ABSTRACT
Expression of the heparan sulfate proteoglycan syndecan-1 is a hallmark of both normal and multiple myeloma (MM) plasma cells. Syndecan-1 could affect plasma cell fate by strengthening integrin-mediated adhesion via its core protein and/or by accommodating and presenting soluble factors via its HS side chains. Here, we show that inducible RNAi-mediated knockdown of syndecan-1 in human MM cells leads to reduced growth rates and a strong increase of apoptosis. Importantly, knockdown of EXT1, a copolymerase critical for HS chain biosynthesis, had similar effects. Using an innovative myeloma xenotransplantation model in Rag-2(-/-)gamma(c)(-/-) mice, we demonstrate that induction of EXT1 knockdown in vivo dramatically suppresses the growth of bone marrow localized myeloma. Our findings provide direct evidence that the HS chains of syndecan-1 are crucial for the growth and survival of MM cells within the bone marrow environment, and indicate the HS biosynthesis machinery as a potential treatment target in MM.
Subject(s)
Cell Proliferation/drug effects , Heparitin Sulfate/physiology , Multiple Myeloma/pathology , N-Acetylglucosaminyltransferases/genetics , RNA, Small Interfering/pharmacology , Animals , Antineoplastic Combined Chemotherapy Protocols/administration & dosage , Antineoplastic Combined Chemotherapy Protocols/pharmacology , Cell Line, Tumor , DNA-Binding Proteins/genetics , Doxycycline/administration & dosage , Drug Delivery Systems , Gene Targeting , Heparitin Sulfate/metabolism , Humans , Immunoglobulin gamma-Chains/genetics , Mice , Mice, Knockout , Multiple Myeloma/drug therapy , Multiple Myeloma/metabolism , N-Acetylglucosaminyltransferases/antagonists & inhibitors , N-Acetylglucosaminyltransferases/physiology , Syndecan-1/genetics , Syndecan-1/metabolism , Xenograft Model Antitumor AssaysABSTRACT
BACKGROUND: Multiple myeloma is a hematologic malignancy characterized by a clonal expansion of malignant plasma cells in the bone marrow, which is accompanied by the development of osteolytic lesions and/or diffuse osteopenia. The intricate bi-directional interaction with the bone marrow microenvironment plays a critical role in sustaining the growth and survival of myeloma cells during tumor progression. Identification and functional analysis of the (adhesion) molecules involved in this interaction will provide important insights into the pathogenesis of multiple myeloma. DESIGN AND METHODS: Multiple myeloma cell lines and patients' samples were analyzed for expression of the adhesion molecule N-cadherin by immunoblotting, flow cytometry, immunofluorescence microscopy, immunohistochemistry and expression microarray. In addition, by means of blocking antibodies and inducible RNA interference we studied the functional consequence of N-cadherin expression for the myeloma cells, by analysis of adhesion, migration and growth, and for the bone marrow microenvironment, by analysis of osteogenic differentiation. RESULTS: The malignant plasma cells in approximately half of the multiple myeloma patients, belonging to specific genetic subgroups, aberrantly expressed the homophilic adhesion molecule N-cad-herin. N-cadherin-mediated cell-substrate or homotypic cell-cell adhesion did not contribute to myeloma cell growth in vitro. However, N-cadherin directly mediated the bone marrow localization/retention of myeloma cells in vivo, and facilitated a close interaction between myeloma cells and N-cadherin-positive osteoblasts. Furthermore, this N-cadherin-mediated interaction contributed to the ability of myeloma cells to inhibit osteoblastogenesis. CONCLUSIONS: Taken together, our data show that myeloma cells frequently display aberrant expression of N-cadherin and that N-cadherin mediates the interaction of myeloma cells with the bone marrow microenvironment, in particular the osteoblasts. This N-cadherin-mediated interaction inhibits osteoblast differentiation and may play an important role in the pathogenesis of myeloma bone disease.