Acquired rituximab resistance is associated with cross -resistance to chemotherapy that is overcome by treatment with bortezomib

Despite recent advances in therapy, acquired resistance is common among patients with B-cell non-Hodgkin's lymphoma (B-NHL). Targeting malignant B-cells using the anti-CD20 monoclonal antibody rituximab has improved the efficacy of chemotherapy used to treat patients with B-NHL. In vitro studies suggest synergistic activity of rituximab and chemotherapy results from modification of the ratio of pro- to anti-apoptotic Bcl-2 family proteins via signaling events triggered by CD20 cross-linking. Despite promising clinical results obtained using rituximab, many patients relapse with therapy resistant disease following rituximab-based treatments. Acquired therapy resistance is an especially significant problem when cross-resistance is developed, that is when resistance to one therapy imparts resistance to other therapies with different mechanisms of action. Pre-clinical and clinical evidence suggests that rituximab/chemotherapy cross-resistance can arise and that alterations in Bcl-2 family proteins may be responsible for this cross-resistance. To explore the molecular basis of therapy resistance, we have created cell line models of rituximab resistance using B-cell non-Hodgkin's lymphoma derived cell lines. This thesis focuses on the sensitivity of RRCL to standard chemotherapeutic agents and the use of the proteasome inhibitor bortezomib (PS-341, Velcade RTM) to overcome therapy resistance in B-NHL.;When exposed to doses of standard chemotherapeutic agents sufficient to induce apoptosis of rituximab-sensitive cell lines (RSCL), RRCL did not undergo apoptosis. Significant decreases in expression of the pro-apoptotic Bcl-2 family proteins Bak and Bax were observed in all RRCL when compared to RSCL. Loss of Bax was due to a common point mutation in a stretch of eight deoxyguanosine residues known as the G(8) tract, while decreased Bak expression occurred at the post-transcriptional level. Exogenous expression of wild-type Bax or Bak led to rapid, caspase-dependent cell death of RRCL. Furthermore, forced expression of Bak in RRCL sensitized them to chemotherapy-induced apoptosis. While a single or limited exposure of lymphoma cells to rituximab may lead to a favorable ratio of pro- to anti-apoptotic Bcl-2 family proteins, repeated exposure to rituximab is associated with a therapy resistant phenotype via modulation of Bax and Bak expression.;It has become increasingly apparent that proteasome inhibitors, such as bortezomib, may act by altering the expression of Bcl-2 family proteins. Treatment of RRCL with bortezomib led to caspase activation and cell death subsequent to enhanced expression of pro-apoptotic Bcl-2 family proteins Bak, Bik and Noxa. Together, these proteins are responsible for bortezomib-induced caspase activation but surprisingly appear dispensable for cell death. Indeed, a caspase-independent cell death mechanism was induced in resistant cells following bortezomib treatment. Together our data suggest that bortezomib is capable of overcoming therapy resistance in non-Hodgkin's lymphoma via caspase-dependent and -independent mechanisms.