Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent activator of apoptotic pathway in a variety of tumor cells but not normal cells. Therefore, TRAIL and its receptors have been considered as possible therapeutic targets in cancer treatment. However, several myeloma cells were resistant to TRAIL-induced apoptosis depending on the expression patterns of TRAIL receptors including TRAIL-R1 and -R2 death receptors, and TRAIL-R3 and -R4 decoy receptors. To explore the contribution of each TRAIL receptor to apoptosis induction of myeloma cells, we generated fully human monoclonal antibodies (MoAbs) that bind specifically to TRAIL-R1 and TRAIL-R2 using KM mice that possess the human chromosome fragments containing human immunoglobulin heavy chain loci and YAC transgene containing human kappa light chain gene. Several myeloma cell lines as well as freshly isolated myeloma cells were cultured with TRAIL or these MoAbs in the presence of F(ab′)2 goat anti-human IgG. Soluble TRAIL (1000 ng/mL) showed more than 80% of cytotoxicity in RPMI 8226 and KMS12-BM cells within 24 hours. However, U266, HS-Sultan, and ARH-77 cells were relatively resistant to TRAIL with maximal cytotoxicity of only 3–31%. In contrast, anti-TRAIL-R1 MoAb (R1-B12) effectively induced apoptosis even in TRAIL-resistant myeloma cell lines in a time- and dose-dependent manner with maximal cytotoxicity of 65-99%. This apoptotic response of myeloma cells was confirmed by caspase activation and annexin V binding. On the other hand, anti-TRAIL-R2 MoAb (R2-E11) was less effective against these myeloma cell lines showing 1–33% of cytotoxicity. Among 10 primary myeloma cells, R1-B12 induced at least 10% of cytotoxicity in 7 samples and R2-E11 showed in 3 samples. Flow cytometric analysis demonstrated that TRAIL-R1 was expressed at a higher level than TRAIL-R2 on these myeloma cell lines and specific mean fluorescence intensity (MFI) was 3.8–7.9 and 1.6–4.2, respectively. TRAIL-R1 and -R2 were also detected on primary myeloma cells at low levels and specific MFI was 1.0–2.0 and 1.0–1.6, respectively. Thus, the sensitivity to R1-B12 and R2-E11 was correlated with the expression level of TRAIL-R1 and -R2 on cell surface. Treatment of proteasome inhibitor bortezomib and histone deacetylase (HDAC) inhibitor suberoylanilide hydroxamic acid (SAHA) did not increase the cell surface expression of TRAIL-R1 and -R2 in myeloma cells. However, bortezomib and SAHA induced reduction of cellular FLICE inhibitory protein (c-FLIP) and synergistically enhanced the effect of R1-B12 but not of R2-E11 on apoptosis induction of TRAIL-resistant U266 cells. These results suggest that TRAIL-R1 mainly contribute to TRAIL-induced apoptosis in myeloma cells and that R1-B12 may have the therapeutic potential in combination with bortezomib or HDAC inhibitors in patients with multiple myeloma.

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