A recessive mutant of the U937 cell line acquired resistance to anti-Fas and anti-p55 tumor necrosis factor receptor antibody-induced apoptosis

Human monocytic leukemia U937 cells readily undergo apoptosis when cells are treated with various stimuli including antitumor agents, tumor necrosis factor (TNF)-α and anti-Fas antibody. However, the signal transduction mechanism resulting in apoptosis is unclear. To study the mechanism of apoptosis, we isolated and characterized a mutant, UK110, from U937 cells, which was resistant to TNF-α- and anti-Fas antibody-induced apoptosis but was less resistant to etoposide-induced apoptosis. TNF-α-induced signals are mediated by two types of TNF receptors (TNFR), p55- and p75-TNFR, and p55- TNFR is homologous to the Fas antigen. Interestingly, UK110 cells showed resistance to apoptosis by agonistic anti-p55-TNFR antibody, indicating that UK110 cells were resistant to Fas-and p55-TNFR-mediated apoptosis. Because expression of apoptosis-associated molecules, such as c-Myc, Bcl-2, and Bax, was similar between U937 and UK110 cells, an undetermined pathway for apoptosis through Fas and p55-TNFR could be mutated in UK110 cells. To clarify the genetic phenotype of UK110 cells, we performed somatic cell hybridization with parental U937 and the UK110 cells. All of the hybrid clones were as sensitive as the parental U937 cells to apoptosis by both anti-Fas and anti-p55-TNFR antibodies, indicating that the apoptosis resistance in UK110 cells resulted from recessive genotype.