Blocking decorin expression or activity attenuates the inhibitory effect of osteoblasts on myeloma cell growth. (A,B) MSCs infected with lentivirus-introduced scrambled (Scram) or decorin (DCN) shRNA or left untreated (mock) were differentiated into osteoblasts. Immunohistochemical staining for decorin (panel A, brown color indicates positive staining, original magnification ×200; see “Immunohistochemistry”) and decorin expression as determined by qRT-PCR (panel B) indicate that osteoblasts produce and express high levels of decorin and that decorin expression and production are markedly reduced by specific shRNA (DCN shRNA). (C) Knockdown of decorin had no effect on the ability of MSCs to differentiate into osteoblasts as determined by Alizarin red staining. (D) Myeloma cells from 4 patients were cocultured with untreated osteoblasts (mock) or with osteoblasts infected with lentivirus-introduced scrambled (Scram) or decorin (DCN) shRNA for approximately 7 days, and then subjected to MTT assay. Note the higher myeloma cell growth rate after decorin knockdown in osteoblasts (P < .03, DCN shRNA vs Scram shRNA or mock). (E) Recombinant human decorin (rhDCN) inhibited the growth of myeloma cells in coculture with osteoclasts, an effect that was completely abrogated by decorin-neutralizing antibody (DCN AB). (F) Myeloma cells from 5 patients were cocultured with osteoclasts in the presence or absence of osteoblast-conditioned medium (OB CM) and decorin-neutralizing antibody (5 μg/mL) before being subjected to MTT assay. Osteoblast-conditioned medium inhibited myeloma cell growth (P < .002), an effect that was partially blocked by decorin-neutralizing antibody (P < .01, OB CM vs OB CM + DCN AB).