Reporter cell lines to screen for inhibitors or regulators of the KRAS-RAF-MEK1/2-ERK1/2 pathway.

The RAS-regulated RAF-MEK1/2-ERK1/2 signalling pathway is activated in cancer due to mutations in RAS proteins (especially KRAS), BRAF, CRAF, MEK1 and MEK2. Whilst inhibitors of KRASG12C (lung adenocarcinoma) and BRAF and MEK1/2 (melanoma and colorectal cancer) are clinically approved, acquired resistance remains a problem. Consequently, the search for new inhibitors (especially of RAS proteins), new inhibitor modalities and regulators of this pathway, which may be new drug targets, continues and increasingly involves cell-based screens with small molecules or genetic screens such as RNAi, CRISPR or protein interference. Here we describe cell lines that exhibit doxycycline-dependent expression KRASG12V or BRAFV600E and harbour a stably integrated EGR1:EmGFP reporter gene that can be detected by flow cytometry, high-content microscopy or immunoblotting. KRASG12V or BRAFV600E-driven EmGFP expression is inhibited by MEK1/2 or ERK1/2 inhibitors (MEKi and ERKi). BRAFi inhibit BRAFV600E-driven EmGFP expression but enhance the response to KRASG12V, recapitulating paradoxical activation of wild type RAF proteins. In addition to small molecules, expression of iDab6, encoding a RAS-specific antibody fragment inhibited KRASG12V- but not BRAFV600E-driven EmGFP expression. Finally, substitution of EmGFP for a bacterial nitroreductase gene allowed KRASG12V or BRAFV600E to drive cell death in the presence of a pro-drug, which may allow selection of pathway inhibitors that promote survival. These cell lines should prove useful for cell-based screens to identify new regulators of KRAS- or BRAF-dependent ERK1/2 signalling (drug target discovery) as well as screening or triaging 'hits' from drug discovery screens.

An mTORC1-to-CDK1 Switch Maintains Autophagy Suppression during Mitosis.

Since nuclear envelope breakdown occurs during mitosis in metazoan cells, it has been proposed that macroautophagy must be inhibited to maintain genome integrity. However, repression of macroautophagy during mitosis remains controversial and mechanistic detail limited to the suggestion that CDK1 phosphorylates VPS34. Here, we show that initiation of macroautophagy, measured by the translocation of the ULK complex to autophagic puncta, is repressed during mitosis, even when mTORC1 is inhibited. Indeed, mTORC1 is inactive during mitosis, reflecting its failure to localize to lysosomes due to CDK1-dependent RAPTOR phosphorylation. While mTORC1 normally represses autophagy via phosphorylation of ULK1, ATG13, ATG14, and TFEB, we show that the mitotic phosphorylation of these autophagy regulators, including at known repressive sites, is dependent on CDK1 but independent of mTOR. Thus, CDK1 substitutes for inhibited mTORC1 as the master regulator of macroautophagy during mitosis, uncoupling autophagy regulation from nutrient status to ensure repression of macroautophagy during mitosis.

Targeting melanoma's MCL1 bias unleashes the apoptotic potential of BRAF and ERK1/2 pathway inhibitors.

BRAF and MEK1/2 inhibitors are effective in melanoma but resistance inevitably develops. Despite increasing the abundance of pro-apoptotic BIM and BMF, ERK1/2 pathway inhibition is predominantly cytostatic, reflecting residual pro-survival BCL2 family activity. Here, we show that uniquely low BCL-XL expression in melanoma biases the pro-survival pool towards MCL1. Consequently, BRAF or MEK1/2 inhibitors are synthetic lethal with the MCL1 inhibitor AZD5991, driving profound tumour cell death that requires BAK/BAX, BIM and BMF, and inhibiting tumour growth in vivo. Combination of ERK1/2 pathway inhibitors with BCL2/BCL-w/BCL-XL inhibitors is stronger in CRC, correlating with a low MCL1:BCL-XL ratio; indeed the MCL1:BCL-XL ratio is predictive of ERK1/2 pathway inhibitor synergy with MCL1 or BCL2/BCL-w/BCL-XL inhibitors. Finally, AZD5991 delays acquired BRAFi/MEKi resistance and enhances the efficacy of an ERK1/2 inhibitor in a model of acquired BRAFi + MEKi resistance. Thus combining ERK1/2 pathway inhibitors with MCL1 antagonists in melanoma could improve therapeutic index and patient outcomes.

MEK1/2 inhibitor withdrawal reverses acquired resistance driven by BRAFV600E amplification whereas KRASG13D amplification promotes EMT-chemoresistance.

Acquired resistance to MEK1/2 inhibitors (MEKi) arises through amplification of BRAFV600E or KRASG13D to reinstate ERK1/2 signalling. Here we show that BRAFV600E amplification and MEKi resistance are reversible following drug withdrawal. Cells with BRAFV600E amplification are addicted to MEKi to maintain a precise level of ERK1/2 signalling that is optimal for cell proliferation and survival, and tumour growth in vivo. Robust ERK1/2 activation following MEKi withdrawal drives a p57KIP2-dependent G1 cell cycle arrest and senescence or expression of NOXA and cell death, selecting against those cells with amplified BRAFV600E. p57KIP2 expression is required for loss of BRAFV600E amplification and reversal of MEKi resistance. Thus, BRAFV600E amplification confers a selective disadvantage during drug withdrawal, validating intermittent dosing to forestall resistance. In contrast, resistance driven by KRASG13D amplification is not reversible; rather ERK1/2 hyperactivation drives ZEB1-dependent epithelial-to-mesenchymal transition and chemoresistance, arguing strongly against the use of drug holidays in cases of KRASG13D amplification.

Control of cell death and mitochondrial fission by ERK1/2 MAP kinase signalling.

The ERK1/2 signalling pathway is best known for its role in connecting activated growth factor receptors to changes in gene expression due to activated ERK1/2 entering the nucleus and phosphorylating transcription factors. However, active ERK1/2 also translocate to a variety of other organelles including the endoplasmic reticulum, endosomes, golgi and mitochondria to access specific substrates and influence cell physiology. In this article, we review two aspects of ERK1/2 signalling at the mitochondria that are involved in regulating cell fate decisions. First, we describe the prominent role of ERK1/2 in controlling the BCL2-regulated, cell-intrinsic apoptotic pathway. In most cases ERK1/2 signalling promotes cell survival by activating prosurvival BCL2 proteins (BCL2, BCL-xL and MCL1) and repressing prodeath proteins (BAD, BIM, BMF and PUMA). This prosurvival signalling is co-opted by oncogenes to confer cancer cell-specific survival advantages and we describe how this information has been used to develop new drug combinations. However, ERK1/2 can also drive the expression of the prodeath protein NOXA to control 'autophagy or apoptosis' decisions during nutrient starvation. We also describe recent studies demonstrating a link between ERK1/2 signalling, DRP1 and the mitochondrial fission machinery and how this may influence metabolic reprogramming during tumorigenesis and stem cell reprogramming. With advances in subcellular proteomics it is likely that new roles for ERK1/2, and new substrates, remain to be discovered at the mitochondria and other organelles.

Visualization of Endogenous ERK1/2 in Cells with a Bioorthogonal Covalent Probe.

The RAS-RAF-MEK-ERK pathway has been intensively studied in oncology, with RAS known to be mutated in ∼30% of all human cancers. The recent emergence of ERK1/2 inhibitors and their ongoing clinical investigation demands a better understanding of ERK1/2 behavior following small-molecule inhibition. Although fluorescent fusion proteins and fluorescent antibodies are well-established methods of visualizing proteins, we show that ERK1/2 can be visualized via a less-invasive approach based on a two-step process using inverse electron demand Diels-Alder cycloaddition. Our previously reported trans-cyclooctene-tagged covalent ERK1/2 inhibitor was used in a series of imaging experiments following a click reaction with a tetrazine-tagged fluorescent dye. Although limitations were encountered with this approach, endogenous ERK1/2 was successfully imaged in cells, and "on-target" staining was confirmed by over-expressing DUSP5, a nuclear ERK1/2 phosphatase that anchors ERK1/2 in the nucleus.

Tumor cells with KRAS or BRAF mutations or ERK5/MAPK7 amplification are not addicted to ERK5 activity for cell proliferation.

ERK5, encoded by MAPK7, has been proposed to play a role in cell proliferation, thus attracting interest as a cancer therapeutic target. While oncogenic RAS or BRAF cause sustained activation of the MEK1/2-ERK1/2 pathway, ERK5 is directly activated by MEK5. It has been proposed that RAS and RAF proteins can also promote ERK5 activation. Here we investigated the interplay between RAS-RAF-MEK-ERK and ERK5 signaling and studied the role of ERK5 in tumor cell proliferation in 2 disease-relevant cell models. We demonstrate that although an inducible form of CRAF (CRAF:ER*) can activate ERK5 in fibroblasts, the response is delayed and reflects feed-forward signaling. Additionally, oncogenic KRAS and BRAF do not activate ERK5 in epithelial cells. Although KRAS and BRAF do not couple directly to MEK5-ERK5, ERK5 signaling might still be permissive for proliferation. However, neither the selective MEK5 inhibitor BIX02189 or ERK5 siRNA inhibited proliferation of colorectal cancer cells harbouring KRAS(G12C/G13D) or BRAF(V600E). Furthermore, there was no additive or synergistic effect observed when BIX02189 was combined with the MEK1/2 inhibitor Selumetinib (AZD6244), suggesting that ERK5 was neither required for proliferation nor a driver of innate resistance to MEK1/2 inhibitors. Finally, even cancer cells with MAPK7 amplification were resistant to BIX02189 and ERK5 siRNA, showing that ERK5 amplification does not confer addiction to ERK5 for cell proliferation. Thus ERK5 signaling is unlikely to play a role in tumor cell proliferation downstream of KRAS or BRAF or in tumor cells with ERK5 amplification. These results have important implications for the role of ERK5 as an anti-cancer drug target.

Adaptation to mTOR kinase inhibitors by amplification of eIF4E to maintain cap-dependent translation.

The mechanistic target of rapamycin (mTOR) protein kinase coordinates responses to nutrients and growth factors and is an anti-cancer drug target. To anticipate how cells will respond and adapt to chronic mTOR complex (mTORC)1 and mTORC2 inhibition, we have generated SW620 colon cancer cells with acquired resistance to the ATP-competitive mTOR kinase inhibitor AZD8055 (SW620:8055R). AZD8055 inhibited mTORC1 and mTORC2 signalling and caused a switch from cap-dependent to internal ribosome entry site (IRES)-dependent translation in parental SW620 cells. In contrast, SW620:8055R cells exhibited a loss of S6K signalling, an increase in expression of the eukaryotic translation initiation factor eIF4E and increased cap-dependent mRNA translation. As a result, the expression of CCND1 and MCL1, proteins encoded by eIF4E-sensitive and cap-dependent transcripts, was refractory to AZD8055 in SW620:8055R cells. RNAi-mediated knockdown of eIF4E reversed acquired resistance to AZD8055 in SW620:8055R cells; furthermore, increased expression of eIF4E was sufficient to reduce sensitivity to AZD8055 in a heterologous cell system. Finally, although the combination of MEK1/2 inhibitors with mTOR inhibitors is an attractive rational drug combination, SW620:8055R cells were actually cross-resistant to the MEK1/2 inhibitor selumetinib (AZD6244). These results exemplify the convergence of ERK1/2 and mTOR signalling at eIF4E, and the key role of eIF4E downstream of mTOR in maintaining cell proliferation. They also have important implications for therapeutic strategies based around mTOR and the MEK1/2-ERK1/2 pathway.

Adaptation to chronic mTOR inhibition in cancer and in aging.

The mTOR [mammalian (or mechanistic) target of rapamycin] protein kinase co-ordinates catabolic and anabolic processes in response to growth factors and nutrients and is a validated anticancer drug target. Rapamycin and related allosteric inhibitors of mTORC1 (mTOR complex 1) have had some success in specific tumour types, but have not exhibited broad anticancer activity, prompting the development of new ATP-competitive mTOR kinase inhibitors that inhibit both mTORC1 and mTORC2. In common with other targeted kinase inhibitors, tumours are likely to adapt and acquire resistance to mTOR inhibitors. In the present article, we review studies that describe how tumour cells adapt to become resistant to mTOR inhibitors. mTOR is a central signalling hub which responds to an array of signalling inputs and activates a range of downstream effector pathways. Understanding how this signalling network is remodelled and which pathways are invoked to sustain survival and proliferation in the presence of mTOR inhibitors can provide new insights into the importance of the various mTOR effector pathways and may suggest targets for intervention to combine with mTOR inhibitors. Finally, since chronic mTOR inhibition by rapamycin can increase lifespan and healthspan in nematodes, fruitflies and mice, we contrast these studies with tumour cell responses to mTOR inhibition.

ERK5 and its role in tumour development.

The MEK5 [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase 5]/ERK5 pathway is the least well studied MAPK signalling module. It has been proposed to play a role in the pathology of cancer. In the present paper, we review the role of the MEK5/ERK5 pathway using the 'hallmarks of cancer' as a framework and consider how this pathway is deregulated. As well as playing a key role in endothelial cell survival and tubular morphogenesis during tumour neovascularization, ERK5 is also emerging as a regulator of tumour cell invasion and migration. Several oncogenes can stimulate ERK5 activity, and protein levels are increased by a novel amplification at chromosome locus 17p11 and by down-regulation of the microRNAs miR-143 and miR-145. Together, these finding underscore the case for further investigation into understanding the role of ERK5 in cancer.

CDK1, not ERK1/2 or ERK5, is required for mitotic phosphorylation of BIMEL.

The pro-apoptotic BH3 only protein BIM(EL) is phosphorylated by ERK1/2 and this targets it for proteasome-dependent degradation. A recent study has shown that ERK5, an ERK1/2-related MAPK, is activated during mitosis and phosphorylates BIM(EL) to promote cell survival. Here we show that treatment of cells with nocodazole or paclitaxel does cause phosphorylation of BIM(EL), which is independent of ERK1/2. However, this was not due to ERK5-catalysed phosphorylation, since it was not reversed by the MEK5 inhibitor BIX02189 and proceeded normally in ERK5-/- fibroblasts. Indeed, although ERK5 is phosphorylated at multiple sites in the C-terminal transactivation region during mitosis, these do not include the activation-loop and ERK5 kinase activity does not increase. Mitotic phosphorylation of BIM(EL) occurred at proline-directed phospho-acceptor sites and was abolished by selective inhibition of CDK1. Furthermore, cyclin B1 was able to interact with BIM and cyclin B1/CDK1 complexes could phosphorylate BIM in vitro. Finally, we show that CDK1-dependent phosphorylation of BIM(EL) drives its polyubiquitylation and proteasome-dependent degradation to protect cells during mitotic arrest. These results provide new insights into the regulation of BIM(EL) and may be relevant to the therapeutic use of agents such as paclitaxel.

ERK1/2, but not ERK5, is necessary and sufficient for phosphorylation and activation of c-Fos.

Growth factor-stimulated expression and activation of c-Fos is regulated by the ERK1/2 pathway. However, recent reports have also suggested a prominent role for the closely related ERK5 pathway in regulating the expression, transcriptional activation and nuclear localization of c-Fos. Here we have compared the role of ERK1/2 and ERK5 in regulating c-Fos using a combination of conditional protein kinases, selective biochemical inhibitors and ERK5 null fibroblasts. We demonstrate that activation of the ERK1/2 pathway, but not ERK5, is sufficient for c-Fos phosphorylation and transcriptional activation. Furthermore, growth factor-dependent expression of c-Fos is blocked by low doses of PD184352 that selectively inhibit the ERK1/2 pathway but proceeds normally in ERK5-/- 3T9 cells; in addition, nuclear localization of c-Fos is normal in ERK5-/- cells. ERK5-/- cells are, however, defective for c-Jun expression but this is reversed by re-expression of ERK5. In addition to ERK5, neither the JNK nor p38 pathways can substitute for ERK1/2 in the regulation of c-Fos transcriptional activity. These results demonstrate that c-Fos transcriptional activity is not regulated by the ERK5 pathway; rather, of all the MAPKs and SAPKs, c-Fos activation appears to be predominantly linked to the ERK1/2 pathway.

ERK1/2-dependent phosphorylation of BimEL promotes its rapid dissociation from Mcl-1 and Bcl-xL.

The proapoptotic protein Bim is expressed de novo following withdrawal of serum survival factors. Here, we show that Bim-/- fibroblasts and epithelial cells exhibit reduced cell death following serum withdrawal in comparison with their wild-type counterparts. In viable cells, Bax associates with Bcl-2, Bcl-x(L) and Mcl-1. Upon serum withdrawal, newly expressed Bim(EL) associates with Bcl-x(L) and Mcl-1, coinciding with the dissociation of Bax from these proteins. Survival factors can prevent association of Bim with pro-survival proteins by preventing Bim expression. However, we now show that even preformed Bim(EL)/Mcl-1 and Bim(EL)/Bcl-x(L) complexes can be rapidly dissociated following activation of ERK1/2 by survival factors. The dissociation of Bim from Mcl-1 is specific for Bim(EL) and requires ERK1/2-dependent phosphorylation of Bim(EL) at Ser(65). Finally, ERK1/2-dependent dissociation of Bim(EL) from Mcl-1 and Bcl-x(L) may play a role in regulating Bim(EL) degradation, since mutations in the Bim(EL) BH3 domain that disrupt binding to Mcl-1 cause increased turnover of Bim(EL). These results provide new insights into the role of Bim in cell death and its regulation by the ERK1/2 survival pathway.

The duration of ERK1/2 activity determines the activation of c-Fos and Fra-1 and the composition and quantitative transcriptional output of AP-1.

The duration of ERK1/2 activation influences the nature of the biological response to agonist. Members of the AP-1 transcription factor family are well known targets of the ERK1/2 pathway and are expressed in a temporally coordinated fashion during cell cycle re-entry. In CCl39 fibroblasts, sustained ERK1/2 activation is required for the expression of Fra-1, Fra-2, c-Jun and JunB, whereas expression of c-Fos is still strongly induced even in response to transient ERK activation. However, the significance of this pattern of expression for AP-1 activity has not been addressed. Here we show that growth factor stimulated activation of the C-terminal c-Fos transactivation domain (TAD) serves as a sensor for ERK1/2 signal duration whereas the c-JunTAD is not responsive to growth factors. In addition, sustained ERK1/2 activation determines the duration of increases in AP-1 DNA binding complexes as well as their qualitative make up. Finally, this is reflected in both the duration and quantitative transcriptional output of stably integrated AP-1 reporter constructs, indicating that AP-1 activity is finely tuned to ERK1/2 signal duration. These results provide new insights into the importance of ERK1/2 signal duration in the regulation of AP-1 and provide an explanation for how differences in signal duration can lead to both quantitative and qualitative changes in gene expression.