Kathryn Balmanno

Tumour cell survival signalling by the ERK1/2 pathway

Several advances in recent years have focused increasing attention on the role of the RAF-MEK-ERK1/2 pathway in promoting cell survival. The demonstration that BRAF is a human oncogene mutated at high frequency in melanoma, thyroid and colon cancer has provided a pathophysiological context, whilst the description of potent and highly selective inhibitors of BRAF or MEK has allowed a more informed and rational intervention in both normal and tumour cells. In addition, separate studies have uncovered new mechanisms by which the ERK1/2 pathway can control the activity or abundance of members of the BCL-2 protein family to promote cell survival. It is now apparent that various oncogenes co-opt ERK1/2 signalling to de-regulate these BCL-2 proteins and this contributes to, and even underpins, survival signalling in some tumours. New oncogene-targeted therapies allow direct or indirect inhibition of ERK1/2 signalling and can cause quite striking tumour cell death. In other cases, inhibition of the ERK1/2 pathway may be more effective in combination with other conventional and novel therapeutics. Here, we review recent advances in our understanding of how the ERK1/2 pathway regulates BCL-2 proteins to promote survival, how this is de-regulated in tumour cells and the opportunities this might afford with the use of new targeted therapies.

Sustained MAP kinase activation is required for the expression of cyclin D1, p21Cip1 and a subset of AP-1 proteins in CCL39 cells.

In CCL39 cells thrombin is a potent growth factor which requires sustained activation of mitogen activated protein kinases (MAPKs) to promote DNA synthesis. Some of the effects of thrombin can be mimicked by peptides based on the new amino terminus of the cleaved receptor; however, these thrombin receptor peptides (TRPs) fail to induce sustained activation of MAPK or DNA synthesis. We have used thrombin, TRP-7 and other agonists which elicit sustained or transient MAPK activation to identify immediate-early and delayed-early genes which are only expressed under conditions of sustained MAPK activation focusing on cyclin D1, p21Cip1 and the AP-1 transcription factor. Of the stimuli tested only FBS and thrombin were able to stimulate a sustained activation of MAPK, expression of cyclin D1, p21Cip1 and cell cycle re-entry. The expression of cyclin D1 was strongly, though not completely, inhibited by the MEK1 inhibitor PD098059. Thrombin stimulated a rapid but transient accumulation of c-Fos whereas the expression of Fra-1, Fra-2, c-Jun and JunB was sustained throughout the G1 phase of the cell cycle. We focussed our analysis on c-Fos (typical of AP-1 genes which are expressed rapidly and transiently) and Fra-1 and JunB (typical of AP-1 genes expressed after a delay but in a sustained manner). The expression of c-Fos, Fra-1 and JunB was dependent upon the activation of MAPK since these responses were inhibited by PD098059. However, a comparison of responses to FBS, thrombin, TRPs, LPA and EGF revealed that Fra-1 and JunB expression required sustained activation of MAPK whereas c-Fos expression was strongly induced even by non-mitogenic stimuli which elicited only transient MAPK activation. The expression of c-Fos (in response to thrombin, TRP or LPA) or Fra-1, JunB and cyclin D1 (thrombin only) was also inhibited by pertussis toxin. This suggests that both early and late AP-1 gene expression is regulated by the same Gi-mediated, MEK-dependent MAPK signalling pathway but that expression of late AP-1 genes and cyclin D1 requires that this pathway be persistently activated. The results suggest that the duration of receptor signalling and therefore MAPK activation is a key determinant of qualitative changes in gene expression during cell cycle re-entry.

Extracellular signal-regulated kinases 1/2 are serum-stimulated "Bim(EL) kinases" that bind to the BH3-only protein Bim(EL) causing its phosphorylation and turnover.

Bim, a “BH3-only” protein, is expressed de novo following withdrawal of serum survival factors and promotes cell death. We have shown previously that activation of the ERK1/2 pathway promotes phosphorylation of BimEL, targeting it for degradation via the proteasome. However, the nature of the kinase responsible for BimEL phosphorylation remained unclear. We now show that BimEL is phosphorylated on at least three sites in response to activation of the ERK1/2 pathway. By using the peptidylprolyl isomerase, Pin1, as a probe for proline-directed phosphorylation, we show that ERK1/2-dependent phosphorylation of BimEL occurs at (S/T)P motifs. ERK1/2 phosphorylates BimEL, but not BimS or BimL, in vitro, and mutation of Ser65 to alanine blocks the phosphorylation of BimEL by ERK1/2 in vitro and in vivo and prevents the degradation of the protein following activation of the ERK1/2 pathway. We also find that ERK1/2, but not JNK, can physically associate with GST-BimEL, but not GST-BimL or GST-BimS, in vitro. ERK1/2 also binds to full-length BimEL in vivo, and we have localized a potential ERK1/2 “docking domain” lying within a 27-amino acid stretch of the BimEL protein. Our findings provide new insights into the post-translational regulation of BimEL and the role of the ERK1/2 pathway in cell survival signaling.

Activation of ERK1/2 by ΔRaf-1 : ER* represses Bim expression independently of the JNK or PI3K pathways

CC139 fibroblasts are one of several model systems in which the Raf→MEK→ERK1/2 pathway can inhibit apoptosis independently of the PI3K pathway; however, the precise mechanism for this protective effect is not known. Serum withdrawal from CC139 fibroblasts resulted in the rapid onset of apoptosis, which was prevented by actinomycin D or cycloheximide. Serum withdrawal promoted the rapid, de novo accumulation of BimEL, a proapoptotic ‘BH3-only’ member of the Bcl-2 protein family. BimEL expression was an early event, occurring several hours prior to caspase activation. In contrast to studies in neurons, activation of the JNK→c-Jun pathway was neither necessary nor sufficient to induce BimEL expression. Selective inhibition of either the ERK pathway (with U0126) or the PI3K pathway (with LY294002) caused an increase in the expression of BimEL. Furthermore, selective activation of the ERK1/2 pathway by ΔRaf-1:ER* substantially reduced BimEL expression, abolished conformational changes in Bax and blocked the appearance of apoptotic cells. The ability of ΔRaf-1:ER* to repress BimEL expression required the ERK pathway but was independent of the PI3K→PDK→PKB pathway. Thus, serum withdrawal-induced expression of BimEL occurs independently of the JNK→c-Jun pathway and can be repressed by the ERK pathway independently of the PI3K pathway. This may contribute to Raf- and Ras-induced cell survival at low serum concentrations.

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‐xL and Mcl‐1. Upon serum withdrawal, newly expressed BimEL associates with Bcl‐xL 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 BimEL/Mcl‐1 and BimEL/Bcl‐xL complexes can be rapidly dissociated following activation of ERK1/2 by survival factors. The dissociation of Bim from Mcl‐1 is specific for BimEL and requires ERK1/2‐dependent phosphorylation of BimEL at Ser65. Finally, ERK1/2‐dependent dissociation of BimEL from Mcl‐1 and Bcl‐xL may play a role in regulating BimEL degradation, since mutations in the BimEL BH3 domain that disrupt binding to Mcl‐1 cause increased turnover of BimEL. These results provide new insights into the role of Bim in cell death and its regulation by the ERK1/2 survival pathway.

Amplification of the Driving Oncogene, KRAS or BRAF, Underpins Acquired Resistance to MEK1/2 Inhibitors in Colorectal Cancer Cells

Resistance to cancer therapeutics targeting the second kinase in a three-kinase cascade involves amplification of the upstream kinase, not the inhibited kinase. Driving Resistance The promise of using targeted small-molecule kinase inhibitors in treating cancer has been shadowed by the development of resistance to these drugs. Here, Little et al. used colorectal cancer cell lines with oncogenic mutations in either KRAS or BRAF—both of which lead to increased signaling through the ERK (extracellular signal–regulated kinase) signaling pathway—to investigate the mechanisms whereby cells developed resistance to AZD6244, a MEK1/2 (mitogen-activated or extracellular signal–regulated protein kinase kinases 1 and 2) inhibitor now in clinical trials. Rather than developing mutations in MEK1/2, cancer cells became resistant to MEK1/2 through amplification of the driving oncogene (oncogenic KRAS or BRAF) and a consequent increase in signaling through the ERK pathway. These observations have implications for the use of MEK1/2 inhibitors—and possibly other inhibitors that target downstream pathway components rather than the driving oncogene itself—in combination with other antineoplastic therapies. The acquisition of resistance to protein kinase inhibitors is a growing problem in cancer treatment. We modeled acquired resistance to the MEK1/2 (mitogen-activated or extracellular signal–regulated protein kinase kinases 1 and 2) inhibitor selumetinib (AZD6244) in colorectal cancer cell lines harboring mutations in BRAF (COLO205 and HT29 lines) or KRAS (HCT116 and LoVo lines). AZD6244-resistant derivatives were refractory to AZD6244-induced cell cycle arrest and death and exhibited a marked increase in ERK1/2 (extracellular signal–regulated kinases 1 and 2) pathway signaling and cyclin D1 abundance when assessed in the absence of inhibitor. Genomic sequencing revealed no acquired mutations in MEK1 or MEK2, the primary target of AZD6244. Rather, resistant lines showed a marked up-regulation of their respective driving oncogenes, BRAF600E or KRAS13D, due to intrachromosomal amplification. Inhibition of BRAF reversed resistance to AZD6244 in COLO205 cells, which suggested that combined inhibition of MEK1/2 and BRAF may reduce the likelihood of acquired resistance in tumors with BRAF600E. Knockdown of KRAS reversed AZD6244 resistance in HCT116 cells as well as reduced the activation of ERK1/2 and protein kinase B; however, the combined inhibition of ERK1/2 and phosphatidylinositol 3-kinase signaling had little effect on AZD6244 resistance, suggesting that additional KRAS effector pathways contribute to this process. Microarray analysis identified increased expression of an 18-gene signature previously identified as reflecting MEK1/2 pathway output in resistant cells. Thus, amplification of the driving oncogene (BRAF600E or KRAS13D) can drive acquired resistance to MEK1/2 inhibitors by increasing signaling through the ERK1/2 pathway. However, up-regulation of KRAS13D leads to activation of multiple KRAS effector pathways, underlining the therapeutic challenge posed by KRAS mutations. These results may have implications for the use of combination therapies.

Intrinsic resistance to the MEK1/2 inhibitor AZD6244 (ARRY-142886) is associated with weak ERK1/2 signalling and/or strong PI3K signalling in colorectal cancer cell lines†

Mutations in KRAS or BRAF frequently manifest in constitutive activation of the MEK1/2‐ERK1/2 signalling pathway. The MEK1/2‐selective inhibitor, AZD6244 (ARRY‐142886), blocks ERK1/2 activation and is currently undergoing clinical evaluation. Tumour cells can vary markedly in their response to MAPK or ERK kinase (MEK) inhibitors, and the presence of a BRAF mutation is thought to predict sensitivity, with the RAS mutations being associated with intrinsic resistance. We analysed cell proliferation in a panel of 19 colorectal cancer cell lines and found no simple correlation between BRAF or KRAS mutation and sensitivity to AZD6244, though cells that harbour neither mutation tended to be resistant. Cells that were sensitive arrested in G1 and/or underwent apoptosis and the presence of BRAF or KRAS mutation was not sufficient to predict either fate. Cell lines that were resistant to AZD6244 exhibited low or no ERK1/2 activation or exhibited coincident activation of ERK1/2 and protein kinase B (PKB), the latter indicative of activation of the PI3K pathway. In cell lines with coincident ERK1/2 and PKB activation, sensitivity to AZD6244 could be re‐imposed by any of the 3 distinct PI3K/mTOR inhibitors. We conclude that AZD6244 is effective in colorectal cancer cell lines with BRAF or KRAS mutations. Sensitivity to MEK1/2 inhibition correlates with a biochemical signature; those cells with high ERK1/2 activity (whether mutant for BRAF or KRAS) evolve a dependency upon that pathway and tend to be sensitive to AZD6244 but this can be offset by high PI3K‐dependent signalling. This may have implications for the use of MEK inhibitors in combination with PI3K inhibitors.

Apoptosis and autophagy: BIM as a mediator of tumour cell death in response to oncogene-targeted therapeutics.

The BCL‐2 homology domain 3 (BH3)‐only protein, B‐cell lymphoma 2 interacting mediator of cell death (BIM) is a potent pro‐apoptotic protein belonging to the B‐cell lymphoma 2 protein family. In recent years, advances in basic biology have provided a clearer picture of how BIM kills cells and how BIM expression and activity are repressed by growth factor signalling pathways, especially the extracellular signal‐regulated kinase 1/2 and protein kinase B pathways. In tumour cells these oncogene‐regulated pathways are used to counter the effects of BIM, thereby promoting tumour cell survival. In parallel, a new generation of targeted therapeutics has been developed, which show remarkable specificity and efficacy in tumour cells that are addicted to particular oncogenes. It is now apparent that the expression and activation of BIM is a common response to these new therapeutics. Indeed, BIM has emerged from this marriage of basic and applied biology as an important mediator of tumour cell death in response to such drugs. The induction of BIM alone may not be sufficient for significant tumour cell death, as BIM is more likely to act in concert with other BH3‐only proteins, or other death pathways, when new targeted therapeutics are used in combination with traditional chemotherapy agents. Here we discuss recent advances in understanding BIM regulation and review the role of BIM as a mediator of tumour cell death in response to novel oncogene‐targeted therapeutics.

ERK1/2 and p38 cooperate to induce a p21CIP1-dependent G1 cell cycle arrest.

To study the mechanisms by which mitogen- and stress-activated protein kinases regulate cell cycle re-entry, we have used a panel of conditional kinases that stimulate defined MAPK or SAPK cascades. Activation of ΔMEKK3:ER* during serum restimulation of quiescent cells causes a strong activation of JNK1 and p38α but only a modest potentiation of serum-stimulated ERK1/2 activity. In CCl39 cells this promoted a sustained G1 arrest that correlated with decreased expression of cyclin D1 and Cdc25A, increased expression of p21CIP1 and inhibition of CDK2 activity. In Rat-1 cells, in which p21CIP1 expression is silenced by methylation, ΔMEKK3:ER* activation caused only a transient delay in the S phase entry rather than a sustained G1 arrest. Furthermore, p21CIP1−/− 3T3 cells were defective for the ΔMEKK3:ER*-induced G1 cell cycle arrest compared to their wild-type counterparts. These results suggest that activated ΔMEKK3:ER* inhibits the G1 → S progression by two kinetically distinct mechanisms, with expression of p21CIP1 being required to ensure a sustained G1 cell cycle arrest. The ERK1/2 and p38αβ pathways cooperated to induce p21CIP1 expression and inhibition of p38αβ caused a partial reversal of the cell cycle arrest. In contrast, selective activation of ERK1/2 by ΔRaf-1:ER* did not inhibit serum stimulated cell cycle re-entry. Finally, selective activation of JNK by ΔMEKK1:ER* failed to inhibit cell cycle re-entry, even in cells that retained wild-type p53, arguing against a major role for JNK alone in antagonizing the G1 → S transition.

Tumour cell responses to new fibroblast growth factor receptor tyrosine kinase inhibitors and identification of a gatekeeper mutation in FGFR3 as a mechanism of acquired resistance

Fibroblast growth factor receptors (FGFRs) can act as driving oncoproteins in certain cancers, making them attractive drug targets. Here we have characterized tumour cell responses to two new inhibitors of FGFR1–3, AZ12908010 and the clinical candidate AZD4547, making comparisons with the well-characterized FGFR inhibitor PD173074. In a panel of 16 human tumour cell lines, the anti-proliferative activity of AZ12908010 or AZD4547 was strongly linked to the presence of deregulated FGFR signalling, indicating that addiction to deregulated FGFRs provides a therapeutic opportunity for selective intervention. Acquired resistance to targeted tyrosine kinase inhibitors is a growing problem in the clinic but has not yet been explored for FGFR inhibitors. To assess how FGFR-dependent tumour cells adapt to long-term FGFR inhibition, we generated a derivative of the KMS-11 myeloma cell line (FGFRY373C) with acquired resistance to AZ12908010 (KMS-11R cells). Basal phosphorylated FGFR and FGFR-dependent downstream signalling were constitutively elevated and refractory to drug in KMS-11R cells. Sequencing of FGFR3 in KMS-11R cells revealed the presence of a heterozygous mutation at the gatekeeper residue, encoding FGFR3V555M; consistent with this, KMS-11R cells were cross-resistant to AZD4547 and PD173074. These results define the selectivity and efficacy of two new FGFR inhibitors and identify a secondary gatekeeper mutation as a mechanism of acquired resistance to FGFR inhibitors that should be anticipated as clinical evaluation proceeds.