CDK4/6-MEK Inhibition in MPNSTs Causes Plasma Cell Infiltration, Sensitization to PD-L1 Blockade, and Tumor Regression

PURPOSE

Malignant peripheral nerve sheath tumors (MPNST) are lethal, Ras-driven sarcomas that lack effective therapies. We investigated effects of targeting cyclin-dependent kinases 4 and 6 (CDK4/6), MEK, and/or programmed death-ligand 1 (PD-L1) in preclinical MPNST models.

EXPERIMENTAL DESIGN

Patient-matched MPNSTs and precursor lesions were examined by FISH, RNA sequencing, IHC, and Connectivity-Map analyses. Antitumor activity of CDK4/6 and MEK inhibitors was measured in MPNST cell lines, patient-derived xenografts (PDX), and de novo mouse MPNSTs, with the latter used to determine anti-PD-L1 response.

RESULTS

Patient tumor analyses identified CDK4/6 and MEK as actionable targets for MPNST therapy. Low-dose combinations of CDK4/6 and MEK inhibitors synergistically reactivated the retinoblastoma (RB1) tumor suppressor, induced cell death, and decreased clonogenic survival of MPNST cells. In immune-deficient mice, dual CDK4/6-MEK inhibition slowed tumor growth in 4 of 5 MPNST PDXs. In immunocompetent mice, combination therapy of de novo MPNSTs caused tumor regression, delayed resistant tumor outgrowth, and improved survival relative to monotherapies. Drug-sensitive tumors that regressed contained plasma cells and increased cytotoxic T cells, whereas drug-resistant tumors adopted an immunosuppressive microenvironment with elevated MHC II-low macrophages and increased tumor cell PD-L1 expression. Excitingly, CDK4/6-MEK inhibition sensitized MPNSTs to anti-PD-L1 immune checkpoint blockade (ICB) with some mice showing complete tumor regression.

CONCLUSIONS

CDK4/6-MEK inhibition induces a novel plasma cell-associated immune response and extended antitumor activity in MPNSTs, which dramatically enhances anti-PD-L1 therapy. These preclinical findings provide strong rationale for clinical translation of CDK4/6-MEK-ICB targeted therapies in MPNST as they may yield sustained antitumor responses and improved patient outcomes.

A Genome-Wide Screen Identifies PDPK1 as a Target to Enhance the Efficacy of MEK1/2 Inhibitors in NRAS Mutant Melanoma

Melanomas frequently harbor activating NRAS mutations. However, limited advance has been made in developing targeted therapy options for patients with NRAS mutant melanoma. MEK inhibitors (MEKi) show modest efficacy in the clinic and their actions need to be optimized. In this study, we performed a genome-wide CRISPR-Cas9-based screen and demonstrated that loss of phosphoinositide-dependent kinase-1 (PDPK1) enhances the efficacy of MEKi. The synergistic effects of PDPK1 loss and MEKi was validated in NRAS mutant melanoma cell lines using pharmacologic and molecular approaches. Combined PDPK1 inhibitors (PDPK1i) with MEKi suppressed NRAS mutant xenograft growth and induced gasdermin E-associated pyroptosis. In an immune-competent allograft model, PDPK1i+MEKi increased the ratio of intratumoral CD8+ T cells, delayed tumor growth, and prolonged survival; the combination treatment was less effective against tumors in immune-deficient mice. These data suggest PDPK1i+MEKi as an efficient immunostimulatory strategy against NRAS mutant melanoma.

SIGNIFICANCE

Targeting PDPK1 stimulates antitumor immunity and sensitizes NRAS mutant melanoma to MEK inhibition, providing rationale for the clinical development of a combinatorial approach for treating patients with melanoma.

Exploiting Allosteric Properties of RAF and MEK Inhibitors to Target Therapy-Resistant Tumors Driven by Oncogenic BRAF Signaling

Current clinical RAF inhibitors (RAFi) inhibit monomeric BRAF (mBRAF) but are less potent against dimeric BRAF (dBRAF). RAFi equipotent for mBRAF and dBRAF have been developed but are predicted to have lower therapeutic index. Here we identify a third class of RAFi that selectively inhibits dBRAF over mBRAF. Molecular dynamic simulations reveal restriction of the movement of the BRAF αC-helix as the basis of inhibitor selectivity. Combination of inhibitors based on their conformation selectivity (mBRAF- plus dBRAF-selective plus the most potent BRAF-MEK disruptor MEK inhibitor) promoted suppression of tumor growth in BRAFV600E therapy-resistant models. Strikingly, the triple combination showed no toxicities, whereas dBRAF-selective plus MEK inhibitor treatment caused weight loss in mice. Finally, the triple combination achieved durable response and improved clinical well-being in a patient with stage IV colorectal cancer. Thus, exploiting allosteric properties of RAF and MEK inhibitors enables the design of effective and well-tolerated therapies for BRAFV600E tumors. SIGNIFICANCE: This work identifies a new class of RAFi that are selective for dBRAF over mBRAF and determines the basis of their selectivity. A rationally designed combination of RAF and MEK inhibitors based on their conformation selectivity achieved increased efficacy and a high therapeutic index when used to target BRAFV600E tumors.See related commentary by Zhang and Bollag, p. 1620.This article is highlighted in the In This Issue feature, p. 1601.

Synthetic Lethal Screens Reveal Cotargeting FAK and MEK as a Multimodal Precision Therapy for GNAQ-Driven Uveal Melanoma

PURPOSE

Uveal melanoma is the most common eye cancer in adults. Approximately 50% of patients with uveal melanoma develop metastatic uveal melanoma (mUM) in the liver, even after successful treatment of the primary lesions. mUM is refractory to current chemo- and immune-therapies, and most mUM patients die within a year. Uveal melanoma is characterized by gain-of-function mutations in GNAQ/GNA11, encoding Gαq proteins. We have recently shown that the Gαq-oncogenic signaling circuitry involves a noncanonical pathway distinct from the classical activation of PLCβ and MEK-ERK. GNAQ promotes the activation of YAP1, a key oncogenic driver, through focal adhesion kinase (FAK), thereby identifying FAK as a druggable signaling hub downstream from GNAQ. However, targeted therapies often activate compensatory resistance mechanisms leading to cancer relapse and treatment failure.

EXPERIMENTAL DESIGN

We performed a kinome-wide CRISPR-Cas9 sgRNA screen to identify synthetic lethal gene interactions that can be exploited therapeutically. Candidate adaptive resistance mechanisms were investigated by cotargeting strategies in uveal melanoma and mUM in vitro and in vivo experimental systems.

RESULTS

sgRNAs targeting the PKC and MEK-ERK signaling pathways were significantly depleted after FAK inhibition, with ERK activation representing a predominant resistance mechanism. Pharmacologic inhibition of MEK and FAK showed remarkable synergistic growth-inhibitory effects in uveal melanoma cells and exerted cytotoxic effects, leading to tumor collapse in uveal melanoma xenograft and liver mUM models in vivo.

CONCLUSIONS

Coupling the unique genetic landscape of uveal melanoma with the power of unbiased genetic screens, our studies reveal that FAK and MEK-ERK cotargeting may provide a new network-based precision therapeutic strategy for mUM treatment.See related commentary by Harbour, p. 2967.

Dual Inhibition of MEK and AXL Targets Tumor Cell Heterogeneity and Prevents Resistant Outgrowth Mediated by the Epithelial-to-Mesenchymal Transition in NSCLC

The epithelial-to-mesenchymal transition (EMT) is a dynamic epigenetic reprogramming event that occurs in a subset of tumor cells and is an initiating step toward invasion and distant metastasis. The process is reversible and gives plasticity to cancer cells to survive under variable conditions, with the acquisition of cancer stem cell-like characteristics and features such as drug resistance. Therefore, understanding survival dependencies of cells along the phenotypic spectrum of EMT will provide better strategies to target the spatial and temporal heterogeneity of tumors and prevent their ability to bypass single-inhibitor treatment strategies. To address this, we integrated the data from a selective drug screen in epithelial and mesenchymal KRAS/p53 (KP)-mutant lung tumor cells with separate datasets including reverse-phase protein array and an in vivo shRNA dropout screen. These orthogonal approaches identified AXL and MEK as potential mesenchymal and epithelial cell survival dependencies, respectively. To capture the dynamicity of EMT, incorporation of a dual fluorescence EMT sensor system into murine KP lung cancer models enabled real-time analysis of the epigenetic state of tumor cells and assessment of the efficacy of single agent or combination treatment with AXL and MEK inhibitors. Both two- and three-dimensional culture systems and in vivo models revealed that this combination treatment strategy of MEK plus AXL inhibition synergistically killed lung cancer cells by specifically targeting each phenotypic subpopulation. In conclusion, these results indicate that cotargeting the specific vulnerabilities of EMT subpopulations can prevent EMT-mediated drug resistance, effectively controlling tumor cell growth and metastasis. SIGNIFICANCE: This study shows that a novel combination of MEK and AXL inhibitors effectively bypasses EMT-mediated drug resistance in KRAS/p53-mutant non-small cell lung cancer by targeting EMT subpopulations, thereby preventing tumor cell survival.

New advances in the clinical management of RAS and BRAF mutant colorectal cancer patients

INTRODUCTION

In colorectal carcinogenesis, genetic alterations in RAS and BRAF oncogenes play an important role for cancer initiation and/or progression and represent a key focus in the search for targeted therapies. Despite many years of research and a great amount of studies, until very recently this pathway was considered extremely hard to downregulate to obtain a significant clinical impact in colorectal cancer patients. But better times are coming with the advent of new promising drugs and combinations strategies.

AREAS COVERED

In this review, we go over the biological characteristics of the MAPK pathway in colorectal tumors, while illustrating the clinical correlation of RAS and BRAF mutations, particularly its prognostic and predictive value. We also present newly data about recent improvements in the treatment strategy for patients harboring these types of tumors.

EXPERT COMMENTARY

With great advances in the knowledge of molecular basis of RAS and BRAF mutant colorectal cancer in conjunction with biotechnology development and the constant effort for improvement, in the near future many new therapeutic options would be available for the management of this group of patient with dismal prognosis.

Safety and efficacy evaluation of encorafenib plus binimetinib for the treatment of advanced BRAF-mutant melanoma patients

INTRODUCTION

Approximately 40-50% of patients with cutaneous melanoma harbor point mutations in BRAF. BRAF and MEK inhibitors in combination are now a standard therapy for advanced BRAF ^V600-mutated melanoma. Nevertheless, survival rates with the combination are limited, highlighting the need for further therapeutic improvement and strategies to overcome primary and acquired resistance.

AREAS COVERED

Encorafenib, a highly selective BRAF inhibitor, was developed in combination with binimetinib, a potent, selective allosteric MEK1/2 inhibitor, to improve efficacy and tolerability over other approved combo-targeted therapies. This novel combination shows peculiar pharmacodynamic properties which translate in a higher on-target potency and paradox index. Consistent survival improvements for encorafenib and binimetinib in BRAF ^V600-mutated melanoma have been confirmed in clinical trials, with over 4 years of median follow up.

EXPERT OPINION

the favorable survival results and the attractive toxicity profile suggest that encorafenib and binimetinib combination is an intriguing standard option when targeted therapies are considered as first line treatment in BRAF mutated melanoma patients. In the near future, results from ongoing clinical trials will provide information on the use of this novel combination in specific situation, including as adjuvant treatment or as a combination strategy.

Trametinib Activity in Patients with Solid Tumors and Lymphomas Harboring BRAF Non-V600 Mutations or Fusions: Results from NCI-MATCH (EAY131)

PURPOSE

Substantial preclinical evidence and case reports suggest that MEK inhibition is an active approach in tumors with BRAF mutations outside the V600 locus, and in BRAF fusions. Thus, Subprotocol R of the NCI-MATCH study tested the MEK inhibitor trametinib in this population.

PATIENTS AND METHODS

The NCI-MATCH study performed genomic profiling on tumor samples from patients with solid tumors and lymphomas progressing on standard therapies or with no standard treatments. Patients with prespecified fusions and non-V600 mutations in BRAF were assigned to Subprotocol R using the NCI-MATCHBOX algorithm. The primary endpoint was objective response rate (ORR).

RESULTS

Among 50 patients assigned, 32 were eligible and received therapy with trametinib. Of these, 1 had a BRAF fusion and 31 had BRAF mutations (13 and 19 with class 2 and 3 mutations, respectively). There were no complete responses; 1 patient (3%) had a confirmed partial response (patient with breast ductal adenocarcinoma with BRAF G469E mutation) and 10 patients had stable disease as best response (clinical benefit rate 34%). Median progression-free survival (PFS) was 1.8 months, and median overall survival was 5.7 months. Exploratory subgroup analyses showed that patients with colorectal adenocarcinoma (n = 8) had particularly poor PFS. No new toxicity signals were identified.

CONCLUSIONS

Trametinib did not show promising clinical activity in patients with tumors harboring non-V600 BRAF mutations, and the subprotocol did not meet its primary endpoint.

Mutant BRAF and MEK Inhibitors Regulate the Tumor Immune Microenvironment via Pyroptosis

Combinations of BRAF inhibitors and MEK inhibitors (BRAFi + MEKi) are FDA-approved to treat BRAF V600E/K-mutant melanoma. Efficacy of BRAFi + MEKi associates with cancer cell death and alterations in the tumor immune microenvironment; however, the links are poorly understood. We show that BRAFi + MEKi caused durable melanoma regression in an immune-mediated manner. BRAFi + MEKi treatment promoted cleavage of gasdermin E (GSDME) and release of HMGB1, markers of pyroptotic cell death. GSDME-deficient melanoma showed defective HMGB1 release, reduced tumor-associated T cell and activated dendritic cell infiltrates in response to BRAFi + MEKi, and more frequent tumor regrowth after drug removal. Importantly, BRAFi + MEKi-resistant disease lacked pyroptosis markers and showed decreased intratumoral T-cell infiltration but was sensitive to pyroptosis-inducing chemotherapy. These data implicate BRAFi + MEKi-induced pyroptosis in antitumor immune responses and highlight new therapeutic strategies for resistant melanoma. SIGNIFICANCE: Targeted inhibitors and immune checkpoint agents have advanced the care of patients with melanoma; however, detailed knowledge of the intersection between these two research areas is lacking. We describe a molecular mechanism of targeted inhibitor regulation of an immune-stimulatory form of cell death and provide a proof-of-principle salvage therapy concept for inhibitor-resistant melanoma.See related commentary by Smalley, p. 176.This article is highlighted in the In This Issue feature, p. 161.

In Vivo ERK1/2 Reporter Predictively Models Response and Resistance to Combined BRAF and MEK Inhibitors in Melanoma

Combined BRAF and MEK inhibition is a standard of care in patients with advanced BRAF-mutant melanoma, but acquired resistance remains a challenge that limits response durability. Here, we quantitated in vivo ERK1/2 activity and tumor response associated with resistance to combined BRAF and MEK inhibition in mutant BRAF xenografts. We found that ERK1/2 pathway reactivation preceded the growth of resistant tumors. Moreover, we detected a subset of cells that not only persisted throughout long-term treatment but restored ERK1/2 signaling and grew upon drug removal. Cell lines derived from combination-resistant tumors (CRT) exhibited elevated ERK1/2 phosphorylation, which were sensitive to ERK1/2 inhibition. In some CRTs, we detected a tandem duplication of the BRAF kinase domain. Monitoring ERK1/2 activity in vivo was efficacious in predicting tumor response during intermittent treatment. We observed maintained expression of the mitotic regulator, polo-like kinase 1 (Plk1), in melanoma resistant to BRAF and MEK inhibitors. Plk1 inhibition induced apoptosis in CRTs, leading to slowed growth of BRAF and MEK inhibitor-resistant tumors in vivo These data demonstrate the utility of in vivo ERK1/2 pathway reporting as a tool to optimize clinical dosing schemes and establish suppression of Plk1 as potential salvage therapy for BRAF inhibitor and MEK inhibitor-resistant melanoma.

FREQUENT SUBCLINICAL MACULAR CHANGES IN COMBINED BRAF/MEK INHIBITION WITH HIGH-DOSE HYDROXYCHLOROQUINE AS TREATMENT FOR ADVANCED METASTATIC BRAF MUTANT MELANOMA: Preliminary Results From a Phase I/II Clinical Treatment Trial

PURPOSE

To assess the potential ocular toxicity of a combined BRAF inhibition (BRAFi) + MEK inhibition (MEKi) + hydroxychloroquine (HCQ) regime used to treat metastatic BRAF mutant melanoma.

METHODS

Patients with stage IV metastatic melanoma and BRAF V600E mutations (n = 11, 31-68 years of age) were included. Treatment was with oral dabrafenib, 150 mg bid, trametinib, 2 mg/day, and HCQ, 400 mg to 600 mg bid. An ophthalmic examination, spectral domain optical coherence tomography, near-infrared and short-wavelength fundus autofluorescence, and static perimetry were performed at baseline, 1 month, and q/6 months after treatment.

RESULTS

There were no clinically significant ocular events; there was no ocular inflammation. The only medication-related change was a separation of the photoreceptor outer segment tip from the apical retinal pigment epithelium that could be traced from the fovea to the perifoveal retina noted in 9/11 (82%) of the patients. There were no changes in retinal pigment epithelium melanization or lipofuscin content by near-infrared fundus autofluorescence and short-wavelength fundus autofluorescence, respectively. There were no inner retinal or outer nuclear layer changes. Visual acuities and sensitivities were unchanged.

CONCLUSION

BRAFi (trametinib) + MEKi (dabrafenib) + HCQ causes very frequent, subclinical separation of the photoreceptor outer segment from the apical retinal pigment epithelium without inner retinal changes or signs of inflammation. The changes suggest interference with the maintenance of the outer retinal barrier and/or phagocytic/pump functions of the retinal pigment epithelium by effective MEK inhibition.

Targeting Alterations in the RAF-MEK Pathway

The MAPK pathway is one of the most commonly mutated oncogenic pathways in cancer. Although RAS mutations are the most frequent MAPK alterations, less frequent alterations in downstream components of the pathway, including the RAF and MEK genes, offer promising therapeutic opportunities. In addition to BRAFV600 mutations, for which several approved therapeutic regimens exist, other alterations in the RAF and MEK genes may provide more rare, but tractable, targets. However, recent studies have illustrated the complexity of MAPK signaling and highlighted that distinct alterations in these genes may have strikingly different properties. Understanding the unique functional characteristics of specific RAF and MEK alterations, reviewed herein, will be critical for developing effective therapeutic approaches for these targets. SIGNIFICANCE: Alterations in the RAF and MEK genes represent promising therapeutic targets in multiple cancer types. However, given the unique and complex signaling biology of the MAPK pathway, the diverse array of RAF and MEK alterations observed in cancer can possess distinct functional characteristics. As outlined in this review, understanding the key functional properties of different RAF and MEK alterations is fundamental to selecting the optimal therapeutic approach.

Nucleocytoplasmic traffic of MAP kinases

MAPK pathways represent a unique extracellular signal response system. An important feature of such a multicomponent system appears to be the spatial intracellular organization of individual components. Recent studies demonstrate that the MAP kinases of such pathways are the molecular link between the plasma membrane sensors and the nuclear transcription factors. Stimulation of several MAPK pathways induces rapid and transient nuclear accumulation of MAP kinases. Investigations on the mode of regulation of this process using higher eukaryotes Erk2 and lower eukaryotes Hog1 and Sty1/Spc1 have revealed that at least three events contribute to signal-induced nuclear localization of these MAP kinases: activation by phosphorylation, regulated nuclear import and export, and nuclear retention.

A modified gene trap approach for improved high-throughput cancer drug discovery

While advances in laboratory automation has dramatically increased throughout of compound screening efforts, development of robust cell-based assays in relevant disease models remain resource-intensive and time-consuming, presenting a bottleneck to drug discovery campaigns. To address this issue, we present a modified gene trap approach to efficiently generate pathway-specific reporters that result in a robust "on" signal when the pathway of interest is inhibited. In this proof-of-concept study, we used vemurafenib and trametinib to identify traps that specifically detect inhibition of the mitogen-activated protein kinase (MAPK) pathway in a model of BRAFV600E driven human malignant melanoma. We demonstrate that insertion of our trap into particular loci results in remarkably specific detection of MAPK pathway inhibitors over compounds targeting any other pathway or cellular function. The accuracy of our approach was highlighted in a pilot screen of ~6000 compounds where 40 actives were detected, including 18 MEK, 10 RAF, and 3 ERK inhibitors along with a few compounds representing previously under-characterized inhibitors of the MAPK pathway. One such compound, bafetinib, a second generation BCR/ABL inhibitor, reduced phosphorylation of ERK and when combined with trametinib, both in vitro and in vivo, reduced growth of vemurafenib resistant melanoma cells. While piloted in a model of BRAF-driven melanoma, our results set the stage for using this approach to rapidly generate reporters against any transcriptionally active pathway across a wide variety of disease-relevant cell-based models to expedite drug discovery efforts.

Combined BRAF, EGFR, and MEK Inhibition in Patients with BRAFV600E-Mutant Colorectal Cancer

Although BRAF inhibitor monotherapy yields response rates >50% in BRAFV600-mutant melanoma, only approximately 5% of patients with BRAFV600E colorectal cancer respond. Preclinical studies suggest that the lack of efficacy in BRAFV600E colorectal cancer is due to adaptive feedback reactivation of MAPK signaling, often mediated by EGFR. This clinical trial evaluated BRAF and EGFR inhibition with dabrafenib (D) + panitumumab (P) ± MEK inhibition with trametinib (T) to achieve greater MAPK suppression and improved efficacy in 142 patients with BRAFV600E colorectal cancer. Confirmed response rates for D+P, D+T+P, and T+P were 10%, 21%, and 0%, respectively. Pharmacodynamic analysis of paired pretreatment and on-treatment biopsies found that efficacy of D+T+P correlated with increased MAPK suppression. Serial cell-free DNA analysis revealed additional correlates of response and emergence of KRAS and NRAS mutations on disease progression. Thus, targeting adaptive feedback pathways in BRAFV600E colorectal cancer can improve efficacy, but MAPK reactivation remains an important primary and acquired resistance mechanism.Significance: This trial demonstrates that combined BRAF + EGFR + MEK inhibition is tolerable, with promising activity in patients with BRAFV600E colorectal cancer. Our findings highlight the MAPK pathway as a critical target in BRAFV600E colorectal cancer and the need to optimize strategies inhibiting this pathway to overcome both primary and acquired resistance. Cancer Discov; 8(4); 428-43. ©2018 AACR.See related commentary by Janku, p. 389See related article by Hazar-Rethinam et al., p. 417This article is highlighted in the In This Issue feature, p. 371.

Modulation of Plasma Metabolite Biomarkers of the MAPK Pathway with MEK Inhibitor RO4987655: Pharmacodynamic and Predictive Potential in Metastatic Melanoma

MAPK pathway activation is frequently observed in human malignancies, including melanoma, and is associated with sensitivity to MEK inhibition and changes in cellular metabolism. Using quantitative mass spectrometry-based metabolomics, we identified in preclinical models 21 plasma metabolites including amino acids, propionylcarnitine, phosphatidylcholines, and sphingomyelins that were significantly altered in two B-RAF-mutant melanoma xenografts and that were reversed following a single dose of the potent and selective MEK inhibitor RO4987655. Treatment of non-tumor-bearing animals and mice bearing the PTEN-null U87MG human glioblastoma xenograft elicited plasma changes only in amino acids and propionylcarnitine. In patients with advanced melanoma treated with RO4987655, on-treatment changes of amino acids were observed in patients with disease progression and not in responders. In contrast, changes in phosphatidylcholines and sphingomyelins were observed in responders. Furthermore, pretreatment levels of seven lipids identified in the preclinical screen were statistically significantly able to predict objective responses to RO4987655. The RO4987655 treatment-related changes were greater than baseline physiological variability in nontreated individuals. This study provides evidence of a translational exo-metabolomic plasma readout predictive of clinical efficacy together with pharmacodynamic utility following treatment with a signal transduction inhibitor. Mol Cancer Ther; 16(10); 2315-23. ©2017 AACR.

MAPK Pathway Inhibitors Sensitize BRAF-Mutant Melanoma to an Antibody-Drug Conjugate Targeting GPNMB

PURPOSE

To determine if BRAF and/or MEK inhibitor-induced GPNMB expression renders melanomas sensitive to CDX-011, an antibody-drug conjugate targeting GPNMB.

EXPERIMENTAL DESIGN

The Cancer Genome Atlas melanoma dataset was interrogated for a panel of MITF-regulated melanosomal differentiation antigens, including GPNMB. BRAF-mutant melanoma cell lines treated with BRAF or MEK inhibitors were assessed for GPNMB expression by RT-qPCR, immunoblot, and FACS analyses. Transient siRNA-mediated knockdown approaches were used to determine if MITF is requirement for treatment-induced GPNMB upregulation. GPNMB expression was analyzed in serial biopsies and serum samples from patients with melanoma taken before, during, and after disease progression on MAPK inhibitor treatment. Subcutaneous injections were performed to test the efficacy of MAPK inhibitors alone, CDX-011 alone, or their combination in suppressing melanoma growth.

RESULTS

A MITF-dependent melanosomal differentiation signature is associated with poor prognosis in patients with this disease. MITF is increased following BRAF and MEK inhibitor treatment and induces the expression of melanosomal differentiation genes, including GPNMB. GPNMB is expressed at the cell surface in MAPK inhibitor-treated melanoma cells and is also elevated in on-treatment versus pretreatment biopsies from melanoma patients receiving MAPK pathway inhibitors. Combining BRAF and/or MEK inhibitors with CDX-011, an antibody-drug conjugate targeting GPNMB, is effective in causing melanoma regression in preclinical animal models and delays the recurrent melanoma growth observed with MEK or BRAF/MEK inhibitor treatment alone.

CONCLUSIONS

The combination of MAPK pathway inhibitors with an antibody-drug conjugate targeting GPNMB is an effective therapeutic option for patients with melanoma. Clin Cancer Res; 22(24); 6088-98. ©2016 AACR.

HDAC Inhibition Overcomes Acute Resistance to MEK Inhibition in BRAF-Mutant Colorectal Cancer by Downregulation of c-FLIPL

PURPOSE

Activating mutations in the BRAF oncogene are found in 8% to 15% of colorectal cancer patients and have been associated with poor survival. In contrast with BRAF-mutant (MT) melanoma, inhibition of the MAPK pathway is ineffective in the majority of BRAFMT colorectal cancer patients. Therefore, identification of novel therapies for BRAFMT colorectal cancer is urgently needed.

EXPERIMENTAL DESIGN

BRAFMT and wild-type (WT) colorectal cancer models were assessed in vitro and in vivo. Small-molecule inhibitors of MEK1/2, MET, and HDAC were used, overexpression and siRNA approaches were applied, and cell death was assessed by flow cytometry, Western blotting, cell viability, and caspase activity assays.

RESULTS

Increased c-MET-STAT3 signaling was identified as a novel adaptive resistance mechanism to MEK inhibitors (MEKi) in BRAFMT colorectal cancer models in vitro and in vivo. Moreover, MEKi treatment resulted in acute increases in transcription of the endogenous caspase-8 inhibitor c-FLIPL in BRAFMT cells, but not in BRAFWT cells, and inhibition of STAT3 activity abrogated MEKi-induced c-FLIPL expression. In addition, treatment with c-FLIP-specific siRNA or HDAC inhibitors abrogated MEKi-induced upregulation of c-FLIPL expression and resulted in significant increases in MEKi-induced cell death in BRAFMT colorectal cancer cells. Notably, combined HDAC inhibitor/MEKi treatment resulted in dramatically attenuated tumor growth in BRAFMT xenografts.

CONCLUSIONS

Our findings indicate that c-MET/STAT3-dependent upregulation of c-FLIPL expression is an important escape mechanism following MEKi treatment in BRAFMT colorectal cancer. Thus, combinations of MEKi with inhibitors of c-MET or c-FLIP (e.g., HDAC inhibitors) could be potential novel treatment strategies for BRAFMT colorectal cancer.

Antitumor activity in RAS-driven tumors by blocking AKT and MEK

PURPOSE

KRAS is the most commonly mutated oncogene in human tumors. KRAS-mutant cells may exhibit resistance to the allosteric MEK1/2 inhibitor selumetinib (AZD6244; ARRY-142886) and allosteric AKT inhibitors (such as MK-2206), the combination of which may overcome resistance to both monotherapies.

EXPERIMENTAL DESIGN

We conducted a dose/schedule-finding study evaluating MK-2206 and selumetinib in patients with advanced treatment-refractory solid tumors. Recommended dosing schedules were defined as MK-2206 at 135 mg weekly and selumetinib at 100 mg once daily.

RESULTS

Grade 3 rash was the most common dose-limiting toxicity (DLT); other DLTs included grade 4 lipase increase, grade 3 stomatitis, diarrhea, and fatigue, and grade 3 and grade 2 retinal pigment epithelium detachment. There were no meaningful pharmacokinetic drug-drug interactions. Clinical antitumor activity included RECIST 1.0-confirmed partial responses in non-small cell lung cancer and low-grade ovarian carcinoma.

CONCLUSION

Responses in KRAS-mutant cancers were generally durable. Clinical cotargeting of MEK and AKT signaling may be an important therapeutic strategy in KRAS-driven human malignancies (Trial NCT number NCT01021748).

Non-Small Cell Lung Cancer beyond Biomarkers: The Evolving Landscape of Clinical Trial Design

The approval of EGFR and ALK directed tyrosine kinase inhibitors materialized the concept of tailoring therapy on the basis of specific biomarkers for treating patients with NSCLC. Research for other biologics, although demonstrating clinical benefit, has been less successful so far for producing biomarkers that predict response. Blocking angiogenesis is the prototype for the agents that belong in the latter group that target specific molecules, yet they are currently approved for relatively unselected groups of patients. In order to meet the goal of personalizing care in the various settings of NSCLC, a wealth of biologics and compounds are currently being tested in clinical trials in different phases of clinical development. In a subset of the relevant studies, a biomarker perspective is appreciated. This review summarizes the clinical rationale of the major ongoing phase II and III NSCLC studies that employ targeting specific molecules with novel agents, as well as innovative strategies, and includes a comparative discussion of the different designs.

Effect of selumetinib vs chemotherapy on progression-free survival in uveal melanoma: a randomized clinical trial

IMPORTANCE

Uveal melanoma is characterized by mutations in GNAQ and GNA11, resulting in mitogen-activated protein kinase pathway activation.

OBJECTIVE

To assess the efficacy of selumetinib, a selective, non-adenosine triphosphate competitive inhibitor of MEK1 and MEK2, in uveal melanoma.

DESIGN, SETTING, AND PARTICIPANTS

Randomized, open-label, phase 2 clinical trial comparing selumetinib vs chemotherapy conducted from August 2010 through December 2013 among 120 patients with metastatic uveal melanoma at 15 academic oncology centers in the United States and Canada.

INTERVENTIONS

One hundred one patients were randomized in a 1:1 ratio to receive selumetinib, 75 mg orally twice daily on a continual basis (n = 50), or chemotherapy (temozolomide, 150 mg/m2 orally daily for 5 of every 28 days, or dacarbazine, 1000 mg/m2 intravenously every 21 days [investigator choice]; n = 51) until disease progression, death, intolerable adverse effects, or withdrawal of consent. After primary outcome analysis, 19 patients were registered and 18 treated with selumetinib without randomization to complete the planned 120-patient enrollment. Patients in the chemotherapy group could receive selumetinib at the time of radiographic progression.

MAIN OUTCOMES AND MEASURES

Progression-free survival, the primary end point, was assessed as of April 22, 2013. Additional end points, including overall survival, response rate, and safety/toxicity, were assessed as of December 31, 2013.

RESULTS

Median progression-free survival among patients randomized to chemotherapy was 7 weeks (95% CI, 4.3-8.4 weeks; median treatment duration, 8 weeks; interquartile range [IQR], 4.3-16 weeks) and among those randomized to selumetinib was 15.9 weeks (95% CI, 8.4-21.1 weeks; median treatment duration, 16.1 weeks; IQR, 8.1-25.3 weeks) (hazard ratio, 0.46; 95% CI, 0.30-0.71; P < .001). Median overall survival time was 9.1 months (95% CI, 6.1-11.1 months) with chemotherapy and 11.8 months (95% CI, 9.8-15.7 months) with selumetinib (hazard ratio, 0.66; 95% CI, 0.41-1.06; P = .09). No objective responses were observed with chemotherapy. Forty-nine percent of patients treated with selumetinib achieved tumor regression, with 14% achieving an objective radiographic response to therapy. Treatment-related adverse events were observed in 97% of patients treated with selumetinib, with 37% requiring at least 1 dose reduction.

CONCLUSIONS AND RELEVANCE

In this hypothesis-generating study of patients with advanced uveal melanoma, selumetinib compared with chemotherapy resulted in a modestly improved progression-free survival and response rate; however, no improvement in overall survival was observed. Improvement in clinical outcomes was accompanied by a high rate of adverse events.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01143402.

Failure to induce apoptosis via BCL-2 family proteins underlies lack of efficacy of combined MEK and PI3K inhibitors for KRAS-mutant lung cancers

Although several groups have demonstrated that concomitant use of MEK and phosphoinositide 3-kinase (PI3K) inhibitors (MEKi/PI3Ki) can induce dramatic tumor regressions in mouse models of KRAS-mutant non-small cell lung cancer (NSCLC), ongoing clinical trials investigating this strategy have been underwhelming to date. While efficacy may be hampered by a narrow therapeutic index, the contribution of biologic heterogeneity in the response of KRAS-mutant NSCLCs to MEKi/PI3Ki has been largely unexplored. In this study, we find that most human KRAS-mutant NSCLC cell lines fail to undergo marked apoptosis in response to MEKi/PI3Ki, which is key for tumor responsiveness in vivo. This heterogeneity of apoptotic response occurs despite relatively uniform induction of growth arrest. Using a targeted short hairpin RNA screen of BCL-2 family members, we identify BIM, PUMA, and BCL-XL as key regulators of the apoptotic response induced by MEKi/PI3Ki, with decreased expression of BIM and PUMA relative to BCL-XL in cell lines with intrinsic resistance. In addition, by modeling adaptive resistance to MEKi/PI3Ki both in vitro and in vivo, we find that, upon the development of resistance, tumors have a diminished apoptotic response due to downregulation of BIM and PUMA. These results suggest that the inability to induce apoptosis may limit the effectiveness of MEKi/PI3Ki for KRAS-mutant NSCLCs by contributing to intrinsic and adaptive resistance to this therapy.

Advances in adjuvant therapy: potential for prognostic and predictive biomarkers

Melanoma is the third most common skin cancer but accounts for the majority of skin cancer-related mortality. The rapidly rising incidence and younger age at diagnosis has made melanoma a leading cause of lost productive years of life and has increased the urgency of finding improved adjuvant therapy for melanoma. Interferon-α was approved for the adjuvant treatment of resected high-risk melanoma following studies that demonstrated improvements in relapse-free survival and overall survival that were commenced nearly 30 years ago. The clinical benefits associated with this agent have been consistently observed across multiple studies and meta-analyses in terms of relapse rate, and to a smaller and less-consistent degree, mortality. However, significant toxicity and lack of prognostic and/or predictive biomarkers that would allow greater risk-benefit ratio have limited the more widespread adoption of this modality.Recent success with targeted agents directed against components of the MAP-kinase pathway and checkpoint inhibitors have transformed the treatment landscape in metastatic disease. Current research efforts are centered around discovering predictive/prognostic biomarkers and exploring the options for more effective regimens, either singly or in combination.

The role of PKC signaling in CRF-induced modulation of startle

RATIONALE

Hypersignaling of corticotropin releasing factor (CRF) has been implicated in stress disorders; however, many of its downstream mechanisms of action remain unclear. In vitro, CRF1 receptor activation initiates multiple cell signaling cascades, including protein kinase A (PKA), protein kinase C (PKC), and mitogen-activated protein kinase kinase MEK1/2 signaling. It is unclear, however, which of these signaling cascades mediate CRF-induced behaviors during stress.

OBJECTIVES

We examined the role of PKA, PKC, and MEK1/2 signaling pathways in CRF-induced anxiety as measured by startle hyperreactivity.

METHODS

Mice treated with intracerbroventricular (ICV) ovine CRF (oCRF) were pretreated with the PKA inhibitor Rp-cAMPS, PKC inhibitor bisindolylmaleimide (BIM), or MEK1/2 inhibitor PD98059 (ICV) and assessed for acoustic startle reactivity.

RESULTS

The PKC inhibitor BIM significantly attenuated CRF-induced increases in startle. BIM was also able to block startle increases induced by oCRF when both compounds were infused directly into the bed nucleus of stria terminalis (BNST). PKA and MEK1/2 inhibition had no significant effects on CRF-induced changes in startle at the dose ranges tested. CRF-induced disruption of prepulse inhibition was not significantly reversed by any of the three pretreatments at the dose ranges tested.

CONCLUSIONS

PKC signaling is required for CRF-induced increases in startle, and this effect is mediated at least in part at the BNST. These findings suggest that PKC signaling cascades (1) may be important for the acute effects of CRF to induce startle hyperreactivity and (2) support further research of the role of PKC signaling in startle abnormalities relevant to disorders such as posttraumatic stress disorder.

Acute tumour response to the MEK1/2 inhibitor selumetinib (AZD6244, ARRY-142886) evaluated by non-invasive diffusion-weighted MRI

BACKGROUND

Non-invasive imaging biomarkers underpin the development of molecularly targeted anti-cancer drugs. This study evaluates tumour apparent diffusion coefficient (ADC), measured by diffusion-weighted magnetic resonance imaging (DW-MRI), as a biomarker of response to the MEK1/2 inhibitor selumetinib (AZD6244, ARRY-142886) in human tumour xenografts.

METHODS

Nude mice bearing human BRAF(V600D) WM266.4 melanoma or BRAF(V600E) Colo205 colon carcinoma xenografts were treated for 4 days with vehicle or selumetinib. DW-MRI was performed before and 2 h after the last dose and excised tumours analysed for levels of phospho-ERK1/2, cleaved caspase 3 (CC3) and necrosis.

RESULTS

Selumetinib treatment induced tumour stasis and reduced ERK1/2 phosphorylation in both WM266.4 and Colo205 tumour xenografts. Relative to day 0, mean tumour ADC was unchanged in the control groups but was significantly increased by up to 1.6-fold in selumetinib-treated WM266.4 and Colo205 tumours. Histological analysis revealed a significant increase in necrosis in selumetinib-treated WM266.4 and Colo205 xenografts and CC3 staining in selumetinib-treated Colo205 tumours relative to controls.

CONCLUSION

Changes in ADC following treatment with the MEK1/2 inhibitor selumetinib in responsive human tumour xenografts were concomitant with induction of tumour cell death. ADC may provide a useful non-invasive pharmacodynamic biomarker for early clinical assessment of response to selumetinib and other MEK-ERK1/2 signalling-targeted therapies.

Signal transduction inhibitors in treatment of myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are a group of hematologic disorders characterized by ineffective hematopoiesis that results in reduced blood counts. Although MDS can transform into leukemia, most of the morbidity experienced by these patients is due to chronically low blood counts. Conventional cytotoxic agents used to treat MDS have yielded some encouraging results but are characterized by many adverse effects in the predominantly elderly patient population. Targeted interventions aimed at reversing the bone marrow failure and increasing the peripheral blood counts would be advantageous in this cohort of patients. Studies have demonstrated over-activated signaling of myelo-suppressive cytokines such as TGF-β, TNF-α and Interferons in MDS hematopoietic stem cells. Targeting these signaling cascades could be potentially therapeutic in MDS. The p38 MAP kinase pathway, which is constitutively activated in MDS, is an example of cytokine stimulated kinase that promotes aberrant apoptosis of stem and progenitor cells in MDS. ARRY-614 and SCIO-469 are p38 MAPK inhibitors that have been used in clinical trials and have shown activity in a subset of MDS patients. TGF-β signaling has been therapeutically targeted by small molecule inhibitor of the TGF-β receptor kinase, LY-2157299, with encouraging preclinical results. Apart from TGF-β receptor kinase inhibition, members of TGF-β super family and BMP ligands have also been targeted by ligand trap compounds like Sotatercept (ACE-011) and ACE-536. The multikinase inhibitor, ON-01910.Na (Rigosertib) has demonstrated early signs of efficacy in reducing the percentage of leukemic blasts and is in advanced stages of clinical testing. Temsirolimus, Deforolimus and other mTOR inhibitors are being tested in clinical trials and have shown preclinical efficacy in CMML. EGF receptor inhibitors, Erlotinib and Gefitinib have shown efficacy in small trials that may be related to off target effects. Cell cycle regulator inhibitors such as Farnesyl transferase inhibitors (Tipifarnib, Lonafarnib) and MEK inhibitor (GSK1120212) have shown acceptable toxicity profiles in small studies and efforts are underway to select mutational subgroups of MDS and AML that may benefit from these inhibitors. Altogether, these studies show that targeting various signal transduction pathways that regulate hematopoiesis offers promising therapeutic potential in this disease. Future studies in combination with high resolution correlative studies will clarify the subgroup specific efficacies of these agents.

MEK inhibition leads to lysosome-mediated Na+/I- symporter protein degradation in human breast cancer cells

The Na(+)/I(-) symporter (NIS (SLC5A5)) is a transmembrane glycoprotein that mediates active iodide uptake into thyroid follicular cells. NIS-mediated iodide uptake in thyroid cells is the basis for targeted radionuclide imaging and treatment of differentiated thyroid carcinomas and their metastases. Furthermore, NIS is expressed in many human breast tumors but not in normal non-lactating breast tissue, suggesting that NIS-mediated radionuclide uptake may also allow the imaging and targeted therapy of breast cancer. However, functional cell surface NIS expression is often low in breast cancer, making it important to uncover signaling pathways that modulate NIS expression at multiple levels, from gene transcription to posttranslational processing and cell surface trafficking. In this study, we investigated NIS regulation in breast cancer by MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK) signaling, an important cell signaling pathway involved in oncogenic transformation. We found that MEK inhibition decreased NIS protein levels in all-trans retinoic acid/hydrocortisone-treated MCF-7 cells as well as human breast cancer cells expressing exogenous NIS. The decrease in NIS protein levels by MEK inhibition was not accompanied by a decrease in NIS mRNA or a decrease in NIS mRNA export from the nucleus to the cytoplasm. NIS protein degradation upon MEK inhibition was prevented by lysosome inhibitors but not by proteasome inhibitors. Interestingly, NIS protein level was correlated with MEK/ERK activation in human breast tumors from a tissue microarray. Taken together, MEK activation appears to play an important role in maintaining NIS protein stability in human breast cancers.

Inhibition of both BRAF and MEK in BRAF(V600E) mutant melanoma restores compromised dendritic cell (DC) function while having differential direct effects on DC properties

PURPOSE

Dendritic cells (DCs) can induce strong tumor-specific T-cell immune responses. Constitutive upregulation of the mitogen-activated protein kinase (MAPK) pathway by a BRAF(V600) mutation, which is present in about 50 % of metastatic melanomas, may be linked to compromised function of DCs in the tumor microenvironment. Targeting both MEK and BRAF has shown efficacy in BRAF(V600) mutant melanoma.

METHODS

We co-cultured monocyte-derived human DCs with melanoma cell lines pretreated with the MEK inhibitor U0126 or the BRAF inhibitor vemurafenib. Cytokine production (IL-12 and TNF-α) and surface marker expression (CD80, CD83, and CD86) in DCs matured with the Toll-like receptor 3/Melanoma Differentiation-Associated protein 5 agonist polyI:C was examined. Additionally, DC function, viability, and T-cell priming capacity were assessed upon direct exposure to U0126 and vemurafenib.

RESULTS

Cytokine production and co-stimulation marker expression were suppressed in polyI:C-matured DCs exposed to melanoma cells in co-cultures. This suppression was reversed by MAPK blockade with U0126 and/or vemurafenib only in melanoma cell lines carrying a BRAF(V600E) mutation. Furthermore, when testing the effect of U0126 directly on DCs, marked inhibition of function, viability, and DC priming capacity was observed. In contrast, vemurafenib had no effect on DC function across a wide range of dose concentrations.

CONCLUSIONS

BRAF(V600E) mutant melanoma cells modulate DC through the MAPK pathway as its blockade can reverse suppression of DC function. MEK inhibition negatively impacts DC function and viability if applied directly. In contrast, vemurafenib does not have detrimental effects on important functions of DCs and may therefore be a superior candidate for combination immunotherapy approaches in melanoma patients.

The synergistic interaction of MEK and PI3K inhibitors is modulated by mTOR inhibition

BACKGROUND

Combined targeting of MAPK and PI3K signalling pathways may be necessary for optimal therapeutic activity in cancer. This study evaluated the MEK inhibitors AZD6244 and PD0325901, alone and in combination with the dual mTOR/PI3K inhibitor NVP-BEZ235 or the PI3K inhibitor GDC-0941, in three colorectal cancer cell lines.

METHODS

Growth inhibition, survival and signal transduction were measured using the Sulforhodamine B assay, clonogenicity and western blotting, respectively, in HCT116, HT29 and DLD1 cell lines.

RESULTS

All MEK/PI3K inhibitor combinations exhibited marked synergistic growth inhibition; however, GDC-0941 displayed greater synergy in combination with either MEK inhibitor. NVP-BEZ235 exhibited stronger inhibition of 4EBP1 phosphorylation, and similar inhibition of S6 and AKT phosphorylation, compared with GDC-0941. Both PD0325901 and AZD6244 inhibited ERK phosphorylation, and with MEK/PI3K inhibitor combinations inhibition of S6 phosphorylation was increased. The reduced synergy exhibited by NVP-BEZ235 in combination with MEK inhibitors, compared with GDC-0941, may be due to inhibition of mTOR, and the addition of the mTORC1/2 inhibitor KU0063794 compromised the synergy of GDC-0941:PD0325901 combinations.

CONCLUSION

These studies confirm that dual targeting of PI3K and MEK can induce synergistic growth inhibition; however, the combination of specific PI3K inhibitors, rather than dual mTOR/PI3K inhibitors, with MEK inhibitors results in greater synergy.

Partners in crime: the TGFβ and MAPK pathways in cancer progression

The TGFβ and Ras-MAPK pathways play critical roles in cell development and cell cycle regulation, as well as in tumor formation and metastasis. In the absence of cellular transformation, these pathways operate in opposition to one another, where TGFβ maintains an undifferentiated cell state and suppresses proliferation, while Ras-MAPK pathways promote proliferation, survival and differentiation. However, in colorectal and pancreatic cancers, the opposing pathways' mechanisms are simultaneously activated in order to promote cancer progression and metastasis. Here, we highlight the roles of the TGFβ and Ras-MAPK pathways in normal and malignant states, and provide an explanation for how the concomitant activation of these pathways drives tumor biology. Finally, we survey potential therapeutic targets in these pathways.

Hydrogen peroxide signaling is required for glucocorticoid-induced apoptosis in lymphoma cells

Glucocorticoid-induced apoptosis is exploited clinically for the treatment of hematologic malignancies. Determining the required molecular events for glucocorticoid-induced apoptosis will identify resistance mechanisms and suggest strategies for overcoming resistance. In this study, we found that glucocorticoid treatment of WEHI7.2 murine thymic lymphoma cells increased the steady-state [H(2)O(2)] and oxidized the intracellular redox environment before cytochrome c release. Removal of glucocorticoids after the H(2)O(2) increase resulted in a 30% clonogenicity; treatment with PEG-CAT increased clonogenicity to 65%. Human leukemia cell lines also showed increased H(2)O(2) in response to glucocorticoids and attenuated apoptosis after PEG-CAT treatment. WEHI7.2 cells that overexpress catalase (CAT2, CAT38) or were selected for resistance to H(2)O(2) (200R) removed enough of the H(2)O(2) generated by glucocorticoids to prevent oxidation of the intracellular redox environment. CAT2, CAT38, and 200R cells showed a 90-100% clonogenicity. The resistant cells maintained pERK survival signaling in response to glucocorticoids, whereas the sensitive cells did not. Treating the resistant cells with a MEK inhibitor sensitized them to glucocorticoids. These data indicate that: (1) an increase in H(2)O(2) is necessary for glucocorticoid-induced apoptosis in lymphoid cells, (2) increased H(2)O(2) removal causes glucocorticoid resistance, and (3) MEK inhibition can sensitize oxidative stress-resistant cells to glucocorticoids.

Silencing of BNIP3 results from promoter methylation by DNA methyltransferase 1 induced by the mitogen-activated protein kinase pathway

We have previously shown that Ras mediates NO-induced BNIP3 expression via the MEK-E RK-HIF-1 pathway i n mouse macrophages, and that NO-induced death results at least in part from the induction of BNIP3. In the present study, we describe another aspect of Ras regulation of BNIP3 expression in pancreatic cancer cells. Human BNIP3 promoter-driven luciferase activity was efficiently induced by activated Ras in AsPC-1, Miapaca-2, PK-1 and PANC-1 cells. However, expression of endogenous BNIP3 was not induced, and BNIP3 up-regulation by hypoxia was also inhibited. Treatment of the cells with the DNMT inhibitor, 5-aza-2-deoxycytidine, restored BNIP3 induction, indicating that DNA methylation of the BNIP3 promoter was responsible for the inhibition of BNIP3 induction. Furthermore, inhibition of the MEK pathway with U0126 reduced DNMT1 expression, but not that of DNMT3a and 3b, and restored the hypoxia-inducibility of BNIP3, suggesting that the DNA methylation of the BNIP3 promoter was mediated by DNMT1 via the MEK pathway.

Activated MEK cooperates with Ink4a/Arf loss or Akt activation to induce gliomas in vivo

The RAS/RAF mitogen-activated protein kinase pathway (MAPK) is highly active in many tumor types including the majority of high-grade gliomas and expression of activated RAS or RAF in neural progenitor cells combined with either AKT activation or Ink4a/Arf loss leads to the development of high-grade gliomas in vivo. This strongly suggests that this pathway is necessary for glioma formation and maintenance. To further define the role of this pathway in the development of high-grade gliomas, we used the established RCAS/TVA glioma mouse model to test the ability of activated MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK), a RAF effector, to induce tumors in vivo in the context of activated AKT or Ink4a/Arf loss. Although expression of activated MEK alone in neural progenitor cells is not sufficient for tumorigenesis, the combination of activated MEK and AKT or MEK with Ink4a/Arf loss is transforming. The data reveal that activation of the classical RAS/MAPK pathway, which is mediated through MEK, leads to the development of high-grade gliomas in vivo and suggest that MEK may be a relevant target for glioma therapy. To test this, we treated both mouse and human glioma cells with the MEK inhibitor PD0325901. Although this treatment induced apoptosis in a significant percentage of the cells, the effect was enhanced by combined treatment with the phosphatidylinositol 3-kinase (PI3K)/mTOR inhibitor NVP-BEZ235. Our results demonstrate that combined inhibition of MEK and PI3K/mTOR is a rational strategy for the treatment of high-grade gliomas and may be an effective adjuvant therapy for this disease.

Acquired resistance to BRAF inhibitors mediated by a RAF kinase switch in melanoma can be overcome by cotargeting MEK and IGF-1R/PI3K

BRAF is an attractive target for melanoma drug development. However, resistance to BRAF inhibitors is a significant clinical challenge. We describe a model of resistance to BRAF inhibitors developed by chronic treatment of BRAF(V)⁶⁰⁰(E) melanoma cells with the BRAF inhibitor SB-590885; these cells are cross-resistant to other BRAF-selective inhibitors. Resistance involves flexible switching among the three RAF isoforms, underscoring the ability of melanoma cells to adapt to pharmacological challenges. IGF-1R/PI3K signaling was enhanced in resistant melanomas, and combined treatment with IGF-1R/PI3K and MEK inhibitors induced death of BRAF inhibitor-resistant cells. Increased IGF-1R and pAKT levels in a post-relapse human tumor sample are consistent with a role for IGF-1R/PI3K-dependent survival in the development of resistance to BRAF inhibitors.

Selective Raf inhibition in cancer therapy

Over the past 5 years, the Raf kinase family has emerged as a promising target for protein-directed cancer therapy development. The goal of this review is to first provide a concise summary of the data validating Raf proteins as high-interest therapeutic targets. The authors then outline the mode of action of Raf kinases, emphasizing how Raf activities and protein interactions suggest specific approaches to inhibiting Raf. The authors then summarize the set of drugs, antisense reagents and antibodies available or in development for therapeutically targeting Raf or Raf-related proteins, as well as existing strategies combining these and other therapeutic agents. Finally, the authors discuss recent results from systems biology analyses that have the potential to increasingly guide the intelligent selection of combination therapies involving Raf-targeting agents and other therapeutics.

Receptor-mediated tobacco toxicity: alterations of the NF-kappaB expression and activity downstream of alpha7 nicotinic receptor in oral keratinocytes

To gain a mechanistic insight into nicotinic receptor-dependent morbidity of tobacco products in the oral cavity, we studied effects of exposures of normal human oral keratinocytes (KCs) for 24 h to environmental tobacco smoke (ETS) vs. equivalent concentration of pure nicotine. The exposed KCs showed a multifold increase of nuclear factor-kappaB (NF-kappaB) at the mRNA and protein levels, which could be significantly (p<0.05) diminished by alpha-bungarotoxin or transfection with anti-alpha7 small interfering RNA. An increased protein-binding activity of NF-kappaB also could be prevented by blocking alpha7 signaling. The use of pathway inhibitors demonstrated that the Ras/Raf-1/MEK1/ERK steps mediated alpha7-dependent upregulation of NF-kappaB. Thus, exposure of KCs to tobacco may lead to the pathobiologic effects via an intracellular signaling pathway downstream of alpha7 that proceeds through the Ras/Raf-1/MEK1/ERK steps leading to upregulated expression and transactivation of NF-kappaB.

Semaphorin 3F antagonizes neurotrophin-induced phosphatidylinositol 3-kinase and mitogen-activated protein kinase kinase signaling: a mechanism for growth cone collapse

Peripheral nerve growth is regulated by the coordinated action of numerous external stimuli, including positively acting neurotrophin-derived growth cues and restrictive semaphorin cues. Here, we show that Semaphorin 3F (Sema 3F) can antagonize nerve growth factor (NGF)-stimulated TrkA (tyrosine receptor kinase A) signaling in sympathetic neurons, thereby apparently contributing to growth cone collapse. Sema 3F suppressed NGF-induced activation of the phosphatidylinositol 3 (PI3)-kinase-Akt and MEK (mitogen-activated protein kinase kinase)-ERK (extracellular signal-regulated kinase) pathways, both of which we show to be required to maintain growth cone structure. Sema 3F-induced growth cone collapse was partially reversed by sustained activation of the PI3-kinase and MEK pathways, which was achieved by overexpression of the Gab-1 (growth-associated binder 1) docking protein. These data indicate that a novel mechanism used by Sema 3F to collapse growth cones in sympathetic neurons is to dampen neurotrophin signaling, providing an intracellular mechanism for cross talk between positive and negative axon growth cues.

Evaluation of clinical significance of TP53, BCL-2, BAX and MEK1 expression in 229 ovarian carcinomas treated with platinum-based regimen

In cell line studies, BCL-2, BAX, as well as novel MEK1 protein levels have strong influence on ovarian cancer response to cisplatin-based chemotherapy. However, such associations have not been demonstrated clinically. We evaluated prognostic/predictive significance of these proteins with regard to TP53 status. Immunohistochemical analysis was performed on 229 ovarian carcinomas FIGO stage IIB-IV treated with platinum-based chemotherapy; the results were analysed by the Cox and logistic regression models. Clinical parameters (residual tumour size, patient age, FIGO stage) were the only indicators of overall survival (OS) and the strongest predictors of complete remission (CR). On the other hand, BAX expression was the strongest (P=0.005) or the only (in FIGO IIIC, P=0.02) prognostic indicator of disease-free survival (DFS) in the TP53(+) group. TP53(+) and TP53(-) ovarian carcinomas differed in clinical and molecular prognostic and predictive factors. Another novel finding is that CR was negatively influenced by high BAX expression in all patients group (P=0.047) and by BCL2 expression in the TP53(-) group (P=0.05). High MEK1 expression was associated with endometrioid and clear cell carcinomas (P=0.049); its loss was found with advancing FIGO stage (P=0.002). Our results suggest that binomial TP53 status divides ovarian carcinomas into two biologically distinct groups. BAX expression is an important factor of DFS in the TP53(+) group. BCL-2 and BAX, but not MEK1 expressions have predictive value in ovarian cancer patients treated with platinum-based chemotherapy.

Mutant K-ras oncogene regulates steroidogenesis of normal human adrenocortical cells by the RAF-MEK-MAPK pathway

The result of our previous study has shown that the K-ras mutant (pK568MRSV) transfected human adrenocortical cells can significantly increase cortisol production and independently cause cell transformation. The aim of this study is to investigate the effect of the active K-ras oncogene on the cortisol production in normal human adrenocortical cells. First we used isopropyl thiogalactoside to induce the inducible mutant K-ras expression plasmid, pK568MRSV, in the stable transfected human adrenocortical cells. The result showed that the increase of RasGTP levels in transfected cells was time-dependent after isopropyl thiogalactoside induction. Additionally, results from Western blot analysis revealed significant elevation in phosphorylation of c-Raf-1 and Mitogen-activated protein kinase. We also detected the levels of mRNA encoding Cholesterol side-chain cleavage enzyme (P450(SCC)), 17alpha-Hydroxylase/17,20-lyase (P450(c17)) and 3beta-Hydroxysteroid dehydrogenase (3betaHSD) were increased in human adrenocortical cells transfected with mutant K-ras after IPTG treatment. The increase of mRNA amount in P450(scc) P450(c17) and 3betaHSD and the elevation of cortisol level were inhibited with a pretreatment of PD098059, a specific extracellular signal-regulated kinase inhibitor. In our previous report, we proved that lovastatin, a pharmacological inhibitor of p21(ras) function, also reversed the increase of cortisol level in mutant K-ras stably transfected human adrenocortical cells. Taken together, these findings proved that the active mutant Ras enhanced not only cell proliferation but also steroidogenesis in steroidogenic phenotype cells by activating Raf-MEK-MAPK related signal transduction pathway. Therefore, we believe that K-ras mutants influence regulation of steroidogenesis in adrenocortical cells through RAF-MEK-MAPK pathway.

Inhibition of the MEK1/ERK pathway reduces arachidonic acid release independently of cPLA2 phosphorylation and translocation

BACKGROUND

The 85-kDa cytosolic phospholipase A2 (cPLA2) mediates arachidonic acid (AA) release in MDCK cells. Although calcium and mitogen-activated protein kinases regulate cPLA2, the correlation of cPLA2 translocation and phosphorylation with MAPK activation and AA release is unclear.

RESULTS

MEK1 inhibition by U0126 inhibited AA release in response to ATP and ionomycin. This directly correlated with inhibition of ERK activation but not with phosphorylation of cPLA2 on Ser505, which was only partially inhibited by ERK inhibition. Inhibition of AA release by U0126 was still observed when stoichiometric phosphorylation of cPLA2 on Ser505 was maintained by activating p38 with anisomycin. Translocation kinetics of wild-type cPLA2 and cPLA2 containing S505A or S727A mutations to Golgi were similar in response to ATP and ionomycin and were not affected by U0126.

CONCLUSIONS

These results suggest that the ability of cPLA2 to hydrolyze membrane phospholipid is reduced by inhibition of the MEK1/ERK pathway and that the reduction in activity is independent of cPLA2 phosphorylation and translocation to membrane. The results also demonstrate that cPLA2 mutated at the phosphorylation sites Ser505 and Ser727 translocated with similar kinetic as wild-type cPLA2.

Kainate receptor-mediated apoptosis in primary cultures of cerebellar granule cells is attenuated by mitogen-activated protein and cyclin-dependent kinase inhibitors

1. Previous studies have suggested that neuronal apoptosis is the result of an abortive attempt to re-enter the cell cycle, and more recently the cyclin-dependent (CDKs) and the mitogen-activated protein (MAP) kinases, two superfamilies of kinases that influence and control cell cycle progression, have been implicated in neuronal apoptosis. 2. Here, to examine whether CDK/MAPK related pathways are involved in excitotoxicity, we studied the actions of various kinase inhibitors on apoptosis induced by the ionotropic glutamate (Glu) receptor agonist, kainate (KA), in primary cultures of murine cerebellar granule cells (CGCs). 3. KA-mediated neurotoxicity was concentration-dependent, as determined by a cell viability assay monitoring the reduction of 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and largely apoptotic in nature, as shown by morphological examination and labelling of DNA fragmentation in situ using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP digoxigenin nick-end labelling (TUNEL). 4. KA-mediated neurotoxicity and apoptosis was completely attenuated by the mixed CDK and MAP kinase inhibitor, olomoucine, in a concentration-dependent manner (50 - 600 microM), and partially by roscovitine (1 - 100 microM), a more selective CDK inihibitor. 5. The p38 MAP kinase inhibitor, SB203580 (1 - 100 microM), partially attenuated KA receptor-mediated apoptosis, as did the MAP kinase kinase inhibitors PD98509 (1 - 100 microM) and U0126 (1 - 100 microM). 6. These findings provide new evidence for a complex network of interacting pathways involving CDK/MAPK that control apoptosis downstream of KA receptor activation in excitotoxic neuronal cell death.

KSR is a scaffold required for activation of the ERK/MAPK module

Mechanisms that regulate signal propagation through the ERK/MAPK pathway are still poorly understood. Several proteins are suspected to play critical roles in this process. One of these is Kinase Suppressor of Ras (KSR), a component previously identified in RAS-dependent genetic screens in Drosophila and Caenorhabditis elegans. Here, we show that KSR functions upstream of MEK within the ERK/MAPK module. In agreement with this, we found that KSR facilitates the phosphorylation of MEK by RAF. We further show that KSR associates independently with RAF and MEK, and that these interactions lead to the formation of a RAF/MEK complex, thereby positioning RAF in close proximity to its substrate MEK. These findings suggest that KSR functions as a scaffold that assembles the RAF/MEK functional pair.

Isolation and characterization of pmk-(1-3): three p38 homologs in Caenorhabditis elegans

p38, a member of the mitogen-activated protein kinase (MAPK) superfamily, is activated in response to a variety of cellular stresses and ligands. Since the genome of the nematode C. elegans has been sequenced, we sought to identify and characterize the nematode homolog of mammalian p38. By sequence analysis and RT-PCR, we isolated cDNAs encoding three kinases, PMK-1, PMK-2, and PMK-3, which we call p38 map kinases due to their high sequence identity with p38. The three genes are contiguous on chromosome IV and comprise an operon. By use of a GFP reporter, we found that the promoter of the pmks is active throughout the intestine. An active form of MAPK/ERK kinase 6 (MEK6) phosphorylated and activated recombinant PMK-1 and PMK-2 in vitro. PMK-1 and PMK-2 phosphorylated activating transcription factor-2 (ATF-2), indicating an activity similar to mammalian p38. When transfected into mammalian cells, these kinases, like p38, are stimulated by osmotic stresses.

Mitogenic effect of oxidized low-density lipoprotein on vascular smooth muscle cells mediated by activation of Ras/Raf/MEK/MAPK pathway

1. It has been demonstrated that oxidized low-density lipoprotein (OX-LDL) is a risk factor in atherosclerosis by stimulating vascular smooth muscle cell (VSMC) proliferation. However, the mechanisms of OX-LDL-induced cell proliferation are not completely understood. Therefore, we investigated the effect of OX-LDL on cell proliferation associated with mitogen-activated protein kinase (MAPK) activation in rat cultured VSMCs. 2. Both native-LDL (N-LDL) and OX-LDL induced a time- and concentration-dependent incorporation of [(3)H]-thymidine in VSMCs. 3. OX-LDL induced time- and concentration-dependent phosphorylation of p42/p44 MAPK. Pretreatment of these cells with pertussis toxin or U73122 attenuated the OX-LDL-induced responses. 4. Pretreatment with PMA for 24 h, preincubation with a PKC inhibitor staurosporine or the tyrosine kinase inhibitors, genistein and herbimycin A for 1 h, substantially reduced [(3)H]-thymidine incorporation and p42/p44 MAPK phosphorylation induced by OX-LDL. 5. Removal of Ca(2+) by BAPTA/AM or depletion of the internal Ca(2+) pool by thapsigargin significantly inhibited OX-LDL-induced [(3)H]-thymidine incorporation and p42/p44 MAPK phosphorylation. 6. OX-LDL-induced [(3)H]-thymidine incorporation and p42/p44 MAPK phosphorylation was inhibited by PD98059 (an inhibitor of MEK1/2) and SB203580 (an inhibitor of p38 MAPK) in a concentration-dependent manner. 7. Overexpression of dominant negative mutants of Ras (H-Ras-15A) and Raf (Raf-N4) significantly suppressed MEK1/2 and p42/p44 MAPK activation induced by OX-LDL and PDGF-BB, indicating that Ras and Raf may be required for activation of these kinases. 8. These results suggest that the mitogenic effect of OX-LDL is mediated through a PTX-sensitive G protein-coupled receptor that involves the activation of the Ras/Raf/MEK/MAPK pathway similar to that of PDGF-BB in rat cultured VSMCs.

A novel 14-kilodalton protein interacts with the mitogen-activated protein kinase scaffold mp1 on a late endosomal/lysosomal compartment

We have identified a novel, highly conserved protein of 14 kD copurifying with late endosomes/lysosomes on density gradients. The protein, now termed p14, is peripherally associated with the cytoplasmic face of late endosomes/lysosomes in a variety of different cell types. In a two-hybrid screen with p14 as a bait, we identified the mitogen-activated protein kinase (MAPK) scaffolding protein MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK) partner 1 (MP1) as an interacting protein. We confirmed the specificity of this interaction in vitro by glutathione S-transferase pull-down assays and by coimmunoprecipitation, cosedimentation on glycerol gradients, and colocalization. Moreover, expression of a plasma membrane-targeted p14 causes mislocalization of coexpressed MP1. In addition, we could reconstitute protein complexes containing the p14-MP1 complex associated with ERK and MEK in vitro.The interaction between p14 and MP1 suggests a MAPK scaffolding activity localized to the cytoplasmic surface of late endosomes/lysosomes, thereby combining catalytic scaffolding and subcellular compartmentalization as means to modulate MAPK signaling within a cell.

Blockade of NGF-induced neurite outgrowth by a dominant-negative inhibitor of the egr family of transcription regulatory factors

Although it is well established that members of the Egr family of transcription regulatory factors are induced in many neuronal plasticity paradigms, it is still unclear what role, if any, they play in this process. Because NGF stimulation of pheochromocytoma 12 cells elicits a robust induction of Egr family members, we have investigated their role in mediating long-term effects elicited by NGF in these cells by using the Egr zinc finger DNA-binding domain as a selective antagonist of Egr family-mediated transcription. We report that expression of this Egr inhibitor construct suppresses neurite outgrowth elicited by NGF but not by dibutyryl cAMP. To check that this Egr inhibitor construct does not act by blocking the MEK/ERK pathway, which is known to mediate NGF-induced neurite outgrowth, we confirmed that the Egr inhibitor construct does not block NGF activation of Elk1-mediated transcription, a response that is dependent on this pathway. Conversely, inhibition of MEK does not impair Egr family-mediated transcription. Thus, we conclude (1) that induction of Egr family members and activation of the MEK/ERK pathway by NGF are mediated by separate signaling pathways and (2) that both are required to trigger neurite outgrowth induced by NGF.

Modulating sensitivity to drug-induced apoptosis: the future for chemotherapy?

Drug resistance is a fundamental problem in the treatment of most common human cancers. Our understanding of the cellular mechanisms underlying death and survival has allowed the development of rational approaches to overcoming drug resistance. The mitogen activated protein kinase family of protein serine/threonine kinases has been implicated in this complex web of signalling, with some members acting to enhance death and other members to prevent it. A recent publication by MacKeigan et al is the first to demonstrate an enhancement of drug-induced cell death by simultaneous blockade of MEK-mediated survival signalling, and offers the potential for targeted adjuvant therapy as a means of overcoming drug resistance.

Lipopolysaccharide enhances bradykinin-induced signal transduction via activation of Ras/Raf/MEK/MAPK in canine tracheal smooth muscle cells

Bacterial lipopolysaccharide (LPS) was found to induce inflammatory responses and to enhance bronchial hyperreactivity to several contractile agonists. However, the implication of LPS in the pathogenesis of bronchial hyperreactivity was not completely understood. Therefore, in this study, we investigated the effect of LPS on mitogen-activated protein kinase (MAPK) activation associated with potentiation of bradykinin (BK)-induced inositol phosphates (IPs) accumulation and Ca(2+) mobilization in canine cultured tracheal smooth muscle cells (TSMCs). LPS stimulated phosphorylation of p42/p44 MAPK in a time- and concentration-dependent manner using a Western blot analysis against a specific phosphorylated form of MAPK antibody. Maximal stimulation of the p42 and p44 MAPK isoforms occurred after 7 min-incubation and the maximal effect was achieved with 100 microg ml(-1) LPS. Pretreatment of TSMCs with LPS potentiated BK-induced IPs accumulation and Ca(2+) mobilization. However, there was no effect on the IPs response induced by endothelin-1, 5-hydroxytryptamine, and carbachol. In addition, pretreatment with PDGF-BB enhanced BK-induced IPs response. These enhancements by LPS and PDGF-BB might be due to an increase in BK B(2) receptor density (B(max)) in TSMCs, characterized by competitive inhibition of [(3)H]-BK binding using B(1) and B(2) receptor-selective reagents. The enhancing effects of LPS and PDGF-BB were attenuated by PD98059, an inhibitor of MAPK kinase (MEK), suggesting that the effect of LPS may share a common signalling pathway with PDGF-BB in TSMCs. Furthermore, overexpression of dominant negative mutants, H-Ras-15A and Raf-N4, significantly suppressed p42/p44 MAPK activation induced by LPS and PDGF-BB, indicating that Ras and Raf may be required for activation of these kinases. These results suggest that the augmentation of BK-induced responses produced by LPS might be, at least in part, mediated through activation of Ras/Raf/MEK/MAPK pathway in TSMCs.

Differential regulation of MAP kinase cascade in human colorectal tumorigenesis

Hyper-activation of mitogen-activated protein kinase (MAPK) has recently been reported in several human cancers and activation of MAPK in those cancers may be associated with carcinogenesis through aberrant cell proliferation. To understand the roles of the MAPK pathway in colorectal tumorigenesis, we examined the status of extracellular signal-regulated protein kinases (ERK1/2) in 21 colorectal tumour specimens and compared it with that of paired normals. The specific MAPK activities were two- to tenfold lower in 71% (15 out of 21 cases) of colorectal tumours compared to those in paired normals. The individual MAPK kinase (MEK) correlated with MAPK activities (P = 0.006). Reduction of the MAPK and MEK activities in colorectal tumours was also observed in adenomas. These results suggested that down-regulation of the MAPK cascade may be caused by early genetic event(s) and that it may be related to the loss of normal growth control. Although MAPK activities were down-regulated both in adenomas and carcinomas, activities of the MAPKs in carcinomas were higher than those of paired adenomas. These results suggested that MAPK activities may be increased in the adenoma-to-carcinoma sequence and that it may play a role in the tumour progression. Observation of the differential regulation of MAPK activities in colorectal tumorigeneis suggested roles for the MAPK pathway in both positive and negative controls of cell growth.