Although osteolysis is a common complication in patients with multiple myeloma (MM), the biologic mechanisms involved in the pathogenesis of MM-induced bone disease are poorly understood. Two factors produced by stromal-osteoblastic cells seem critical to the regulation of bone resorption: osteoprotegerin (OPG) and its ligand (OPGL). OPGL stimulates osteoclast differentiation and activity, whereas OPG inhibits these processes.

The proteasome inhibitor bortezomib may increase osteoblast-related markers in multiple myeloma (MM) patients; however, its potential osteoblastic stimulatory effect is not known. In this study, we show that bortezomib significantly induced a stimulatory effect on osteoblast markers in human mesenchymal cells without affecting the number of osteoblast progenitors in bone marrow cultures or the viability of mature osteoblasts. Consistently we found that bortezomib significantly increased the transcription factor Runx2/Cbfa1 activity in human osteoblast progenitors and osteoblasts without affecting nuclear and cytoplasmatic active ␤-catenin levels. IntroductionMultiple myeloma (MM) is a plasma cell malignancy characterized by the high capacity to induce osteolytic bone lesions. 1 Hystomorphometric studies showed that osteoblast formation and function are profoundly impaired in MM patients and critical in the bone lesion development. [1][2][3] Several mechanisms are involved in MMinduced osteoblast suppression 1,4,7 including the production of Wnt inhibitors as DKK-1 or sFRP-2 5,6 or the impairment of osteoblast formation and differentiation through the block of the critical osteoblast transcription factor Runx2/Cbfa1. 7 In turn, osteoblastic cells also regulate myeloma cell growth 8,9 and the increase of bone formation in mice results in a reduction of tumoral burden. 10 Recent data suggest that ubiquitin-proteasome pathway, which is the major cellular degradative system and therapeutic target in myeloma cells, 11 also regulates osteoblast differentiation. [12][13][14] The ubiquitin-proteasome pathway can modulate the BMP-2 expression, 12 which can induce osteoblast differentiation through the Wnt signaling 13 and regulates the proteolytic degradation of the osteoblast transcription factor Runx2/ Cbfa1. 14 Recently, Garrett et al 12 demonstrated that proteasome inhibitors as PS1 and epoximicin stimulate bone formation in neonatal murine calvarial bones and in vivo in mice. 12 A strong correlation between the capacity of these compounds to inhibit proteasomal activity in osteoblasts and their bone-forming activity was also demonstrated. 12 Preliminary observations obtained in MM patients treated with the proteasome inhibitor bortezomib show an increase of serum bone-specific alkaline phosphatase and other osteoblast related markers suggesting a potential osteoblast stimulatory effect of this drug. [15][16][17][18] Currently it is not known whether the proteasome inhibitor bortezomib may have a direct effect on osteoblast differentiation and formation in vitro in human cultures and in vivo in MM patients. Patients, materials, and methods DrugsBortezomib was purchased from Janssen-Cilag (Milan, Italy). For in vitro studies, the drug was reconstituted in DMSO at a stock concentration of 50 mM, and this stock was diluted in medium just before use, so that the concentration of DMSO never exceeded 0.1%. The proteasome inhibitors MG-132 and MG-262 were purchased from BIOMOL International (Plymouth Meeting, PA; DBA srl, M...

Key Points• Myeloma cells produce ammonium in the presence of glutamine, showing high glutaminase and low glutamine synthetase expression.• Myeloma cells show high expression of glutamine transporters and inhibition of ASCT2 transporter hinders myeloma growth.The importance of glutamine (Gln) metabolism in multiple myeloma (MM) cells and its potential role as a therapeutic target are still unknown, although it has been reported that human myeloma cell lines (HMCLs) are highly sensitive to Gln depletion. In this study, we found that both HMCLs and primary bone marrow (BM) CD138 1 cells produced large amounts of ammonium in the presence of Gln. MM patients have lower BM plasma Gln with higher ammonium and glutamate than patients with indolent monoclonal gammopathies. Interestingly, HMCLs expressed glutaminase (GLS1) and were sensitive to its inhibition, whereas they exhibited negligible expression of glutamine synthetase (GS). High GLS1 and low GS expression were also observed in primary CD138 1 cells. Gln-free incubation or treatment with the glutaminolytic enzyme L-asparaginase depleted the cell contents of Gln, glutamate, and the anaplerotic substrate 2-oxoglutarate, inhibiting MM cell growth. Consistent with the dependence of MM cells on extracellular Gln, a gene expression profile analysis, on both proprietary and published datasets, showed an increased expression of the Gln transporters SNAT1, ASCT2, and LAT1 by CD138 1 cells across the progression of monoclonal gammopathies. Among these transporters, only ASCT2 inhibition in HMCLs caused a marked decrease in Gln uptake and a significant fall in cell growth. Consistently, stable ASCT2 downregulation by a lentiviral approach inhibited HMCL growth in vitro and in a murine model. In conclusion, MM cells strictly depend on extracellular Gln and show features of Gln addiction. Therefore, the inhibition of Gln uptake is a new attractive therapeutic strategy for MM. (Blood. 2016;128(5):667-679)