Subversion of Pkca and Pkcbii Upregulation Play an Essential Role in Chronic Lymphocytic Leukaemia Development, Inducing Constitutive B Cell Receptor-Mediated Signals

B cell receptor (BCR)-mediated signals orchestrate key events during the life cycle of B lymphocytes enabling normal B cell development and maturation. Chronic lymphocytic leukaemia (CLL), an incurable malignancy of mature B cells, displays deregulated BCR-mediated signalling, the intensity of which correlates disease pathogenesis. As such, signals generated upon BCR engagement represent promising targets for novel therapies. Of note, the expression profile of selected protein kinase C (PKC) isoforms, which link proximal BCR mediated signals with downstream pathways, exhibit altered expression patterns in CLL cells: upregulation of PKCβII, PKCε, PKCζ and downregulation of PKCα and PKCβI compared with normal B cells. We previously demonstrated that introduction of a kinase inactive PKCα (PKCα-KR) construct in mouse lymphoid progenitor cells resulted in development of a CLL-like disease both in vitro and in vivo. This model resembles the more aggressive subset of CLL, exhibiting an upregulation of ZAP-70 and elevated ERK-MAPK-mTOR signalling resulting in enhanced proliferation and increased tumor load in the lymphoid organs. Interestingly, reduced PKCα function resulted in PKCβII upregulation, a key pathogenic feature of CLL. Inhibition of PKCβII in these cells with enzastaurin resulted in cell cycle arrest in vitro, reduced tumour load and elevated apoptosis in vivo indicating that PKCβII plays a vital role in maintaining cell survival in our model. To further elucidate the role of PKCβ in leukaemogenesis, we have performed sequential prkcb knockdown (KD) and PKCα-KR expression in the lymphoid progenitor cells. prkcb KD resulted in reduced proliferation and survival concurrent with reduced expression of leukaemic markers (CD23 and CD5) compared to control cells indicating that prkcb plays an essential role in initiation of leukaemogenesis in our model. To identify targets responsible for the regulation for prkcb transcription we performed global gene analysis (Affymetrix GeneChip mouse gene 1.0 ST) comparing MIEV (empty vector control) and PKCα-KR transduced cells at successive time-points mapping critical stages of B-cell transformation, pre- and post PKCβII upregulation. MetaCore^TM analysis revealed that upon upregulation of PKCβII, the BCR-mediated signalling pathway was significantly upregulated in PKCα-KR expressing cells. At the protein level, key hubs proximal to the BCR - Lyn, Btk and Akt - were upregulated, indicating constitutive activation of BCR signalling in the CLL-like PKCα-KR expressing cells. Additionally, proliferative signals downstream of the BCR (mTOR, NF-kB and c-myc) were also upregulated. Treatment of PKCα-KR expressing cells with the Btk inhibitor Ibrutinib (PCI-32765), which has recently been approved for the treatment of CLL, reduced cellular proliferation and inhibited phosphorylation of Btk^Y223, Akt^S473 and S6^S235/236. Overall, we demonstrate that PKCβII expression is essential for leukaemogenesis and identify key signalling pathways that drive the initiation/development of CLL in the PKCα-KR model.