TRAIL-induced apoptosis of authentic myeloma cells does not correlate with the procaspase-8/cFLIP ratio.

We read with interest Mitsiades et al’s recent paper1 purporting to define the intracellular factors regulating tumor necrosis factor– related apoptosis-inducing ligand (TRAIL) activity in myeloma cells. They demonstrated quite clearly the crucial role for procaspase-8 activation in initiating TRAIL-induced apoptosis and the clear correlation between the efficiency of procaspase-8 activation and the degree of TRAIL-induced apoptosis. Furthermore, they were able to demonstrate that maneuvers capable of inhibiting antiapoptotic proteins or artificially elevating the levels of intracellular procaspase-8 enhanced the apoptosis-inducing capability of TRAIL. This preliminary data demonstrating the “sensitization” of previously TRAIL-resistant myeloma cells with novel agents are very interesting and, if it can be confirmed that these strategies do not also sensitize nonmalignant cells, may well provide a rationale for early-phase clinical trials. Of concern, however, is the claim that the degree of TRAIL resistance of the cell lines studied was associated with a low procaspase-8/cFLIP (FLICE inhibitory protein) ratio. No data are actually presented to support this assertion. Scrutiny of the data that are presented shows that there is no obvious correlation between the physiologic levels of cFLIP and/or procaspase-8 and the degree of TRAIL-induced apoptosis for the cell lines studied. This is clearly exemplified by the “sensitive” cell lines MM1S and RPMI showing quite different immunoblot findings, with high procaspase8/low cFLIP ratios and low procaspase-8/high cFLIP ratios, respectively. Our own data from 5 authentic myeloma cell lines confirm the crucial relationship between the efficiency of procaspase-8 activation and TRAIL-induced apoptosis (Figure 1) and also show the lack of correlation between the physiologic procaspase-8/cFLIP levels and TRAIL-induced apoptosis (Figure 2). There are 4 known surface receptors for TRAIL, 2 of which (TRAIL-R1 and TRAIL-R2) appear to be capable of inducing apoptosis upon binding of TRAIL.2 Our understanding of TRAIL– TRAIL receptor interactions beyond this is limited. But the earlier belief that the 2 “decoy” TRAIL receptors (TRAIL-R3 and TRAIL-R4) lacking intracellular death domains somehow protect cells from TRAIL has been shown by our group and others to be incorrect.3-5 For TRAIL to be exploited maximally as an antitumor agent, how it works must be more thoroughly understood, particularly the exact roles of the known TRAIL receptors. Acceptance of the perhaps premature assertion that myeloma-cell TRAIL sensitivity is regulated by physiologic levels of procaspase-8 and/or cFLIP will do little to improve this lack of understanding.