SB431542, a TGF-Beta Receptor Kinase Inhibitor, Restores Bone Formation Which Ameliorates Myeloma-Induced Microenvironment.

Multiple myeloma (MM) cells stimulate bone resorption and concomitantly suppress bone formation, leading to devastating bone destruction. TGF-beta, a potent inhibitor for terminal osteoblasts (OB) maturation and mineralization, is abundantly produced and released from bone tissues through enhanced bone resorption, and activated by osteoclasts (OC)-derived acids and MMPs in MM bone lesions. In the present study we investigated the impact of TGF-beta inhibition on induction of bone formation in MM as well as the effects of matured OB on MM growth. TGF-beta completely suppressed BMP-2-induced mineralized nodule formation by OB cultured in osteogenic media supplemented with beta-glycerophosphate and vitamin C. SB431542, an inhibitor of TGF-beta type I receptor kinase, potently suppressed induction of Smad6 by TGF-beta, which inhibits BMP-2 signaling. SB431542 along with BMP-2 abolished such TGF-beta actions and enhanced mineralized nodule formation more than BMP-2 alone. Notably, addition of SB431542 antagonized the inhibitory effects of conditioned media from MM cell lines (RPMI8226 and U266) and bone marrow plasma from patients with MM, resulting in restoration of the BMP-induced mineralized nodule formation. Furthermore, MC3T3-E1 cells matured by BMP-2 enough to exhibit mineralized nodules suppressed the proliferation of 5TGM1 MM cells in sharp contrast to stromal cells as well as untreated or TGF-beta-treated undifferentiated MC3T3-E1 cells that promote MM cell growth and survival. Interestingly, 5TGM1 MM cell growth was also potently suppressed by MC3T3-E1 cells cultured with BMP-2 in the absence of beta-glycerophosphate to achieve OB maturation without formation of mineralized nodules, suggesting a responsible role for OB-derived non-mineralized factors associated with terminal differentiation of OB. In addition, the induction of OB maturation down-regulated the production by OB of IL-6 and up-regulated osteoprotegerin, an inhibitor for osteoclastogenesis. Taken together, blockade of TGF-beta actions release OB from the differentiational arrest in MM bone disease and thus can be a good therapeutic maneuver restoring bone formation as well as suppressing osteoclastogenesis to ameliorate bone destruction and at the same time suppressing MM expansion by disrupting the MM-induced microenvironment which can be called as MM niche.