Previously we have reported a differential expression of CD95/CD95L and Bcl-2 family of genes in multidrug resistant tumor cells. TRAIL, a member of the TNF receptor family, induces apoptosis in many tumor cells by binding to DR4 (TRAIL receptor 1) and DR5 (TRAIL receptor 2). In contrast, TRAIL-induced apoptosis is prevented by a decoy receptor (DcR1, TRID or TRAIL receptor 3). In the present study, we compared the expression of TRAIL, DR4, DR5, and TRID between a drug sensitive HL60, a myeloid leukemia cell line, and its multidrug resistant (MDR) sublines that either overexpressed MDR 1 gene (HL60/Tax) or MRP gene (HL60/AR), using RT-PCR. TRAIL mRNA was expressed in HL60 cells but was present in low levels in HL60/AR cells and was completely lacking in HL60/Tax cells. Both DR4 and DR5 were undetectable in HL60/Tax but were present at comparable levels in HL60/AR and drug sensitive HL60 cells. TRID were absent in HL60 and HL60/Tax cells, but was present in low but comparable levels in peripheral blood mononuclear cells and HL60/AR cells. These data suggest that the multidrug resistance in MDR HL60 cell lines, regardless of overexpression of MDR 1 or MRP, may be due to different mechanisms. In HL60/AR cells it appears that MDR may be due to decreased expression of TRAIL and constitutive expression of TRID, whereas in HL60/Tax cells, MDR could be due to the absence of TRAIL and/or DR4 and DR5.

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