Molecular Mechanism For PBX3-mediated Cell Proliferation In Colorectal Cancer Cells

Cancer has been the leading cause of death since 2010.Cancer deaths account for 23.91% of all deaths in China and have become a very important public health problem in China.Colorectal cancer,the third most common malignant tumor in the world,is the most common digestive tract malignant tumor,and its incidence rate and mortality rate in China isrising.However,the molecular mechanism of colorectal cancer progression is very complex,and thus has not been fully elucidated.Pre-B-cell leukemia transcription factor 3(PBX3)belongs to the highly conserved PBX protein family.PBX3 is not only widely expressed in normal cells in mammals but also in various tumors,including leukemia and solid tumors,such as gastric cancer,lung cancer,and prostate cancer.Previous researches have shown that PBX3 is highly expressed in several solid tumor tissues and plays crucial role in promoting tumor cells proliferation.However,the role of PBX3 and its molecular mechanismin colon cancer development has not been clarified.Tumor suppressor gene p53 is the most studied tumor suppressor which was discovered in 1979.Although it has beendiscovered for 40 years,its biological functions and molecular mechanisms have not been fully elucidated.Wild-type p53,which promotes tumor cell apoptosis and inhibits tumor growth,is encoded by the TP53 gene.In about 50% of tumors,TP53 is mutated in the DNA domain encoding p53,thereby losing its biological function in suppressing tumorigenesis;however,in tumors with wild-type TP53,the expression of p53 is also frequently down-regulated,indicating that the regulatory mechanism of its expression is also closely related with tumor development.p53 is a sequence-specific DNA-binding protein that regulates transcription.Its protein consists of an N-terminal transactivation domain and a C-terminal encoding nuclear localization domain,as well as a central proline-rich DNA binding domain.Previous researches have shown that p53 could regulate genome stability and induce DNA repair,and furthermore,could induce the expression of cell-cycle inhibitor p21 at its transcriptional level which then leads to cell cycle arrest,inhibition of cell proliferation and the induction of apoptosis.In this study,we investigated the effect of PBX3 on the proliferation and the apoptosis of colon cancer cells,as well as its molecular mechanism.We used RNA interference(RNAi)to knockdown PBX3,and found that knocking down PBX3 results in the significant increase of p21 expression,both in mRNA and protein levels.By using clinical human colon carcinoma tissues,we then found that compared to normal adjacent tissue,the expression level of PBX3 in colon cancer tissue is aberrantly high.These results suggest that PBX3 is closely related with colon cancer progression.We also investigated the effects of PBX3 on colon cancer cell proliferation,apoptosis and colony formation potentials,and found that PBX3 silencing grossly suppressed the increase of colon cancer cell proliferation and colony formation potential,and simultenously,induces apoptosis.In order to further elucidate the mechanism of PBX3 regulation on p21 expression,we examined the role of PBX3 the expression level of on p21 upstream regulator p53.Our experimental results showed that the expression levels of p53 mRNA and protein in colon cancer cells increase significantly after knockdown of PBX3;while knocking down both PBX3 and p53 simultaneously abolishedthe increase of p21 expression level induced by PBX3 knockdown;and furthermore,diminishes the suppressive effect of PBX3 knockdown on cell number and colony formation potential.These results suggest that PBX3 could negatively regulate p53/p21 axis,thereby promotes colon cancer cells proliferation and colony formation potential.Moreover,by using p53 promoter-driven luciferase reporter assay,we also demonstrated the negative regulation of PBX3 on p53 transcriptional level.Taken together,through molecular and cellular experiments,we have demonstrated that PBX3 could promote the proliferation and colony formation potentials of colon cancer cells by negativelyregulates p53/p21 signaling pathway,indicating its crucial role in promoting colon cancer progression.This study not only reveals a new mechanism of PBX3 in regulating tumorigenesis,but also provides new insights regarding the regulation of p53/p21 axis.