C-RAF activation promotes BAD poly-ubiquitylation and turn-over by the proteasome

Comprehensive review of Bcl-2 family proteins in cancer apoptosis: Therapeutic strategies and promising updates of natural bioactive compounds and small molecules

Cancer has a considerably higher fatality rate than other diseases globally and is one of the most lethal and profoundly disruptive ailments. The increasing incidence of cancer among humans is one of the greatest challenges in the field of healthcare. A significant factor in the initiation and progression of tumorigenesis is the dysregulation of physiological processes governing cell death, which results in the survival of cancerous cells. B-cell lymphoma 2 (Bcl-2) family members play important roles in several cancer-related processes. Drug research and development have identified various promising natural compounds that demonstrate potent anticancer effects by specifically targeting Bcl-2 family proteins and their associated signaling pathways. This comprehensive review highlights the substantial roles of Bcl-2 family proteins in regulating apoptosis, including the intricate signaling pathways governing the activity of these proteins, the impact of reactive oxygen species, and the crucial involvement of proteasome degradation and the stress response. Furthermore, this review discusses advances in the exploration and potential therapeutic applications of natural compounds and small molecules targeting Bcl-2 family proteins and thus provides substantial scientific information and therapeutic strategies for cancer management.

Wild-type C-Raf gene dosage and dimerization drive prostate cancer metastasis

Mutated Ras and Raf kinases are well-known to promote cancer metastasis via flux through the Ras/Raf/MEK/ERK (mitogen-activated protein kinase [MAPK]) pathway. A role for non-mutated Raf in metastasis is also emerging, but the key mechanisms remain unclear. Elevated expression of any of the three wild-type Raf family members (C, A, or B) can drive metastasis. We utilized an in vivo model to show that wild-type C-Raf overexpression can promote metastasis of immortalized prostate cells in a gene dosage-dependent manner. Analysis of the transcriptomic and phosphoproteomic landscape indicated that C-Raf-driven metastasis is accompanied by upregulated MAPK signaling. Use of C-Raf mutants demonstrated that the dimerization domain, but not its kinase activity, is essential for metastasis. Endogenous Raf monomer knockouts revealed that C-Raf's ability to form dimers with endogenous Raf molecules is important for promoting metastasis. These data identify wild-type C-Raf heterodimer signaling as a potential target for treating metastatic disease.

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.

Bad phosphorylation as a target of inhibition in oncology

Bcl-2 agonist of cell death (BAD) is a BH3-only member of the Bcl-2 family which possesses important regulatory function in apoptosis. BAD has also been shown to possess many non-apoptotic functions closely linked to cancer including regulation of glycolysis, autophagy, cell cycle progression and immune system development. Interestingly, BAD can be either pro-apoptotic or pro-survival depending on the phosphorylation state of three specific serine residues (human S75, S99 and S118). Expression of BAD and BAD phosphorylation patterns have been shown to influence tumor initiation and progression and play a predictive role in disease prognosis, drug response and chemosensitivity in various cancers. This review aims to summarize the current evidence on the functional role of BAD phosphorylation in human cancer and evaluate the potential utility of modulating BAD phosphorylation in cancer.

Anti-c-Met monoclonal antibody ABT-700 breaks oncogene addiction in tumors with MET amplification

Background

c-Met is the receptor tyrosine kinase for hepatocyte growth factor (HGF) encoded by the MET proto-oncogene. Aberrant activation of c-Met resulting from MET amplification and c-Met overexpression is associated with poor clinical outcome in multiple malignancies underscoring the importance of c-Met signaling in cancer progression. Several c-Met inhibitors have advanced to the clinic; however, the development of inhibitory c-Met-directed therapeutic antibodies has been hampered by inherent agonistic activity.

Method

We generated and tested a bivalent anti-c-Met monoclonal antibody ABT-700 in vitro for binding potency and antagonistic activity and in vivo for antitumor efficacy in human tumor xenografts. Human cancer cell lines and gastric cancer tissue microarrays were examined for MET amplification by fluorescence in situ hybridization (FISH).

Results

ABT-700 exhibits a distinctive ability to block both HGF-independent constitutive c-Met signaling and HGF-dependent activation of c-Met. Cancer cells addicted to the constitutively activated c-Met signaling driven by MET amplification undergo apoptosis upon exposure to ABT-700. ABT-700 induces tumor regression and tumor growth delay in preclinical tumor models of gastric and lung cancers harboring amplified MET. ABT-700 in combination with chemotherapeutics also shows additive antitumor effect. Amplification of MET in human cancer tissues can be identified by FISH.

Conclusions

The preclinical attributes of ABT-700 in blocking c-Met signaling, inducing apoptosis and suppressing tumor growth in cancers with amplified MET provide rationale for examining its potential clinical utility for the treatment of cancers harboring MET amplification.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2138-z) contains supplementary material, which is available to authorized users.

PAK1 limits the expression of the pro-apoptotic protein Bad in pancreatic islet β-cells

Human type 2 diabetes is associated with β-cell apoptosis, and human islets from diabetic donors are ∼80% deficient in PAK1 protein. Toward addressing linkage of PAK1 to β-cell survival, PAK1–siRNA targeted MIN6 pancreatic β-cells were found to exhibit increased caspase-3 cleavage, cytosolic cytochrome-C and the pro-apoptotic protein Bad. PAK1^+/− heterozygous mouse islets recapitulated the upregulation of Bad protein expression, as did hyperglycemic treatment of human or mouse islets; Bad levels were exacerbated most in PAK1^+/− islets subjected to hyperglycemic stress. These data implicate PAK1 in β-cell survival via quenching of Bad protein expression, and suggest PAK1 as potential molecular target to preserve β-cell mass.

The ubiquitin/proteasome system-dependent control of mitochondrial steps in apoptosis

Insights into the role of ubiquitin-dependent signaling in the regulation of apoptosis have provided one of the most significant breakthroughs in recent years for cell death research. It has been revealed that all steps in the apoptotic cascade, including transcriptional regulation of apoptotic gene expression, outer mitochondrial membrane permeabilization and caspase activation, are under the control of the ubiquitin/proteasome system. This makes ubiquitin signaling one on the most critical life and death decision checkpoints in mammalian cells. Here we discuss the ubiquitylation-dependent regulation of the mitochondrial steps in apoptosis, with a focus on the role of regulated protein degradation in this process. The newly identified ubiquitylation-dependent processes in the Bcl-2 family-regulated outer mitochondrial membrane permeabilization, as well as the role of mitochondria-associated ubiquitin ligases and other molecular components of the ubiquitin/proteasome system in the control of mitochondrial steps in apoptosis, are discussed.

The cytokine tumor necrosis factor-like weak inducer of apoptosis and its receptor fibroblast growth factor-inducible 14 have a neuroprotective effect in the central nervous system

Background

Cerebral cortical neurons have a high vulnerability to the harmful effects of hypoxia. However, the brain has the ability to detect and accommodate to hypoxic conditions. This phenomenon, known as preconditioning, is a natural adaptive process highly preserved among species whereby exposure to sub-lethal hypoxia promotes the acquisition of tolerance to a subsequent lethal hypoxic injury. The cytokine tumor necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor fibroblast growth factor-inducible 14 (Fn14) are found in neurons and their expression is induced by exposure to sub-lethal hypoxia. Accordingly, in this work we tested the hypothesis that the interaction between TWEAK and Fn14 induces tolerance to lethal hypoxic and ischemic conditions.

Methods

Here we used in vitro and in vivo models of hypoxic and ischemic preconditioning, an animal model of transient middle cerebral artery occlusion and mice and neurons genetically deficient in TWEAK, Fn14, or tumor necrosis factor alpha (TNF-α) to investigate whether treatment with recombinant TWEAK or an increase in the expression of endogenous TWEAK renders neurons tolerant to lethal hypoxia. We used enzyme-linked immunosorbent assay to study the effect of TWEAK on the expression of neuronal TNF-α, Western blot analysis to investigate whether the effect of TWEAK was mediated by activation of mitogen-activated protein kinases and immunohistochemical techniques and quantitative real-time polymerase chain reaction analysis to study the effect of TWEAK on apoptotic cell death.

Results

We found that either treatment with recombinant TWEAK or an increase in the expression of TWEAK and Fn14 induce hypoxic and ischemic tolerance in vivo and in vitro. This protective effect is mediated by neuronal TNF-α and activation of the extracellular signal-regulated kinases 1 and 2 pathway via phosphorylation and inactivation of the B-cell lymphoma 2-associated death promoter protein.

Conclusions

Our work indicate that the interaction between TWEAK and Fn14 triggers the activation of a cell signaling pathway that results in the induction of tolerance to lethal hypoxia and ischemia. These data indicate that TWEAK may be a potential therapeutic strategy to protect the brain from the devastating effects of an ischemic injury.

Upregulation of BAD, a pro-apoptotic protein of the BCL2 family, in vascular smooth muscle cells exposed to uremic conditions

Chronic kidney disease (CKD) has recently emerged as a major risk factor for cardiovascular pathology. CKD patients display accelerated atherosclerotic process, leading to circulatory complications. However, it is currently not clear how uremic conditions accelerate atherosclerosis. Apoptosis is an important homeostatic regulator of vascular smooth cells under pathological conditions. In the present study, we explored the regulation of apoptosis in cells of the vascular wall in the uremic context. We analysed the expression and regulation of the proteins of the BCL2 family that play an essential role in apoptosis. Our results, obtained in mice and primary human smooth muscle cells exposed to two uremic toxins, point to the existence of an alteration in expression and function of one pro-apoptotic member of this family, the protein BAD. We explore the regulation of BAD by uremic toxins and report the sensitization of vascular smooth muscle cells to apoptosis upon BAD induction.

The Bcl-2 homology domain 3 (BH3) mimetic ABT-737 reveals the dynamic regulation of bad, a proapoptotic protein of the Bcl-2 family, by Bcl-xL

The proteins of the B-cell lymphoma 2 (Bcl-2) family are important regulators of apoptosis under normal and pathological conditions. Chemical compounds that block the antiapoptotic proteins of this family have been introduced, such as 4-[4-[(4'-Chloro[1,1'-biphenyl]-2-yl)methyl]-1-piperazinyl]-N-[[4-[[(1R)-3-(dimethylamino)-1-[(phenylthio)methyl]propyl]amino]-3-nitrophenyl]sulfonyl]benzamide (ABT-737), a BH3-mimetic that neutralizes Bcl-2 and Bcl-xL. In this study, we used ABT-737 to explore the dynamic regulation of Bcl-2 proteins in living cells of different origins. Using ABT-737 as well as RNA interference or the application of growth factors, we examined the impact of the functional availability of the antiapoptotic proteins Bcl-2 and Bcl-2-extra large (Bcl-xL) on the Bcl-2 network. We report that ABT-737 increases the expression of Bcl-2-associated death promoter (Bad), a proapoptotic partner of the proteins Bcl-2 and Bcl-xL. Our observations indicate that Bad overexpression induced by ABT-737 results from the control of its normally rapid protein turnover, leading to the stabilization of this protein. We demonstrate the relevance of Bad post-translational regulation by Bcl-xL to the physiological setting using RNA interference against Bcl-xL as well as the application of epidermal growth factor, a growth factor that promotes the dissociation of Bad from Bcl-xL. Our results highlight a new facet of the mode of action of the antiapoptotic proteins Bcl-2 and Bcl-xL consisting of the regulation of the stability of the protein Bad. Finally, our results shed light on the mode of action of ABT-737, currently the best characterized inhibitor of the antiapoptotic proteins of the Bcl-2 family, and bear important implications regarding its use as an anticancer drug.

Raf family kinases: old dogs have learned new tricks

First identified in the early 1980s as retroviral oncogenes, the Raf proteins have been the objects of intense research. The discoveries 10 years later that the Raf family members (Raf-1, B-Raf, and A-Raf) are bona fide Ras effectors and upstream activators of the ubiquitous ERK pathway increased the interest in these proteins primarily because of the central role that this cascade plays in cancer development. The important role of Raf in cancer was corroborated in 2002 with the discovery of B-Raf genetic mutations in a large number of tumors. This led to intensified drug development efforts to target Raf signaling in cancer. This work yielded not only recent clinical successes but also surprising insights into the regulation of Raf proteins by homodimerization and heterodimerization. Surprising insights also came from the hunt for new Raf targets. Although MEK remains the only widely accepted Raf substrate, new kinase-independent roles for Raf proteins have emerged. These include the regulation of apoptosis by suppressing the activity of the proapoptotic kinases, ASK1 and MST2, and the regulation of cell motility and differentiation by controlling the activity of Rok-α. In this review, we discuss the regulation of Raf proteins and their role in cancer, with special focus on the interacting proteins that modulate Raf signaling. We also describe the new pathways controlled by Raf proteins and summarize the successes and failures in the development of efficient anticancer therapies targeting Raf. Finally, we also argue for the necessity of more systemic approaches to obtain a better understanding of how the Ras-Raf signaling network generates biological specificity.

Identification of novel in vivo phosphorylation sites of the human proapoptotic protein BAD: pore-forming activity of BAD is regulated by phosphorylation

BAD is a proapoptotic member of the Bcl-2 protein family that is regulated by phosphorylation in response to survival factors. Although much attention has been devoted to the identification of phosphorylation sites in murine BAD, little data are available with respect to phosphorylation of human BAD protein. Using mass spectrometry, we identified here besides the established phosphorylation sites at serines 75, 99, and 118 several novel in vivo phosphorylation sites within human BAD (serines 25, 32/34, 97, and 124). Furthermore, we investigated the quantitative contribution of BAD targeting kinases in phosphorylating serine residues 75, 99, and 118. Our results indicate that RAF kinases represent, besides protein kinase A, PAK, and Akt/protein kinase B, in vivo BAD-phosphorylating kinases. RAF-induced phosphorylation of BAD was reduced to control levels using the RAF inhibitor BAY 43-9006. This phosphorylation was not prevented by MEK inhibitors. Consistently, expression of constitutively active RAF suppressed apoptosis induced by BAD and the inhibition of colony formation caused by BAD could be prevented by RAF. In addition, using the surface plasmon resonance technique, we analyzed the direct consequences of BAD phosphorylation by RAF with respect to association with 14-3-3 and Bcl-2/Bcl-X(L) proteins. Phosphorylation of BAD by active RAF promotes 14-3-3 protein association, in which the phosphoserine 99 represented the major binding site. Finally, we show here that BAD forms channels in planar bilayer membranes in vitro. This pore-forming capacity was dependent on phosphorylation status and interaction with 14-3-3 proteins. Collectively, our findings provide new insights into the regulation of BAD function by phosphorylation.

Mitochondrial accumulation of polyubiquitinated proteins and differential regulation of apoptosis by polyubiquitination sites Lys-48 and -63

Proteins tagged with lysine (Lys, K) 48 polyubiquitins chains are destined for degradation by the 26S proteasomal system. Impairment of the ubiquitin proteasome system (UPS) function culminates in the accumulation of polyubiquitinated proteins in many neurodegenerative conditions including Parkinson's disease (PD). Nevertheless, the cellular mechanisms underlying cell death induced by an impaired UPS are still not clear. Intriguingly, recent studies indicate that several proteins associated with familial PD are capable of promoting the assembly of Lys-63 polyubiquitin chains. Therefore, the objective of this study was to examine the role of K48 and K63 ubiquitination in mitochondria-mediated apoptosis in in vitro models of dopaminergic degeneration. Exposure of the widely used proteasome inhibitor MG-132 to dopaminergic neuronal cell line (N27) induced a rapid accumulation of polyubiquitinated proteins in the mitochondria. This appears to result in the preferential association of ubiquitin conjugates in the outer membrane and polyubiquitination of outer membrane proteins. Interestingly, the ubiquitin(K48R) mutant effectively rescued cells from MG-132-induced mitochondrial apoptosis without altering the antioxidant status of cells; whereas the ubiquitin(K63R) mutant augmented the proapoptotic effect of MG-132. Herein, we report a novel conclusion that polyubiquitinated proteins, otherwise subjected to proteasomal degradation, preferentially accumulate in the mitochondria during proteolytic stress; and that polyubiquitination of Lys-48 and Lys-63 are key determinants of mitochondria-mediated cell death during proteasomal dysfunction. Together, these findings yield novel insights into a crosstalk between the UPS and mitochondria in dopaminergic neuronal cells.

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.