The Type III Transforming Growth Factor (TGF)-β Receptor Regulates Growth, Proliferation and Adhesion in Multiple Myeloma Cells

The TGF-β signaling pathway has an important role in regulating normal hematopoeisis, inhibiting proliferation while stimulating differentiation when appropriate. TGF-β ligands regulate cellular processes by binding to three high affinity cell surface receptors: the type I TGF-β receptor (TβRI), the type II TGF-β receptor (TβRII) and the type III TGF-β receptor (TβRIII or betaglycan). TβRIII has essential and non-redundant roles in mediating and regulating TGF-β signaling. Our laboratory has demonstrated that TβRIII serves as a novel tumor suppressor in epithelial-derived solid tumors. Like most human cancers, multiple myeloma (MM) cells develop resistance to the homeostatic functions of the TGF-β signaling pathway. While loss of TβRIII expression correlates with tumor progression in many epithelial-derived tumors, the role of this receptor in MM is unknown. Analysis of TβRIII expression in MM specimens revealed loss of TβRIII expression, which correlated with disease progression from monoclonal gammopathy of undetermined significance to MM. Consistent with results in MM specimens, TβRIII expression was decreased at the mRNA and protein levels in MP399, NCI-H929, OPM2, and RPMI-8226 human MM cell lines, with only the U266 MM cell line expressing significant levels of TβRIII. To investigate whether decreased expression of TβRIII contributes to the pathogenesis of MM, we examined the effect of TβRIII on cell growth, apoptosis, proliferation, and cell-matrix adhesion. Restoring TβRIII expression in RPMI-8226 cells by adenoviral-mediated infection significantly (>50%, p<0.05) inhibited cell growth. While apoptosis was not affected, restoring TβRIII expression inhibited the proliferation of RPMI-8226 cells. Mechanistically, TβRIII-mediated inhibition of proliferation was associated with increased levels of the cyclin-dependent kinase inhibitors p21, p27, p15 and p16. In a reciprocal manner, short hairpin RNA-mediated silencing of endogenous TβRIII in U266 cells significantly (>50%, p<0.05) increased cell growth and proliferation. In accordance, knockdown of TβRIII decreased levels of p21, p27, p15 and p16. To investigate whether these cellular effects of TβRIII are ligand-dependent, cells were treated with TGF-β1 or BMP-2. Both TGF-β1 and BMP-2 enhanced TβRIII-mediated inhibition of proliferation in a time- and dose-dependent manner. As MM pathogenesis is dependent on the interaction of MM cells with the stroma, we examined the effects of TβRIII on adhesion to the extracellular matrix. Restoring TβRIII expression in RPMI-8226 cells increased their adhesion to fibronectin (>30%, p<0.05). Taken together, these findings indicate that loss of TβRIII expression during the pathogenesis of MM may contribute to disease progression by promoting increased proliferation and decreasing adhesion of MM cells to the extracellular matrix. Current studies are underway to elucidate the cellular mechanisms for these TβRIII functions and to validate these findings in primary MM cells and bone marrow stroma obtained from MM patients.