The induction of KLF5 transcription factor by progesterone contributes to progesterone-induced breast cancer cell proliferation and dedifferentiation

Epigenetic regulation of mRNA mediates the phenotypic plasticity of cancer cells during metastasis and therapeutic resistance (Review)

Plasticity, the ability of cancer cells to transition between differentiation states without genomic alterations, has been recognized as a major source of intratumoral heterogeneity. It has a crucial role in cancer metastasis and treatment resistance. Thus, targeting plasticity holds tremendous promise. However, the molecular mechanisms of plasticity in cancer cells remain poorly understood. Several studies found that mRNA, which acts as a bridge linking the genetic information of DNA and protein, has an important role in translating genotypes into phenotypes. The present review provided an overview of the regulation of cancer cell plasticity occurring via changes in the transcription and editing of mRNAs. The role of the transcriptional regulation of mRNA in cancer cell plasticity was discussed, including DNA‑binding transcriptional factors, DNA methylation, histone modifications and enhancers. Furthermore, the role of mRNA editing in cancer cell plasticity was debated, including mRNA splicing and mRNA modification. In addition, the role of non‑coding (nc)RNAs in cancer plasticity was expounded, including microRNAs, long intergenic ncRNAs and circular RNAs. Finally, different strategies for targeting cancer cell plasticity to overcome metastasis and therapeutic resistance in cancer were discussed.

New insights into KLFs and SOXs in cancer pathogenesis, stemness, and therapy

Despite the development of cancer therapies, the success of most treatments has been impeded by drug resistance. The crucial role of tumor cell plasticity has emerged recently in cancer progression, cancer stemness and eventually drug resistance. Cell plasticity drives tumor cells to reversibly convert their cell identity, analogous to differentiation and dedifferentiation, to adapt to drug treatment. This phenotypical switch is driven by alteration of the transcriptome. Several pluripotent factors from the KLF and SOX families are closely associated with cancer pathogenesis and have been revealed to regulate tumor cell plasticity. In this review, we particularly summarize recent studies about KLF4, KLF5 and SOX factors in cancer development and evolution, focusing on their roles in cancer initiation, invasion, tumor hierarchy and heterogeneity, and lineage plasticity. In addition, we discuss the various regulation of these transcription factors and related cutting-edge drug development approaches that could be used to drug "undruggable" transcription factors, such as PROTAC and PPI targeting, for targeted cancer therapy. Advanced knowledge could pave the way for the development of novel drugs that target transcriptional regulation and could improve the outcome of cancer therapy.

DANCR Induces Cisplatin Resistance of Triple-Negative Breast Cancer by KLF5/p27 Signaling

An increasing body of evidence suggests that long noncoding RNAs play critical roles in human cancer. Breast cancer is a heterogeneous disease and the potential involvement of long noncoding RNAs in breast cancer remains poorly understood. Herein, the study identified a long noncoding RNA, DANCR, which promotes cisplatin chemoresistance in triple-negative breast cancer (TNBC) cells. Mechanistically, binding of DANCR to Krüppel-like factor 5 (KLF5) induced acetylation of KLF5 at lysine 369 (K369), and DANCR knockdown resulted in down-regulation of KLF5 protein levels. Furthermore, DANCR/KLF5 signaling pathway induced hypersensitivity to cisplatin in chemoresistant patients by inhibiting p27 transcription. In summary, this study reinforced the potential presence of a growth regulatory network in TNBC cells, and documented a DANCR/KLF5/p27 signaling pathway mediating cisplatin chemoresistance in TNBC.

A multi-tissue transcriptomic landscape of female mice in estrus and diestrus provides clues for precision medicine

Female reproductive cycle, also known as menstrual cycle or estrous cycle in primate or non-primate mammals, respectively, dominates the reproductive processes in non-pregnant state. However, in addition to reproductive tissues, reproductive cycle could also perform global regulation because the receptors of two major female hormones fluctuating throughout the cycle, estrogen and progesterone, are widely distributed. Therefore, a multi-tissue gene expression landscape is in continuous demand for better understanding the systemic changes during the reproductive cycle but remains largely undefined. Here we delineated a transcriptomic landscape covering 15 tissues of C57BL/6J female mice in two phases of estrous cycle, estrus and diestrus, by RNA-sequencing. Then, a number of genes, pathways, and transcription factors involved in the estrous cycle were revealed. We found the estrous cycle could widely regulate the neuro-functions, immuno-functions, blood coagulation and so on. And behind the transcriptomic alteration between estrus and diestrus, 13 transcription factors may play important roles. Next, bioinformatics modeling with 1,263 manually curated gene signatures of various physiological and pathophysiological states systematically characterized the beneficial/deleterious effects brought by estrus/diestrus on individual tissues. We revealed that the estrous cycle has a significant effect on cardiovascular system (aorta, heart, vein), in which the anti-hypertensive pattern in aorta induced by estrus is one of the most striking findings. Inspired by this point, we validated that two hypotensive drugs, felodipine and acebutolol, could exhibit significantly enhanced efficacy in estrus than diestrus by mouse and rat experiments. Together, this study provides a valuable data resource for investigating reproductive cycle from a transcriptomic perspective, and presents models and clues for investigating precision medicine associated with reproductive cycle.

The role of transcription factors in the acquisition of the four latest proposed hallmarks of cancer and corresponding enabling characteristics

With the recent description of the molecular and cellular characteristics that enable acquisition of both core and new hallmarks of cancer, the consequences of transcription factor dysregulation in the hallmarks scheme has become increasingly evident. Dysregulation or mutation of transcription factors has long been recognized in the development of cancer where alterations in these key regulatory molecules can result in aberrant gene expression and consequential blockade of normal cellular differentiation. Here, we provide an up-to-date review of involvement of dysregulated transcription factor networks with the most recently reported cancer hallmarks and enabling characteristic properties. We present some illustrative examples of the impact of dysregulated transcription factors, specifically focusing on the characteristics of phenotypic plasticity, non-mutational epigenetic reprogramming, polymorphic microbiomes, and senescence. We also discuss how new insights into transcription factor dysregulation in cancer is contributing to addressing current therapeutic challenges.

Inhibitory effect of the novel tyrosine kinase inhibitor DCC-2036 on triple-negative breast cancer stem cells through AXL-KLF5 positive feedback loop

Triple-negative breast cancer (TNBC), an aggressive histological subtype of breast cancer, exhibits a high risk of early recurrence rate and a poor prognosis, and it is primarily associated with the abundance of cancer stem cells (CSCs). At present, the strategies for effectively eradicating or inhibiting TNBC CSCs are still limited, which makes the development of novel drugs with anti-CSCs function be of great value for the treatment of TNBC, especially the refractory TNBC. In this study, we found that the small-molecule tyrosine kinase inhibitor DCC-2036 suppressed TNBC stem cells by inhibiting the tyrosine kinase AXL and the transcription factor KLF5. DCC-2036 downregulated the expression of KLF5 by decreasing the protein stability of KLF5 via the AXL-Akt-GSK3β signal axis, and in turn, the downregulation of KLF5 further reduced the expression of AXL via binding to its promotor (-171 to -162 bp). In addition, p-AXL/AXL levels were positively correlated with KLF5 expression in human TNBC specimens. These findings indicated that DCC-2036 is able to suppress the CSCs in TNBC by targeting the AXL-KLF5 positive feedback loop. Moreover, our findings indicated that DCC-2036 increased the sensitivity of TNBC chemotherapy. Therefore, this study proposes a potential drug candidate and several targets for the treatment of refractory TNBC.

YB-1 is a positive regulator of KLF5 transcription factor in basal-like breast cancer

Y-box binding protein 1 (YB-1) is a well-known oncogene highly expressed in various cancers, including basal-like breast cancer (BLBC). Beyond its role as a transcription factor, YB-1 is newly defined as an epigenetic regulator involving RNA 5-methylcytosine. However, its specific targets and pro-cancer functions are poorly defined. Here, based on clinical database, we demonstrate a positive correlation between Kruppel-like factor 5 (KLF5) and YB-1 expression in breast cancer patients, but a negative correlation with that of Dachshund homolog 1 (DACH1). Mechanistically, YB-1 enhances KLF5 expression not only through transcriptional activation that can be inhibited by DACH1, but also by stabilizing KLF5 mRNA in a RNA 5-methylcytosine modification-dependent manner. Additionally, ribosomal S6 kinase 2 (RSK2) mediated YB-1 phosphorylation at Ser102 promotes YB-1/KLF5 transcriptional complex formation, which co-regulates the expression of BLBC specific genes, Keratin 16 (KRT16) and lymphocyte antigen 6 family member D (Ly6D), to promote cancer cell proliferation. The RSK inhibitor, LJH685, suppressed BLBC cell tumourigenesis in vivo by disturbing YB-1-KLF5 axis. Our data suggest that YB-1 positively regulates KLF5 at multiple levels to promote BLBC progression. The novel RSK2-YB-1-KLF5-KRT16/Ly6D axis provides candidate diagnostic markers and therapeutic targets for BLBC.

Novel insights linking BRCA1-IRIS role in mammary gland development to formation of aggressive PABCs: the case for longer breastfeeding

Pregnancy-associated breast cancer (PABC) is diagnosed during or shortly after pregnancy. Although rare, PABC is a serious occurrence often of the triple negative (TNBC) subtype. Here we show progesterone, prolactin, and RANKL upregulate BRCA1-IRIS (IRIS) in separate and overlapping subpopulations of human mammary epithelial cell lines, which exacerbates the proliferation, survival, and the TNBC-like phenotype in them. Conversely, vitamin D₃ reduces IRIS expression in TNBC cell lines, which attenuates growth, survival, and the TNBC-like phenotype in them. In the mouse, Brca1-Iris (Iris, mouse IRIS homolog) is expressed at low-level in nulliparous mice, increases ~10-fold in pregnant/lactating mice, to completely disappear in involuting mice, and reappears at low-level in regressed glands. Mice underwent 3 constitutive pregnancies followed by a forced involution (after 5 days of lactation) contained ~10-fold higher Iris in their mammary glands compared to those underwent physiological involution (after 21 days of lactation). While protein extracts from lactating glands promote proliferation in IRIS^low and IRIS overexpressing (IRISOE) cells, extracts from involuting glands promote apoptosis in IRIS^low, and aneuploidy in IRISOE cells. In a cohort of breast cancer patients, lack of breastfeeding was associated with formation of chemotherapy resistant, metastatic IRISOE breast cancers. We propose that terminal differentiation triggered by long-term breastfeeding reduces IRIS expression in mammary cells allowing their elimination by the inflammatory microenvironment during physiological involution. No/short-term breastfeeding retains in the mammary gland IRISOE cells that thrive in the inflammatory microenvironment during forced involution to become precursors for aggressive breast cancers shortly after pregnancy.

KLF5, a Novel Therapeutic Target in Squamous Cell Carcinoma

Squamous cell carcinomas (SCCs) are the most common ectodermal cancers, and result in more than 300,000 deaths per year. The Krüppel-like family of transcription factors play a critical role in cancer pathogenesis. The Krüppel-like factor 5 gene (KLF5), which is a member of Krüppel-like family, has been reported to promote cancer cell proliferation and tumorigenesis. In this review, we discuss the roles of KLF5 in different SCCs and the mechanisms by which KLF5 transcriptionally regulates its target gene expression in the pathogenesis and progression of SCCs. Due to its significant functions in cell proliferation and differentiation, KLF5 could be a novel diagnostic biomarker and therapeutic target for the treatment of SCCs.

Roles and Mechanisms of Deubiquitinases (DUBs) in Breast Cancer Progression and Targeted Drug Discovery

Deubiquitinase (DUB) is an essential component in the ubiquitin—proteasome system (UPS) by removing ubiquitin chains from substrates, thus modulating the expression, activity, and localization of many proteins that contribute to tumor development and progression. DUBs have emerged as promising prognostic indicators and drug targets. DUBs have shown significant roles in regulating breast cancer growth, metastasis, resistance to current therapies, and several canonical oncogenic signaling pathways. In addition, specific DUB inhibitors have been identified and are expected to benefit breast cancer patients in the future. Here, we review current knowledge about the effects and molecular mechanisms of DUBs in breast cancer, providing novel insight into treatments of breast cancer-targeting DUBs.

The roles and regulation of the KLF5 transcription factor in cancers

Krüppel‐like factor 5 (KLF5) is a member of the KLF family. Recent studies have suggested that KLF5 regulates the expression of a large number of new target genes and participates in diverse cellular functions, such as stemness, proliferation, apoptosis, autophagy, and migration. In response to multiple signaling pathways, various transcriptional modulation and posttranslational modifications affect the expression level and activity of KLF5. Several transgenic mouse models have revealed the physiological and pathological functions of KLF5 in different cancers. Studies of KLF5 will provide prognostic biomarkers, therapeutic targets, and potential drugs for cancers.

Rictor promotes cell migration and actin polymerization through regulating ABLIM1 phosphorylation in Hepatocellular Carcinoma

As one of the most ominous malignancies, hepatocellular carcinoma (HCC) is frequently diagnosed at an advanced stage, owing to its aggressive invasion and metastatic spread. Emerging evidence has demonstrated that Rictor, as a unique component of the mTORC2, plays a role in cell migration, as it is dysregulated in various cancers, including HCC. However, the underlying molecular mechanism has not been well-characterized. Here, evaluation on a tissue-array panel and bioinformatics analysis revealed that Rictor is highly expressed in HCC tissues. Moreover, increased Rictor expression predicts poor survival of HCC patients. Rictor knockdown significantly suppressed cell migration and actin polymerization, thereby leading to decreased nuclear accumulation of MKL1 and subsequent inactivation of SRF/MKL1-dependent gene transcription, i.e. Arp3 and c-Fos. Mechanistically, we identified ABLIM1 as a previously unknown phosphorylation target of Rictor. Rictor interacts with ABLIM1 and regulates its serine phosphorylation in HCC cells. We generated ABLIM1 knockout cell lines of HCC, in which dominant negative mutations of Ser 214 and Ser 431 residues inhibited the ABLIM1-mediated actin polymerization and the MKL1 signaling pathway. Overall, ABLIM1 phosphorylation induced by Rictor plays an important role in controlling actin polymerization in HCC cells.

Mifepristone Derivative FZU-00,003 Suppresses Triple-negative Breast Cancer Cell Growth partially via miR-153-KLF5 axis

Triple-negative breast cancer (TNBC) is one of the most malignant breast cancers lacking targeted therapeutics currently. We recently reported that mifepristone (MIF), a drug regularly used for abortion, suppresses TNBC cell growth by inhibiting KLF5 expression via inducing miR-153. However, its anticancer efficacy is only modest at high dose. In order to enhance the anticancer activities, a focused compound library containing 17 compounds by altering the sensitive metabolic region of mifepristone has been designed and synthesized. We first tested the cell growth inhibitory effects of these compounds in TNBC cell lines. Among them, FZU-00,003 displayed the most potent efficiency. FZU-00,003 suppresses TNBC cell growth, cell cycle progression and induces apoptosis more effectively than MIF does. Consistently, FZU-00,003 induces miR-153 expression and suppressed KLF5 expression at much lower dosages than MIF does. Furthermore, FZU-00,003 inhibits tumor growth more potently than MIF does. Taken together, the MIF derivative, FZU-00,003 may serve as a better therapeutic compound for TNBC than MIF.

Krüppel-like factors in breast cancer: Function, regulation and clinical relevance

Breast cancer has accounted for the leading cause of cancer-related mortality among women worldwide. Although the progress in its diagnosis and treatment has come at a remarkable pace during the past several decades, there are still a wide array of problems regarding its progression, metastasis and treatment resistance that have not yet been fully clarified. Recently, an increasing number of studies have revealed that some members of Krüppel-like factors(KLFs) are significantly associated with cell proliferation, apoptosis, metastasis, cancer stem cell regulation and prognostic and predictive value for patients in breast cancer, indicating their promising prognostic and predictive potential for breast cancer survival and outcome. In this review, we will summarize our current knowledge of the functions, regulations and clinical relevance of KLFs in breast cancer.

USP3 promotes breast cancer cell proliferation by deubiquitinating KLF5

The Krüppel-like factor 5 (KLF5) transcription factor is highly expressed in basal type breast cancer and promotes breast cancer cell proliferation, survival, migration, and tumorigenesis. KLF5 protein stability is regulated by ubiquitination. In this study, ubiquitin-specific protease 3 (USP3) was identified as a new KLF5 deubiquitinase by genome-wide siRNA library screening. We demonstrated that USP3 interacts with KLF5 and stabilizes KLF5 via deubiquitination. USP3 knockdown inhibits breast cancer cell proliferation in vitro and tumorigenesis in vivo, which can be partially rescued by ectopic expression of KLF5. Furthermore, we observed a positive correlation between USP3 and KLF5 protein expression levels in human breast cancer samples. These findings suggest that USP3 is a new KLF5 deubiquitinase and that USP3 may represent a potential therapeutic target for breast cancer.

Inhibition of super enhancer downregulates the expression of KLF5 in basal-like breast cancers

The transcription factor KLF5 (Krüpple-like factor 5) is highly expressed in basal-like breast cancer (BLBC), which promotes cell proliferation, survival, migration and stemness, serving as a potential therapeutic target. In the current study, a super-enhancer (SE) was identified to be located downstream of the KLF5 gene in BLBC cell lines, HCC1806 and HCC1937. JQ-1, a BRD4 inhibitor, inhibits the expression and activity of KLF5 in both HCC1806 and HCC1937 cells in a time- and dose-dependent manner. Compound 870, an in-house BRD4 inhibitor, exhibited higher potency than JQ-1 to inhibit KLF5 and BLBC growth by arresting cells in G1 phase. Additionally, THZ1, a CDK7 inhibitor, also inhibits KLF5 and BLBC growth in a similar manner. Our findings suggested that KLF5 is regulated by SE, and modulation of SE could be an effective therapeutic strategy for treating BLBC.

Krüpple-like factor 5 is essential for mammary gland development and tumorigenesis

Krüpple-like factor 5 (KLF5) is required for the development of the embryo and multiple organs, such as the lung and intestine. KLF5 plays a pro-proliferative and oncogenic role in several carcinomas, including breast cancer. However, its role in normal mammary gland development and oncogenesis has not been elucidated in vivo. In this study, we used mammary gland-specific Klf5 conditional knockout mice derived by mating Klf5-LoxP and MMTV-Cre mice. The genetic ablation of Klf5 suppresses mammary gland ductal elongation and lobuloalveolar formation. Klf5 deficiency inhibits mammary epithelial cell proliferation, survival, and stem cell maintenance. Klf5 promotes mammary stemness, at least partially, by directly promoting the transcription of Slug. Finally, Klf5 depletion suppressed PyMT-induced mammary gland tumor cell stemness, tumor initiation, and growth in vivo. Slug also mediated these functions of Klf5 in vivo. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Discovery of novel mifepristone derivatives via suppressing KLF5 expression for the treatment of triple-negative breast cancer

Triple-negative breast cancer (TNBC) is one of the most malignant breast cancers currently with a lack of targeted therapeutic drugs. Accumulating evidence supports that KLF5 represents a novel therapeutic target for the treatment of basal TNBC. Our previous studies revealed that mifepristone is capable of suppressing TNBC cell proliferation and promoting cancer cell apoptosis by inhibiting KLF5 expression. Nevertheless, its anticancer efficacy is only modest with high dose. Moreover, its main metabolite N-desmethyl mifepristone with the removal of one methyl moiety results in a significant loss of antiproliferative activity, indicating an important pharmacophore domain around this methyl moiety. To improve the pharmacokinetic properties including metabolic stability and enhance the anticancer activities, a focused compound library by altering this sensitive metabolic region of mifepristone has been designed and synthesized for scaffold repurposing and structural optimization. Compound 17 (FZU-00,004) has been identified with an attractive anticancer profile against TNBC via suppressing KLF5 expression.

Mithramycin A suppresses basal triple-negative breast cancer cell survival partially via down-regulating Krüppel-like factor 5 transcription by Sp1

As the most malignant breast cancer subtype, triple-negative breast cancer (TNBC) does not have effective targeted therapies clinically to date. As a selective Sp1 inhibitor, Mithramycin A (MIT) has been reported to have anti-tumor activities in multiple cancers. However, the efficacy and the mechanism of MIT in breast cancer, especially TNBC, have not been studied. In this study, we demonstrated that MIT suppressed breast cancer cell survival in a dosage-dependent manner. Interestingly, TNBC cells were more sensitive to MIT than non-TNBC cells. MIT inhibited TNBC cell proliferation and promoted apoptosis in vitro in time- and dosage-dependent manners. MIT suppressed TNBC cell survival, at least partially, by transcriptionally down-regulating KLF5, an oncogenic transcription factor specifically expressed in basal TNBC. Finally, MIT suppressed TNBC cell growth in a xenograft mouse model. Taken together, our findings suggested that MIT inhibits basal TNBC via the Sp1/KLF5 axis and that MIT may be used for TNBC treatment.

Spatiotemporally Controlled Ablation of Klf5 Results in Dysregulated Epithelial Homeostasis in Adult Mouse Corneas

Purpose

Corneal epithelial (CE) homeostasis requires coordination between proliferation and differentiation. Here we examine the role of cell proliferation regulator Krüppel-like factor 5 (Klf5) in adult mouse CE homeostasis.

Methods

Klf5 was ablated in a spatiotemporally restricted manner by inducing Cre expression in 8-week-old ternary transgenic Klf5LoxP/LoxP/Krt12rtTA/rtTA/Tet-O-Cre (Klf5Δ/ΔCE) mouse CE by administering doxycycline via chow. Normal chow-fed ternary transgenic siblings served as controls. The control and Klf5Δ/ΔCE corneal (1) histology, (2) cell proliferation, and (3) Klf5-target gene expression were examined using (1) periodic acid Schiff reagent-stained sections, (2) Ki67 expression, and (3) quantitative PCR and immunostaining, respectively. The effect of KLF4, KLF5, and OCT1 on gastrokine-1 (GKN1) promoter activity was determined by transient transfection in human skin keratinocyte NCTC-2544 cells.

Results

Klf5 expression was decreased to 23% of the controls in Klf5Δ/ΔCE corneas, which displayed increased fluorescein uptake, downregulation of tight junction proteins Tjp1 and Gkn1, desmosomal Dsg1a, and basement membrane Lama3 and Lamb1, suggesting defective permeability barrier. In transient transfection assays, KLF5 and OCT1 synergistically stimulated GKN1 promoter activity. Klf5Δ/ΔCE CE displayed significantly fewer cell layers and Ki67+ proliferative cells coupled with significantly decreased cyclin-D1, and elevated phospho(Ser-10) p27/Kip1 expression. Expression of Krt12, E-cadherin, and β-catenin remained unaltered in Klf5Δ/ΔCE corneas.

Conclusions

Klf5 contributes to adult mouse CE homeostasis by promoting (1) permeability barrier function through upregulation of Tjp1, Gkn1, Dsg1a, Lama3, and Lamb1, and (2) basal cell proliferation through upregulation of cyclin-D1 and suppression of phospho(Ser-10) p27/Kip1, without significantly affecting the expression of epithelial markers Krt12, E-cadherin, and β-catenin.

Krüppel-like factors in mammalian stem cells and development

Krüppel-like factors (KLFs) are a family of zinc-finger transcription factors that are found in many species. Recent studies have shown that KLFs play a fundamental role in regulating diverse biological processes such as cell proliferation, differentiation, development and regeneration. Of note, several KLFs are also crucial for maintaining pluripotency and, hence, have been linked to reprogramming and regenerative medicine approaches. Here, we review the crucial functions of KLFs in mammalian embryogenesis, stem cell biology and regeneration, as revealed by studies of animal models. We also highlight how KLFs have been implicated in human diseases and outline potential avenues for future research.

miR-217 inhibits triple-negative breast cancer cell growth, migration, and invasion through targeting KLF5

Triple negative breast cancer (TNBC) is one of the most aggressive breast cancers without effective targeted therapies. Numerous studies have implied that KLF5 plays an important roles in TNBC. How is KLF5 regulated by microRNAs has not been well studied. Here, we demonstrated that miR-217 down-regulates the expression of KLF5 and KLF5's downstream target gene FGF-BP and Cyclin D1 in TNBC cell lines HCC1806 and HCC1937. Consequently, miR-217 suppresses TNBC cell growth, migration, and invasion. MiR-217 suppresses TNBC, at least partially, through down-regulating the KLF5 expression. These results suggest that the miR-217-KLF5 axis might serve as a potential target for treatment of TNBC.

Dexamethasone induces docetaxel and cisplatin resistance partially through up-regulating Krüppel-like factor 5 in triple-negative breast cancer

PURPOSE

Dexamethasone (Dex), a glucocorticoid (GC), is used as a pretreatment drug in cancer patients undergoing chemotherapy. Dex functions by binding to the glucocorticoid receptor (GR) to prevent allergic reactions and severe chemotherapeutic side effects such as nausea and vomiting. However, the mechanisms by which Dex causes chemoresistance remain unknown.

METHODS

We used docetaxel and cisplatin to treat triple-negative breast cancer (TNBC) cells with or without Dex and assessed cell proliferation using a sulforhodamine B colorimetric (SRB) assay. Additionally, Western blotting was employed to measure Krüppel-like factor 5 (KLF5), GR and several apoptosis-related proteins. To determine how the GR regulates KLF5, we used qRT-PCR, luciferase reporter assays and ChIP assays. Finally, we detected the involvement of Dex in TNBC chemotherapeutic resistance using HCC1806 xenograft model in vivo.

RESULTS

In this study, we demonstrated that Dex induces docetaxel and cisplatin resistance in TNBC cells in vitro and in vivo. Dex up-regulates pro-survival transcription factor KLF5 expression at both mRNA and protein levels dependent on GR. Importantly, Dex failed to promote cancer cell survival and tumor growth when KLF5 induction was blocked.

CONCLUSIONS

We conclude that KLF5 is a Dex-induced gene that contributes to Dex-mediated drug chemoresistance, providing a potential novel target for TNBC treatment.

Ataxin-3 like (ATXN3L), a member of the Josephin family of deubiquitinating enzymes, promotes breast cancer proliferation by deubiquitinating Krüppel-like factor 5 (KLF5)

The Krüppel-like factor 5 (KLF5) has been suggested to promote breast cell proliferation, survival and tumorigenesis. KLF5 protein degradation is increased by several E3 ubiquitin ligases, including WWP1 and SCFFbw7, through the ubiquitin-proteasome pathway. However, the deubiquitinase (DUB) of KLF5 has not been demonstrated. In this study, we identified ATXN3L as a KLF5 DUB by genome-wide siRNA screening. ATXN3L directly binds to KLF5, decreasing its ubiquitination and thus degradation. Functionally, knockdown of ATXN3L inhibits breast cancer cell proliferation partially through KLF5. These findings reveal a previously unrecognized role of ATXN3L in the regulation of KLF5 stability in breast cancer. ATXN3L might be a therapeutic target for breast cancer.

Mifepristone Suppresses Basal Triple-Negative Breast Cancer Stem Cells by Down-regulating KLF5 Expression

Triple-negative breast cancer (TNBC) is currently the most malignant subtype of breast cancers without effective targeted therapies. Mifepristone (MIF), a drug regularly used for abortion, has been reported to have anti-tumor activity in multiple hormone-dependent cancers, including luminal type breast cancers. In this study, we showed that MIF suppressed tumor growth of the TNBC cell lines and patient-derived xenografts in NOD-SCID mice. Furthermore, MIF reduced the TNBC cancer stem cell (CSC) population through down-regulating KLF5 expression, a stem cell transcription factor over-expressed in basal type TNBC and promoting cell proliferation, survival and stemness. Interestingly, MIF suppresses the expression of KLF5 through inducing the expression of miR-153. Consistently, miR-153 decreases CSC and miR-153 inhibitor rescued MIF-induced down-regulation of the KLF5 protein level and CSC ratio. Taken together, our findings suggest that MIF inhibits basal TNBC via the miR-153/KLF5 axis and MIF may be used for the treatment of TNBC.

Estrogen Exhibits a Biphasic Effect on Prostate Tumor Growth through the Estrogen Receptor β-KLF5 Pathway

Estrogens are effective in the treatment of prostate cancer; however, the effects of estrogens on prostate cancer are enigmatic. In this study, we demonstrated that estrogen (17β-estradiol [E2]) has biphasic effects on prostate tumor growth. A lower dose of E2 increased tumor growth in mouse xenograft models using DU145 and PC-3 human prostate cancer cells, whereas a higher dose significantly decreased tumor growth. We found that anchorage-independent apoptosis in these cells was inhibited by E2 treatment. Similarly, in vivo angiogenesis was suppressed by E2. Interestingly, these effects of E2 were abolished by knockdown of either estrogen receptor β (ERβ) or Krüppel-like zinc finger transcription factor 5 (KLF5). Ιn addition, E2 suppressed KLF5-mediated transcription through ERβ, which inhibits proapoptotic FOXO1 and proangiogenic PDGFA expression. Furthermore, we revealed that a nonagonistic ER ligand GS-1405 inhibited FOXO1 and PDGFA expression through the ERβ-KLF5 pathway and regulated prostate tumor growth without ERβ transactivation. Therefore, these results suggest that E2 biphasically modulates prostate tumor formation by regulating KLF5-dependent transcription through ERβ and provide a new strategy for designing ER modulators, which will be able to regulate prostate cancer progression with minimal adverse effects due to ER transactivation.

CRTC2 Is a Coactivator of GR and Couples GR and CREB in the Regulation of Hepatic Gluconeogenesis

Glucocorticoid hormones play essential roles in the regulation of gluconeogenesis in the liver, an adaptive response that is required for the maintenance of circulating glucose levels during fasting. Glucocorticoids do this by cooperating with glucagon, which is secreted from pancreatic islets to activate the cAMP-signaling pathway in hepatocytes. The cAMP-response element-binding protein (CREB)-regulated transcription coactivator 2 (CRTC2) is a coactivator known to be specific to CREB and plays a central role in the glucagon-mediated activation of gluconeogenesis in the early phase of fasting. We show here that CRTC2 also functions as a coactivator for the glucocorticoid receptor (GR). CRTC2 strongly enhances GR-induced transcriptional activity of glucocorticoid-responsive genes. CRTC2 physically interacts with the ligand-binding domain of the GR through a region spanning amino acids 561-693. Further, CRTC2 is required for the glucocorticoid-associated cooperative mRNA expression of the glucose-6-phosphatase, a rate-limiting enzyme for hepatic gluconeogenesis, by facilitating the attraction of GR and itself to its promoter region already occupied by CREB. CRTC2 is required for the maintenance of blood glucose levels during fasting in mice by enhancing the GR transcriptional activity on both the G6p and phosphoenolpyruvate carboxykinase (Pepck) genes. Finally, CRTC2 modulates the transcriptional activity of the progesterone receptor, indicating that it may influence the transcriptional activity of other steroid/nuclear receptors. Taken together, these results reveal that CRTC2 plays an essential role in the regulation of hepatic gluconeogenesis through coordinated regulation of the glucocorticoid/GR- and glucagon/CREB-signaling pathways on the key genes G6P and PEPCK.

KLF5 promotes breast cancer proliferation, migration and invasion in part by upregulating the transcription of TNFAIP2

The Kruppel-like factor 5 (KLF5) transcription factor is highly expressed in high-grade and basal-like breast cancers. However, the mechanism by which KLF5 promotes cell migration and invasion is still not completely understood. In this study, we demonstrate that TNFAIP2, a tumor necrosis factor-α (TNFα)-induced gene, is a direct KLF5 target gene. The expression of TNFAIP2 is highly correlated with the expression of KLF5 in breast cancers. The manipulation of KLF5 expression positively alters TNFAIP2 expression levels. KLF5 directly binds to the TNFAIP2 gene promoter and activates its transcription. Functionally, KLF5 promotes cancer cell proliferation, migration and invasion in part through TNFAIP2. TNFAIP2 interacts with the two small GTPases Rac1 and Cdc42, thereby increasing their activities to change actin cytoskeleton and cell morphology. These findings collectively suggest that TNFAIP2 is a direct KLF5 target gene, and both KLF5 and TNFAIP2 promote breast cancer cell proliferation, migration and invasion through Rac1 and Cdc42.

The interplay between TEAD4 and KLF5 promotes breast cancer partially through inhibiting the transcription of p27Kip1

Growing evidence suggests that YAP/TAZ are mediators of the Hippo pathway and promote breast cancer. However, the roles of YAP/TAZ transcription factor partners TEADs in breast cancer remain unclear. Here we found that TEAD4 was expressed in breast cancer cell lines, especially in triple negative breast cancers (TNBC) cell lines. TEAD4 binds to KLF5. Knockdown of either TEAD4 or KLF5 in HCC1937 and HCC1806 cells induced the expression of CDK inhibitor p27. Depletion of either TEAD4 or KLF5 activated the p27 gene promoter and increased the p27 mRNA levels. Depletion of p27 partially prevents growth inhibition caused by TEAD4 and KLF5 knockdown. TEAD4 overexpression stimulated proliferation in vitro and tumor growth in mice, while stable knockdown of TEAD4 inhibited proliferation in vitro and tumor growth in mice. Thus TEAD4 and KLF5, in collaboration, promoted TNBC cell proliferation and tumor growth in part by inhibiting p27 gene transcription. TEAD4 is a potential target and biomarker for the development of novel therapeutics for breast cancer.

Steroid hormones, steroid receptors, and breast cancer stem cells

The ovarian hormones progesterone and estrogen play important roles in breast cancer etiology, proliferation, and treatment. Androgens may also contribute to breast cancer risk and progression. In recent years, significant advances have been made in defining the roles of these steroid hormones in stem cell homeostasis in the breast. Stem cells are potential origins of breast cancer and may dictate tumor phenotype. At least a portion of breast cancers are proposed to be driven by cancer stem cells (CSCs), cells that mimic the self-renewing and repopulating properties of normal stem cells, and can confer drug resistance. Progesterone has been identified as the critical hormone regulating normal murine mammary stem cell (MaSC) populations and normal human breast stem cells. Synthetic progestins increase human breast cancer risk; one theory speculates that this occurs through increased stem cells. Progesterone treatment also increases breast CSCs in established breast cancer cell lines. This is mediated in part through progesterone regulation of transcription factors, signal transduction pathways, and microRNAs. There is also emerging evidence that estrogens and androgens can regulate breast CSC numbers. The evolving concept that a breast CSC phenotype is dynamic and can be influenced by cell signaling and external cues emphasizes that steroid hormones could be crucial players in controlling CSC number and function. Here we review recent studies on steroid hormone regulation of breast CSCs, and discuss mechanisms by which this occurs.

KLF15 in breast cancer: a novel tumor suppressor?

PURPOSE

Krüppel-like factor 15 (KLF15) is a transcription factor that is involved in various biological processes, including cellular proliferation, differentiation and death. In addition, KLF15 has recently been implicated in the development of several human malignancies, including breast cancer. In vitro breast cancer studies have pointed at a putative role in the regulation of cell proliferation. As yet, however, KLF15 expression analyses in primary human breast cancers have not been reported. Here, we set out to investigate the clinical and biological significance of KLF15 expression in human breast cancers.

METHODS

KLF15 expression was evaluated by immunohistochemistry in 54 primary invasive ductal breast carcinomas, and its status was correlated with various clinicopathological parameters. We also assessed KLF15 expression in vitro in 4 breast cancer-derived cell lines using Western blotting, and examined the effects of exogenous KLF15 expression on cell cycle progression using flow cytometry. Concomitant (changes in) p21, p27 and TOPO2A expression levels were examined using real-time RT-PCR and immunocytochemistry, respectively.

RESULTS

In ~90% of the primary breast carcinoma tissues tested, KLF15 was found to be expressed and localized in either the cytoplasm, the nucleus or both. Predominant nuclear immunoreactivity was found to be associated with clinicopathological factors predicting a better clinical outcome (i.e., ER positive, HER2 negative, low grade, low Ki-67 expression). The breast cancer-derived cell lines tested showed a low KLF15 expression with a predominant cytoplasmic localization. Subsequent exogenous KLF15 over-expression resulted in a predominant nuclear localization and a concomitant decreased cellular proliferation and an arrest at the G0/G1 phase of the cell cycle. In addition, we found that nuclear KLF15 expression results in up-regulation of p21, a pivotal suppressor of the G1 to S phase transition of the cell cycle.

CONCLUSIONS

Our results indicate that nuclear KLF15 expression suppresses breast cancer cell proliferation at least partially through p21 up-regulation and subsequent cell cycle arrest. This is a first study addressing the role of KLF15 in breast cancer development.

Krüppel-like Factor 4 Promotes Esophageal Squamous Cell Carcinoma Differentiation by Up-regulating Keratin 13 Expression

Squamous cell differentiation requires the coordinated activation and repression of genes specific to the differentiation process; disruption of this program accompanies malignant transformation of epithelium. The exploration of genes that control epidermal proliferation and terminal differentiation is vital to better understand esophageal carcinogenesis. KLF4 is a member of the KLF family of transcription factors and is involved in both cellular proliferation and differentiation. This study using immunohistochemistry analysis of KLF4 in clinical specimens of esophageal squamous cell carcinoma (ESCC) demonstrated that decreased KLF4 was substantially associated with poor differentiation. Moreover, we determined that both KLF4 and KRT13 levels were undoubtedly augmented upon sodium butyrate-induced ESCC differentiation and G1 phase arrest. Conversely, silencing of KLF4 and KRT13 abrogated the inhibition of G1-S transition induced by sodium butyrate. Molecular investigation demonstrated that KLF4 transcriptionally regulated KRT13 and the expression of the two molecules appreciably correlated in ESCC tissues and cell lines. Collectively, these results suggest that KLF4 transcriptionally regulates KRT13 and is invovled in ESCC cell differentiation.

Regulation of anti-apoptotic signaling by Kruppel-like factors 4 and 5 mediates lapatinib resistance in breast cancer

The Kruppel-like transcription factors (KLFs) 4 and 5 (KLF4/5) are coexpressed in mouse embryonic stem cells, where they function redundantly to maintain pluripotency. In mammary carcinoma, KLF4/5 can each impact the malignant phenotype, but potential linkages to drug resistance remain unclear. In primary human breast cancers, we observed a positive correlation between KLF4/5 transcript abundance, particularly in the human epidermal growth factor receptor 2 (HER2)-enriched subtype. Furthermore, KLF4/5 protein was rapidly upregulated in human breast cancer cells following treatment with the HER2/epidermal growth factor receptor inhibitor, lapatinib. In addition, we observed a positive correlation between these factors in the primary tumors of genetically engineered mouse models (GEMMs). In particular, the levels of both factors were enriched in the basal-like tumors of the C3(1) TAg (SV40 large T antigen transgenic mice under control of the C3(1)/prostatein promoter) GEMM. Using tumor cells derived from this model as well as human breast cancer cells, suppression of KLF4 and/or KLF5 sensitized HER2-overexpressing cells to lapatinib. Indicating cooperativity, greater effects were observed when both genes were depleted. KLF4/5-deficient cells had reduced basal mRNA and protein levels of the anti-apoptotic factors myeloid cell leukemia 1 (MCL1) and B-cell lymphoma-extra large (BCL-XL). Moreover, MCL1 was upregulated by lapatinib in a KLF4/5-dependent manner, and enforced expression of MCL1 in KLF4/5-deficient cells restored drug resistance. In addition, combined suppression of KLF4/5 in cultured tumor cells additively inhibited anchorage-independent growth, resistance to anoikis and tumor formation in immunocompromised mice. Consistent with their cooperative role in drug resistance and other malignant properties, KLF4/5 levels selectively stratified human HER2-enriched breast cancer by distant metastasis-free survival. These results identify KLF4 and KLF5 as cooperating protumorigenic factors and critical participants in resistance to lapatinib, furthering the rationale for combining anti-MCL1/BCL-XL inhibitors with conventional HER2-targeted therapies.

Hippo pathway in mammary gland development and breast cancer

Accumulated evidence suggests that the Hippo signaling pathway plays crucial roles in mammary gland development and breast cancer. Key components of the Hippo pathway regulate breast epithelial cell proliferation, migration, invasion, and stemness. Additionally, the Hippo pathway regulates breast tumor growth, metastasis, and drug resistance. It is expected that the Hippo pathway will provide novel therapeutic targets for breast cancer. This review will discuss and summarize the roles of several core components of the Hippo pathway in mammary gland development and breast cancer.

WWOX suppresses KLF5 expression and breast cancer cell growth

The WW domain-containing oxidoreductase (WWOX) is a tumor suppressor in a variety of cancers, including breast cancer. Reduced WWOX expression is associated with the basal-like subtype and a relatively poor disease-free survival rate among breast cancer patients. Though several WWOX partners have been identified, the functional mechanisms of WWOX's role in cancers have not been fully addressed to date. In the current study, we found WWOX suppresses expression of KLF5-an oncogenic transcription factor-at protein level, and suppresses cancer cell proliferation in both bladder and breast cancer cell lines. Furthermore, we demonstrated that WWOX physically interacts with KLF5 via the former's WW domains and the latter's PY motifs. Interestingly, we found the expression of WWOX negatively correlates with KLF5 expression in a panel of breast cancer cell lines. Taken together, we conjecture that WWOX may suppress cancer cell proliferation partially by reducing the expression of KLF5.

Krüppel-like factor 9 inhibits glioma cell proliferation and tumorigenicity via downregulation of miR-21

Krüppel-like factors (KLFs) are zinc finger-containing transcription factors that play key roles in the regulation of differentiation and development as well as biological processes central to the development of malignancies. Increasing evidence indicates that Krüppel-like factor 9 (KLF9) plays a critical role in regulating tumorigenesis. However, the biological role and molecular mechanism of KLF9 in glioma progression remain unclear. Herein, we found that KLF9 expression was strongly reduced in gliomas. Reduced KLF9 expression promoted glioma cell proliferation. Importantly, re-constitution of KLF9 expression inhibited glioma cell proliferation and tumor growth in vivo. Furthermore, we determined that KLF9 interacted with the miR-21 promoter, leading to suppression of miR-21 expression and cell cycle arrest. Taken together, our findings indicate a novel mechanism for KLF function in tumorigenesis and may also suggest new targets for clinical intervention in human cancer.

Krüppel-like factors in cancer

Krüppel-like factors (KLFs) are a family of DNA-binding transcriptional regulators with diverse and essential functions in a multitude of cellular processes, including proliferation, differentiation, migration, inflammation and pluripotency. In this Review, we discuss the roles and regulation of the 17 known KLFs in various cancer-relevant processes. Importantly, the functions of KLFs are context dependent, with some KLFs having different roles in normal cells and cancer, during cancer development and progression and in different cancer types. We also identify key questions for the field that are likely to lead to important new translational research and discoveries in cancer biology.

Kruppel-like factor 5 transcription factor promotes microsomal prostaglandin E2 synthase 1 gene transcription in breast cancer

The KLF5 (Krüppel-like factor 5) transcription factor is specifically expressed in a subset of estrogen receptor α-negative breast cancers. Although KLF5 promotes breast cancer cell cycle progression, survival, and tumorigenesis, the mechanism by which KLF5 promotes breast cancer is still not entirely understood. Here, we demonstrate that mPGES1, encoding microsomal prostaglandin E2 synthase 1 (mPGES1), is a KLF5 direct downstream target gene. KLF5 overexpression or knockdown positively altered the levels of mPGES1 mRNA and protein in multiple breast cell lines. 12-O-Tetradecanoylphorbol-13-acetate induced the expression of both KLF5 and mPGES1 in dosage- and time-dependent manners. The induction of KLF5 was essential for 12-O-tetradecanoylphorbol-13-acetate to induce mPGES1 expression. Additionally, KLF5 bound to the mPGES1 gene proximal promoter and activated its transcription. Both KLF5 and mPGES1 promoted prostaglandin E2 production; regulated p21, p27, and Survivin downstream gene expression; and likewise stimulated cell proliferation. Overexpression of mPGES1 partially rescued the KLF5 knockdown-induced downstream gene expression changes and growth arrest in MCF10A cells. Finally, we demonstrate that the expression of mPGES1 was positively correlated with the estrogen receptor α/progesterone receptor/HER2 triple-negative status. These findings suggest that mPGES1 is a target gene of KLF5, making it a new biomarker and a potential therapeutic target for triple-negative breast cancers.

Prolactin suppresses a progestin-induced CK5-positive cell population in luminal breast cancer through inhibition of progestin-driven BCL6 expression

Prolactin controls the development and function of milk-producing breast epithelia but also supports growth and differentiation of breast cancer, especially luminal subtypes. A principal signaling mediator of prolactin, Stat5, promotes cellular differentiation of breast cancer cells in vitro, and loss of active Stat5 in tumors is associated with antiestrogen therapy failure in patients. In luminal breast cancer, progesterone induces a cytokeratin-5 (CK5)-positive basal cell-like population. This population possesses characteristics of tumor stem cells including quiescence, therapy resistance and tumor-initiating capacity. Here we report that prolactin counteracts induction of the CK5-positive population by the synthetic progestin (Pg) R5020 in luminal breast cancer cells both in vitro and in vivo. CK5-positive cells were chemoresistant as determined by fourfold reduced rate of apoptosis following docetaxel exposure. Pg-induction of CK5 was preceded by marked upregulation of BCL6, an oncogene and transcriptional repressor critical for the maintenance of leukemia-initiating cells. Knockdown of BCL6 prevented induction of CK5-positive cell population by Pg. Prolactin suppressed Pg-induced BCL6 through Jak2-Stat5 but not Erk- or Akt-dependent pathways. In premenopausal but not postmenopausal patients with hormone receptor-positive breast cancer, tumor protein levels of CK5 correlated positively with BCL6, and high BCL6 or CK5 protein levels were associated with unfavorable clinical outcome. Suppression of Pg-induction of CK5-positive cells represents a novel prodifferentiation effect of prolactin in breast cancer. The present progress may have direct implications for breast cancer progression and therapy as loss of prolactin receptor-Stat5 signaling occurs frequently and BCL6 inhibitors currently being evaluated for lymphomas may have value for breast cancer.

Mechanisms of implantation: strategies for successful pregnancy

Physiological and molecular processes initiated during implantation for pregnancy success are complex but highly organized. This review primarily highlights adverse ripple effects arising from defects during the peri-implantation period that perpetuate throughout pregnancy. These defects are reflected in aberrations in embryo spacing, decidualization, placentation and intrauterine embryonic growth, manifesting in preeclampsia, miscarriages and/or preterm birth. Understanding molecular signaling networks that coordinate strategies for successful implantation and decidualization may lead to approaches to improve the outcome of natural pregnancy and pregnancy conceived from in vitro fertilization.

Role of KLF5 in hormonal signaling and breast cancer development

Steroid hormones, including ovarian steroid hormones progesterone and estrogen and androgen, play vital roles in the development of normal mammary gland and breast cancer via their receptors. How these hormones regulate these physiological and pathological processes remains to be elucidated. Krüppel-like factor 5 (KLF5) is a transcription factor playing significant roles in breast carcinogenesis, whose expression has been shown to be regulated by hormones. In this review, the relationships among hormonal signaling, KLF5, and breast cancer are summarized and discussed.

Krüppel-like factor 5 in human breast carcinoma: a potent prognostic factor induced by androgens

Krüppel-like factor 5 (intestinal) or Krüppel-like factor 5 (KLF5) is a zinc finger-containing transcription factor and involved in important biological processes including cell proliferation and differentiation. However, clinical significance of KLF5 protein has remained largely unknown in breast cancer. Therefore, in this study, we immunolocalized KLF5 in 113 human breast carcinoma cases. KLF5 immunoreactivity was frequently detected in the nuclei of breast carcinoma cells, and median value of the ratio of KLF5-positive carcinoma cells was 30% and was positively associated with the status of androgen receptor. KLF5 immunoreactivity was also significantly associated with increased risk of recurrence and worse clinical outcome in breast cancer patients by univariate analyses, and subsequent multivariate analyses demonstrated that KLF5 immunoreactivity was an independent prognostic factor for both disease-free and breast cancer-specific survival of the patients. We then examined possible regulation of KLF5 by androgen using MCF-7 breast carcinoma cells. KLF5 mRNA was induced by biologically active androgen 5α-dihydrotestosterone in a dose- and time-dependent manner in MCF-7 cells. In addition, results of transfection experiments demonstrated that proliferation activity of MCF-7 cells was significantly associated with the KLF5 expression level. These findings suggest that KLF5 is an androgen-responsive gene in human breast carcinomas and play important roles in the progression of breast carcinomas. KLF5 immunoreactivity is therefore considered a potent prognostic factor in human breast cancers.

Interpretation of interlocking key issues of cancer stem cells in malignant solid tumors

OBJECTIVE

In this review, several interlinking issues related to cancer stem cells (CSCs) in malignant solid tumors are sequentially discussed.

METHODS

A literature search was performed using PubMed, Web of Science and the Cochrane library, combining the words CSCs, solid tumor, isolation, identification, origination, therapy, target and epithelial-mesenchymal transition.

RESULTS

Because a primary problem is the isolation of CSCs, we first analyzed the advantages and disadvantages of recently used methods, which were mostly based on the physical or immunochemical characteristics of CSCs. Once CSCs are isolated, they should be identified by their stem cell properties. Here, we suggest how to establish a standard identification strategy. We also focused on the origination hypotheses of CSCs. The supporting molecular mechanisms for each theory were thoroughly analyzed and integrated. Especially, epithelial- mesenchymal transition is an increasingly recognized mechanism to generate CSCs that are endowed with a more invasive and metastatic phenotype. Finally, we discuss putative strategies of eliminating CSCs as effective cancer therapies.

CONCLUSION

After several interlocking issues of CSCs are thoroughly clarified, these CSCs in solid malignant tumors may specifically be targeted, which raises a new hope for eliminating these tumors.

Progestin suppression of miR-29 potentiates dedifferentiation of breast cancer cells via KLF4

The female hormone progesterone (P4) promotes the expansion of stem-like cancer cells in estrogen receptor (ER)- and progesterone receptor (PR)-positive breast tumors. The expanded tumor cells lose expression of ER and PR, express the tumor-initiating marker CD44, the progenitor marker cytokeratin 5 (CK5) and are more resistant to standard endocrine and chemotherapies. The mechanisms underlying this hormone-stimulated reprogramming have remained largely unknown. In the present study, we investigated the role of microRNAs in progestin-mediated expansion of this dedifferentiated tumor cell population. We demonstrate that P4 rapidly downregulates miR-29 family members, particularly in the CD44(+) cell population. Downregulation of miR-29 members potentiates the expansion of CK5(+) and CD44(+) cells in response to progestins, and results in increased stem-like properties in vitro and in vivo. We demonstrate that miR-29 directly targets Krüppel-like factor 4 (KLF4), a transcription factor required for the reprogramming of differentiated cells to pluripotent stem cells, and for the maintenance of breast cancer stem cells. These results reveal a novel mechanism, whereby progestins increase the stem cell-like population in hormone-responsive breast cancers, by decreasing miR-29 to augment PR-mediated upregulation of KLF4. Elucidating the mechanisms whereby hormones mediate the expansion of stem-like cells furthers our understanding of the progression of hormone-responsive breast cancers.

Kruppel-like factor 5 (KLF5) is critical for conferring uterine receptivity to implantation

A blastocyst will implant only when the uterus becomes receptive. Following attachment, luminal epithelial cells undergo degeneration at the site of the blastocyst. Although many genes critical for uterine receptivity are primarily regulated by ovarian hormones, Kruppel-like factor 5 (KLF5), a zinc finger-containing transcription factor, is persistently expressed in epithelial cells independently of ovarian hormones. Loss of uterine Klf5 causes female infertility due to defective implantation. Cox2 is normally expressed in the luminal epithelium and stroma at the site of blastocyst attachment, but luminal epithelial COX2 expression is absent with loss of Klf5. This is associated with the retention of the epithelium around the implantation chamber with arrested embryonic growth. These results suggest that Klf5 is indispensable for normal implantation.

TAZ antagonizes the WWP1-mediated KLF5 degradation and promotes breast cell proliferation and tumorigenesis

Krüppel-like factor 5 (KLF5) is a PY motif-containing transcription factor promoting breast cell proliferation. The KLF5 protein is rapidly degraded through the proteasome after ubiquitination by E3 ubiquitin ligases, such as WWP1 and SCF(Fbw7). In this study, we demonstrated that a transcriptional co-activator with the PDZ-binding motif (TAZ) upregulated the KLF5 expression through antagonizing the WWP1-, but not Fbw7-, mediated KLF5 ubiquitination and degradation. TAZ interacted with KLF5 through the WW domain of TAZ and the PY motif of KLF5, which is the binding site for WWP1. TAZ inhibited WWP1-KLF5 protein interaction and WWP1-mediated KLF5 ubiquitination and degradation in a WW domain-dependent manner. Overexpression of TAZ upregulated the protein levels of KLF5 and FGF-BP, which is a well-established KLF5 target gene. In addition, depletion of TAZ in both 184A1 and HCC1937 breast cells downregulated protein levels of KLF5 and FGF-BP and inhibited cell growth. Furthermore, stable depletion of either TAZ or KLF5 significantly suppressed HCC1937 xenograft growth in immunodeficient mice. Knockdown of LATS1, a TAZ upstream inhibitory kinase, up-regulated the protein levels of KLF5 and FGF-BP in 184A1 and promoted cell growth through TAZ. Finally, both KLF5 and TAZ were co-expressed in a subset of estrogen receptor α-negative breast cell lines. These results, for the first time, suggest that TAZ promotes breast cell growth partially through protecting KLF5 from WWP1-mediated degradation and enhancing KLF5's activities.