OBJECTIVETo investigate the effects of myeloma cells on the differentiation of osteoclast precursors (pOCs) into OCs in different culture systems in vitro and the interaction between OCs and myeloma cells.

METHODSMyeloma cell lines 8226, XG1 and XG7 and pOCs were cocultured in different culture system. OCs was examined by TRAP staining. RT-PCR was used to evaluate the expression of receptor activator of NF-kappaB ligand (RANKL) and osteoprotegerin (OPG) of myeloma cells and the effects of myeloma cells on RANKL/OPG expression in coculture. The role of OCs in myeloma cells cycle was measured by FCM with PI staining. The supportive effects of OCs on myeloma cells survival were determined by FCM with double staining for annexin V and PI.

RESULTS8226 and XG1 cells could directly stimulate the differentiation of pOCs into TRAP+ multinuclear mature OCs. Myeloma cells, which expressed neither RANKL nor OPG, upregulated RANKL expression and decreased OPG expression in mouse primary bone marrow stromal cells (pBMSC). When OCs were co-cultured with myeloma cells, all OCs apparently remained alive after 7 days while devoid of sRANKL and M-CSF. OCs stimulated the proliferation of myeloma cells in co-culture systems,the cell number increased to (3.8 +/- 0.1) x 10(5)/well, (3.9 +/- 0.1) x 10(5)/well, (4.0 +/- 0.1) x 10(5)/well, and to (8.7 +/- 0.1) x 10(5)/well, (9.1 +/- 0.1) x 10(5)/well, (9.0 +/- 0.1 ) x 10(5)/well after co-culture for 3 days and 7 days for XG1 cells, XG7 cells and 8226 cells, respectively (P <0.01). However, OCs could counteract cytotoxic effects of dexamethasone. The proportion of Annexin V-/PI- cells were 57.71%, 82.18% and 90.92% for 8226 cells, XG1 and XG7 cells after co-culture with OCs (P <0.01).

CONCLUSIONMyeloma cells stimulated the differentiation of pOCs into TRAP+ multinuclear mature OCs by directly and/or indirectly disrupting the balance of RANKL/OPG, OCs promoted MM cells growth and survival, thus maintaining a vicious circle between myeloma cells and osteoclasts.