Abstract
Rituximab is a human chimeric monoclonal antibody that targets the CD20 receptor uniquely expressed on normal and malignant B-cells. While it is particularly useful for follicular lymphoma, Rituximab has recently been used for other types of B-cell proliferative disorders. Four major mechanisms have been proposed to account for the activity of Rituximab in B-cell lymphoproliferative disorders: 1) apoptosis and proliferation inhibition via intracellular signaling, 2) increased sensitivity to other chemotherapeutic drugs, 3) complement-dependent cytotoxicity (CDC), and 4) antibody-dependent cellular cytotoxicity (ADCC). A combination of all four mechanisms has also been proposed as the mechanism of action. However, the intracellular events that occur in B-cells in response to Rituximab are largely unknown.
We have employed a global quantitative proteomic profiling approach to study the effects of Rituximab on a t(14;18)+ B cell lymphoma (SUDHL-4). A decrease in viability of 31.3 ± 2.9% was observed after treatment with 10μg/mL of Rituximab for 48 hours. We used isotope-coded affinity tagging (ICAT™) in combination with microcapillary liquid chromatography and tandem mass spectrometry (μLC-MS/MS) to quantitatively determine the proteomic differences between equal amounts of untreated and treated total cell lystates. Protein lysates were labeled with either heavy or light cleavable ICAT™ reagents, digested with trypsin, and purified using avidin affinity chromatography. The labeled peptides were then fractionated using off-line strong cation exchange (SCX) and each fraction was subjected to μLC-MS/MS analysis. Several proteins previously unknown to be expressed in B-cells were identified using this approach. Of the 123 positively identified proteins, 59 proteins were differentially expressed by greater than or less than 1.5 fold (25 upregulated, 34 downregulated). Analysis of differentially expressed proteins identified several functional groups, including integral membrane proteins (Integrin alpha-7 precursor), transcription factors (Zinc finger proteins 445 and 20), proteins involved in migration and adhesion (Astrotactin1), proteins associated with degradation and turnover (26S proteasome non-ATPase regulatory subunit 9), proteins affecting calcium-induced signaling (Tumor-associated calcium signal transducer 1 precursor), proteins associated with lipid rafts (Gemin7), and components of the phosphoinositol (PI4K-alpha) and NF-κB pathways (IKK-E). Our studies reveal the proteomic consequences of Rituximab administration to neoplastic B-cells, and provide novel insights into the mechanism of the drug’s efficacy in the treatment of susceptible B-cell lymphoproliferative disorders.
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