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 approximately 28% or approximately 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.