Lipids and Their Effects in Chronic Lymphocytic Leukemia

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Abstract

Chronic lymphocytic leukemia (CLL) is an incurable common B-cell malignancy with a spectrum of clinical outcomes. Over the past decade, our increasing understanding of the drivers of CLL progression has led to the development and use of novel therapeutics. For example, B-cell re-ceptor (BCR) signaling was shown to be overactive in CLL, and subse-quently the kinase inhibitors ibrutinib (BTK inhibitor) and idelalisib (PI3K delta inhibitor) were found to have clinical efficacy in this malig-nancy (Byrd et al., 2013; Furman et al., 2014). Despite these new treat-ments, CLL remains incurable and there remains a need to identify new therapeutic targets. The therapeutic target of interest for McCaw et al. (2017) in their EBioMedicine article is lipid metabolism. It has been appreciated for many years that lipids have importance in CLL progression and out-comes. Most notably, lipoprotein lipase is a well-known (although not routinely measured clinically) prognostic factor in CLL, with higher levels associated with inferior clinical outcomes. LPL is not expressed in normal lymphocytes, but its expression is increased in CLL cells, par-ticularly in the IGHV unmutated subset (Heintel et al., 2005). LPL cata-lyzes hydrolysis of VLDL and chylomicrons, releasing fatty acids. LPL also has non-catalytic functions, for example co-localizing with lipopro-teins at the cell surface. In CLL cells, the exact function of LPL and the reason for its overexpression compared to normal B-cells is not fully un-derstood. However, recent work has demonstrated that inhibition of LPL with orlistat induces CLL apoptosis, and that LPL expression is increased by BCR cross-linking, by binding of STAT3 to the LPL promoter, and by certain CLL stimulants that induce demethylation of the LPL gene (Moreno et al., 2013; Pallasch et al., 2008; Rozovski et al., 2015). Togeth-er, this previous work has suggested that free fatty acids, liberated by LPL, may be a protective factor for CLL lymphocytes. Within this context, McCaw et al. (2017) provide a compelling argu-ment for the role of lipids in inducing second messenger signaling in CLL. The authors were intrigued by a recent case-control study in Cana-da that demonstrated that CLL patients have more dyslipidemia than age-matched controls, and that CLL patients who took HMG-CoA reduc-tase inhibitors (" statins ") had improved survival compared to CLL pa-tients who did not take these medications, which confirmed similar results in smaller CLL cohorts (Chae et al., 2014; Friedman et al., 2010; Mozessohn et al., 2017). Together with the story regarding lipoprotein lipase, these clinical data beg the question of if and how LDLs affect CLL cells. In their paper, McCaw et al. (2017) focus on LDL potentiation of cy-tokine-induced STAT3 phosphorylation. The authors demonstrate that LDLs are able to increase STAT3 phosphorylation within the context of cytokine stimulation, not BCR cross-linking. The induced STAT3 phos-phorylation was suppressed by anti-IL10 antibodies and by small mole-cule JAK inhibition, suggesting overlapping pathways with IL10 and JAK mediated signaling. The authors evaluated which of the different com-ponents of LDL contributed to the effect on STAT3 phosphorylation, and they found that long-chain fatty acids and free cholesterol were the main actors. Lastly, the authors found a negative correlation be-tween the extent of LDL-potentiated STAT3 phosphorylation and HMGCoA reductase expression. Since HMGCoA reductase is the rate limiting step in cholesterol synthesis, this suggests that the subset of CLL cells with lower intracellular cholesterol synthesis are affected more by LDL incubation, and that this mechanism may be important for disease progression amongst these patients. McCaw et al.'s work (McCaw et al., 2017) adds important infor-mation to the growing knowledge regarding the effect of lipids on CLL cell biology, however numerous unknowns remain. For exam-ple, molecular prognostic markers in the CLL patients in these exper-iments are not fully detailed, LPL levels are unknown, and serum lipid levels are unknown. These could affect the in vitro findings observed. Second, the relevance of the results in this manuscript within the context of research related to LPL is not explored. Third, it would be helpful to investigate LDL-induced effects on a broader representation of relevant CLL signaling pathways including other chemokines, TNF family members (BAFF, APRIL), and TLR agonists. This would provide insight into the relative importance of lipopro-tein metabolism in different aspects of CLL cell biology. Fourth, as more attention is paid to the CLL microenvironment, it would be interesting to learn if and how lipids and lipoproteins modulate the interaction between CLL cells and nurse-like cells. Lastly, from a therapeutic perspective, do lipid-lowering medications, such as statins, synergize with BTK or PI3K inhibitors in CLL? EBioMedicine 15 (2017) 2–3

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APA

Friedman, D. R. (2017, February 1). Lipids and Their Effects in Chronic Lymphocytic Leukemia. EBioMedicine. Elsevier B.V. https://doi.org/10.1016/j.ebiom.2016.12.001

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