br Methods br Results br Discussion
Discussion Analyses of basal syndecan-1 expression showed differences in syndecan-1 levels as well as staining pattern for this proteoglycan in the breast cancer cell lines that were employed. Different localization of syndecan-1 have been found in previous studies showing lysosomal syndecan-1 in poorly differentiated cells, while a membranous pattern was present in more differentiated cell lines [17,18]. This may also be the case in our study. Despite numerous studies addressing the role of syndecan-1 in breast cancer its function in this tumor entity remains incompletely understood and only few regulators of syndecan-1 are known [1,3,5–7,15,19–22]. In this study, we identified dexamethasone as an inducer of membranous syndecan-1 expression in the MCF-7 cell line. The other tested substances zoledronic aa-dutp and the aromatase inhibitor did not alter syndecan-1 in any of the breast cancer cell lines investigated. Interestingly, a recently published study reported a significant down-regulation of syndecan-1 in MCF-7 as well as MDA-MB-231 cells by zoledronic acid when used at a concentration of 20µM . The use of a lower concentration of zoledronic acid in our study could be a reason for this discrepancy. Because of high levels of basal syndecan-1 expression and the dexamethasone-mediated syndecan-1 up-regulation in MCF-7 cells, further experiments were focused on this cell line. Next experiments addressed the functional relevance of dexamethasone-mediated syndecan-1 up-regulation in MCF-7 cells. Since membrane-bound syndecan-1 was reported to support proliferation and inhibit invasion of breast cancer cells , we analyzed the cell viability and migration in MCF-7 cells after dexamethasone treatment. We found a decrease in cell viability after dexamethasone treatment whereas migration was not altered. Because of this unexpected result we performed syndecan-1 knock-down experiments in all three cell lines and subsequently measured viability. Only the hormone receptor-positive cell lines MCF-7 and T-47D responded with a decrease in cell viability after syndecan-1 knock-down, but not the triple-negative MDA-MB-231 cells. Thus, a certain threshold of syndecan-1 may be necessary for the viability of hormone-responsive breast cancer cells but increased levels do not further stimulate cell viability. The fact that only the hormone receptor-positive cell lines responded to the syndecan-1 knock-down raised the question whether there is a link between the progesterone receptor status and syndecan-1 expression. However, the data from the tissue microarray did not support this assumption, as no correlation between the syndecan-1 expression and progesterone receptor status was found. This result is in line with two previous studies showing no correlation between syndecan-1 and the progesterone receptor status [9,10]. Nevertheless, another study found that a high syndecan-1 expression correlated with a negative progesterone receptor status . In two of these three studies lobular and other types of carcinomas were also included besides ductal carcinomas, which may be the reason for these conflicting results [8,9]. However, studies with a greater sample size are needed to resolve this discrepancy. The direct impact of soluble syndecan-1 on the bone compartment was previously shown in two studies [14,15]. In a mouse model of breast cancer, where heparanase-overexpressing MDA-Met cells were used, osteoclastogenesis was enhanced and shown to be dependent on the presence of syndecan-1 expression . In another study using a mouse model, syndecan-1-expressing multiple myeloma cells produced fewer osteolytic lesions compared to cells that did not express syndecan-1. Additional in vitro experiments in murine bone marrow cells showed that the syndecan-1 ectodomain inhibits osteoclast formation and promotes osteoblast development . An indirect effect of syndecan-1 was also discussed for multiple myeloma where the protein interaction with OPG was proposed as a cause for lower OPG levels in patient with osteolytic lesions compared to patients without this complication . In this study, we found that OPG expression was up-regulated after syndecan-1 knock-down in MCF-7 cells. Thus, showing that syndecan-1 does not only alter OPG function at the protein level, but also at transcriptional level. Moreover, the modulation of OPG by syndecan-1 knock-down was sufficient to alter osteoclast biology. Thus, our study supports the osteoclast-promoting effects of syndecan-1 and identified it as a novel direct regulator of OPG.