TP53 Positively Regulates Cell Death through TRAILR2 (DR5) but Negatively through TRAILR1 (DR4) In Myeloma Cells

Abstract 136

Mapatumumab and lexatumumab are human antibodies that bind and activate death receptors TRAILR1/TNFSF10A/DR4 and TRAILR2/TNFSF10B/DR5, respectively. Treatment of primary myeloma cells and myeloma cell lines with these mAbs induced cell death. Mapatumumab induced cell death more effectively than lexatumumab in a panel of 30 human myeloma cell lines (HMCLs). Interestingly, sensitivity to mapatumumab and lexatumumab was mutually exclusive and related to TP53 status (p=0.006). Indeed, wildtype TP53 HMCLs (n=9) were sensitive to lexatumumab (mean of death 40%) but resistant to mapatumumab (mean of death 7%). In contrast, abnormal (n=21) TP53 HMCLs were resistant to lexatumumab (mean of death 7%) but sensitive to mapatumumab (mean of death 44%). Of note, killing by lexatumumab was correlated to TRAILR2 expression while no correlation was found for TRAILR1 expression and lapatumumab killing. Transcriptomic analysis of 30 HMCLs revealed that HMCLs with abnormal TP53 status underexpressed 4 well-known p53 target genes, MDM2, CDKN1A, Bax and TRAILR2 (p<0.01). To activate p53 pathway in myeloma cells, we used melphalan at low doses. Melphalan treatment of wildtype HMCLs, but not of TP53 abnormal HMCLs, increased TRAILR2 expression and cell death mediated by lexatumumab. In contrast, melphalan did not alter TRAILR1 expression or mapatumumab-induced killing, suggesting that TRAILR2 but not TRAILR1 is a p53 target in myeloma cells. Silencing TP53 significantly increased mapatumumab apoptosis (>50% p<0.01). In good agreement with Bax underexpression in TP53 abnormal HMCLs, extensive silencing of key molecules of both extrinsic (caspase 8, caspase 3) and intrinsic pathways of apoptosis (caspase 9, Bid, Bim, Bax) showed that mapatumumab killing was dependent on the extrinsic pathway of apoptosis only: only silencing of caspase 8 or caspase 3 inhibited mapatumumab killing. Altogether, these data show that killing through TRAIL receptors is differentially regulated by p53 in myeloma cells, positively for TRAILR2 but negatively for TRAILR1. Interestingly, myeloma cells with an abnormal p53 that are more resistant to all drugs are more sensitive than wt ones to killing through TRAILR1 making this pathway very attractive for p53 deficient myeloma cells.