Syndecan-1 Is Required for Robust Growth, Vascularization, and Metastasis of Myeloma Tumors in Vivo

Abstract

Myeloma tumors are characterized by high expression of syndecan-1 (CD138), a heparan sulfate proteoglycan present on the myeloma cell surface and shed into the tumor microenvironment. High levels of shed syndecan-1 in the serum of patients are an indicator of poor prognosis, and numerous studies have implicated syndecan-1 in promoting the growth and progression of this cancer. In the present study we directly addressed the role of syndecan-1 in myeloma by stable knockdown of its expression using RNA interference. Knockdown cells that were negative for syndecan-1 expression became apoptotic and failed to grow in vitro. Knockdown cells expressing syndecan-1 at ∼28% or ∼14% of normal levels survived and grew well in vitro but formed fewer and much smaller subcutaneous tumors in mice compared with tumors formed by cells expressing normal levels of syndecan-1. When injected intravenously into mice (experimental metastasis model), knockdown cells formed very few metastases as compared with controls. This indicates that syndecan-1 may be required for the establishment of multi-focal metastasis, a hallmark of this cancer. One mechanism of syndecan-1 action occurs via stimulation of tumor angiogenesis because tumors formed by knockdown cells exhibited diminished levels of vascular endothelial growth factor and impaired development of blood vessels. Together, these data indicate that the effects of syndecan-1 on myeloma survival, growth, and dissemination are due, at least in part, to its positive regulation of tumor-host interactions that generate an environment capable of sustaining robust tumor growth. Myeloma tumors are characterized by high expression of syndecan-1 (CD138), a heparan sulfate proteoglycan present on the myeloma cell surface and shed into the tumor microenvironment. High levels of shed syndecan-1 in the serum of patients are an indicator of poor prognosis, and numerous studies have implicated syndecan-1 in promoting the growth and progression of this cancer. In the present study we directly addressed the role of syndecan-1 in myeloma by stable knockdown of its expression using RNA interference. Knockdown cells that were negative for syndecan-1 expression became apoptotic and failed to grow in vitro. Knockdown cells expressing syndecan-1 at ∼28% or ∼14% of normal levels survived and grew well in vitro but formed fewer and much smaller subcutaneous tumors in mice compared with tumors formed by cells expressing normal levels of syndecan-1. When injected intravenously into mice (experimental metastasis model), knockdown cells formed very few metastases as compared with controls. This indicates that syndecan-1 may be required for the establishment of multi-focal metastasis, a hallmark of this cancer. One mechanism of syndecan-1 action occurs via stimulation of tumor angiogenesis because tumors formed by knockdown cells exhibited diminished levels of vascular endothelial growth factor and impaired development of blood vessels. Together, these data indicate that the effects of syndecan-1 on myeloma survival, growth, and dissemination are due, at least in part, to its positive regulation of tumor-host interactions that generate an environment capable of sustaining robust tumor growth. Multiple myeloma is an aggressive and deadly hematologic malignancy of plasma cells that resides predominantly in the bone marrow (1Barlogie B. Shaughnessy J. Epstein J. Sanderson R. Anaissie E. Walker R. Tricot G. Lichtman M.A. Beutler E. Kipps T.J. Seligsohn U. Kaushansky K. Prchal J.T. Williams Hematology. McGraw-Hill, New York2006: 1501-1533Google Scholar). The term “multiple” refers to the multifocal appearance of myeloma throughout the skeleton, which identifies the intrinsic ability of myeloma cells to metastasize extensively. Despite significant progress made in the last 20 years, myeloma remains incurable, often requiring aggressive therapeutic approaches leading to a multitude of adverse effects. New biologically based therapies such as the proteasome inhibitor Velcade, bisphosphonates, and thalidomide have proven effective in some patients. Success of these therapies, at least in part, is due to their impact on the tumor microenvironment (2Anderson K.C. Exp. Hematol. 2007; 35: 155-162Abstract Full Text Full Text PDF PubMed Scopus (104) Google Scholar). Interactions between myeloma cells and the bone clearly drive the progression of this cancer and are also important in mediating drug resistance (3Roodman G.D. J. Bone Miner. Res. 2002; 17: 1921-1925Crossref PubMed Scopus (111) Google Scholar, 4Caers J. Van Valckenborgh E. Menu E. Van Camp B. Vanderkerken K. Bull. Cancer. 2008; 95: 301-313PubMed Google Scholar, 5Sanderson R.D. Yang Y. Clin. Exp. Metastasis. 2008; 25: 149-159Crossref PubMed Scopus (99) Google Scholar, 6Perez L.E. Parquet N. Shain K. Nimmanapalli R. Alsina M. Anasetti C. Dalton W. J. Immunol. 2008; 180: 1545-1555Crossref PubMed Scopus (47) Google Scholar). Thus, understanding the myeloma microenvironment is key to devising new strategies for therapeutic intervention. Heparan sulfate proteoglycans are known to regulate the initiation and progression of some cancers (7Alexander C.M. Reichsman F. Hinkes M.T. Lincecum J. Becker K.A. Cumberledge S. Bernfield M. Nat. Genet. 2000; 25: 329-332Crossref PubMed Scopus (308) Google Scholar, 8Maeda T. Alexander C.M. Friedl A. Cancer Res. 2004; 64: 612-621Crossref PubMed Scopus (115) Google Scholar, 9Aikawa T. Whipple C.A. Lopez M.E. Gunn J. Young A. Lander A.D. Korc M. J. Clin. Invest. 2008; 118: 89-99Crossref PubMed Scopus (143) Google Scholar). Syndecan-1 is a cell surface heparan sulfate-bearing proteoglycan that plays an important role in regulating myeloma (5Sanderson R.D. Yang Y. Clin. Exp. Metastasis. 2008; 25: 149-159Crossref PubMed Scopus (99) Google Scholar). Syndecan-1 is expressed by all myeloma tumors within the bone marrow and is present in relatively high levels on the surface of most myeloma tumor cells (10Wijdenes J. Vooijs W.C. Clément C. Post J. Morard F. Vita N. Laurent P. Sun R.X. Klein B. Dore J.M. Br. J. Haematol. 1996; 94: 318-323Crossref PubMed Scopus (327) Google Scholar, 11Bayer-Garner I.B. Sanderson R.D. Dhodapkar M.V. Owens R.B. Wilson C.S. Mod. Pathol. 2001; 14: 1052-1058Crossref PubMed Scopus (108) Google Scholar). The extracellular domain of this proteoglycan can be cleaved from the cell surface by sheddases, and high levels of shed syndecan-1 correlate with poor prognosis in myeloma patients (12Seidel C. Børset M. Hjertner O. Cao D. Abildgaard N. Hjorth-Hansen H. Sanderson R.D. Waage A. Sundan A. Blood. 2000; 96: 3139-3146Crossref PubMed Google Scholar). Shed syndecan-1 remains biologically active and can participate in regulating many cellular behaviors, including myeloma growth (13Bartlett A.H. Hayashida K. Park P.W. Mol. Cells. 2007; 24: 153-166PubMed Google Scholar, 14Yang Y. Yaccoby S. Liu W. Langford J.K. Pumphrey C.Y. Theus A. Epstein J. Sanderson R.D. Blood. 2002; 100: 610-617Crossref PubMed Scopus (163) Google Scholar). Much of syndecan-1 function is mediated by its heparan sulfate chains that bind to, and regulate the activity of, many of the factors known to influence myeloma growth (e.g. IL-6, 3The abbreviations used are: ILinterleukinELISAenzyme-linked immunosorbent assayFACSfluorescence-activated cell sortingGFPgreen fluorescent proteinHGFhepatocyte growth factorPBSphosphate-buffered salineRTreverse transcriptionSCIDsevere combined immunodeficientshRNAsmall hairpin RNAVEGFvascular endothelial growth factorERKextracellular signal-regulated kinase. 3The abbreviations used are: ILinterleukinELISAenzyme-linked immunosorbent assayFACSfluorescence-activated cell sortingGFPgreen fluorescent proteinHGFhepatocyte growth factorPBSphosphate-buffered salineRTreverse transcriptionSCIDsevere combined immunodeficientshRNAsmall hairpin RNAVEGFvascular endothelial growth factorERKextracellular signal-regulated kinase. IL-7, IL-8, VEGF, HGF, fibroblast growth factor 2, and fibroblast growth factor family ligands). Signaling events propagated by these growth factors, particularly those events occurring between tumor cell and bone marrow components, are critical to the growth and development of myeloma (15Anderson K.C. Dalton W.S. Mol. Cancer Ther. 2002; 1: 1361-1365PubMed Google Scholar). In addition, syndecan-1 becomes lodged within fibrotic regions of bone marrow following treatment of patients (11Bayer-Garner I.B. Sanderson R.D. Dhodapkar M.V. Owens R.B. Wilson C.S. Mod. Pathol. 2001; 14: 1052-1058Crossref PubMed Scopus (108) Google Scholar). This residual syndecan-1 may retain growth factors that aid in forming niches that facilitate tumor relapse. Thus, both on the cell surface and within the extracellular matrix, syndecan-1 is strategically placed to act as an important moderator of cross-talk between tumor and host cells, thereby promoting the growth and maintenance of the tumor as an “organ” and contributing to development of refractory disease. interleukin enzyme-linked immunosorbent assay fluorescence-activated cell sorting green fluorescent protein hepatocyte growth factor phosphate-buffered saline reverse transcription severe combined immunodeficient small hairpin RNA vascular endothelial growth factor extracellular signal-regulated kinase. interleukin enzyme-linked immunosorbent assay fluorescence-activated cell sorting green fluorescent protein hepatocyte growth factor phosphate-buffered saline reverse transcription severe combined immunodeficient small hairpin RNA vascular endothelial growth factor extracellular signal-regulated kinase. We previously demonstrated in a limited study that knockdown of syndecan-1 expression inhibited growth of subcutaneous myeloma tumors (16Yang Y. MacLeod V. Dai Y. Khotskaya-Sample Y. Shriver Z. Venkataraman G. R. A. G. B. Epstein J. Yaccoby S. Shaughnessy B. Sanderson R.D. Blood. 2007; PubMed Scopus Google Scholar). This is in the present using a and a myeloma cell we that of syndecan-1 expression of the of angiogenesis and When tumors from cells syndecan-1 angiogenesis is but to that tumor growth is limited by blood myeloma cells syndecan-1 expression are impaired in their ability to following of cells, that syndecan-1 may a key role in the in all myeloma patients. into regulation of myeloma tumor growth by syndecan-1. from a myeloma bone marrow at the Cancer M. Hjertner O. Yaccoby S. Epstein J. Sanderson R.D. Blood. 2000; 96: PubMed Google and myeloma cell were in with and The cells were with for a or syndecan-1 as previously (16Yang Y. MacLeod V. Dai Y. Khotskaya-Sample Y. Shriver Z. Venkataraman G. R. A. G. B. Epstein J. Yaccoby S. Shaughnessy B. Sanderson R.D. Blood. 2007; PubMed Scopus Google Scholar). The were at in with The were as and the of syndecan-1 knockdown cells, myeloma cells were with the syndecan-1 S. M. S. Bernfield M. J. PubMed Scopus Google Scholar). cells were with and syndecan-1 expression by cells for or the cells were by and in The cell were in syndecan-1 or syndecan-1 M. H. A. N. Bernfield M. J. PubMed Scopus Google or or from of at in and on for The cells were in and with to on for the cells were in and in sorting by or The were on the cell by by on the green fluorescent protein cells to that cell sorting or limited to cells with the cell the cells were in in on and with for at and were using a with for with a or with and for cell an of myeloma cells were by stimulation with of for from an of cells were as previously A. Yang Y. Sanderson R.D. J. 2008; Full Text Full Text PDF PubMed Scopus Google on or and with or tumor were in on and for syndecan-1 J. 2004; PubMed Scopus Google by of or were by because of the of its heparan sulfate as a and the between the and RNA from myeloma cells using and using a to the The used to syndecan-1 and were previously (16Yang Y. MacLeod V. Dai Y. Khotskaya-Sample Y. Shriver Z. Venkataraman G. R. A. G. B. Epstein J. Yaccoby S. Shaughnessy B. Sanderson R.D. Blood. 2007; PubMed Scopus Google Scholar). The were on by and the were by were for syndecan-1 and for of and syndecan-1 knockdown cells were in growth and the were at in for were and with to cells, which were on a of and syndecan-1 knockdown cells were in and the were at in for using the assay The assay from W. C.A. M.T. J. Y. Blood. 2004; PubMed Scopus Google myeloma cells were in in of of and in The were at in for at which of cells were of and syndecan-1 knockdown myeloma cells were in growth the cells were by and the by the myeloma cells were The levels of were using an to into all of the cell were and to be negative for using a the subcutaneous tumor or or syndecan-1 knockdown cells were injected into the of severe combined immunodeficient this myeloma growth in bone the of myeloma used to study myeloma tumor growth and to myeloma W. K.C. Clin. Cancer Res. PubMed Scopus Google Scholar). In addition, we have that to tumor growth using the subcutaneous those from the of myeloma Y. Yaccoby S. Liu W. Langford J.K. Pumphrey C.Y. Theus A. Epstein J. Sanderson R.D. Blood. 2002; 100: 610-617Crossref PubMed Scopus (163) Google Scholar, Y. MacLeod V. Dai Y. Khotskaya-Sample Y. Shriver Z. Venkataraman G. R. A. G. B. Epstein J. Yaccoby S. Shaughnessy B. Sanderson R.D. Blood. 2007; PubMed Scopus Google Scholar). The were for of tumor cells and at the metastasis cells expressing were injected intravenously via the into The were on an cell and The were on mice at of of The were on mice placed in the and for from to on the cells were The serum and the were tumor cell to the were injected with the and following of to the of bone with of the were and by the of at and to or levels were in to tumor studies were at and by in as previously Y. Yaccoby S. Liu W. Langford J.K. Pumphrey C.Y. Theus A. Epstein J. Sanderson R.D. Blood. 2002; 100: 610-617Crossref PubMed Scopus (163) Google Scholar). of tumor protein for levels of syndecan-1 by of syndecan-1 as of of syndecan-1 in of The data are from a study with and syndecan-1 knockdown on the were and a of and by in The activity by the in and with serum in The were at with syndecan-1 or syndecan-1 M. H. A. N. Bernfield M. J. PubMed Scopus Google for negative the the were and in were using from the cells were using a at and in were by serum in The were at with the syndecan-1 or for with and with of the were with and were using a with a using the of Myeloma cells were in and in RNA using and were on by the of at Cancer The data were using for The with were the effects of syndecan-1 in we to syndecan-1 expression in and myeloma cell via with In of and cells syndecan-1 expression were and placed in within Y. and R. syndecan-1 in cells using and the cells were and placed in cell became apoptotic within and failed to grow Thus, syndecan-1 expression at some is required for of these myeloma cell in vitro. We the of cells that expressed but at levels from that of myeloma with for syndecan-1 as by syndecan-1 expression to ∼14% and ∼28% of the normal levels present in cells with knockdown of syndecan-1 the of these cells, we cell surface expression on the the and knockdown cells were positive for expression of a present on cells of and but from In addition, the cells were negative for a expressed throughout the development of with the of and plasma and knockdown cells were also negative for a that and of but is from most plasma that the knockdown cells retain their plasma cell and of of and expression that these were knockdown of syndecan-1 studies have demonstrated that is expressed in cells Y. V. Theus A. F. Shaughnessy J. E. Sanderson R.D. J. 2007; Full Text Full Text PDF PubMed Scopus Google Scholar). The in vitro growth of were also to as by cell forming and cell This growth that cell to cell growth in vitro are by diminished syndecan-1 We this by cells with HGF, a myeloma growth factor that syndecan-1 for its via the P.W. M. Blood. 2002; PubMed Scopus Google Scholar). The that levels of are in and knockdown cells This that syndecan-1 expression is is syndecan-1 present to of the between the and knockdown cells were from which of In to the in syndecan-1 and a in expression between and knockdown cells of these were these data indicate that ∼14% of the normal of syndecan-1 retain their plasma cell and have growth in vitro that are to cells expressing levels of syndecan-1. We the ability of syndecan-1 knockdown cells to tumors in limited studies with the cells indicate have a ability to grow injected in mice as compared with cells (16Yang Y. MacLeod V. Dai Y. Khotskaya-Sample Y. Shriver Z. Venkataraman G. R. A. G. B. Epstein J. Yaccoby S. Shaughnessy B. Sanderson R.D. Blood. 2007; PubMed Scopus Google Scholar). When we compared and cells in the of their growth with the of syndecan-1 expressed syndecan-1 levels tumor that the of syndecan-1 expression the of in tumor growth. of of tumors formed by the cells as the knockdown cells positive for syndecan-1 following their into mice of the for syndecan-1 a from that of the of syndecan-1 by the tumor cells with syndecan-1 within the for that the cells forming the tumors were from cells with the of the levels of syndecan-1 by and that the tumors formed by the cells syndecan-1 present as compared with and The also that the of the syndecan-1 proteoglycan in tumors formed by knockdown and cells is This indicates that knockdown of syndecan-1 the of present on the syndecan-1 to with cells, with the levels of syndecan-1 expression in cells with the cells, knockdown of syndecan-1 in cells on their in vitro growth subcutaneous of and knockdown cells into a small tumor in injected with the knockdown cells In of injected with cells the tumor from the knockdown cells, we levels of in the to that tumors formed metastases cells and high levels of this that very levels of were in the of injected with knockdown data with the cells are to those with the cell that knockdown of syndecan-1 growth of myeloma cell in the of syndecan-1 knockdown cells in an myeloma is and the or knockdown cells expressing were injected and the were for that the syndecan-1 knockdown cells were capable of forming tumors were of were present in injected with cells as compared with in injected with knockdown cells 20 This by levels of in the of the levels were in the and in the knockdown well with many of the injected with cells tumors in the were also of to of following of that of tumor were present in the and of the injected with In tumors in of injected with knockdown cells a few tumors in their bone in bone in these data that cells expressing levels of syndecan-1 in the bone marrow microenvironment. that the in the in growth of knockdown and cells due to effects of the the knockdown cells were with a for the syndecan-1 of the syndecan-1 and cells were injected into the of The that expression of the syndecan-1 the ability of the knockdown cells to grow in to levels to those of cells expressing levels of syndecan-1 are in or growth between knockdown and cells in vitro 2, but between their growth in this that the growth promoting effects of syndecan-1 in may be to its on the tumor microenvironment. Thus, because of tumors is required for their growth and because heparan sulfate is known to angiogenesis via its interactions with growth factors, we the in tumors formed by and knockdown cells we in a in In tumors formed by the cells, were and in tumors formed by knockdown cells were small and of that on in tumors formed by syndecan-1 knockdown cells were those in the This is with studies in that syndecan-1 by myeloma cells can the and of endothelial T. and R. is an important of tumor for both the initiation of and M. J. Pathol. Google we its expression in tumors formed from and knockdown in levels with the tumors formed by syndecan-1 knockdown cells much of levels in vitro that the knockdown cells cells levels in vitro were in and knockdown cells, that regulation of is and R. these data indicate that levels of may to the impaired of tumors and the poor tumor growth. The that robust growth, and metastasis of myeloma tumors in are on expression of levels of syndecan-1. myeloma cell syndecan-1 expression or we made the following cells expressing syndecan-1 became apoptotic in vitro and cells in which syndecan-1 to as as of syndecan-1 levels their plasma cell and exhibited in vitro growth to those of cells expressing levels of in to in vitro growth, growth of injected myeloma cells on the of syndecan-1 expression with the syndecan-1 expression with poor tumor and growth of including growth within the bone also on the of syndecan-1 and tumors formed by cells diminished levels of syndecan-1 exhibited levels of and impaired vascular development as compared with tumors expressing high levels of syndecan-1. be that both and cell used in the present study grow in of the bone The that knockdown of syndecan-1 expression can have such a on and growth of tumors from these aggressive cells to the of syndecan-1 in myeloma progression in critical role for syndecan-1 in regulating the growth of myeloma tumors by for myeloma growth via and growth factor family is on syndecan-1 heparan sulfate P.W. M. Blood. 2002; PubMed Scopus Google Scholar, K. T. M. M. J. G. G. J. M. P. H. D. Klein B. 25: PubMed Scopus Google myeloma cells to syndecan-1 grow and metastasize in cells expressing levels of syndecan-1 Y. Yaccoby S. Liu W. Langford J.K. Pumphrey C.Y. Theus A. Epstein J. Sanderson R.D. Blood. 2002; 100: 610-617Crossref PubMed Scopus (163) Google the growth of tumor cells from myeloma bone marrow inhibited in with an that heparan sulfate (16Yang Y. MacLeod V. Dai Y. Khotskaya-Sample Y. Shriver Z. Venkataraman G. R. A. G. B. Epstein J. Yaccoby S. Shaughnessy B. Sanderson R.D. Blood. 2007; PubMed Scopus Google expression of or that sulfate from heparan myeloma tumor growth in Y. Yang Y. MacLeod V. Shriver Z. Venkataraman G. R. Sanderson R.D. J. Full Text Full Text PDF PubMed Scopus (115) Google all myeloma tumors from the bone marrow are positive for syndecan-1 and most of the cells within the tumors are positive for syndecan-1 (11Bayer-Garner I.B. Sanderson R.D. Dhodapkar M.V. Owens R.B. Wilson C.S. Mod. Pathol. 2001; 14: 1052-1058Crossref PubMed Scopus (108) Google and to in myeloma the of cells syndecan-1 expression within a tumor are M. M. E. M. J. J. J. Klein B. Br. J. Haematol. 100: PubMed Scopus Google Scholar). Thus, present are with these syndecan-1 and heparan sulfate for the the for syndecan-1 for robust myeloma growth, and that studies have that heparan and as myeloma growth in at least of the of syndecan-1 knockdown is due to the in tumor heparan In of syndecan-1 expression all heparan sulfate from myeloma cells that syndecan-1 as their heparan sulfate proteoglycan Y. V. Theus A. F. Shaughnessy J. E. Sanderson R.D. J. 2007; Full Text Full Text PDF PubMed Scopus Google Scholar). In addition, is also that of the syndecan-1 protein tumor growth via J. 2004; PubMed Scopus Google Scholar). key of the present is that cells in syndecan-1 expression following In cells expressing levels of syndecan-1 including growth within is a myeloma metastases are by tumor cells or by myeloma cells by is that plasma cells are In the of myeloma tumor cells into the blood and can be by 1996; PubMed Scopus Google Scholar). demonstrated that these plasma cells from the blood of myeloma patients can a bone and that is of myeloma S. B. Epstein J. Blood. PubMed Google Scholar, S. Epstein J. Blood. 94: PubMed Google Scholar). plasma cells in patients to new tumor leading to a hallmark of myeloma Vanderkerken K. Van Camp B. Van Clin. Exp. Metastasis. 2008; 25: PubMed Scopus Google Scholar). the mediating metastasis of myeloma tumors are well of a of and Vanderkerken K. Van Camp B. Van Clin. Exp. Metastasis. 2008; 25: PubMed Scopus Google Scholar). indicate that syndecan-1 plays a critical role in that tumor cells following their into the role for syndecan-1 in myeloma metastasis also demonstrated in a study we that of expression of shed syndecan-1 metastasis from bone to bone Y. Yaccoby S. Liu W. Langford J.K. Pumphrey C.Y. Theus A. Epstein J. Sanderson R.D. Blood. 2002; 100: 610-617Crossref PubMed Scopus (163) Google Scholar). we syndecan-1 to the events following of tumor the of interactions between syndecan-1 heparan sulfate and growth factors, and (5Sanderson R.D. Yang Y. Clin. Exp. Metastasis. 2008; 25: 149-159Crossref PubMed Scopus (99) Google syndecan-1 be of cells in the of cells to of the cells have to the of the mechanism of syndecan-1 indicate that syndecan-1 on tumor cells of myeloma patients may or tumor growth. is that a in syndecan-1 levels to as as of normal levels on myeloma growth in growth in This that diminished syndecan-1 expression have a on tumor cells but may impact tumor-host interactions required for tumor and growth. This is by that levels within the tumors formed by syndecan-1 knockdown cells are and that angiogenesis is is a critical of myeloma and plays a role in the D. B. A. 25: PubMed Scopus Google Scholar, D. A. 2008; PubMed Scopus Google Scholar). that myeloma cells and VEGF, which to and of endothelial cells A. R. D. F. V. F. F. Google Scholar). Thus, following knockdown of the impaired that we may be due to the levels of may also be because some of bind to heparan the diminished of cell surface or shed syndecan-1 within the tumor the of within the tumor leading to the impaired demonstrated that can and the endothelial and S. H. Y. K. J. Full Text Full Text PDF PubMed Scopus (143) Google and that heparan sulfate in can the leading to angiogenesis J. K. Full Text Full Text PDF PubMed Scopus Google Scholar). Thus, is that in a with syndecan-1 heparan sulfate thereby promoting a robust that myeloma cells the myeloma cell because as compared with cells W. C.A. M.T. J. Y. Blood. 2004; PubMed Scopus Google Scholar, A. A. Dhodapkar K. A. D. R. S. Dhodapkar M.V. J. Exp. PubMed Scopus Google Scholar). In cells with normal levels of syndecan-1 and knockdown cells expressing as as of the normal levels of syndecan-1 exhibited in their forming We the of cells that were in forming but that these cells became apoptotic and failed to in of the and of the myeloma cells, the of myeloma tumor cells in patients are plasma cells may the for as the myeloma tumor by the that bone marrow of plasma cells tumors following their directly into model), the myeloma cells present in the blood or bone marrow tumors S. B. Epstein J. Blood. PubMed Google Scholar, S. Epstein J. Blood. 94: PubMed Google Scholar). Thus, therapeutic of the of myeloma tumor cells may of the a in syndecan-1 levels within the tumor that are mediated by syndecan-1 heparan sulfate as data tumor growth and We of at for and of at for with the metastasis for and of for to syndecan-1.