Reduced expression of the tumor suppressor PHLPP1 enhances the antiapoptotic B-cell receptor signal in chronic lymphocytic leukemia B-cells

Abstract

The PI3K/Akt pathway is activated in response to various microenvironmental stimuli that regulate the survival and proliferation of chronic lymphocytic leukemia (CLL) B-cells, including triggering of the B-cell receptor (BCR). Although this pathway is frequently targeted in cancer, no significant alterations have yet been identified in CLL. We now show that the phosphatase PH domain leucin-rich repeat protein phosphatase (PHLPP1), a recently identified tumor suppressor and negative regulator of the Akt kinase, is absent or expressed at substantially reduced levels in CLL B-cells. To determine what the consequences of PHLPP1 loss on BCR signaling are, we downregulated or re-expressed PHLPP1 in lymphoma cell lines and primary CLL B-cells, respectively. Downregulation of PHLPP1 increased BCR-induced phosphorylation and activation of the Akt, GSK3 and ERK kinases, whereas re-expression had the opposite effect. Importantly, re-expression of PHLPP1 in primary CLL cells prevented upregulation of Mcl-1 and inhibited the increase in leukemic cell viability induced by sustained BCR engagement. Enforced expression of PHLPP1 also affected the response to other microenvironmental stimuli, particularly in terms of ERK phosphorylation. Collectively, these data show that CLL cells lack an important negative regulator of the Akt and ERK pathways, which could confer them a growth advantage by facilitating the propagation of crucial microenvironment-derived stimuli.

Overexpression of the autoimmunity-associated phosphatase PTPN22 promotes survival of antigen-stimulated CLL cells by selectively activating AKT

“…Sustained activation of AKT is known to increase the survival of antigen-stimulated CLL and lymphoma B cells, 33,44,45 whereas p38MAPK has been shown to induce apoptosis in murine splenocytes and human lymphoma B-cell lines 34,35 and was found to display similar activity in primary human CLL cells in the current study. Based on these observations, it would be expected that PTPN22 overexpression would increase the survival of BCR-stimulated CLL cells by enhancing AKT activation and protect them from activation-induced cell death by inhibiting p38MAPK.…”

Mechanisms and consequences of the loss of PHLPP1 phosphatase in chronic lymphocytic leukemia (CLL)

“…A similar correlation between PHLPP1 protein and mRNA expression levels were observed in various CLL cell lines as well (Supplementary Figure 1). These results suggest that the mechanism for PHLPP1 reduction could be due to genetic mutation(s), epigenetic modification (for example, DNA methylation) or alterations in mRNA stability rather than decreased protein stability; this is consistent with findings from Suljagic et al 10 in which calpain and proteasomal inhibitors had no effect on PHLPP1 protein accumulation in CLL cells.…”

“…10 Similarly, we observed a loss of PHLPP1 expression in 39 out of 43 CLL samples (loss in >90% of patients), although it was detected in all B cells from healthy donors ( n =6) and was highly expressed in the Ramos Burkitt's lymphoma line. However, PHLPP2 and PTEN expression were detected in all CLL cells probed (representative blot Figure 1a).…”

“Role of the B-cell receptor and the microenvironment in chronic lymphocytic leukemia’’

“…Once encountering the antigen, the activated BCR recruits kinases such as spleen tyrosine kinase (SYK) and LYN, which phosphorylate the immunoreceptor tyrosine-based motifs receptors of Igα/Igβ. 13 Phosphorylation of these induces a cascade of downstream events, which include that activation of BTK and PI3K, 14 which subsequently induce the mobilization and activation of other downstream kinases such as protein kinase C-β, mammalian target of rapamycin and mitogen-activated protein kinase ERK. 15 The activation of this cascade promotes survival and proliferation of B cells through the upregulation of transcription factors such as nuclear factor κ β (NF-κβ).…”