The role of glutamine synthetase in the regulation of lipid droplet accumulation and lipogenic pathways in liver and breast cancer cells under insulin or glutamine deprivation

Abstract

In recent years many studies have shown that oncogenesis is closely related to dysregulated metabolism in cells Cancer cells have the characteristics of consuming a large amount of glucose for lactic fermentation to quickly produce energy Cancer cells also uptake glutamine as a nutrient source to fill up the tricarboxylic acid cycle and as a precursor for protein nucleotide and lipid synthesis Glutamine synthetase (GS) can synthesize glutamate and ammonia into glutamine In previous studies it is proved that endogenous glutamine synthesized by GS can promote protein and nucleotides synthesis but whether it can participate in the regulation of lipogenesis remains unclear Many enzymes are involved in lipogenesis The citrate in the tricarboxylic acid cycle is first decomposed into acetyl-CoA and then acetyl-CoA is converted into malonyl-CoA through acetyl-CoA carboxylase 1 (ACC1) Malonyl-CoA is further converted into fatty acids through fatty acid synthase (FAS) and sterol regulatory element-binding protein 1 ( SREBP1) is a transcription factor of ACC1 and FAS which can upregulate their expression to promote lipogenesis Besides it has been shown that adding a high concentration of glutamine in the medium increases the O-GlcNAcylation of the transcription factor specificity protein 1 (Sp1) of SREBP1 and then upregulates the expression of SREBP1 Therefore in this study we want to explore the role of GS in the regulation of lipid droplet accumulation and lipogenic pathways Our results show that in HepG2 and MCF7 cells overexpression of GS induces lipid droplet accumulation and the expression of SREBP1 ACC1 and O-GlcNAc-Sp1 are also increased in HepG2 cells Under insulin treatment knockdown or inhibiting GS in HepG2 and MCF7 cells reduces the lipid droplet accumulation caused by insulin and the protein expression of SREBP1 ACC1 and O-GlcNAc-Sp1 are decreased in HepG2 cells Furthermore we find that glutamine deprivation can induce the expression of GS with upregulated SREBP1 ACC1 and O-GlcNAc-Sp1 to promote lipid droplets formation in HepG2 and MCF7 cells On the contrary knocking down or inhibiting GS in HepG2 and MCF7 cells reduces the accumulation of lipid droplets caused by glutamine deprivation and the protein expression of SREBP1 ACC1 and O-GlcNAc-Sp1 are decreased in HepG2 cells Besides we also find that under glutamine deprivation knocking down GS can decrease cancer cell growth Moreover adding back glutamine to the HepG2 and MCF7 cell reduces the accumulation of lipid droplets caused by glutamine deprivation and the protein expression of SREBP1 and ACC1 in HepG2 cells are also decreased All the above results show that GS can promote the production of lipid droplets and protect cancer cells to survive under glutamine deprivation

Date of Award	2021
Original language	English
Supervisor	I-Chen Peng (Supervisor)