TRAILR Triggering by Human Antibodies Induces Apoptosis through an Early Cleavage of Mcl-1 and Caspase 3 in Myeloma Cells.

We investigated TRAILR expression and sensitivity of myeloma cells in vitro. This study was done using a panel of 20 myeloma cell lines that are representative of primary myeloma cells (14q chromosomal translocation, IL-6 dependency, phenotype, oncogenes mutation). TRAILR were stimulated with agonistic human antibodies directed against either TRAIL-R1/DR4 (HGS-ETR1, mapatumumab) or TRAIL-R2/DR5 (HGS-ETR2), provided by Human Genome Sciences, Rockville, MD. This approach allowed us to analyze the contribution of each receptor separately. We show that a wide majority of cell lines, 16 of 20 were killed upon either TRAIL-R1 or R2 stimulation in the presence or absence of IL-6. However, 4 cell lines were resistant to HGS-ETR1 and 6 to HGS-ETR2 and 3 to both. Activation of both caspase 8 and Bid has been extensively described as being associated with TRAIL response. Indeed, we observed an activation of both caspase 8 and Bid. Cleaved molecules were detected 6 to 18h after antibody addition but after detection of cellular apoptosis. However, we show that Mcl-1L, a key molecule for myeloma survival, was downregulated and cleaved as soon as 3h after Ab addition. The cleaved form of Mcl-1 has been shown to behave like a proapoptotic molecule. Since caspase 3 has been reported to cleave Mcl-1, we looked at caspase 3 activation. Indeed, we observed that caspase 3 cleavage occured early and concomitantly to the one of Mcl-1. Our data show that in a wide majority of myeloma cell lines (80%) TRAILR triggering induces massive apoptosis that was not prevented by IL-6, the major myeloma cell growth and survival factor. Moreover, apoptosis induced upon TRAILR triggering was fully correlated to an early cleavage of both caspase 3 and Mcl-1 and to a delayed one of both caspase 8 and Bid.