A role for K-ras in conferring resistance to the MEK inhibitor, CI-1040

KRAS and NRAS Translation Is Increased upon MEK Inhibitors-Induced Processing Bodies Dissolution

Overactivation of the mitogen-activated protein kinase (MAPK) pathway is a critical driver of many human cancers. However, therapies directly targeting this pathway lead to cancer drug resistance. Resistance has been linked to compensatory RAS overexpression, but the mechanisms underlying this response remain unclear. Here, we find that MEK inhibitors (MEKi) are associated with an increased translation of the KRAS and NRAS oncogenes through a mechanism involving dissolution of processing body (P-body) biocondensates. This effect is seen across different cell types and is extremely dynamic since removal of MEKi and ERK reactivation result in reappearance of P-bodies and reduced RAS-dependent signaling. Moreover, we find that P-body scaffold protein levels negatively impact RAS expression. Overall, we describe a new feedback loop mechanism involving biocondensates such as P-bodies in the translational regulation of RAS proteins and MAPK signaling.

UV-Induced Somatic Mutations Driving Clonal Evolution in Healthy Skin, Nevus, and Cutaneous Melanoma

Introduction: Due to its aggressiveness, cutaneous melanoma (CM) is responsible for most skin cancer-related deaths worldwide. The origin of CM is closely linked to the appearance of UV-induced somatic mutations in melanocytes present in normal skin or in CM precursor lesions (nevi or dysplastic nevi). In recent years, new NGS studies performed on CM tissue have increased the understanding of the genetic somatic changes underlying melanomagenesis and CM tumor progression. Methods: We reviewed the literature using all important scientific databases. All articles related to genomic mutations in CM as well as normal skin and nevi were included, in particular those related to somatic mutations produced by UV radiation. Conclusions: CM development and progression are strongly associated with exposure to UV radiation, although each melanoma subtype has different characteristic genetic alterations and evolutionary trajectories. While BRAF and NRAS mutations are common in the early stages of tumor development for most CM subtypes, changes in CDKN2A, TP53 and PTEN, together with TERT promoter mutations, are especially common in advanced stages. Additionally, large genome duplications, loss of heterozygosity, and copy number variations are hallmarks of metastatic disease. Finally, the mutations driving melanoma targeted-therapy drug resistance are also summarized. The complete sequential stages of clonal evolution leading to CM onset from normal skin or nevi are still unknown, so further studies are needed in this field to shed light on the molecular pathways involved in CM malignant transformation and in melanoma acquired drug resistance.

Harnessing Reversed Allosteric Communication: A Novel Strategy for Allosteric Drug Discovery

Allostery is a fundamental and extensive mechanism of intramolecular signal transmission. Allosteric drugs possess several unique pharmacological advantages over traditional orthosteric drugs, including greater selectivity, better physicochemical properties, and lower off-target toxicity. However, owing to the complexity of allosteric regulation, experimental approaches for the development of allosteric modulators are traditionally serendipitous. Recently, the reversed allosteric communication theory has been proposed, providing a feasible tool for the unbiased detection of allosteric sites. Herein, we review the latest research on the reversed allosteric communication effect using the examples of sirtuin 6, epidermal growth factor receptor, 3-phosphoinositide-dependent protein kinase 1, and Related to A and C kinases (RAC) serine/threonine protein kinase B and recapitulate the methodologies of reversed allosteric communication strategy. The novel reversed allosteric communication strategy greatly expands the horizon of allosteric site identification and allosteric mechanism exploration and is expected to accelerate an end-to-end framework for drug discovery.

Resistance to Molecularly Targeted Therapies in Melanoma

Malignant melanoma is the most aggressive type of skin cancer with invasive growth patterns. In 2021, 106,110 patients are projected to be diagnosed with melanoma, out of which 7180 are expected to die. Traditional methods like surgery, radiation therapy, and chemotherapy are not effective in the treatment of metastatic and advanced melanoma. Recent approaches to treat melanoma have focused on biomarkers that play significant roles in cell growth, proliferation, migration, and survival. Several FDA-approved molecular targeted therapies such as tyrosine kinase inhibitors (TKIs) have been developed against genetic biomarkers whose overexpression is implicated in tumorigenesis. The use of targeted therapies as an alternative or supplement to immunotherapy has revolutionized the management of metastatic melanoma. Although this treatment strategy is more efficacious and less toxic in comparison to traditional therapies, targeted therapies are less effective after prolonged treatment due to acquired resistance caused by mutations and activation of alternative mechanisms in melanoma tumors. Recent studies focus on understanding the mechanisms of acquired resistance to these current therapies. Further research is needed for the development of better approaches to improve prognosis in melanoma patients. In this article, various melanoma biomarkers including BRAF, MEK, RAS, c-KIT, VEGFR, c-MET and PI3K are described, and their potential mechanisms for drug resistance are discussed.

Inhibition of AURKA Reduces Proliferation and Survival of Gastrointestinal Cancer Cells With Activated KRAS by Preventing Activation of RPS6KB1

BACKGROUND & AIMS

Activation of KRAS signaling and overexpression of the aurora kinase A (AURKA) are often detected in luminal gastrointestinal cancers. We investigated regulation of ribosomal protein S6 kinase B1 (RPS6KB1) by AURKA and the effects of alisertib, an AURKA inhibitor, in mice xenograft tumors grown from human gastrointestinal cancer cells with mutant, activated forms of KRAS.

METHODS

We tested the effects of alisertib or AURKA overexpression or knockdown in 10 upper gastrointestinal or colon cancer cell lines with KRAS mutations or amplifications using the CellTiter-Glo luminescence and clonogenic cell survival assays. We used the proximity ligation in situ assay to evaluate protein co-localization and immunoprecipitation to study protein interactions. Nude mice with xenograft tumors grown from HCT116, SNU-601, SW480, or SNU-1 cells were given oral alisertib (40 mg/kg, 5 times/wk) for 4 weeks. Tumor samples were collected and analyzed by immunoblots and immunohistochemistry. Tissue microarrays from 151 paraffin-embedded human colon tumors, with adjacent normal and adenoma tissues, were analyzed by immunohistochemistry for levels of AURKA.

RESULTS

Alisertib reduced proliferation and survival of the cell lines tested. AURKA knockdown or inhibition with alisertib reduced levels of phosphorylated RPS6KB1 (at T389) and increased levels of proteins that induce apoptosis, including BIM, cleaved PARP, and cleaved caspase 3. AURKA co-localized and interacted with RPS6KB1, mediating RPS6KB1 phosphorylation at T389. We detected AURKA-dependent phosphorylation of RPS6KB1 in cell lines with mutations in KRAS but not in cells with wild-type KRAS. Administration of alisertib to mice with xenograft tumors significantly reduced tumor volumes (P < .001). Alisertib reduced phosphorylation of RPS6KB1 and Ki-67 and increased levels of cleaved caspase 3 in tumor tissues. In analyses of tissue microarrays, we found significant overexpression of AURKA in gastrointestinal tumor tissues compared with non-tumor tissues (P = .0003).

CONCLUSION

In studies of gastrointestinal cancer cell lines with activated KRAS, we found AURKA to phosphorylate RPS6KB1, promoting cell proliferation and survival and growth of xenograft tumors in mice. Agents that inhibit AURKA might slow the growth of gastrointestinal tumors with activation of KRAS.

MEK-inhibitor PD184352 enhances the radiosensitizing effect of the Hsp90 inhibitor NVP-AUY922: the role of cell type and drug-irradiation schedule

Targeting MEK protein in cancer cells usually leads to acquired resistance to MEK inhibitors and activation of the prosurvival protein Akt. Since both MEK and Akt are clients of the Hsp90 chaperone system, the present study explores the responses of irradiated lung carcinoma A549 and glioblastoma SNB19 cell lines to combined MEK and Hsp90 inhibition. Unexpectedly, the MEK inhibitor PD184352 administered 24 h prior to irradiation, enhanced cell survival through upregulation of not only MEK and Erk1/2 but also of Akt. In contrast, PD184352 added 1 h before irradiation strongly reduced the expression of Erk and did not upregulate Akt in both cell lines. As a result, the MEK inhibitor increased the radiosensitizing effect of the Hsp90 inhibitor NVP-AUY922 in glioblastoma SNB19 cells. Possible reasons for the enhanced cell killing under this short-term pretreatment schedule may be a down-regulation of Erk during or directly after irradiation, increased DNA damage and/or a strong G₂/M arrest 24 h after irradiation. In addition, an 1-h pretreatment with PD184352 and/or NVP-AUY922 under schedule II induced neither G₁ arrest nor up-regulation of p-Akt in both cell lines as it did under schedule I. Yet, a long-term treatment with the MEK inhibitor alone caused a strong cytostatical effect. We conclude that the duration of drug pretreatment before irradiation plays a key role in the targeting of MEK in tumor cells. However, due to an aberrant activation of prosurvival proteins, the therapeutic window needs to be carefully defined, or a combination of inhibitors should be considered.

Induction of immunoglobulin transcription factor 2 and resistance to MEK inhibitor in melanoma cells

Primary or acquired resistance to MEK inhibitors has been a barrier to successful treatment with MEK inhibitors in many tumors. In this study, we analyzed genome-wide gene expression profiling data from 6 sensitive and 6 resistant cell lines to identify candidate genes whose expression changes are associated with responses to a MEK inhibitor, selumetinib (AZD6244). Of 62 identified differentially expressed genes, we selected Immunoglobulin Transcription Factor 2, also known as transcription factor 4 as a potential drug resistance marker for further analysis. This was because the ITF-2 expression increase in resistant cell lines was relatively high and a previous study has suggested that ITF-2 functions as an oncogene in human colon cancers. We also established an AZD6244 resistant cell line (M14/AZD-3) from an AZD6244 sensitive M14 cell line. The expression of the ITF-2 was elevated both in primary AZD6244 resistant cell line, LOX-IMVI and acquired resistant cell line, M14/AZD-3. Targeted silencing of ITF-2 by siRNA significantly enhanced susceptibility to AZD6244 in resistant cells. Wnt/β-catenin pathway was activated through direct interaction of p-ERK and GSK3β. Our results suggest that up-regulation of the ITF-2 gene expression is associated with cellular resistance to MEK inhibitors, and activation of Wnt signaling pathway through interaction of p-ERK and GSK3β seems to be a mechanism for increase of ITF-2.

Ras Dimer Formation as a New Signaling Mechanism and Potential Cancer Therapeutic Target

The K-, N-, and HRas small GTPases are key regulators of cell physiology and are frequently mutated in human cancers. Despite intensive research, previous efforts to target hyperactive Ras based on known mechanisms of Ras signaling have been met with little success. Several studies have provided compelling evidence for the existence and biological relevance of Ras dimers, establishing a new mechanism for regulating Ras activity in cells additionally to GTP-loading and membrane localization. Existing data also start to reveal how Ras proteins dimerize on the membrane. We propose a dimer model to describe Ras-mediated effector activation, which contrasts existing models of Ras signaling as a monomer or as a 5-8 membered multimer. We also discuss potential implications of this model in both basic and translational Ras biology.

Dual Inhibition of MEK and PI3K Pathway in KRAS and BRAF Mutated Colorectal Cancers

Colorectal cancer (CRC) is a heterogeneous disease with multiple underlying causative genetic mutations. Genetic mutations in the phosphatidylinositol-3 kinase (PI3K) and the mitogen activated protein kinase (MAPK) pathways are frequently implicated in CRC. Targeting the downstream substrate MEK in these mutated tumors stands out as a potential target in CRC. Several selective inhibitors of MEK have entered clinical trial evaluation; however, clinical activity with single MEK inhibitors has been rarely observed and acquired resistance seems to be inevitable. Amplification of the driving oncogene KRAS(13D), which increases signaling through the ERK1/2 pathway, upregulation of the noncanonical wingless/calcium signaling pathway (Wnt), and coexisting PIK3CA mutations have all been implicated with resistance against MEK inhibitor therapy in KRAS mutated CRC. The Wnt pathway and amplification of the oncogene have also been associated with resistance to MEK inhibitors in CRCs harboring BRAF mutations. Thus, dual targeted inhibition of MEK and PI3K pathway effectors (mTOR, PI3K, AKT, IGF-1R or PI3K/mTOR inhibitors) presents a potential strategy to overcome resistance to MEK inhibitor therapy. Many clinical trials are underway to evaluate multiple combinations of these pathway inhibitors in solid tumors.

Allosteric MEK1/2 inhibitor refametinib (BAY 86-9766) in combination with sorafenib exhibits antitumor activity in preclinical murine and rat models of hepatocellular carcinoma

OBJECTIVE

The objectives of the study were to evaluate the allosteric mitogen-activated protein kinase kinase (MEK) inhibitor BAY 86-9766 in monotherapy and in combination with sorafenib in orthotopic and subcutaneous hepatocellular carcinoma (HCC) models with different underlying etiologies in two species.

DESIGN

Antiproliferative potential of BAY 86-9766 and synergistic effects with sorafenib were studied in several HCC cell lines. Relevant pathway signaling was studied in MH3924a cells. For in vivo testing, the HCC cells were implanted subcutaneously or orthotopically. Survival and mode of action (MoA) were analyzed.

RESULTS

BAY 86-9766 exhibited potent antiproliferative activity in HCC cell lines with half-maximal inhibitory concentration values ranging from 33 to 762 nM. BAY 86-9766 was strongly synergistic with sorafenib in suppressing tumor cell proliferation and inhibiting phosphorylation of the extracellular signal-regulated kinase (ERK). BAY 86-9766 prolonged survival in Hep3B xenografts, murine Hepa129 allografts, and MH3924A rat allografts. Additionally, tumor growth, ascites formation, and serum alpha-fetoprotein levels were reduced. Synergistic effects in combination with sorafenib were shown in Huh-7, Hep3B xenografts, and MH3924A allografts. On the signaling pathway level, the combination of BAY 86-9766 and sorafenib led to inhibition of the upregulatory feedback loop toward MEK phosphorylation observed after BAY 86-9766 monotreatment. With regard to the underlying MoA, inhibition of ERK phosphorylation, tumor cell proliferation, and microvessel density was observed in vivo.

CONCLUSION

BAY 86-9766 shows potent single-agent antitumor activity and acts synergistically in combination with sorafenib in preclinical HCC models. These results support the ongoing clinical development of BAY 86-9766 and sorafenib in advanced HCC.

Enhanced inhibition of ERK signaling by a novel allosteric MEK inhibitor, CH5126766, that suppresses feedback reactivation of RAF activity

Tumors with mutant RAS are often dependent on extracellular signal-regulated kinase (ERK) signaling for growth; however, MEK inhibitors have only marginal antitumor activity in these tumors. MEK inhibitors relieve ERK-dependent feedback inhibition of RAF and cause induction of MEK phosphorylation. We have now identified a MEK inhibitor, CH5126766 (RO5126766), that has the unique property of inhibiting RAF kinase as well. CH5126766 binding causes MEK to adopt a conformation in which it cannot be phosphorylated by and released from RAF. This results in formation of a stable MEK/RAF complex and inhibition of RAF kinase. Consistent with this mechanism, this drug does not induce MEK phosphorylation. CH5126766 inhibits ERK signaling output more effectively than a standard MEK inhibitor that induces MEK phosphorylation and has potent antitumor activity as well. These results suggest that relief of RAF feedback limits pathway inhibition by standard MEK inhibitors. CH5126766 represents a new type of MEK inhibitor that causes MEK to become a dominant-negative inhibitor of RAF and that, in doing so, may have enhanced therapeutic activity in ERK-dependent tumors with mutant RAS.

Quantitative NMR analysis of Erk activity and inhibition by U0126 in a panel of patient-derived colorectal cancer cell lines

We comparatively analyzed the basal activity of extra-cellular signal-regulated kinase (Erk1/2) in lysates of 10 human colorectal cancer cell lines by semi-quantitative Western blotting and time-resolved NMR spectroscopy. Both methods revealed heterogeneous levels of endogenous Erk1/2 activities in a highly consistent manner. Upon treatment with U0126, an inhibitor of mitogen-activated protein kinase kinase (MEK) acting upstream of Erk1/2, Western-blotting and NMR congruently reported specific modulations of cellular phospho-Erk levels that translated into reduced kinase activities. Results obtained in this study highlight the complementary nature of antibody- and NMR-based phospho-detection techniques. They further exemplify the usefulness of time-resolved NMR measurements in providing fast and quantitative readouts of kinase activities and kinase inhibitor efficacies in native cellular environments. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Impact of oncogenic driver mutations on feedback between the PI3K and MEK pathways in cancer cells

Inhibition of the PI3K (phosphoinositide 3-kinase)/Akt/mTORC1 (mammalian target of rapamycin complex 1) and Ras/MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase]/ERK pathways for cancer therapy has been pursued for over a decade with limited success. Emerging data have indicated that only discrete subsets of cancer patients have favourable responses to these inhibitors. This is due to genetic mutations that confer drug insensitivity and compensatory mechanisms. Therefore understanding of the feedback mechanisms that occur with respect to specific genetic mutations may aid identification of novel biomarkers that predict patient response. In the present paper, we show that feedback between the PI3K/Akt/mTORC1 and Ras/MEK/ERK pathways is cell-line-specific and highly dependent on the activating mutation of K-Ras or overexpression c-Met. We found that cell lines exhibited differential signalling and apoptotic responses to PD184352, a specific MEK inhibitor, and PI103, a second-generation class I PI3K inhibitor. We reveal that feedback from the PI3K/Akt/mTORC1 to the Ras/MEK/ERK pathway is present in cancer cells harbouring either K-Ras activating mutations or amplification of c-Met but not the wild-type counterparts. Moreover, we demonstrate that inhibition of protein phosphatase activity by OA (okadaic acid) restored PI103-mediated feedback in wild-type cells. Together, our results demonstrate a novel mechanism for feedback between the PI3K/Akt/mTORC1 and the Ras/MEK/ERK pathways that only occurs in K-Ras mutant and c-Met amplified cells but not the isogenic wild-type cells through a mechanism that may involve inhibition of a specific endogenous phosphatase(s) activity. We conclude that monitoring K-Ras and c-Met status are important biomarkers for determining the efficacy of PI103 and other PI3K/Akt inhibitors in cancer therapy.

BAY61-3606 affects the viability of colon cancer cells in a genotype-directed manner

Background

K-RAS mutation poses a particularly difficult problem for cancer therapy. Activating mutations in K-RAS are common in cancers of the lung, pancreas, and colon and are associated with poor response to therapy. As such, targeted therapies that abrogate K-RAS-induced oncogenicity would be of tremendous value.

Methods

We searched for small molecule kinase inhibitors that preferentially affect the growth of colorectal cancer cells expressing mutant K-RAS. The mechanism of action of one inhibitor was explored using chemical and genetic approaches.

Results

We identified BAY61-3606 as an inhibitor of proliferation in colorectal cancer cells expressing mutant forms of K-RAS, but not in isogenic cells expressing wild-type K-RAS. In addition to its anti-proliferative effects in mutant cells, BAY61-3606 exhibited a distinct biological property in wild-type cells in that it conferred sensitivity to inhibition of RAF. In this context, BAY61-3606 acted by inhibiting MAP4K2 (GCK), which normally activates NFκβ signaling in wild-type cells in response to inhibition of RAF. As a result of MAP4K2 inhibition, wild-type cells became sensitive to AZ-628, a RAF inhibitor, when also treated with BAY61-3606.

Conclusions

These studies indicate that BAY61-3606 exerts distinct biological activities in different genetic contexts.

Mouse genetics suggests cell-context dependency for Myc-regulated metabolic enzymes during tumorigenesis

c-Myc (hereafter called Myc) belongs to a family of transcription factors that regulates cell growth, cell proliferation, and differentiation. Myc initiates the transcription of a large cast of genes involved in cell growth by stimulating metabolism and protein synthesis. Some of these, like those involved in glycolysis, may be part of the Warburg effect, which is defined as increased glucose uptake and lactate production in the presence of adequate oxygen supply. In this study, we have taken a mouse-genetics approach to challenge the role of select Myc-regulated metabolic enzymes in tumorigenesis in vivo. By breeding λ-Myc transgenic mice, Apc(Min) mice, and p53 knockout mice with mouse models carrying inactivating alleles of Lactate dehydrogenase A (Ldha), 3-Phosphoglycerate dehydrogenase (Phgdh) and Serine hydroxymethyltransferase 1 (Shmt1), we obtained offspring that were monitored for tumor development. Very surprisingly, we found that these genes are dispensable for tumorigenesis in these genetic settings. However, experiments in fibroblasts and colon carcinoma cells expressing oncogenic Ras show that these cells are sensitive to Ldha knockdown. Our genetic models reveal cell context dependency and a remarkable ability of tumor cells to adapt to alterations in critical metabolic pathways. Thus, to achieve clinical success, it will be of importance to correctly stratify patients and to find synthetic lethal combinations of inhibitors targeting metabolic enzymes.

Autoregulation of kinase dephosphorylation by ATP binding in AGC protein kinases

AGC kinases, including the three Akt (protein kinase B) isoforms, protein kinase A (PKA) and all protein kinase C (PKC) isoforms, require activation loop phosphorylation (threonine 308 in Akt1) as well as phosphorylation of a C-terminal residue (serine 473 in Akt1) for catalytic activity and phosphorylation of downstream targets. Conversely, phosphatases reverse these phosphorylations. Virtually all cellular processes are affected by AGC kinases, a circumstance that has led to intense scrutiny of the molecular mechanisms that regulate phosphorylation of these kinases. Here, we review a new layer of control of phosphorylation in Akt, PKA and PKC pointing to ATP binding pocket occupancy as a means to decelerate dephosphorylation of these and, potentially, other kinases. This additional level of kinase regulation opens the door to search for new functional motifs for the rational design of non- ATP-competitive kinase inhibitors that discriminate within and between protein kinase families.

Tumour cell responses to MEK1/2 inhibitors: acquired resistance and pathway remodelling

The Raf/MEK1/2 [mitogen-activated protein kinase/ERK (extracellular-signal-regulated kinase) kinase 1/2]/ERK1/2 signalling pathway is frequently activated in human tumours due to mutations in BRAF or KRAS. B-Raf and MEK1/2 inhibitors are currently undergoing clinical evaluation, but their ultimate success is likely to be limited by acquired drug resistance. We have used colorectal cancer cell lines harbouring mutations in B-Raf or K-Ras to model acquired resistance to the MEK1/2 inhibitor selumetinib (AZD6244). Selumetinib-resistant cells were refractory to other MEK1/2 inhibitors in cell proliferation assays and exhibited a marked increase in MEK1/2 and ERK1/2 activity and cyclin D1 abundance when assessed in the absence of inhibitor. This was driven by a common mechanism in which resistant cells exhibited an intrachromosomal amplification of their respective driving oncogene, B-Raf V600E or K-RasG13D. Despite the increased signal flux from Raf to MEK1/2, resistant cells maintained in drug actually exhibited the same level of ERK1/2 activity as parental cells, indicating that the pathway is remodelled by feedback controls to reinstate the normal level of ERK1/2 signalling that is required and sufficient to maintain proliferation in these cells. These results provide important new insights into how tumour cells adapt to new therapeutics and highlight the importance of homoeostatic control mechanisms in the Raf/MEK1/2/ERK1/2 signalling cascade.

Preclinical activity of the rational combination of selumetinib (AZD6244) in combination with vorinostat in KRAS-mutant colorectal cancer models

PURPOSE

Despite the availability of several active combination regimens for advanced colorectal cancer (CRC), the 5-year survival rate remains poor at less than 10%, supporting the development of novel therapeutic approaches. In this study, we focused on the preclinical assessment of a rationally based combination against KRAS-mutated CRC by testing the combination of the MEK inhibitor, selumetinib, and vorinostat, a histone deacetylase (HDAC) inhibitor.

EXPERIMENTAL DESIGN

Transcriptional profiling and gene set enrichment analysis (baseline and posttreatment) of CRC cell lines provided the rationale for the combination. The activity of selumetinib and vorinostat against the KRAS-mutant SW620 and SW480 CRC cell lines was studied in vitro and in vivo. The effects of this combination on tumor phenotype were assessed using monolayer and 3-dimensional cultures, flow cytometry, apoptosis, and cell migration. In vivo, tumor growth inhibition, (18)F-fluoro-deoxy-glucose positron emission tomography (FDG-PET), and proton nuclear magnetic resonance were carried out to evaluate the growth inhibitory and metabolic responses, respectively, in CRC xenografts.

RESULTS

In vitro, treatment with selumetinib and vorinostat resulted in a synergistic inhibition of proliferation and spheroid formation in both CRC cell lines. This inhibition was associated with an increase in apoptosis, cell-cycle arrest in G(1), and reduced cellular migration and VEGF-A secretion. In vivo, the combination resulted in additive tumor growth inhibition. The metabolic response to selumetinib and vorinostat consisted of significant inhibition of membrane phospholipids; no significant changes in glucose uptake or metabolism were observed in any of the treatment groups.

CONCLUSION

These data indicate that the rationally based combination of the mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor, selumetinib, with the HDAC inhibitor vorinostat results in synergistic antiproliferative activity against KRAS-mutant CRC cell lines in vitro. In vivo, the combination showed additive effects that were associated with metabolic changes in phospholipid turnover, but not on FDG-PET, indicating that the former is a more sensitive endpoint of the combination effects.

Resistance of Akt kinases to dephosphorylation through ATP-dependent conformational plasticity

Phosphorylation of a threonine residue (T308 in Akt1) in the activation loop of Akt kinases is a prerequisite for deregulated Akt activity frequently observed in neoplasia. Akt phosphorylation in vivo is balanced by the opposite activities of kinases and phosphatases. Here we describe that targeting Akt kinase to the cell membrane markedly reduced sensitivity of phosphorylated Akt to dephosphorylation by protein phosphatase 2A. This effect was amplified by occupancy of the ATP binding pocket by either ATP or ATP-competitive inhibitors. Mutational analysis revealed that R273 in Akt1 and the corresponding R274 in Akt2 are essential for shielding T308 in the activation loop against dephosphorylation. Thus, occupancy of the nucleotide binding pocket of Akt kinases enables intramolecular interactions that restrict phosphatase access and sustain Akt phosphorylation. This mechanism provides an explanation for the "paradoxical" Akt hyperphosphorylation induced by ATP-competitive inhibitor, A-443654. The lack of phosphatase resistance further contributes insight into the mechanism by which the human Akt2 R274H missense mutation may cause autosomal-dominant diabetes mellitus.

Resistance to MEK inhibitors: should we co-target upstream?

Aberrant activation of the ERK pathway is common in human tumors. This pathway consists of a three-tiered kinase module [comprising the kinases RAF, mitogen-activated protein kinase (MAPK) kinase (MEK), and extracellular signal-regulated kinase (ERK)] that functions as a negative feedback amplifier to confer robustness and stabilization of pathway output. Because this pathway is frequently dysregulated in human cancers, intense efforts are under way to develop selective inhibitors of the ERK pathway as anticancer drugs. Although promising results have been reported in early trials for inhibitors of RAF or MEK, resistance invariably occurs. Amplification of the upstream oncogenic driver of ERK signaling has been identified as a mechanism for MEK inhibitor resistance in cells with mutant BRAF or KRAS. Increased abundance of the oncogenic driver (either KRAS or BRAF in the appropriate cellular context) in response to prolonged drug treatment results in increased flux through the ERK pathway and restoration of ERK activity above the threshold required for cell growth. For patients with BRAF mutant tumors, the results suggest that the addition of a RAF inhibitor to a MEK inhibitor may delay or overcome drug resistance. The data thus provide a mechanistic basis for ongoing trials testing concurrent treatment with RAF and MEK inhibitors.

Activation of the MEK-S6 pathway in high-grade ovarian cancers

The primary objective of this study is to show the activation and analyze the regulation of the MEK- S6 kinase pathway in high-grade ovarian cancer. Phospho-ERK (pERK), a direct substrate of MEK and 2 phosphorylation sites on the ribosomal protein, S6, Ser235/236, and Ser240/244, which are both targeted by the MEK and PI3-kinase/AKT pathways, were analyzed in 13 cell lines, 28 primary cancers and 8 cases of cancer cells from ascites. In primary cancers, ERK and S6 phosphorylation was measured by immunohistochemistry (IHC). pERK, pS6, pAKT, and p4EBP1 were also measured by Western blotting (WB). The regulation of S6 phosphorylation by the MEK and PI3-kinase pathways was determined in ovarian cancer cell lines. We observed frequent pERK expression in primary ovarian cancers (100% by WB, 75% by IHC) but not in ovarian cancer cells from ascites (25% of cases by WB). The activation of the AKT pathway, measured by pAKT expression occurred in 7 cases of primary ovarian cancer by WB, but in none of the ascites samples. In ovarian cancer cell lines, the MEK pathway had a greater effect on S6 phosphorylation in cells without hyperactive AKT signaling. Our data suggest that MEK is a potential drug target in high-grade ovarian cancer, however, cancer cells with hyperactive AKT and cancer cells in ascites may be less responsive to MEK inhibition. The phosphorylation of S6 as a specific biomarker for either MEK or PI3-kinase pathway activation should be used with caution.

Identification of predictive markers of response to the MEK1/2 inhibitor selumetinib (AZD6244) in K-ras-mutated colorectal cancer

Mutant K-ras activity leads to the activation of the RAS/RAF/MEK/ERK pathway in approximately 44% of colorectal cancer (CRC) tumors. Accordingly, several inhibitors of the MEK pathway are under clinical evaluation in several malignancies including CRC. The aim of this study was to develop and characterize predictive biomarkers of response to the MEK1/2 inhibitor AZD6244 in CRC in order to maximize the clinical utility of this agent. Twenty-seven human CRC cell lines were exposed to AZD6244 and classified according to the IC(50) value as sensitive (≤ 0.1 μmol/L) or resistant (>1 μmol/L). All cell lines were subjected to immunoblotting for effector proteins, K-ras/BRAF mutation status, and baseline gene array analysis. Further testing was done in cell line xenografts and K-ras mutant CRC human explants models to develop a predictive genomic classifier for AZD6244. The most sensitive and resistant cell lines were subjected to differential gene array and pathway analyses. Members of the Wnt signaling pathway were highly overexpressed in cell lines resistant to AZD6244 and seem to be functionally involved in mediating resistance by shRNA knockdown studies. Baseline gene array data from CRC cell lines and xenografts were used to develop a k-top scoring pair (k-TSP) classifier, which predicted with 71% accuracy which of a test set of patient-derived K-ras mutant CRC explants would respond to AZD6244, providing the basis for a patient-selective clinical trial. These results also indicate that resistance to AZD6244 may be mediated, in part, by the upregulation of the Wnt pathway, suggesting potential rational combination partners for AZD6244 in CRC.

Molecular, cellular and physiological characterization of the cancer cachexia-inducing C26 colon carcinoma in mouse

BACKGROUND

The majority of cancer patients experience dramatic weight loss, due to cachexia and consisting of skeletal muscle and fat tissue wasting. Cachexia is a negative prognostic factor, interferes with therapy and worsens the patients' quality of life by affecting muscle function. Mice bearing ectopically-implanted C26 colon carcinoma are widely used as an experimental model of cancer cachexia. As part of the search for novel clinical and basic research applications for this experimental model, we characterized novel cellular and molecular features of C26-bearing mice.

METHODS

A fragment of C26 tumor was subcutaneously grafted in isogenic BALB/c mice. The mass growth and proliferation rate of the tumor were analyzed. Histological and cytofluorometric analyses were used to assess cell death, ploidy and differentiation of the tumor cells. The main features of skeletal muscle atrophy, which were highlighted by immunohistochemical and electron microscopy analyses, correlated with biochemical alterations. Muscle force and resistance to fatigue were measured and analyzed as major functional deficits of the cachectic musculature.

RESULTS

We found that the C26 tumor, ectopically implanted in mice, is an undifferentiated carcinoma, which should be referred to as such and not as adenocarcinoma, a common misconception. The C26 tumor displays aneuploidy and histological features typical of transformed cells, incorporates BrdU and induces severe weight loss in the host, which is largely caused by muscle wasting. The latter appears to be due to proteasome-mediated protein degradation, which disrupts the sarcomeric structure and muscle fiber-extracellular matrix interactions. A pivotal functional deficit of cachectic muscle consists in increased fatigability, while the reported loss of tetanic force is not statistically significant following normalization for decreased muscle fiber size.

CONCLUSIONS

We conclude, on the basis of the definition of cachexia, that ectopically-implanted C26 carcinoma represents a well standardized experimental model for research on cancer cachexia. We wish to point out that scientists using the C26 model to study cancer and those using the same model to study cachexia may be unaware of each other's works because they use different keywords; we present strategies to eliminate this gap and discuss the benefits of such an exchange of knowledge.

Recovery of phospho-ERK activity allows melanoma cells to escape from BRAF inhibitor therapy

Background:

Resistance to BRAF inhibitors is an emerging problem in the melanoma field. Strategies to prevent and overcome resistance are urgently required.

Methods:

The dynamics of cell signalling, BrdU incorporation and cell-cycle entry after BRAF inhibition was measured using flow cytometry and western blot. The ability of combined BRAF/MEK inhibition to prevent the emergence of resistance was demonstrated by apoptosis and colony formation assays and in 3D organotypic cell culture.

Results:

BRAF inhibition led to a rapid recovery of phospho-ERK (pERK) signalling. Although most of the cells remained growth arrested in the presence of drug, a minor population of cells retained their proliferative potential and escaped from BRAF inhibitor therapy. A function for the rebound pERK signalling in therapy escape was demonstrated by the ability of combined BRAF/MEK inhibition to enhance the levels of apoptosis and abrogate the onset of resistance.

Conclusion:

Combined BRAF/MEK inhibition may be one strategy to prevent the emergence of drug resistance in BRAF-V600E-mutated melanomas.

KRAS/BRAF mutation status and ERK1/2 activation as biomarkers for MEK1/2 inhibitor therapy in colorectal cancer

Phase II clinical trials of mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitors are ongoing and ERK1/2 activation is frequently used as a biomarker. In light of the mutational activation of BRAF and KRAS in colorectal cancer, inhibitors of the Raf-MEK-ERK mitogen-activated protein kinase are anticipated to be promising. Previous studies in pancreatic cancer have found little correlation between BRAF/KRAS mutation status and ERK1/2 activation, suggesting that identifying biomarkers of MEK inhibitor response may be more challenging than previously thought. The purpose of this study was to evaluate the effectiveness of MEK inhibitor therapy for colorectal cancer and BRAF/KRAS mutation status and ERK1/2 activation as biomarkers for MEK inhibitor therapy. First, we found that MEK inhibitor treatment impaired the anchorage-independent growth of nearly all KRAS/BRAF mutant, but not wild-type, colorectal cancer cells. There was a correlation between BRAF, but not KRAS, mutation status and ERK1/2 activation. Second, neither elevated ERK1/2 activation nor reduction of ERK1/2 activity correlated with MEK inhibition of anchorage-independent growth. Finally, we validated our cell line observations and found that ERK1/2 activation correlated with BRAF, but not KRAS, mutation status in 190 patient colorectal cancer tissues. Surprisingly, we also found that ERK activation was elevated in normal colonic epithelium, suggesting that normal cell toxicity may be a complication for colorectal cancer treatment. Our results suggest that although MEK inhibitors show promise in colorectal cancer, KRAS/BRAF mutation status, but not ERK activation as previously thought, may be useful biomarkers for MEK inhibitor sensitivity.

The future of targeted therapy approaches in melanoma

BACKGROUND

The past 30 years have seen little improvement in the survival of patients with stage IV melanoma. Following the discovery of activating BRAF mutations in most melanomas, a wealth of preclinical experimentation has validated the BRAF/MAPK pathway as an excellent therapeutic target in melanoma. Despite these encouraging results, early clinical trials on BRAF/MAPK inhibition have been disappointing.

OBJECTIVE

In the current review, we discuss how differences between the preclinical and clinical settings may influence the response of melanoma cells to BRAF/MEK inhibition. As the BRAF/MEK signaling pathway is not solely responsible for the growth and survival of melanoma cells, we further discuss the therapeutic utility of inhibiting the PI3K/AKT and mTOR pathways both alone and in combination with BRAF/MEK.

CONCLUSION

In looking ahead to the future, it is likely that new advances in melanoma biology, such as the identification of melanoma stem cells and a greater understanding of intratumoral heterogeneity, may play a role in the design of any future melanoma targeted therapy.

Targeting the RAF-MEK-ERK pathway in cancer therapy

The clinical success of selective kinase inhibitors, such as imatinib and erlotinib, as therapeutic agents for several human cancers has prompted substantial interest in the further development and clinical testing of such inhibitors for a wide variety of malignancies. While much of this effort has been focused on the receptor tyrosine kinases, including EGFR, HER2, PDGF receptor, c-KIT, and MET, inhibitors of serine/threonine kinases are also beginning to emerge within discovery pipelines. Among these kinases, the RAF and MEK kinases have received substantial attention, owing largely to the relatively high frequency of activating mutations of RAS ( approximately 20% of all human cancers), an upstream activator of the well established RAF-MEK-ERK signaling cascade, as well as frequent activating mutations in the BRAF kinase ( approximately 7% of all human cancers). Here, we summarize the current state of development of kinase inhibitors directed at this signaling pathway, a few of which have already demonstrating favorable toxicity profiles as well as promising activity in early phase clinical studies.

Integrating BRAF/MEK inhibitors into combination therapy for melanoma

The discovery of BRAF mutations in melanoma has not yet translated into clinical success, suggesting that BRAF/MEK inhibitors will need to be combined with other agents. In the current review, we discuss other pathways likely to be important for melanoma progression and suggest possible drug combinations for future clinical testing.

Overcoming resistance to molecularly targeted anticancer therapies: Rational drug combinations based on EGFR and MAPK inhibition for solid tumours and haematologic malignancies

Accumulating evidence suggests that cancer can be envisioned as a "signaling disease", in which alterations in the cellular genome affect the expression and/or function of oncogenes and tumour suppressor genes. This ultimately disrupts the physiologic transmission of biochemical signals that normally regulate cell growth, differentiation and programmed cell death (apoptosis). From a clinical standpoint, signal transduction inhibition as a therapeutic strategy for human malignancies has recently achieved remarkable success. However, as additional drugs move forward into the clinical arena, intrinsic and acquired resistance to "targeted" agents becomes an issue for their clinical utility. One way to overcome resistance to targeted agents is to identify genetic and epigenetic aberrations underlying sensitivity/resistance, thus enabling the selection of patients that will most likely benefit from a specific therapy. Since resistance often ensues as a result of the concomitant activation of multiple, often overlapping, signaling pathways, another possibility is to interfere with multiple, cross-talking pathways involved in growth and survival control in a rational, mechanism-based, fashion. These concepts may be usefully applied, among others, to agents that target two major signal transduction pathways: the one initiated by epidermal growth factor receptor (EGFR) signaling and the one converging on mitogen-activated protein kinase (MAPK) activation. Here, we review the molecular mechanisms of sensitivity/resistance to EGFR inhibitors, as well as the rationale for combining them with other targeted agents, in an attempt to overcome resistance. In the second part of the paper, we review MAPK-targeted agents, focusing on their therapeutic potential in haematologic malignancies, and examine the prospects for combinations of MAPK inhibitors with cytotoxic agents or other signal transduction-targeted agents to obtain synergistic anti-tumour effects.

Ras oncogenes and their downstream targets

RAS proteins are small GTPases, which serve as master regulators of a myriad of signaling cascades involved in highly diverse cellular processes. RAS oncogenes have been originally discovered as retroviral oncogenes, and ever since constitutively activating RAS mutations have been identified in human tumors, they are in the focus of intense research. In this review, we summarize the biochemical properties of RAS proteins, trace down the evolution of RAS signaling and present an overview of the spatio-temporal activation of major RAS isoforms. We further discuss RAS effector pathways, their role in normal and transformed cell physiology and summarize ongoing attempts to interfere with aberrant RAS signaling. Finally, we comment on the role of micro RNAs in modulating RAS expression, contribution of RAS to stem cell function and on high-throughput analyses of RAS signaling networks.

Elevated expression of ERK 2 in human tumor cells chronically treated with PD98059

We examined the effect of chronic exposure of tumor cells to a mitogen-activated protein kinase/extracellular signal-regulated kinases (ERK) kinase inhibitor, PD98059, on cell proliferation was investigated. Human renal carcinoma cells (ACHN) and prostatic carcinoma cells (DU145) were cultured in the presence of PD98059 for more than 4 weeks (denoted ACHN (PD) cells and DU145 (PD) cells, respectively) and proliferation and signal transduction pathways were examined. PD98059 significantly inhibited the proliferation of parental cells. However, PD98059 failed to inhibit proliferation of ACHN (PD) and DU145 (PD) cells significantly. Expression of ERK 1 and 2 was elevated in these cells. These phenotypes were reversible. Downregulation of ERK 2, but not ERK 1, by small interfering RNA significantly inhibited the proliferation of ACHN (PD) and DU145 (PD) cells. Taken together, chronic exposure of tumor cells to PD98059 induced elevated expression of ERK 2, which was associated with decreased sensitivity of cellular proliferation to PD98059.

Mechanisms of drug inhibition of signalling molecules

The emergence of tumour-specific, molecularly targeted agents signifies a paradigm shift in cancer therapy, with less reliance on drugs that non-discriminately kill tumour and host cells. Although the diversity of targets giving rise to this new generation of anticancer drugs has expanded, many challenges persist in the design of effective treatment regimens. The complex interplay of signal-transduction pathways further complicates the customization of cancer treatments to target single mechanisms. However, despite uncertainty over precise or dominant mechanisms of action, especially for compounds targeting multiple gene products, emerging agents are producing significant therapeutic advances against a broad range of human cancers.