Large granular lymphocyte leukemia: from dysregulated pathways to therapeutic targets

Abstract

Large granular lymphocyte (LGL) leukemia is a clonal lymphoproliferative disorder of cytotoxic lymphocytes characterized by an expansion of CD3(+) cytotoxic T lymphocytes or CD3(-) natural killer cells. Patients present with various cytopenias including neutropenia, anemia and thrombocytopenia. In addition, there is an association of T-cell large granular lymphocytic leukemia with rheumatoid arthritis. It is believed that LGL leukemia begins as an antigen-driven immune response with subsequent constitutive activation of cytotoxic T lymphocytes or natural killer cells through PDGF and IL-15 contributing to their survival. Consequently, this leads to a dysregulation of apoptosis and dysfunction of the activation-induced cell death pathway. Treatment of LGL leukemia is based on a low-dose immunosuppressive regimen using methotrexate or cyclophosphamide. However, no standard of therapy has been established, as large prospective trials have not been conducted. In addition, some patients are refractory to treatment. The lack of a curative therapy for LGL leukemia means that new treatment options are needed. Insight into the various dysregulated signaling pathways in LGL leukemia may provide novel therapeutic treatment modalities.

Figure 1. Dysregulated survival pathways in large granular lymphocyte leukemia

The intact Fas–FasL-mediated apoptosis pathway is shown. Elevated levels of the soluble form of Fas or FasL contribute to apoptotic resistance in leukemic LGLs by acting as receptor or ligand decoys. MMP inhibitors are proposed for therapeutic intervention. Other contributing signaling pathways that are essential in LGL leukemia pathogenesis include the signals that are mediated by IL-15, PDGF and NF-κB. IL-15 promotes LGL survival either by activating the JAK–STAT3 signaling pathway to upregulate gene expression of antiapoptotic factor or by targeting Bid to the proteasome for degradation. IL-15 signaling can be inhibited by a Mikβ1 monoclonal antibody targeting the IL-15R. PDGF functions through PI3K–Akt and Ras–MEK1–ERK signaling cascades, leading to cell proliferation and growth. PDGF signaling can be inhibited by RTK inhibitors such as imatinib. PI3K–Akt signaling can be blocked by using pan-PI3K inhibitors. Ras–MEK1–ERK signaling can be targeted using farnesyltransferase inhibitors to prevent farnesylation of Ras or by using MEK1 inhibitors such as PD98059 and U0216. NF-κB complex upregulates the transcription of survival genes resulting in survival of leukemic LGLs. Bortezomib, a proteasome inhibitor, can be used to prevent NF-κB signaling. FasL: Fas ligand; IκB: Inhibitor of NF-κB; IKK: IκB kinase; IL-15R: IL-15 receptor; LGL: Large granular lymphocyte; MMP: Matrix metalloproteinase; RTK: Receptor tyrosine kinase.

Figure 2. The STAT signaling pathway

STATs serve as the converging point for many signaling pathways that are mediated through cytokine receptors, growth factor receptors and nonreceptor tyrosine kinases, which are commonly activated in cancer cells. First, stimulation of cells with growth factors or cytokines results in the activation of intrinsic receptor tyrosine kinase activity or of receptor-associated kinases. These kinases in turn phosphorylate the receptor cytoplasmid tail to provide docking sites for STAT monomers via their SH2 domain, resulting in phosphorylation of STATs by JAKs or Src kinases. Oncogenic Src and Abl can also phosphorylate STATs in a receptor-independent manner. Second, the phosphorylated STAT monomers dimerize and translocate to the nucleus. Finally, STATs bind to specific STAT DNA-response elements and directly regulate the expression of a variety of gene expression that are critical for cell proliferation (e.g., cyclin D1 and cyclin D2), survival (e.g., survivin), angiogenesis (e.g., VEGF) and immune evasion (e.g., immune-suppressing factor). In normal cells, STAT activation is tightly regulated; however, constitutive activation of STATs, in particular STAT3, is associated with human cancer development. Consequently, STAT3 emerges as an attractive target for cancer therapy. Proposed therapeutic interventions include JAK inhibitors (e.g., CYT387), preventing STAT3 dimerization (e.g., OPB-31121) and preventing STAT3 nuclear translocation (e.g., P(pTyr)LKTK and C48).

Figure 3. Dysregulation of sphingolipid metabolism and signaling in leukemic large granular lymphocytes

Decreased antiproliferative and apoptotic activities mediated by ceramide and sphingosine are illustrated. The upregulated enzyme activities or gene function in leukemic large granular lymphocytes are shown. Inhibition of ASAH1 can be accomplished using NOE. S1P signaling can be blocked with fingolomid (FTY720). C₆-ceramide nanoliposome is an effective method of delivering ceramide into a cell. C1PP: Ceramide 1-phosphate phosphatase; S1P: Sphingosine 1-phosphate; S1PP: Sphingosine 1-phosphate phosphatase; S1PR: Sphingosine 1-phosphate receptor.