EGFR-targeted TRAIL and a Smac mimetic synergize to overcome apoptosis resistance in KRAS mutant colorectal cancer cells

Dipeptidyl-peptidase 9 regulates the dynamics of tumorigenesis and metastasis in breast cancer

The cytosolic dipeptidyl-aminopeptidase 9 (DPP9) cleaves protein N-termini post-proline or -alanine. Our analysis of DPP9 mRNA expression from the TCGA 'breast cancer' data set revealed that low/intermediate DPP9 levels are associated with poor overall survival of breast cancer patients. To unravel the impact of DPP9 on breast cancer development and progression, the transgenic MMTV-PyMT mouse model of metastasizing breast cancer was used. In addition, tissue- and time-controlled genetic deletion of DPP9 by the Cre-loxP recombination system was done. Despite a delay of tumor onset, a higher number of lung metastases were measured in DPP9-deficient mice compared to controls. In human mammary epithelial cells with oncogenic RAS pathway activation, DPP9 deficiency delayed tumorigenic transformation and accelerated TGF-β1 induced epithelial-to-mesenchymal transition (EMT) of spheroids. For further analysis of the mechanism, primary breast tumor cells were isolated from the MMTV-PyMT model. DPP9 deficiency in these cells caused cancer cell migration and invasion accompanied by EMT. In absence of DPP9, the EMT transcription factor ZEB1 was stabilized due to insufficient degradation by the proteasome. In summary, low expression of DPP9 appears to decelerate mammary tumorigenesis but favors EMT and metastasis, which establishes DPP9 as a novel dynamic regulator of breast cancer initiation and progression.

Analysis of RAS and drug induced homo- and heterodimerization of RAF and KSR1 proteins in living cells using split Nanoluc luciferase

The dimerization of RAF kinases represents a key event in their activation cycle and in RAS/ERK pathway activation. Genetic, biochemical and structural approaches provided key insights into this process defining RAF signaling output and the clinical efficacy of RAF inhibitors (RAFi). However, methods reporting the dynamics of RAF dimerization in living cells and in real time are still in their infancy. Recently, split luciferase systems have been developed for the detection of protein-protein-interactions (PPIs), incl. proof-of-concept studies demonstrating the heterodimerization of the BRAF and RAF1 isoforms. Due to their small size, the Nanoluc luciferase moieties LgBiT and SmBiT, which reconstitute a light emitting holoenzyme upon fusion partner promoted interaction, appear as well-suited to study RAF dimerization. Here, we provide an extensive analysis of the suitability of the Nanoluc system to study the homo- and heterodimerization of BRAF, RAF1 and the related KSR1 pseudokinase. We show that KRAS^G12V promotes the homo- and heterodimerization of BRAF, while considerable KSR1 homo- and KSR1/BRAF heterodimerization already occurs in the absence of this active GTPase and requires a salt bridge between the CC-SAM domain of KSR1 and the BRAF-specific region. We demonstrate that loss-of-function mutations impairing key steps of the RAF activation cycle can be used as calibrators to gauge the dynamics of heterodimerization. This approach identified the RAS-binding domains and the C-terminal 14-3-3 binding motifs as particularly critical for the reconstitution of RAF mediated LgBiT/SmBiT reconstitution, while the dimer interface was less important for dimerization but essential for downstream signaling. We show for the first time that BRAF^V600E, the most common BRAF oncoprotein whose dimerization status is controversially portrayed in the literature, forms homodimers in living cells more efficiently than its wildtype counterpart. Of note, Nanoluc activity reconstituted by BRAF^V600E homodimers is highly sensitive to the paradox-breaking RAFi PLX8394, indicating a dynamic and specific PPI. We report the effects of eleven ERK pathway inhibitors on RAF dimerization, incl. third-generation compounds that are less-defined in terms of their dimer promoting abilities. We identify Naporafenib as a potent and long-lasting dimerizer and show that the split Nanoluc approach discriminates between type I, I^1/2 and II RAFi. Video Abstract.

A novel tumor-homing TRAIL variant eradicates tumor xenografts of refractory colorectal cancer cells in combination with tumor cell-targeted photodynamic therapy

Multidrug resistance (MDR), which is common in colorectal cancer (CRC), induces high mortality in patients. Due to its robust and selective apoptosis induction in some CRC cells with MDR, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is attractive as a novel tool for CRC therapy. However, TRAIL is limited by its poor tumor-homing ability and inefficient apoptosis induction in CRC cells expressing low levels of death receptor (DR). Here, the tumor-homing RGR peptide (CRGRRST) was fused to TRAIL to produce RGR-TRAIL. Compared with TRAIL, RGR-TRAIL showed greater cell binding and cytotoxicity in CRC cells. In addition, RGR-TRAIL exerted significantly enhanced tumor uptake and growth suppression in mice bearing CRC tumor xenografts. Notably, RGR-TRAIL eradicated all tumor xenografts of DR-overexpressing COLO205 cells. However, TRAIL only showed mild tumor growth suppression under the same conditions, indicating that RGR fusion significantly increased the antitumor effect of TRAIL in DR-overexpressing CRC cells by improving tumor homing. Nevertheless, RGR fusion did not significantly enhance the antitumor effect of TRAIL in HT29 cells expressing low levels of DR. We found that DR expression in HT29 cells was enhanced by epidermal growth factor receptor (EGFR)-targeted photodynamic therapy (PDT). Moreover, both the in vitro and in vivo antitumor effects of RGR-TRAIL were significantly improved by combination with PDT. HT29 tumor xenografts (∼20%) were even eradicated by combination therapy. These results indicate that it is valuable to further evaluate the combination therapy of RGR-TRAIL and tumor-targeted PDT for clinical therapy of CRC with MDR.

Major Molecular Signaling Pathways in Oral Cancer Associated With Therapeutic Resistance

Oral cancer is a sub-category of head and neck cancers that primarily initiates in the oral cavity. The primary treatment option for oral cancer remains surgery but it is associated with massive disfigurement, inability to carry out normal oral functions, psycho-social stress and exhaustive rehabilitation. Other treatment options such as chemotherapy and radiotherapy have their own limitations in terms of toxicity, intolerance and therapeutic resistance. Immunological treatments to enhance the body's ability to recognize cancer tissue as a foreign entity are also being used but they are new and underdeveloped. Although substantial progress has been made in the treatment of oral cancer, its complex heterogeneous nature still needs to be explored, to elucidate the molecular basis for developing resistance to therapeutic agents and how to overcome it, with the aim of improving the chances of patients' survival and their quality of life. This review provides an overview of up-to-date information on the complex role of the major molecules and associated signaling, epigenetic changes, DNA damage repair systems, cancer stem cells and micro RNAs in the development of therapeutic resistance and treatment failure in oral cancer. We have also summarized the current strategies being developed to overcome these therapeutic challenges. This review will help not only researchers but also oral oncologists in the management of the disease and in developing new therapeutic modalities.

Artemisinin Derivatives Stimulate DR5-Specific TRAIL-Induced Apoptosis by Regulating Wildtype P53

Simple Summary

The risk of developing colorectal cancer at a younger age has increased, but current therapies are either risky or limited. We aim to demonstrate that the combination treatment of artemisinin derivatives and TRAIL could be a potential therapy to kill colon cancer cells. We found that artemisinin derivatives increase death receptor production and further sensitize colon cancer cells to TRAIL-induced apoptosis. Furthermore, we explored the role of P53 in response to artemisinin derivatives, which transactivates Death Receptor 5 (DR5) and the cyclin-dependent kinase inhibitor P21. Finally, a 3D tumor spheroid model also confirmed the efficacy of the combination treatment.

Abstract

Artemisinin derivatives, widely known as commercial anti-malaria drugs, may also have huge potential in treating cancer cells. It has been reported that artemisinin derivatives can overcome resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in liver and cervical cancer cells. In our study, we demonstrated that artesunate (ATS) and dihydroartemisinin (DHA) are more efficient in killing colon cancer cells compared to artemisinin (ART). ATS/DHA induces the expression of DR5 in a P53 dependent manner in HCT116 and DLD-1 cells. Both ATS and DHA overcome the resistance to DHER-induced apoptosis in HCT116, mainly through upregulating death receptor 5 (DR5). We also demonstrate that DHA sensitizes HCT116 cells to DHER-induced apoptosis via P53 regulated DR5 expression in P53 knockdown assays. Nevertheless, a lower effect was observed in DLD-1 cells, which has a single Ser²⁴¹Phe mutation in the P53 DNA binding domain. Thus, the status of P53 could be one of the determinants of TRAIL resistance in some cancer cells. Finally, the combination treatment of DHA and the TRAIL variant DHER increases cell death in 3D colon cancer spheroid models, which shows its potential as a novel therapy.

Customizing a Tridomain TRAIL Variant to Achieve Active Tumor Homing and Endogenous Albumin-Controlled Release of the Molecular Machine In Vivo

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is an attractive antitumor drug candidate for precision cancer therapy due to its superior selective cytotoxicity in a variety of tumor cells. However, the clinical application of TRAIL in cancer therapy has been limited by its poor tumor-homing capacities and short half-life. Herein, we designed a tridomain TRAIL variant, Z-ABD-TRAIL, by sequentially fusing the platelet-derived growth factor receptor beta (PDGFRβ)-specific affibody Z_PDGFRβ and an albumin-binding domain (ABD) to the N-terminus of TRAIL. The fusion protein Z-ABD-TRAIL was produced as a soluble protein with high yield in Escherichia coli (E. coli). The Z_PDGFRβ domain provided Z-ABD-TRAIL with PDGFRβ-binding properties and thus promoted its tumor homing via the engagement of PDGFRβ-expressing pericytes on tumor microvessels. ABD-mediated binding of Z-ABD-TRAIL to albumin in the blood endowed TRAIL with long-lasting (>72 h for Z-ABD-TRAIL vs <0.5 h for TRAIL) abilities to kill tumor cells. Although the in vitro cytotoxicity of Z-ABD-TRAIL in tumor cells was similar to that of the parent TRAIL, the in vivo tumor uptake, apoptosis-inducing ability, and antitumor effect of Z-ABD-TRAIL were much greater than those of TRAIL, indicating that Z_PDGFRβ-mediated tumor homing and ABD-introduced albumin binding significantly improved the pharmacodynamics of TRAIL. In addition, repeated injection of high-dose Z-ABD-TRAIL showed no obvious acute toxicity in mice. These results demonstrate that the newly designed tridomain Z-ABD-TRAIL is a promising agent for precision cancer therapy.

Bcl-2-mediated control of TRAIL-induced apoptotic response in the non-small lung cancer cell line NCI-H460 is effective at late caspase processing steps

Dysregulation of the mitochondrial signaling pathway of apoptosis induction represents a major hurdle in tumor therapy. The objective of the presented work was to investigate the role of the intrinsic (mitochondrial) apoptotic pathway in the non-small lung cancer cell line NCI-H460 upon induction of apoptosis using the highly bioactive TRAIL derivative Db-scTRAIL. NCI-H460 cells were TRAIL sensitive but an only about 3 fold overexpression of Bcl-2 was sufficient to induce a highly TRAIL resistant phenotype, confirming that the mitochondrial pathway is crucial for TRAIL-induced apoptosis induction. TRAIL resistance was paralleled by a strong inhibition of caspase-8, -9 and -3 activities and blocked their full processing. Notably, especially the final cleavage steps of the initiator caspase-8 and the executioner caspase-3 were effectively blocked by Bcl-2 overexpression. Caspase-9 knockdown failed to protect NCI-H460 cells from TRAIL-induced cell death, suggesting a minor role of this initiator caspase in this apoptotic pathway. Rather, knockdown of the XIAP antagonist Smac resulted in enhanced caspase-3 degradation after stimulation of cells with TRAIL. Of note, downregulation of XIAP had only limited effects on TRAIL sensitivity of wild-type NCI-H460 cells, but resensitized Bcl-2 overexpressing cells for TRAIL-induced apoptosis. In particular, XIAP knockdown in combination with TRAIL allowed the final cleavage step of caspase-3 to generate the catalytically active p17 fragment, whose production was otherwise blocked in Bcl-2 overexpressing cells. Together, our data strongly suggest that XIAP-mediated inhibition of final caspase-3 processing is the last and major hurdle in TRAIL-induced apoptosis in NCI-H460 cells, which can be overcome by Smac in a Bcl-2 level dependent manner. Quantitative investigation of the XIAP/Smac interplay using a mathematical model approach corroborates our experimental data strengthening the suggested roles of XIAP and Smac as critical determinants for TRAIL sensitivity.

IgG-single-chain TRAIL fusion proteins for tumour therapy

Single-chain formats of TNF-related apoptosis inducing ligand (scTRAIL) can serve as effector components of tumour-associated antigen-targeted as well as non-targeted fusion proteins, being characterized by high tumour cell-specific induction of apoptosis through death receptor activation. We studied the suitability of immunoglobulin G as a scaffold for oligovalent and bispecific TRAIL fusion proteins. Thus, we developed novel targeted hexa- and dodecavalent IgG-scTRAIL molecules by fusing scTRAIL to the C-terminus of either light (LC-scTRAIL) or heavy immunoglobulin chain (HC-scTRAIL), or to both ends (LC/HC-scTRAIL) of the anti-EGFR IgG antibody hu225. The binding specificity to EGFR and death receptors was retained in all IgG-scTRAIL formats and translated into high antigen-specific bioactivity on EGFR-positive Colo205, HCT116 and WM1366 tumour cell lines, with or without sensitization to apoptosis by bortezomib. In vivo, therapeutic potential was assessed for one of the targeted variants, HC-scTRAIL, compared to the non-targeted Fc-scTRAIL. Both molecules showed a significant reduction of tumour volume and synergism with a Smac mimetic in a Colo205 xenograft tumour model. The IgG-scTRAIL format allows directing a defined, highly bioactive form of TRAIL to a wide variety of tumour antigens, enabling customized solutions for a patient-specific targeted cancer therapy with a reduced risk of side effects.

Targeting scFv-Fc-scTRAIL fusion proteins to tumor cells

Fusion proteins combining hexavalent TRAIL with antibody fragments allow for a targeted delivery and efficient apoptosis induction in tumor cells. Here, we analyzed scFv-Fc-scTRAIL molecules directed against EGFR, HER2, HER3, and EpCAM as well as an untargeted Fc-scTRAIL fusion protein for their potentials to induce cell death both in vitro and in a xenograft tumor model in vivo. The scFv-Fc-scTRAIL fusion protein directed against EGFR as well as the fusion protein directed against EpCAM showed targeting effects on the two tested colorectal carcinoma cell lines Colo205 and HCT116, while a fusion protein targeting HER3 was more effective than untargeted Fc-scTRAIL only on Colo205 cells. Interestingly, another anti-HER3 scFv-Fc-scTRAIL fusion protein exhibiting approximately 10-fold weaker antigen binding as well as the HER2-directed molecule were unable to increase cytotoxicity compared to Fc-scTRAIL. A comparison of EC50 values of cell death induction and antigen binding supports the assumption that high affinity antigen binding is one of the requirements for in vitro targeting effects. Furthermore, a minimal number of expressed target antigens might be required for increased cytotoxicity of targeted compared to non-targeted molecules. In a Colo205 s.c. xenograft tumor model, strongest antitumor activity was observed for the anti-HER3 scFv-Fc-scTRAIL fusion protein based on antibody 3-43, with complete tumor remissions after six twice-weekly injections. Surprisingly, a similar in vivo activity was also observed for untargeted Fc-scTRAIL in this tumor model, indicating that additional factors contribute to the potent efficacy of targeted as well as untargeted hexavalent Fc-scTRAIL fusion proteins in vivo.

Oncogenic Ras triggers hyperproliferation and impairs polarized colonic morphogenesis by autocrine ErbB3 signaling

Here we study the effects of inducible oncogenic K-Ras (G12V) expression on the polarized morphogenesis of colonic epithelial cells. We provide evidence that the autocrine production of heregulins, ligands for the ErbB3 receptor tyrosine kinase, is responsible for the hyperproliferation and aberrant 3D morphogenesis upon oncogenic K-Ras expression. This is in line with results obtained in primary intestinal organoid cultures, in which exogenous heregulin is shown to interfere with normal tissue architecture. Importantly, ErbB3 inhibition and heregulin gene silencing rescued K-RasG12V-induced features of cell transformation. Together with the increased ErbB3 positivity detected in human high-grade primary colorectal cancers, our findings provide support for an autocrine signaling loop engaged by oncogenic K-Ras involving ErbB3 that contributes to the dedifferentiation of the intestinal epithelium during tumor initiation and progression.

KRAS promotes tumor metastasis and chemoresistance by repressing RKIP via the MAPK-ERK pathway in pancreatic cancer

Oncogenic KRAS plays a crucial role in pancreatic ductal adenocarcinoma (PDAC) development and progression. However, the mechanism has not been clearly elucidated. RKIP is a tumor repressor, and loss of RKIP has been shown in PDAC. Here, we found that KRAS expression was inversely correlated with RKIP expression in PDAC fresh tissue regardless of the KRAS mutant status. The negative correlation between KRAS and RKIP was further confirmed in our PDAC tissue microarray. KRAS overexpression and RKIP downregulation were associated with poor clinical outcomes. Knockdown or overexpression of KRAS in PDAC cell lines robustly increased or decreased, respectively, RKIP protein and mRNA levels. Furthermore, the MAPK-ERK pathway was involved in the regulation of RKIP. KRAS-regulated RKIP expression, which in turn affected the expression of pivotal epithelial-mesenchymal transition (EMT) and apoptosis factors. The biological function of the KRAS-RKIP axis was demonstrated in human pancreatic cancer cells in vitro and in vivo. KRAS knockdown increased RKIP expression and inhibited metastasis and chemoresistance. Moreover, the feature of metastasis and chemoresistance was rescued in the KRAS-knockdown cells through the inhibition of RKIP by RNA interference. In conclusion, our studies demonstrate how KRAS inhibits the tumor suppressor RKIP, thus offering novel justification for targeting RKIP as a strategy to overcome KRAS-induced tumor metastasis and chemoresistance in PDAC.

Therapeutic opportunities based on caspase modulation

Caspases are a family of proteolytic enzymes that play a critical role in the regulation of programmed cell death via apoptosis. Activation of caspases is frequently impaired in human cancers, contributing to cancer formation, progression and therapy resistance. A better understanding of the molecular mechanisms regulating caspase activation in cancer cells is therefore highly important. Thus, targeted modulation of caspase activation and apoptosis represents a promising approach for the development of new therapeutic options to elucidate cancer cell death.

Superior Properties of Fc-comprising scTRAIL Fusion Proteins

The TNF-related apoptosis-inducing ligand (TRAIL) has been considered as a promising molecule for cancer treatment. However, clinical studies with soluble TRAIL failed to show therapeutic activity, which resulted in subsequent development of more potent TRAIL-based therapeutics. In this study, we applied defined oligomerization and tumor targeting as strategies to further improve the activity of a single-chain version of TRAIL (scTRAIL). We compared three different formats of EGF receptor (EGFR)-targeting dimeric scTRAIL fusion proteins [Diabody (Db)-scTRAIL, scFv-IgE heavy chain domain 2 (EHD2)-scTRAIL, scFv-Fc-scTRAIL] as well as two nontargeted dimeric scTRAIL molecules (EHD2-scTRAIL, Fc-scTRAIL) to reveal the influence of targeting and protein format on antitumor activity. All EGFR-targeted dimeric scTRAIL molecules showed similar binding properties and comparable cell death induction in vitro, exceeding the activity of the respective nontargeted dimeric format and monomeric scTRAIL. Superior properties were observed for the Fc fusion proteins with respect to production and in vivo half-life. In vivo studies using a Colo205 xenograft model revealed potent antitumor activity of all EGFR-targeting formats and Fc-scTRAIL and furthermore highlighted the higher efficacy of fusion proteins comprising an Fc part. Despite enhanced in vitro cell death induction of targeted scTRAIL molecules, however, comparable antitumor activities were found for the EGFR-targeting scFv-Fc-scTRAIL and the nontargeting Fc-scTRAIL in vivoMol Cancer Ther; 16(12); 2792-802. ©2017 AACR.

RGD and NGR modified TRAIL protein exhibited potent anti-metastasis effects on TRAIL-insensitive cancer cells in vitro and in vivo

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been considered to be a promising anti-tumor agent since the discovery of TRAIL-mediated apoptosis specifically on cancer cells. However, TRAIL resistance of tumor cells and patients remains to be an insurmountable obstacle for its clinical application. Here, we expressed TRAIL-related recombinant protein RGD-TRAIL, TRAIL-NGR, and RGD-TRAIL-NGR by fusing tumor targeting peptides RGD and (or) NGR at the N-terminus and C-terminus, respectively, to not only induce apoptosis of cancer cells but also inhibit metastasis. The fusion proteins possessed potent cytotoxicity with approximative IC50 in H460 and A549 cells, while TRAIL-NGR and RGD-TRAIL-NGR appeared to be more effective in HT1080 and PANC-1 cells which were relatively insensitive to TRAIL. A low concentration of fusion proteins, especially RGD-TRAIL-NGR, could inhibit migration of A549 and HT1080 cells in vitro and lung metastasis in HT1080^LUC experimental model in vivo, indicating that the recombinant protein maintained the function of both TRAIL and targeting peptide RGD and NGR, which improved the sensitivity of tumor cells to TRAIL.

Applying phasor approach analysis of multiphoton FLIM measurements to probe the metabolic activity of three-dimensional in vitro cell culture models

Fluorescence lifetime imaging microscopy (FLIM) can measure and discriminate endogenous fluorophores present in biological samples. This study seeks to identify FLIM as a suitable method to non-invasively detect a shift in cellular metabolic activity towards glycolysis or oxidative phosphorylation in 3D Caco-2 models of colorectal carcinoma. These models were treated with potassium cyanide or hydrogen peroxide as controls, and epidermal growth factor (EGF) as a physiologically-relevant influencer of cell metabolic behaviour. Autofluorescence, attributed to nicotinamide adenine dinucleotide (NADH), was induced by two-photon laser excitation and its lifetime decay was analysed using a standard multi-exponential decay approach and also a novel custom-written code for phasor-based analysis. While both methods enabled detection of a statistically significant shift of metabolic activity towards glycolysis using potassium cyanide, and oxidative phosphorylation using hydrogen peroxide, employing the phasor approach required fewer initial assumptions to quantify the lifetimes of contributing fluorophores. 3D Caco-2 models treated with EGF had increased glucose consumption, production of lactate, and presence of ATP. FLIM analyses of these cultures revealed a significant shift in the contribution of protein-bound NADH towards free NADH, indicating increased glycolysis-mediated metabolic activity. This data demonstrate that FLIM is suitable to interpret metabolic changes in 3D in vitro models.

Smac Mimetics to Therapeutically Target IAP Proteins in Cancer

Inhibitor of Apoptosis (IAP) proteins are overexpressed in a variety of human cancers. Therefore, they are considered as promising targets for the design of therapeutic strategies. Smac mimetics mimic the endogenous mitochondrial protein Smac that antagonizes IAP proteins upon its release into the cytosol. Multiple preclinical studies have documented the ability of Smac mimetics to either directly induce cell death of cancer cells or to prime them to agents that trigger cell death. At present, several Smac mimetics are being evaluated in early clinical trials. The current review provides an overview on the potential of Smac mimetics as cancer therapeutics to target IAP proteins for cancer therapy.

Discrete cytosolic macromolecular BRAF complexes exhibit distinct activities and composition

As a central element within the RAS/ERK pathway, the serine/threonine kinase BRAF plays a key role in development and homeostasis and represents the most frequently mutated kinase in tumors. Consequently, it has emerged as an important therapeutic target in various malignancies. Nevertheless, the BRAF activation cycle still raises many mechanistic questions as illustrated by the paradoxical action and side effects of RAF inhibitors. By applying SEC-PCP-SILAC, we analyzed protein-protein interactions of hyperactive BRAF^V600E and wild-type BRAF (BRAF^WT). We identified two macromolecular, cytosolic BRAF complexes of distinct molecular composition and phosphorylation status. Hyperactive BRAF^V600E resides in large complexes of higher molecular mass and activity, while BRAF^WT is confined to smaller, slightly less active complexes. However, expression of oncogenic K-Ras^G12V, either by itself or in combination with RAF dimer promoting inhibitors, induces the incorporation of BRAF^WT into large, active complexes, whereas pharmacological inhibition of BRAF^V600E has the opposite effect. Thus, the quaternary structure of BRAF complexes is shaped by its activation status, the conformation of its kinase domain, and clinically relevant inhibitors.

Effects of RAF inhibitors on PI3K/AKT signalling depend on mutational status of the RAS/RAF signalling axis

Targeted therapies within the RAS/RAF/MEK/ERK signalling axis become increasingly popular, yet cross-talk and feedbacks in the signalling network lead to unexpected effects. Here we look systematically into how inhibiting RAF and MEK with clinically relevant inhibitors result in changes in PI3K/AKT activation. We measure the signalling response using a bead-based ELISA, and use a panel of three cell lines, and isogenic cell lines that express mutant forms of the oncogenes KRAS and BRAF to interrogate the effects of the MEK and RAF inhibitors on signalling. We find that treatment with the RAF inhibitors have opposing effects on AKT phosphorylation depending on the mutational status of two important oncogenes, KRAS and BRAF. If these two genes are in wildtype configuration, RAF inhibitors reduce AKT phosphorylation. In contrast, if BRAF or KRAS are mutant, RAF inhibitors will leave AKT phosphorylation unaffected or lead to an increase of AKT phosphorylation. Down-regulation of phospho-AKT by RAF inhibitors also extends to downstream transcription factors, and correlates with apoptosis induction. Our results show that oncogenes rewire signalling such that targeted therapies can have opposing effects on parallel pathways, which depend on the mutational status of the cell.

The polarity protein Scribble positions DLC3 at adherens junctions to regulate Rho signaling

The spatial regulation of cellular Rho signaling by GAP proteins is still poorly understood. By performing mass spectrometry, we here identify the polarity protein Scribble as a scaffold for the RhoGAP protein DLC3 (also known as StarD8) at cell-cell adhesions. This mutually dependent interaction is mediated by the PDZ domains of Scribble and a PDZ ligand (PDZL) motif in DLC3. Both Scribble depletion and PDZL deletion abrogated DLC3 junctional localization. Using a RhoA biosensor and a targeted GAP domain, we demonstrate that DLC3 activity locally regulates RhoA-ROCK signaling at and Scribble localization to adherens junctions, and is required for their functional integrity. In a 3D model of cyst development, we furthermore show that DLC3 depletion impairs polarized morphogenesis, phenocopying the effects observed upon Scribble knockdown. We thus propose a new function for Scribble in Rho regulation that entails positioning of DLC3 GAP activity at cell junctions in polarized epithelial cells.

Uncoupling of EGFR-RAS signaling and nuclear localization of YBX1 in colorectal cancer

The transcription factor YBX1 can act as a mediator of signals transmitted via the EGFR-RAS-MAPK axis. YBX1 expression has been associated with tumor progression and prognosis in multiple types of cancer. Immunohistochemical studies have revealed dependency between YBX1 expression and individual EGFR family members. We analyzed YBX1 and EGFR family proteins in a colorectal cancer (CRC) cohort and provide functional analyses of YBX1 in the context of EGFR-RAS-MAPK signaling. Immunohistochemistry for YBX1 and EGFR family receptors with two antibodies for YBX1 and EGFR were performed and related to clinicopathological data. We employed Caco2 cells expressing an inducible KRASV12 gene to determine effects on localization and levels of YBX1. Mouse xenografts of Caco2-KRASV12 cells were used to determine YBX1 dynamics in a tissue context. The two different antibodies against YBX1 showed discordant immunohistochemical stainings in cell culture and clinical specimens. Expression of YBX1 and EGFR family members were not correlated in CRC. Analysis of Caco2 xenografts displayed again heterogeneity of YBX1 staining with both antibodies. Our results suggest that YBX1 is controlled via complex regulatory mechanisms involving tumor stroma interaction and signal transduction processes. Our study highlights that YBX1 antibodies have different specificities, advocating their use in a combined manner.

Oncogenic KRAS sensitizes premalignant, but not malignant cells, to Noxa-dependent apoptosis through the activation of the MEK/ERK pathway

KRAS is mutated in about 20-25% of all human cancers and especially in pancreatic, lung and colorectal tumors. Oncogenic KRAS stimulates several pro-survival pathways, but it also triggers the trans-activation of pro-apoptotic genes. In our work, we show that G13D mutations of KRAS activate the MAPK pathway, and ERK2, but not ERK1, up-regulates Noxa basal levels. Accordingly, premalignant epithelial cells are sensitized to various cytotoxic compounds in a Noxa-dependent manner. In contrast to these findings, colorectal cancer cell sensitivity to treatment is independent of KRAS status and Noxa levels are not up-regulated in the presence of mutated KRAS despite the fact that ERK2 still promotes Noxa expression. We therefore speculated that other survival pathways are counteracting the pro-apoptotic effect of mutated KRAS and found that the inhibition of AKT restores sensitivity to treatment, especially in presence of oncogenic KRAS. In conclusion, our work suggests that the pharmacological inhibition of the pathways triggered by mutated KRAS could also switch off its oncogene-activated pro-apoptotic stimulation. On the contrary, the combination of chemotherapy to inhibitors of specific pro-survival pathways, such as the one controlled by AKT, could enhance treatment efficacy by exploiting the pro-death stimulation derived by oncogene activation.