Handling of LDLs is also heterogenous

Handling of LDLs is also heterogenous in CLL cells. For example, signaling processes in CLL Sulfo-NHS-LC-Biotin appear less dependent on exogenous LDLs in a group of patients with more aggressive disease, indicated by higher circulating leukemia cell numbers and rapid doubling times in vivo (Fig. 6A, B). An interpretation of these findings may be that relatively early-stage CLL cells with indolent growth are more dependent on LDLs. However, as they become more aggressive in terms of rate of proliferation, they change their metabolism to become more like mitogen-activated normal lymphocytes (Owens et al., 1990) or acute leukemia cells, i.e. they become less responsive to exogenous cholesterol (Fig. 2, Fig. 6A, B) and decrease their intracellular cholesterol levels (Fig. 6C, D). Multivariate analysis of our prior clinical study revealed that the protective effects of statins did not apply to patients with high-risk cytogenetic lesions associated with aggressive disease (Chow et al., 2016). However, we could not identify common cytogenetic abnormalities in group 2 patients (Table 1) and querying Oncomine (Rhodes et al., 2004) with the search terms: Gene: “HMGCR” and Cancer Type: Chronic Lymphocytic Leukemia did not reveal any correlation of aggressive prognostic markers (i.e. unmutated IGVH genes or 11q and 17 deletions) with HMGCR expression. Taken together, the observations in this manuscript suggest there is a cohort of CLL patients whose leukemia cells internalize LDLs and amplify signaling properties and for whom LDLs are tumor promoters. By lowering LDL levels, statins may reduce signaling and slow the growth of these cells, explaining their ability to slow time to first treatment and increase overall survival. (Chow et al., 2016; Mozessohn et al., 2016) However, statins may also have direct anti-tumor effects that are independent of plasma LDL-concentrations (Chapman-Shimshoni et al., 2003). A clinical trial comparing dietary strategies to lower cholesterol (Jenkins et al., 2011) with statins, especially in patients with elevated baseline LDL-cholesterol levels and relatively indolent disease (Fig. 6), is needed to determine the relevance of the mechanism proposed here that statins exert their beneficial effects by lowering intracellular cholesterol, antioxidants, and growth promoting signal transduction in CLL cells (Figs. 5A, 6E). Appropriate therapeutic targets for LDL-cholesterol to prevent tumor progression are unknown, in contrast to target levels required to prevent cardiovascular disease (Stone and Lloyd-Jones, 2015). Moreover, the kinase inhibitor Ibrutinib lowers cytokine levels in CLL patients (Bachireddy and Wu, 2016) and prevents STAT3-phosphorylation in the pseudofollicle model regardless of LDLs (Supplementary Fig. 5). An arm of the trial should also compare kinase inhibitors that block tumor-promoting signaling directly with lowering LDL plasma levels to decrease signaling-strength in the watch-and-wait phase of CLL management. The following are the supplementary data related to this article.
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Conflict of Interest
Author Contributions LM and DS designed the research, analyzed the data, and wrote the paper. YS and GW helped design and perform experiments. Y-JL generated PPARDhi Daudi cells.
Acknowledgements This work was supported by the Leukemia and Lymphoma Society of Canada (LLSC) and CIHR grants MOP130479 and 110952 (DS). We thank David Andrews (Sunnybrook Research Institute) for providing PFO1 and Dana Martin (Synageva BioPharma, Lexington, MA) for providing Lalistat and Gordon Duncan for providing anti-Ig antibodies.
Introduction The ubiquitin-proteasome system (UPS) serves a critical function by orchestrating protein-protein interactions, protein stability, protein subcellular localization, and activation/deactivation of functional proteins among other cellular activities (Skaar et al., 2014; Lipkowitz and Weissman, 2011). Malfunction of the UPS leads to various human diseases including tumor development, from initiation to invasion (Lipkowitz and Weissman, 2011; Popovic et al., 2014). While ubiquitin protein E3 ligase targets functional proteins for ubiquitylation followed by degradation, deubiquitinases (DUBs) catalyze the removal of the ubiquitin chain attached to the substrate protein, thereby preventing the degradation, or promoting the stabilization of the targeted protein (Christianson and Ye, 2014; Clague et al., 2013). The activity of UPS has been tightly linked to tumorigenesis through its impact on a variety of cellular processes, including modulation of oncogenic or tumor suppressing signaling, cell cycle control, genome stability, apoptosis, immune surveillance, and angiogenesis (Clague et al., 2013; Lipkowitz and Weissman, 2011; Skaar et al., 2014; Hanahan and Weinberg, 2011). While the role of a large number of ubiquitin-protein ligases in regulating carcinogenesis has been extensively studied, the importance of various DUBs in tumor initiation and invasion has not drawn the same amount of attention in the field until recently, especially in breast cancer (Clague et al., 2013). To systematically determine the pivotal role that DUBs play in initiating mammary tumorigenesis, we screened a non-biased library of DUBs using a mammary gland cell malignant transformation assay. This endeavor led to the identification of ubiquitin-specific peptidase 11 (USP11) as a critical DUB that promotes mammary tumor initiation and progression. Using a TAP-purification coupled with mass spectrometry, we further identified X-linked inhibitor of apoptosis protein (XIAP) to be a target for USP11.