Molecular Mechanism Of YY1 Regulation On Lipid Metabolism In Hepatocarcinoma Cells

Yin Yang 1(YY1)is a GLI-Kruppel class protein,which is highly conserved,widely expressed and involved in the regulation of various genes.Human YY1 is composed of 414 amino acids with,and it has various functional areas including DNA binding site.Numerous studies have shown that YY1 plays important roles in various physiological processes,such as cell proliferation,differentiation,migration,infiltration.Recent studies had shown that YY1 overexpression could be found in various tumor cells,and could be closely linked with tumor development.YY1 could downregulate tumor suppressor p53,and thus act as an oncogene that promotes tumor cell proliferation.Our previous study showed that YY1 could also involved in tumor development in a p53-independent manner.We demonstrated that irrespective to the status of p53,YY1 could stabilize hypoxia inducible factor 1?,which is a key regulator for tumor angiogenesis and cell adaptation to hypoxia.Rapid cell proliferation places tumor cells in a hypoxic condition.On the other hand,to support their rapid proliferation,tumor cells need huge amount of energy and nutrient supply.Thus,to answer this necessity of energy,and to adapt to the hypoxic condition,tumor cells alter their metabolism.Lipid is one of the major source of energy,and lipid metabolism is a crucial metabolic pathway of living cells.Recent researches showed the importance of lipid metabolism in tumor cell proliferation and tumor development,however,whether YY1 is involved in tumor lipid metabolism remained unknown.In this study,we investigated the role of YY1 on lipid metabolism in hepatocellular carcinoma cells under hypoxia condition.Using hepatocellular carcinoma cell line HepG2,we showed that YY1 could positively regulate lipid accumulation and triglyceride synthesis under hypoxic condition,indicating that YY1 might play a crucial role in tumor lipid metabolism.Through screening of factors involved with lipid metabolism,we found that YY1-silencing restored the expression of MCAD(Medium-chain acyl-CoA Dehydrogenase)and LCAD(Long-chain acyl-CoA Dehydrogenase),two crucial factors that promote fatty acid ?-oxidation and degradation,which were downregulated by the hypoxic condition.Consistently,YY1 overexpression suppressed their expression levels.Furthermore,double knockdown of YY1 and MCAD or LCAD resulted in the accumulation of lipid and triglycerides.These facts indicate that YY1 plays a crucial role in tumor lipid metabolism through negative regulation on MCAD and LCAD.In an effort to further elucidate the detail mechanism of YY1 regulation on tumor lipid metabolism,we investigated its regulatory effect on PGC-1?,an upstream regulator of MCAD and LCAD.We showed that YY1 could downregulate PGC-1? expression level;while double knockdown of YY1 and PGC-1? suppressed the expression of MCAD and LCAD,and consequently,lipid and triglyceride accumulation induced by YY1 knockdown.PGC-1b was reported to be regulated by hypoxia induclibe factor(HIF)-1?.However,we found that YY1 regulation on PGC-1? could occur in a HIF-independent manner.Furthermore,through bioinformatic approach and Ch IP assay,we determine a consensus YY1 binding site in the chr5:149,109,300 to 149,109,650 region of PGC-1b promoter.We also showed that YY1 could suppress the luciferase activity of reporter vector bringing the wild-type PGC-1? promoter;however,this effect was cancelled by mutations in YY1 binding site.Collectively,here we determined YY1 as a novel,critical factor in regulating tumor lipid metabolism under hypoxic condition,as it could bind to PGC-1? promoter,downregulate its activity,suppress fatty acid ?-oxidation,promote lipid and triglyceride accumulation,and subsequently,enhance tumor cell proliferation and tumor development.Thus,our findings not only unravel a novel biological function of YY1,but also provide new insights regarding the molecular mechanism of the aberrant lipid metabolism in tumor cells.Furthermore,these facts also provide new evidences that YY1 could be involved in tumor development through various biological aspects,and thus,might be a potential therapeutic target for cancer.