute myeloid leukemia (AML) and largely dispensable for basal hematopoiesis, plays an important role in facilitating leukemia stem cell adhesion to bone marrow vasculature and is a potential therapeutic target in AML. Since the “hematopoietic niche concept” was first postulated, our knowledge of the key cellular and molecular determinants that regulate normal hematopoietic development has significantly improved. Bone marrow (BM) niche cells, including endothelial cells, osteoblasts, mesenchymal stromal cells, and others, provide multiple signals whose contributions to hematopoietic stem cell (HSC) functions are being elucidated. These include secreted factors such as the chemokine SDF-1 (CXCL12) and its receptor CXCR4, adhesion molecules such as vascular cell adhesion molecule 1 (VCAM-1), multiple cytokines, various selectins, and extracellular matrix (ECM) proteins such as fibronectin and hyaluronic acid, which are key regulators of HSC homing and anchoring in the niche (Calvi and Link, 2015Calvi L.M. Link D.C. Blood. 2015; 126: 2443-2451Crossref PubMed Scopus (144) Google Scholar). Similar to HSCs, acute myeloid leukemia (AML) cells also depend on the BM microenvironment, where they interact both anatomically and functionally with stromal elements. Leukemia stem cells (LSCs) not only co-opt normal niches but also can alter them to their own advantage (Schepers et al., 2013Schepers K. Pietras E.M. Reynaud D. Flach J. Binnewies M. Garg T. Wagers A.J. Hsiao E.C. Passegué E. Cell Stem Cell. 2013; 13: 285-299Abstract Full Text Full Text PDF PubMed Scopus (440) Google Scholar), and they use niche-derived signals for survival, maintenance, and expansion. In human AML, less is known about critical leukemia cell-niche interactions, although xenotransplantation models comparing mouse to humanized BM suggest that AML cells may be more dependent on niche signals for homing and engraftment, compared to HSCs (Reinisch et al., 2016Reinisch A. Thomas D. Corces M.R. Zhang X. Gratzinger D. Hong W.J. Schallmoser K. Strunk D. Majeti R. Nat. Med. 2016; 22: 812-821Crossref PubMed Scopus (141) Google Scholar). Understanding the differences between normal and malignant BM niches and, in particular, elucidating specific leukemia cell-niche interactions, has the potential to lead to the development of novel niche-targeted therapies. Thus far, pre-clinical models and early-phase clinical trials suggest that dislodging leukemic cells from their protective BM niches by targeting AML-stromal interactions through inhibition of CXCR4-SDF-1 with plerixafor (AMD3100) (Uy et al., 2012Uy G.L. Rettig M.P. Motabi I.H. McFarland K. Trinkaus K.M. Hladnik L.M. Kulkarni S. Abboud C.N. Cashen A.F. Stockerl-Goldstein K.E. et al.Blood. 2012; 119: 3917-3924Crossref PubMed Scopus (308) Google Scholar) or by blocking the binding of adhesion molecules such as VLA-4 or CD44 to their ligands VCAM-1 and hyaluronic acid are promising strategies either as single agents or as “chemosensitizers” together with intensive chemotherapy. In this issue of Cancer Cell, Bajaj et al., 2016Bajaj J. Konuma T. Lytle N.K. Kwon H.Y. Ablack J.N. Cantor J.M. Rizzieri D. Chuah C. Oehler V.G. Broome E.H. et al.Cancer Cell. 2016; 30 (this issue): 792-805Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar add to our molecular understanding of leukemia cell-niche interactions by elegantly showing that CD98 expressed on leukemia cells and LSCs mediates adhesive interactions with the microenvironment that are critical for leukemia establishment and propagation. In addition, they show that targeting this interaction with an anti-CD98 antibody has activity against human AML, suggesting a further addition to the toolbox of niche-targeting therapeutics for AML (Figure 1). CD98 is a heterodimeric protein consisting of a single-pass transmembrane heavy chain (CD98hc, also known as SLC3A2) disulfide-linked to a multi-pass light chain. The heavy chain binds to cytoplasmic tails of integrin-β chains and mediates adhesive signals, while the light chain can be any one of six amino acid transporters. CD98 has been implicated in the pathogenesis of multiple hematologic and solid malignancies, and its elevated expression has been correlated with poor prognosis (Holte et al., 1987Holte H. Davies C.D. Kvaløy S. Smeland E.B. Foss-Abrahamsen A. Kaalhus O. Marton P.F. Godal T. Int. J. Cancer. 1987; 39: 590-594Crossref PubMed Scopus (25) Google Scholar). Employing a mouse model that allows Cre-mediated conditional deletion of CD98hc, Bajaj and colleagues report that complete deletion of CD98 severely limits engraftment of HSCs after transplantation but that it is dispensable under steady-state conditions. This suggests that CD98 facilitates signals permitting HSCs to engraft, rapidly proliferate, and regenerate hematopoiesis under stress conditions, consistent with previous data showing that CD98 is required for rapid clonal expansion of lymphocytes (Cantor et al., 2011Cantor J. Slepak M. Ege N. Chang J.T. Ginsberg M.H. J. Immunol. 2011; 187: 851-860Crossref PubMed Scopus (53) Google Scholar). The authors then explore two AML mouse models initiated by retroviral expression of MLL-AF9 or AML-ETO9a, together with NRASG12V, and find that CD98 deletion led to a decrease in phenotypically defined LSCs, abrogated leukemia initiation, and serial transplantation efficiency. Furthermore, in mice with established leukemia—a setting more faithfully reflecting the clinical scenario—deletion of CD98 resulted in a survival benefit, altogether demonstrating CD98 to be important for leukemia initiation, self-renewal, and propagation. Because CD98 regulates amplification of integrin signaling and transport of amino acids, both of these functions could contribute to leukemia cell survival and proliferation. To investigate the relative contribution of each of these functions to AML, the authors engineered chimeric CD98-CD69 proteins, in which the domains responsible for integrin signaling or amino acid transport were replaced with the analogous domains from CD69 lacking these functions. While integrin function alone could partially rescue leukemia colony formation in vitro, amino acid transport alone was not sufficient, suggesting that CD98-mediated integrin signaling is more critically required for leukemia cell growth than amino acid transport. Interestingly, full rescue could only by achieved when both functions were intact, potentially implicating some leukemia cell dependency on CD98-mediated amino acid transport. To gain further mechanistic insight into CD98 function in AML, integrin-mediated adhesive signals provided by BM sinusoidal endothelial cells were studied through in vivo BM imaging. The authors showed that CD98-deficient leukemia cells were impaired in their ability to firmly adhere (longer than 25 min) to the BM vasculature, and in vitro integrin-blocking experiments confirmed that CD98-mediated integrin signaling in leukemic cells promoted their survival and maintenance of phenotypic LSC characteristics. Lastly, the authors conducted several studies with human AML cells. First, they showed that RNAi-mediated CD98hc knockdown in human AML cell lines, and a limited number of primary patient samples, confirmed a decrease in adherence to a surrogate in vitro model using endothelial cells (HUVECs), reduced colony-forming potential in methylcellulose, and reduced engraftment. Finally, blocking studies with a non-humanized monoclonal anti-CD98 antibody abolished leukemia engraftment and, more importantly, reduced leukemia burden in mice with established disease using patient-derived xenografts. Importantly, CD98 blockade had no adverse effect on engraftment of normal CD34+ HSPCs, suggesting a potential therapeutic window for clinical application of this novel reagent. These exciting pre-clinical data support the strategy of targeting CD98 as an approach to the treatment of AML. However, there are several issues that require further investigation. In the xenograft treatment studies of established disease, anti-CD98 antibody caused a statistically significant reduction in leukemia in the peripheral blood, but not in the bone marrow. These results are consistent with a recent phase I clinical trial of the same humanized monoclonal antibody against CD98 utilized here (IGN523) in relapsed or refractory AML that demonstrated only modest evidence of anti-leukemic activity in 3 out of 19 patients as a single agent (Bixby et al., 2015Bixby D. Wieduwilt M.J. Akard L.P. Khoury H.J. Becker P.S. Van Der Horst E.H. Ho W. Cortes J.E. Blood. 2015; 126: 3809Google Scholar). Moreover, in the experiments reported here, optimal activity of the CD98 antibody was dependent on Fc crosslinking in vitro, consistent with published results suggesting that it may act through other mechanisms in addition to blocking the interaction of leukemia cells with BM sinusoidal endothelial cells, such as antibody-dependent cell-mediated cytotoxicity (ADCC) (Hayes et al., 2015Hayes G.M. Chinn L. Cantor J.M. Cairns B. Levashova Z. Tran H. Velilla T. Duey D. Lippincott J. Zachwieja J. et al.Int. J. Cancer. 2015; 137: 710-720Crossref PubMed Scopus (38) Google Scholar). Although highly expressed on BM sinusoidal endothelial cells, VCAM-1 is also expressed by other niche cell types, including osteoblasts and mesenchymal stromal cells, suggesting that a similar CD98-mediated mechanism may be used at different anatomic locations to engage leukemia cells with the niche. It would be interesting to see whether CD98 deletion or blockade is similarly efficient in targeting chemoresistant LSCs tightly engaged with osteoblasts at endosteal regions in refractory leukemia or in the setting of minimal residual disease after chemotherapy (Ishikawa et al., 2007Ishikawa F. Yoshida S. Saito Y. Hijikata A. Kitamura H. Tanaka S. Nakamura R. Tanaka T. Tomiyama H. Saito N. et al.Nat. Biotechnol. 2007; 25: 1315-1321Crossref PubMed Scopus (736) Google Scholar). The concept of treating AML by focusing novel therapeutics on leukemia-niche interactions is an exciting new strategy that might eventually be used to reduce or replace chemotherapy. The discovery of CD98 as a critical mediator of such interactions adds to our understanding of AML pathogenesis and has the potential to lead to new treatments. Such niche-targeting therapeutics might find a role in the treatment of patients in remission, as residual LSCs likely reside in close contact and dependence on niche interactions. CD98-Mediated Adhesive Signaling Enables the Establishment and Propagation of Acute Myelogenous LeukemiaBajaj et al.Cancer CellOctober 27, 2016In BriefBajaj et al. demonstrate the importance of CD98-mediated adhesion for survival of acute myelogenous leukemia (AML) and show that genetic deletion of CD98 in mice or use of a therapeutic CD98 antibody in patient-derived xenografts blocks AML growth. Full-Text PDF Open Archive