Characterization of the Resistance Mechanisms of HL60 Leukemia Cell Derivatives to Apo2L/TRAIL.

Tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) is a recently identified member of the TNF superfamily. Recombinant Apo2L/TRAIL is a promising immunotherapeutic agent for treating malignant diseases since this molecule preferentially induces apoptosis in a variety of tumor cells with apparently little toxicity to normal cells. However, it has also been shown that some tumor cells are resistant to this molecule. We hypothesized that resistance to Apo2L/TRAIL occurs through defects in the Apo2L/TRAIL-mediated apoptotic signaling pathway. To address this hypothesis, we developed several Apo2L/TRAIL-resistant HL60 derivatives (HL60/TR) by exposure of Apo2L/TRAIL-sensitive HL60 human leukemia cells to escalating levels of Apo2L/TRAIL, followed by subcloning. Two of these resistant clones (a moderately resistant clone-R1 and a highly resistant clone-R3) were selected for further study. Molecules in the Apo2L/TRAIL-mediated apoptotic pathway of R1 and R3 cells were analyzed by Western blot analysis, flow cytometry and gene sequencing and compared to those in parental HL60 cells. In the R1 cells, the activation of caspase-8 and -10 by Apo2L/TRAIL was significantly inhibited. However, R1 cells were still sensitive to Fas agonistic monoclonal antibody treatment, indicating that the FAS-mediated apoptosis-inducing pathway was intact. In the R3 cells, caspase-8 expression was completely lost and activation of caspase-10 in response to Apo2L/TRAIL was totally inhibited; R3 cells were therefore also resistant to FAS antibody treatment. Although the total protein level of death receptors DR4 and DR5 was equal in HL60 cells and in the Apo2L/TRAIL-resistant derivatives, the cell surface levels of DR4 were significantly decreased in both R1 and R3 cells, while the surface expression level of DR5 in these two clones was comparable to that on HL60 cells. No mutation in either the DR4 or DR5 genes was found in these cells. These results suggest that defective targeting of DR4 molecules to the cell surface occurs in these Apo2L/TRAIL resistant cells. Blocking cell surface DR4 significantly attenuated the sensitivity of parental HL60 cells to Apo2L/TRAIL, indicating that cell surface expression of DR4 plays a crucial role in regulating susceptibility of tumor cells to Apo2L/TRAIL. Taken together, our results demonstrate that malignant cells can develop resistance to Apo2L/TRAIL by several different mechanisms and multiple resistance mechanisms may develop in a single tumor cell (such as R3 cells). Understanding the basis of Apo2L/TRAIL resistance will help to predict sensitivity and to develop strategies to circumvent resistance.