ABSTRACT
B cell antigen receptor (BCR) signalling competence is critical for the pathogenesis of chronic lymphocytic leukaemia (CLL). Defining key proteins that facilitate these networks aid in the identification of targets for therapeutic exploitation. We previously demonstrated that reduced PKCα function in mouse hematopoietic stem/progenitor cells (HPSCs) resulted in PKCßII upregulation and generation of a poor-prognostic CLL-like disease. Here, prkcb knockdown in HSPCs leads to reduced survival of PKCα-KR-expressing CLL-like cells, concurrent with reduced expression of the leukemic markers CD5 and CD23. SP1 promotes elevated expression of prkcb in PKCα-KR expressing cells enabling leukemogenesis. Global gene analysis revealed an upregulation of genes associated with B cell activation in PKCα-KR expressing cells, coincident with upregulation of PKCßII: supported by activation of key signalling hubs proximal to the BCR and elevated proliferation. Ibrutinib (BTK inhibitor) or enzastaurin (PKCßII inhibitor) treatment of PKCα-KR expressing cells and primary CLL cells showed similar patterns of Akt/mTOR pathway inhibition, supporting the role for PKCßII in maintaining proliferative signals in our CLL mouse model. Ibrutinib or enzastaurin treatment also reduced PKCα-KR-CLL cell migration towards CXCL12. Overall, we demonstrate that PKCß expression facilitates leukemogenesis and identify that BCR-mediated signalling is a key driver of CLL development in the PKCα-KR model.
ABSTRACT
PURPOSE: To determine whether inhibition of mTOR kinase-mediated signaling represents a valid therapeutic approach for chronic lymphocytic leukemia (CLL). EXPERIMENTAL DESIGN: Stratification of mTOR activity was carried out in patients with primary CLL samples and an aggressive CLL-like mouse model. The potency of dual mTOR inhibitor AZD8055 to induce apoptosis in primary CLL cells was assessed in the presence/absence of B-cell receptor (BCR) ligation. Furthermore, we addressed the molecular and functional impact of dual mTOR inhibition in combination with BTK inhibitor ibrutinib. RESULTS: Differential regulation of basal mTORC1 activity was observed in poor prognostic CLL samples, with elevated p4EBP1T37/46 and decreased p70S6 kinase activity, suggesting that dual mTORC1/2 inhibitors may exhibit improved response in poor prognostic CLL compared with rapalogs. AZD8055 treatment of primary CLL cells significantly reduced CLL survival in vitro compared with rapamycin, preferentially targeting poor prognostic subsets and overcoming BCR-mediated survival advantages. Furthermore, AZD8055, and clinical analog AZD2014, significantly reduced CLL tumor load in mice. AKT substrate FOXO1, while overexpressed in CLL cells of poor prognostic patients in LN biopsies, peripheral CLL cells, and mouse-derived CLL-like cells, appeared to be inactive. AZD8055 treatment partially reversed FOXO1 inactivation downstream of BCR crosslinking, significantly inhibiting FOXO1T24 phosphorylation in an mTORC2-AKT-dependent manner, to promote FOXO1 nuclear localization, activity, and FOXO1-mediated gene regulation. FOXO1 activity was further significantly enhanced on combining AZD8055 with ibrutinib. CONCLUSIONS: Our studies demonstrate that dual mTOR inhibitors show promise as future CLL therapies, particularly in combination with ibrutinib.
Subject(s)
Forkhead Box Protein O1/metabolism , Leukemia, Lymphocytic, Chronic, B-Cell/metabolism , Leukemia, Lymphocytic, Chronic, B-Cell/mortality , Mechanistic Target of Rapamycin Complex 2/antagonists & inhibitors , Protein Kinase Inhibitors/pharmacology , Proto-Oncogene Proteins c-akt/antagonists & inhibitors , Receptors, Antigen, B-Cell/metabolism , Animals , Cell Line, Tumor , Cell Proliferation/drug effects , Disease Models, Animal , Drug Synergism , Female , Forkhead Box Protein O1/genetics , Gene Expression Regulation, Neoplastic/drug effects , Humans , Leukemia, Lymphocytic, Chronic, B-Cell/drug therapy , Leukemia, Lymphocytic, Chronic, B-Cell/genetics , Male , Mechanistic Target of Rapamycin Complex 2/metabolism , Mice , Mice, Transgenic , Prognosis , Protein Kinase Inhibitors/therapeutic use , Proto-Oncogene Proteins c-akt/metabolism , Signal Transduction/drug effects , Treatment Outcome , Xenograft Model Antitumor AssaysABSTRACT
Mechanistic target for rapamycin (mTOR) is a serine/threonine protein kinase that forms two distinct complexes mTORC1 and mTORC2, integrating mitogen and nutrient signals to regulate cell survival and proliferation; processes which are commonly deregulated in human cancers. mTORC1 and mTORC2 have divergent molecular associations and cellular functions: mTORC1 regulates in mRNA translation and protein synthesis, while mTORC2 is involved in the regulation of cellular survival and metabolism. Through AKT phosphorylation/activation, mTORC2 has also been reported to regulate cell migration. Recent attention has focused on the aberrant activation of the PI3K/mTOR pathway in B cell malignancies and there is growing evidence for its involvement in disease pathogenesis, due to its location downstream of other established novel drug targets that intercept B cell receptor (BCR) signals. Shared pharmacological features of BCR signal inhibitors include a striking "lymphocyte redistribution" effect whereby patients experience a sharp increase in lymphocyte count on initiation of therapy followed by a steady decline. Chronic lymphocytic leukemia (CLL) serves as a paradigm for migration studies as lymphocytes are among the most widely travelled cells in the body, a product of their role in immunological surveillance. The subversion of normal lymphocyte movement in CLL is being elucidated; this review aims to describe the migration impairment which occurs as part of the wider context of cancer cell migration defects, with a focus on the role of mTOR in mediating migration effects downstream of BCR ligation and other microenvironmental signals.