Nuclear translocation of EGFR has been shown to be important for tumor cell growth, survival, and therapeutic resistance. Previously, we detected the association of EGFR with Keap1 in the nucleus. Keap1 is a Kelch-like ECH-associated protein, which plays an important role in cellular response to chemical and oxidative stress by regulating Nrf2 protein stability and nuclear translocation. In this study, we investigate the role of EGFR in regulating Keap1/Nrf2 cascade in the nucleus and provide evidence to show that nuclear EGFR interacts with and phosphorylates nuclear Keap1 to reduce its nuclear protein level. The reduction of nuclear Keap1 consequently stabilizes nuclear Nrf2 and increases its transcriptional activity in cancer cells, which contributes to tumor cell resistance to chemotherapy.
EGFR; Keap1; Nrf2; cancer; ubiquitination
Breast cancer is the second-leading cause of oncology-related death in US women. Of all invasive breast cancers, patients with tumors lacking expression of the estrogen and progesterone hormone receptors and overexpression of human epidermal growth factor receptor 2 have the poorest clinical prognosis. These referred to as triple-negative breast cancer (TNBC) represent an aggressive form of disease that is marked by early-onset metastasis, high tumor recurrence rate, and low overall survival during the first three years post-diagnosis. In this report, we discuss a novel model of early-onset TNBC metastasis to bone and lungs, derived from MDA-MB-231 cells. Breast cancer cells injected intravenously produced rapid, osteolytic metastases in long bones and spines of athymic nude mice, with concurrent metastasis to lungs, liver, and soft tissues. From the bone metastases, we developed a highly metastatic luciferase-tagged cell line variant named MDA-231-LUC Met. In this report, we demonstrate that the Akt/mTOR/S6K1 axis is hyperactivated in these cells, leading to a dramatic increase in phosphorylation of S6 ribosomal protein at Ser235/236. Lastly, we provide evidence that inhibition of the furthest downstream kinase in the mTOR pathway, S6K1, with a highly specific inhibitor PF-4708671 inhibits cell migration, and thus may provide a potent anti-metastatic adjuvant therapy approach.
TNBC; metastasis; bone; S6K1; S6
Polycomb repressive complex 2 (PRC2) is the epigenetic regulator that induces histone H3 lysine 27 methylation (H3K27me3) and silences specific gene transcription. Enhancer of zeste homolog 2 (EZH2) is an enzymatic subunit of PRC2, and evidence shows that EZH2 plays an essential role in cancer initiation, development, progression, metastasis, and drug resistance. EZH2 expression is indeed regulated by various oncogenic transcription factors, tumor suppressor miRNAs, and cancer-associated non-coding RNA. EZH2 activity is also controlled by post-translational modifications, which are deregulated in cancer. The canonical role of EZH2 is gene silencing through H3K27me3, but accumulating evidence shows that EZH2 methlyates substrates other than histone and has methylase-independent functions. These non-canonical functions of EZH2 are shown to play a role in cancer progression. In this review, we summarize current information on the regulation and roles of EZH2 in cancer. We also discuss various therapeutic approaches to targeting EZH2.
EZH2; PRC2; Neoplasms; Genetic transcription; Untranslated RNA; MicroRNAs; Post-translational protein processing
Results of multiple clinical trials suggest that EGFR tyrosine kinase inhibitors (TKIs) exhibit negative effects on platinum-based chemotherapy in lung cancer patients with wild type (wt) EGFR, but the underlying molecular mechanisms are still uncertain. Studies that identify the mechanism of how TKIs negatively affect patients with wt EGFR are important for future development of effective strategies to target lung cancer. Thus, we returned to in vitro study to investigate and determine a possible explanation for this phenomenon.
We investigated the effects of TKIs and cisplatin on caspase-independent cell death (CID) and the role of CID in the efficacy of each drug and the combination. Furthermore, we studied the mechanism how EGFR signaling pathway is involved in CID. Finally, based on the identified mechanism, we tested the combinational effects of cisplatin plus SAHA or erastin on CID.
We found that gefitinib inhibited cisplatin-induced CID but not caspase-dependent apoptotic cell death. In wt EGFR cells, gefitinib not only inhibited CID but also failed to induce apoptosis, therefore, compromising the efficacy of cisplatin. Inhibition of EGFR-ERK/AKT by gefitinib activates FOXO3a which in turn reduces reactive oxygen species (ROS) and ROS-mediated CID. To overcome this, we showed that SAHA and erastin, the inducers of ROS-mediated CID, strongly enhance the effect of cisplatin in wt EGFR cells.
TKI-mediated inhibition of CID plays an important role of the efficacy of chemotherapy. Moreover, FOXO3a is a key factor in the negative effects of TKI by eliminating cisplatin-induced ROS.
Clinical correlation studies have clearly shown that obesity is associated with breast cancer risk and patient survival. Although several potential mechanisms linking obesity and cancers have been proposed, the detailed molecular mechanism of obesity-mediated breast tumorigenesis has not yet been critically evaluated. In this study, we evaluated the effects of obesity on mammary tumor initiation and progression using mice with genetic and diet-induced obesity bearing mammary tumor xenografts and mouse mammary tumor virusneu transgenic mice that were fed a high-fat diet. We show that obesity promoted mammary tumor growth and development in these animal models. Moreover, the expressions of TNFα, VEGF, IKKβ, and mTOR are upregulated in mammary tumors of obese mice, suggesting that the IKKβ/ mTOR/VEGF signaling pathway is activated by TNFα in the tumors of obese mice. More importantly, inhibitors (rapamycin, bevacizumab, and aspirin) that target members of the pathway suppressed tumorigenesis and prolonged survival more effectively in obese mice than in nonobese mice. Here, we not only identified a specific signaling pathway that contributes to mammary tumorigenesis in obese mice but also a strategy for treating obesity-mediated breast cancer.
Antiangiogenesis is a promising antitumor strategy that inhibits tumor vascular formation to suppress tumor growth. Specifically, targeting VEGF has shown therapeutic benefits in many cancer types, leading to its approval as the first antiangiogenic drug by the Food and Drug Administration in the United States. It is known, however, that patients will experience unfavorable side effects as the VEGF and/or VEGF receptor signaling pathway is also required for homeostasis in normal tissues. Moreover, due to the cytostatic nature of antiangiogenic, cancer cells that are not killed by these drugs later develop an even more malignant phenotype, resulting in tumor invasion and metastasis. Although there have been many attempts to reduce drug resistance and increase therapeutic efficacy by combining antiangiogenic drugs with chemotherapy, the cumulative toxicity of antiangiogenic combinations limits their feasibility as treatments, as chronic angiogenesis inhibition typically reduces the antitumor effect of the co-administered chemotherapeutics. To overcome these problems, it is critical to explore new strategies that limit tumor resistance and side effects and also increase the exposure of chemotherapy drugs at the tumor site. Here, we review current understanding of antiangiogenic drugs and introduce a new combination strategy that links direct antiangiogenic protein and enzyme prodrug system with dual-targeting antiangiogenic and antiproliferative therapeutic effect in tumor microenvironment. This strategy has the potential to overcome these clinical hindrances and may serve as a paradigm for the next generation of antiangiogenic drugs.
Antiangiogenesis; bevacizumab; chemotherapy; 5-fluorouracil; endothelial cell-targeting
Sodium/glucose co-transporter 1 (SGLT1), which actively and energy-dependently uptakes glucose, plays critical roles in the development of various diseases including diabetes mellitus and cancer, and has been viewed as a promising therapeutic target for these diseases. Protein-protein interaction with EGFR has been shown to regulate the expression and activity of SGLT1. Exogenous expression of SGLT1 is one of the essential approaches to characterize its functions; however, exogenously expressed SGLT1 is not firmly detectable by Western blot at its calculated molecular weight, which creates a hurdle for further understanding the molecular events by which SGLT1 is regulated. In this study, we demonstrated that exogenous SGLT1 functions in glucose-uptake normally but is consistently detected near the interface between stacking gel and running gel rather than at the calculated molecular weight in Western blot analysis, suggesting that the overexpressed SGLT1 forms SDS-resistant aggregates, which cannot be denatured and effectively separated on SDS-PAGE. Co-expression of EGFR enhances both the glucose-uptake activity and protein level of the SGLT1. However, fusion with Flag or HA tag at its carboxy- but not its amino-terminus abolished the glucose-uptake activity of exogenous SGLT1 without affecting its protein level. Furthermore, the solubility of SGLT1 aggregates was not affected by other detergents but was partially improved by inhibition of o-link glycosylation. These findings suggested exogenous overexpression of SGLT1 can function normally but may not be consistently detectable at its formula weight due to its gel-shift behavior by forming the SDS-resistant aggregates.
Sodium/glucose cotranspoter 1; epidermal growth factor receptor; protein aggregation; glucose uptake; o-link glycosylation
Epigenetic regulation plays an important role in stem cell self-renewal, maintenance and lineage differentiation. The epigenetic profiles of stem cells are related to their transcriptional signature. Enhancer of Zeste homlog 2 (EZH2), a catalytic subunit of epigenetic regulator Polycomb repressive complex 2 (PRC2), has been shown to be a key regulator in controlling cellular differentiation. EZH2 is a histone methyltransferase that not only methylates histone H3 on Lys 27 (H3K27me3) but also interacts with and recruits DNA methyltransferases to methylate CpG at certain EZH2 target genes to establish firm repressive chromatin structures, contributing to tumor progression and the regulation of development and lineage commitment both in embryonic stem cells (ESCs) and adult stem cells. In addition to its well-recognized epigenetic gene silencing function, EZH2 also directly methylates nonhistone targets such as the cardiac transcription factor, GATA4, resulting in attenuated GATA4 transcriptional activity and gene repression. This review addresses recent progress toward the understanding of the biological functions and regulatory mechanisms of EZH2 and its targets as well as their roles in stem cell maintenance and cell differentiation.
EZH2; polycomb repressive complex; embryonic stem cells; adult stem cells; chromatin modification; methylation
Several antiangiogenic drugs targeting VEGF/VEGFR approved by the FDA for many cancer types including colorectal and lung cancer can effectively reduce tumor growth. However, targeting the VEGF signaling pathway will likely influence the normal function of endothelial cells in maintaining homeostasis and cause unwanted adverse effects. Indeed, emerging experimental evidence suggests that VEGF-targeting therapy induced less tumor cell–specific cytotoxicity, allowing residual cells to become more resistant and eventually develop a more malignant phenotype. We report an antitumor therapeutic EndoCD fusion protein developed by linking endostatin (Endo) to cytosine deaminase and uracil phosphoribosyl transferase (CD). Specifically, Endo possesses tumor antiangiogenesis activity that targets tumor endothelial cells, followed by CD, which converts the nontoxic prodrug 5-fluorocytosine (5-FC) to the cytotoxic antitumor drug 5-fluorouracil (5-FU) in the local tumor area. Moreover, selectively targeting of tumor sites allows an increasing local intratumoral concentration of 5-FU, thus providing high levels of cytotoxic activity. We demonstrated that treatment with EndoCD plus 5-FC, compared with bevacizumab plus 5-FU treatment significantly increased the 5-FU concentration around tumor sites and suppressed tumor growth and metastasis in human breast and colorectal orthotropic animal models. In addition, in contrast to treatment with bevacizumab/5-FU, EndoCD/5-FC did not induce cardiotoxicity leading to heart failure in mice after long-term treatment. Our results showed that compared with currently used antiangiogenic drugs, EndoCD possesses potent anticancer activity with virtually no toxic effects and does not increase tumor invasion or metastasis. Together, these findings suggest that EndoCD/5-FC could become an alternative option for future antiangiogenesis therapy.
antiangiogenesis; bevacizumab; chemotherapy; 5-fluorouracil; endothelial cell-targeting
Aberrant regulation of ribosomal RNA (rRNA) synthesis and translation control can facilitate tumorigenesis. The ErbB2 growth factor receptor is overexpressed in many human tumors and has been detected in the nucleus, but the role of nuclear ErbB2 is obscure. In this study, we defined a novel function of nuclear ErbB2 in enhancing rRNA gene transcription by RNA polymerase-I (RNA Pol I). Nuclear ErbB2 physically associates with β-actin and RNA Pol I, coinciding with active RNA Pol I transcription sites in nucleoli. RNAi-mediated knockdown of ErbB2 reduced pre-rRNA and protein synthesis. In contrast, wild-type ErbB2 augmented pre-rRNA level, protein production and cell size/cell growth, but not by an ErbB2 mutant which is defective in nuclear translocation. Chromatin immunoprecipitation assays revealed that ErbB2 enhances binding of RNA Pol I to rDNA. Additionally, ErbB2 associated with rDNA, RNA Pol I and β-actin, suggesting how it could stimulate rRNA production, protein synthesis and increased cell size and cell growth. Lastly, ErbB2-potentiated RNA Pol I transcription could be stimulated by ligand and was not substantially repressed by inhibition of PI3-K and MEK/ERK, the main ErbB2 effector signaling pathways. Together, our findings indicate that nuclear ErbB2 functions as a regulator of rRNA synthesis and cellular translation, which may contribute to tumor development and progression.
ErbB-2; ribosomal RNA; RNA polymerase I; translation
With rapid development of sequencing technologies such as deep sequencing and whole genome high-density tiling array, we now know that most of the “junk” genomic sequences are transcribed as non-coding RNAs (ncRNAs). A large number of long ncRNA transcripts (> 200bp) have been identified, and these long ncRNAs (LncRNAs) are found to be crucial regulators for epigenetic modulation, transcription, and translation. In this review, we briefly summarize the regulatory function of LncRNAs with a particular focus on the underlying mechanisms of LncRNAs in oncogenesis, tumor metastasis and suppression.
Long non-coding RNA (LncRNA); epigenetic regulation; competitive endogenous RNA (ceRNA); oncogenic lncRNA; pseudogene transcript; natural antisense RNA (NAT)
Alteration of epidermal growth factor receptor (EGFR) is involved in various human cancers and has been intensively investigated. A plethora of evidence demonstrates that posttranslational modifications of EGFR play a pivotal role in controlling its function and metabolism. Here, we show that EGFR can be acetylated by CREB binding protein (CBP) acetyltransferase. Interestingly, EGFR acetylation affects its tyrosine phosphorylation, which may contribute to cancer cell resistance to histone deacetylase inhibitors (HDACIs). Since there is an increasing interest in using HDACIs to treat various cancers in the clinic, our current study provides insights and rationale for selecting effective therapeutic regimen. Consistent with the previous reports, we also show that HDACI combined with EGFR inhibitors achieves better therapeutic outcomes and provides a molecular rationale for the enhanced effect of combination therapy. Our results unveil a critical role of EGFR acetylation that regulates EGFR function, which may have an important clinical implication.
The ubiquitin–proteasome system is essential for multiple physiological processes via selective degradation of target proteins and has been shown to plays a critical role in human cancer. Activation of oncogenic factors and inhibition of tumor suppressors have been shown to be essential for cancer development, and protein ubiquitination has been linked to the regulation of oncogenic factors and tumor suppressors. Three kinases, AKT, extracellular signal-regulated kinase, and IκB kinase, we refer to as oncokinases, are activated in multiple human cancers. We and others have identified several key downstream targets that are commonly regulated by these oncokinases, some of which are regulated directly or indirectly via ubiquitin-mediated proteasome degradation, including FOXO3, β-catenin, myeloid cell leukemia-1, and Snail. In this review, we summarize these findings from our and other groups and discuss potential future studies and applications in the clinic.
AKT; ERK; IKK; FOXO3; β-catenin; Mcl-1; snail
Colorectal cancer is the second leading cause of death from cancer in the United States. Metastases in the liver, the most common metastatic site for colorectal cancer, are found in one-third of the patients who die of colorectal cancer. Currently, the genes and molecular mechanisms that are functionally critical in modulating colorectal cancer hepatic metastasis remain unclear. Here, we report our studies using functional selection in an orthotopic mouse model of colorectal cancer to identify a set of genes that play an important role in mediating colorectal cancer liver metastasis. These genes included APOBEC3G, CD133, LIPC, and S100P. Clinically, we found these genes to be highly expressed in a cohort of human hepatic metastasis and their primary colorectal tumors, suggesting that it might be possible to use these genes to predict the likelihood of hepatic metastasis. We have further revealed what we believe to be a novel mechanism in which APOBEC3G promotes colorectal cancer hepatic metastasis through inhibition of miR-29–mediated suppression of MMP2. Together, our data elucidate key factors and mechanisms involved in colorectal cancer liver metastasis, which could be potential targets for diagnosis and treatment.
Drug resistance is a central challenge of cancer therapy that ultimately leads to treatment failure. In this study, we characterized a mechanism of drug resistance that arises to AZD6244, an established mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) 1/2 inhibitor currently being evaluated in cancer clinical trials. AZD6244 enhanced the expression of transcription factor FOXO3a, which suppressed cancer cell proliferation. In AZD6244-resistant cancer cells, we observed the impaired nuclear localization of FOXO3a, reduced FOXO3a-mediated transcriptional activity, and decreased the expression of FOXO3a target gene Bim after cell treatment with AZD6244. Resistant cells could be sensitized by phosphoinositide 3-kinase (PI3K)/AKT inhibitors, which are known to enhance FOXO3a nuclear translocation. Our findings define FOXO3a as candidate marker to predict the clinical efficacy of AZD6244. Furthermore, they suggest a mechanism of resistance to MEK inhibitors that may arise in the clinic yet can be overcome by cotreatment with PI3K/AKT inhibitors.
Enhancer of zeste homolog 2 (EZH2), a catalytic component of polycomb repressive complex 2 (PRC2), epigenetically regulates chromatin structure and gene expressions through tri-methylation at histone H3K27 and recruitment of DNA methyltransferases for gene silencing. Despite extensive studies of the role of EZH2 in cancer progression and malignancy, increasing evidence also suggest that EZH2 plays a critical role in stem cells renewal, maintenance, and differentiation into specific cell lineages. Here, we review the updated information regarding how EZH2 contributes to stem cell maintenance, cell lineage determination, including myogenesis, adipogenesis, osteogenesis, neurogenesis, hematopoiesis, lymphopoiesis, epidermal differentiation and hepatogenesis, and how EZH2 is regulated by phosphorylation and microRNAs in these processes.
EZH2; stem cells; cell lineage; differentiation
The transcription factor hypoxia-inducible factor 1α (HIF-1α) is regulated by oxygen availability as well as various inflammatory mediators, including tumor necrosis factor α (TNFα). Early work suggested that the phosphatidylinositol-3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling pathways are involved in TNFα-mediated HIF-1α accumulation and activation under normoxic conditions. Here, we provide evidence showing that IκBkinase β (IKKβ) is required for HIF-1α regulation by TNFα. We found that TNFα enhances HIF-1α protein expression in various breast cancer cell lines under either normoxic or hypoxia-mimicking conditions, but has little effect on the HIF-1α mRNA level. Increased HIF-1α expression was found in IKKβ stable clones and transient transfectants, and depletion of IKKβ consistently reduced the amount of HIF-1α protein. Treatment of cells with the IKKβ inhibitor Bay 11-7082 reduced the TNFα-induced HIF-1α expression, suggesting that IKKβ is required in this signaling pathway. Decreased expression of vascular endothelial growth factor (VEGF), a direct target of HIF-1α, was shown in IKKβ-knockout mouse embryonic fibroblast cells. We further demonstrated a positive correlation between IKKβ and VEGF expression in primary human breast cancer specimens. Our findings indicate that TNFα-induced HIF-1α accumulation is IKKβ dependent, and may enable further understanding of the HIF-1α regulation by inflammatory signals.
Tumor necrosis factor α; Hypoxia-inducible factor 1α; IκBkinase β; Vascular endothelial growth factor; Breast cancer
IκB kinase β (IKKβ), a major kinase downstream of various proinflammatory signals, mediates multiple cellular functions through phosphorylation and regulation of its substrates. On the basis of protein sequence analysis, we identified arrest-defective protein 1 (ARD1), a protein involved in apoptosis and cell proliferation processes in many human cancer cells, as a new IKKβ substrate. We provided evidence showing that ARD1 is indeed a bona fide substrate of IKKβ. IKKβ physically associated with ARD1 and phosphorylated it at Ser209. Phosphorylation by IKKβ destabilized ARD1 and induced its proteasome-mediated degradation. Impaired growth suppression was observed in ARD1 phosphorylation-mimic mutant (S209E)-transfected cells as compared with ARD1 non-phosphorylatable mutant (S209A)-transfected cells. Our findings of molecular interactions between ARD1 and IKKβ may enable further understanding of the upstream regulation mechanisms of ARD1 and of the diverse functions of IKKβ.
Phosphorylation; Arrest-defective protein 1; IκB kinase β; Destabilization; Degradation
Currently, an effective gene therapy strategy, which not only retains cancer-specific expression but also limits toxicity, has yet to be developed for ovarian cancer. Mounting reports over the years have shown that human telomerase activity is significantly elevated in cancer cells compared with normal cells. In this study, we evaluated the hTERT promoter and showed that it can direct target gene expression preferentially in ovarian cancer cells. However, its promoter (hTERT) activity is much lower than that of CMV, a commonly used non-specific promoter. To overcome this problem, we have integrated the hTERT promoter into our recently developed VISA system (VP16-Gal4-WPRE integrated systemic amplifier) and dramatically enhanced transgene expression. In addition, to further develop this cancer-specific promoter gene expression system into an applicable therapeutic vector, we expressed E1A (an adenoviral type 5 transcription factor which possesses anti-cancer properties) through this novel VISA platform. We demonstrated that the hTERT-VISA system specifically targeted E1A’s expression to ovarian cancer cells at a level greater than or comparable to the commonly used CMV promoter, yet remained nearly silent in normal cells, and thus making this a suitable gene therapy construct. By using this cancer-specific promoter, which limits target gene expression in normal cells/tissues, potential toxicity induced by the CMV promoter would be prevented. More importantly, we showed significant antitumor activity with much less toxicity in animal models via intravenous delivery of hTERT-VISA-E1A:liposomal nanoparticles, suggesting a promising role of hTERT-VISA-E1A for ovarian cancer treatment under a gene therapy setting.
Human telomerase; gene therapy; non-viral vector; liposome; E1A
Forkhead O transcription factors (FOXO) playa pivotal role in the regulation of a myriad of cellular functions including cell cycle arrest, cell death, and protection from stress stimuli. Activation of cell survival pathways such as phosphoinositide-3-kinase/AKT/IKK or RAS/mitogen-activated protein kinase are known to phosphorylate FOXOs at different sites which cause FOXOs nuclear exclusion and degradation, resulting in the suppression of FOXO's transcriptional activity. Perturbation of FOXO's function leads to deregulated cell proliferation and accumulation of DNA damage, resulting in diseases such as cancer. Emerging evidence shows that active FOXO proteins are crucial for keeping cells in check; and inactivation of FOXO proteins is associated with tumorigenesis, including breast cancer, prostate cancer, glioblastoma, rhabdomyosarcoma, and leukemia. Moreover, clinically used drugs like paclitaxel, imatinib, and doxorubicin have been shown to achieve their therapeutic effects through activation of FOXO3a and FOXO3a targets. In this review, we will focus the novel functions of FOXOs revealed in recent studies and further highlight FOXOs as new therapeutic targets in a broad spectrum of cancers.
The serine/threonine protein kinase B (PKB, also known as Akt) constitutes an important node in diverse signaling cascades downstream of growth factor receptor tyrosine kinases. Akt plays an essential role in cell survival, growth, migration, proliferation, polarity, and metabolism (lipid and glucose); cell cycle progression; muscle and cardiomyocyte contractility; angiogenesis; and self-renewal of stem cells. Altered Akt activity has been associated with cancer and other disease conditions, such as diabetes mellitus, neurodegenerative diseases, and muscle hypotrophy. In the past decade, the upstream signals that lead to Akt activation, the downstream substrates that exert the effects of Akt, and the secondary binding proteins that regulate Akt activation have been well documented. Recent reports from our group and others have revealed how the stability of Akt protein is regulated through phosphorylation on its Thr-Pro motifs. This literature review details findings of those reports and others relevant to the regulation of Akt activation by its upstream kinases, with a focus on mammalian target of rapamycin complexes (mTORCs) and inactivation by PHLDA3 and the protein phosphatases PP2A and pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP). Reports on ubiquitin-dependent Akt degradation, caspase-dependent cleavage, and the roles of molecular chaperone heat shock protein 90 (Hsp90) in the regulation of Akt stability are summarized. The highlight will be on the role of “turn motif” phosphorylation and an isomerase, Pin1, in the regulation of Akt stability. We also discuss issues related to the intricate mTORC2-AktmTORC1 loop and the contradictory regulation of Akt phosphorylation and stabilization of Akt by mTORC2. Finally, we offer perspective on potential future directions for investigation, particularly on translating the knowledge we learned on the regulation of Akt stability into therapeutic intervention on human cancer with Akt alteration.
Serine/threonine protein kinase B; PKB; Akt; growth factor receptor; tyrosine kinases; physiological activity regulation; stability; mammalian target of rapamycin complexes; mTORCs; Pin1; caspase; DEPTOR; PP2A; pleckstrin homology domain; PH domain; PHLPPs; PHLDAs; heat shock protein
IκB kinases (IKK) and IKK-related kinases play critical roles in regulating the immune response through nuclear factor-κB and IFN regulatory factor – dependent signaling transduction cascades. Recently, these kinases have been implicated in the pathogenesis of many human diseases, including cancer. In fact, dysregulation of IKK activities promotes tumor survival, proliferation, migration, metastasis, and angiogenesis—common characteristics of many types of human cancers. Because of their oncogenic effects in human cancer development, targeting IKK and IKK-related kinases is becoming an increasingly popular avenue for the development of novel therapeutic interventions for cancer. This review will briefly cover the recent discovery of the downstream substrates of IKK and IKK-related kinases, and outline the strategies used for targeting IKK as a therapeutic intervention for cancer.
Myeloid cell leukemia-1 (Mcl-1), a Bcl-2–like antiapoptotic protein, plays a role in cell immortalization and chemoresistance in a number of human malignancies. A peptidyl-prolyl cis/trans isomerase, Pin1 is involved in many cellular events, such as cell cycle progression, cell proliferation, and differentiation through isomerizing prophosphorylated substrates. It has been reported that down-regulation of Pin1 induces apoptosis, and that Erk phosphorylates and up-regulates Mcl-1; however, the underlying mechanisms for the two phenomena are not clear yet. Here, we showed that Pin 1 stabilizes Mcl-1, which is required for Mcl-1 posphorylation by Erk. First, we found expression of Mcl-1 and Pin1 were positively correlated and associated with poor survival in human breast cancer. We then showed that Erk could phosphorylate Mcl-1 at two consensus residues, Thr 92 and 163, which is required for the association of Mcl-1 and Pin1, resulting in stabilization of Mcl-1. Moreover, Pin1 is also required for the up-regulation of Mcl-1 by Erk activation. Based on this newly identified mechanism of Mcl-1 stabilization, two strategies were used to overcome Mcl-1–mediated chemoresistance: inhibiting Erk by Sorafenib, an approved clinical anticancer drug, or knocking down Pin1 by using a SiRNA technique. In conclusion, the current report not only unravels a novel mechanism to link Erk/Pin1 pathway and Mcl-1–mediated chemoresistance but also provides a plausible combination therapy, Taxol (Paclitaxel) plus Sorafenib, which was shown to be effective in killing breast cancer cells.
Barrett’s esophagus, a columnar metaplasia of the lower esophagus epithelium related to gastroesophageal reflux disease, is the strongest known risk factor for the development of esophageal adenocarcinoma (EAC). Understanding the signal transduction events involved in esophageal epithelium carcinogenesis may provide insights into the origins of EAC and may suggest new therapies. To elucidate the molecular pathways of bile acid–induced tumorigenesis, the newly identified inflammation-associated signaling pathway involving IκB kinases β (IKKβ), tuberous sclerosis complex 1 (TSC1), and mammalian target of rapamycin (mTOR) downstream effector S6 kinase (S6K1) was confirmed to be activated in immortalized Barrett’s CPC-A and CPC-C cells and esophageal cancer SEG-1 and BE3 cells. Phosphorylation of TSC1 and S6K1 was induced in response to bile acid stimulation. Treatment of these cells with the mTOR inhibitor rapamycin or the IKKβ inhibitor Bay 11-7082 suppressed bile acid–induced cell proliferation and anchorage-independent growth. We next used an orthotopic rat model to evaluate the role of bile acid in the progression of Barrett’s esophagus to EAC. Of interest, we found high expression of phosphorylated IKKβ (pIKKβ) and phosphorylated S6K1 (pS6K1) in tumor tissues and the Barrett’s epithelium compared with normal epithelium. Furthermore, immunostaining of clinical EAC tissue specimens revealed that pIKKβ expression was strongly correlated with pS6K1 level. Together, these results show that bile acid can deregulate TSC1/mTOR through IKKβ signaling, which may play a critical role in EAC progression. In addition, Bay 11-7082 and rapamycin may potentially be chemopreventive drugs against Barrett’s esophagus–associated EAC.
The epidermal growth factor receptor (EGFR) has been shown to be able to translocate to the nucleus where it is involved in many cellular process including transcriptional regulation and DNA repair. Recently, it has been shown that the DNA damage-inducing agents ionizing radiation (IR) and cisplatin are able to induce EGFR nuclear localization, and this nuclear localization is correlated with increased DNA-PK activity, which plays an essential role in DNA double stranded repair. Here we sought to determine if there is a causal relationship between nuclear EGFR and DNA repair activity. We found that mutation in the nuclear localization signal (NLS) of EGFR (mNLS), known to be unable to translocate to the nucleus, released EGFR induced resistance to cisplatin. Re-introduction of an NLS in the C-terminal allowed EGFR to re-enter the nucleus and the cells regained resistance to cisplatin. In addition, we show that the re-expression of a functional nuclear localization sequence in EGFR (mNLS-R) is not only able to restore its resistance to cisplatin, but also reduced the DNA damage caused by cisplatin, and restored DNA repair activity. Thus, we demonstrate here that nuclear EGFR is required for DNA repair and resistance to cisplatin treatment.
Epidermal growth factor receptor; nuclear localization; dna damage; DNA repair; cisplatin