YY2 Inhibits Proliferation Of Colorectal Carcinoma Cell Through Suppressing The Serine Synthesis Pathway Via Phosphoglycerate Dehydrogenase

Yin Yang 2（YY2）is a recently discovered transcription factor containing zinc finger domains in YY family.YY2 first appeared after the evolution of placental mammals,and is a reverse transcription copy of Yin Yang 1（YY1）of YY family.YY2is highly homologous with YY1 in their nucleotides and amino acids sequences.YY1has been well known to participate in the regulation of a various biological processes as a transcription factor;however,while recent studies revealed that YY2 is involved in immunity,development,and in suppressing tumorigenesis,our knowledge regarding its biological and pathological functions remains very limited.Metabolic reprogramming is one of the characteristics of tumor cells which is crucial for supporting their rapid proliferation and survival under severe tumor microenvironment.Recent studies revealed that beside glucose and lipid metabolic reprogramming,tumor cells also shift their amino acids reprogramming.For example,tumor cells up-regulate glutamate transporters to promote glutamine metabolism,which replenishes the tricarboxylic acid（TCA）cycle and provides energy for tumor cells,as well as producing the important reducing agent reduced glutathione（GSH）to remove the reactive oxygen species（ROS）that accumulated during tumor cells rapid proliferation.In addition,tumor cells can also promote serine metabolism by promoting the expression of enzymes which involve in serine metabolism,thus providing tumor cells with precursors of nucleic acid synthesis and producing abundant reducing substances nicotinamide adenine dinucleotide phosphate（NADPH）to maintain intracellular redox balance.These alterations of amino acid metabolism promote tumorigenesis.On the other hand,while YY2 has been indicated as a tumor suppressor that could inhibit cell cycle and tumorigenesis,the molecular mechanism underlying it has not been totally elucidated.Furthermore,whether it is involved in tumor cells metabolic reprogramming,especially tumor cells amino acids metabolic reprogramming,remains unknown.First of all,using RNA-sequencing and KEGG analysis,we found that YY2overexpression led to the enrichment of serine metabolism pathway.This was further confirmed by the decrease of serine metabolites.Furthermore,we found that YY2inhibitory effect on serine metabolism led to the decrease of proliferation potential of colon cancer cells.Further in-depth analysis of RNA-sequencing results and western blotting assay showed that YY2 significantly decreased the m RNA and protein expression levels of 3-phosphoglycerate dehydrogenase（PHGDH）,the key rate-limiting enzyme in serine anabolism that converts 3-phosphoglycerate（3PG）to3-phosphohydropyruvate（3PHP）,which subsequently forms 3-phosphoserine（3PS）catalyzed by phosphoserine aminotransferase 1（PSAT1）,finally,3-phosphate serine is converted to serine by phospho-serine phosphatase（PSPH）to completes the de novo synthesis of serine.In addition,through the detection of serine metabolites and analysis of the proliferation potential of colon cancer cells,we found that overexpression of both YY2 and PHGDH cancelled the suppressive effect of YY2 on serine metabolism as well as tumor cells proliferation,indicating that YY2 regulation on PHDGH is crucial for its role in suppressing tumor cells proliferation through regulation of amino acids metabolism.Meanwhile,previous studies showed that YY2 regulates the expression of tumor suppressor p53,which could negatively regulate the expression of PHGDH.In order to clarify whether the regulation of YY2 on PHGDH expression is dependent on p53,we analyzed the effect of overexpressing YY2 on PHGDH expression level,as well as the serine metabolite level in p53-null HCT116^p53null cells.Our results revealed that regulation of YY2 on PHGDH occurs in a p53-independent level.To further unravel the specific molecular mechanism of YY2 regulation on PHGDH transcription,we performed dual luciferase reporter assay and chromatin immunoprecipitation assay（Ch IP assay）.Our results showed that YY2 binds directly to PHGDH promoter and suppressed its transcriptional activity.Finally,through xenograft experiments using immunodeficient mice,we proved that while YY2-overexpression suppressed tumorigenesis potential of HCT116^p53null cells,overexpression of PHGDH restored it.This result indicated that YY2/PHGDH axis is crucial for tumor suppressive effect of YY2.Together,this study unraveled a novel role of YY2 in suppressing tumorigenesis potential through negative regulation of PHGDH transcriptional activity and thus suppresses tumor cells serine metabolism.This study not only links YY2 and tumor cells amino acids metabolic reprogramming,but also suggest the potential of YY2 as a target of anti-tumor therapeutic strategy.