In vitro susceptibility to TRAIL-induced apoptosis of acute leukemia cellsin the context of TRAIL receptor gene expression and constitutive NF-kappaB activity

The TNF-related apoptosis-inducing ligand (TRAIL) is currently under evalua tion as a possible (co-)therapeutic in cancer treatment. We therefore exami ned 129 cell samples from patients with de novo acute leukemia as to their constitutive susceptibility to TRAIL-induced apoptosis in vitro. Only 21 (1 6%) cell samples revealed at least 10% TRAIL-susceptible cells/sample as de tected by flow cytometric annexinV staining after 24 h culture compared wit h medium control. Precursor B cell ALL samples (11 (27%) of 41) were more T RAIL-susceptible compared with AML (5 (9%) of 54; P < 0.05) but not compare d with precursor T cell ALL (5 (15%) of 34; P= 0.20). Furthermore, we exami ned constitutive mRNA expression levels of TRAIL receptors R1-R4 by semi-qu antitative RT-PCR (n=58). Expression levels were heterogeneous, however, th ere was no significant correlation between the expression of the signal-tra nsducing receptors (R1, R2) as well as of the decoy receptors (R3, R4) and TRAIL sensitivity in this series. Constitutive NF-<kappa>B activity has bee n shown to influence TRAIL susceptibility of leukemic cells. In 39 leukemic cell samples examined, we found a generally high NF-kappaB activity as det ected by electrophoretic mobility shift assay which did not differ between TRAIL-susceptible and TRAIL-resistant cases. Finally, 49 acute leukemic cel l samples were coincubated with doxorubicin in vitro. Doxorubicin sensitize d four of 35 initially TRAIL-resistant samples and augmented TRAIL-induced apoptosis in two of 14 TRAIL-susceptible samples. In summary, constitutive TRAIL susceptibility differs between leukemia subtypes and does not correla te with mRNA expression levels of the TRAIL receptors R1-R4 as well as cons titutive NF-kappaB activation status. The observed sensitization of leukemi c cells to TRAIL by doxorubicin in vitro indicates that TRAIL should be fur ther evaluated as to its possible role as an in vivo cotherapeutic in acute leukemia.