The Functional Impact of PKCβ/PI3K/AKT Signalling on Translational Initiation in MCL

Introduction: Mantle cell lymphoma (MCL) is an aggressive form of B-cell non-Hodgkin’s lymphoma (NHL). It is characterized by the t(11;14)(q13;q32) translocation, which results in the overexpression of cyclinD1, a cyclin regulated by the PI3K/AKT pathway. Activation of the PI3K/AKT pathway has been shown to be involved in the pathogenesis of MCL. In addition overexpression of the protein kinase C beta (PKCβ) has been described for most cases of MCL, inhibited by enzastaurin which in turn induces apoptosis and reduces proliferation through the PKCβ/PI3K/AKT pathways. 4EBP1 is described as one of the downstream targets of PI3K/mTOR pathway linking translation initiation with PI3K/mTOR signalling as a EIF4E binding protein and playing therefore critical role in the control of protein synthesis, survival and cell growth. Targeting 4EBP1 and/or EIF4E via the PI3K/AKt/mTOR signalling or directly will affect tumor tissue.

Aim of the study: The aim of the study was to determine the functional impact of PKCβ/PI3K/AKt/mTOR signaling on the translation initiation factor EIF4E, its binding protein and regulated proteins in MCL cell lines.

Methods: MCL cell lines were treated with inhibitors of the PKCβ/PI3K/AKt/mTOR pathways (enzastaurin, LY294002, rapamycin) for up to 48h.The impact of the drugs on the proliferation rate of the cells was accessed after 48h by WST-assay and/or cell count. mRNA expression levels were determined using Taqmanassays. Protein phosphorylation status and protein expression were identified by westernblot. For downregulation of EIF4E in the cells sodium arsenite was used. Specific silencing of EIF4E was achieved by transfection of cells with siRNA against EIF4E.

Results: The MCL celllines (5) responded to the treatment with the inhibitors of the PI3K/AKt/mTOR pathway at a IC50 for rapamycin between 5nM-50nM and for the PI3Kinhibitor between 0,31μM-5μM. Treatment of the cells with the PI3K/AKt/mTOR inhibitors induced dephosphorylation of 4EBP1 in a time-and dosedependent manner while a potential effect of the PI3K and mTOR inhibitors on the EIF4E expression and its target genes (cyclinD1, BCL2) could not be shown consistently. 4 out of 5 MCL cell lines were susceptible to enzastaurin with an IC50 between 2μM-5μM. In the not responding to enzastaurin and most resistant to rapamycin cell line (Rec-1) no 4EBP1 proteinexpression was detectable. Dephosphorylation of 4EBP1 achieved by treatment of the cells with sodium arsenit was accompanied by downregulation of EIF4E, cyclinD1 and BCL2 proteins but also stop of proliferation. The potential involvement of eIF4E gene expression in the NaAsO2-induced cytotoxicity and cell death in MCL cell lines was shown by silencing the expression of the eIF4E gene by transfection with siRNA specifically targeting the eIF4E gene expression leading to downregulation of cyclinD1, 4EBP1 proteins and cell proliferation.

Conclusion: Eventhough treatment of the cells with the PI3K/AKt/mTOR inhibitors induced dephosphorylation of 4EBP1 a potential effect of the PI3K and mTOR inhibitors on the EIF4E expression and its target genes (cyclinD1, BCL2) could not be shown consistently. Instead dephosphorylation of 4EBP1 accomponied by downregulation of EIF4E or targeted downregulation of eIF4E gene expression lead to downregulation of cyclinD1 and BCL2 proteins as well as cell death in MCL. Therefore targeting the downstream targets of the PI3K/AKt/mTOR signalling 4EBP1 and/or EIF4E directly seems to be a promising anticancer strategy.