The Anti-CD 20 Mab R603 Induces PTEN and RKIP Expression in B-NHL Cells: Role in Inhibition of Constitutively Activated Akt and NF-ΚB Survival Anti-Apoptotic Pathways.

Abstract 1970

Poster Board I-993

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is associated with the human cancer susceptibility locus at 10q23. PTEN is one of the most frequently mutated genes in human cancer. Cells deficient in PTEN exhibit increased proliferation, reduced apoptosis and enhanced migration. PTEN primarily converts phosphatidylinositol-3,4,5,-triphosphate (PIP3) in the cytoplasm to phophatidylinositol-4,5-biphosphate (PIP2), thereby directly antagonizing the activity of PI3 kinase (PI3K). Its inactivation results in constitutive activation of the PI3K/Akt/mTOR survival pathway, normally associated with cancer development and progression. In contrast, overexpression of wild type PTEN in cancer cells induces apoptosis and blocks cell-cycle progression, colony formation and cell migration. Therefore, agents that can induce the expression of PTEN in cancer cells are needed and that can be used alone or in combination with cytotoxic drugs in the treatment of resistant cancers. We have reported that rituximab (chimeric anti-CD 20 mAb) inhibits the PI3K/Akt pathway in B-NHL cells and inhibition of this pathway contributed to rituximab-mediated sensitization of resistant tumor cells to apoptosis by chemotherapeutic drugs (Suzuki et. al., Oncogene 2007; 26:6184). It has also been reported that low levels of PTEN expression in DLBCL is a poor prognostic factor. The underlying mechanism by which the anti-CD 20 mAb inhibits the PI3K/Akt pathway is not known and is the subject of the present investigation. We hypothesized that anti-CD 20 mAb-mediated inhibition of the PI3K/Akt pathway might result from the derepression and expression of PTEN. PTEN has been reported to be inhibited by the activation of NF-ΚB and NF-ΚB has been shown to be inhibited by anti-CD 20 mAb. We also hypothesized that inhibition of NF-ΚB by anti-CD 20 mAb was due, in part, to the induction of the NF-ΚB inhibitor, Raf-1-kinase inhibitor protein (RKIP). These two hypotheses were tested using a novel anti-CD 20 mAb, namely R603 (derived from LFB). We expected that R603 treatment of B-NHL cells will result in the induction of both PTEN and RKIP expression through its inhibitory effect on NF-ΚB. Experimentally, Ramos cells were treated with various concentrations of R603 (5-40 μg/ml) for 18 hours and the lysates were prepared and examined for gene products expression by western. The findings revealed that R603 induces significantly the expression of both PTEN and RKIP and the levels of expression were a function of the antibody concentration used. The induction of PTEN and RKIP was paralleled by the inhibition of both PI3K/Akt and NF-ΚB activated pathways. Our studies in non-lymphoid cancers (prostate carcinoma and melanoma) revealed that the transcription repressor Snail, downstream of NF-ΚB, negatively regulates the transcription and expression of PTEN and RKIP and, thus, the inhibition of NF-ΚB by anti-CD 20 mAb should result in the inhibition of Snail. These findings demonstrate a novel mechanism by which R603 mediates its signaling modification in B-NHL through the induction of PTEN and RKIP and, consequently, regulates the sensitiviy of B-NHL cells to various cytotoxic drugs.