Rituximab-Mediated Upregulation of PTEN Expression and Inhibition of Both the PI3K/AKT Pathway and Pleitrophin (PTN) Expression in B-NHL: Roles in Cell Growth Inhibition and Chemosensitization.

Abstract

We have recently reported that rituximab treatment of B-NHL cell lines, like Ramos, inhibited the PI3K/AKT signaling pathway and downregulated Bcl-xL expression. The role of the AKT pathway in chemosensitization was corroborated by treating Ramos cells with the AKT inhibitor, LY294002, and resulted in sensitization of the cells to drug-induced apoptosis. We have investigated a potential underlying mechanism responsible for the rituximab-mediated inhibition of the AKT pathway. PTEN (phosphatase and tensin homologue deleted on chromosome 10) is a tumor suppressor phosphatase and functions as a negative regulator of the PI3K pathway through its phosphatase activity. We hypothesized that rituximab may upregulate PTEN expression and thus, inhibiting the AKT pathway. Treatment of Ramos cells with rituximab (20μg/ml for 20h) resulted in significant upregulation of PTEN expression (as assessed by both Western and RT-PCR). Time kinetic analysis showed that PTEN is upregulated as early as 6–9h post rituximab treatment. In addition, since rituximab inhibits cell proliferation and cell growth, we have examined the effect of rituximab on the expression of the growth factor pleitrophin (PTN). PTN is a heparin-binding and secreted growth differentiation factor that mediates various functions such as cell motility and migration, survival, growth and differentiation. Treatment of Ramos cells with rituximab inhibited PTN expression as early as 12h post treatment. The present findings suggested that rituximab-mediated induction of PTEN expression and inhibition of both the AKT pathway and PTN epxression may be interrelated and play an important role in rituximab-mediated cell growth inhibition and chemosensitization. A recent report by Li et al., (JBC, 281:10663,2006) demonstrated that PTEN null cells exhibited upregulation of PTN expression and activation of the PI3K/AKT pathway. Further, inhibition of PTN resulted in inhibition of the AKT pathway, thus establishing a feedback mechanism. Our findings with rituximab are consistent with the Li's findings'. The mechanism by which rituximab inhibits PTN expression is not clear. Reported studies have implicated the role of AP-1 in PTN transcription and in agreement, our findings have also demonstrated that rituximab inhibits AP-1. Overall, the present studies suggest novel targets modified by rituximab namely, PTN and PTEN, which can be considered for therapeutic intervention in the treatment of both rituximab-sensitive and rituximab-resistant tumors.