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

The BCL2 family: from apoptosis mechanisms to new advances in targeted therapy

The B cell lymphoma 2 (BCL2) protein family critically controls apoptosis by regulating the release of cytochrome c from mitochondria. In this cutting-edge review, we summarize the basic biology regulating the BCL2 family including canonical and non-canonical functions, and highlight milestones from basic research to clinical applications in cancer and other pathophysiological conditions. We review laboratory and clinical development of BH3-mimetics as well as more recent approaches including proteolysis targeting chimeras (PROTACs), antibody-drug conjugates (ADCs) and tools targeting the BH4 domain of BCL2. The first BCL2-selective BH3-mimetic, venetoclax, showed remarkable efficacy with manageable toxicities and has transformed the treatment of several hematologic malignancies. Following its success, several chemically similar BCL2 inhibitors such as sonrotoclax and lisaftoclax are currently under clinical evaluation, alone and in combination. Genetic analysis highlights the importance of BCL-XL and MCL1 across different cancer types and the possible utility of BH3-mimetics targeting these proteins. However, the development of BH3-mimetics targeting BCL-XL or MCL1 has been more challenging, with on-target toxicities including thrombocytopenia for BCL-XL and cardiac toxicities for MCL1 inhibitors precluding clinical development. Tumor-specific BCL-XL or MCL1 inhibition may be achieved by novel targeting approaches using PROTACs or selective drug delivery strategies and would be transformational in many subtypes of malignancy. Taken together, we envision that the targeting of BCL2 proteins, while already a success story of translational research, may in the foreseeable future have broader clinical applicability and improve the treatment of multiple diseases.

Repurposing bosentan as an anticancer agent: EGFR/ERK/c-Jun modulation inhibits NSCLC tumor growth

Drug repurposing of well-established drugs to be targeted against lung cancer has been a promising strategy. Bosentan is an endothelin 1 (ET-1) blocker widely used in pulmonary hypertension. The current experiment intends to inspect the anticancer and antiangiogenic mechanism of bosentan targeting epidermal growth factor receptor (EGFR) /extra-cellular Signal Regulated Kinase (ERK) /c-Jun/vascular endothelial growth factor (VEGF) carcinogenic pathway. BALB/c mice were randomized into four groups, the first received the vehicle, the second received 100 mg/kg oral bosentan alone, the third has non-small cell lung cancer (NSCLC) induced by two doses of 1.5 g/kg urethane i.p. and finally the fourth has NSCLC received bosentan. To determine the anti-proliferative impact of bosentan, cytokeratin 19 fragments (CYFRA 21-1) level was assessed, and Ki-67 positive cells were counted by immunohistochemical (IHC). Molecular expression of EGFR via IHC, relative expression of p-ERK1/2 and p-c-Jun via western blotting and caspase 3, Bcl-2 Associated X-protein (BAX)/B-cell lymphoma 2 (Bcl-2) ratio and VEGF via ELISA were quantified. Bosentan showed pronounced improvement in lung index and histopathological examinations. Bosentan exerted a noticeable arrest of lung cancer growth indicated by the attenuation of CYFRA 21-1 and Ki-67 positive cell counts besides the boost of BAX/Bcl-2 ratio and caspase 3. Bosentan induced a remarkable decline of EGFR, T-ERK1/2/p-ERK1/2, T-c-Jun/p-c-Jun, and VEGF. Bosentan induced cytotoxic and anti-angiogenic impact through regulation of EGFR/ERK/c-Jun/VEGF axis suggesting its potential therapeutic impact against lung cancer.

Enhanced MAPK signaling induced by CSF3R mutants confers dependence to DUSP1 for leukemic transformation

ABSTRACT

Elevated MAPK and the JAK-STAT signaling play pivotal roles in the pathogenesis of chronic neutrophilic leukemia and atypical chronic myeloid leukemia. Although inhibitors targeting these pathways effectively suppress the diseases, they fall short in providing enduring remission, largely attributed to the cytostatic nature of these drugs. Even combinations of these drugs are ineffective in achieving sustained remission. Enhanced MAPK signaling besides promoting proliferation and survival triggers a proapoptotic response. Consequently, malignancies reliant on elevated MAPK signaling use MAPK feedback regulators to intricately modulate the signaling output, prioritizing proliferation and survival while dampening the apoptotic stimuli. Herein, we demonstrate that enhanced MAPK signaling in granulocyte colony-stimulating factor 3 receptor (CSF3R)-driven leukemia upregulates the expression of dual specificity phosphatase 1 (DUSP1) to suppress the apoptotic stimuli crucial for leukemogenesis. Consequently, genetic deletion of Dusp1 in mice conferred synthetic lethality to CSF3R-induced leukemia. Mechanistically, DUSP1 depletion in leukemic context causes activation of JNK1/2 that results in induced expression of BIM and P53 while suppressing the expression of BCL2 that selectively triggers apoptotic response in leukemic cells. Pharmacological inhibition of DUSP1 by BCI (a DUSP1 inhibitor) alone lacked antileukemic activity due to ERK1/2 rebound caused by off-target inhibition of DUSP6. Consequently, a combination of BCI with a MEK inhibitor successfully cured CSF3R-induced leukemia in a preclinical mouse model. Our findings underscore the pivotal role of DUSP1 in leukemic transformation driven by enhanced MAPK signaling and advocate for the development of a selective DUSP1 inhibitor for curative treatment outcomes.

Ubiquitin-proteasome pathway-mediated regulation of the Bcl-2 family: effects and therapeutic approaches

Proteasomal degradation of proteins represents an important regulatory mechanism in maintaining healthy homeostasis in cells. Deregulation of the ubiquitin-proteasome system is associated with various diseases as it controls protein abundance and turnover in cells. Furthermore, proteasomal regulation of protein turnover rate can determine a cell's response to external stimuli. The Bcl-2 family of proteins is an important group of proteins involved in mediating cell survival or cell death in response to external stimuli. Aberrant overexpression of anti-apoptotic proteins or deletion of pro-apoptotic proteins can lead to the development of cancer. Unsurprisingly, proteasomal degradation of Bcl-2 proteins also serves as an important factor regulating the level of Bcl-2 proteins and thereby affecting the functional outcome of cell death. This review aims to highlight the regulation of the Bcl-2 family of proteins with particular emphasis on proteasomal-mediated degradation pathways and the current literature on the therapeutic approaches targeting the proteasome system.

ROS production by mitochondria: function or dysfunction?

In eukaryotic cells, ATP generation is generally viewed as the primary function of mitochondria under normoxic conditions. Reactive oxygen species (ROS), in contrast, are regarded as the by-products of respiration, and are widely associated with dysfunction and disease. Important signaling functions have been demonstrated for mitochondrial ROS in recent years. Still, their chemical reactivity and capacity to elicit oxidative damage have reinforced the idea that ROS are the products of dysfunctional mitochondria that accumulate during disease. Several studies support a different model, however, by showing that: (1) limited oxygen availability results in mitochondria prioritizing ROS production over ATP, (2) ROS is an essential adaptive mitochondrial signal triggered by various important stressors, and (3) while mitochondria-independent ATP production can be easily engaged by most cells, there is no known replacement for ROS-driven redox signaling. Based on these observations and other evidence reviewed here, we highlight the role of ROS production as a major mitochondrial function involved in cellular adaptation and stress resistance. As such, we propose a rekindled view of ROS production as a primary mitochondrial function as essential to life as ATP production itself.

Pro-Apoptotic Activity of MCL-1 Inhibitor in Trametinib-Resistant Melanoma Cells Depends on Their Phenotypes and Is Modulated by Reversible Alterations Induced by Trametinib Withdrawal

BACKGROUND

Although BRAFV600/MEK inhibitors improved the treatment of melanoma patients, resistance is acquired almost inevitably.

METHODS

Trametinib withdrawal/rechallenge and MCL-1 inhibition in trametinib-resistance models displaying distinct p-ERK1/2 levels were investigated.

RESULTS

Trametinib withdrawal/rechallenge caused reversible changes in ERK1/2 activity impacting the balance between pro-survival and pro-apoptotic proteins. Reversible alterations were found in MCL-1 levels and MCL-1 inhibitors, BIM and NOXA. Taking advantage of melanoma cell dependency on MCL-1 for survival, we used S63845. While it was designed to inhibit MCL-1 activity, we showed that it also significantly reduced NOXA levels. S63845-induced apoptosis was detected as the enhancement of Annexin V-positivity, caspase-3/7 activation and histone H2AX phosphorylation. Percentages of Annexin V-positive cells were increased most efficiently in trametinib-resistant melanoma cells displaying the p-ERK1/2low/MCL-1low/BIMhigh/NOXAlow phenotype with EC50 values at concentrations as low as 0.1 μM. Higher ERK1/2 activity associated with increased MCL-1 level and reduced BIM level limited pro-apoptotic activity of S63845 further influenced by a NOXA level.

CONCLUSIONS

Our study supports the notion that the efficiency of an agent designed to target a single protein can largely depend on the phenotype of cancer cells. Thus, it is important to define appropriate phenotype determinants to stratify the patients for the novel therapy.

Cotargeting a MYC/eIF4A-survival axis improves the efficacy of KRAS inhibitors in lung cancer

Despite the success of KRAS G12C inhibitors in non-small cell lung cancer (NSCLC), more effective treatments are needed. One preclinical strategy has been to cotarget RAS and mTOR pathways; however, toxicity due to broad mTOR inhibition has limited its utility. Therefore, we sought to develop a more refined means of targeting cap-dependent translation and identifying the most therapeutically important eukaryotic initiation factor 4F complex-translated (eIF4F-translated) targets. Here, we show that an eIF4A inhibitor, which targets a component of eIF4F, dramatically enhances the effects of KRAS G12C inhibitors in NSCLCs and together these agents induce potent tumor regression in vivo. By screening a broad panel of eIF4F targets, we show that this cooperativity is driven by effects on BCL-2 family proteins. Moreover, because multiple BCL-2 family members are concomitantly suppressed, these agents are broadly efficacious in NSCLCs, irrespective of their dependency on MCL1, BCL-xL, or BCL-2, which is known to be heterogeneous. Finally, we show that MYC overexpression confers sensitivity to this combination because it creates a dependency on eIF4A for BCL-2 family protein expression. Together, these studies identify a promising therapeutic strategy for KRAS-mutant NSCLCs, demonstrate that BCL-2 proteins are the key mediators of the therapeutic response in this tumor type, and uncover a predictive biomarker of sensitivity.

Capitalizing on paradoxical activation of the mitogen-activated protein kinase pathway for treatment of Imatinib-resistant mast cell leukemia

Prevention of fatal side effects during cancer therapy of cancer patients with high-dosed pharmacological inhibitors is to date a major challenge. Moreover, the development of drug resistance poses severe problems for the treatment of patients with leukemia or solid tumors. Particularly drug-mediated dimerization of RAF kinases can be the cause of acquired resistance, also called "paradoxical activation." In the present work we re-analyzed the effects of different tyrosine kinase inhibitors (TKIs) on the proliferation, metabolic activity, and survival of the Imatinib-resistant, KIT V560G, D816V-expressing human mast cell (MC) leukemia (MCL) cell line HMC-1.2. We observed that low concentrations of the TKIs Nilotinib and Ponatinib resulted in enhanced proliferation, suggesting paradoxical activation of the MAPK pathway. Indeed, these TKIs caused BRAF-CRAF dimerization, resulting in ERK1/2 activation. The combination of Ponatinib with the MEK inhibitor Trametinib, at nanomolar concentrations, effectively suppressed HMC-1.2 proliferation, metabolic activity, and induced apoptotic cell death. Effectiveness of this drug combination was recapitulated in the human KIT D816V MC line ROSAKIT D816V and in KIT D816V hematopoietic progenitors obtained from patient-derived induced pluripotent stem cells (iPS cells) and systemic mastocytosis patient samples. In conclusion, mutated KIT-driven Imatinib resistance and possible TKI-induced paradoxical activation can be efficiently overcome by a low concentration Ponatinib and Trametinib co-treatment, potentially reducing the negative side effects associated with MCL therapy.

Arsenic trioxide promotes ERK1/2-mediated phosphorylation and degradation of BIMEL to attenuate apoptosis in BEAS-2B cells

Inorganic arsenic is highly toxic, widely distributed in the human environment and may result in multisystem diseases and several types of cancers. The BCL-2-interacting mediator of cell death protein (BIM) is a key modulator of the intrinsic apoptosis pathway. Interestingly, in the present study, we found that arsenic trioxide (As₂O₃) decreased BIM_EL levels in human bronchial epithelial cell line BEAS-2B and increased BIM_EL levels in human lung carcinoma cell line A549 and mouse Sertoli cell line TM4. Mechanismly, the 26S proteasome inhibitors MG132 and bortezomib could effectively inhibit BIM_EL degradation induced by As₂O₃ in BEAS-2B cells. As₂O₃ activated extracellular signal-regulated kinase (ERK) 1/2, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) signaling pathways, but only the ERK1/2 MAPK inhibitor PD98059 blocked BIM_EL degradation induced by As₂O₃. Furthermore, As₂O₃ induced-phosphorylation of BIM_EL at multiple sites was inhibited by ERK1/2 MAPK inhibitor PD98059. Inhibition of As₂O₃-induced ERK1/2 MAPK phosphorylation increased the levels of BIM_EL and cleaved-caspase-3 proteins and decreased BEAS-2B cell viability. As₂O₃ also markedly mitigated tunicamycin-induced apoptosis of BEAS-2B cells by increasing ERK1/2 phosphorylation and BIM_EL degradation. Our results suggest that As₂O₃-induced activation of the ERK1/2 MAPK pathway increases phosphorylation of BIM_EL and promotes BIM_EL degradation, thereby alleviating the role of apoptosis in As₂O₃-induced cell death. This study provides new insights into how to maintain the survival of BEAS-2B cells before malignant transformation induced by high doses of As₂O₃.

Genotype-Tailored ERK/MAPK Pathway and HDAC Inhibition Rewires the Apoptotic Rheostat to Trigger Colorectal Cancer Cell Death

The EGFR/RAS/MEK/ERK signaling pathway (ERK/MAPK) is hyperactivated in most colorectal cancers. A current limitation of inhibitors of this pathway is that they primarily induce cytostatic effects in colorectal cancer cells. Nevertheless, these drugs do induce expression of proapoptotic factors, suggesting they may prime colorectal cancer cells to undergo apoptosis. As histone deacetylase inhibitors (HDACis) induce expression of multiple proapoptotic proteins, we examined whether they could synergize with ERK/MAPK inhibitors to trigger colorectal cancer cell apoptosis. Combined MEK/ERK and HDAC inhibition synergistically induced apoptosis in colorectal cancer cell lines and patient-derived tumor organoids in vitro, and attenuated Apc-initiated adenoma formation in vivo. Mechanistically, combined MAPK/HDAC inhibition enhanced expression of the BH3-only proapoptotic proteins BIM and BMF, and their knockdown significantly attenuated MAPK/HDAC inhibitor-induced apoptosis. Importantly, we demonstrate that the paradigm of combined MAPK/HDAC inhibitor treatment to induce apoptosis can be tailored to specific MAPK genotypes in colorectal cancers, by combining an HDAC inhibitor with either an EGFR, KRASG12C or BRAFV600 inhibitor in KRAS/BRAFWT; KRASG12C, BRAFV600E colorectal cancer cell lines, respectively. These findings identify a series of ERK/MAPK genotype-tailored treatment strategies that can readily undergo clinical testing for the treatment of colorectal cancer.

Mechanisms of EGFR-TKI-Induced Apoptosis and Strategies Targeting Apoptosis in EGFR-Mutated Non-Small Cell Lung Cancer

Homeostasis is achieved by balancing cell survival and death. In cancer cells, especially those carrying driver mutations, the processes and signals that promote apoptosis are inhibited, facilitating the survival and proliferation of these dysregulated cells. Apoptosis induction is an important mechanism underlying the therapeutic efficacy of epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors (TKIs) for EGFR-mutated non-small cell lung cancer (NSCLC). However, the mechanisms by which EGFR-TKIs induce apoptosis have not been fully elucidated. A deeper understanding of the apoptotic pathways induced by EGFR-TKIs is essential for the developing novel strategies to overcome resistance to EGFR-TKIs or to enhance the initial efficacy through therapeutic synergistic combinations. Recently, therapeutic strategies targeting apoptosis have been developed for cancer. Here, we review the state of knowledge on EGFR-TKI-induced apoptotic pathways and discuss the therapeutic strategies for enhancing EGFR-TKI efficiency. We highlight the great progress achieved with third-generation EGFR-TKIs. In particular, combination therapies of EGFR-TKIs with anti-vascular endothelial growth factor/receptor inhibitors or chemotherapy have emerged as promising therapeutic strategies for patients with EGFR-mutated NSCLC. Nevertheless, further breakthroughs are needed to yield an appropriate standard care for patients with EGFR-mutated NSCLC, which requires gaining a deeper understanding of cancer cell dynamics in response to EGFR-TKIs.

Cell Signaling Pathways That Promote Radioresistance of Cancer Cells

Radiation therapy (RT) is a standard treatment for solid tumors and about 50% of patients with cancer, including pediatric cancer, receive RT. While RT has significantly improved the overall survival and quality of life of cancer patients, its efficacy has still been markedly limited by radioresistance in a significant number of cancer patients (intrinsic or acquired), resulting in failure of the RT control of the disease. Radiation eradicates cancer cells mainly by causing DNA damage. However, radiation also concomitantly activates multiple prosurvival signaling pathways, which include those mediated by ATM, ATR, AKT, ERK, and NF-κB that promote DNA damage checkpoint activation/DNA repair, autophagy induction, and/or inhibition of apoptosis. Furthermore, emerging data support the role of YAP signaling in promoting the intrinsic radioresistance of cancer cells, which occurs through its activation of the transcription of many essential genes that support cell survival, DNA repair, proliferation, and the stemness of cancer stem cells. Together, these signaling pathways protect cancer cells by reducing the magnitude of radiation-induced cytotoxicity and promoting radioresistance. Thus, targeting these prosurvival signaling pathways could potentially improve the radiosensitivity of cancer cells. In this review, we summarize the contribution of these pathways to the radioresistance of cancer cells.

Autophagy Inhibition in BRAF-Driven Cancers

Simple Summary

BRAF is a protein kinase that is frequently mutationally activated in cancer. Mutant BRAF can be pharmacologically inhibited, which in combination with blockade of its direct effector, MEK1/2, is an FDA-approved therapeutic strategy for several BRAF-mutated cancer patients, such as melanoma, non-small-cell lung carcinoma, and thyroid cancer. However, therapy resistance is a major clinical challenge, highlighting the need for comprehensive investigations on the biological causes of such resistance, as well as to develop novel therapeutic strategies to improve patient survival. Autophagy is a cellular recycling process, which has been shown to allow cancer cells to escape from BRAF inhibition. Combined blockade of autophagy and BRAF signaling is a novel therapeutic strategy that is currently being tested in clinical trials. This review describes the relationship between BRAF-targeted therapy and autophagy regulation and discusses possible future treatment strategies.

Abstract

Several BRAF-driven cancers, including advanced BRAF^V600E/K-driven melanoma, non-small-cell lung carcinoma, and thyroid cancer, are currently treated using first-line inhibitor combinations of BRAF^V600E plus MEK1/2. However, despite the success of this vertical inhibition strategy, the durability of patient response is often limited by the phenomenon of primary or acquired drug resistance. It has recently been shown that autophagy, a conserved cellular recycling process, is increased in BRAF-driven melanoma upon inhibition of BRAF^V600E signaling. Autophagy is believed to promote tumor progression of established tumors and also to protect cancer cells from the cytotoxic effects of chemotherapy. To this end, BRAF inhibitor (BRAFi)-resistant cells often display increased autophagy compared to responsive lines. Several mechanisms have been proposed for BRAFi-induced autophagy, such as activation of the endoplasmic reticulum (ER) stress gatekeeper GRP78, AMP-activated protein kinase, and transcriptional regulation of the autophagy regulating transcription factors TFEB and TFE3 via ERK1/2 or mTOR inhibition. This review describes the relationship between BRAF-targeted therapy and autophagy regulation, and discusses possible future treatment strategies of combined inhibition of oncogenic signaling plus autophagy for BRAF-driven cancers.

Dexmedetomidine reduces the apoptosis of rat hippocampal neurons via mediating ERK1/2 signal pathway by targeting miR-155

Rat hippocampal neurons were isolated and divided into Normal, oxygen glucose deprivation/reoxygenation (OGD/R), OGD/R + DEX, OGD/R + NC mimic, OGD/R + miR-155 mimic and OGD/R + DEX + miR-155 mimic groups. In OGD/R group, LDH, ROS and MDA levels and apoptosis rate was increased, with up-regulations of miR-155, Cyt c and Bax/Bcl-2 ratio, but decreases of SOD, GSH-Px and MMP levels, as well as down-regulations of p-ERK1/2/ERK1/2. As compared to the OGD/R group, parameters above in the OGD/R + DEX group were ameliorated evidently, while OGD/R + miR-155 mimic group manifested the opposite changes. Besides, miR-155 mimic could abolish the protective effect of DEX on the hippocampal neurons under OGD/R. DEX, via down-regulating the expression of miR-155, could activate the ERK1/2 pathway, thereby mitigating the apoptosis and oxidative stress injury and increasing the MMP, thereby protecting hippocampal cells from OGD/R injury.

Irreversible JNK blockade overcomes PD-L1-mediated resistance to chemotherapy in colorectal cancer

Colorectal cancer (CRC) cells have low or absent tumor cell PD-L1 expression that we previously demonstrated can confer chemotherapy resistance. Here, we demonstrate that PD-L1 depletion enhances JNK activity resulting in increased Bim^Thr116 phosphorylation and its sequestration by MCL-1 and BCL-2. Activated JNK signaling in PD-L1-depeted cells was due to reduced mRNA stability of the CYLD deubiquitinase. PD-L1 was found to compete with the ribonuclease EXOSC10 for binding to CYLD mRNA. Thus, loss of PD-L1 promoted binding and degradation of CYLD mRNA by EXOSC10 which enhanced JNK activity. An irreversible JNK inhibitor (JNK-IN-8) reduced Bim^Thr116 phosphorylation and unsequestered Bim from MCL-1 and BCL-2 to promote apoptosis. In cells lacking PD-L1, treatment with JNK-IN-8, an MCL-1 antagonist (AZD5991), or their combination promoted apoptosis and reduced long-term clonogenic survival by anticancer drugs. Similar effects of the JNK inhibitor on cell viability were observed in CRC organoids with suppression of PD-L1. These data indicate that JNK inhibition may represent a promising strategy to overcome drug resistance in CRC cells with low or absent PD-L1 expression.

RAF-MEK-ERK pathway in cancer evolution and treatment

The RAF-MEK-ERK signaling cascade is a well-characterized MAPK pathway involved in cell proliferation and survival. The three-layered MAPK signaling cascade is initiated upon RTK and RAS activation. Three RAF isoforms ARAF, BRAF and CRAF, and their downstream MEK1/2 and ERK1/2 kinases constitute a coherently orchestrated signaling module that directs a range of physiological functions. Genetic alterations in this pathway are among the most prevalent in human cancers, which consist of numerous hot-spot mutations such as BRAF^V600E. Oncogenic mutations in this pathway often override otherwise tightly regulated checkpoints to open the door for uncontrolled cell growth and neoplasia. The crosstalk between the RAF-MEK-ERK axis and other signaling pathways further extends the proliferative potential of this pathway in human cancers. In this review, we summarize the molecular architecture and physiological functions of the RAF-MEK-ERK pathway with emphasis on its dysregulations in human cancers, as well as the efforts made to target the RAF-MEK-ERK module using small molecule inhibitors.

Inhibition of MEK1/2 Forestalls the Onset of Acquired Resistance to Entrectinib in Multiple Models of NTRK1-Driven Cancer

NTRK1 gene fusions are actionable drivers of numerous human malignancies. Here, we show that expression of the TPR-NTRK1 fusion kinase in immortalized mouse pancreatic ductal epithelial (IMPE) (pancreas) or mouse lung epithelial (MLE-12) cells is sufficient to promote rapidly growing tumors in mice. Both tumor models are exquisitely sensitive to targeted inhibition with entrectinib, a tropomyosin-related kinase A (TRKA) inhibitor. Initial regression of NTRK1-driven tumors is driven by induced expression of BIM, such that BIM silencing leads to a diminished response to entrectinib in vivo. However, the emergence of drug-resistant disease limits the long-term durability of responses. Based on the reactivation of RAF>MEK>ERK signaling observed in entrectinib-treated tumors, we show that the combination of entrectinib plus the MEK1/2 inhibitor cobimetinib dramatically forestalls the onset of drug resistance in vivo. Collectively, these data provide a mechanistic rationale for rapid clinical deployment of combined inhibition of TRKA plus MEK1/2 in NTRK1-driven cancers.

ERK signalling: a master regulator of cell behaviour, life and fate

The proteins extracellular signal-regulated kinase 1 (ERK1) and ERK2 are the downstream components of a phosphorelay pathway that conveys growth and mitogenic signals largely channelled by the small RAS GTPases. By phosphorylating widely diverse substrates, ERK proteins govern a variety of evolutionarily conserved cellular processes in metazoans, the dysregulation of which contributes to the cause of distinct human diseases. The mechanisms underlying the regulation of ERK1 and ERK2, their mode of action and their impact on the development and homeostasis of various organisms have been the focus of much attention for nearly three decades. In this Review, we discuss the current understanding of this important class of kinases. We begin with a brief overview of the structure, regulation, substrate recognition and subcellular localization of ERK1 and ERK2. We then systematically discuss how ERK signalling regulates six fundamental cellular processes in response to extracellular cues. These processes are cell proliferation, cell survival, cell growth, cell metabolism, cell migration and cell differentiation.

MARCH5 mediates NOXA-dependent MCL1 degradation driven by kinase inhibitors and integrated stress response activation

MCL1 has critical antiapoptotic functions and its levels are tightly regulated by ubiquitylation and degradation, but mechanisms that drive this degradation, particularly in solid tumors, remain to be established. We show here in prostate cancer cells that increased NOXA, mediated by kinase inhibitor activation of an integrated stress response, drives the degradation of MCL1, and identify the mitochondria-associated ubiquitin ligase MARCH5 as the primary mediator of this NOXA-dependent MCL1 degradation. Therapies that enhance MARCH5-mediated MCL1 degradation markedly enhance apoptosis in response to a BH3 mimetic agent targeting BCLXL, which may provide for a broadly effective therapy in solid tumors. Conversely, increased MCL1 in response to MARCH5 loss does not strongly sensitize to BH3 mimetic drugs targeting MCL1, but instead also sensitizes to BCLXL inhibition, revealing a codependence between MARCH5 and MCL1 that may also be exploited in tumors with MARCH5 genomic loss.

Dynamics and Regulations of BimEL Ser65 and Thr112 Phosphorylation in Porcine Granulosa Cells during Follicular Atresia

BimEL protein is involved in follicular atresia by regulating granulosa cell apoptosis, but the dynamic changes of BimEL phosphorylation during follicular atresia are poorly understood. The aim of this study was to explore the changes of key BimEL phosphorylation sites and their upstream regulatory pathways. First, the levels of BimEL-Ser65 and BimEL-Thr112 phosphorylation (p-BimEL-S65, p-BimEL-T112) in granulosa cells (GC) from healthy (H), slightly-atretic (SA), and atretic (A) follicles and in cultured GC after different treatments were detected by Western blotting. Next, the effects of the corresponding site mutations of BIM on apoptosis of GC were investigated. Finally, the pathways of two phosphorylation sites were investigated by kinase inhibitors. The results revealed that p-BimEL-S65 levels were higher in GC from H than SA and A, whereas p-BimEL-T112 was reversed. The prosurvival factors like FSH and IGF-1 upregulated the level of p-BimEL-S65, while the proapoptotic factor, heat stress, increased the level of p-BimEL-T112 in cultured GC. Compared with the overexpression of wild BimEL, the apoptotic rate of the GC overexpressed BimEL-S65A (replace Ser65 with Ala) mutant was significantly higher, but the apoptotic rate of the cells overexpressing BimEL-T112A did not differ. In addition, inhibition of the ERK1/2 or JNK pathway by specific inhibitors reduced the levels of p-BimEL-S65 and p-BimEL-T112. In conclusion, the levels of p-BimEL-S65 and p-BimEL-T112 were reversed during follicular atresia. Prosurvival factors promote p-BimEL-S65 levels via ERK1/2 to inhibit GC apoptosis, whereas proapoptotic factor upregulates the level of p-BimEL-T112 via JNK to induce GC apoptosis.

BIK ubiquitination by the E3 ligase Cul5-ASB11 determines cell fate during cellular stress

The BH3-only pro-apoptotic protein BIK is regulated by the ubiquitin-proteasome system. However, the mechanism of this regulation and its physiological functions remain elusive. Here, we identify Cul5-ASB11 as the E3 ligase targeting BIK for ubiquitination and degradation. ER stress leads to the activation of ASB11 by XBP1s during the adaptive phase of the unfolded protein response, which stimulates BIK ubiquitination, interaction with p97/VCP, and proteolysis. This mechanism of BIK degradation contributes to ER stress adaptation by promoting cell survival. Conversely, genotoxic agents down-regulate this IRE1α-XBP1s-ASB11 axis and stabilize BIK, which contributes in part to the apoptotic response to DNA damage. We show that blockade of this BIK degradation pathway by an IRE1α inhibitor can stabilize a BIK active mutant and increase its anti-tumor activity. Our study reveals that different cellular stresses regulate BIK ubiquitination by ASB11 in opposing directions, which determines whether or not cells survive, and that blocking BIK degradation has the potential to be used as an anti-cancer strategy.

Rotundic Acid Induces DNA Damage and Cell Death in Hepatocellular Carcinoma Through AKT/mTOR and MAPK Pathways

Hepatocellular carcinoma (HCC) is the fourth largest cause of cancer-related deaths worldwide with limited therapeutic interventions. Renewed interest in natural products as drug leads has resulted in a paradigm shift toward the rapid screening of medicinal plants for the discovery of new chemical entities. Rotundic acid (RA), a plant-derived triterpenoid, has been anecdotally reported to possess anti-inflammatory and cardio-protective abilities. The present study highlights the anti-cancer efficacy of RA on HCC in vitro and in vivo. The inhibitory effects of RA on HCC cell viability was determined by MTT. Soft agar colony formation and clonogenic assays also showed that RA inhibited HCC cell proliferation. Flow cytometry, confocal, and western blot results further indicated that RA induced cell cycle arrest, DNA damage, and apoptosis by modulating the AKT/mTOR and MAPK pathways. Besides the suppression of migration and invasion, tube formation and VEGF-ELISA revealed the anti-angiogenic abilities of RA on HCC. Moreover, RA also inhibited tumor growth in a HepG2 xenograft mouse model. To our best knowledge, this is the first extensive study of the anticancer activity of RA on HCC. The results demonstrate that RA could be a potential drug candidate for HCC treatment.

The established and the predicted roles of dynein light chain in the regulation of mitochondrial apoptosis

The mitochondrial pathway of apoptosis is regulated by the interplay between the members of Bcl-2 family. Within this family, BH3-only proteins are the sensors of apoptotic stimuli and can trigger apoptosis either by inhibiting the anti-apoptotic Bcl-2-family proteins or by directly activating the effectors Bax and Bak. An expanding body of research suggests that a number of non-Bcl-2 proteins can also interact with Bcl-2 proteins and contribute to the decision of cell fate. Dynein light chain (LC8, DYNLL or DLC), a hub protein and a dimerizing engine has been proposed to regulate the pro-apoptotic activity of two BH3-only proteins, Bim and Bmf. Our recent work has provided insight into the mechanisms through which DLC1 (DYNLL1) modulates Bim activity. Here we discuss the present day understanding of Bim-DLC interaction and endeavor to evaluate this interaction in the light of information from studies of DLC with other binding partners.

Phosphorylation alters Bim-mediated Mcl-1 stabilization and priming

Mcl-1 is a highly labile protein, subject to extensive post-translational regulation. This distinguishes Mcl-1 from other antiapoptotic proteins and necessitates further study to better understand how interactions with proapoptotic Bcl-2 proteins affect its regulation. One such protein, Bim, is known to stabilize Mcl-1, and Bim phosphorylation has been associated with increased Mcl-1 binding. Consequently, we investigated the potential impact of Bim phosphorylation on Mcl-1 stability. We found that Bim stabilizes and primes Mcl-1 in RPCI-WM1 cells and is constitutively phosphorylated. Additionally, introduction of several phospho-mimetic and unphosphosphorylateable Bim mutations resulted in altered Mcl-1 stability and distinct Bim binding to antiapoptotic proteins. These findings suggest Bim phosphorylation not only regulates Mcl-1 stability but also is a potential mechanism for enforcing Mcl-1 dependence.

The combination of TRAIL and MG-132 induces apoptosis in both TRAIL-sensitive and TRAIL-resistant human follicular lymphoma cells

We have previously shown that the human follicular lymphoma cell line, HF28GFP, is sensitive to TRAIL-mediated apoptosis. Nevertheless, when the same cells overexpress anti-apoptotic Bcl-2 family protein, Bcl-xL (HF28Bcl-xL), they become resistant to TRAIL. Thus, these cell lines help us to investigate the action of novel apoptosis inducing candidate drugs. In the present study, we examined the effects of MG-132 (a proteasome inhibitor), LiCl (a glycogen synthase kinase-3 inhibitor) and/or TRAIL on pro-apoptotic Bcl-2 family proteins such as Bim and Bid. Here we demonstrate that the combination of MG-132 and TRAIL induced significant apoptotic cell death in both cell lines, HF28GFP and HF28BclxL. Apoptosis correlated with a decrease of phospho-ERK1/2, the accumulation of Bim and translocation of truncated Bid (tBid) and jBid. In addition, the combination of MG-132 and TRAIL seemed to target other apoptotic factors, which led to the accumulation of active capsase-3. Furthermore, co-stimulation of LiCl and TRAIL induced apoptosis in HF28GFP cells. However, HF28Bcl-xL cells were far less sensitive to the combinatorial effects of LiCl and TRAIL. Interestingly, we observed that LiCl did not target Bim and Bid proteins. In conclusion, these data show that targeting of pro-apoptotic Bcl-2 family proteins simultaneously through a selective proteasome inhibition might help to overcome TRAIL resistance caused by overexpression of anti-apoptotic Bcl-2 family proteins. Moreover, the data may provide new strategies to develop targeted therapies against lymphomas.

Epithelial-to-Mesenchymal Transition Antagonizes Response to Targeted Therapies in Lung Cancer by Suppressing BIM

Purpose: Epithelial-to-mesenchymal transition (EMT) confers resistance to a number of targeted therapies and chemotherapies. However, it has been unclear why EMT promotes resistance, thereby impairing progress to overcome it.Experimental Design: We have developed several models of EMT-mediated resistance to EGFR inhibitors (EGFRi) in EGFR-mutant lung cancers to evaluate a novel mechanism of EMT-mediated resistance.Results: We observed that mesenchymal EGFR-mutant lung cancers are resistant to EGFRi-induced apoptosis via insufficient expression of BIM, preventing cell death despite potent suppression of oncogenic signaling following EGFRi treatment. Mechanistically, we observed that the EMT transcription factor ZEB1 inhibits BIM expression by binding directly to the BIM promoter and repressing transcription. Derepression of BIM expression by depletion of ZEB1 or treatment with the BH3 mimetic ABT-263 to enhance "free" cellular BIM levels both led to resensitization of mesenchymal EGFR-mutant cancers to EGFRi. This relationship between EMT and loss of BIM is not restricted to EGFR-mutant lung cancers, as it was also observed in KRAS-mutant lung cancers and large datasets, including different cancer subtypes.Conclusions: Altogether, these data reveal a novel mechanistic link between EMT and resistance to lung cancer targeted therapies. Clin Cancer Res; 24(1); 197-208. ©2017 AACR.

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.

Dynein light chain 1 induces assembly of large Bim complexes on mitochondria that stabilize Mcl-1 and regulate apoptosis

In this study, Singh et al. investigated Bim structure and activity and show that Bim is regulated by the formation of large protein complexes containing dynein light chain 1 (DLC1). Their findings demonstrate that control of apoptosis at mitochondria extends beyond the interaction of monomers of proapoptotic and anti-apoptotic Bcl-2 family members and involves more complex structures of proteins at the mitochondrial outer membrane.

The Bcl-2 family protein Bim triggers mitochondrial apoptosis. Bim is expressed in nonapoptotic cells at the mitochondrial outer membrane, where it is activated by largely unknown mechanisms. We found that Bim is regulated by formation of large protein complexes containing dynein light chain 1 (DLC1). Bim rapidly inserted into cardiolipin-containing membranes in vitro and recruited DLC1 to the membrane. Bim binding to DLC1 induced the formation of large Bim complexes on lipid vesicles, on isolated mitochondria, and in intact cells. Native gel electrophoresis and gel filtration showed Bim-containing mitochondrial complexes of several hundred kilodaltons in all cells tested. Bim unable to form complexes was consistently more active than complexed Bim, which correlated with its substantially reduced binding to anti-apoptotic Bcl-2 proteins. At endogenous levels, Bim surprisingly bound only anti-apoptotic Mcl-1 but not Bcl-2 or Bcl-X_L, recruiting only Mcl-1 into large complexes. Targeting of DLC1 by RNAi in human cell lines induced disassembly of Bim–Mcl-1 complexes and the proteasomal degradation of Mcl-1 and sensitized the cells to the Bcl-2/Bcl-X_L inhibitor ABT-737. Regulation of apoptosis at mitochondria thus extends beyond the interaction of monomers of proapoptotic and anti-apoptotic Bcl-2 family members but involves more complex structures of proteins at the mitochondrial outer membrane, and targeting complexes may be a novel therapeutic strategy.

BH3-only protein BIM: An emerging target in chemotherapy

BH3-only proteins constitute major proportion of pro-apoptotic members of B-cell lymphoma 2 (Bcl-2) family of apoptotic regulatory proteins and participate in embryonic development, tissue homeostasis and immunity. Absence of BH3-only proteins contributes to autoimmune disorders and tumorigenesis. Bim (Bcl-2 Interacting Mediator of cell death), most important member of BH3-only proteins, shares a BH3-only domain (9-16 aa) among 4 domains (BH1-BH4) of Bcl-2 family proteins and highly pro-apoptotic in nature. Bim initiates the intrinsic apoptotic pathway under both physiological and patho-physiological conditions. Reduction in Bim expression was found to be associated with tumor promotion and autoimmunity, while overexpression inhibited tumor growth and drug resistance as cancer cells suppress Bim expression and stability. Apart from its role in normal homeostasis, Bim has emerged as a central player in regulation of tumorigenesis, therefore gaining attention as a plausible target for chemotherapy. Regulation of Bim expression and stability is complicated and regulated at multiple levels viz. transcriptional, post-transcriptional, post-translational (preferably by phosphorylation and ubiquitination), epigenetic (by promoter acetylation or methylation) including miRNAs. Furthermore, control over Bim expression and stability may be exploited to enhance chemotherapeutic efficacy, overcome drug resistance and select anticancer drug regimen as various chemotherapeutic agents exploit Bim as an executioner of cell death. Owing to its potent anti-tumorigenic activity many BH3 mimetics e.g. ABT-737, ABT-263, obatoclax, AT-101and A-1210477 have been developed and entered in clinical trials. It is more likely that in near future strategies commanding Bim expression and stability ultimately lead to Bim based therapeutic regimen for cancer treatment.

ERK1/2 signalling protects against apoptosis following endoplasmic reticulum stress but cannot provide long-term protection against BAX/BAK-independent cell death

Disruption of protein folding in the endoplasmic reticulum (ER) causes ER stress. Activation of the unfolded protein response (UPR) acts to restore protein homeostasis or, if ER stress is severe or persistent, drive apoptosis, which is thought to proceed through the cell intrinsic, mitochondrial pathway. Indeed, cells that lack the key executioner proteins BAX and BAK are protected from ER stress-induced apoptosis. Here we show that chronic ER stress causes the progressive inhibition of the extracellular signal-regulated kinase (ERK1/2) signalling pathway. This is causally related to ER stress since reactivation of ERK1/2 can protect cells from ER stress-induced apoptosis whilst ERK1/2 pathway inhibition sensitises cells to ER stress. Furthermore, cancer cell lines harbouring constitutively active BRAFV600E are addicted to ERK1/2 signalling for protection against ER stress-induced cell death. ERK1/2 signalling normally represses the pro-death proteins BIM, BMF and PUMA and it has been proposed that ER stress induces BIM-dependent cell death. We found no evidence that ER stress increased the expression of these proteins; furthermore, BIM was not required for ER stress-induced death. Rather, ER stress caused the PERK-dependent inhibition of cap-dependent mRNA translation and the progressive loss of pro-survival proteins including BCL2, BCLXL and MCL1. Despite these observations, neither ERK1/2 activation nor loss of BAX/BAK could confer long-term clonogenic survival to cells exposed to ER stress. Thus, ER stress induces cell death by at least two biochemically and genetically distinct pathways: a classical BAX/BAK-dependent apoptotic response that can be inhibited by ERK1/2 signalling and an alternative ERK1/2- and BAX/BAK-independent cell death pathway.

Duvelisib treatment is associated with altered expression of apoptotic regulators that helps in sensitization of chronic lymphocytic leukemia cells to venetoclax (ABT-199)

Duvelisib, an oral dual inhibitor of PI3K-δ and PI3K-γ, is in phase III trials for the treatment of chronic lymphocytic leukemia (CLL) and indolent non-Hodgkin's lymphoma. In CLL, duvelisib monotherapy is associated with high iwCLL (International Workshop on Chronic Lymphocytic Leukemia) and nodal response rates, but complete remissions are rare. To characterize the molecular effect of duvelisib, we obtained samples from CLL patients on the duvelisib phase I trial. Gene expression studies (RNAseq, Nanostring, Affymetrix array and real-time RT-PCR) demonstrated increased expression of BCL2 along with several BH3-only pro-apoptotic genes. In concert with induction of transcript levels, reverse phase protein arrays and immunoblots confirmed increase at the protein level. The BCL2 inhibitor venetoclax induced greater apoptosis in ex vivo-cultured CLL cells obtained from patients on duvelisib compared with pre-treatment CLL cells from the same patients. In vitro combination of duvelisib and venetoclax resulted in enhanced apoptosis even in CLL cells cultured under conditions that simulate the tumor microenvironment. These data provide a mechanistic rationale for testing the combination of duvelisib and venetoclax in the clinic. Such combination regimen (NCT02640833) is being evaluated for patients with B-cell malignancies including CLL.

HSPB1 facilitates ERK-mediated phosphorylation and degradation of BIM to attenuate endoplasmic reticulum stress-induced apoptosis

BIM, a pro-apoptotic BH3-only protein, is a key regulator of the intrinsic (or mitochondrial) apoptosis pathway. Here, we show that BIM induction by endoplasmic reticulum (ER) stress is suppressed in rat PC12 cells overexpressing heat shock protein B1 (HSPB1 or HSP27) and that this is due to enhanced proteasomal degradation of BIM. HSPB1 and BIM form a complex that immunoprecipitates with p-ERK1/2. We found that HSPB1-mediated proteasomal degradation of BIM is dependent on MEK-ERK signaling. Other studies have shown that several missense mutations in HSPB1 cause the peripheral neuropathy, Charcot-Marie-Tooth (CMT) disease, which is associated with nerve degeneration. Here we show that cells overexpressing CMT-related HSPB1 mutants exhibited increased susceptibility to ER stress-induced cell death and high levels of BIM. These findings identify a novel function for HSPB1 as a negative regulator of BIM protein stability leading to protection against ER stress-induced apoptosis, a function that is absent in CMT-associated HSPB1 mutants.

Mitosis and mitochondrial priming for apoptosis

Cell division is a period of danger for cells, as inaccurate segregation of chromosomes can lead to loss of cell viability or aneuploidy. In order to protect against these dangers, cells ultimately initiate mitochondrial apoptosis if they are unable to correctly exit mitosis. A number of important chemotherapeutics exploit this response to delayed mitotic exit, but despite this, the molecular mechanism of the apoptotic timer in mitosis has proved elusive. Some recent studies have now shed light on this, showing how passage through the cell cycle fine-tunes a cell's apoptotic sensitivity such that it can respond appropriately when errors arise.

Repositioning of amprenavir as a novel extracellular signal-regulated kinase-2 inhibitor and apoptosis inducer in MCF-7 human breast cancer

Computational drug repositioning by virtually screening existing drugs for additional therapeutic usage could efficiently accelerate anticancer drug discovery. Herein, a library of 1447 Food and Drug Administration (FDA)-approved small molecule drugs was screened in silico for inhibitors of extracellular signal-regulated kinase 2 (ERK2). Then, in vitro kinase assay demonstrated amprenavir, a HIV-1 protease inhibitor, as a potential kinase inhibitor of ERK2. The in vivo kinase assay indicated that amprenavir could inhibit ERK2-mediated phosphorylation of BimEL at Ser69. Amprenavir could suppress this phosphorylation in MCF-7 cells, which may further facilitate the association of BimEL with several pro-survival molecules. Additionally, inhibition of ERK2-BimEL signaling pathway by amprenavir could contribute to its anti-proliferative and apoptosis-inducing activity in MCF-7 cells. Finally, in vivo tumor growth and immunohistochemical studies confirmed that amprenavir remarkably suppressed tumor proliferation and induce apoptosis in MCF-7 xenografts. Taken together, amprenavir can effectively inhibit the kinase activity of ERK2, and thus induces apoptosis and inhibits tumor growth in human MCF-7 cancer cells both in vitro and in vivo, making amprenavir a promising candidate for future anticancer therapeutics.

The paradox role of caspase cascade in ionizing radiation therapy

Radiotherapy alone or in combination with chemotherapy/surgery is widely used for treatment of cancers. It reduces tumor growth and prevents metastasis. While ionizing radiation activates caspase cascade resulted in apoptosis in cancer cells, it also stimulates tumor cell re-population that leads to reduce the effectiveness of the radiation therapy. This review describes the mechanisms for paradox role of caspase cascade in cancer therapy and discusses the logical and practical strategies for improvement the therapeutic index of radiotherapy through enhancement of radiosensitivity and decreasing the rate of tumor recurrence.

Splicing repression allows the gradual emergence of new Alu-exons in primate evolution

Alu elements are retrotransposons that frequently form new exons during primate evolution. Here, we assess the interplay of splicing repression by hnRNPC and nonsense-mediated mRNA decay (NMD) in the quality control and evolution of new Alu-exons. We identify 3100 new Alu-exons and show that NMD more efficiently recognises transcripts with Alu-exons compared to other exons with premature termination codons. However, some Alu-exons escape NMD, especially when an adjacent intron is retained, highlighting the importance of concerted repression by splicing and NMD. We show that evolutionary progression of 3' splice sites is coupled with longer repressive uridine tracts. Once the 3' splice site at ancient Alu-exons reaches a stable phase, splicing repression by hnRNPC decreases, but the exons generally remain sensitive to NMD. We conclude that repressive motifs are strongest next to cryptic exons and that gradual weakening of these motifs contributes to the evolutionary emergence of new alternative exons.

Insulin-like growth factor 1 rescues R28 retinal neurons from apoptotic death through ERK-mediated BimEL phosphorylation independent of Akt

Insulin-like growth factor 1 (IGF-1) can provide long-term neurotrophic support by activation of Akt, inhibition of FoxO nuclear localization and suppression of Bim gene transcription in multiple neuronal systems. However, MEK/ERK activation can also promote neuron survival through phosphorylation of BimEL. We explored the contribution of the PI3K/Akt/FoxO and MEK/ERK/BimEL pathways in IGF-1 stimulated survival after serum deprivation (SD) of R28 cells differentiated to model retinal neurons. IGF-1 caused rapid activation of Akt leading to FoxO1/3-T32/T24 phosphorylation, and prevented FoxO1/3 nuclear translocation and Bim mRNA upregulation in response to SD. IGF-1 also caused MAPK/MEK pathway activation as indicated by ERK1/2-T202/Y204 and Bim-S65 phosphorylation. Overexpression of FoxO1 increased Bim mRNA expression and amplified the apoptotic response to SD without shifting the serum response curve. Inhibition of Akt activation with LY294002 or by Rictor knockdown did not block the protective effect of IGF-1, while inhibition of MEK activity with PD98059 prevented Bim phosphorylation and blocked IGF-1 protection. In addition, knockdown of Bim expression was protective during SD, while co-silencing of FoxO1 and Fox03 expression had little effect. Thus, the PI3K/Akt/FoxO pathway was not essential for protection from SD-induced apoptosis by IGF-1 in R28 cells. Instead, IGF-1 protection was dependent on activation of the MEK/ERK pathway leading to BimEL phosphorylation, which is known to prevent Bax/Bak oligomerization and activation of the intrinsic mitochondrial apoptosis pathway. These studies demonstrate the requirement of the MEK/ERK pathway in a model of retinal neuron cell survival and highlight the cell specificity for IGF-1 signaling in this response.

The deubiquitinase Usp27x stabilizes the BH3-only protein Bim and enhances apoptosis

Bim is a pro-apoptotic Bcl-2 family member of the BH3-only protein subgroup. Expression levels of Bim determine apoptosis susceptibility in non-malignant and in tumour cells. Bim protein expression is downregulated by proteasomal degradation following ERK-dependent phosphorylation and ubiquitination. Here, we report the identification of a deubiquitinase, Usp27x, that binds Bim upon its ERK-dependent phosphorylation and can upregulate its expression levels. Overexpression of Usp27x reduces ERK-dependent Bim ubiquitination, stabilizes phosphorylated Bim, and induces apoptosis in PMA-stimulated cells, as well as in tumour cells with a constitutively active Raf/ERK pathway. Loss of endogenous Usp27x enhances the Bim-degrading activity of oncogenic Raf. Overexpression of Usp27x induces low levels of apoptosis in melanoma and non-small cell lung cancer (NSCLC) cells and substantially enhances apoptosis induced in these cells by the inhibition of ERK signalling. Finally, deletion of Usp27x reduces apoptosis in NSCLC cells treated with an EGFR inhibitor. Thus, Usp27x can trigger via its proteolytic activity the deubiquitination of Bim and enhance its levels, counteracting the anti-apoptotic effects of ERK activity, and therefore acts as a tumour suppressor.

Identification of DYRK1B as a substrate of ERK1/2 and characterisation of the kinase activity of DYRK1B mutants from cancer and metabolic syndrome

The dual-specificity tyrosine-phosphorylation-regulated kinase, DYRK1B, is expressed de novo during myogenesis, amplified or mutated in certain cancers and mutated in familial cases of metabolic syndrome. DYRK1B is activated by cis auto-phosphorylation on tyrosine-273 (Y273) within the activation loop during translation but few other DYRK1B phosphorylation sites have been characterised to date. Here, we demonstrate that DYRK1B also undergoes trans-autophosphorylation on serine-421 (S421) in vitro and in cells and that this site contributes to DYRK1B kinase activity. Whilst a DYRK1B(S421A) mutant was completely defective for p-S421 in cells, DYRK1B inhibitors caused only a partial loss of p-S421 suggesting the existence of an additional kinase that could also phosphorylate DYRK1B S421. Indeed, a catalytically inactive DYRK1B(D239A) mutant exhibited very low levels of p-S421 in cells but this was increased by KRAS(G12V). In addition, selective activation of the RAF-MEK1/2-ERK1/2 signalling pathway rapidly increased p-S421 in cells whereas activation of the stress kinases JNK or p38 could not. S421 resides within a Ser-Pro phosphoacceptor motif that is typical for ERK1/2 and recombinant ERK2 phosphorylated DYRK1B at S421 in vitro. Our results show that DYRK1B is a novel ERK2 substrate, uncovering new links between two kinases involved in cell fate decisions. Finally, we show that DYRK1B mutants that have recently been described in cancer and metabolic syndrome exhibit normal or reduced intrinsic kinase activity.

miR-634 restores drug sensitivity in resistant ovarian cancer cells by targeting the Ras-MAPK pathway

BACKGROUND

Drug resistance hampers the efficient treatment of malignancies, including advanced stage ovarian cancer, which has a 5-year survival rate of only 30 %. The molecular processes underlying resistance have been extensively studied, however, not much is known about the involvement of microRNAs.

METHODS

Differentially expressed microRNAs between cisplatin sensitive and resistant cancer cell line pairs were determined using microarrays. Mimics were used to study the role of microRNAs in drug sensitivity of ovarian cancer cell lines and patient derived tumor cells. Luciferase reporter constructs were used to establish regulation of target genes by microRNAs.

RESULTS

MiR-634 downregulation was associated with cisplatin resistance. Overexpression of miR-634 affected cell cycle progression and enhanced apoptosis in ovarian cancer cells. miR-634 resensitized resistant ovarian cancer cell lines and patient derived drug resistant tumor cells to cisplatin. Similarly, miR-634 enhanced the response to carboplatin and doxorubicin, but not to paclitaxel. The cell cycle regulator CCND1, and Ras-MAPK pathway components GRB2, ERK2 and RSK2 were directly repressed by miR-634 overexpression. Repression of the Ras-MAPK pathway using a MEK inhibitor phenocopied the miR-634 effects on viability and chemosensitivity.

CONCLUSION

miR-634 levels determine chemosensitivity in ovarian cancer cells. We identify miR-634 as a therapeutic candidate to resensitize chemotherapy resistant ovarian tumors.

Pathophysiological mechanisms of death resistance in colorectal carcinoma

Colon cancers develop adaptive mechanisms to survive under extreme conditions and display hallmarks of unlimited proliferation and resistance to cell death. The deregulation of cell death is a key factor that contributes to chemoresistance in tumors. In a physiological context, balance between cell proliferation and death, and protection against cell damage are fundamental processes for maintaining gut epithelial homeostasis. The mechanisms underlying anti-death cytoprotection and tumor resistance often bear common pathways, and although distinguishing them would be a challenge, it would also provide an opportunity to develop advanced anti-cancer therapeutics. This review will outline cell death pathways (i.e., apoptosis, necrosis, and necroptosis), and discuss cytoprotective strategies in normal intestinal epithelium and death resistance mechanisms of colon tumor. In colorectal cancers, the intracellular mechanisms of death resistance include the direct alteration of apoptotic and necroptotic machinery and the upstream events modulating death effectors such as tumor suppressor gene inactivation and pro-survival signaling pathways. The autocrine, paracrine and exogenous factors within a tumor microenvironment can also instigate resistance against apoptotic and necroptotic cell death in colon cancers through changes in receptor signaling or transporter uptake. The roles of cyclooxygenase-2/prostaglandin E2, growth factors, glucose, and bacterial lipopolysaccharides in colorectal cancer will be highlighted. Targeting anti-death pathways in the colon cancer tissue might be a promising approach outside of anti-proliferation and anti-angiogenesis strategies for developing novel drugs to treat refractory tumors.

Regulation of Bim in Health and Disease

The BH3-only Bim protein is a major determinant for initiating the intrinsic apoptotic pathway under both physiological and pathophysiological conditions. Tight regulation of its expression and activity at the transcriptional, translational and post-translational levels together with the induction of alternatively spliced isoforms with different pro-apoptotic potential, ensure timely activation of Bim. Under physiological conditions, Bim is essential for shaping immune responses where its absence promotes autoimmunity, while too early Bim induction eliminates cytotoxic T cells prematurely, resulting in chronic inflammation and tumor progression. Enhanced Bim induction in neurons causes neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. Moreover, type I diabetes is promoted by genetically predisposed elevation of Bim in β-cells. On the contrary, cancer cells have developed mechanisms that suppress Bim expression necessary for tumor progression and metastasis. This review focuses on the intricate network regulating Bim activity and its involvement in physiological and pathophysiological processes.

EGFR inhibition evokes innate drug resistance in lung cancer cells by preventing Akt activity and thus inactivating Ets-1 function

Nonsmall cell lung cancer (NSCLC) is the leading cause of cancer death worldwide. About 14% of NSCLCs harbor mutations in epidermal growth factor receptor (EGFR). Despite remarkable progress in treatment with tyrosine kinase inhibitors (TKIs), only 5% of patients achieve tumor reduction >90%. The limited primary responses are attributed partly to drug resistance inherent in the tumor cells before therapy begins. Recent reports showed that activation of receptor tyrosine kinases (RTKs) is an important determinant of this innate drug resistance. In contrast, we demonstrate that EGFR inhibition promotes innate drug resistance despite blockade of RTK activity in NSCLC cells. EGFR TKIs decrease both the mitogen-activated protein kinase (MAPK) and Akt protein kinase pathways for a short time, after which the Ras/MAPK pathway becomes reactivated. Akt inhibition selectively blocks the transcriptional activation of Ets-1, which inhibits its target gene, dual specificity phosphatase 6 (DUSP6), a negative regulator specific for ERK1/2. As a result, ERK1/2 is activated. Furthermore, elevated c-Src stimulates Ras GTP-loading and activates Raf and MEK kinases. These observations suggest that not only ERK1/2 but also Akt activity is essential to maintain Ets-1 in an active state. Therefore, despite high levels of ERK1/2, Ets-1 target genes including DUSP6 and cyclins D1, D3, and E2 remain suppressed by Akt inhibition. Reduction of DUSP6 in combination with elevated c-Src renews activation of the Ras/MAPK pathway, which enhances cell survival by accelerating Bim protein turnover. Thus, EGFR TKIs evoke innate drug resistance by preventing Akt activity and inactivating Ets-1 function in NSCLC cells.

The BCL2 Family: Key Mediators of the Apoptotic Response to Targeted Anticancer Therapeutics

The ability of cancer cells to suppress apoptosis is critical for carcinogenesis. The BCL2 family proteins comprise the sentinel network that regulates the mitochondrial or intrinsic apoptotic response. Recent advances in our understanding of apoptotic signaling pathways have enabled methods to identify cancers that are "primed" to undergo apoptosis, and have revealed potential biomarkers that may predict which cancers will undergo apoptosis in response to specific therapies. Complementary efforts have focused on developing novel drugs that directly target antiapoptotic BCL2 family proteins. In this review, we summarize the current knowledge of the role of BCL2 family members in cancer development and response to therapy, focusing on targeted therapeutics, recent progress in the development of apoptotic biomarkers, and therapeutic strategies designed to overcome deficiencies in apoptosis.

SIGNIFICANCE

Apoptosis, long known to be important for response to conventional cytotoxic chemotherapy, has more recently been shown to be essential for the efficacy of targeted therapies. Approaches that increase the likelihood of a cancer to undergo apoptosis following therapy may help improve targeted treatment strategies. Cancer Discov; 5(5); 475-87. ©2015 AACR.

Targeting novel signaling pathways for resistant acute myeloid leukemia

Acute myeloid leukemia (AML) is a hematologic malignancy that is the most common type of acute leukemia diagnosed in adults and the second most common type in children. The overall survival is poor and treatment is associated with significant complications and even death. In addition, a significant number of patients will not respond to therapy or relapse. In this review, several new signaling proteins aberrantly regulated in AML are described, including CREB, Triad1, Bcl-2 family members, Stat3, and mTOR/MEK. Identifying more effective and less toxic agents will provide novel approaches to treat AML.

Radiation-induced signaling pathways that promote cancer cell survival (review)

Radiation therapy is a staple cancer treatment approach that has significantly improved local disease control and the overall survival of cancer patients. However, its efficacy is still limited by the development of radiation resistance and the presence of residual disease after therapy that leads to cancer recurrence. Radiation impedes cancer cell growth by inducing cytotoxicity, mainly caused by DNA damage. However, radiation can also simultaneously induce multiple pro-survival signaling pathways, such as those mediated by AKT, ERK and ATM/ATR, which can lead to suppression of apoptosis, induction of cell cycle arrest and/or initiation of DNA repair. These signaling pathways act conjointly to reduce the magnitude of radiation-induced cytotoxicity and promote the development of radioresistance in cancer cells. Thus, targeting these pro-survival pathways has great potential for the radiosensitization of cancer cells. In the present review, we summarize the current literature on how these radiation‑activated signaling pathways promote cancer cell survival.