Whole body loss of ACLY is early embryonic
Whole-body loss of ACLY is early embryonic lethal, indicating that it serves non-redundant roles during development (Beigneux et al., 2004). Silencing or inhibition of ACLY suppresses the proliferation of many cancer cell lines and impairs tumor growth (Bauer et al., 2005, Hanai et al., 2012, Hatzivassiliou et al., 2005, Migita et al., 2008, Shah et al., 2016, Zaidi et al., 2012). Depending on the context, ACLY silencing or inhibition can also promote senescence (Lee et al., 2015), induce differentiation (Hatzivassiliou et al., 2005), or suppress cancer stemness (Hanai et al., 2013), further pointing to its potential as a target for cancer therapy. Inhibition of ACLY in adult animals and humans is reasonably well tolerated and produces blood lipid-lowering effects (Ballantyne et al., 2013, Gutierrez et al., 2014, Pearce et al., 1998). Thus, there may be a therapeutic window for ACLY inhibition in treatment of cancer and/or metabolic diseases; although the extent to which cells could leverage other compensatory mechanisms upon reduced ACLY function is not clear.
In this study, we aimed to elucidate two questions: first, does use of glucose-derived carbon for fatty Nicotine synthesis and histone acetylation require ACLY; and second, can cells compensate for ACLY deficiency and, if so, by which mechanisms or pathways? To address these questions, we generated a conditional mouse model of Acly deficiency (Acly mice), as well as immortalized mouse embryonic fibroblast (MEF) cell lines (Acly MEFs). As a complement to these models, we used CRISPR-Cas9 genome editing to delete ACLY from human glioblastoma cells. ACLY deficiency in both MEFs and glioblastoma cells potently impaired proliferation and suppressed histone acetylation levels. Both lipid synthesis and histone acetylation from glucose-derived carbon were severely impaired in ACLY-deficient MEFs. Cells partially compensated for the absence of ACLY by upregulating ACSS2, and ACLY-deficient MEFs became dependent on exogenous acetate for viability. Acetate was used to supply acetyl-CoA for both lipid synthesis and histone acetylation, although global histone acetylation levels remained low unless cells were supplemented with high levels of acetate. ACSS2 upregulation in the absence of ACLY was also observed in vivo upon deletion of Acly from adipocytes in mice. Acly mice exhibited normal body weight and adipose tissue architecture, and production of acetyl-CoA and malonyl-CoA from acetate was enhanced in ACLY-deficient adipocytes. Upon deuterated-water (D2O) labeling of wild-type (WT) and Acly mice, we observed that de novo synthesized fatty acids were present in white adipose tissue (WAT) in both genotypes, although some differences between depots were apparent. Visceral (epididymal) WAT (VWAT) exhibited no significant differences between WT and Acly mice in quantities of de novo synthesized fatty acids, while synthesized saturated fatty acids were reduced in subcutaneous (inguinal) WAT (SWAT) of Acly mice. Histone acetylation levels were also significantly altered in Acly SWAT. Taken together, this study demonstrates that ACLY is required for glucose-dependent fatty acid synthesis and histone acetylation and that a major, albeit partial, compensatory mechanism for ACLY deficiency involves engagement of acetate metabolism.
Discussion From a clinical perspective, prior study of PET (positron emission tomography) imaging in human hepatocellular carcinoma patients using 11C-acetate and 18F-fluorodeoxyglucose (FDG) revealed a dichotomy between acetate and glucose uptake. Patient tumors, or regions within tumors, with high 11C-acetate uptake demonstrated low 18F-FDG uptake and vice versa. Moreover, tumors with high 18F-FDG uptake were more proliferative (Yun et al., 2009). These data support the concept that mammalian cells—cancer cells, in particular—possess an intrinsic flexibility in their ability to acquire acetyl-CoA from different sources to adjust to changing metabolic environments in vivo. Further elucidation of the mechanisms connecting ACLY and ACSS2, as well as the differential phenotypes observed downstream of their activity, could point toward synthetic lethal strategies for cancer therapy or improved tumor imaging protocols.