Pre-clinical studies have demonstrated that rituximab triggers apoptotic signals in B-cell lymphoma cells upon biding to CD20 antigen. Downstream signaling events observed in lymphoma cells following in vitro exposure of rituximab or chemotherapy include: 1) activation of the intrinsic apoptotic pathway and 2) increased mitogen activated protein kinase (MAPK) activity. In addition, pre-clinical and clinical studies strongly suggest that rituximab may sensitize lymphoma cells to apoptotic effects of various drugs used to treat NHL. Despite its anti-tumor activity, many patients relapse after initial response to rituximab-based therapy and demonstrate variable degrees of rituximab resistance. To further study the impact of rituximab resistance in cellular responses to chemotherapy we developed several rituximab resistant cell lines (RRCL) derived from Raji, SU-DHL-4 and RL cells by exposing cells to escalating doses of rituximab with (4RH cells) or without (2R cells) human serum. The rituximab resistance of each RRCL was confirmed by immunological assays. Subsequently, lymphoma cells (parental and RRCL) were exposed to various chemotherapeutic agents (cisplatin, doxorubicin, paclitaxel, or vincristine) for up to 48 hours. Detection of cell death was determined by trypan blue and/or propidium iodine staining. Caspase-3 activity was measured by PhiPhi Lux G1D2 enzymatic cleavage. Bcl-2 expression was determined by Western blotting. Chemotherapy resistance to all agents tested was observed in RRCL when compared to parental Raji, SU-DHL-4 and RL cell lines. In addition, caspase-3 activity was lower in RRCL following chemotherapy exposure than in parental cell lines. A significantly lower percent of RRCL cells within sub-G0/G1 peaks in cell cycle histograms confirmed that RRCL were less sensitive to chemotherapy-induced apoptosis. Additionally, an increase in Bcl-2 expression was observed in RRCL when compared parental cell lines. Our data strongly suggest that chemotherapy resistance emerges concomitantly with the acquirement of rituximab resistance in lymphoma cells. Chronic exposure to rituximab appears to cause overexpression of Bcl-2, which likely renders RRCL less susceptible to the apoptotic effects of chemotherapy agents. Ongoing studies are aimed at identifying and overcoming rituximab/chemotherapy shared cellular pathways of resistance.

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