Rituximab-Induced Inhibition of YY1 and Bcl-xL Expression in Ramos Non-Hodgkin’s Lymphoma Cell Line via Inhibition of NF- B Activity: Role of YY1 and Bcl-xL in Fas Resistance and Chemoresistance, Respectively

Rituximab treatment of B non-Hodgkin’s lymphoma (NHL) cell lines inhibits the constitutive NFB activity and results in the sensitization of tumor cells to both chemotherapy and Fas-induced apoptosis. Cells expressing dominant active I B or treated with NFB-specific inhibitors were sensitive to both drugs and Fas agonist mAb (CH-11)-induced apoptosis. Down-regulation of Bcl-xL expression via inhibition of NFB activity correlated with chemosensitivity. The direct role of Bcl-xL in chemoresistance was demonstrated by the use of Bcl-xL-overexpressing Ramos cells, Ramos hemagglutinin (HA)-Bcl-x, which were not sensitized by rituximab to drug-induced apoptosis. However, inhibition of Bcl-xL in Ramos HA-Bcl-x resulted in sensitization to drug-induced apoptosis. The role of Bcl-xL expression in the regulation of Fas resistance was not apparent; Ramos HA-Bcl-x cells were as sensitive as the wild type to CH-11-induced apoptosis. Several lines of evidence support the direct role of the transcription repressor yin-yang 1 (YY1) in the regulation of resistance to CH-11-induced apoptosis. Inhibition of YY1 activity by either rituximab or the NO donor DETANONOate or after transfection with YY1 small interfering RNA resulted in up-regulation of Fas expression and sensitization to CH-11-induced apoptosis. These findings suggest two mechanisms underlying the chemosensitization and immunosensitization of B-NHL cells by rituximab via inhibition of NFB. The regulation of chemoresistance by NFB is mediated via Bcl-xL expression, whereas the regulation of Fas resistance by NFB is mediated via YY1 expression and activity. The potential clinical significance of these findings is discussed. The Journal of Immunology, 2005, 175: 2174–2183.