Abstract
Syndecan-1 is a transmembrane heparan sulfate-bearing proteoglycan expressed on the surface of most myeloma tumor cells as well as some other tumors (e.g., breast cancer). The extracellular domain of syndecan-1 is shed from the surface of tumor cells by proteolytic enzymes and accumulates in the extracellular matrix and in the blood. High levels of soluble syndecan-1 in the blood are an indicator of poor prognosis for myeloma patients. Human heparanase-1 (heparanase) is an enzyme that releases biologically active fragments of heparan sulfate chains. In addition, growth factors within the tumor microenvironment that are bound to heparan sulfate are released by heparanase activity. In previous in vivo studies we demonstrated that enhanced expression of heparanase or soluble syndecan-1 by myeloma cells dramatically increases tumor growth and upregulates their spontaneous metastasis. We have now discovered that an increase in heparanase expression on tumor cells leads to enhanced expression, shedding and accumulation of syndecan-1 within the tumor microenvironment. One myeloma cell line and two breast cancer cell lines transfected with the cDNA for heparanase exhibit a dramatic increase in shed syndecan-1 as compared to equal number of control cells that were transfected with empty vector (2.7-fold, 6.3-fold and 17-fold increase over controls, respectively). This accumulation of syndecan-1 in the culture media was not accompanied by an increase in cell surface syndecan-1 levels as assessed by flow cytometry. Gene array analysis demonstrates that following transfection of the myeloma cell line with heparanase, the expression of the syndecan-1 gene is upregulated 1.4-fold. Together these findings suggest that expression of heparanase elevates syndecan-1 transcription and rate of shedding from the cell surface. To examine this further, ARH-77 cells, a B-lymphoblastoid cell line lacking significant expression of either syndecan-1 or heparanase, were transfected with the cDNA for heparanase. Following selection and confirmation that heparanase was stably expressed, the cells were analyzed by gene array and flow cytometry for syndecan-1 expression. Results show that expression of heparanase stimulates initiation of syndecan-1 transcription and expression on the cell surface. Karyotyping and analysis with a series of phenotypic markers for B cells show that the transfected ARH-77 cells maintain their general phenotype when syndecan-1 is upregulated by heparanase. Together these findings indicate that in addition to its role in cleaving heparan sulfate chains, the expression of heparanase upregulates the expression and shedding of syndecan-1 in tumor cells. Thus, the promotion by heparanase of tumor growth, angiogenesis and metastasis may, at least in part, be due to its positive effects on syndecan-1 expression and shedding which are also known to promote tumor progression in myeloma. Inhibitors of heparanase now being tested clinically may thus have the dual effect of blocking heparanase enzyme activity and decreasing syndecan-1 expression, both which could negatively affect tumor growth and metastasis.
Loss of Cell-Surface Laminin Anchoring Promotes Tumor Growth and Is Associated with Poor Clinical Outcomes
