Abstract 4406: Cullin3-KLHL25 ubiquitin ligase targets ACLY for degradation to inhibit lipid synthesis and tumor progression
Authors: Zhang Cen , Liu Juan , Zhao Yuhan , Yue Xuetian , Wu Hao , Li Jun , Shen Zhiyuan , Haffty Bruce , Hu Wenwei , Feng Zhaohui 
Published: July 2017

Abstract
Increased lipid synthesis is a key characteristic of many cancers, which is critical for cancer progression. As a key enzyme in lipid synthesis, ACLY (ATP citrate lyase) converts citrate in the cytosol to acetyl-CoA, which is a precursor for lipid synthesis. ACLY is frequently overexpressed or activated in cancer, which plays an important role in promoting lipid synthesis and cancer progression. Currently, the mechanism underlying ACLY overexpression and increased lipid synthesis in cancer is poorly understood. Cullin3 (CUL3) is a core protein for CUL3-RING ubiquitin ligase complex. CUL3 has been recently reported to be a tumor suppressor and its expression is frequently down-regulated in lung cancer. The mechanism of CUL3 in tumor suppression is not well-understood. In this study, we found that CUL3 interacted with ACLY through its adaptor protein KLHL25 to ubiquitinate and degrade ACLY in cells. Ectopic CUL3 expression in lung cancer cells greatly inhibited lipid synthesis, cell proliferation and tumor growth, whereas knockdown of endogenous CUL3 in lung cancer cells greatly promoted lipid synthesis, cell proliferation and tumor growth. Furthermore, negative regulation of ACLY contributes greatly to these functions of CUL3. Supplementation of lipid metabolites in culture medium, such as oleic acid, mevalonate and acetate, significantly reduced the inhibitory effect of CUL3 on proliferation and colony formation of cancer cells. SB-204990, a small-molecule ACLY inhibitor greatly abolished the promoting effect of CUL3 down-regulation on lipid synthesis, cell proliferation and tumor growth. In addition, we identified K540, K546 and K554 of ALCY as major ubiquitination sites of CUL3. Mutations of K540, K546 and K554 (K540R/K546R/K554R) greatly reduced CUL3-mediated ubiquitination and degradation of ACLY, and greatly compromised the inhibitory effect of CUL3 on tumor growth as well. Importantly, low CUL3 expression is significantly associated with high ACLY expression in clinical lung cancer specimens, and poor prognosis in lung cancer patients. In summary, our results identified CUL3-KLHL25 ubiquitin ligase as a novel negative regulator for ACLY and lipid synthesis, and demonstrated that decreased CUL3 expression is an important mechanism for increased ACLY expression and lipid synthesis in lung cancer. These results also revealed that negative regulation of ACLY and lipid synthesis is a novel and critical mechanism for CUL3 in tumor suppression.
Citation Format: Cen Zhang, Juan Liu, Yuhan Zhao, Xuetian Yue, Hao Wu, Jun Li, Zhiyuan Shen, Bruce Haffty, Wenwei Hu, Zhaohui Feng. Cullin3-KLHL25 ubiquitin ligase targets ACLY for degradation to inhibit lipid synthesis and tumor progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4406. doi:10.1158/1538-7445.AM2017-4406