Chronic lymphocytic leukemia (CLL) cells with aggressive clinical properties express lipoprotein lipase (LPL), which generates activating ligands for the nuclear receptor peroxisome proliferator activated receptor (PPAR)α and allows fatty acids to be used as fuel. However, the role of PPARα in CLL is unclear. PPARα was found to be expressed by circulating CLL cells and highly associated with advanced stage disease. Consistent with this observation, palmitate oxidation rates in circulating CLL cells were similar to more conventional fat-burning cells such as muscle. Transgenic expression of PPARα in CD5(+) Daudi cells increased both their expression of immunosuppressive factors (that is, interleukin (IL)10 and phospho-STAT3) and resistance to metabolic and cytotoxic stressors. In contrast, marked downregulation of PPARα expression accompanied immunogenic death of proliferating CLL cells. The PPARα antagonist MK886 killed circulating CLL cells directly, caused proliferating CLL cells to enter an immunogenic death pathway and cleared CLL xenografts from immunodeficient mice. These results suggest that PPARα is a biological mediator of CLL and MK886 is a clinically relevant agent with activity against CLL.

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Recent studies suggest there is a high incidence of elevated low-density lipoprotein (LDL) levels in Chronic Lymphocytic Leukemia (CLL) patients and a survival benefit from cholesterol-lowering statin drugs. The mechanisms of these observations and the kinds of patients they apply to are unclear. Using an in vitro model of the pseudofollicles where CLL cells originate, LDLs were found to increase plasma membrane cholesterol, signaling molecules such as tyrosine-phosphorylated STAT3, and activated CLL cell numbers. The signaling effects of LDLs were not seen in normal lymphocytes or glycolytic lymphoma cell-lines but were restored by transduction with the nuclear receptor PPARδ, which mediates metabolic activity in CLL cells. Breakdown of LDLs in lysosomes was required for the amplification effect, which correlated with down-regulation of HMGCR expression and long lymphocyte doubling times (LDTs) of 53.6 ± 10.4 months. Cholesterol content of circulating CLL cells correlated directly with blood LDL levels in a subgroup of patients. These observations suggest LDLs may enhance proliferative responses of CLL cells to inflammatory signals. Prospective clinical trials are needed to confirm the therapeutic potential of lowering LDL concentrations in CLL, particularly in patients with indolent disease in the “watch-and-wait” phase of management.

Key Points• IL-6 from splenic stromal cells prevents CLL cells from responding strongly to TLR ligands.• IL-6-signaling inhibitors enhance TLR-mediated responses of CLL cells in vitro and in vivo.The regulation of toll-like receptor (TLR) signaling in a tumor microenvironment is poorly understood despite its importance in cancer biology. To address this problem, TLR7-responses of chronic lymphocytic leukemia (CLL) cells were studied in the presence and absence of a human stromal cell-line derived from a leukemic spleen. CLL cells alone produced high levels of tumor necrosis factor (TNF)-a and proliferated in response to TLR7-agonists. A signal transducer and activator of transcription 3 -activating stromal factor, identified as interleukin (IL)-6, was found to upregulate microRNA (miR)-17 and miR-19a, target TLR7 and TNFA messenger RNA, and induce a state of tolerance to TLR7-agonists in CLL cells. Overexpression of the miR-17-92 cluster tolerized CLL cells directly and miR-17 and miR-19a antagomiRs restored TLR7-signaling. Inhibition of IL-6 signaling with antibodies or small-molecule Janus kinase inhibitors reversed tolerization and increased TLR7-stimulated CLL cell numbers in vitro and in NOD-SCIDg c null mice. These results suggest IL-6 can act as tumor suppressor in CLL by inhibiting TLR-signaling. (Blood. 2015;126(6):766-778)