The Regulation And Function Of KLF4 In Tumorigenesis And Endocrine Resistance

Krüppel-like factor 4(KLF4)is a transcription factor that contributes to diverse cellular processes,including cell growth,proliferation,differentiation,as well as stem cell self-renewal and maintenance of pluripotency.KLF4 plays a complex role in human cancers,acting as both a tumor suppressor and oncogene depending on the tissue type.The tumor suppressive function of KLF4 in lung cancer has been reported in several reports.Our previous studies demonstrated that KLF4 was reduced in primary lung cancer tissues and regulated cancer development and progression via the transcriptional downregulation of h TERT(Human telomerase reverse transcriptase)expression and telomerase activity.In breast cancer,the role of KLF4 remains less clear and controversial,and the underlying mechanism remains unclear.Here,in this dissertation,based on KLF4 gene,we explored the related downstream targets and signaling pathway of KLF4 in lung cancer and the roles of KLF4 in tamoxifen resistance of breast cancer.Research one: The underlying mechanism and function of KLF4/PLAC8 regulatory pathway in affecting lung cancer growthBackgroud: Accumulating evidence suggests that placenta-specific 8(PLAC8)plays an important role in normal cellular process and human diseases,including multiple types of human tumors,and its role is highly relied upon in cellular and physiologic contexts.As a zinc-finger transcription factor,KLF4 enhance or suppress the downstream target gene transcription.However,there are no reports on its expression profile and biological roles during lung cancer development.Materials and methods: In the current study,both the clinical implications and biological effects of PLAC8 in lung cancer(LC)progression were investigated,and we identified and described the novel Krüppel-like factor 4(KLF4)/PLAC8 regulatory pathway in cancer progression.Elevated PLAC8 levels were positively correlated with tumor size,histological grade,and tumor node metasis(TNM)stage,and LC patients with high PLAC8 expression suffered poor outcomes.In vitro and in vivo assays further revealed that endogenous PLAC8 promoted cell proliferation and tumor formation.We also found downregulated PLAC8 protein in several LC cell lines following the induction of KLF4,and immunohistochemistry analysis of LC tissues by microarray indicated a potential inverse correlation between PLAC8 and KLF4 expression.Luciferase reporter analysis and chromatin immunoprecipitation assays determined that KLF4 negatively regulated PLAC8 promoter activity via directly binding to the promoter region.Furthermore,the growth inhibition resulting from KLF4 overexpression was partially rescued by ectopic PLAC8 expression.Results:1.PLAC8 overexpression predictes poor outcome in LC patients,and PLAC8 expresssion is related to LC tumor size,TMN stage and lymph node metastasis.2.In vitro and in vivo assay confirm that PLAC8 overexprssion promote LC cell growth and tumor formation.3.There is an inverse correlation between PLAC8 and KLF4 in LC,and overexpression of KLF4 inhibits PLAC8 transcription.4.KLF4 directly binds to the promoter region of PLAC8,and inhibits PLAC8 expression.Together,our data uncovered a previously unidentified role of PLAC8 as a central mediator in LC progression.PLAC8 was transcriptionally repressed by KLF4,and the novel KLF4/PLAC8 axis may act as a promising candidate target for LC diagnosis and therapy.Research two:KLF4 overcomes endocrine resistance by suppressing MAPK signaling pathwayBackgroud: Endocrine therapy remains the most effective treatment for estrogen receptor(ER)-positive breast cancer,but the therapeutic efficacy is limited by intrinsically resistant to this therapy or acquired endocrine resistance.Tamoxifen(TAM)resistance represents a daunting challenge to the successful treatment for breast cancer.Significant efforts should be undertaken to resolve the molecular mechanisms that lead to endocrine resistance.KLF4 has critical roles in the development and progression of breast cancer,but its expression,function and regulation in the efficacy of TAM therapy in breast cancer have yet to be investigated.Materials and methods: Here,we examined the clinical significance and biologic effects of KLF4 in breast cancer.Firstly,higher expression of KLF4 correlated with increased TAM sensitivity in breast cancer cells,and analysis of GEO datasets indicated that KLF4 expression was positively correlated with ERα and enhanced expression of KLF4 sensitized breast cancer patients to endocrine therapy.Knockdown of KLF4 in MCF-7 and BCAP37 cells led to increased TAM resistance,while ectopic KLF4 expression promoted the responsiveness to TAM in T47 D and TAM-resistant MCF-7/TAM cells.Secondly,ectopic KLF4 overexpression suppressed MCF-7/TAM cell growth,invasion and migration.Moreover,KLF4 expression was down-regulated in breast cancer tumor tissues and high expression of KLF4 was associated with favorable outcomes.Mechanistically,KLF4 may enhance the responsiveness of breast cancer cells to TAM through suppressing mitogen-activated protein kinase(MAPK)signaling pathway.We found that ERK and p38 were more activated in MCF-7/TAM compared with MCF-7,and treatment with MAPK-specific inhibitors significantly suppressed cell viability.Knockdown of KLF4 activated ERK and p38 and drove MCF-7 cells to become resistant to TAM.Conversely,overexpression of KLF4 in MCF-7/TAM cells suppressed ERK and p38 signaling and resulted in increased sensitivity to TAM.Therefore,our findings suggested that KLF4 contributed to TAM sensitivity in breast cancer via phosphorylation modification of ERK and p38 signaling.Results:1.KLF4 expresssion is downregualted in breast cancer,which is associated with patients’ better prognosis.2.Breast cancer patients with highly-expressed KLF4 is more likely to benefit from endocrine therapy,and KLF4 could be used as a molecular marker for predicting complete pathological remission after endocrine therapy for breast cancer patients3.KLF4 enhance TAM sensitivity of breast cancer cells by suppressing MAPK signaling pathway.4.The MAPK signaling pathway is aberrantly activated in TAM-resistant breast cancer cells,and the specific small molecule inhibitors targeting MAPK signaling pathway have significant killing effect on TAM-resistant breast cancer cells,which providing a new potential theoretical basis for clinically reversing the endocrine therapy resistance of breast cancer.In summary,this study highlighted the significance of KLF4/MAPK signal interaction in regulating TAM resistance of breast cancer,and suggested that targeting KLF4/MAPK signaling may be a potential therapeutic strategy for breast cancer treatment,especially for the TAM-resistant patients.