c-Jun regulates phosphoinositide-dependent kinase 1 transcription: implication for Akt and protein kinase C activities and melanoma tumorigenesis

Crosstalk of LncRNA HOTAIR and SP1-mediated repression of PDK1 contributes to β-Elemene-inhibited proliferation of hepatocellular carcinoma cells

ETHNOPHARMACOLOGICAL RELEVANCE

Hepatocellular carcinoma (HCC) is a liver malignancy which lacks effective treatment and has a poor prognosis. β-Elemene refers to a natural Curcuma wenyujin-derived single molecular entity, which exhibits various biological activities, and is especially well-known for it's antitumor properties.

AIM OF THE RESEARCH

LncRNA HOTAIR, SP1, and PDK1 have displayed oncogenic roles in many tumors, participating in the initiation and progression of cancers by mediating multiple signaling pathways. However, there are only a few reports about their roles and mutual relationship in the growth of HCC cells. Therefore, this study aimed to investigate the expression of LncRNA HOTAIR, SP1, and PDK1 and their interaction with β-Elemene in HCC cells.

MATERIALS AND METHODS

MTT, a Colony formation assay, and flow cytometry were employed to evaluate the growth of HCC and LO2 cells under β-Elemene. LncRNA HOTAIR, SP1 and PDK1 plasmids were transfected into HCC cells by a transient transfection assay, and the expression and interaction of LncRNA HOTAIR, SP1 and PDK1 were assessed via qRT-PCR and western blotting.

RESULTS

β-Elemene suppressed HCC cell growth through the downregulation of LncRNA HOTAIR, SP1 and PDK1. The results demonstrated a reciprocal interaction among LncRNA HOTAIR, SP1 and PDK1. Exogenous overexpression LncRNA HOTAIR or SP1 eliminated the suppressive effects of β-Elemene on them, and both of which regulated PDK1 expression in HCC cells. Additionally, exogenously overexpressed SP1 or LncRNA HOTAIR prevented β-Elemene inhibition of the protein-level expression of PDK1, whereas overexpressing PDK1 had no effect on SP1, though it still weakened the inhibition of cell growth and LncRNA HOTAIR expression by β-Elemene.

CONCLUSION

β-Elemene suppresses HCC cell proliferation via through the regulation of LncRNA HOTAIR, SP1, PDK1 and their interaction.

LncRNA AC093818.1 accelerates gastric cancer metastasis by epigenetically promoting PDK1 expression

Gastric cancer (GC) is a highly prevalent type of metastatic tumor. The mechanisms underlying GC metastasis are poorly understood. Some long noncoding RNAs (lncRNAs) reportedly play key roles in regulating metastasis of GC. However, the biological roles of five natural antisense lncRNAs (AC093818.1, CTD-2541M15.1, BC047644, RP11-597M12.1, and RP11-40A13.1) in GC metastasis remain unclear. In this study, the expression of these lncRNAs was measured by quantitative reverse transcription-polymerase chain reaction. Migration and invasion were evaluated by wound-healing and the Transwell assay, respectively. Stable cells were injected into the tail veins of nude mice. Sections of collected lung and liver tissues were stained using hematoxylin and eosin. Protein expression was analyzed by western blot. RNA immunoprecipitation (RIP) assay was used to verify whether the STAT3 and SP1 transcription factors bound to AC093818.1 in GC cells. Expression levels of the five lncRNAs, especially AC093818.1, were significantly upregulated in metastatic GC tissues relative to those in nonmetastatic GC tissues. AC093818.1 expression was correlated with invasion, lymphatic metastasis, distal metastasis, and tumor-node-metastasis stage. AC093818.1 expression was highly sensitive and specific in the diagnosis of metastatic or nonmetastatic GC. AC093818.1 overexpression promoted GC migration and invasion in vitro and in vivo. AC093818.1 overexpression increased PDK1, p-AKT1, and p-mTOR expression levels. AC093818.1 silencing decreased these expressions. AC093818.1 bound to transcription factors STAT3 and SP1, and SP1 or STAT3 silencing could alleviated the effect of AC093818.1 overexpression. The data demonstrate that lncRNA AC093818.1 accelerates gastric cancer metastasis by epigenetically promoting PDK1 expression. LncRNA AC093818.1 may be a potential therapeutic target for metastatic GC.

Identification of Key Pathways and Genes in the Orai2 Mediated Classical and Mesenchymal Subtype of Glioblastoma by Bioinformatic Analyses

Background

Ca²⁺ release-activated Ca²⁺ channels (CRAC) are the main Ca²⁺ entry pathway regulating intracellular Ca²⁺ concentration in a variety of cancer types. Orai2 is the main pore-forming subunit of CRAC channels in central neurons. To explore the role of Orai2 in glioblastoma (GBM), we investigated the key pathways and genes in Orai2-mediated GBM by bioinformatic analyses.

Methods

Via The Cancer Genome Atlas (TCGA), French, Sun, and Gene Expression Omnibus (GEO) (GDS3885) datasets, we collected 1231 cases with RNA-seq data and analyzed the functional annotation of Orai2 by gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Univariate and multivariate survival analyses were applied to 823 patients with survival data.

Results

We discovered that Orai2 was markedly upregulated in GBM compared to normal brain samples and lower-grade gliomas (LGG). Survival analysis found that higher expression of Orai2 was independently associated with a worse prognosis of patients with the classical and mesenchymal subtypes of GBM. Simultaneously, Orai2 expression was higher in tumors of the classical and mesenchymal subtypes than other subtypes and was significantly correlated with classical- and mesenchymal-related genes. GO and KEGG pathway analysis revealed that genes significantly correlated with Orai2 were involved in the JNK pathway. Through screening transcriptomic data, we found a strong association between Orai2 and apoptosis, stemness, and an epithelial-mesenchymal transition- (EMT-) like phenotype.

Conclusion

As a prognostic factor, Orai2 is obviously activated in the classical and mesenchymal subtypes of GBM and promotes glioma cell self-renewal, apoptosis, and EMT-like by the JNK pathway. These findings indicate that Orai2 could be a candidate prognostic and therapeutic target, especially for the classical and mesenchymal subtypes of GBM.

Aberrant upregulation of PDK1 in ovarian cancer cells impairs CD8+ T cell function and survival through elevation of PD-L1

Blockade of the programmed cell death 1(PD-1)/PD-1 ligand-1(PD-L1) pathway has been exploited therapeutically in many cancer types. Upregulation of PD-L1 in tumor cells contributes to malignancy through suppression of the T cell-mediated antitumor response. Pyruvate dehydrogenase kinase 1 (PDK1), a glycolytic gate-keeping enzyme, is also known to promote tumor development. Here, we have uncovered a mechanism of regulation of PD-L1 by PDK1 through activation of c-Jun-NH₂-kinase (JNK)-c-Jun in ovarian cancer cells. Elevated PDK1 expression was correlated with that of PD-L1 in the TCGA ovarian cancer dataset and ovarian cancer tissue array. Overexpression of PDK1 in ovarian cancer cells impaired CD8⁺ T cell function by suppressing IFN-γ secretion through the PD-1/PD-L1 pathway. Conversely, knockdown of PDK1 in ovarian cancer cells relieved suppression of CD8⁺ T cell function. CD8⁺ T cell apoptosis induced by binding of PD-1 with PD-L1 was increased after co-culture with ovarian cancer cells overexpressing PDK1, while depletion of PDK1 exerted the opposite effect. In vivo experiments revealed synergistic improved overall survival and enhanced inhibition of tumor growth upon co-treatment with dichloroacetate (DCA), a PDK inhibitor, and PD-L1 antibody, accompanied by increased IFN-γ secretion by monocytes infiltrating tumor islets. Moreover, PDK1 expression and CD8⁺ T cell infiltration were inversely correlated in the ovarian cancer tissue array. Our collective findings provide a novel explanation of how PDK1 contributes to upregulation of PD-L1 in ovarian cancer and highlight its potential as a target therapeutic molecule that cooperates with the immune checkpoint blockade.

C-Jun drives melanoma progression in PTEN wild type melanoma cells

Due to the critical impact of active AP-1 transcription factors in melanoma, it is important to define their target genes and to identify and ultimately inhibit oncogenic signals. Here we mapped the genome-wide occupancy of the AP-1 family member c-Jun in different melanoma cells and correlated AP-1 binding with transcriptome data to detect genes in melanoma regulated by c-Jun. Our analysis shows that c-Jun supports the malignant phenotype by deregulating genes in cancer-relevant signaling pathways, such as mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI3K) pathways. Moreover, we demonstrate that the importance of c-Jun depends on melanoma stage and mutation status of the tumor suppressor PTEN. Our study reveals that activation of c-Jun overrules the tumor suppressive effect of PTEN in early melanoma development. These findings help to understand the relevance of c-Jun within cancer pathways in different melanoma cell types, especially in relation to MAPK and PI3K pathways, which are commonly deregulated in melanomas. Consequently, targeting c-Jun in PTEN⁺ melanoma cells may represent a promising therapeutic strategy to inhibit survival of melanoma cells to prevent the development of a metastatic phenotype.

STAT3 enhances the constitutive activity of AGC kinases in melanoma by transactivating PDK1

Background

The PI3K/Akt and the STAT3 pathways are functionally associated in many tumor types. Both in vitro and in vivo studies have revealed that either biochemical or genetic manipulation of the STAT3 pathway activity induce changes in the same direction in Akt activity. However, the implicated mechanism has been poorly characterized. Our goal was to characterize the precise mechanism linking STAT3 with the activity of Akt and other AGC kinases in cancer using melanoma cells as a model.

Results

We show that active STAT3 is constitutively bound to the PDK1 promoter and positively regulate PDK1 transcription through two STAT3 responsive elements. Transduction of WM9 and UACC903 melanoma cells with STAT3-small hairpin RNA decreased both PDK1 mRNA and protein levels. STAT3 knockdown also induced a decrease of the phosphorylation of AGC kinases Akt, PKC, and SGK. The inhibitory effect of STAT3 silencing on Akt phosphorylation was restored by HA-PDK1. Along this line, HA-PDK1 expression significantly blocked the cell death induced by dacarbazine plus STAT3 knockdown. This effect might be mediated by Bcl2 proteins since HA-PDK1 rescued Bcl2, Bcl-XL, and Mcl1 levels that were down-regulated upon STAT3 silencing.

Conclusions

We show that PDK1 is a transcriptional target of STAT3, linking STAT3 pathway with AGC kinases activity in melanoma. These data provide further rationale for the ongoing effort to therapeutically target STAT3 and PDK1 in melanoma and, possibly, other malignancies.

Electronic supplementary material

The online version of this article (10.1186/s13578-018-0265-8) contains supplementary material, which is available to authorized users.

Divergent roles of lysyl oxidase family members in ornithine decarboxylase- and RAS-transformed mouse fibroblasts and human melanoma cells

We have previously shown that proto-oncoprotein c-Jun is activated in ornithine decarboxylase (ODC)- and RAS-transformed mouse fibroblasts, and that the transformed morphology of these cells can be reversed by expressing the transactivation domain deletion mutant of c-Jun (TAM67). Here, we found that lysyl oxidase (Lox), encoding an extracellular matrix-modifying enzyme, is downregulated in a c-Jun-dependent manner in ODC-transformed fibroblasts (Odc cells). In addition to Lox, the Lox family members Lox-like 1 and 3 (Loxl1 and Loxl3) were found to be downregulated in Odc as well as in RAS-transformed fibroblasts (E4), whereas Lox-like 4 (Loxl4) was upregulated in Odc and downregulated in E4 cells compared to normal N1 fibroblasts. Tetracycline-regulatable LOX re-expression in Odc cells led to inhibition of cell growth and invasion in three-dimensional Matrigel in an activity-independent manner. On the contrary, LOX and especially LOXL2, LOXL3, and LOXL4 were found to be upregulated in several human melanoma cell lines, and LOX inhibitor B-aminopropionitrile inhibited the invasive growth of these cells particularly when co-cultured with fibroblasts in Matrigel. Knocking down the expression of LOX and especially LOXL2 in melanoma cells almost completely abrogated the invasive growth capability. Further, LOXL2 was significantly upregulated in clinical human primary melanomas compared to benign nevi, and high expression of LOXL2 in primary melanomas was associated with formation of metastases and shorter survival of patients. Thus, our studies reveal that inactive pro-LOX (together with Lox propeptide) functions as a tumor suppressor in ODC- and RAS-transformed murine fibroblasts by inhibiting cell growth and invasion, and active LOX and LOXL2 as tumor promoters in human melanoma cells by promoting their invasive growth.

Phosphoprotein patterns predict trametinib responsiveness and optimal trametinib sensitisation strategies in melanoma

Malignant melanoma is a highly aggressive form of skin cancer responsible for the majority of skin cancer-related deaths. Recent insight into the heterogeneous nature of melanoma suggests more personalised treatments may be necessary to overcome drug resistance and improve patient care. To this end, reliable molecular signatures that can accurately predict treatment responsiveness need to be identified. In this study, we applied multiplex phosphoproteomic profiling across a panel of 24 melanoma cell lines with different disease-relevant mutations, to predict responsiveness to MEK inhibitor trametinib. Supported by multivariate statistical analysis and multidimensional pattern recognition algorithms, the responsiveness of individual cell lines to trametinib could be predicted with high accuracy (83% correct predictions), independent of mutation status. We also successfully employed this approach to case specifically predict whether individual melanoma cell lines could be sensitised to trametinib. Our predictions identified that combining MEK inhibition with selective targeting of c-JUN and/or FAK, using siRNA-based depletion or pharmacological inhibitors, sensitised resistant cell lines and significantly enhanced treatment efficacy. Our study indicates that multiplex proteomic analyses coupled with pattern recognition approaches could assist in personalising trametinib-based treatment decisions in the future.

GSK3 and its interactions with the PI3K/AKT/mTOR signalling network

Glycogen Synthase Kinase-3 (GSK3 or GSK-3) is a promiscuous protein kinase and its phosphorylation of its diverse substrates has major influences on many areas of physiology and pathology, including cellular metabolism, lineage commitment and neuroscience. GSK3 was one of the first identified substrates of the heavily studied oncogenic kinase AKT, phosphorylation by which inhibits GSK3 activity via the formation of an autoinhibitory pseudosubstrate sequence. This has led to investigation of the role of GSK3 inhibition as a key component of the cellular responses to growth factors and insulin, which stimulate the class I PI 3-Kinases and in turn AKT activity and GSK3 phosphorylation. GSK3 has been shown to phosphorylate several upstream and downstream components of the PI3K/AKT/mTOR signalling network, including AKT itself, RICTOR, TSC1 and 2, PTEN and IRS1 and 2, with the potential to apply feedback control within the network. However, it has been clear for some time that functionally distinct, insulated pools of GSK3 exist which are regulated independently, so that for some GSK3 substrates such as β-catenin, phosphorylation by GSK3 is not controlled by input from PI3K and AKT. Instead, as almost all GSK3 substrates require a priming phosphorylated residue to be 4 amino acids C-terminal to the Ser/Thr phosphorylated by GSK3, the predominant form of regulation of the activity of GSK3 often appears to be through control over these priming events, specific to individual substrates. Therefore, a major role of GSK3 can be viewed as an amplifier of the electrostatic effects on protein function which are caused by these priming phosphorylation events. Here we discuss these different aspects to GSK3 regulation and function, and the functions of GSK3 as it integrates with signalling through the PI3K-AKT-mTOR signalling axis.

The AP-1 transcription factor FOSL1 causes melanocyte reprogramming and transformation

The MAPK pathway is activated in the majority of melanomas and is the target of therapeutic approaches. Under normal conditions, it initiates the so-called immediate early response, which encompasses the transient transcription of several genes belonging to the AP-1 transcription factor family. Under pathological conditions, such as continuous MAPK pathway overactivation due to oncogenic alterations occurring in melanoma, these genes are constitutively expressed. The consequences of a permanent expression of these genes are largely unknown. Here, we show that FOSL1 is the main immediate early AP-1 member induced by melanoma oncogenes. We first examined its role in established melanoma cells. We found that FOSL1 is involved in melanoma cell migration as well as cell proliferation and anoikis-independent growth, which is mediated by the gene product of its target gene HMGA1, encoding a multipotent chromatin modifier. As FOSL1 expression is increased in patient melanoma samples compared to nevi, we investigated the effect of enhanced FOSL1 expression on melanocytes. Intriguingly, we found that FOSL1 acts oncogenic and transforms melanocytes, enabling subcutaneous tumor growth in vivo. During the process of transformation, FOSL1 reprogrammed the melanocytes and downregulated MITF in a HMGA1-dependent manner. At the same time, AXL was upregulated, leading to a shift in the MITF/AXL balance. Furthermore, FOSL1 re-enforced pro-tumorigenic transcription factors MYC, E2F3 and AP-1. Together, this led to the enhancement of several growth-promoting processes, such as ribosome biogenesis, cellular detachment and pyrimidine metabolism. Overall, we demonstrate that FOSL1 is a novel reprogramming factor for melanocytes with potent tumor transformation potential.

Timp1 Promotes Cell Survival by Activating the PDK1 Signaling Pathway in Melanoma

High TIMP1 expression is associated with poor prognosis in melanoma, where it can bind to CD63 and β1 integrin, inducing PI3-kinase pathway and cell survival. Phosphatidylinositol (3,4,5)-trisphosphate (PIP3), generated under phosphatidylinositol-3-kinase (PI3K) activation, enables the recruitment and activation of protein kinase B (PKB/AKT) and phosphoinositide-dependent kinase 1 (PDK1) at the membrane, resulting in the phosphorylation of a host of other proteins. Using a melanoma progression model, we evaluated the impact of Timp1 and AKT silencing, as well as PI3K, PDK1, and protein kinase C (PKC) inhibitors on aggressiveness characteristics. Timp1 downregulation resulted in decreased anoikis resistance, clonogenicity, dacarbazine resistance, and in vivo tumor growth and lung colonization. In metastatic cells, pAKT^Thr308 is highly expressed, contributing to anoikis resistance. We showed that PDK1^Ser241 and PKCβII^Ser660 are activated by Timp1 in different stages of melanoma progression, contributing to colony formation and anoikis resistance. Moreover, simultaneous inhibition of Timp1 and AKT in metastatic cells resulted in more effective anoikis inhibition. Our findings demonstrate that Timp1 promotes cell survival with the participation of PDK1 and PKC in melanoma. In addition, Timp1 and AKT act synergistically to confer anoikis resistance in advanced tumor stages. This study brings new insights about the mechanisms by which Timp1 promotes cell survival in melanoma, and points to novel perspectives for therapeutic approaches.

JUN dependency in distinct early and late BRAF inhibition adaptation states of melanoma

A prominent mechanism of acquired resistance to BRAF inhibitors in BRAF (V600) -mutant melanoma is associated with the upregulation of receptor tyrosine kinases. Evidences suggested that this resistance mechanism is part of a more complex cellular adaptation process. Using an integrative strategy, we found this mechanism to invoke extensive transcriptomic, (phospho-) proteomic and phenotypic alterations that accompany a cellular transition to a de-differentiated, mesenchymal and invasive state. Even short-term BRAF-inhibitor exposure leads to an early adaptive, differentiation state change-characterized by a slow-cycling, persistent state. The early persistent state is distinct from the late proliferative, resistant state. However, both differentiation states share common signaling alterations including JUN upregulation. Motivated by the similarities, we found that co-targeting of BRAF and JUN is synergistic in killing fully resistant cells; and when used up-front, co-targeting substantially impairs the formation of the persistent subpopulation. We confirmed that JUN upregulation is a common response to BRAF inhibitor treatment in clinically treated patient tumors. Our findings demonstrate that events shared between early- and late-adaptation states provide candidate up-front co-treatment targets.

PI3K p110β isoform synergizes with JNK in the regulation of glioblastoma cell proliferation and migration through Akt and FAK inhibition

BACKGROUND

Glioblastoma multiforme is the most aggressive malignant primary brain tumor, characterized by rapid growth and extensive infiltration to neighboring normal brain parenchyma. Both PI3K/Akt and JNK pathways are essential to glioblastoma cell survival, migration and invasion. Due to their hyperactivation in glioblastoma cells, PI3K and JNK are promising targets for glioblastoma treatment.

METHODS

To investigate the combination effects of class IA PI3K catalytic isoforms (p110α, p110β and p110δ) and JNK inhibition on tumor cell growth and motility, glioblastoma cells and xenografts in nude mice were treated with isoform-selective PI3K inhibitors in combination with JNK inhibitor.

RESULTS

We showed that combined inhibition of these PI3K isoforms and JNK exerted divergent effects on the proliferation, migration and invasion of glioblastoma cells in vitro. Pharmacological inhibition of p110β or p110δ, but not p110α, displayed synergistic inhibitory effect with JNK inhibition on glioblastoma cell proliferation and migration through decreasing phosphorylation of Akt, FAK and zyxin, leading to blockade of lamellipodia and membrane ruffles formation. No synergistic effect on invasion was observed in all the combination treatment. In vivo, combination of p110β and JNK inhibitors significantly reduced xenograft tumor growth compared with single inhibitor alone.

CONCLUSION

Concurrent inhibition of p110β and JNK exhibited synergistic effects on suppressing glioblastoma cell proliferation and migration in vitro and xenograft tumor growth in vivo. Our data suggest that combined inhibition of PI3K p110β isoform and JNK may serve as a potent and promising therapeutic approach for glioblastoma multiforme.

FGFR and PTEN signaling interact during lens development to regulate cell survival

Lens epithelial cells express many receptor tyrosine kinases (RTKs) that stimulate PI3K-AKT and RAS-RAF-MEK-ERK intracellular signaling pathways. These pathways ultimately activate the phosphorylation of key cellular transcription factors and other proteins that control proliferation, survival, metabolism, and differentiation in virtually all cells. Among RTKs in the lens, only stimulation of fibroblast growth factor receptors (FGFRs) elicits a lens epithelial cell to fiber cell differentiation response in mammals. Moreover, although the lens expresses three different Fgfr genes, the isolated removal of Fgfr2 at the lens placode stage inhibits both lens cell survival and fiber cell differentiation. Phosphatase and tensin homolog (PTEN), commonly known as a tumor suppressor, inhibits ERK and AKT activation and initiates both apoptotic pathways, and cell cycle arrest. Here, we show that the combined deletion of Fgfr2 and Pten rescues the cell death phenotype associated with Fgfr2 loss alone. Additionally, Pten removal increased AKT and ERK activation, above the levels of controls, in the presence or absence of Fgfr2. However, isolated deletion of Pten failed to stimulate ectopic fiber cell differentiation, and the combined deletion of Pten and Fgfr2 failed to restore differentiation-specific Aquaporin0 and DnaseIIβ expression in the lens fiber cells.

Prostaglandin E2 stimulates normal bronchial epithelial cell growth through induction of c-Jun and PDK1, a kinase implicated in oncogenesis

Background

Cyclooxygenase-2-derived prostaglandin E₂ (PGE₂), a bioactive eicosanoid, has been implicated in many biological processes including reproduction, inflammation and tumor growth. We previously showed that PGE₂ stimulated lung cancer cell growth and progression through PGE₂ receptor EP2/EP4-mediated kinase signaling pathways. However, the role of PGE₂ in controlling lung airway epithelial cell phenotype remains unknown. We evaluated the effects of c-Jun and 3-phosphoinositede dependent protein kinase-1 (PDK1) in mediating epithelial cell hyperplasia induced by PGE₂.

Method

The bronchial epithelial cell lines BEAS-2B and HBEc14-KT were cultured and then treated with PGE₂. PDK1 small interfering RNA (siRNA) and a PDK1 inhibitor, an antagonist of the PGE₂ receptor subtype EP4 and EP4 siRNA, c-Jun siRNA, and overexpressions of c-Jun and PDK1 have been used to evaluate the effects on cell proliferation.

Results

We demonstrated that PGE₂ increased normal bronchial epithelial cell proliferation through induction of PDK1, an ankyrin repeat-containing Ser/Thr kinase implicated in the induction of apoptosis and the suppression of tumor growth. PDK1 siRNA and a PDK1 inhibitor blocked the effects of PGE₂ on normal cell growth. The PGE₂-induced PDK1 expression was blocked by an antagonist of the PGE₂ receptor subtype EP4 and by EP4 siRNA. In addition, we showed that induction of PDK1 by PGE₂ was associated with induction of the transcription factor, c-Jun protein. Silencing of c-Jun using siRNA and point mutations of c-Jun sites in the PDK1 gene promoter resulted in blockade of PDK1 expression and promoter activity induced by PGE₂. In contrast, overexpression of c-Jun induced PDK1 gene promoter activity and expression followed increased cell proliferation.

Conclusion

PGE₂ increases normal bronchial epithelial cell proliferation through increased PDK1 gene expression that is dependent on EP4 and induction of c-Jun. Therewith, our data suggest a new role of c-Jun and PDK1 in mediating epithelial cell hyperplasia induced by PGE₂.

Systematic analysis of BRAF(V600E) melanomas reveals a role for JNK/c-Jun pathway in adaptive resistance to drug-induced apoptosis

Drugs that inhibit RAF/MEK signaling, such as vemurafenib, elicit profound but often temporary anti-tumor responses in patients with BRAF^V600E melanoma. Adaptive responses to RAF/MEK inhibition occur on a timescale of hours to days, involve homeostatic responses that reactivate MAP kinase signaling and compensatory mitogenic pathways, and attenuate the anti-tumor effects of RAF/MEK inhibitors. We profile adaptive responses across a panel of melanoma cell lines using multiplex biochemical measurement, single-cell assays, and statistical modeling and show that adaptation involves at least six signaling cascades that act to reduce drug potency (IC₅₀) and maximal effect (i.e., E_max ≪ 1). Among these cascades, we identify a role for JNK/c-Jun signaling in vemurafenib adaptation and show that RAF and JNK inhibitors synergize in cell killing. This arises because JNK inhibition prevents a subset of cells in a cycling population from becoming quiescent upon vemurafenib treatment, thereby reducing drug E_max. Our findings demonstrate the breadth and diversity of adaptive responses to RAF/MEK inhibition and a means to identify which steps in a signaling cascade are most predictive of phenotypic response.

Mitogen-activated protein kinase (MAPK) hyperactivation and enhanced NRAS expression drive acquired vemurafenib resistance in V600E BRAF melanoma cells

Although targeting the V600E activating mutation in the BRAF gene, the most common genetic abnormality in melanoma, has shown clinical efficacy in melanoma patients, response is, invariably, short lived. To better understand mechanisms underlying this acquisition of resistance to BRAF-targeted therapy in previously responsive melanomas, we induced vemurafenib resistance in two V600E BRAF+ve melanoma cell lines, A375 and DM443, by serial in vitro vemurafenib exposure. The resulting approximately 10-fold more vemurafenib-resistant cell lines, A375rVem and D443rVem, had higher growth rates and showed differential collateral resistance to cisplatin, melphalan, and temozolomide. The acquisition of vemurafenib resistance was associated with significantly increased NRAS levels in A375rVem and D443rVem, increased activation of the prosurvival protein, AKT, and the MAPKs, ERK, JNK, and P38, which correlated with decreased levels of the MAPK inhibitor protein, GSTP1. Despite the increased NRAS, whole exome sequencing showed no NRAS gene mutations. Inhibition of all three MAPKs and siRNA-mediated NRAS suppression both reversed vemurafenib resistance significantly in A375rVem and DM443rVem. Together, the results indicate a mechanism of acquired vemurafenib resistance in V600E BRAF+ve melanoma cells that involves increased activation of all three human MAPKs and the PI3K pathway, as well as increased NRAS expression, which, contrary to previous reports, was not associated with mutations in the NRAS gene. The data highlight the complexity of the acquired vemurafenib resistance phenotype and the challenge of optimizing BRAF-targeted therapy in this disease. They also suggest that targeting the MAPKs and/or NRAS may provide a strategy to mitigate such resistance in V600E BRAF+ve melanoma.

Mechanisms of tumour vascularization in cutaneous malignant melanoma: clinical implications

Malignant melanoma represents < 10% of all skin cancers but is responsible for the majority of skin-cancer-related deaths. Metastatic melanoma has historically been considered as one of the most therapeutically challenging malignancies. Fortunately, for the first time after decades of basic research and clinical investigation, new drugs have produced major clinical responses. Angiogenesis has been considered an important target for cancer treatment. Initial efforts have focused primarily on targeting endothelial and tumour-related vascular endothelial growth factor signalling. Here, we review different mechanisms of tumour vascularization described in melanoma and discuss the potential clinical implications.

Tumor-specific activation of the C-JUN/MELK pathway regulates glioma stem cell growth in a p53-dependent manner

Accumulated evidence suggests that glioma stem cells (GSCs) may contribute to therapy resistance in high-grade glioma (HGG). Although recent studies have shown that the serine/threonine kinase maternal embryonic leucine-zipper kinase (MELK) is abundantly expressed in various cancers, the function and mechanism of MELK remain elusive. Here, we demonstrate that MELK depletion by shRNA diminishes the growth of GSC-derived mouse intracranial tumors in vivo, induces glial fibrillary acidic protein (+) glial differentiation of GSCs leading to decreased malignancy of the resulting tumors, and prolongs survival periods of tumor-bearing mice. Tissue microarray analysis with 91 HGG tumors demonstrates that the proportion of MELK (+) cells is a statistically significant indicator of postsurgical survival periods. Mechanistically, MELK is regulated by the c-Jun NH(2)-terminal kinase (JNK) signaling and forms a complex with the oncoprotein c-JUN in GSCs but not in normal progenitors. MELK silencing induces p53 expression, whereas p53 inhibition induces MELK expression, indicating that MELK and p53 expression are mutually exclusive. Additionally, MELK silencing-mediated GSC apoptosis is partially rescued by both pharmacological p53 inhibition and p53 gene silencing, indicating that MELK action in GSCs is p53 dependent. Furthermore, irradiation of GSCs markedly elevates MELK mRNA and protein expression both in vitro and in vivo. Clinically, recurrent HGG tumors following the failure of radiation and chemotherapy exhibit a statistically significant elevation of MELK protein compared with untreated newly diagnosed HGG tumors. Together, our data indicate that GSCs, but not normal cells, depend on JNK-driven MELK/c-JUN signaling to regulate their survival, maintain GSCs in an immature state, and facilitate tumor radioresistance in a p53-dependent manner.

The role of mitogen- and stress-activated protein kinase pathways in melanoma

Recent discoveries have increased our comprehension of the molecular signaling events critical for melanoma development and progression. Many oncogenes driving melanoma have been identified, and most of them exert their oncogenic effects through the activation of the RAF/MEK/ERK mitogen-activated protein kinase (MAPK) pathway. The c-Jun N-terminal kinase (JNK) and p38 MAPK pathways are also important in melanoma, but their precise role is not clear yet. This review summarizes our current knowledge on the role of the three main MAPK pathways, extracellular regulated kinase (ERK), JNK, and p38, and their impact on melanoma biology. Although the results obtained with BRAF inhibitors in melanoma patients are impressive, several mechanisms of acquired resistance have emerged. To overcome this obstacle constitutes the new challenge in melanoma therapy. Given the major role that MAPKs play in melanoma, understanding their functions and the interconnection among them and with other signaling pathways represents a step forward toward this goal.

Pre-symptomatic activation of antioxidant responses and alterations in glucose and pyruvate metabolism in Niemann-Pick Type C1-deficient murine brain

Niemann-Pick Type C (NPC) disease is an autosomal recessive neurodegenerative disorder caused in most cases by mutations in the NPC1 gene. NPC1-deficiency is characterized by late endosomal accumulation of cholesterol, impaired cholesterol homeostasis, and a broad range of other cellular abnormalities. Although neuronal abnormalities and glial activation are observed in nearly all areas of the brain, the most severe consequence of NPC1-deficiency is a near complete loss of Purkinje neurons in the cerebellum. The link between cholesterol trafficking and NPC pathogenesis is not yet clear; however, increased oxidative stress in symptomatic NPC disease, increases in mitochondrial cholesterol, and alterations in autophagy/mitophagy suggest that mitochondria play a role in NPC disease pathology. Alterations in mitochondrial function affect energy and neurotransmitter metabolism, and are particularly harmful to the central nervous system. To investigate early metabolic alterations that could affect NPC disease progression, we performed metabolomics analyses of different brain regions from age-matched wildtype and Npc1^-/- mice at pre-symptomatic, early symptomatic and late stage disease by ¹H-NMR spectroscopy. Metabolic profiling revealed markedly increased lactate and decreased acetate/acetyl-CoA levels in Npc1^-/- cerebellum and cerebral cortex at all ages. Protein and gene expression analyses indicated a pre-symptomatic deficiency in the oxidative decarboxylation of pyruvate to acetyl-CoA, and an upregulation of glycolytic gene expression at the early symptomatic stage. We also observed a pre-symptomatic increase in several indicators of oxidative stress and antioxidant response systems in Npc1^-/- cerebellum. Our findings suggest that energy metabolism and oxidative stress may present additional therapeutic targets in NPC disease, especially if intervention can be started at an early stage of the disease.

Genetic inactivation or pharmacological inhibition of Pdk1 delays development and inhibits metastasis of Braf(V600E)::Pten(-/-) melanoma

Phosphoinositide-dependent kinase-1 (PDK1) is a serine/threonine protein kinase that phosphorylates members of the conserved AGC kinase superfamily, including AKT and protein kinase C (PKC), and is implicated in important cellular processes including survival, metabolism and tumorigenesis. In large cohorts of nevi and melanoma samples, PDK1 expression was significantly higher in primary melanoma, compared with nevi, and was further increased in metastatic melanoma. PDK1 expression suffices for its activity, owing to auto-activation, or elevated phosphorylation by phosphoinositide 3'-OH-kinase (PI3K). Selective inactivation of Pdk1 in the melanocytes of Braf(V600E)::Pten(-/-) or Braf(V600E)::Cdkn2a(-/-)::Pten(-/-) mice delayed the development of pigmented lesions and melanoma induced by systemic or local administration of 4-hydroxytamoxifen. Melanoma invasion and metastasis were significantly reduced or completely prevented by Pdk1 deletion. Administration of the PDK1 inhibitor GSK2334470 (PDKi) effectively delayed melanomagenesis and metastasis in Braf(V600E)::Pten(-/-) mice. Pdk1(-/-) melanomas exhibit a marked decrease in the activity of AKT, P70S6K and PKC. Notably, PDKi was as effective in inhibiting AGC kinases and colony forming efficiency of melanoma with Pten wild-type (WT) genotypes. Gene expression analyses identified Pdk1-dependent changes in FOXO3a-regulated genes, and inhibition of FOXO3a restored proliferation and colony formation of Pdk1(-/-) melanoma cells. Our studies provide direct genetic evidence for the importance of PDK1, in part through FOXO3a-dependent pathway, in melanoma development and progression.

The activation of MAPK in melanoma cells resistant to BRAF inhibition promotes PD-L1 expression that is reversible by MEK and PI3K inhibition

PURPOSE

Selective BRAF inhibition (BRAFi) provides a paradigm shift for melanoma treatment. The duration of benefit is typically limited before resistance develops. Interest remains in combining targeted and immune therapies to overcome resistance and improve durability of clinical benefit. One mechanism of evading immune destruction is programmed death-1-ligand 1 (PD-L1) expression by tumors that results in potent antitumor immune suppression.

EXPERIMENTAL DESIGN

BRAFi-resistant melanoma cells were examined for changes in PD-L1 expression by immunoblot and flow cytometry. Signaling pathways involved in altering PD-L1 expression were examined. Strategies to maximize the effect of the BRAFi therapy were studied including MEKi, MEKi combinations, and additional pathways including phosphoinositide-3 kinase (PI3K).

RESULTS

Melanoma cells resistant to BRAFi exhibit increased MAPK signaling and promotion of PD-L1 expression. PD-L1 expression is transcriptionally modulated by c-Jun and augmented by STAT3. MEK inhibition (MEKi) regains downregulation of MAPK signaling and suppresses the production of PD-L1. MEKi in melanoma cells shows dual therapeutic effects with simultaneous suppression of PD-L1 expression and induction of apoptosis. By combining MEKi with BRAFi, an additive effect on the inhibition of PD-L1 expression results.

CONCLUSIONS

We report a novel mechanism that suppresses preexisting immune responses in patients with melanoma receiving BRAFi therapy. BRAFi resistance leads to increased expression of PD-L1 in melanoma cells, mediated by c-Jun and STAT3. MEKi may be feasible to counteract BRAFi resistance of MAPK reactivation and also for the additive effect of PD-L1 suppression. Potential therapeutic benefits of combining targeted inhibitors and immune modulation to improve patient outcomes should be investigated.

Specifying protein kinase C functions in melanoma

The protein kinase C (PKC) family of serine/threonine protein kinases is a heterogeneous group of enzymes receiving and integrating signals involved in both normal melanocyte biology and melanoma pathology. Alterations in PKC enzyme expression and activation contribute to the malignant phenotype of melanoma in both oncogenic and tumor suppressive roles. Delineating the diverse and often context-dependent functions of PKC enzymes in melanocyte/melanoma biology is key to capitalize on these kinases as drug targets. This review summarizes several of the diverse functions of PKC in melanocyte and melanoma biology with a focus on PKC enzyme regulation and function.

Lactotransferrin acts as a tumor suppressor in nasopharyngeal carcinoma by repressing AKT through multiple mechanisms

LTF (lactotransferrin, also known as lactoferrin) is a key component of innate immune defense. It has recently been found to have anti-tumor and anti-metastatic activity in different cancers. We previously reported LTF to be the most significantly downregulated gene in nasopharyngeal carcinoma (NPC) specimens relative to normal nasopharyngeal epithelial tissues, and it was also negatively associated with the progression and metastasis of NPC. However, the mechanism underlying this remains unclear. In the current study, we revealed that LTF can suppress 3-phosphoinositide-dependent protein kinase 1 expression via the mitogen-activated protein kinase/c-Jun pathway and thus repress AKT signaling. We also showed that LTF interacts with keratin 18 (K18) and so blocks the formation of the K18-14-3-3 complex, leading to downregulation of K18-mediated AKT activation. Thus, LTF suppresses AKT signaling by two separate mechanisms, leading to inhibition of NPC tumorigenesis. This is the first report on the tumor suppressive effects of LTF through repression of AKT signaling in NPC. It suggests that both LTF and AKT signaling merit further study in the field of NPC research.

ETS-1/RhoC signaling regulates the transcription factor c-Jun in melanoma

Recently, we discovered that the loss of E-cadherin induces c-Jun protein expression, which is a member of the AP-1 transcription factor family and a key player in the processes of cell proliferation and tumor development and also found in elevated levels in melanomas. Notably, the mRNA level of c-Jun was not affected, suggesting that c-Jun is regulated at post-transcriptional level. Here, we present data that suggest that the dynamic cytoskeletal network, linked to E-cadherin, is involved in the regulation of the c-Jun protein and transcriptional activity. In a signaling cascade, the loss of E-cadherin activates the transcriptional regulator ETS-1 and consequently leads to the induction of RhoC expression that stabilizes c-Jun in melanoma. The link between RhoC and c-Jun seems to be indirect via the cytoskeleton. We conclude that the loss of E-cadherin mediated cell-adhesion induces c-Jun protein expression in a multistep process, offering several possibilities for therapeutic intervention.

Identification of a c-Jun N-terminal kinase-2-dependent signal amplification cascade that regulates c-Myc levels in ras transformation

Ras is one of the most frequently activated oncogenes in cancer. Two mitogen-activated protein kinases (MAPKs) are important for ras transformation: extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase 2 (JNK2). Here we present a downstream signal amplification cascade that is critical for ras transformation in murine embryonic fibroblasts. This cascade is coordinated by ERK and JNK2 MAPKs, whose Ras-mediated activation leads to the enhanced levels of three oncogenic transcription factors, namely, c-Myc, activating transcription factor 2 (ATF2) and ATF3, all of which are essential for ras transformation. Previous studies show that ERK-mediated serine 62 phosphorylation protects c-Myc from proteasomal degradation. ERK is, however, not alone sufficient to stabilize c-Myc but requires the cooperation of cancerous inhibitor of protein phosphatase 2A (CIP2A), an oncogene that counteracts protein phosphatase 2A-mediated dephosphorylation of c-Myc. Here we show that JNK2 regulates Cip2a transcription via ATF2. ATF2 and c-Myc cooperate to activate the transcription of ATF3. Remarkably, not only ectopic JNK2, but also ectopic ATF2, CIP2A, c-Myc and ATF3 are sufficient to rescue the defective ras transformation of JNK2-deficient cells. Thus, these data identify the key signal converging point of JNK2 and ERK pathways and underline the central role of CIP2A in ras transformation.

Light at night activates IGF-1R/PDK1 signaling and accelerates tumor growth in human breast cancer xenografts

Regulation of diurnal and circadian rhythms and cell proliferation are coupled in all mammals, including humans. However, the molecular mechanisms by which diurnal and circadian rhythms regulate cell proliferation are relatively poorly understood. In this study, we report that tumor growth in nude rats bearing human steroid receptor-negative MCF-7 breast tumors can be significantly accelerated by exposing the rats to light at night (LAN). Under normal conditions of an alternating light/dark cycle, proliferating cell nuclear antigen (PCNA) levels in tumors were maximal in the early light phase but remained at very low levels throughout the daily 24-hour cycle period monitored. Surprisingly, PCNA was expressed in tumors continually at a high level throughout the entire 24-hour period in LAN-exposed nude rats. Daily fluctuations of Akt and mitogen activated protein kinase activation in tumors were also disrupted by LAN. These fluctuations did not track with PCNA changes, but we found that activation of the Akt stimulatory kinase phosphoinositide-dependent protein kinase 1 (PDK1) directly correlated with PCNA levels. Expression of insulin-like growth factor 1 receptor (IGF-1R), an upstream signaling molecule for PDK1, also correlated with fluctuations of PDK1/PCNA in the LAN group. In addition, circulating IGF-1 concentrations were elevated in LAN-exposed tumor-bearing nude rats. Finally, RNAi-mediated knockdown of PDK1 led to a reduction in PCNA expression and cell proliferation in vitro and tumor growth in vivo, indicating that PDK1 regulates breast cancer growth in a manner correlated with PCNA expression. Taken together, our findings demonstrate that LAN exposure can accelerate tumor growth in vivo, in part through continuous activation of IGF-1R/PDK1 signaling.

A role for ATF2 in regulating MITF and melanoma development

The transcription factor ATF2 has been shown to attenuate melanoma susceptibility to apoptosis and to promote its ability to form tumors in xenograft models. To directly assess ATF2's role in melanoma development, we crossed a mouse melanoma model (Nras(Q61K)::Ink4a⁻/⁻) with mice expressing a transcriptionally inactive form of ATF2 in melanocytes. In contrast to 7/21 of the Nras(Q61K)::Ink4a⁻/⁻ mice, only 1/21 mice expressing mutant ATF2 in melanocytes developed melanoma. Gene expression profiling identified higher MITF expression in primary melanocytes expressing transcriptionally inactive ATF2. MITF downregulation by ATF2 was confirmed in the skin of Atf2⁻/⁻ mice, in primary human melanocytes, and in 50% of human melanoma cell lines. Inhibition of MITF transcription by MITF was shown to be mediated by ATF2-JunB-dependent suppression of SOX10 transcription. Remarkably, oncogenic BRAF (V600E)-dependent focus formation of melanocytes on soft agar was inhibited by ATF2 knockdown and partially rescued upon shMITF co-expression. On melanoma tissue microarrays, a high nuclear ATF2 to MITF ratio in primary specimens was associated with metastatic disease and poor prognosis. Our findings establish the importance of transcriptionally active ATF2 in melanoma development through fine-tuning of MITF expression.

Gene expression analysis after receptor tyrosine kinase activation reveals new potential melanoma proteins

BACKGROUND

Melanoma is an aggressive tumor with increasing incidence. To develop accurate prognostic markers and targeted therapies, changes leading to malignant transformation of melanocytes need to be understood. In the Xiphophorus melanoma model system, a mutated version of the EGF receptor Xmrk (Xiphophorus melanoma receptor kinase) triggers melanomagenesis. Cellular events downstream of Xmrk, such as the activation of Akt, Ras, B-Raf or Stat5, were also shown to play a role in human melanomagenesis. This makes the elucidation of Xmrk downstream targets a useful method for identifying processes involved in melanoma formation.

METHODS

Here, we analyzed Xmrk-induced gene expression using a microarray approach. Several highly expressed genes were confirmed by realtime PCR, and pathways responsible for their induction were revealed using small molecule inhibitors. The expression of these genes was also monitored in human melanoma cell lines, and the target gene FOSL1 was knocked down by siRNA. Proliferation and migration of siRNA-treated melanoma cell lines were then investigated.

RESULTS

Genes with the strongest upregulation after receptor activation were FOS-like antigen 1 (Fosl1), early growth response 1 (Egr1), osteopontin (Opn), insulin-like growth factor binding protein 3 (Igfbp3), dual-specificity phosphatase 4 (Dusp4), and tumor-associated antigen L6 (Taal6). Interestingly, most genes were blocked in presence of a SRC kinase inhibitor. Importantly, we found that FOSL1, OPN, IGFBP3, DUSP4, and TAAL6 also exhibited increased expression levels in human melanoma cell lines compared to human melanocytes. Knockdown of FOSL1 in human melanoma cell lines reduced their proliferation and migration.

CONCLUSION

Altogether, the data show that the receptor tyrosine kinase Xmrk is a useful tool in the identification of target genes that are commonly expressed in Xmrk-transgenic melanocytes and melanoma cell lines. The identified molecules constitute new possible molecular players in melanoma development. Specifically, a role of FOSL1 in melanomagenic processes is demonstrated. These data are the basis for future detailed analyses of the investigated target genes.

Disruption of IGF-1R signaling increases TRAIL-induced apoptosis: a new potential therapy for the treatment of melanoma

Resistance of cancer cells to apoptosis is dependent on a balance of multiple genetic and epigenetic mechanisms, which up-regulate efficacy of the surviving growth factor-receptor signaling pathways and suppress death-receptor signaling pathways. The Insulin-like Growth Factor-1 Receptor (IGF-1R) signaling pathway is highly active in metastatic melanoma cells by mediating downstream activation of PI3K-AKT and MAPK pathways and controlling general cell survival and proliferation. In the present study, we used human melanoma lines with established genotypes that represented different phases of cancer development: radial-growth-phase WM35, vertical-growth-phase WM793, metastatic LU1205 and WM9 [1]. All these lines have normal NRAS. WM35, WM793, LU1205 and WM9 cells have mutated BRAF (V600E). WM35 and WM9 cells express normal PTEN, while in WM793 cells PTEN expression is down-regulated; finally, in LU1205 cells PTEN is inactivated by mutation. Cyclolignan picropodophyllin (PPP), a specific inhibitor of IGF-1R kinase activity, strongly down-regulated the basal levels of AKT activity in WM9 and in WM793 cells, modestly does so in LU1205, but has no effect on AKT activity in the early stage WM35 cells that are deficient in IGF-1R. In addition, PPP partially down-regulated the basal levels of active ERK1/2 in all lines used, highlighting the role of an alternative, non-BRAF pathway in MAPK activation. The final result of PPP treatment was an induction of apoptosis in WM793, WM9 and LU1205 melanoma cells. On the other hand, dose-dependent inhibition of IGF-1R kinase activity by PPP at a relatively narrow dose range (near 500 nM) has different effects on melanoma cells versus normal cells, inducing apoptosis in cancer cells and G2/M arrest of fibroblasts. To further enhance the pro-apoptotic effects of PPP on melanoma cells, we used a combined treatment of TNF-Related Apoptosis-Inducing Ligand (TRAIL) and PPP. This combination substantially increased death by apoptosis for WM793 and WM9 cells, but did so only modestly for LU1205 cells with very high basal activity of AKT. The ultimate goal of this direction of research is the discovery of a new treatment method for highly resistant human metastatic melanomas. Our findings provide the rationale for further preclinical evaluation of this novel treatment.