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

Bim and Mcl-1 exert key roles in regulating JAK2V617F cell survival

Background

The JAK2^V617F mutation plays a major role in the pathogenesis of myeloproliferative neoplasms and is found in the vast majority of patients suffering from polycythemia vera and in roughly every second patient suffering from essential thrombocythemia or from primary myelofibrosis. The V617F mutation is thought to provide hematopoietic stem cells and myeloid progenitors with a survival and proliferation advantage. It has previously been shown that activated JAK2 promotes cell survival by upregulating the anti-apoptotic STAT5 target gene Bcl-xL. In this study, we have investigated the role of additional apoptotic players, the pro-apoptotic protein Bim as well as the anti-apoptotic protein Mcl-1.

Methods

Pharmacological inhibition of JAK2/STAT5 signaling in JAK2^V617F mutant SET-2 and MB-02 cells was used to study effects on signaling, cell proliferation and apoptosis by Western blot analysis, WST-1 proliferation assays and flow cytometry. Cells were transfected with siRNA oligos to deplete candidate pro- and anti-apoptotic proteins. Co-immunoprecipitation assays were performed to assess the impact of JAK2 inhibition on complexes of pro- and anti-apoptotic proteins.

Results

Treatment of JAK2^V617F mutant cell lines with a JAK2 inhibitor was found to trigger Bim activation. Furthermore, Bim depletion by RNAi suppressed JAK2 inhibitor-induced cell death. Bim activation following JAK2 inhibition led to enhanced sequestration of Mcl-1, besides Bcl-xL. Importantly, Mcl-1 depletion by RNAi was sufficient to compromise JAK2^V617F mutant cell viability and sensitized the cells to JAK2 inhibition.

Conclusions

We conclude that Bim and Mcl-1 have key opposing roles in regulating JAK2^V617F cell survival and propose that inactivation of aberrant JAK2 signaling leads to changes in Bim complexes that trigger cell death. Thus, further preclinical evaluation of combinations of JAK2 inhibitors with Bcl-2 family antagonists that also tackle Mcl-1, besides Bcl-xL, is warranted to assess the therapeutic potential for the treatment of chronic myeloproliferative neoplasms.

Glucocorticoid-mediated repression of the oncogenic microRNA cluster miR-17~92 contributes to the induction of Bim and initiation of apoptosis

Synthetic glucocorticoids were one of the first effective treatments for lymphoid malignancies because of their ability to induce apoptosis and are still used in combination with other chemotherapeutic agents. Up-regulation of Bim, a proapoptotic member of the B-cell lymphoma-2 family, is an important mediator of glucocorticoid-induced apoptosis. Although glucocorticoids are known to elevate Bim mRNA and protein, little is known about the mechanism. Here, we report that glucocorticoids repress the expression of the microRNA cluster miR-17∼92, which results in elevated Bim protein expression as a mechanism by which glucocorticoids induce Bim. Using a luciferase-Bim 3' untranslated region construct, we demonstrate that glucocorticoids mediate Bim induction posttranscriptionally after miR-17∼92 repression, resulting in increased Bim protein expression. Overexpression of miR-17∼92 microRNAs decreases Bim induction and attenuates glucocorticoid-mediated apoptosis. Conversely, knockdown of miR-17∼92 increases Bim protein expression and glucocorticoid-mediated apoptosis. These findings indicate that endogenous levels of miR-17∼92 repress Bim expression in T-cell lymphoid malignancies and that glucocorticoids induce Bim expression via down-regulation of the miR-17∼92 microRNA cluster. Our findings present a novel mechanism that contributes to the up-regulation of Bim and induction of apoptosis in lymphocytes after glucocorticoid treatment. Furthermore, our work demonstrating that inhibition of miR-17∼92 increases glucocorticoid-induced apoptosis highlights the potential importance of miR-17∼92 as a therapeutic target in leukemias and lymphomas.

The essential role of evasion from cell death in cancer

The link between evasion of apoptosis and the development of cellular hyperplasia and ultimately cancer is implicitly clear if one considers how many cells are produced each day and, hence, how many cells must die to make room for the new ones (reviewed in Raff, 1996). Furthermore, cells are frequently experiencing noxious stimuli that can cause lesions in their DNA and faults in DNA replication can occur during cellular proliferation. Such DNA damage needs to be repaired efficiently or cells with irreparable damage must be killed to prevent subsequent division of aberrant cells that may fuel tumorigenesis (reviewed in Weinberg, 2007). The detection of genetic lesions in human cancers that activate prosurvival genes or disable proapoptotic genes have provided the first evidence that defects in programmed cell death can cause cancer (Tagawa et al., 2005; Tsujimoto et al., 1984; Vaux, Cory, and Adams, 1988) and this concept was proven by studies with genetically modified mice (Egle et al., 2004b; Strasser et al., 1990a). It is therefore now widely accepted that evasion of apoptosis is a requirement for both neoplastic transformation and sustained growth of cancer cells (reviewed in Cory and Adams, 2002; Hanahan and Weinberg, 2000; Weinberg, 2007). Importantly, apoptosis is also a major contributor to anticancer therapy-induced killing of tumor cells (reviewed in Cory and Adams, 2002; Cragg et al., 2009). Consequently, a detailed understanding of apoptotic cell death will help to better comprehend the complexities of tumorigenesis and should assist with the development of improved targeted therapies for cancer based on the direct activation of the apoptotic machinery (reviewed in Lessene, Czabotar, and Colman, 2008).

BRAF, a target in melanoma: implications for solid tumor drug development

The successful translation of therapies targeting signal-transduction pathways that are activated by oncogenes has provided a model for molecularly targeted therapy, and the identification of mutations in v-raf murine sarcoma viral oncogene homolog B1 (BRAF), a serine/threonine kinase, has turned the attention of the melanoma field toward this concept. The current review indicated that BRAF represents an important target in cancer, in part because it is present in 7% of all cancers and also because it represents the first intracellular signaling molecule that is activated by point mutations for which single-agent therapy appears to have efficacy. Therapy for advanced melanoma has progressed slowly over the past 3 decades, although significant advances have been made in other cancers with the application of cytotoxic chemotherapy and targeted therapies. However, in melanoma, cytotoxic chemotherapies have severe limits, chemotherapy does not convincingly improve on the natural history of metastatic disease and has no role in the adjuvant setting, and cytokine therapy may have a niche in both the adjuvant and metastatic settings but confers only a modest benefit to a small proportion of patients at the cost of severe toxicity. Thus, there are few other cancers in which completely novel therapies are so highly prioritized in clinical research. Understanding network of signal-transduction pathways and how that network may adapt to BRAF inhibition or mitogen-activated protein kinase kinase inhibition will point to the next generation of clinical trials investigating rational combination regimens. The current investigations in melanoma will create a set of hypotheses to be tested in each cancer that harbors BRAF mutations.

Cyclic-AMP-dependent protein kinase A regulates apoptosis by stabilizing the BH3-only protein Bim

The proapoptotic Bcl2 homology domain 3(BH3)-only protein Bim is controlled by stringent post-translational regulation, predominantly through alterations in phosphorylation status. To identify new kinases involved in its regulation, we carried out a yeast two-hybrid screen using a non-spliceable variant of the predominant isoform--Bim(EL)--as the bait and identified the regulatory subunit of cyclic-AMP-dependent protein kinase A--PRKAR1A--as an interacting partner. We also show that protein kinase A (PKA) is a Bim(EL) isoform-specific kinase that promotes its stabilization. Inhibition of PKA or mutation of the PKA phosphorylation site within Bim(EL) resulted in its accelerated proteasome-dependent degradation. These results might have implications for human diseases that are characterized by abnormally increased PKA activity, such as the Carney complex and dilated cardiomyopathy.

Phosphorylation of Puma modulates its apoptotic function by regulating protein stability

Puma is a potent BH3-only protein that antagonises anti-apoptotic Bcl-2 proteins, promotes Bax/Bak activation and has an essential role in multiple apoptotic models. Puma expression is normally kept very low, but can be induced by several transcription factors including p53, p73, E2F1 and FOXO3a, whereby it can induce an apoptotic response. As Puma can to bind and inactivate all anti-apoptotic members of the Bcl-2 family, its activity must be tightly controlled. We report here, for the first time, evidence that Puma is subject to post-translational control through phosphorylation. We show that Puma is phosphorylated at multiple sites, with the major site of phosphorylation being serine 10. Replacing serine 10 with alanine causes reduced Puma turnover and enhanced cell death. Interestingly, Puma turnover occurs through the proteasome, and substitution of serine 10 causes elevated Puma levels independently of macroautophagy, Bcl-2 family member binding, caspase activity and apoptotic death. We conclude, therefore, that phosphorylation of Puma at serine 10 promotes Puma turnover, represses Puma's cell death potential and promotes cell survival. Owing to the highly pro-apoptotic nature of Puma, these studies highlight an important additional regulatory step in the determination of cellular life or death.

Lipoxin A4 impairment of apoptotic signaling in macrophages: implication of the PI3K/Akt and the ERK/Nrf-2 defense pathways

Lipoxin A(4) (LXA(4)) is an endogenous lipid mediator that requires transcellular metabolic traffic for its synthesis. The targets of LXA(4) on neutrophils are well described, contributing to attenuation of inflammation. However, effects of lipoxins on macrophage are less known, particularly the action of LXA(4) on the regulation of apoptosis of these cells. Our data show that pretreatment of human or murine macrophages with LXA(4) at the concentrations prevailing in the course of resolution of inflammation (nanomolar range) significantly inhibits the apoptosis induced by staurosporine, etoposide and S-nitrosoglutathione or by more pathophysiological stimuli, such as LPS/IFNgamma challenge. The release of mitochondrial mediators of apoptosis and the activation of caspases was abrogated in the presence of LXA(4). In addition to this, the synthesis of reactive oxygen species induced by staurosporine was attenuated and antiapoptotic proteins of the Bcl-2 family accumulated in the presence of lipoxin. Analysis of the targets of LXA(4) identified an early activation of the PI3K/Akt and ERK/Nrf-2 pathways, which was required for the observation of the antiapoptotic effects of LXA(4). These data suggest that the LXA(4), released after the recruitment of neutrophils to sites of inflammation, exerts a protective effect on macrophage viability that might contribute to a better resolution of inflammation.

The proapoptotic BH3-only protein Bim is downregulated in a subset of colorectal cancers and is repressed by antiapoptotic COX-2/PGE(2) signalling in colorectal adenoma cells

Overexpression of cyclooxygenase-2 (COX-2) and elevated levels of its enzymatic product prostaglandin E2 (PGE(2)) occur in the majority of colorectal cancers and have important roles in colorectal tumorigenesis. However, despite the established prosurvival role of PGE(2) in cancer, the underlying mechanisms are not fully understood. Here, we have shown that PGE(2) suppresses apoptosis via repression of the proapoptotic BH3-only protein Bim in human colorectal adenoma cells. Repression of Bim expression was dependent upon PGE(2)-mediated activation of the Raf-MEK-ERK1/2 pathway, which promoted Bim phosphorylation and proteasomal degradation. Reduction of Bim expression using RNA interference reduced spontaneous apoptosis in adenoma cells and abrogated PGE(2)-dependent apoptosis suppression. Treatment of COX-2-expressing colorectal carcinoma cells with COX-2-selective NSAIDs-induced Bim expression, suggesting that Bim repression via PGE(2) signalling may be opposed by COX-2 inhibition. Examination of Bim expression in two established in vitro models of the adenoma-carcinoma sequence revealed that downregulation of Bim expression was associated with tumour progression towards an anchorage-independent phenotype. Finally, immunohistochemical analyses revealed that Bim expression is markedly reduced in approximately 40% of human colorectal carcinomas in vivo. These observations highlight the COX-2/PGE(2) pathway as an important negative regulator of Bim expression in colorectal tumours and suggest that Bim repression may be an important step during colorectal cancer tumorigenesis.

Crossing paths: interactions between the cell death machinery and growth factor survival signals

Cytokines and growth factors play a crucial role in the maintenance of haematopoietic homeostasis. They transduce signals that regulate the competing commitments of haematopoietic stem cells, quiescence or proliferation, retention of stem cell pluripotency or differentiation, and survival or demise. When the balance between these commitments and the requirements of the organisms is disturbed, particularly when it favours survival and proliferation, cancer may result. Cell death provoked by loss of growth factor signalling is regulated by the Bcl-2 family of apoptosis regulators, and thus survival messages transduced by growth factors must regulate the activity of these proteins. Many aspects of direct interactions between cytokine signalling and regulation of apoptosis remain elusive. In this review, we explore the mechanisms by which cytokines, in particular Interleukin-3 and granulocyte-macrophage colony-stimulating factor, promote cell survival and suppress apoptosis as models of how cytokine signalling and apoptotic pathways intersect.

Intrinsically disordered proteins in bcl-2 regulated apoptosis

Intrinsic cell death is mediated by interaction between pro-apoptotic and pro-survival proteins of the B-cell lymphoma-2 (Bcl-2) family. Members of this family are either intrinsically disordered or contain intrinsically disordered regions/domains that are critical to their function. Alternate splicing and post-translational modifications can determine the extent of these disordered regions and are critical for regulating Bcl-2 proteins. Conformational plasticity and structural transitions characterize the interactions within the Bcl-2 family, with conserved sequence motifs on both binding partners required for their molecular recognition.

Apoptosis induced by JAK2 inhibition is mediated by Bim and enhanced by the BH3 mimetic ABT-737 in JAK2 mutant human erythroid cells

The activating mutation JAK2 V617F plays a central role in the pathogenesis of polycythemia vera, essential thrombocythemia, and primary myelofibrosis. Inhibition of JAK2 activity leads to growth inhibition and apoptosis in cells with mutated JAK2. However, the proapoptotic proteins involved in JAK2 inhibition-induced apoptosis remain unclear. In this study, we show that JAK2 inhibition-induced apoptosis correlated with up-regulation of the nonphosphorylated form of the BH3-only protein Bim in hematopoietic cell lines bearing JAK2 mutations. Knockdown of Bim dramatically inhibited apoptosis induced by JAK2 inhibition, which was reversed by the BH3 mimetic agent ABT-737. In addition, ABT-737 enhanced the apoptosis induced by JAK2 inhibition in JAK2 V617F(+) HEL and SET-2 cells. The combination of JAK inhibitor I and ABT-737 reduced the number of erythroid colonies derived from CD34(+) cells isolated from JAK2 V617F(+) polycythemia vera patients more efficiently than either drug alone. These data suggest that Bim is a key effector molecule in JAK2 inhibition-induced apoptosis and that targeting this apoptotic pathway could be a novel therapeutic strategy for patients with activating JAK2 mutations.

Pem renders tumor cells resistant to apoptotic cell death induced by a CD8+ T cell-mediated immune response or anticancer drug treatment

Pem, a member of homeobox genes, is an oncofetal gene which is preferentially expressed in reproductive tissues and in multiple tumor cell lines. However, the function of Pem in tumor cell lines has not been elucidated. Herein we report that the ectopic expression of Pem in TC-1, a human papillomavirus type 16 (HPV-16) E7-expressing surrogate cervical tumor cell line, demonstrated a significant increase in extracellular signal-regulated kinase (ERK) activity and multiple resistance to various apoptotic pressures from an E7-specific CD8(+) T cell-mediated immune response and anticancer drug treatment. The observed resistance to apoptotic death of the Pem-over-expressing TC-1 tumor cells (TC-1/Pem) was associated with the down-regulation of a pro-apoptotic molecule, such as BIM, and up-regulation of an anti-apoptotic molecule, such as Bcl-2 protein, which mediated ERK activation. We also observed that the intratumoral injection of an ERK inhibitor enhanced the therapeutic efficacy of E7-specific CD8(+) T cell adoptive transfer or anticancer drug treatment against the resistant TC-1/Pem tumor. This is the first evidence demonstrating an association between Pem and a signaling pathway, namely the ERK-mediated survival signal transduction pathway. Thus, our data indicate that activation of the ERK pathway represents a new mechanism of Pem-mediated multiple resistances and the present research will contribute to the development of a novel strategy in cancer therapy against Pem-over-expressing tumor cells.

Inhibition of MEK/ERK1/2 sensitizes lymphoma cells to sorafenib-induced apoptosis

Interactions between the multi-kinase inhibitor sorafenib and MEK1/2 inhibitors were investigated in DLBCL cells. Sorafenib (3-10 microM) triggered apoptosis in multiple GC and ABC lymphoma cells. Unexpectedly, sorafenib did not cause sustained ERK1/2 inactivation, and in SUDHL-6 and -16 cells, triggered ERK1/2 activation. Marginally toxic MEK1/2 inhibitor concentrations (5 microM PD184352) abrogated ERK1/2 activation in sorafenib-treated cells and synergistically potentiated apoptosis. MEK1 shRNA transfection also significantly increased sorafenib-mediated lethality. Sorafenib/PD184352 co-administration accelerated Mcl-1 down-regulation without up-regulating Bim(EL). Finally, ectopic Mcl-1 expression attenuated sorafenib/PD184352-mediated apoptosis. Together, these findings provide a theoretical basis for potentiating sorafenib anti-lymphoma activity by MEK1/2 inhibitors.

Exploiting cellular pathways to develop new treatment strategies for AML

The standard approaches to the treatment of acute myeloid leukemia (AML) have been predominantly based on cytarabine and anthracyclines. Yet, the outcomes associated with AML continue to be poor, especially for those patients who are older or carry higher-risk disease. In recent years, extensive research has led to the development and study of novel agents which target AML by diverse and varied mechanisms. Among these are targeted therapeutics such as kinase inhibitors and oligonucleotide constructs. These aim to suppress the production or activity of proteins, such as FLT3 and BCL2, among others, and thus disrupt related signaling cascades essential for leukemogenesis and proliferation. In addition, other agents like flavopiridol appear to target the myeloid blast by various mechanisms including suppression of cyclin-dependent kinases and interference with nucleotide synthesis. Another class of novel therapies includes inhibitors of histone deacetylase, which cause growth arrest and apoptosis through histone acetylation and resultant conformational changes. Clinical trials are now studying these and other agents alone and in combination with traditional cytotoxic therapies, with some encouraging results. In this review, we aim to provide a summary of the preclinical and clinical investigations of selected promising agents currently under study.

Sorafenib inhibits ERK1/2 and MCL-1(L) phosphorylation levels resulting in caspase-independent cell death in malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is an aggressive, rapidly progressive malignancy without effective therapy. We evaluate sorafenib efficacy and impact on the cellular pro-survival machinery in vitro, efficacy of sorafenib as monotherapy and in combination with the naturally occurring death receptor agonist, TRAIL using human MPM cell lines, MSTO-211H, M30, REN, H28, H2052 and H2452. In vitro studies of the six MPM lines demonstrated single agent sensitivity to the multikinase inhibitor sorafenib and resistance to TRAIL. H28 and H2452 demonstrated augmented apoptosis with the addition of TRAIL to sorafenib in vitro. Treated cell lines demonstrated sorafenib-induced rapid dephosphorylation of AKT followed shortly by near complete dephosphorylation of the constitutively phosphorylated ERK1/2. Sorafenib therapy also decreased phosphorylation of B-Raf and mTOR in several cell lines. Within 3 h of sorafenib treatment, a number of known pro-survival molecules were dephosphorylated and/or downregulated in expression including MCL-1(L), c-FLIP(L), survivin and cIAP(1). These changes and eventual cell death did not elicit significant caspase-3 activation or PARP cleavage and pretreatment with the pan-caspase inhibitor, Z-VAD-FMK, did not block sorafenib efficacy but did block the effect of TRAIL monotherapy. Pre-treatment with Z-VAD-FMK did not block the synergistic effect of TRAIL and sorafenib in H28. In summary, single agent treatment with sorafenib results in widespread inhibition of the pro-survival machinery in vitro leading to cell death via a primarily caspase-independent mechanism. Combining sorafenib therapy with TRAIL, may be useful in order to provide a more efficient death signal and this synergistic effect appears to be caspase-independent. Pilot in vivo data demonstrates promising evidence of therapeutic efficacy in human tumor bearing xenograft nu/nu mice. We document single agent activity of sorafenib against MPM, unravel novel effects of sorafenib on anti-apoptotic signaling mediators, and suggest the combination of sorafenib plus TRAIL as possible therapy for clinical testing in MPM.

Bim upregulation by histone deacetylase inhibitors mediates interactions with the Bcl-2 antagonist ABT-737: evidence for distinct roles for Bcl-2, Bcl-xL, and Mcl-1

The Bcl-2 antagonist ABT-737 kills transformed cells in association with displacement of Bim from Bcl-2. The histone deactetylase (HDAC) inhibitor suberoyl bis-hydroxamic acid (SBHA) was employed to determine whether and by what mechanism ABT-737 might interact with agents that upregulate Bim. Expression profiling of BH3-only proteins indicated that SBHA increased Bim, Puma, and Noxa expression, while SBHA concentrations that upregulated Bim significantly potentiated ABT-737 lethality. Concordance between SBHA-mediated Bim upregulation and interactions with ABT-737 was observed in various human leukemia and myeloma cells. SBHA-induced Bim was largely sequestered by Bcl-2 and Bcl-x(L), rather than Mcl-1; ABT-737 attenuated these interactions, thereby triggering Bak/Bax activation and mitochondrial outer membrane permeabilization. Knockdown of Bim (but not Puma or Noxa) by shRNA or ectopic overexpression of Bcl-2, Bcl-x(L), or Mcl-1 diminished Bax/Bak activation and apoptosis. Notably, ectopic expression of these antiapoptotic proteins disabled death signaling by sequestering different proapoptotic proteins, i.e., Bim by Bcl-2, both Bim and Bak by Bcl-x(L), and Bak by Mcl-1. Together, these findings indicate that HDAC inhibitor-inducible Bim is primarily neutralized by Bcl-2 and Bcl-x(L), thus providing a mechanistic framework by which Bcl-2 antagonists potentiate the lethality of agents, such as HDAC inhibitors, which upregulate Bim.

Interleukin-7 regulates Bim proapoptotic activity in peripheral T-cell survival

Interleukin-7 (IL-7) is critical for T-cell development and peripheral T-cell homeostasis. The survival of pro-T cells and mature T cells requires IL-7. The survival function of IL-7 is accomplished partly through induction of the antiapoptotic protein Bcl-2 and inhibition of proapoptotic proteins Bax and Bad. We show here that the proapoptotic protein Bim, a BH3-only protein belonging to the Bcl-2 family, also plays a role in peripheral T-cell survival. Deletion of Bim partially protected an IL-7-dependent T-cell line and peripheral T cells, especially cells with an effector memory phenotype, from IL-7 deprivation. However, T-cell development in the thymus was not restored in IL-7(-/-) Rag2(-/-) mice reconstituted with Bim(-/-) bone marrow. IL-7 withdrawal altered neither the intracellular location of Bim, which was constitutively mitochondrial, nor its association with Bcl-2; however, a reduction in its association with the prosurvival protein Mcl-1 was observed. IL-7 withdrawal did not increase Bim mRNA or protein expression but did induce changes in the isoelectric point of Bim(EL) and its reactivity with an antiphosphoserine antibody. Our findings suggest that the maintenance of peripheral T cells by IL-7 occurs partly through inhibition of Bim activity at the posttranslational level.

How adhesion signals reach a mitochondrial conclusion--ECM regulation of apoptosis

A fundamental aspect in metazoans is the ability of a cell to recognise its positional context within a tissue. This is important in both development and homeostasis, where cell proliferation, differentiation and apoptosis are strictly controlled to form and maintain tissues. Much information has been generated on how cells receive and interpret adhesion-mediated signals. The non-receptor tyrosine kinase, Fak (focal adhesion kinase) has received much attention with regard to adhesion mediated signalling, including its role in survival. Survival signals are required to suppress the default pathway of apoptosis. The ultimate outcome of apoptotic signalling is the release of factors from the mitochondria into the cytosol. How the defined signalling pathways that control apoptosis converge on the mitochondria is an area with many unresolved questions.

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.

The BH3-only protein Bim plays a critical role in leukemia cell death triggered by concomitant inhibition of the PI3K/Akt and MEK/ERK1/2 pathways

Mechanisms underlying apoptosis induced by concomitant interruption of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 (MEK/ERK1/2) and phosphatidylinositol 3-kinase (PI3K)/Akt pathways were investigated in human leukemia cells. Inhibition of these pathways using the MEK inhibitor PD184352 or U0126 and the PI3K/Akt inhibitor perifosine strikingly induced apoptosis in multiple malignant human hematopoietic cells, and substantially reduced the colony-forming capacity of primary acute myeloblastic leukemia, but not normal CD34+ cells. These events were associated with pronounced Bim up-regulation, Mcl-1 down-regulation, marked Bak/Bax conformational change accompanied by Bax membrane translocation, and a pronounced increase in Bax/Bak association. Molecular studies using tet-inducible Akt, constitutively active MEK1, dominant-negative Akt, and MEK1 small interfering RNA revealed that inhibition of both MEK/ERK1/2 and Akt pathways plays a critical functional role in perifosine/PD184352-mediated lethality. Ectopic Mcl-1 expression potently inhibited perifosine/PD184352-induced apoptosis, as did Bak or Bax knockdown. Notably, knockdown of Bim, but not Bad, blocked Bak and Bax conformational change, inhibited Bax membrane translocation, diminished Bax/Bak binding, and sharply attenuated perifosine/PD184352-induced apoptosis. Finally, enforced expression of Bim significantly enhanced apoptosis induced by PI3K/Akt inhibitors, analogous to the effects of MEK1/2 inhibitors. Collectively, these findings suggest that Bim, and Mcl-1, but not Bad, integrate death signaling triggered by concomitant disruption of the PI3K/Akt and MEK1/2/ERK1/2 pathways in human leukemia cells.

Neuroprotective effects of the complement terminal pathway during demyelination: implications for oligodendrocyte survival

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system that is mediated by activated lymphocytes, macrophages/microglia, and complement. In MS, the myelin-forming oligodendrocytes (OLGs) are the targets of the immune attack. Experimental evidence indicates that C5b-9 plays a role in demyelination during the acute phase of experimental allergic encephalomyelitis (EAE). Terminal complement C5b-9 complexes are capable of protecting OLGs from apoptosis. During chronic EAE complement C5 promotes axonal preservation, remyelination and provides protection from gliosis. These findings indicate that the activation of complement and C5b-9 assembly can also have protective roles during demyelination.

MEK/ERK-mediated phosphorylation of Bim is required to ensure survival of T and B lymphocytes during mitogenic stimulation

Survival and death of lymphocytes are regulated by the balance between pro- and antiapoptotic members of the Bcl-2 family; this is coordinated with the control of cell cycling and differentiation. Bim, a proapoptotic BH3-only member of the Bcl-2 family, can be regulated by MEK/ERK-mediated phosphorylation, which affects its binding to pro-survival Bcl-2 family members and its turnover. We investigated Bim modifications in mouse B and T lymphoid cells after exposure to apoptotic stimuli and during mitogenic activation. Treatment with ionomycin or cytokine withdrawal caused an elevation in Bim(EL), the most abundant Bim isoform. In contrast, in mitogenically stimulated T and B cells, Bim(EL) was rapidly phosphorylated, and its levels declined. Pharmacological inhibitors of MEK/ERK signaling prevented both of these changes in Bim, reduced proliferation, and triggered apoptosis of mitogen-stimulated T and B cells. Loss of Bim prevented this cell killing but did not restore cell cycling. These results show that during mitogenic stimulation of T and B lymphocytes MEK/ERK signaling is critical for two distinct processes, cell survival, mediated (at least in part) through phosphorylation and consequent inhibition of Bim, and cell cycling, which proceeds independently of Bim inactivation.

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.

New insights into the regulation and function of serine/threonine kinases in T lymphocytes

The development of T lymphocytes in the thymus and the function of mature T cells in adaptive immune responses are choreographed by antigen receptors, costimulatory molecules, adhesion molecules, cytokines, and chemokines. These extrinsic stimuli are coupled to a diverse network of signal transduction pathways that control the transcriptional and metabolic programs that determine T-cell function. At the core of T-lymphocyte signal transduction is the regulated metabolism of inositol phospholipids and the production of two key lipid second messengers: polyunsaturated diacylglycerols (DAGs) and phosphatidylinositol (3-5) triphosphate [PI-(3-5)-P(3)]. The object of the present review is to discuss facts, controversies, and unresolved issues about DAG and PI-(3,4,5)-P(3) production in T lymphocytes and to discuss some of the serine/threonine kinases that control unique aspects of T-lymphocyte biology and coordinate T-cell participation in adaptive immune responses.

Transforming growth factor beta (TGFbeta)-induced apoptosis: the rise & fall of Bim

Transforming growth factor beta (TGFbeta) regulates essential cellular functions such as cellular proliferation, differentiation and apoptosis. Multiple apoptotic mediators and signaling pathways have been implicated in TGFbeta-induced apoptosis. Bim, a BH3-only protein, is critical for apoptosis in a variety of cell types. In resting cells, BimEL expression levels, the major and most abundant isoform, are controlled by Erk1/2-mediated phosphorylation, which targets BimEL for ubiquitination and degradation. We previously reported that TGFbeta induces the expression of the pro-apoptotic protein Bim through a Smad3-dependent mechanism to induce cell death in B-lymphocytes. A number of studies have shown TGFbeta to cause transcriptional induction of Bim in many cell types. Recently, we demonstrated that, in addition to its transcriptional effects on Bim, TGFbeta induces a MAPK phosphatase (MKP), MKP2/DUSP4, to rapidly increase BimEL levels by inactivation of Erk1/2, resulting in dephosphorylation and escape of BimEL from ubiquitin-mediated degradation. Our findings are of importance not only in the context that we implicate TGFbeta to increase BimEL levels through both an immediate post-translational regulatory mechanism and a long-term effect through transcriptional induction, but also in the context of implicating MKPs as regulatory players in apoptosis. Here we summarize these recent findings and their significance to our understanding of how TGFbeta mediates apoptosis, and we explore the possible regulatory mechanisms controlling Bim expression levels.

A protein kinase Cdelta-dependent protein kinase D pathway modulates ERK1/2 and JNK1/2 phosphorylation and Bim-associated apoptosis by asbestos

Inhalation of asbestos and oxidant-generating pollutants causes injury and compensatory proliferation of lung epithelium, but the signaling mechanisms that lead to these responses are unclear. We hypothesized that a protein kinase (PK)Cdelta-dependent PKD pathway was able to regulate downstream mitogen-activated protein kinases, affecting pro- and anti-apoptotic responses to asbestos. Elevated levels of phosphorylated PKD (p-PKD) were observed in distal bronchiolar epithelial cells of mice inhaling asbestos. In contrast, PKCdelta-/- mice showed significantly lower levels of p-PKD in lung homogenates and in situ after asbestos inhalation. In a murine lung epithelial cell line, asbestos caused significant increases in the phosphorylation of PKCdelta-dependent PKD, ERK1/2, and JNK1/2/c-Jun that occurred with decreases in the BH3-only pro-apoptotic protein, Bim. Silencing of PKCdelta, PKD, and use of small molecule inhibitors linked the ERK1/2 pathway to the prevention of Bim-associated apoptosis as well as the JNK1/2/c-Jun pathway to the induction of apoptosis. Our studies are the first to show that asbestos induces PKD phosphorylation in lung epithelial cells both in vivo and in vitro. PKCdelta-dependent PKD phosphorylation by asbestos is causally linked to a cellular pathway that involves the phosphorylation of both ERK1/2 and JNK1/2, which play opposing roles in the apoptotic response induced by asbestos.

Colorectal cancer cells with the BRAF(V600E) mutation are addicted to the ERK1/2 pathway for growth factor-independent survival and repression of BIM

The RAF-mitogen-activated protein kinase kinase 1/2-extracellular signal-regulated kinase 1/2 (RAF-MEK1/2-ERK1/2) pathway is activated in many human tumours and can protect cells against growth factor deprivation; however, most such studies have relied upon overexpression of RAF or MEK constructs that are not found in tumours. Here we show that expression of the endogenous BRAF(V600E) allele in mouse embryonic fibroblasts from conditional knock-in transgenic mice activates ERK1/2, represses the BH3-only protein BIM and protects cells from growth factor withdrawal. Human colorectal cancer (CRC) cell lines harbouring BRAF(V600E) are growth factor independent for the activation of ERK1/2 and survival. However, treatment with the MEK1/2 inhibitors U0126, PD184352 or the novel clinical candidate AZD6244 (ARRY-142886) overcomes growth factor independence, causing CRC cell death. BIM is de-phosphorylated and upregulated following MEK1/2 inhibition in all CRC cell lines studied and knockdown of BIM reduces cell death, indicating that repression of BIM is a major part of the ability of BRAF(V600E) to confer growth factor-independent survival. We conclude that a single endogenous BRAF(V600E) allele is sufficient to repress BIM and prevent death arising from growth factor withdrawal, and CRC cells with BRAF(V600E) mutations are addicted to the ERK1/2 pathway for repression of BIM and growth factor-independent survival.

Treatment of B-RAF mutant human tumor cells with a MEK inhibitor requires Bim and is enhanced by a BH3 mimetic

B-RAF is frequently mutated in solid tumors, resulting in activation of the MEK/ERK signaling pathway and ultimately tumor cell growth and survival. MEK inhibition in these cells results in cell cycle arrest and cytostasis. Here, we have shown that MEK inhibition also triggers limited apoptosis of human tumor cell lines with B-RAF mutations and that this effect was dependent on upregulation and dephosphorylation of the proapoptotic, Bcl-2 homology 3-only (BH3-only) Bcl-2 family member Bim. However, upregulation of Bim was insufficient for extensive apoptosis and was countered by overexpression of Bcl-2. To overcome apoptotic resistance, we treated the B-RAF mutant cells both with MEK inhibitors and with the BH3 mimetic ABT-737, resulting in profound synergism and extensive tumor cell death. This treatment was successful because of both efficient antagonism of the prosurvival Bcl-2 family member Mcl-1 by Bim and inhibition of Bcl-2 and Bcl-x(L) by ABT-737. Critically, addition of ABT-737 converted the predominantly cytostatic effect of MEK inhibition to a cytotoxic effect, causing long-term tumor regression in mice xenografted with human tumor cell lines. Thus, the therapeutic efficacy of MEK inhibition requires concurrent unleashing of apoptosis by a BH3 mimetic and represents a potent combination treatment for tumors harboring B-RAF mutations.

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.

Multisite phosphorylation regulates Bim stability and apoptotic activity

The proapoptotic BH3-only protein Bim is established to be an important mediator of signaling pathways that induce cell death. Multisite phosphorylation of Bim by several members of the MAP kinase group is implicated as a regulatory mechanism that controls the apoptotic activity of Bim. To test the role of Bim phosphorylation in vivo, we constructed mice with a series of mutant alleles that express phosphorylation-defective Bim proteins. We show that mutation of the phosphorylation site Thr-112 causes decreased binding of Bim to the antiapoptotic protein Bcl2 and can increase cell survival. In contrast, mutation of the phosphorylation sites Ser-55, Ser-65, and Ser-73 can cause increased apoptosis because of reduced proteasomal degradation of Bim. Together, these data indicate that phosphorylation can regulate Bim by multiple mechanisms and that the phosphorylation of Bim on different sites can contribute to the sensitivity of cellular apoptotic responses.

PINCH-1 regulates the ERK-Bim pathway and contributes to apoptosis resistance in cancer cells

Resistance to apoptosis is a hallmark of cancer cells. We report here that PINCH-1, a cytoplasmic component of cell-extracellular matrix adhesions, is required for protection of multiple types of cancer cells from apoptosis. Furthermore, using HT-1080 fibrosarcoma cells as a model system, we have investigated the signaling pathway through which PINCH-1 contributes to apoptosis resistance. Loss of PINCH-1 markedly increases the level of Bim and promotes Bim translocation to mitochondria, resulting in activation of the intrinsic apoptosis pathway. Depletion of Bim completely blocked apoptosis induced by the loss of PINCH-1. Thus, PINCH-1 contributes to apoptosis resistance through suppression of Bim. Mechanistically, PINCH-1 suppresses Bim not only transcriptionally but also post-transcriptionally. PINCH-1 promotes activating phosphorylation of Src family kinase and ERK1/2. Consistent with this, ERK1/2-mediated Ser(69) phosphorylation of Bim, a key signal for turnover of Bim, is suppressed by the removal of PINCH-1. Our results demonstrate a strong dependence of multiple types of apoptosis-resistant cancer cells on PINCH-1 and provide new insights into the molecular mechanism by which cancer cells are protected from apoptosis.

c-Cbl is not required for ERK1/2-dependent degradation of BimEL

Bim(EL) the most abundant Bim splice variant, is subject to ERK1/2-catalysed phosphorylation, which targets it for ubiquitination and proteasome-dependent destruction. In contrast, inactivation of ERK1/2, following withdrawal of survival factors, promotes stabilization of Bim(EL). It has been proposed that the RING finger protein Cbl binds to Bim(EL) and serves as its E3 ubiquitin ligase. However, this is controversial since most Cbl substrates are tyrosine phosphoproteins and yet Bim(EL) is targeted for destruction by ERK1/2-catalysed serine phosphorylation. Consequently, a role for Cbl could suggest a second pathway for Bim(EL) turnover, regulated by direct tyrosine phosphorylation, or could suggest that Bim(EL) is a coincidence detector, requiring phosphorylation by ERK1/2 and a tyrosine kinase. Here we show that degradation of Bim(EL) does not involve its tyrosine phosphorylation; indeed, Bim(EL) is not a tyrosine phosphoprotein. Furthermore, Bim(EL) fails to interact with Cbl and growth factor-stimulated, ERK1/2-dependent Bim(EL) turnover proceeds normally in Cbl-containing or Cbl-/- fibroblasts. These results indicate that Cbl is not required for ERK1/2-dependent Bim(EL) turnover in fibroblasts and epithelial cells and any role it has in other cell types is likely to be indirect.

What do we know about the mechanisms of elimination of autoreactive T and B cells and what challenges remain

Tolerance to self-antigens within the adaptive immune system is safeguarded, at least in part, through deletion of autoreactive T and B lymphocytes. This deletion can occur during the development of these cells in primary lymphoid organs, the thymus or bone marrow, respectively, or at the mature stage in peripheral lymphoid tissues. Deletion of autoreactive lymphocytes is achieved to a large extent through apoptotic cell death. This review describes current understanding of the mechanisms that mediate apoptosis of autoreactive lymphocytes during their development in primary lymphoid organs and during their activation in the periphery. In particular, we discuss the roles of the proapoptotic Bcl-2 family member Bim and the small family of Nur77-related transcriptional regulators in lymphocyte negative selection. Finally, we speculate on the processes that may lead to the activation of Bim when antigen receptors are activated on autoreactive T or B cells.