KLF6 allelic loss is associated with tumor recurrence and markedly decreased survival in head and neck squamous cell carcinoma

Network Analysis of Biomarkers Associated with Occupational Exposure to Benzene and Malathion

Complex diseases are associated with the effects of multiple genes, proteins, and biological pathways. In this context, the tools of Network Medicine are compatible as a platform to systematically explore not only the molecular complexity of a specific disease but may also lead to the identification of disease modules and pathways. Such an approach enables us to gain a better understanding of how environmental chemical exposures affect the function of human cells, providing better perceptions about the mechanisms involved and helping to monitor/prevent exposure and disease to chemicals such as benzene and malathion. We selected differentially expressed genes for exposure to benzene and malathion. The construction of interaction networks was carried out using GeneMANIA and STRING. Topological properties were calculated using MCODE, BiNGO, and CentiScaPe, and a Benzene network composed of 114 genes and 2415 interactions was obtained. After topological analysis, five networks were identified. In these subnets, the most interconnected nodes were identified as: IL-8, KLF6, KLF4, JUN, SERTAD1, and MT1H. In the Malathion network, composed of 67 proteins and 134 interactions, HRAS and STAT3 were the most interconnected nodes. Path analysis, combined with various types of high-throughput data, reflects biological processes more clearly and comprehensively than analyses involving the evaluation of individual genes. We emphasize the central roles played by several important hub genes obtained by exposure to benzene and malathion.

Circular RNA circ-ABCB10 Promotes Proliferation and Inhibits Apoptosis of Laryngeal Carcinoma by Inhibiting KLF6

Objective

To explore the effect of circular RNA circ-ABCB10 on the proliferation and apoptosis of laryngeal carcinoma via inhibiting KLF6.

Methods

RT-qPCR assay was adopted to detect the expression of circ-ABCB10 and KFL6 in laryngeal carcinoma tissues and cell lines. Cell counting kit-8 (CCK-8) and clone formation assay were employed to detect laryngeal cancer cell viability and proliferation when circ-ABCB10 was silenced or upregulated. In this study, the apoptosis rate was detected by flow cytometry and the protein expression was detected by Western blotting. Wound healing and cross-hole invasion were used to study the migration and invasion of laryngeal cancer cells when circ-ABCB10 was silenced or upregulated.

Results

The results of RT-qPCR detection indicated that the expression of circ-ABCB10 in all three laryngeal carcinoma cells was downregulated by 3.2 times compared with that of HaCat cells. There is low expression of circ-ABCB10 in most laryngeal carcinoma tissues, the diagnostic cutoff value of circ-ABCB10 is 0.0008, the area under the curve is 0.718, the sensitivity is 0.981, and the specificity is 0.556. The expression level of KLF6 in laryngeal carcinoma is on the rise, which is significantly higher compared to healthy tissues (P < 0.05); 48 hours after transfection, RT-qPCR analysis confirmed the transfection efficiency, and upregulation of circ-ABCB10 could significantly promote cell proliferation. Compared with the control group, silencing circ-MTCL1 could inhibit cell proliferation, overexpression of circ-ABCB10 promoted cell migration, and downregulation of circ-ABCB10 significantly inhibited cell movement (P < 0.001). Upregulation of circ-ABCB10 significantly enhanced the invasiveness and motility of laryngeal cancer cells, while downregulation of circ-ABCB10 was the opposite. Compared with the KLF6 NC group, KLF6 level increased significantly in the KLF6 group, while cell viability, colony formation, scratch healing rate, invasive cell number, and Bcl-2 expression level decreased significantly in the KLF6 group, while apoptosis rate and Bax expression level increased significantly (P < 0.05). KLF6 level in the si-circ-ABCB10+anti-KLF6 group was significantly lower than that in the si-circ-ABCB10+anti-KLF6-NC group (P < 0.05). Meanwhile, the cell activity, colony formation number, cell scratch healing rate, number of invaded cells, and Bcl-2 all indicated an upward trend, while the cell apoptosis rate and Bax expression indicated a downward trend (P < 0.05).

Conclusion

The expression of circ-ABCB10 in laryngeal carcinoma was significantly higher compared to that in paracancerous tissues. Silencing circ-ABCB10 could significantly inhibit the growth and proliferation of laryngeal adenocarcinoma cells, while overexpression of circ-ABCB10 could significantly promote the growth of laryngeal adenocarcinoma cells, probably by inhibiting KLF6 to enhance the proliferation of laryngeal carcinoma and inhibit apoptosis.

Can Radiographic Tumor Volume of Oral Squamous Cell Carcinoma Help Predict Clinical and Pathological Tumor Features?

PURPOSE

Radiographic tumor volume (RTV) of oral squamous cell carcinoma (SCC) is seldom measured in practice. Aims of the study are to estimate RTV of SCC and to investigate its relationship with clinical and pathological stage, tumor margin status, recurrence, and need for chemo/radiation.

METHODS

The Design is a retrospective cohort study. The predictor variable is SCC RTV. The primary outcome variables are clinical and pathological tumor size. The secondary outcomes are margin status and postoperative chemo/radiation. Tumor dimensions were measured on preoperative maxillofacial or neck computer tomography images with contrast. Information on patient and tumor characteristics was obtained. Pearson correlation, t test, ANOVA and log rank test were used for statistical analysis. The significance level was set at .05.

RESULTS

Thirty-Six subjects aged 36 to 86 were included in the study. Positive association was found between clinical T stage and RTV (P = .0003) and between pathologic T stage and RTV (P = .002). Mean value of RTV was significantly higher in group with positive margins (P = .0004). RTV was significantly higher in cancers requiring adjuvant chemo/radiation (P = .033). Mean RTV for patients with recurrence was 1.86 cm³ as compared to 1.29 cm³ for patients with no recurrence. Higher tumor volumes were more likely to be associated with recurrence.

CONCLUSIONS

RTV is a variable that is readily available to head and neck surgeons. RTV is associated with clinical and pathological tumor sizes, margin status, need for adjuvant chemo/radiation and tumor recurrence.

Predictive value of transcriptional expression of Krüppel-like factor-6 (KLF6) in head and neck carcinoma patients treated with radiotherapy

PURPOSE

To analyse the relationship between the transcriptional expression of Krüppel-like factor-6 (KLF6) and local response to treatment with radiotherapy in patients with head and neck squamous cell carcinoma (HNSCC).

METHODS

We determined the transcriptional expression of KLF6 in tumour biopsies obtained before treatment with radiotherapy in 83 HNSCC patients. The KLF6 expression was categorized according to the local control of the disease with a recursive partitioning analysis.

RESULTS

During the follow-up period, 27 patients (32.5%) had a local recurrence of the tumour. Patients with local recurrence had significantly higher levels of KLF6 expression than patients in which radiotherapy achieved local control of the disease (P = 0.029). Five-year local recurrence-free survival for patients with a high transcriptional expression of KLF6 (n = 46) was 51.1% (95% CI 36.4-66.2%), and for patients with low expression it was 85.6% (95% CI 73.9-97.3%) (P = 0.0001). The results of a multivariate analysis showed that patients with a high KLF6 expression had a 3.8 times higher risk of local recurrence after treatment with radiotherapy (95% CI 1.4-10.5, P = 0.008).

CONCLUSION

Transcriptional expression of KLF6 was significantly related to local control in HNSCC patients treated with radiotherapy. Patients with high levels of KLF6 expression had a significantly higher risk of local recurrence after treatment.

Artesunate induces mitochondria-mediated apoptosis of human retinoblastoma cells by upregulating Kruppel-like factor 6

Retinoblastoma (RB) is the most common primary intraocular malignancy in children. Intravitreal chemotherapy achieves favorable clinical outcomes in controlling RB vitreous seeds, which are a common reason for treatment failure. Thus, a novel, effective and safe intravitreal chemotherapeutic drug is urgently required. The malaria drug artesunate (ART) recently demonstrated remarkable anticancer effects with mild side effects. The purpose of this study is to investigate the anti-RB efficacy, the underlying mechanism and the intraocular safety of ART. Herein, we verified that ART inhibits RB cell viability and induces cell apoptosis in a dose- and time-dependent manner. Microarray analysis revealed that Kruppel-like factor 6 (KLF6) was upregulated after ART treatment, and this was further confirmed by real-time PCR and western blot assays. Silencing of KLF6 expression significantly reversed ART-induced RB cell growth inhibition and apoptosis. Furthermore, ART activated mitochondria-mediated apoptosis of RB cells, while silencing KLF6 expression significantly inhibited this effect. In murine xenotransplantation models of RB, we further confirmed that ART inhibits RB tumor growth, induces tumor cell apoptosis and upregulates KLF6 expression. In addition, KLF6 silencing attenuates ART-mediated inhibition of tumor growth in vivo. Furthermore, we proved that intravitreal injection of ART in Sprague-Dawley (SD) rats is safe, with no obvious retinal function damage or structural disorders observed by electrophysiology (ERG), fundal photographs, fundus fluorescein angiography (FFA) or optical coherence tomography (OCT) examinations. Collectively, our study revealed that ART induces mitochondrial apoptosis of RB cells via upregulating KLF6, and our results may extend the application of ART to the clinic as an effective and safe intravitreal chemotherapeutic drug to treat RB, especially RB with vitreous seeds.

Biomarkers in Head and Neck Cancer an Update

The study is aimed at establishing the purpose of tumour markers, their application, classification, diagnostic and therapeutic roles in the management of head and neck cancer. A literature review using Medline, Scopus, Google Scholar, the Cochrane Database of Systematic Reviews and the Cochrane central register of controlled trials for articles published between 1993 and 2016 on tumour markers and their role in head and neck cancer was performed. A broader search of prognostic markers in head and neck cancer was also carried out to avoid missing other pertinent markers. Natural history, tumour biology, stage and prognostic factors influence the outcome of management in patients with Head and Neck Squamous cell carcinoma (HNSCC). Evaluation of the cellular lineage and histogenic origin of diverse neoplasms can be done using tumour biomarkers. Identifying predictive tumour markers can lead to improvement in preventive management of HNSCC. There has been remarkable advancement in molecular technology with gene expression and proteomic profiling. Integration of specific tumour markers into routine clinical practice requires substantiation through well designed clinical trials. The investigation of tumour markers is imperative as they influence the prognosis of HNSCC and provide the potential to improve outcomes of treatment through targeted therapy. We have outlined recent tumour biomarkers in this review which have significant role in diagnosis, screening and prognostication in HNSCC. Recent advancement in clinical applications, therapeutic strategies of tumour markers has been highlighted.

KLF6-SV1 is a new prognostic biomarker in postoperative patients with non-small cell lung cancer

Objectives

Non-small cell lung cancer (NSCLC) is aggressive and associated with a poor prognosis. Recent studies have revealed that several genes are involved in the origin and progression of NSCLC. Kruppel-like factor 6 (KLF6) inactivation has been shown in some malignant tumors. KLF6-SV1, as one of the alternatively spliced KLF6 isoforms, has been found to be correlated with metastatic potential and poor survival in some cancers. The purpose of this study was to investigate the clinical and prognostic significance of KLF6-SV1 expression in NSCLC patients after curative resection.

Patients and methods

A total of 79 patients were enrolled in this study. Enumeration data were analyzed using the chi-squared test or Fisher’s exact probability test. Measurement data were represented as average±SD and t-test (homoscedasticity) or t’-test (homoscedasticity uneven). Univariate analysis was performed by modeling Kaplan–Meier survival curves. The log-rank test was used to calculate the survival rate. Multivariate analysis was carried out by the use of the Cox proportional hazard model.

Results

KLF6-SV1 expression was correlated with pN (P<0.05) and pTNM stage (P<0.05). The expression of KLF6-SV1 in the adenocarcinoma group was significantly higher than that in the squamous cell carcinoma group (P<0.05). The 5-year survival rate for 79 NSCLC patients was 40.5%, and it was significantly associated with differentiation (P<0.05), pN (P<0.01), pTNM stage (P<0.01) and high expression of KLF6-SV1 (P<0.01). Cox multivariate regression demonstrated that differentiation, pN and KLF6-SV1 expression were independent factors for the 5-year survival rate.

Conclusion

KLF6-SV1 expression in adenocarcinoma was significantly higher than that in the squamous cell carcinoma, and high expression of KLF6-SV1 was significantly associated with pN and pTNM stage and poor survival in NSCLC patients.

Overexpression of the Oncogenic Variant (KLF6-SV1) in Young NPC Patients and Correlation with Lack of E-Cadherin

Purpose

The transcription factor Krüppel-like factor 6 (KLF6) regulates various cellular functions, such as metabolism, cell proliferation, and differentiation. KLF6 plays a key role in the development and progression of multiple human cancers.

Methods

Fifty primary biopsies and 10 normal nasopharyngeal mucosae were used to analyze by RT-QPCR the expression and the copy number of wtKLF6 and the spliced variants (KLF6-SV1, KLF6-SV2, and KLF6-SV3) in Tunisian patients with nasopharyngeal carcinoma. The expression analysis of E-cadherin and cyclin D1 was conducted by RT-QPCR and Western blot, respectively.

Results

The wtKLF6 was significantly downexpressed in tumors compared to normal tissues (p = 0.0015), whereas KLF6-SV1 and KLF6-SV2 were overexpressed in tumors compared to wtKLF6 and KLF6-SV3 (p < 0.0001). Copy number variation was reduced in tumors compared to normal tissues (p = 0.0071). Interestingly, KLF6-SV1 is associated with the juvenile form (p = 0.0003) which is more aggressive than the adult form of NPC. Furthermore, the oncogenic variant KLF6-SV1 was overexpressed in tumors lacking the expression of E-cadherin (p = 0.0022) suggesting its role in metastasis and tumor progression. The wtKLF6 is associated negatively with cyclin D1 in tumor tissues (p = 0.048).

Conclusion

The wtKLF6 was downexpressed in contrast with the oncogenic variants. Overexpression of KLF6-SV1 is associated with young patients, and loss of E-cadherin suggests that this variant correlated with the aggressiveness of NPC.

MicroRNA-543 promotes the proliferation and invasion of clear cell renal cell carcinoma cells by targeting Krüppel-like factor 6

MicroRNA-543 (miR-543) has been suggested as an important regulator of the development and progression of various cancer types. However, the role and biological function of miR-543 in clear cell renal cell carcinoma (ccRCC) remains unclear. Here, we found that miR-543 expression was significantly increased in tumor tissues from ccRCC patients and ccRCC cell lines. We found that overexpression of miR-543 markedly promoted the proliferation and invasion of ccRCC cells, whereas suppression of miR-543 had the opposite effects. Krüppel-like factor 6 (KLF6) was identified as a target gene of miR-543. Furthermore, we found that miR-543 negatively regulates the expression of KLF6 and p21 in ccRCC cells. Overexpression of KLF6 markedly attenuated the oncogenic effect of miR-543 overexpression. Moreover, knockdown of KLF6 significantly reversed the antitumor effect of miR-543 inhibition. Overall, our results demonstrate that miR-543 promotes the proliferation and invasion of ccRCC cells by targeting KLF6 and suggest that miR-543 may serve as a potential therapeutic target for treatment of ccRCC.

KLF6 inhibited oral cancer migration and invasion via downregulation of mesenchymal markers and inhibition of MMP-9 activities

Krüppel-like factors can bind to specific DNA motifs and regulate various cellular functions, such as metabolism, cell proliferation, and differentiation. Krüppel-like factor 6 (KLF6), a member of this family, is downregulated in human cancers. Oral cancer is a highly prevalent type in Taiwan. Although KLF6 overexpression in human cancer cells inhibits cell proliferation, induces apoptosis, and attenuates cell migration, the effects of KLF6 on oral cancer remains poorly elucidated. This study investigated the role of KLF6 in oral cancer tumorigenesis. Immunohistochemical staining revealed that nuclear KLF6 level was significantly and inversely associated with tumor size and stages. KLF6 overexpression attenuated the migration and invasion of oral cancer SAS cells. Zymography assay demonstrated that KLF6 inhibited the activities of matrix metalloproteinase 9 (MMP-9) and weakened the expression of mesenchymal markers, such as snail, slug, and vimentin. Our study is the first to provide demonstrate that KLF6 functions as a tumor suppressor gene and prevents the metastasis of oral cancer cells.

Transcriptomic characterization of differential gene expression in oral squamous cell carcinoma: a meta-analysis of publicly available microarray data sets

Oral squamous cell carcinoma (OSCC) is a highly prevalent cancer worldwide, and OSCC often goes undiagnosed until advanced disease is present, which contributes to a low survival rate for OSCC patients. The identification of biomarkers for the early detection OSCC and novel therapeutic targets for OSCC treatment is an important research objective. We performed bioinformatics analyses of the gene expression profile of OSCC using microarray data to identify genes that contribute to the development of OSCC. We also predicted the transcription factors involved in the regulation of differential gene expression in OSCC. Our results showed that PI3K, EGFR, STAT1, and CPBP are important contributors to the changes in cellular physiology that occur during the development of OSCC. Therefore, these genes represent potential diagnostic biomarkers and therapeutic targets for OSCC.

Role of Krüppel-like factors in cancer stem cells

Cancer stem cells (CSCs), or cancer cells with stem cell properties, are a rare population of tumor bulk and are recognized to be responsible for cancer recurrence, drug resistance, and metastasis. However, the molecular mechanisms of how to regulate the differentiation and self-renewing of CSCs are poorly understood. Krüppel-like factors (KLFs) are essential DNA-binding transcriptional regulators with diverse functions in various cellular processes, including differentiation, proliferation, inflammation, migration, and pluripotency. Recent progress has highlighted the significance of KLFs in tumor progression and CSCs. The regulatory functions of KLFs in the development of cancer and CSCs have become a burgeoning area of intense research. In this review, we summarize the current understanding and progress of the transcriptional regulation of KLFs in CSCs and discuss the functional implications of targeting CSCs by KLFs for cancer therapeutics.

Mutations and Krüppel-like factor 6 (KLF6) expression levels in breast cancer

The transcription factor KLF6 gene has been identified as a tumor suppressor because of its inactivation in several types of cancers by different mechanisms. However, there are no data in the literature investigating the KLF6 mutation rate and expression levels in breast cancer. Therefore, the present study was conducted in order to investigate whether genetic alterations of KLF6 in association with the KLF6 mRNA expression levels may play a role in breast carcinogenesis. For this purpose, we analyzed alterations of the KLF6 gene by direct sequencing and the mRNA levels by reverse transcription-PCR (RT-PCR). In addition to four different non-coding alterations, one missense and two silent alterations were identified in the coding sequence. Reduced KLF6 expression was observed in 41 (83.67 %) of the 49 breast cancer tumors. These findings suggest that the mutation profile of the KLF6 gene in breast tumors is similar to other cancer types. However, these mutations do not exert any effect on the gene expression rate. Downregulation of KLF6 during the progression of breast cancer is independent of the mutations and occurs by a different mechanism.

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.

KLF6-SV1 drives breast cancer metastasis and is associated with poor survival

Metastasis is the major cause of cancer mortality. A more thorough understanding of the mechanisms driving this complex multistep process will aid in the identification and characterization of therapeutically targetable genetic drivers of disease progression. We demonstrate that KLF6-SV1, an oncogenic splice variant of the KLF6 tumor suppressor gene, is associated with increased metastatic potential and poor survival in a cohort of 671 lymph node-negative breast cancer patients. KLF6-SV1 overexpression in mammary epithelial cell lines resulted in an epithelial-to-mesenchymal-like transition and drove aggressive multiorgan metastatic disease in multiple in vivo models. Additionally, KLF6-SV1 loss-of-function studies demonstrated reversion to an epithelial and less invasive phenotype. Combined, these findings implicate KLF6-SV1 as a key driver of breast cancer metastasis that distinguishes between indolent and lethal early-stage disease and provides a potential therapeutic target for invasive breast cancer.

A small interfering RNA targeting the KLF6 splice variant, KLF6-SV1, as gene therapy for gastric cancer

BACKGROUND

Accumulating evidence suggests that the tumor suppressor gene Kruppel-like factor 6 (KLF6) and its dominant-negative splice form KLF6-SV1 play important roles in both the development and progression of cancer. However, the role of KLF6-SV1 in gastric cancer remains largely unknown.

METHODS

KLF6-SV1 expression was detected in various human gastric cancer cell lines and gastric cancer patient samples by reverse transcriptase polymerase chain reaction (RT-PCR) and Western blotting. Small interfering RNA (siRNA) was used to inhibit KLF6-SV1 expression in BGC-823 and SGC-7901 cell lines. The effects of downregulation of KLF6-SV1 by siRNA on cell proliferation, migration, invasion, and tumor growth were examined in vitro and in vivo.

RESULTS

Overexpression of KLF6-SV1 was detected in tumor samples from gastric cancer patients, and in various differentiated gastric cancer cell lines. In vitro downregulation of KLF6-SV1 by siRNA inhibited BGC-823 and SGC-7901 cell proliferation, anchorage-independent growth, migration, and invasion through the altered expression of Ki-67, vascular endothelial growth factor (VEGF), E-cadherin, and matrix metalloproteinase (MMP)-9. Also, KLF6-SV1 silencing promoted caspase-dependent apoptosis of BGC-823 and SGC-7901 cells via the regulation of phosphatidylinositol 3-OH kinase (PI3K)/Akt activity and Bcl-2-related protein expression. In vivo animal studies showed that KLF6-SV1 siRNA significantly inhibited the tumorigenicity of BGC-823 and SGC-7901 cells. Gene therapy with polyethylenimine/si-SV1 intratumoral injection also resulted in the suppression of tumor growth and prolonged animal survival in an established xenograft tumor model.

CONCLUSION

These data demonstrate that KLF6-SV1 is an important regulator of the growth, migration, invasion, and survival of gastric cancer cells, and downregulation of KLF6-SV1 by siRNA may offer a new potential gene therapy approach for gastric cancer.

Energy restriction-mimetic agents induce apoptosis in prostate cancer cells in part through epigenetic activation of KLF6 tumor suppressor gene expression

Although energy restriction has been recognized as an important target for cancer prevention, the mechanism by which energy restriction-mimetic agents (ERMAs) mediate apoptosis remains unclear. By using a novel thiazolidinedione-derived ERMA, CG-12 (Wei, S., Kulp, S. K., and Chen, C. S. (2010) J. Biol. Chem. 285, 9780-9791), vis-à-vis 2-deoxyglucose and glucose deprivation, we obtain evidence that epigenetic activation of the tumor suppressor gene Kruppel-like factor 6 (KLF6) plays a role in ERMA-induced apoptosis in LNCaP prostate cancer cells. KLF6 regulates the expression of many proapoptotic genes, and shRNA-mediated KLF6 knockdown abrogated the ability of ERMAs to induce apoptosis. Chromatin immunoprecipitation analysis indicates that this KLF6 transcriptional activation was associated with increased histone H3 acetylation and histone H3 lysine 4 trimethylation occupancy at the promoter region. Several lines of evidence demonstrate that the enhancing effect of ERMAs on these active histone marks was mediated through transcriptional repression of histone deacetylases and H3 lysine 4 demethylases by down-regulating Sp1 expression. First, putative Sp1-binding elements are present in the promoters of the affected histone-modifying enzymes, and luciferase reporter assays indicate that site-directed mutagenesis of these Sp1 binding sites significantly diminished the promoter activities. Second, shRNA-mediated knockdown of Sp1 mimicked the repressive effect of energy restriction on these histone-modifying enzymes. Third, ectopic Sp1 expression protected cells from the repressive effect of CG-12 on these histone-modifying enzymes, thereby abolishing the activation of KLF6 expression. Together, these findings underscore the intricate relationship between energy restriction and epigenetic regulation of tumor suppressor gene expression, which has therapeutic relevance to foster novel strategies for prostate cancer therapy.

Mammalian Krüppel-like factors in health and diseases

The Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes that include proliferation, differentiation, growth, development, survival, and responses to external stress. Seventeen mammalian KLFs have been identified, and numerous studies have been published that describe their basic biology and contribution to human diseases. KLF proteins have received much attention because of their involvement in the development and homeostasis of numerous organ systems. KLFs are critical regulators of physiological systems that include the cardiovascular, digestive, respiratory, hematological, and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer, and inflammatory conditions. Furthermore, KLFs play an important role in reprogramming somatic cells into induced pluripotent stem (iPS) cells and maintaining the pluripotent state of embryonic stem cells. As research on KLF proteins progresses, additional KLF functions and associations with disease are likely to be discovered. Here, we review the current knowledge of KLF proteins and describe common attributes of their biochemical and physiological functions and their pathophysiological roles.

Nucleo-cytoplasmic localization domains regulate Krüppel-like factor 6 (KLF6) protein stability and tumor suppressor function

Background

The tumor suppressor KLF6 and its oncogenic cytoplasmic splice variant KLF6-SV1 represent a paradigm in cancer biology in that their antagonistic cancer functions are encoded within the same gene. As a consequence of splicing, KLF6-SV1 loses both the C-terminus C₂H₂ three zinc finger (ZF) domain, which characterizes all KLF proteins, as well as the adjacent 5′ basic region (5BR), a putative nuclear localization signal (NLS). It has been hypothesized that this NLS is a functional domain critical to direct the distinct subcellular localization of the tumor suppressor and its splice variant.

Methodology/Principal Findings

In this study, we demonstrate using EGFP fusion constructs that KLF6/KLF6-SV1 nucleo-cytoplasmic transport is not regulated by the 5′ basic region but activated by a novel NLS encoded within the ZF domain, and a nuclear export signal (NES) located in the first 16 amino acids of the shared N-terminus sequence. We demonstrate KLF6 nuclear export to be Crm1-dependent. The dysregulation of nucleo-cytoplasmic transport when disrupting the KLF6 NLS using site-directed mutagenesis showed that its integrity is necessary for appropriate protein stability. Moreover, these mutations impaired transcriptional induction of two KLF6 well-characterized target genes, E-cadherin and p21, as shown by RT-PCR and luciferase promoter assays. The addition of the ZF domain to KLF6-SV1 results in its nuclear localization and a markedly decreased half-life similar to wild type KLF6.

Conclusions/Significance

We describe the domains that control KLF6 nucleo-cytoplasmic shuttling and how these domains play a role in KLF6 protein half-life and tumor suppressor function. The results begin to mechanistically explain, at least in part, the opposing functions of KLF6 and KLF6-SV1 in cancer.

Emerging roles of Kruppel-like factor 6 and Kruppel-like factor 6 splice variant 1 in ovarian cancer progression and treatment

Epithelial ovarian cancer is one of the most lethal gynecologic cancers and the fifth most frequent cause of female cancer deaths in the United States. Despite dramatic treatment successes in other cancers through the use of molecular agents targeted against genetically defined events driving cancer development and progression, very few insights into epithelial ovarian cancer have been translated from the laboratory to the clinic. If advances are to be made in the early diagnosis, prevention, and treatment of this disease, it will be critical to characterize the common and private (personalized) genetic defects underlying the development and spread of epithelial ovarian cancer. The tumor suppressor Kruppel-like factor 6 and its alternatively spliced, oncogenic isoform, Kruppel-like factor 6 splice variant 1, are members of the Kruppel-like zinc finger transcription factor family of proteins, which have diverse roles in cellular differentiation, development, proliferation, growth-related signal transduction, and apoptosis. Inactivation of Kruppel-like factor 6 and overexpression of Kruppel-like factor 6 splice variant 1 have been associated with the progression of a number of human cancers and even with patient survival. This article summarizes our recent findings demonstrating that a majority of epithelial ovarian cancer tumors have Kruppel-like factor 6 allelic loss and decreased expression coupled with increased expression of Kruppel-like factor 6 splice variant 1. The targeted reduction of Kruppel-like factor 6 in ovarian cancer cell lines results in marked increases in cell proliferation, invasion, tumor growth, angiogenesis, and intraperitoneal dissemination in vivo. In contrast, the inhibition of Kruppel-like factor 6 splice variant 1 decreases cellular proliferation, invasion, angiogenesis, and tumorigenicity; this provides the rationale for its potential therapeutic application. These results and our recent demonstration that the inhibition of Kruppel-like factor 6 splice variant 1 can dramatically prolong survival in a preclinical mouse model of ovarian cancer are reviewed and discussed.

Decreased expression of KLF6 and its significance in gastric carcinoma

To study the expression of the Krüppel-like transcription factor 6 (KLF6) in human gastric carcinoma and normal gastric mucosa tissues, and to explore the role of KLF6 in the carcinogenesis and tumor progression and its clinical significance. Expression of KLF6, P21WAF1 and PCNA was investigated by immunohistochemistry for 69 surgically resected gastric carcinoma tissues and corresponding normal gastric mucosa tissues, respectively. The correlations of KLF6 expression with clinicopathological characteristics, P21WAF1 and PCNA were examined. Positive-expression of KLF6 was 64 out of 69 cases (92.8%) in normal gastric mucosa and only 23 cases (33.3%) in gastric carcinoma. Expression of KLF6 in the gastric carcinoma was remarkably lower than normal gastric mucosa. Decreased expression of KLF6 in gastric carcinoma was significantly associated with histological differentiation (P<0.01), TNM stage (P<0.05), lymph node metastasis (P<0.01) and distant metastasis (P<0.05). There was no significant correlation between KLF6 expression and sex, age. Meanwhile, expression of KLF6 was associated with expression of P21WAF1 in both normal gastric mucosa and gastric carcinoma (P<0.05). In addition, decreased expression of KLF6 in gastric carcinoma was positively associated with PCNA level (r=0.719, P<0.01) by association analysis. Down-regulation of KLF6 might play an important role in the carcinogenesis and development of human gastric carcinoma and have significant clinical value.

The role of KLF6 and its splice variants in cancer therapy

The Krüppel-like zinc finger transcription factor (KLF6) gene encodes a family of proteins generated through alternative splicing involved in the regulation of cancer development and progression. Alternative splicing of the KLF6 gene results in the production of at least four alternatively spliced isoforms, two of which are extensively discussed in this review. The full length form of the KLF6 gene is a tumor suppressor gene that is frequently inactivated by loss of heterozygozity (LOH), somatic mutation, and/or decreased expression in human cancer. While the exact mechanisms underlying KLF6's tumor suppressor roles are not completely known, a number of highly relevant, overlapping pathways have been described: transactivation of p21 in a p53-independent manner, reduction of cyclin D1/cdk4 complexes via interaction with cyclin D1, inhibition of c-Jun proto-oncoprotein activities, decreased VEGF expression, and induction of apoptosis. Kruppel-like factor 6 splice variant 1 (KLF6-SV1) is an oncogenic splice variant of the KLF6 tumor suppressor gene that is specifically overexpressed in a number of human cancers. Increased KLF6-SV1 expression is associated with poor prognosis in prostate, lung, and ovarian cancer. Furthermore, KLF6-SV1 has been shown to be biologically active, antagonizing the tumor suppressor function of KLF6 and promoting tumor growth and dissemination in both ovarian and prostate cancer models. In addition, a common germline polymorphism in the KLF6 gene associated with increased prostate cancer risk in a large multi-institutional study of 3411 men results in increased expression of KLF6-SV1. Furthermore, recent studies have demonstrated that targeted reduction of KLF6-SV1 results in the induction of spontaneous apoptosis in cell culture, synergizes with chemotherapeutic agents like cisplatin, and results in significant tumor regression in vivo. Combined, these data make the KLF6 gene family a compelling therapeutic target for both the treatment of localized as well as metastatic cancer.

Identification of ARHGEF17, DENND2D, FGFR3, and RB1 mutations in melanoma by inhibition of nonsense-mediated mRNA decay

Gene identification by nonsense-mediated mRNA decay inhibition (GINI) has proven to be a strategy for genome-wide discovery of genes containing inactivating mutations in colon and prostate cancers. Here, we present the first study of inhibition of the nonsense-mediated mRNA decay (NMD) pathway in melanoma. We used a combination of emetine and actinomycin D treatment to stabilize mRNAs containing premature termination codons (PTCs), followed by microarray analysis and sequencing to identify novel tumor suppressor genes (TSGs) in a panel of 12 melanoma cell lines. Stringent analysis of the array data was used to select 35 candidate genes for sequencing. Of these, 4 (11%) were found to carry PTCs, including ARHGEF17, DENND2D, FGFR3, and RB1. While RB1 mutations have previously been described in melanoma, the other three genes represent potentially novel melanoma; TSGs. ARHGEF17 showed a G1865A mutation leading to W622X in a cell line derived from a mucosal melanoma; in RB1 a C1411T base change resulting in Q471X was discovered in a cell line derived from an acral melanoma; and the FGFR3 and DENND2D genes had intronic insertions leading to PTCs in cell lines derived from superficially spreading melanomas. We conclude that although the false positive rate is high, most likely due to the lack of DNA mismatch repair gene defects, the GINI protocol is one approach to discover novel TSGs in melanoma.

KLF6 and TP53 mutations are a rare event in prostate cancer: distinguishing between Taq polymerase artifacts and true mutations

Krüppel-like factor 6 (KLF6) has been reported to act as a tumor suppressor gene involved in the regulation of the cell cycle by activating p21 in a p53-independent manner. Many studies suggest that KLF6 is inactivated by allelic loss and somatic mutation. However, there is a high variability in the reported frequency of mutations (from 1 to 55%). TP53 also regulates the cell cycle through the activation of p21. In prostate cancer, the reported frequency of TP53 mutations ranges from 3 to 42%. In all these reports, there is a considerable degree of methodological heterogeneity. Our aim was to determine the frequency of KLF6 and TP53 mutations in a well-defined group of prostate tumors with different stages and Gleason grades. The four exons of KLF6 and exons 4-9 of TP53 were studied in 103 cases, including 90 formalin-fixed, paraffin-embedded (FFPE) and 13 frozen samples. All tumors were analyzed through PCR and direct sequencing. All changes found were confirmed by a second independent PCR and sequencing reaction. For KLF6, mutation (E227G) was only detected in one tumor (1%) and for TP53, three different mutations (L130H, H214R, and Y234C) were detected in five tumors (5%). This low mutation index is in keeping with recent papers on the subject. Our study strongly supports the notion that KLF6 and TP53 mutations are not frequent events in prostate cancer. When using FFPE tissues, it is mandatory to perform at least two independent rounds of PCR and sequencing to confirm mutations and exclude Taq polymerase-induced artifacts.

Insulin-like growth factor-i regulates Kruppel-like factor-6 gene expression in a p53-dependent manner

High-circulating IGF-I concentrations are associated with an increased risk for breast, prostate, and colorectal cancer. Krüppel-like factor-6 (KLF6) is a zinc finger tumor suppressor inactivated in prostate and other types of cancer. We have previously demonstrated that KLF6 is a potent transactivator of the IGF-I receptor promoter. The aim of the present study was to examine the potential regulation of KLF6 gene expression by IGF-I. The human colon cancer cell lines HCT116 +/+ and -/- (with normal and disrupted p53, respectively) were treated with IGF-I. Western blots, quantitative RT-PCR, and transfection assays were used to evaluate the effect of IGF-I on KLF-6 production. Signaling pathway inhibitors were used to identify the mechanisms responsible for regulation of KLF6 expression. Small interfering RNA against p53 and KLF6 was used to assess the role of p53 in regulation of KLF6 expression by IGF-I and to evaluate KLF6 involvement in cell cycle control. Results obtained showed that IGF-I stimulated KLF-6 transcription in cells with normal, but not disrupted, p53, suggesting that KLF6 is a downstream target for IGF-I action. Stimulation of KLF6 expression by IGF-I in a p53-dependent manner may constitute a novel mechanism of action of IGF-I, with implications in normal cell cycle progression and cancer biology.

Genetic and genomic analysis modeling of germline c-MYC overexpression and cancer susceptibility

BACKGROUND

Germline genetic variation is associated with the differential expression of many human genes. The phenotypic effects of this type of variation may be important when considering susceptibility to common genetic diseases. Three regions at 8q24 have recently been identified to independently confer risk of prostate cancer. Variation at 8q24 has also recently been associated with risk of breast and colorectal cancer. However, none of the risk variants map at or relatively close to known genes, with c-MYC mapping a few hundred kilobases distally.

RESULTS

This study identifies cis-regulators of germline c-MYC expression in immortalized lymphocytes of HapMap individuals. Quantitative analysis of c-MYC expression in normal prostate tissues suggests an association between overexpression and variants in Region 1 of prostate cancer risk. Somatic c-MYC overexpression correlates with prostate cancer progression and more aggressive tumor forms, which was also a pathological variable associated with Region 1. Expression profiling analysis and modeling of transcriptional regulatory networks predicts a functional association between MYC and the prostate tumor suppressor KLF6. Analysis of MYC/Myc-driven cell transformation and tumorigenesis substantiates a model in which MYC overexpression promotes transformation by down-regulating KLF6. In this model, a feedback loop through E-cadherin down-regulation causes further transactivation of c-MYC.

CONCLUSION

This study proposes that variation at putative 8q24 cis-regulator(s) of transcription can significantly alter germline c-MYC expression levels and, thus, contribute to prostate cancer susceptibility by down-regulating the prostate tumor suppressor KLF6 gene.