Extracellular signal-regulated kinase 7, a regulator of hormone-dependent estrogen receptor destruction

Atypical MAPKs in cancer

Impaired kinase signalling leads to various diseases, including cancer. At the same time, kinases make up the majority of the druggable genome and targeting kinase activity has proven to be a successful first-line therapy for many cancers. Among the best-studied kinases are the mitogen-activated protein kinases (MAPKs), which regulate cell proliferation, differentiation, motility, and survival. However, the MAPK family also contains the atypical members ERK3 (MAPK6), ERK4 (MAPK4), ERK7/ERK8 (MAPK15), and NLK that are functionally and structurally different from their conventional family members and have long been neglected. Nevertheless, in recent years, important roles in carcinogenesis, actin cytoskeleton regulation and the immune system have been discovered, underlining the physiological importance of atypical MAPKs and the need to better understand their functions. This review highlights the distinctive features of the atypical MAPKs and summarizes the evidence on their regulation, physiological roles, and potential targeting strategies for cancer therapies.

Mitogen-Activated Protein Kinase 15 Is a New Predictive Biomarker and Potential Therapeutic Target for Ovarian Cancer

Mitogen-activated protein kinase 15 (MAPK15) has been reported to be associated with several cancers. This study aimed to explore for the first time on the relationship between MAPK15 expression and cancer progression/drug responsiveness in ovarian carcinoma. To this end, MAPK15 expression level was examined by immunohistochemistry (IHC) staining of an ovarian tissue array (10 normal and 70 malignant samples). Drug sensitivity of ovarian cancer cell lines (including OVCAR3 and SKOV3) was measured by MTS assay. The modulation of MAPK15 expression in OVCAR3 and SKOV3 was verified by immunoblot and real-time PCR analyses. The prognostic value of MAPK15 in ovarian cancer patients was assessed using the Kaplan-Meier Plotter database and Gene Expression Omnibus (GEO) datasets. The IHC results showed that MAPK15 expression was negatively associated with tumor grade, TNM stage, tumor size, and regional lymph node metastasis of ovarian carcinoma. Importantly, overexpressing MAPK15 increased cisplatin toxicity in ovarian carcinoma cells and online database analysis indicated that patients with high MAPK15 expression had favorable prognosis with/without chemotherapy. Taken together, our results indicate that a decreased MAPK15 expression is associated with advanced-stage ovarian cancer and unfavorable survival outcomes. MAPK15 may be a new biomarker for ovarian cancer, and the encouraging therapeutic strategy would be found by combining the regulation of MAPK15 expression.

Targeting the RAS/RAF/MAPK pathway for cancer therapy: from mechanism to clinical studies

Metastatic dissemination of solid tumors, a leading cause of cancer-related mortality, underscores the urgent need for enhanced insights into the molecular and cellular mechanisms underlying metastasis, chemoresistance, and the mechanistic backgrounds of individuals whose cancers are prone to migration. The most prevalent signaling cascade governed by multi-kinase inhibitors is the mitogen-activated protein kinase (MAPK) pathway, encompassing the RAS-RAF-MAPK kinase (MEK)-extracellular signal-related kinase (ERK) pathway. RAF kinase is a primary mediator of the MAPK pathway, responsible for the sequential activation of downstream targets, such as MEK and the transcription factor ERK, which control numerous cellular and physiological processes, including organism development, cell cycle control, cell proliferation and differentiation, cell survival, and death. Defects in this signaling cascade are associated with diseases such as cancer. RAF inhibitors (RAFi) combined with MEK blockers represent an FDA-approved therapeutic strategy for numerous RAF-mutant cancers, including melanoma, non-small cell lung carcinoma, and thyroid cancer. However, the development of therapy resistance by cancer cells remains an important barrier. Autophagy, an intracellular lysosome-dependent catabolic recycling process, plays a critical role in the development of RAFi resistance in cancer. Thus, targeting RAF and autophagy could be novel treatment strategies for RAF-mutant cancers. In this review, we delve deeper into the mechanistic insights surrounding RAF kinase signaling in tumorigenesis and RAFi-resistance. Furthermore, we explore and discuss the ongoing development of next-generation RAF inhibitors with enhanced therapeutic profiles. Additionally, this review sheds light on the functional interplay between RAF-targeted therapies and autophagy in cancer.

Atypical MAP kinases - new insights and directions from amoeba

Mitogen-activated protein kinases (MAPKs) have been the focus of many studies over the past several decades, but the understanding of one subgroup of MAPKs, orthologs of MAPK15, known as atypical MAPKs, has lagged behind others. In most organisms, specific activating signals or downstream responses of atypical MAPK signaling pathways have not yet been identified even though these MAPKs are associated with many eukaryotic processes, including cancer and embryonic development. In this Review, we discuss recent studies that are shedding new light on both the regulation and function of atypical MAPKs in different organisms. In particular, the analysis of the atypical MAPK in the amoeba Dictyostelium discoideum has revealed important roles in chemotactic responses and gene regulation. The rapid and transient phosphorylation of the atypical MAPK in these responses suggest a highly regulated activation mechanism in vivo despite the ability of atypical MAPKs to autophosphorylate in vitro. Atypical MAPK function can also impact the activation of other MAPKs in amoeba. These advances are providing new perspectives on possible MAPK roles in animals that have not been previously considered, and this might lead to the identification of potential targets for regulating cell movement in the treatment of diseases.

Cellular and physiological roles of the conserved atypical MAP kinase ERK7

Extracellular signal-regulated kinase 7 (ERK7), also known as ERK8 and MAPK15, is an atypical member of the MAP kinase family. Compared with other MAP kinases, the biological roles of ERK7 remain poorly understood. Recent work, however, has revealed several novel functions for ERK7. These include a highly conserved essential role in ciliogenesis, the ability to control cell growth, metabolism and autophagy, as well as the maintenance of genomic integrity. ERK7 functions through phosphorylation-dependent and -independent mechanisms and it is activated by cellular stressors, including DNA-damaging agents, and nutrient deprivation. Here, we summarize recent developments in understanding ERK7 function, emphasizing its conserved roles in cellular and physiological regulation.

Transcriptional upregulation of MAPK15 by NF-κB signaling boosts the efficacy of combination therapy with cisplatin and TNF-α

The efficacy of cisplatin in treating advanced non-small cell lung cancer is limited mainly because of insensitivity and/or acquired resistance. MAPK15, previously shown by us to enhance the sensitivity of the anti-cancer drug arsenic trioxide, could also enhance the sensitivity of other anti-cancer drugs. Here, we explore the potential role of MAPK15 in chemosensitivity to cisplatin in human lung cancer cells. Our results indicated that the expression level of MAPK15 was positively correlated with cisplatin sensitivity through affecting the DNA repair capacity of cisplatin-treated cells. The expression of MAPK15 was transcriptionally regulated by the TNF-α-activated NF-κB signaling pathway, and TNF-α synergized with cisplatin, in a MAPK15-dependent manner, to exert cytotoxicity in vitro and in vivo. Therefore, levels of TNF-α dictate the responsiveness/sensitivity of lung cancer cells to cisplatin by transcriptionally upregulating MAPK15 to enhance chemosensitivity, suggesting manipulation of MAPK15 as a strategy to improve the therapeutic efficacy of chemotherapeutic drugs.

The Revelation of Continuously Organized, Co-Overexpressed Protein-Coding Genes with Roles in Cellular Communications in Breast Cancer

Many human cancers, including breast cancer, are polygenic and involve the co-dysregulation of multiple regulatory molecules and pathways. Though the overexpression of genes and amplified chromosomal regions have been closely linked in breast cancer, the notion of the co-upregulation of genes at a single locus remains poorly described. Here, we describe the co-overexpression of 34 continuously organized protein-coding genes with diverse functions at 8q.24.3(143437655-144326919) in breast and other cancer types, the CanCord34 genes. In total, 10 out of 34 genes have not been reported to be overexpressed in breast cancer. Interestingly, the overexpression of CanCord34 genes is not necessarily associated with genomic amplification and is independent of hormonal or HER2 status in breast cancer. CanCord34 genes exhibit diverse known and predicted functions, including enzymatic activities, cell viability, multipotency, cancer stem cells, and secretory activities, including extracellular vesicles. The co-overexpression of 33 of the CanCord34 genes in a multivariant analysis was correlated with poor survival among patients with breast cancer. The analysis of the genome-wide RNAi functional screening, cell dependency fitness, and breast cancer stem cell databases indicated that three diverse overexpressed CanCord34 genes, including a component of spliceosome PUF60, a component of exosome complex EXOSC4, and a ribosomal biogenesis factor BOP1, shared roles in cell viability, cell fitness, and stem cell phenotypes. In addition, 17 of the CanCord34 genes were found in the microvesicles (MVs) secreted from the mesenchymal stem cells that were primed with MDA-MB-231 breast cancer cells. Since these MVs were important in the chemoresistance and dedifferentiation of breast cancer cells into cancer stem cells, these findings highlight the significance of the CanCord34 genes in cellular communications. In brief, the persistent co-overexpression of CanCord34 genes with diverse functions can lead to the dysregulation of complementary functions in breast cancer. In brief, the present study provides new insights into the polygenic nature of breast cancer and opens new research avenues for basic, preclinical, and therapeutic studies in human cancer.

Not your Mother's MAPKs: Apicomplexan MAPK function in daughter cell budding

Reversible phosphorylation by protein kinases is one of the core mechanisms by which biological signals are propagated and processed. Mitogen-activated protein kinases, or MAPKs, are conserved throughout eukaryotes where they regulate cell cycle, development, and stress response. Here, we review advances in our understanding of the function and biochemistry of MAPK signaling in apicomplexan parasites. As expected for well-conserved signaling modules, MAPKs have been found to have multiple essential roles regulating both Toxoplasma tachyzoite replication and sexual differentiation in Plasmodium. However, apicomplexan MAPK signaling is notable for the lack of the canonical kinase cascade that normally regulates the networks, and therefore must be regulated by a distinct mechanism. We highlight what few regulatory relationships have been established to date, and discuss the challenges to the field in elucidating the complete MAPK signaling networks in these parasites.

Forskolin Stimulates Estrogen Receptor (ER) α Transcriptional Activity and Protects ER from Degradation by Distinct Mechanisms

Estradiol action is mediated by estrogen receptors (ERs), a and ß. Estradiol binding initiates ER-mediated transcription and ER degradation, the latter of which occurs via the ubiquitin-proteasome pathway. Inhibition of proteasome activity prevents estradiol-induced ERα degradation and transactivation. In ER-positive GH3 cells (a rat pituitary prolactinoma cell line), forskolin, acting via protein kinase A (PKA), stimulates ERα transcriptional activity without causing degradation, and proteasome inhibition does not block forskolin-stimulated transcription. Forskolin also protects liganded ERα from degradation. In the current study, we first examined ERα and ERβ transcriptional activity in ER-negative HT22 cells and found that forskolin stimulated ERα-, but not ERβ-dependent transcription, through the ligand-binding domain (LBD). We also identified four mutations (L396R, D431Y, Y542F, and K534E/M548V) on the ERα LBD that selectively obliterated the response to forskolin. In GH3 cells, transfected ERα mutants and ERβ were protected from degradation by forskolin. Ubiquitination of ERα and ERβ was increased by forskolin or estradiol. ERα ubiquitination was diminished by a mutated ubiquitin (K48R) that prevents elongation of polyubiquitin chains for targeting the proteasome. Increased ERα ubiquitination was not affected by the deletion of the A/B domain but significantly diminished in the F domain deletion mutant. Our results indicate distinct and novel mechanisms for forskolin stimulation of ERα transcriptional activity and protection from ligand-induced degradation. It also suggests a unique mechanism by which forskolin increases unliganded and liganded ERα and ERβ ubiquitination but uncouples them from proteasome-mediated degradation regardless of their transcriptional responses to forskolin.

Calcineurin regulates the stability and activity of estrogen receptor α

Estrogen receptor α (ER-α) mediates estrogen-dependent cancer progression and is expressed in most breast cancer cells. However, the molecular mechanisms underlying the regulation of the cellular abundance and activity of ER-α remain unclear. We here show that the protein phosphatase calcineurin regulates both ER-α stability and activity in human breast cancer cells. Calcineurin depletion or inhibition down-regulated the abundance of ER-α by promoting its polyubiquitination and degradation. Calcineurin inhibition also promoted the binding of ER-α to the E3 ubiquitin ligase E6AP, and calcineurin mediated the dephosphorylation of ER-α at Ser²⁹⁴ in vitro. Moreover, the ER-α (S294A) mutant was more stable and activated the expression of ER-α target genes to a greater extent compared with the wild-type protein, whereas the extents of its interaction with E6AP and polyubiquitination were attenuated. These results suggest that the phosphorylation of ER-α at Ser²⁹⁴ promotes its binding to E6AP and consequent degradation. Calcineurin was also found to be required for the phosphorylation of ER-α at Ser¹¹⁸ by mechanistic target of rapamycin complex 1 and the consequent activation of ER-α in response to β-estradiol treatment. Our study thus indicates that calcineurin controls both the stability and activity of ER-α by regulating its phosphorylation at Ser²⁹⁴ and Ser¹¹⁸ Finally, the expression of the calcineurin A-α gene (PPP3CA) was associated with poor prognosis in ER-α-positive breast cancer patients treated with tamoxifen or other endocrine therapeutic agents. Calcineurin is thus a promising target for the development of therapies for ER-α-positive breast cancer.

IGF1R and MAPK15 Emerge as Potential Targets of Pentabromobenzylisothioureas in Lung Neuroendocrine Neoplasms

Pentabromobenzylisothioureas are antitumor agents with diverse properties, including the inhibition of MAPK15, IGF1R and PKD1 kinases. Their dysregulation has been implicated in the pathogenesis of several cancers, including bronchopulmonary neuroendocrine neoplasms (BP-NEN). The present study assesses the antitumor potential of ZKKs, a series of pentabromobenzylisothioureas, on the growth of the lung carcinoid H727 cell line. It also evaluates the expression of MAPK15, IGF1R and PKD1 kinases in different BP-NENs. The viability of the H727 cell line was assessed by colorimetric MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) and its proliferation by BrdU (5-bromo-2′-deoxyuridine) assay. Tissue kinase expression was measured using TaqMan-based RT-PCR and immunohistochemistry. ZKKs (10⁻⁴ to 10⁻⁵ M) strongly inhibited H727 cell viability and proliferation and their antineoplastic effects correlated with their concentrations (p < 0.001). IGF1R and MAPK15 were expressed at high levels in all subtypes of BP-NENs. In addition, the SCLC (small cell lung carcinoma) patients demonstrated higher mRNA levels of IGF1R (p = 0.010) and MAPK15 (p = 0.040) than the other BP-NEN groups. BP-NENs were characterized by low PKD1 expression, and lung neuroendocrine cancers demonstrated lower PKD1 mRNA levels than carcinoids (p = 0.003). ZKKs may suppress BP-NEN growth by inhibiting protein kinase activity. Our results suggest also a possible link between high IGF1R and MAPK15 expression and the aggressive phenotype of BP-NEN tumors.

TRIM8 inhibits breast cancer proliferation by regulating estrogen signaling

Breast cancer (BC) is the most common female malignancy worldwide, and 70% of which are estrogen receptor α (ERα) positive. Endocrine treatment, such as tamoxifen, is a primary adjuvant therapy for patients with ER-positive BC. However, some patients will develop acquired resistance following long-time treatment. Further research on estrogen signaling is important to improve the therapy of these patients. In this study, we report that the E3 ubiquitin ligase tripartite motif 8 (TRIM8) acts as a novel regulator of ERα signaling. TRIM8 is downregulated in BC and is associated with poor prognosis. In addition, the protein level of TRIM8 is negatively correlated with ERα. RNA sequencing revealed that estrogen signaling maybe a potential target of TRIM8. Moreover, knockdown of TRIM8 can significantly enhance BC cell proliferation and migration both in vitro and in vivo. And this effect can be reversed by ERα depletion. Further mechanistic studies showed that TRIM8 interacts with AF1 domain of ERα via its RING domain in the cytoplasm and increases poly-ubiquitination of the ERα protein. In conclusion, our study reveals an interesting post-translational mechanism between ERα and TRIM8 in ER-positive BC, which suggests that TRIM8 may be a potential therapeutic target in the treatment of BC.

Extracellular-Signal Regulated Kinase: A Central Molecule Driving Epithelial-Mesenchymal Transition in Cancer

Epithelial-mesenchymal transition (EMT) is a reversible cellular process, characterized by changes in gene expression and activation of proteins, favoring the trans-differentiation of the epithelial phenotype to a mesenchymal phenotype. This process increases cell migration and invasion of tumor cells, progression of the cell cycle, and resistance to apoptosis and chemotherapy, all of which support tumor progression. One of the signaling pathways involved in tumor progression is the MAPK pathway. Within this family, the ERK subfamily of proteins is known for its contributions to EMT. The ERK subfamily is divided into typical (ERK 1/2/5), and atypical (ERK 3/4/7/8) members. These kinases are overexpressed and hyperactive in various types of cancer. They regulate diverse cellular processes such as proliferation, migration, metastasis, resistance to chemotherapy, and EMT. In this context, in vitro and in vivo assays, as well as studies in human patients, have shown that ERK favors the expression, function, and subcellular relocalization of various proteins that regulate EMT, thus promoting tumor progression. In this review, we discuss the mechanistic roles of the ERK subfamily members in EMT and tumor progression in diverse biological systems.

Polyubiquitination inhibition of estrogen receptor alpha and its implications in breast cancer

Estrogen receptor alpha (ERα) is detected in more than 70% of the cases of breast cancer. Nuclear activity of ERα, a transcriptional regulator, is linked to the development of mammary tumors, whereas the extranuclear activity of ERα is related to endocrine therapy resistance. ERα polyubiquitination is induced by the estradiol hormone, and also by selective estrogen receptor degraders, resulting in ERα degradation via the ubiquitin proteasome system. Moreover, polyubiquitination is related to the ERα transcription cycle, and some E3-ubiquitin ligases also function as coactivators for ERα. Several studies have demonstrated that ERα polyubiquitination is inhibited by multiple mechanisms that include posttranslational modifications, interactions with coregulators, and formation of specific protein complexes with ERα. These events are responsible for an increase in ERα protein levels and deregulation of its signaling in breast cancers. Thus, ERα polyubiquitination inhibition may be a key factor in the progression of breast cancer and resistance to endocrine therapy.

MAPK15 upregulation promotes cell proliferation and prevents DNA damage in male germ cell tumors

Germ cell tumors (GCT) are the most common malignancies in males between 15 and 35 years of age. Despite the high cure rate, achieved through chemotherapy and/or surgery, the molecular basis of GCT etiology is still largely obscure. Here, we show a positive correlation between MAPK15 (ERK8; ERK7) expression and specific GCT subtypes, with the highest levels found in the aggressive embryonal carcinomas (EC). Indeed, in corresponding cellular models for EC, MAPK15 enhanced tumorigenicity in vivo and promoted cell proliferation in vitro, supporting a role for this kinase in human GCT. At molecular level, we demonstrated that endogenous MAPK15 is necessary to sustain cell cycle progression of EC cells, by limiting p53 activation and preventing the triggering of p53-dependent mechanisms resulting in cell cycle arrest.

To understand MAPK15-dependent mechanisms impinging on p53 activation, we demonstrate that this kinase efficiently protects cells from DNA damage. Moreover, we show that the ability of MAPK15 to control the autophagic process is necessary for basal management of DNA damage and for tumor formation controlled by the kinase.

In conclusion, our findings suggest that MAPK15 overexpression may contribute to the malignant transformation of germ cells by controlling a “stress support” autophagic pathway, able to prevent DNA damage and the consequent activation of the p53 tumor suppressor. Moreover, in light of these results, MAPK15-specific inhibitors might represent new tools to enhance the therapeutic index of cytotoxic therapy in GCT treatment, and to increase the sensitivity to DNA-damaging drugs in other chemotherapy-resistant human tumors.

Regulation of human mitogen-activated protein kinase 15 (extracellular signal-regulated kinase 7/8) and its functions: A recent update

Mitogen-activated protein kinase 15 (MAPK15), originally also known as extracellular signal-regulated kinase 7/8, is the most recently identified atypical MAPK and the least studied so far. Examinations of the role of MAPK15 in various cell lines and model systems indicate that MAPK15 participates in a variety of cellular activities such as promoting cell proliferation, cell transformation, and apoptosis; stimulating autophagy; regulating cell division, ciliogenesis, and protein secretion; and maintaining genome stability. As multiple roles of MAPK15 were observed among these studies, therefore, it remains unclear whether MAPK15 acts as a proto-oncogene or tumor suppressor. Here, the recent literature on human MAPK15 and the resulting functions will be discussed.

Ligand Receptor-Mediated Regulation of Growth in Plants

Growth and development of multicellular organisms are coordinately regulated by various signaling pathways involving the communication of inter- and intracellular components. To form the appropriate body patterns, cellular growth and development are modulated by either stimulating or inhibiting these pathways. Hormones and second messengers help to mediate the initiation and/or interaction of the various signaling pathways in all complex multicellular eukaryotes. In plants, hormones include small organic molecules, as well as larger peptides and small proteins, which, as in animals, act as ligands and interact with receptor proteins to trigger rapid biochemical changes and induce the intracellular transcriptional and long-term physiological responses. During the past two decades, the availability of genetic and genomic resources in the model plant species, Arabidopsis thaliana, has greatly helped in the discovery of plant hormone receptors and the components of signal transduction pathways and mechanisms used by these immobile but highly complex organisms. Recently, it has been shown that two of the most important plant hormones, auxin and abscisic acid (ABA), act through signaling pathways that have not yet been recognized in animals. For example, auxins stimulate cell elongation by bringing negatively acting transcriptional repressor proteins to the proteasome to be degraded, thus unleashing the gene expression program required for increasing cell size. The "dormancy" inducing hormone, ABA, binds to soluble receptor proteins and inhibits a specific class of protein phosphatases (PP2C), which activates phosphorylation signaling leading to transcriptional changes needed for the desiccation of the seeds prior to entering dormancy. While these two hormone receptors have no known animal counterparts, there are also many similarities between animal and plant signaling pathways. For example, in plants, the largest single gene family in the genome is the protein kinase family (approximately 5% of the protein coding genes), although the specific function for only a few dozen of these kinases is clearly established. Recent comparative genomics studies have revealed that parasitic nematodes and pathogenic microbes produce plant peptide hormone mimics that target specific plant plasma membrane receptor-like protein kinases, thus usurping endogenous signaling pathways for their own pathogenic purposes. With biochemical, genetic, and physiological analyses of the regulation of hormone receptor signal pathways, we are thus just now beginning to understand how plants optimize the development of their body shape and cope with constantly changing environmental conditions.

MAPK15 mediates BCR-ABL1-induced autophagy and regulates oncogene-dependent cell proliferation and tumor formation

A reciprocal translocation of the ABL1 gene to the BCR gene results in the expression of the oncogenic BCR-ABL1 fusion protein, which characterizes human chronic myeloid leukemia (CML), a myeloproliferative disorder considered invariably fatal until the introduction of the imatinib family of tyrosine kinase inhibitors (TKI). Nonetheless, insensitivity of CML stem cells to TKI treatment and intrinsic or acquired resistance are still frequent causes for disease persistence and blastic phase progression experienced in patients after initial successful therapies. Here, we investigated a possible role for the MAPK15/ERK8 kinase in BCR-ABL1-dependent autophagy, a key process for oncogene-induced leukemogenesis. In this context, we showed the ability of MAPK15 to physically recruit the oncogene to autophagic vesicles, confirming our hypothesis of a biologically relevant role for this MAP kinase in signal transduction by this oncogene. Indeed, by modeling BCR-ABL1 signaling in HeLa cells and taking advantage of a physiologically relevant model for human CML, i.e. K562 cells, we demonstrated that BCR-ABL1-induced autophagy is mediated by MAPK15 through its ability to interact with LC3-family proteins, in a LIR-dependent manner. Interestingly, we were also able to interfere with BCR-ABL1-induced autophagy by a pharmacological approach aimed at inhibiting MAPK15, opening the possibility of acting on this kinase to affect autophagy and diseases depending on this cellular function. Indeed, to support the feasibility of this approach, we demonstrated that depletion of endogenous MAPK15 expression inhibited BCR-ABL1-dependent cell proliferation, in vitro, and tumor formation, in vivo, therefore providing a novel "druggable" link between BCR-ABL1 and human CML.

Mechanisms that Increase Stability of Estrogen Receptor Alpha in Breast Cancer

Estrogen receptor alpha (ER) is a transcriptional regulator that controls the expression of genes related to cellular proliferation and differentiation in normal mammary tissue. However, the expression, abundance, and activity of this receptor are increased in 70% of breast cancers. The ER upregulation is facilitated by several molecular mechanisms, including protein stability, which represents an important strategy to maintain an active and functional repertoire of ER. Several proteins interact and protect ER from degradation by the ubiquitin-proteasome system. Through diverse mechanisms, these proteins prevent polyubiquitination and degradation of ER, leading to an increase in ER protein levels; consequently, estrogen signaling and its physiologic effects are enhanced in breast cancer cells. Thus, increased protein stability seems to be one of the main reasons that ER is upregulated in breast cancer. Here, we highlight findings on the proteins and mechanisms that participate directly or indirectly in ER stability and their relevance to breast cancer.

Ubiquitylation of nuclear receptors: new linkages and therapeutic implications

The nuclear receptor (NR) superfamily is a group of transcriptional regulators that control multiple aspects of both physiology and pathology and are broadly recognized as viable therapeutic targets. While receptor-modulating drugs have been successful in many cases, the discovery of new drug targets is still an active area of research, because resistance to NR-targeting therapies remains a significant clinical challenge. Many successful targeted therapies have harnessed the control of receptor activity by targeting events within the NR signaling pathway. In this review, we explore the role of NR ubiquitylation and discuss how the expanding roles of ubiquitin could be leveraged to identify additional entry points to control receptor function for future therapeutic development.

The kinome associated with estrogen receptor-positive status in human breast cancer

Estrogen receptor alpha (ERα) regulates and is regulated by kinases involved in several functions associated with the hallmarks of cancer. The following literature review strongly suggests that distinct kinomes exist for ERα-positive and -negative human breast cancers. Importantly, consistent with the known heterogeneity of ERα-positive cancers, different subgroups exist, which can be defined by different kinome signatures, which in turn are correlated with clinical outcome. Strong evidence supports the interplay of kinase networks, suggesting that targeting a single node may not be sufficient to inhibit the network. Therefore, identifying the important hubs/nodes associated with each clinically relevant kinome in ER+ tumors could offer the ability to implement the best therapy options at diagnosis, either endocrine therapy alone or together with other targeted therapies, for improved overall outcome.

ERK8 is a negative regulator of O-GalNAc glycosylation and cell migration

ER O-glycosylation can be induced through relocalisation GalNAc-Transferases from the Golgi. This process markedly stimulates cell migration and is constitutively activated in more than 60% of breast carcinomas. How this activation is achieved remains unclear. Here, we screened 948 signalling genes using RNAi and imaging. We identified 12 negative regulators of O-glycosylation that all control GalNAc-T sub-cellular localisation. ERK8, an atypical MAPK with high basal kinase activity, is a strong hit and is partially localised at the Golgi. Its inhibition induces the relocation of GalNAc-Ts, but not of KDEL receptors, revealing the existence of two separate COPI-dependent pathways. ERK8 down-regulation, in turn, activates cell motility. In human breast and lung carcinomas, ERK8 expression is reduced while ER O-glycosylation initiation is hyperactivated. In sum, ERK8 appears as a constitutive brake on GalNAc-T relocalisation, and the loss of its expression could drive cancer aggressivity through increased cell motility.

DOI:http://dx.doi.org/10.7554/eLife.01828.001

The likelihood of an individual being able to recover from cancer depends on: where the cancer is within the body, how quickly the disease is detected and how quickly treatment is started. Cancers that have spread from their original location to another part of the body are particular challenging to treat, and cause the vast majority of cancer deaths every year.

Treatments that can recognize and eradicate cancer cells, while leaving nearby healthy cells untouched, are still needed—and so there has been a lot of research into identifying the key differences between healthy cells and cancer cells. For several decades, researchers have been aware that cancer cells have more proteins coated with modified sugars on their cell surfaces than healthy cells. This is caused by the enzymes that add these sugars to the proteins relocating from one location within the cell, the Golgi apparatus, to another, called the endoplasmic reticulum. These specific ‘sugar-coated’ proteins are known to encourage cancer cells to migrate and invade new tissues, but the mechanisms that regulate the addition of these sugar molecules to proteins remains poorly understood.

Now Chia et al. have discovered 12 molecules that regulate this process, including an enzyme called ERK8 that is found at the Golgi apparatus. ERK8 is shown to prevent the relocation of the sugar-adding enzymes from the Golgi to the endoplasmic reticulum, thereby restricting the production of sugar-coated proteins that help the cancer cells to spread within the body. By identifying 12 potential targets for new therapeutics aimed at preventing the spread of cancer, the work of Chia et al. could ultimately help to improve the chances of patients recovering from certain cancers.

DOI:http://dx.doi.org/10.7554/eLife.01828.002

Lenticular mitoprotection. Part B: GSK-3β and regulation of mitochondrial permeability transition for lens epithelial cells in atmospheric oxygen

PURPOSE

Loss of integrity of either the inner or outer mitochondrial membrane results in the dissipation of the mitochondrial electrochemical gradient that leads to mitochondrial membrane permeability transition (mMPT). This study emphasizes the role of glycogen synthase kinase 3beta (GSK-3β) in maintaining mitochondrial membrane potential, thus preventing mitochondrial depolarization (hereafter termed mitoprotection). Using 3-(2,4-dichlorophenyl)-4-(1-methyl-1H-indol-3-yl)-1H-pyrrole-2,5-dione (SB216763), an inhibitor of GSK-3β, and drawing a distinction between it and 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene (UO126), an inhibitor of extracellular-signal-regulated kinase (ERK) phosphorylation, the means by which GSK-3β influences mitoprotection in cultured human lens epithelial (HLE-B3) cells and normal, secondary cultures of bovine lens epithelial cells, maintained in atmospheric oxygen, was investigated.

METHODS

Virally transfected human lens epithelial cells (HLE-B3) and normal cultures of bovine lens epithelial cells were exposed to acute hypoxic conditions (about 1% O2) followed by exposure to atmospheric oxygen (about 21% O2). Specific antisera and western blot analysis was used to examine the state of phosphorylation of ERK and GSK-3β, as well as the phosphorylation of a downstream substrate of GSK-3β, glycogen synthase (GS, useful in monitoring GSK-3β activity). The potentiometric dye, 1H-benzimidazolium-5,6-dichloro-2-[3-(5,6-dichloro-1,3-diethyl-1,3-dihydro-2H-benzimidazol-2-ylidene)-1-propenyl]-1,3-diethyl-iodide (JC-1), was used to monitor mitochondrial depolarization upon exposure of inhibitor treatment relative to the control cells (mock inhibition) in atmospheric oxygen. Caspase-3 activation was scrutinized to determine whether mitochondrial depolarization inevitably leads to apoptosis.

RESULTS

Treatment of HLE-B3 cells with SB216763 (12 µM) inactivated GSK-3β activity as verified by the enzyme's inability to phosphorylate its substrate, GS. SB216763-treated cells were not depolarized relative to the control cells as demonstrated with JC-1 fluorescent dye analysis. The HLE-B3 cells treated with UO126, which similarly blocked phosphorylation of GS, were nevertheless prone to mMPT relative to the control cells. Western blot analysis determined that Bcl-2-associated X (BAX) levels were unchanged for SB216763-treated or UO126-treated HLE-B3 cells when compared to their respective control cells. However, unlike the SB216763-treated cells, the UO126-treated cells showed a marked absence of Bcl-2, as well as phosphorylated Bcl-2 relative to the controls. UO126 treatment of bovine lens epithelial cells showed similar results with pBcl-2 levels, while the Bcl-2 content appeared unchanged relative to the control cells. HLE-B3 and normal bovine lens cell cultures showed susceptibility to mMPT associated with the loss of pBcl-2 by UO126 treatment.

CONCLUSIONS

MITOCHONDRIAL DEPOLARIZATION MAY OCCUR BY ONE OF TWO KEY OCCURRENCES: interruption of the electrochemical gradient across the inner mitochondrial membrane resulting in mMPT or by disruption of the integrity of the inner or outer mitochondrial membrane. The latter scenario is generally tightly regulated by members of the Bcl-2 family of proteins. Inhibition of GSK-3β activity by SB216763 blocks mMPT by preventing the opening of the mitochondrial permeability transition pore. UO126, likewise, inhibits GSK-3β activity, but unlike SB216763, inhibition of ERK phosphorylation induces the loss of intracellular pBcl-2 levels under conditions where intracellular BAX levels remain constant. These results suggest that the lenticular mitoprotection normally afforded by the inactivation of GSK-3β activity may, however, be bypassed by a loss of pBcl-2, an anti-apoptotic member of the Bcl-2 family. Bcl-2 prevents the translocation of BAX to the mitochondrial outer membrane inhibiting depolarization by disrupting the normal electrochemical gradient leading to mMPT.

MAPK15/ERK8 stimulates autophagy by interacting with LC3 and GABARAP proteins

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved catabolic process necessary for normal recycling of cellular constituents and for appropriate response to cellular stress. Although several genes belonging to the core molecular machinery involved in autophagosome formation have been discovered, relatively little is known about the nature of signaling networks controlling autophagy upon intracellular or extracellular stimuli. We discovered ATG8-like proteins (MAP1LC3B, GABARAP and GABARAPL1) as novel interactors of MAPK15/ERK8, a MAP kinase involved in cell proliferation and transformation. Based on the role of these proteins in the autophagic process, we demonstrated that MAPK15 is indeed localized to autophagic compartments and increased, in a kinase-dependent fashion, ATG8-like proteins lipidation, autophagosome formation and SQSTM1 degradation, while decreasing LC3B inhibitory phosphorylation. Interestingly, we also identified a conserved LC3-interacting region (LIR) in MAPK15 responsible for its interaction with ATG8-like proteins, for its localization to autophagic structures and, consequently, for stimulation of the formation of these compartments. Furthermore, we reveal that MAPK15 activity was induced in response to serum and amino-acid starvation and that this stimulus, in turn, required endogenous MAPK15 expression to induce the autophagic process. Altogether, these results suggested a new function for MAPK15 as a regulator of autophagy, acting through interaction with ATG8 family proteins. Also, based on the key role of this process in several human diseases, these results supported the use of this MAP kinase as a potential novel therapeutic target.

A novel prenylflavone restricts breast cancer cell growth through AhR-mediated destabilization of ERα protein

There is concern that ingestion of dietary phytoestrogens may increase risk of estrogen receptor alpha (ERα)-positive breast cancer. The prenylflavone icaritin, a phytoestrogen consumed in East Asian societies for its perceived beneficial effects on bone health, stimulated the growth of breast cancer (MCF-7) cells at low concentrations. Although acting like an estrogenic ligand, icaritin exerted an unexpected suppressive effect on estrogen-stimulated breast cancer cell proliferation and gene expression at higher concentrations. Like estradiol, icaritin could dose-dependently destabilize ERα protein. However, destabilization of ERα by the estradiol/icaritin combination was profound and greater than that observed for either compound alone. Microarray gene expression analyses implicated aryl hydrocarbon receptor (AhR) signaling for this suppressive effect of icaritin. Indeed, icaritin was an AhR agonist that competitively reduced specific binding of a potent AhR agonist and increased expression of the AhR-regulated gene CYP1A1. When AhR was knocked down by small interfering RNA, the suppressive effect of icaritin on estradiol-stimulated breast cancer cell growth and gene expression was abolished, and ERα protein stability was partially restored. Similarly in an athymic nude mouse model, icaritin restricted estradiol-stimulated breast cancer xenograft growth and strongly reduced ERα protein levels. Overall, our data support the feasibility for the development of dual agonists like icaritin, which are estrogenic but yet, through activating AhR-signaling, can destabilize ERα protein to restrict ERα-positive breast cancer cell growth.

p38γ mitogen-activated protein kinase (MAPK) confers breast cancer hormone sensitivity by switching estrogen receptor (ER) signaling from classical to nonclassical pathway via stimulating ER phosphorylation and c-Jun transcription

Estrogen receptor (ER) α promotes breast cancer growth by regulating gene expression through classical estrogen response element (ERE) binding and nonclassical (interaction with c-Jun at AP-1 sites) pathways. ER is the target for anti-estrogens such as tamoxifen (TAM). However, the potential for classical versus nonclassical ER signaling to influence hormone sensitivity is not known. Moreover, anti-estrogens frequently activate several signaling cascades besides the target ER, and the implications of these "off-target" signaling events have not been explored. Here, we report that p38γ MAPK is selectively activated by treatment with TAM. This results in both phosphorylation of ER at Ser-118 and stimulation of c-Jun transcription, thus switching ER signaling from the classical to the nonclassical pathway leading to increased hormone sensitivity. Unexpectedly, phosphorylation at Ser-118 is required for ER to bind both p38γ and c-Jun, thereby promoting ER relocation from ERE to AP-1 promoter sites. Thus, ER/Ser-118 phosphorylation serves as a central mechanism by which p38γ regulates signaling transduction of ER with its inhibitor TAM.

Regulation of estrogen receptor α N-terminus conformation and function by peptidyl prolyl isomerase Pin1

Estrogen receptor alpha (ERα), a key driver of growth in the majority of breast cancers, contains an unstructured transactivation domain (AF1) in its N terminus that is a convergence point for growth factor and hormonal activation. This domain is controlled by phosphorylation, but how phosphorylation impacts AF1 structure and function is unclear. We found that serine 118 (S118) phosphorylation of the ERα AF1 region in response to estrogen (agonist), tamoxifen (antagonist), and growth factors results in recruitment of the peptidyl prolyl cis/trans isomerase Pin1. Phosphorylation of S118 is critical for Pin1 binding, and mutation of S118 to alanine prevents this association. Importantly, Pin1 isomerizes the serine118-proline119 bond from a cis to trans isomer, with a concomitant increase in AF1 transcriptional activity. Pin1 overexpression promotes ligand-independent and tamoxifen-inducible activity of ERα and growth of tamoxifen-resistant breast cancer cells. Pin1 expression correlates with proliferation in ERα-positive rat mammary tumors. These results establish phosphorylation-coupled proline isomerization as a mechanism modulating AF1 functional activity and provide insight into the role of a conformational switch in the functional regulation of the intrinsically disordered transactivation domain of ERα.

ERK7 is a negative regulator of protein secretion in response to amino-acid starvation by modulating Sec16 membrane association

RNAi screening for kinases regulating the functional organization of the early secretory pathway in Drosophila S2 cells has identified the atypical Mitotic-Associated Protein Kinase (MAPK) Extracellularly regulated kinase 7 (ERK7) as a new modulator. We found that ERK7 negatively regulates secretion in response to serum and amino-acid starvation, in both Drosophila and human cells. Under these conditions, ERK7 turnover through the proteasome is inhibited, and the resulting higher levels of this kinase lead to a modification in a site within the C-terminus of Sec16, a key ER exit site component. This post-translational modification elicits the cytoplasmic dispersion of Sec16 and the consequent disassembly of the ER exit sites, which in turn results in protein secretion inhibition. We found that ER exit site disassembly upon starvation is TOR complex 1 (TORC1) independent, showing that under nutrient stress conditions, cell growth is not only inhibited at the transcriptional and translational levels, but also independently at the level of secretion by inhibiting the membrane flow through the early secretory pathway. These results reveal the existence of new signalling circuits participating in the complex regulation of cell growth.

Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Extracellular signal-regulated kinase 8 (ERK8) controls estrogen-related receptor α (ERRα) cellular localization and inhibits its transcriptional activity

ERK8 (MAPK15) is a large MAP kinase already implicated in the regulation of the functions of different nuclear receptors and in cellular proliferation and transformation. Here, we identify ERRα as a novel ERK8-interacting protein. As a consequence of such interaction, ERK8 induces CRM1-dependent translocation of ERRα to the cytoplasm and inhibits its transcriptional activity. Also, we identify in ERK8 two LXXLL motifs, typical of agonist-bound nuclear receptor corepressors, as necessary features for this MAP kinase to interact with ERRα and to regulate its cellular localization and transcriptional activity. Ultimately, we demonstrate that ERK8 is able to counteract, in immortalized human mammary cells, ERRα activation induced by the EGF receptor pathway, often deregulated in breast cancer. Altogether, these results reveal a novel function for ERK8 as a bona fide ERRα corepressor, involved in control of its cellular localization by nuclear exclusion, and suggest a key role for this MAP kinase in the regulation of the biological activities of this nuclear receptor.

Activation and Function of the MAPKs and Their Substrates, the MAPK-Activated Protein Kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

The MAP kinase signaling cascades: a system of hundreds of components regulates a diverse array of physiological functions

Sequential activation of kinases within the mitogen-activated protein (MAP) kinase (MAPK) cascades is a common, and evolutionary-conserved mechanism of signal transduction. Four MAPK cascades have been identified in the last 20 years and those are usually named according to the MAPK components that are the central building blocks of each of the cascades. These are the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-Terminal kinase (JNK), p38, and ERK5 cascades. Each of these cascades consists of a core module of three tiers of protein kinases termed MAPK, MAPKK, and MAP3K, and often two additional tiers, the upstream MAP4K and the downstream MAPKAPK, which can complete five tiers of each cascade in certain cell lines or stimulations. The transmission of the signal via each cascade is mediated by sequential phosphorylation and activation of the components in the sequential tiers. These cascades cooperate in transmitting various extracellular signals and thus control a large number of distinct and even opposing cellular processes such as proliferation, differentiation, survival, development, stress response, and apoptosis. One way by which the specificity of each cascade is regulated is through the existence of several distinct components in each tier of the different cascades. About 70 genes, which are each translated to several alternatively spliced isoforms, encode the entire MAPK system, and allow the wide array of cascade's functions. These components, their regulation, as well as their involvement together with other mechanisms in the determination of signaling specificity by the MAPK cascade is described in this review. Mis-regulation of the MAPKs signals usually leads to diseases such as cancer and diabetes; therefore, studying the mechanisms of specificity-determination may lead to better understanding of these signaling-related diseases.

Ligand interaction scan (LIScan) in the study of ERK8

ERK8 is the most recent addition for the MAPK family, and its mechanism of activation and function are not yet known, mainly due to the lack of any known physiological stimulator. In this report, we describe the preparation of reagents for the use of a novel method, the ligand interaction scan (LIScan), to study the function of this protein kinase. We generated a set of mutants of ERK8, and identified inhibited as well as stimulated forms. By specifically inhibiting or stimulating the mutants of ERK8, we show that the ERK8-induced inhibition of proliferation is altered. Moreover, we used the developed mutants to show for the first time that ERK8 translocates to the nucleus upon activation. The use of methods such as the ligand interaction scan may thus promote the analyses of the functions of uncharacterized proteins such as ERK8, and possibly help in controlling the activity of target proteins in various experimental systems and applications.

Extracellular signal-regulated kinase 8-mediated c-Jun phosphorylation increases tumorigenesis of human colon cancer

Extracellular signal-regulated kinase 8 (ERK8), a recently discovered member of the mitogen-activated protein kinase protein family, has been less studied than other family members, leaving its physiologic functions mostly unknown. The biological consequences of overexpression of ERK8 in JB6 Cl41 epidermal skin cells or knockdown of ERK8 in HCT15 colorectal cancer cells was studied. Kinase assays and transient transfection experiments were performed to study the signaling pathway between ERK8 and c-Jun. We found that ERK8 is relatively highly expressed in HCT15 human colorectal cancer cells and plays an important role in the promotion and progression of colorectal cancer. ERK8 promoted neoplastic transformation, and knockdown of ERK8 in HCT15 colorectal cancer cells reduced the tumorigenic properties of these cell lines. Furthermore, a direct interaction between ERK8 and c-Jun was shown. With epidermal growth factor treatment, overexpression of ERK8 in JB6 Cl41 cells caused an increased phosphorylation of c-Jun at Ser(63) and Ser(73), resulting in increased activator protein-1 transactivation. In contrast, knockdown of ERK8 in HCT15 colorectal cancer cells blocked c-Jun phosphorylation. The interaction between ERK8 and c-Jun seems to increase the tumorigenic properties of HCT15 colorectal cancer cells. Thus, ERK8-regulated signaling might serve as a potential therapeutic target in colorectal cancer.

[Molecular mechanisms of regulation of estrogen receptor a expression level in breast cancer]

Estrogen receptor alpha (ERalpha) plays an important role in breast cancer development and progression and thus becomes a useful molecular target for breast cancer therapy. ERalpha is differentially expressed in breast cancer patients. Moreover, ERalpha expression levels may change at different stages of breast cancer even for the same patient. ERalpha expression is closely associated with the effect of endocrine therapy and prognosis. The mechanisms underlying ERalpha expression are complicated, because ERalpha expression is regulated at different levels, including chromatin, transcriptional, post- transcriptional, translational, and post-translational levels. Many proteins modulate the transcription of ERalpha gene at the chromatin and transcriptional levels through direct or indirect interaction with the ERa promoter. Some microRNAs decrease ERalpha levels possibly by induction of the degradation of ERalpha mRNA and/or repression of the mRNA translation. At the post-translational level, many proteins regulate ERalpha protein levels through ubiquitin-proteosome pathway. This review focuses on molecular mechanisms of regulation of ERalpha expression at different levels.

CK1delta modulates the transcriptional activity of ERalpha via AIB1 in an estrogen-dependent manner and regulates ERalpha-AIB1 interactions

Oncogenesis in breast cancer often requires the overexpression of the nuclear receptor coactivator AIB1/SRC-3 acting in conjunction with estrogen receptor-α (ERα). Phosphorylation of both ERα and AIB1 has been shown to have profound effects on their functions. In addition, proteasome-mediated degradation plays a major role by regulating their stability and activity. CK1δ, a member of the ubiquitous casein kinase-1 family, is implicated in the progression of breast cancer. In this study, we show that both ERα and AIB1 are substrates for CK1δ in vitro, and identify a novel AIB1 phosphorylation site (S601) targeted by CK1δ, significant for the co-activator function of AIB1. CK1δ is able to interact with ERα and AIB1 in vivo, while overexpression of CK1δ in breast cancer cells results in an increased association of ERα with AIB1 as confirmed by co-immunoprecipitation assays from cell lysates. Using an siRNA-based approach, luciferase reporter assays and qRT-PCR, we observe that silencing of CK1δ leads to reduced ERα transcriptional activity, despite increased ERα levels, similarly to proteasome inhibition. We provide evidence that AIB1 protein levels are reduced by CK1δ silencing, in an estradiol-dependent manner; such destabilization can be inhibited by pre-treatment with the proteasome inhibitor MG132. We propose that differing activities adopted by ERα and AIB1 as a consequence of their interactions with and phosphorylation by CK1δ, particularly AIB1 stabilization, influence the transcriptional activity of ERα, and therefore have a role in breast cancer development.

Signal transducer and activator of transcription 5b: a new target of breast tumor kinase/protein tyrosine kinase 6

Introduction

Signal transducers and activators of transcription (STATs) are mediators of cytokine and growth factor signaling. In recent years, STAT5b has emerged as a key regulator of tumorigenesis. STAT5b phosphorylation and activation is mediated by several kinases known to be overexpressed in breast cancer, such as epidermal growth factor receptor, HER2, and c-Src. Breast tumor kinase (Brk), also known as protein tyrosine kinase 6, is a nonreceptor tyrosine kinase expressed in more than 60% of breast cancers. Only a few substrates of the Brk tyrosine kinase have been identified, the most recent being STAT3. In the present article we investigate the potential role of Brk in the phosphorylation and activation STAT5b.

Methods

To determine whether Brk can phosphorylate STAT5b, transient transfection and in vitro kinase assays were performed. Luciferase reporter assays were used to measure Brk-induced STAT5b transcriptional activity. siRNA technology was utilized to investigate the biological significance of Brk-induced activation of STAT5b in breast cancer cell models.

Results

Phosphospecific antibodies, mutational analysis, and in vitro kinase assays demonstrated that Brk specifically mediated STAT5b phosphorylation at the activating tyrosine, Y699. Transient transfection of Brk into the Brk-negative BT-549 breast cancer cell line enhanced STAT5b transcriptional activity, as measured by a STAT5-specific luciferase reporter. Furthermore, overexpression of kinase active c-Src enhanced Brk-induced STAT5b transcriptional activity. In Brk-positive breast cancer cell lines BT-20 and SKBr3, knockdown of Brk protein or of STAT5b protein using siRNA methodology resulted in a decrease in DNA synthesis. Knockdown of Brk and STAT5b together did not further decrease DNA synthesis compared with each alone, suggesting that Brk and STAT5b converge on the same pathway, ultimately leading to cellular proliferation.

Conclusion

Our studies demonstrate that Brk phosphorylates STAT5b on Y699, leading to increased STAT5b transcriptional activity. Furthermore, analysis of DNA synthesis suggests that STAT5b and Brk are converging upon the same proproliferative signaling pathway in breast cancer cells. We propose that Brk, like other tyrosine kinases, signals downstream to STAT5b to mediate proliferation of breast cancer cells. These results further establish STAT5b as well as Brk as potential targets for breast cancer therapy.

RNA suppression of ERK2 leads to collapse of mitochondrial membrane potential with acute oxidative stress in human lens epithelial cells

17beta-Estradiol (E(2)) reduces oxidative stress-induced depolarization of mitochondrial membrane potential (MMP) in cultured human lens epithelial cells (HLE-B3). The mechanism by which the nongenomic effects of E(2) contributed to the protection against mitochondrial membrane depolarization was investigated. Mitochondrial membrane integrity is regulated by phosphorylation of BAD, and it is known that phosphorylation of Ser(112) inactivates BAD and prevents its participation in the mitochondrial death pathway. We found that E(2) rapidly increased both the phosphorylation of ERK2 and Ser(112) in BAD. Ser(112) is phosphorylated by p90 ribosomal S6 kinase (RSK), a Ser/Thr kinase, which is a downstream effector of ERK1/2. Inhibition of RSK by the RSK-specific inhibitor SL0101 did not reduce the level of E(2)-induced phosphorylation of Ser(112). Silencing BAD using small interfering RNA did not alter mitochondrial membrane depolarization elicited by peroxide insult. However, under the same conditions, silencing ERK2 dramatically increased membrane depolarization compared with the control small interfering RNA. Therefore, ERK2, functioning through a BAD-independent mechanism regulates MMP in humans lens epithelial cells. We propose that estrogen-induced activation of ERK2 acts to protect cells from acute oxidative stress. Moreover, despite the fact that ERK2 plays a regulatory role in mitochondrial membrane potential, estrogen was found to block mitochondrial membrane depolarization via an ERK-independent mechanism.

Differential Regulation of Estrogen Receptor α Turnover and Transactivation by Mdm2 and Stress-Inducing Agents

In mammalian cells, the level of estrogen receptor alpha (ERalpha) is rapidly decreased upon estrogen treatment, and this regulation involves proteasome degradation. Using different approaches, we showed that the Mdm2 oncogenic ubiquitin-ligase directly interacts with ERalpha in a ternary complex with p53 and is involved in the regulation of ERalpha turnover (both in the absence or presence of estrogens). Several lines of evidence indicated that this effect of Mdm2 required its ubiquitin-ligase activity and involved the ubiquitin/proteasome pathway. Moreover, in MCF-7 human breast cancer cells, various p53-inducing agents (such as UV irradiation) or treatment with RITA (which inhibits the interaction of p53 with Mdm2) stabilized ERalpha and abolished its 17beta-estradiol-dependent turnover. Interestingly, our data indicated that ligand-dependent receptor turnover was not required for efficient transactivation. Altogether, our results indicate that the Mdm2 oncoprotein and stress-inducing agents complexly and differentially regulate ERalpha stability and transcriptional activity in human cancer cells.

Ubiquitin-interaction motifs of RAP80 are critical in its regulation of estrogen receptor alpha

In this study, we demonstrate that receptor-associated protein 80 (RAP80) interacts with estrogen receptor alpha (ERα) in an agonist-dependent manner. The interaction is specific for ERα as ERβ and several other nuclear receptors tested did not interact with RAP80. Interaction between RAP80 and ERα was supported by mammalian two-hybrid, GST pull-down, and co-immunoprecipitation analyses. The hinge/ligand-binding domain of ERα is sufficient for interaction with RAP80. RAP80 overexpression reduces ERα polyubiquitination, increases the level of ERα protein, and enhances ERα-mediated transactivation. Knockdown of endogenous RAP80 expression by small-interfering RNA (siRNA) reduced ERα protein level and the E2-dependent induction of pS2. In this study, we also demonstrate that RAP80 contains two functional ubiquitin-interaction motifs (UIMs) that are able to bind ubiquitin and to direct monoubiquitination of RAP80. Deletion of these UIMs does not affect the ability of RAP80 to interact with ERα, but eliminates the effects of RAP80 on ERα polyubiquitination, the level of ERα protein, and ERα-mediated transcription. These data indicate that the UIMs in RAP80 are critical for the function of RAP80. Our study identifies ERα as a new RAP80-interacting protein and suggests that RAP80 may be an important modulator of ERα activity.

In vivo functions of mitogen-activated protein kinases: conclusions from knock-in and knock-out mice

Multicellular organisms achieve intercellular communication by means of signalling molecules whose effect on the target cell is mediated by signal transduction pathways. Such pathways relay, amplify and integrate signals to elicit appropriate biological responses. Protein kinases form crucial intermediate components of numerous signalling pathways. One group of protein kinases, the mitogen-activated protein kinases (MAP kinases) are kinases involved in signalling pathways that respond primarily to mitogens and stress stimuli. In vitro studies revealed that the MAP kinases are implicated in several cellular processes, including cell division, differentiation, cell survival/apoptosis, gene expression, motility and metabolism. As such, dysfunction of specific MAP kinases is associated with diseases such as cancer and immunological disorders. However, the genuine in vivo functions of many MAP kinases remain elusive. Genetically modified mouse models deficient in a specific MAP kinase or expressing a constitutive active or a dominant negative variant of a particular MAP kinase offer valuable tools for elucidating the biological role of these protein kinases. In this review, we focus on the current status of MAP kinase knock-in and knock-out mouse models and their phenotypes. Moreover, examples of the application of MAP kinase transgenic mice for validating therapeutic properties of specific MAP kinase inhibitors, and for investigating the role of MAP kinase in pathogen-host interactions will be discussed.

Atypical mitogen-activated protein kinases: structure, regulation and functions

Mitogen-activated protein (MAP) kinases are a family of serine/threonine kinases that play a central role in transducing extracellular cues into a variety of intracellular responses ranging from lineage specification to cell division and adaptation. Fourteen MAP kinase genes have been identified in the human genome, which define 7 distinct MAP kinase signaling pathways. MAP kinases can be classified into conventional or atypical enzymes, based on their ability to get phosphorylated and activated by members of the MAP kinase kinase (MAPKK)/MEK family. Conventional MAP kinases comprise ERK1/ERK2, p38s, JNKs, and ERK5, which are all substrates of MAPKKs. Atypical MAP kinases include ERK3/ERK4, NLK and ERK7. Much less is known about the regulation, substrate specificity and physiological functions of atypical MAP kinases.

Targeting the ubiquitin-proteasome pathway in breast cancer therapy

The ubiquitin-proteasome pathway (UPP) is the major eukaryotic mechanism for regulated intracellular proteolysis. Targeting this pathway with proteasome inhibitors has been validated as a rational strategy against hematologic malignancies, but for most solid tumor populations, including breast cancer, such agents have not shown encouraging activity. However, there is an increasing body of evidence showing that UPP dysregulation plays an important role in mammary tumorigenesis. Moreover, modulation of ubiquitin-proteasome function is emerging as a rational strategy to enhance chemosensitivity and overcome chemoresistance. Taken together, these facts suggest that we are only beginning to appreciate the relevance of this pathway for the current and future therapy of patients with breast cancer. This review provides an overview of the biology of the UPP, its role in the malignant process, the current state of knowledge regarding clinical heat shock protein and proteasome inhibition, and some likely future directions that may enhance our ability to exploit this pathway therapeutically.

ERK8 down-regulates transactivation of the glucocorticoid receptor through Hic-5

Extracellular signal-regulated kinase 8 (ERK8) is the most recently identified member of the ERK subfamily of MAPKs. Although other members of the ERK subfamily are established regulators of signaling pathways involved in cell growth and/or differentiation, less is known about ERK8. To understand the cellular function of ERK8, a yeast two-hybrid screen of a human lung library was performed to identify binding partners. One binding partner identified was Hic-5 (also known as ARA55), a multiple LIM domain containing protein implicated in focal adhesion signaling and the regulation of specific nuclear receptors, including the androgen receptor and the glucocorticoid receptor (GR). Co-immunoprecipitation experiments in mammalian cells confirmed the interaction between Hic-5 and both ERK8 and its rodent ortholog ERK7. The C-terminal region of ERK8 was not required for the interaction. Although the LIM3 and LIM4 domains of Hic-5 were sufficient and required for this interaction, the specific zinc finger motifs in these domains were not. Transcriptional activation reporter assays revealed that ERK8 can negatively regulate transcriptional co-activation of androgen receptor and GRalpha by Hic-5 in a kinase-independent manner. Knockdown of endogenous ERK8 in human airway epithelial cells enhanced dexamethasone-stimulated transcriptional activity of endogenous GR. Transcriptional regulation of GRalpha and interaction with its ligand binding domain by ERK8 were dependent on the presence of Hic-5. These results provide the first physiological function for human ERK8 as a negative regulator of human GRalpha, acting through Hic-5, and suggest a broader role for ERK8 in the regulation of nuclear receptors beyond estrogen receptor alpha.

Characterization of the reversible phosphorylation and activation of ERK8

ERK8 (extracellular-signal-regulated protein kinase 8) expressed in Escherichia coli or insect cells was catalytically active and phosphorylated at both residues of the Thr-Glu-Tyr motif. Dephosphorylation of the threonine residue by PP2A (protein serine/threonine phosphatase 2A) decreased ERK8 activity by over 95% in vitro, whereas complete dephosphorylation of the tyrosine residue by PTP1B (protein tyrosine phosphatase 1B) decreased activity by only 15-20%. Wild-type ERK8 expressed in HEK-293 cells was over 100-fold less active than the enzyme expressed in bacteria or insect cells, but activity could be increased by exposure to hydrogen peroxide, by incubation with the protein serine/threonine phosphatase inhibitor okadaic acid, or more weakly by osmotic shock. In unstimulated cells, ERK8 was monophosphorylated at Tyr-177, and exposure to hydrogen peroxide induced the appearance of ERK8 that was dually phosphorylated at both Thr-175 and Tyr-177. IGF-1 (insulin-like growth factor 1), EGF (epidermal growth factor), PMA or anisomycin had little effect on activity. In HEK-293 cells, phosphorylation of the Thr-Glu-Tyr motif of ERK8 was prevented by Ro 318220, a potent inhibitor of ERK8 in vitro. The catalytically inactive mutants ERK8[D154A] and ERK8[K42A] were not phosphorylated in HEK-293 cells or E. coli, whether or not the cells had been incubated with protein phosphatase inhibitors or exposed to hydrogen peroxide. Our results suggest that the activity of ERK8 in transfected HEK-293 cells depends on the relative rates of ERK8 autophosphorylation and dephosphorylation by one or more members of the PPP family of protein serine/threonine phosphatases. The major residue in myelin basic protein phosphorylated by ERK8 (Ser-126) was distinct from that phosphorylated by ERK2 (Thr-97), demonstrating that, although ERK8 is a proline-directed protein kinase, its specificity is distinct from ERK1/ERK2.

Role of the proteasome in the regulation of estrogen receptor alpha turnover and function in MCF-7 breast carcinoma cells

Estrogen receptor alpha (ER) turnover in MCF-7 cells was assessed by pulse chase analysis and measurement of ER steady-state level. In untreated cells, degradation of (35)S-labeled ER was characterized by a slow phase followed by a more rapid decline. Without ligand, ER elimination was totally compensated by synthesis which maintained receptor homeostasis. Estradiol (E(2)) and the pure antiestrogen RU 58,668 abolished the slow phase of ER breakdown and enhanced the degradation of neosynthesized ER, producing a low ER steady-state level. By contrast, the partial antiestrogen OH-Tam was ineffective in this respect and caused ER accumulation. Regardless of the conditions, ER breakdown was abolished by proteasome inhibition (MG-132). ER ligands decreased cell capacity to bind [(3)H]E(2), even in the presence of MG-132, indicating that the regulation of ER level and E(2) binding capacity occurs through distinct mechanisms. MG-132 partially blocked the basal transcription of an ERE-dependent reporter gene and modified the ability of E(2) to induce the expression of the latter: the hormone was unable to restore the transactivation activity measured without MG-132. RU 58,668 and OH-Tam failed to enhance the inhibitory action of MG-132, suggesting that a loss of basal ER-mediated transactivation mainly affects the stimulatory effect of estrogens. Overall, our findings reveal that ER steady state level, ligand binding capacity and transactivation potency fit in a complex regulatory scheme involving distinct mechanisms, which may be dissociated from each other under various treatments.

Counting on mitogen-activated protein kinases—ERKs 3, 4, 5, 6, 7 and 8

Signal transduction pathways in eukaryotic cells integrate diverse extracellular signals, and regulate complex biological responses such as growth, differentiation and death. One group of proline-directed Ser/Thr protein kinases, the mitogen-activated protein kinases (MAPKs), plays a central role in these signalling pathways. Much attention has focused in recent years on three subfamilies of MAPKs, the extracellular signal regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. However, the ERK family is broader than the ERK1 and ERK2 proteins that have been the subject of most studies in this area. Here we overview the work on ERKs 3 to 8, emphasising where possible their biological activities as well as distinctive biochemical properties. It is clear from these studies that these additional ERKs show similarities to ERK1 and ERK2, but with some interesting differences that challenge the paradigm of the archetypical ERK1/2 MAPK pathway.

ERK7 expression and kinase activity is regulated by the ubiquitin-proteosome pathway

ERK7 is a unique member of the extracellular signal-regulated kinase (ERK) subfamily of MAP kinases. Although ERK7 shares a TEY motif in the activation loop of the kinase, it displays constitutive activation, nuclear localization, and growth inhibitory properties that are regulated by its C-terminal domain. Because ERK7 is expressed at low levels compared with ERK2 and its activity is dependent upon its expression level, we investigated the mechanism by which ERK7 expression is regulated. We now show that ERK7 expression is regulated by ubiquitination and rapid proteosomal turnover. Furthermore, both the kinase domain and the C-terminal tail are independently degraded at a rate comparable with that of the intact protein. Analysis of a series of chimeras between ERK2 and ERK7 reveal that the N-terminal 20 amino acids of the kinase domain are a primary determinant of ERK7 degradation. Fusion of the N-terminal 20 amino acids is both necessary and sufficient to cause proteolytic degradation of both ERK2 and green fluorescent protein. Finally, ERK7 is stabilized by an N-terminal mutant of Cullin-1 suggesting that ERK7 is ubiquitinated by the Skip1-Cullin-F box complex. These results indicate that ERK7 is a highly regulated enzyme whose cellular expression and kinase activation level is tightly controlled by the ubiquitin-proteosome pathway.

Estrogen receptors ER alpha and ER beta in proliferation in the rodent mammary gland

Most evidence supports the view that ER alpha is responsible for estrogen (ovarian estradiol, E(2))-induced proliferation in the epithelial cells of the mammary gland, but despite this, proliferating epithelial cells do not express ER alpha. We have examined this apparent paradox by studying the role of ER alpha and ER beta in E(2)-induced proliferation in mammary glands (measured by BrdUrd incorporation into DNA) in mice with intact ER beta (WT mice) and those in which the ER beta gene has been inactivated (ER beta(-/-) mice). On treatment of ER beta(-/-) mice with E(2) or ovariectomized WT mice with E(2), tamoxifen, or a specific ER beta agonist (BAG), the number of BrdUrd-labeled cells in mammary glands increased from 3.4% in controls to 28-38% in the treated mice. This indicates that both ER alpha and ER beta can mediate E(2)-induced proliferation independently of each other. With specific antibodies, ER beta was found in both epithelial and stromal cells, whereas ER alpha was strictly epithelial. Within 4 h of a single dose of E(2), ER alpha was lost from the nuclei of epithelial cells. In WT mice, ER alpha reappeared by 24 h, but in ER beta(-/-) mice, return to the nucleus was delayed by 24 h. At 4 h after E(2), neither ER alpha nor progesterone receptor was detectable in BrdUrd-labeled nuclei but by 48 h after E(2), 29% of the BrdUrd-labeled cells expressed ER alpha, and 21-38% expressed progesterone receptor. During 3 weeks of continuous E(2) treatment, ER beta remained in the nucleus, but there was no detectable ER alpha. With tamoxifen treatment, ER alpha remained in the nucleus, but ER beta was lost. From these results, we conclude that ER alpha receives the proliferation signal from E(2), initiates DNA synthesis, and is then lost from cells. The subsequent steps in proliferation can proceed in the absence of either ER alpha or ER beta. ER beta facilitates the return of ER alpha to the nucleus and restores responsiveness to E(2). By down-regulating ER beta, tamoxifen may prolong refractoriness to E(2) in mammary epithelium.