The Role Of PBX3 In The Progression Of Glioma Malignant Biology And Related Mechanisms

Part I Silencing Pre-B-cell leukemia homeobox 3 decreases the proliferation of human glioma cells in vitro and in vivoAmong primary brain tumors,gliomas are the most common and most aggressive,with a poor prognosis and limited treatment options.Thus,it is essential to determine the mechanisms involved in glioma development to develop effective therapies for glioma patients.PBX3,a critical member of the PBX family,is frequently overexpressed in multiple human malignancies.However,the expression patterns and biological functions as well as the involved molecular functions of PBX3 in human gliomas remain largely unknown.In this study,we demonstrate that PBX3 expression is increased in both human glioma tissues and cell lines compared with their normal counterparts.These results suggested that PBX3 might be involved in glioma progression.Thus,the role of PBX3 in glioma cell proliferation was investigated using genetic knockdown and overexpression methods.The results showed that PBX3 knockdown inhibited glioma cell proliferation and induced apoptosis,while PBX3 overexpression significantly promoted glioma cell proliferation.Mechanistically,we found that PBX3 promoted cell proliferation by modulating cell cycle progression.A xenograft LN229 model was used to confirm that PBX3 depletion decreased tumor growth in vivo.In summary,our findings reveal that PBX3 may be a potential therapeutic target in gliomas.Part II MicroRNA-98 attenuates cell migration and invasion in glioma by directly targeting Pre-B-cell leukemia homeobox 3GBM is the most common primary brain tumor in adults.The extraordinary invasion of human GBM into adjacent normal brain tissues contributes to treatment failure.However,the mechanisms that control this process remain poorly understood.Increasing evidence has demonstrated that miRNAs are strongly implicated in the migration and invasion of GBM.In this study,we found that miR-98 was markedly downregulated in human glioma tissues and cell lines.Functional experiments indicated that restored expression of miR-98 attenuated glioma cell invasion and migration,whereas depletion of miR-98 promoted glioma cell invasion and migration.Subsequent investigation showed that PBX3,an important transcription factor that controls tumor invasion,was a direct and functional target of miR-98 in glioma cells.Consistently,an orthotopic mouse model also demonstrated the suppressive effects of miR-98 overexpression on tumor invasion and PBX3 expression.Silencing of PBX3 using small interfering RNA inhibited the migratory and invasive capacities of glioma cells,whereas reintroduction of PBX3 into glioma cells reversed the anti-invasive function of miR-98.Furthermore,depletion of PBX3 phenocopied the effects of miR-98 overexpression in vivo.Finally,qRT-PCR results showed that miR-98 was negatively correlated with PBX3 expression in 24 glioma tissues.Thus,we propose that PBX3 modulation by miR-98 has an important role in regulating glioma invasion and may serve as therapeutic target for glioma.Part III PBX3/MEK/ERK1/2/LIN28/let-7b positive feedback loop enhances mesenchymal phenotype to promote glioma cell migration and invasionBrain invasion by GBM determines recurrence and prognosis in patients,which is,in part,attributed to increased mesenchymal transition.It is,therefore,essential to identify master regulators that control the invasive nature and mesenchymal transition of GBM and target them therapeutically.In this study,we report evidence favoring such a role for the PBX family member PBX3.We found that PBX3 expression levels positively correlate with glioma mesenchymal markers.Ectopic expression of PBX3 in glioma cells promoted an invasive phenotype and triggered the expression of mesenchymal markers,whereas depletion of PBX3 dramatically reduced glioma cell invasive ability and decreased the expression of mesenchymal markers.In addition,inhibition of PBX3 attenuated TGFp-induced glioma mesenchymal transition.Further mechanistic studies revealed that PBX3 mediated glioma mesenchymal transition through activation of MEK/ERK1/2,leading to increased expression of LIN28 by c-myc.Enhanced expression of LIN28 inhibited let-7b biogenesis in glioma cells,which then promoted the pro-invasive genes,such as HMGA2 and IL-6.Furthermore,let-7b suppressed PBX3 by directly targeting 3'-UTR of PBX3.Thus,repressed let-7b by PBX3 amplifying PBX3 signaling and forms a positive feedback loop to promote glioma mesenchymal transition.These data highlight the importance of PBX3 as a key driver of mesenchymal transition and potential therapeutic target.