Bortezomib blocks Bax degradation in malignant B cells during treatment with TRAIL

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.

Interplay between proteasome inhibitors and NF-κB pathway in leukemia and lymphoma: a comprehensive review on challenges ahead of proteasome inhibitors

The current scientific literature has extensively explored the potential role of proteasome inhibitors (PIs) in the NF-κB pathway of leukemia and lymphoma. The ubiquitin-proteasome system (UPS) is a critical component in regulating protein degradation in eukaryotic cells. PIs, such as BTZ, are used to target the 26S proteasome in hematologic malignancies, resulting in the prevention of the degradation of tumor suppressor proteins, the activation of intrinsic mitochondrial-dependent cell death, and the inhibition of the NF-κB signaling pathway. NF-κB is a transcription factor that plays a critical role in the regulation of apoptosis, cell proliferation, differentiation, inflammation, angiogenesis, and tumor migration. Despite the successful use of PIs in various hematologic malignancies, there are limitations such as resistant to these inhibitors. Some reports suggest that PIs can induce NF-κB activation, which increases the survival of malignant cells. This article discusses the various aspects of PIs' effects on the NF-κB pathway and their limitations. Video Abstract.

The E3 ubiquitin ligase TRIM17 promotes gastric cancer survival and progression via controlling BAX stability and antagonizing apoptosis

Tripartite motif 17 (TRIM17) belongs to a subfamily of the RING-type E3 ubiquitin ligases, and regulates several cellular processes and pathological conditions including cancer. However, its potential function in gastric cancer (GC) remains obscure. Here, we have found TRIM17 mRNA and protein levels are both upregulated in human GC compared with normal specimens, and TRIM17 upregulation indicates poor survival for GC patients. Functionally, TRIM17 was found to act as an oncogene by promoting the proliferation and survival of GC cell lines AGS and HGC-27. Mechanistically, TRIM17 acts to interact with BAX and promote its ubiquitination and proteasomal degradation, leading to a deficiency in BAX-dependent apoptosis in GC cells in the absence and presence of apoptosis stimuli. Moreover, TRIM17 and BAX expression levels are inversely correlated in human GC specimens. Our data thus suggest TRIM17 contributes to gastric cancer survival through regulating BAX protein stability and antagonizing apoptosis, which provides a promising therapeutic target for GC treatment and a biomarker for prognosis.

Killing by Degradation: Regulation of Apoptosis by the Ubiquitin-Proteasome-System

Apoptosis is a cell suicide process that is essential for development, tissue homeostasis and human health. Impaired apoptosis is associated with a variety of human diseases, including neurodegenerative disorders, autoimmunity and cancer. As the levels of pro- and anti-apoptotic proteins can determine the life or death of cells, tight regulation of these proteins is critical. The ubiquitin proteasome system (UPS) is essential for maintaining protein turnover, which can either trigger or inhibit apoptosis. In this review, we will describe the E3 ligases that regulate the levels of pro- and anti-apoptotic proteins and assisting proteins that regulate the levels of these E3 ligases. We will provide examples of apoptotic cell death modulations using the UPS, determined by positive and negative feedback loop reactions. Specifically, we will review how the stability of p53, Bcl-2 family members and IAPs (Inhibitor of Apoptosis proteins) are regulated upon initiation of apoptosis. As increased levels of oncogenes and decreased levels of tumor suppressor proteins can promote tumorigenesis, targeting these pathways offers opportunities to develop novel anti-cancer therapies, which act by recruiting the UPS for the effective and selective killing of cancer cells.

A secondary role for hypoxia and HIF1 in the regulation of (IFNγ-induced) PD-L1 expression in melanoma

Cancer cells are able to escape immune surveillance by upregulating programmed death ligand 1 (PD-L1). A key regulator of PD-L1 expression is transcriptional stimulation by the IFNγ/JAK/STAT pathway. Recent studies suggest that hypoxia can induce PD-L1 expression. As hypoxia presents a hallmark of solid tumor development, hypoxic control of PD-L1 expression may affect the efficacy of cancer immunotherapy. This study aims to explore the hypoxic regulation of PD-L1 expression in human melanoma, and its interaction with IFNγ-induced PD-L1 expression. Analysis of the cutaneous melanoma dataset from the cancer genome atlas revealed a significant correlation of the HIF1-signaling geneset signature with PD-L1 mRNA expression. However, this correlation is less pronounced than other key pathways known to control PD-L1 expression, including the IFNγ/JAK/STAT pathway. This secondary role of HIF1 in PD-L1 regulation was confirmed by analyzing single-cell RNA-sequencing data of 33 human melanoma tissues. Interestingly, PD-L1 expression in these melanoma tissues was primarily found in macrophages. However, also in these cells STAT1, and not HIF1, displayed the most pronounced correlation with PD-L1 expression. Moreover, we observed that hypoxia differentially affects PD-L1 expression in human melanoma cell lines. Knockdown of HIF1 expression indicated a minor role for HIF1 in regulating PD-L1 expression. A more pronounced influence of hypoxia was found on IFNγ-induced PD-L1 mRNA expression, which is controlled at a 952 bp PD-L1 promoter fragment. These findings, showing the influence of hypoxia on IFNγ-induced PD-L1 expression, are relevant for immunotherapy, as both IFNγ and hypoxia are frequently present in the tumor microenvironment.

Dysregulation of the Ubiquitin Proteasome System in Human Malignancies: A Window for Therapeutic Intervention

Simple Summary

The ubiquitin proteasome system (UPS) governs the non-lysosomal degradation of oxidized, damaged, or misfolded proteins in eukaryotic cells. Dysregulation of the UPS results in loss of ability to maintain protein quality through proteolysis, and is closely related to the development of various malignancies and tumorigenesis. Here, we provide a comprehensive general overview on the regulation and roles of UPS and discuss the mechanisms linking dysregulated UPS to human malignancies. Inhibitors developed against components of the UPS, which include U.S. Food and Drug Administration FDA-approved and those currently undergoing clinical trials, are also presented in this review.

Abstract

The ubiquitin proteasome system (UPS) governs the non-lysosomal degradation of oxidized, damaged, or misfolded proteins in eukaryotic cells. This process is tightly regulated through the activation and transfer of polyubiquitin chains to target proteins which are then recognized and degraded by the 26S proteasome complex. The role of UPS is crucial in regulating protein levels through degradation to maintain fundamental cellular processes such as growth, division, signal transduction, and stress response. Dysregulation of the UPS, resulting in loss of ability to maintain protein quality through proteolysis, is closely related to the development of various malignancies and tumorigenesis. Here, we provide a comprehensive general overview on the regulation and roles of UPS and discuss functional links of dysregulated UPS in human malignancies. Inhibitors developed against components of the UPS, which include U.S. Food and Drug Administration FDA-approved and those currently undergoing clinical trials, are also presented in this review.

Bortezomib induces HSV-1 lethality in mice with neutrophil deficiency

Bortezomib suppressing NF-κB activity is an effective therapy for patients with myeloma or lymphoma. However, this drug can cause adverse effects, neutropenia, and recurrent infections of herpes viruses. Among herpes viruses, HSV-1 can reactivate to induce mortality. The important issues regarding how bortezomib diminishes neutrophils, whether bortezomib can induce HSV-1 reactivation, and how bortezomib exacerbates HSV-1 infection, need investigation. Using the murine model, we found that bortezomib induced HSV-1 reactivation. Bortezomib diminished neutrophil numbers in organs of uninfected and HSV-1-infected mice and turned a nonlethal infection to lethal with elevated tissue viral loads. In vitro results showed that bortezomib and HSV-1 collaborated to enhance the death and apoptosis of mouse neutrophils. The leukocyte deficiency induced by chemotherapies is generally believed to be the cause for aggravating virus infections. Here we show the potential of pathogen to exacerbate chemotherapy-induced leukocyte deficiency.

Increased autocrine interleukin-6 production is significantly associated with worse clinical outcome in patients with chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) remains incurable with current standard therapy. We have previously reported that an increased expression of interleukin‐6 (IL‐6) receptor CD126 leads to resistance of CLL cells to chemotherapy and worse prognosis for patients with CLL. In this study, we determine whether autocrine IL‐6 production by CLL B cells is associated with poor clinical outcome and explore IL‐6‐mediated survival mechanism in primary CLL cells. Our results demonstrate that higher levels of autocrine IL‐6 are significantly associated with shorter absolute lymphocyte doubling time, patients received treatment, without complete remission, advanced Binet stages, 17p/11q deletion, and shorter time to first time treatment and progression‐free survival. IL‐6 activated both STAT3 and nuclear factor kappa B (NF‐κB) in primary CLL cells. Blocking IL‐6 receptor and JAK2 inhibited IL‐6‐mediated activation of STAT3 and NF‐κB. Our study demonstrates that an increased autocrine IL‐6 production by CLL B‐cells are associated with worse clinical outcome for patients with CLL. IL‐6 promotes CLL cell survival by activating both STAT3 and NF‐κB through diverse signaling cascades. Neutralizing IL‐6 or blocking IL‐6 receptor might contribute overcoming the resistance of CLL cells to chemotherapy. We propose that the measurement of autocrine IL‐6 could be a useful approach to predict clinical outcome.

Delineating Crosstalk Mechanisms of the Ubiquitin Proteasome System That Regulate Apoptosis

Regulatory functions of the ubiquitin-proteasome system (UPS) are exercised mainly by the ubiquitin ligases and deubiquitinating enzymes. Degradation of apoptotic proteins by UPS is central to the maintenance of cell health, and deregulation of this process is associated with several diseases including tumors, neurodegenerative disorders, diabetes, and inflammation. Therefore, it is the view that interrogating protein turnover in cells can offer a strategy for delineating disease-causing mechanistic perturbations and facilitate identification of drug targets. In this review, we are summarizing an overview to elucidate the updated knowledge on the molecular interplay between the apoptosis and UPS pathways. We have condensed around 100 enzymes of UPS machinery from the literature that ubiquitinates or deubiquitinates the apoptotic proteins and regulates the cell fate. We have also provided a detailed insight into how the UPS proteins are able to fine-tune the intrinsic, extrinsic, and p53-mediated apoptotic pathways to regulate cell survival or cell death. This review provides a comprehensive overview of the potential of UPS players as a drug target for cancer and other human disorders.

STAT3 and NF-κB cooperatively control in vitro spontaneous apoptosis and poor chemo-responsiveness in patients with chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is an adult disease characterized by in vivo accumulation of mature CD5/CD19/CD23 triple positive B cells and is currently incurable. CLL cells undergo spontaneous apoptosis in response to in vitro cell culture condition but the underlying mechanism is unclear. We hypothesize that the sensitivity of CLL cells to spontaneous apoptosis may be associated with the constitutive activities of transcription factors STAT3 and/or NF-κB. We now show that the sensitivity of fresh CLL cells to spontaneous apoptosis is highly variable among different patients during 48 hours’ cell culture and inversely correlated with in vivo constitutively activated STAT3 and NF-κB (p < 0.001). Both activated STAT3 and NF-κB maintain the levels of anti-apoptotic protein Mcl-1/Bcl-xL and autocrine IL-6 production. CLL cells with higher susceptibility to in vitro spontaneous apoptosis show the greatest chemosensitivity (p < 0.001), which is reflected clinically as achieving a complete response (CR) (p < 0.001), longer lymphocyte doubling times (p < 0.01), time to first treatment (p < 0.01), and progression free survival (p < 0.05). Our data suggest that the sensitivity of CLL cells to in vitro spontaneous apoptosis is co-regulated by constitutively activated STAT3 and NF-κB and reflects the in vivo chemo-responsiveness and clinical outcomes.

Ubiquitination and the Regulation of Membrane Proteins

Newly synthesized transmembrane proteins undergo a series of steps to ensure that only the required amount of correctly folded protein is localized to the membrane. The regulation of protein quality and its abundance at the membrane are often controlled by ubiquitination, a multistep enzymatic process that results in the attachment of ubiquitin, or chains of ubiquitin to the target protein. Protein ubiquitination acts as a signal for sorting, trafficking, and the removal of membrane proteins via endocytosis, a process through which multiple ubiquitin ligases are known to specifically regulate the functions of a number of ion channels, transporters, and signaling receptors. Endocytic removal of these proteins through ubiquitin-dependent endocytosis provides a way to rapidly downregulate the physiological outcomes, and defects in such controls are directly linked to human pathologies. Recent evidence suggests that ubiquitination is also involved in the shedding of membranes and associated proteins as extracellular vesicles, thereby not only controlling the cell surface levels of some membrane proteins, but also their potential transport to neighboring cells. In this review, we summarize the mechanisms and functions of ubiquitination of membrane proteins and provide specific examples of ubiquitin-dependent regulation of membrane proteins.

Andrographolide reversed 5-FU resistance in human colorectal cancer by elevating BAX expression

5-FU is the first line therapy for colorectal cancer, however, treatment effect is often hampered by the development of drug resistance or toxicity at high doses. Andrographolide is a natural diterpenoid from Andrographis paniculata which has anti-bacterial, anti-antiviral and anti-inflammation activities. In the current study, we test the hypothesis that Andrographolide reverses 5-FU resistance in colorectal cancer and examine the underlying mechanism. In vitro and vivo studies indicated that Andrographolide treatment significantly re-sensitizes HCT116/5-FUR cells (HCT116 cells which are 5-FU resistant) to cytotoxicity of 5-FU. Mechanism analysis showed that Andrographolide/5-FU co-treatment elevated apoptosis level of HCT116/5-FUR cells with highly increased level of BAX. By using biotin-Andrographolide pull down and cellular thermal shift assay, we found out that Andrographolide can directly target to BAX. Andrographolide-BAX interaction prevented BAX degradation, enhancing mitochondria-mediated apoptosis thus reversed 5-FU resistance while BAX silence diminished this effect. Further, by analyzing patient samples who received 5-FU involved chemotherapy, we found that expression level of BAX is correlated with PFS. Our results here provide a novel combination treatment strategy, especially for patients with 5-FU-resistant tumors expressing low level of BAX. Meanwhile, we also proposed that BAX expression may be a predicted and prognosis marker of 5-FU involved chemotherapy.

Dynamin-related protein Drp1 is required for Bax translocation to mitochondria in response to irradiation-induced apoptosis

Translocation of the pro-apoptotic protein Bax from the cytosol to the mitochondria is a crucial step in DNA damage-mediated apoptosis, and is also found to be involved in mitochondrial fragmentation. Irradiation-induced cytochrome c release and apoptosis was associated with Bax activation, but not mitochondrial fragmentation. Both Bax and Drp1 translocated from the cytosol to the mitochondria in response to irradiation. However, Drp1 mitochondrial translocation and oligomerization did not require Bax, and failed to induce apoptosis in Bax deficient diffuse large B-cell lymphoma (DLBCL) cells. Using fluorescent microscopy and the intensity correlation analysis, we demonstrated that Bax and Drp1 were colocalized and the levels of colocalization were increased by UV irradiation. Using co-immuno-precipitation, we confirmed that Bax and Drp1 were binding partners. Irradiation induced a time-associated increase in the interaction between active Bax and Drp1. Knocking down Drp1 using siRNA blocked UV irradiation-mediated Bax mitochondrial translocation. In conclusion, our findings demonstrate for the first time, that Drp1 is required for Bax mitochondrial translocation, but Drp1-induced mitochondrial fragmentation alone is not sufficient to induce apoptosis in DLBCL cells.

Phase II open-label study of recombinant circularly permuted TRAIL as a single-agent treatment for relapsed or refractory multiple myeloma

Background

Despite the recent development of new therapies, multiple myeloma (MM) remains an incurable disease. Thus, new, effective treatments are urgently needed, particularly for relapsed or refractory MM (RRMM). In an earlier phase I study, a novel form of recombinant human Apo2L/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) that is currently in clinical development for the treatment of hematologic malignancies, i.e., circularly permuted TRAIL (CPT), was well tolerated at a dose of 2.5 mg/kg per day and showed promising preliminary activity in patients with RRMM. This phase II, open-label, multicenter study further investigated the efficacy and safety of 2.5-mg/kg per day CPT as single-agent therapy for patients with RRMM.

Methods

Patients with RRMM were treated once daily with CPT (2.5 mg/kg, intravenously) for 14 consecutive days for each 21-day cycle. Clinical response and toxicity were assessed after each treatment cycle.

Results

Twenty-seven patients received CPT. Using the European Group for Blood and Marrow Transplantation criteria, we calculated the overall response rate of 33.3% with 1 near-complete response (nCR) and 8 partial responses (PRs). The clinical benefit rate (48.1%) included 1 nCR, 8 PRs, and 4 minimal responses. The most common treatment-related adverse events (TRAEs) were fever, aspartate aminotransferase elevation, alanine aminotransferase elevation, leucopenia, rash, neutropenia, and thrombocytopenia. We graded toxicity using the Common Toxicity Criteria for Adverse Events, version 3.0, and determined that 37.0% of patients had at least 1 grade 3–4 TRAE.

Conclusions

CPT as a single agent can elicit a response in patients with RRMM and is well tolerated. Further clinical investigation is warranted.

Trial Registration ChiCTR-ONC-12002065 http://www.chictr.org/cn

Rituximab-induced HMGB1 release is associated with inhibition of STAT3 activity in human diffuse large B-cell lymphoma

Treatment with rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) has greatly improved clinical outcomes in patients with diffuse large B-cell lymphoma (DLBCL) compared with CHOP. The mechanism of rituximab-induced cell death is poorly understood. We found that rituximab does not enhance the directly killing efficacy of CHOP, as tested on a panel of DLBCL cell lines. Rituximab induced a rapid release of HMGB1 (High mobility group protein B 1). This release is independent of cell death but significantly correlated with an inhibition on STAT3 activity. In the resting state, HMGB1 co-localizes and interacts with STAT3 in the nucleus of DLBCL cells. Treatment with rituximab breaks this binding and triggers HMGB1 release. Treatment with R-CHOP but not CHOP significantly increased plasma HMGB1 and decreased IL-10 concentrations in DLBCL patients compared with controls. The conditioned medium from rituximab-treated DLBCL cells is able to trigger dendritic cell maturation, phagocytosis, and IFN-g secretion by cytotoxic T cells. In conclusion, our results demonstrate that rituximab induces an inhibition on STAT3 activity, leading to increased HMGB1 release and decreased IL-10 secretion, which elicits immune responses, suggesting that indirect effects on the immune system rather than direct killing contribute to elimination of DLBCL.

Use of class I histone deacetylase inhibitor romidepsin in combination regimens

Histone deacetylase (HDAC) inhibitors are epigenetic-modifying agents that have shown promise as anticancer therapies. Several HDAC inhibitors have been approved by the US Food and Drug Administration (FDA) as single-agent therapies to treat T-cell lymphoma. The synergistic combination of HDAC inhibitors with other anticancer agents has the potential to constitute treatment regimens with enhanced efficacy. Romidepsin is a structurally unique, potent, bicyclic class 1 selective HDAC inhibitor approved by the FDA for the treatment of patients with peripheral T-cell lymphoma who have had at least 1 prior therapy and patients with cutaneous T-cell lymphoma who have had at least 1 prior systemic therapy. Here, we review data that support the use of romidepsin in combination with other anticancer agents for the treatment of various malignancies. Promising results have emerged from early clinical studies, supporting the potential for romidepsin combination regimens to constitute safe and effective treatments for cancer.

Redox Homeostasis and Cellular Antioxidant Systems: Crucial Players in Cancer Growth and Therapy

Reactive oxygen species (ROS) and their products are components of cell signaling pathways and play important roles in cellular physiology and pathophysiology. Under physiological conditions, cells control ROS levels by the use of scavenging systems such as superoxide dismutases, peroxiredoxins, and glutathione that balance ROS generation and elimination. Under oxidative stress conditions, excessive ROS can damage cellular proteins, lipids, and DNA, leading to cell damage that may contribute to carcinogenesis. Several studies have shown that cancer cells display an adaptive response to oxidative stress by increasing expression of antioxidant enzymes and molecules. As a double-edged sword, ROS influence signaling pathways determining beneficial or detrimental outcomes in cancer therapy. In this review, we address the role of redox homeostasis in cancer growth and therapy and examine the current literature regarding the redox regulatory systems that become upregulated in cancer and their role in promoting tumor progression and resistance to chemotherapy.

Drugging the undruggables: exploring the ubiquitin system for drug development

Dynamic modulation of protein levels is tightly controlled in response to physiological cues. In mammalian cells, much of the protein degradation is carried out by the ubiquitin-proteasome system (UPS). Similar to kinases, components of the ubiquitin system are often dysregulated, leading to a variety of diseases, including cancer and neurodegeneration, making them attractive drug targets. However, so far there are only a handful of drugs targeting the ubiquitin system that have been approved by the FDA. Here, we review possible therapeutic intervention nodes in the ubiquitin system, analyze the challenges, and highlight the most promising strategies to target the UPS.

Aqueous Extract of Anticancer Drug CRUEL Herbomineral Formulation Capsules Exerts Anti-proliferative Effects in Renal Cell Carcinoma Cell Lines

PURPOSE

Anti-cancer activity evaluation of aqueous extract of CRUEL (herbomineral formulation) capsules on renal cell carcinoma cell lines, and exploration of mechanisms of cell death.

MATERIALS AND METHODS

To detect the cytotoxic dose concentration in renal cell carcinoma (RCC) cells, MTT assays were performed and morphological changes after treatment were observed by inverted microscopy. Drug effects against RCC cell lines were assessed with reference to cell cycle distribution (flow cytometry), anti-metastatic potential (wound healing assay) and autophagy(RT-PCR).

RESULTS

CRUEL showed anti-proliferative effects against RCC tumor cell lines with an IC50 value of approximately 4mg/mL in vitro, while inducing cell cycle arrest at S-phase of the cell cycle and inhibiting wound healing. LC3 was found to be up-regulated after drug treatment by RT-PCR resulting in an autophagy mode of cell death.

CONCLUSIONS

This study provides experimental validation for antitumor activity of CRUEL.

Direct Activation of Bax Protein for Cancer Therapy

Bax, a central cell death regulator, is an indispensable gateway to mitochondrial dysfunction and a major proapoptotic member of the B-cell lymphoma 2 (Bcl-2) family proteins that control apoptosis in normal and cancer cells. Dysfunction of apoptosis renders the cancer cell resistant to treatment as well as promotes tumorigenesis. Bax activation induces mitochondrial membrane permeabilization, thereby leading to the release of apoptotic factor cytochrome c and consequently cancer cell death. A number of drugs in clinical use are known to indirectly activate Bax. Intriguingly, recent efforts demonstrate that Bax can serve as a promising direct target for small-molecule drug discovery. Several direct Bax activators have been identified to hold promise for cancer therapy with the advantages of specificity and the potential of overcoming chemo- and radioresistance. Further investigation of this new class of drug candidates will be needed to advance them into the clinic as a novel means to treat cancer.

Trailing TRAIL Resistance: Novel Targets for TRAIL Sensitization in Cancer Cells

Resistance to chemotherapeutic drugs is the major hindrance in the successful cancer therapy. The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor (TNF) family of ligands, which initiates apoptosis in cancer cells through interaction with the death receptors DR4 and DR5. TRAIL is perceived as an attractive chemotherapeutic agent as it specifically targets cancer cells while sparing the normal cells. However, TRAIL therapy has a major limitation as a large number of the cancer develop resistance toward TRAIL and escape from the destruction by the immune system. Therefore, elucidation of the molecular targets and signaling pathways responsible for TRAIL resistance is imperative for devising effective therapeutic strategies for TRAIL resistant cancers. Although, various molecular targets leading to TRAIL resistance are well-studied, recent studies have implicated that the contribution of some key cellular processes toward TRAIL resistance need to be fully elucidated. These processes primarily include aberrant protein synthesis, protein misfolding, ubiquitin regulated death receptor expression, metabolic pathways, epigenetic deregulation, and metastasis. Novel synthetic/natural compounds that could inhibit these defective cellular processes may restore the TRAIL sensitivity and combination therapies with such compounds may resensitize TRAIL resistant cancer cells toward TRAIL-induced apoptosis. In this review, we have summarized the key cellular processes associated with TRAIL resistance and their status as therapeutic targets for novel TRAIL-sensitizing agents.

DRAM1 regulates apoptosis through increasing protein levels and lysosomal localization of BAX

DRAM1 (DNA damage-regulated autophagy modulator 1) is a TP53 target gene that modulates autophagy and apoptosis. We previously found that DRAM1 increased autophagy flux by promoting lysosomal acidification and protease activation. However, the molecular mechanisms by which DRAM1 regulates apoptosis are not clearly defined. Here we report a novel pathway by which DRAM1 regulates apoptosis involving BAX and lysosomes. A549 or HeLa cells were treated with the mitochondrial complex II inhibitor, 3-nitropropionic acid (3NP), or an anticancer drug, doxorubicin. Changes in the protein and mRNA levels of BAX and DRAM1 and the role of DRAM1 in BAX induction were determined. The interaction between DRAM1 and BAX and its effect on BAX degradation, BAX lysosomal localization, the release of cathepsin B and cytochrome c by BAX and the role of BAX in 3NP- or doxorubicin-induced cell death were studied. The results showed that BAX, a proapoptotic protein, was induced by DRAM1 in a transcription-independent manner. BAX was degraded by autophagy under basal conditions; however, its degradation was inhibited when DRAM1 expression was induced. There was a protein interaction between DRAM1 and BAX and this interaction prolonged the half-life of BAX. Furthermore, upregulated DRAM1 recruited BAX to lysosomes, leading to the release of lysosomal cathepsin B and cleavage of BID (BH3-interacting domain death agonist). BAX mediated the release of mitochondrial cytochrome c, activation of caspase-3 and cell death partially through the lysosome-cathepsin B-tBid pathway. These results indicate that DRAM1 regulates apoptosis by inhibiting BAX degradation. In addition to mitochondria, lysosomes may also be involved in BAX-initiated apoptosis.

XIAP-targeting drugs re-sensitize PIK3CA-mutated colorectal cancer cells for death receptor-induced apoptosis

Mutations in the oncogenic PIK3CA gene are found in 10–20% of colorectal cancers (CRCs) and are associated with poor prognosis. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and agonistic TRAIL death receptor antibodies emerged as promising anti-neoplastic therapeutics, but to date failed to prove their capability in the clinical setting as especially primary tumors exhibit high rates of TRAIL resistance. In our study, we investigated the molecular mechanisms underlying TRAIL resistance in CRC cells with a mutant PIK3CA (PIK3CA-mut) gene. We show that inhibition of the constitutively active phosphatidylinositol-3 kinase (PI3K)/Akt signaling pathway only partially overcame TRAIL resistance in PIK3CA-mut-protected HCT116 cells, although synergistic effects of TRAIL plus PI3K, Akt or cyclin-dependent kinase (CDK) inhibitors could be noted. In sharp contrast, TRAIL triggered full-blown cell death induction in HCT116 PIK3CA-mut cells treated with proteasome inhibitors such as bortezomib and MG132. At the molecular level, resistance of HCT116 PIK3CA-mut cells against TRAIL was reflected by impaired caspase-3 activation and we provide evidence for a crucial involvement of the E3-ligase X-linked inhibitor of apoptosis protein (XIAP) therein. Drugs interfering with the activity and/or the expression of XIAP, such as the second mitochondria-derived activator of caspase mimetic BV6 and mithramycin-A, completely restored TRAIL sensitivity in PIK3CA-mut-protected HCT116 cells independent of a functional mitochondrial cell death pathway. Importantly, proteasome inhibitors and XIAP-targeting agents also sensitized other CRC cell lines with mutated PIK3CA for TRAIL-induced cell death. Together, our data suggest that proteasome- or XIAP-targeting drugs offer a novel therapeutic approach to overcome TRAIL resistance in PIK3CA-mutated CRC.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

Why bortezomib cannot go with 'green'?

Eat more ‘green’ or eat ‘five a day’ is one of the most important healthy lifestyle behaviours in the 21 century. Aiming to fight cancer effectively, more than half patients use vitamins or herbs concurrently with conventional anticancer treatment. Flavonoids or polyphenols existing in vegetables, fruits and green tea are common plant pigments with antioxidant properties and considered acting as cancer preventing or anti-cancer agents. Recently it was found that some flavonoids and vitamin C in diet or supplements have antagonistic effect with the anti-cancer drug bortezomib. Bortezomib is a specific inhibitor for proteasome and is currently used for treatment of relapsed and refractory multiple myeloma. Despite its successful rates in treating multiple myeloma and other solid tumors, it is unable to kill leukemic cells in the blood. It was recently revealed that some flavonoids and vitamin C present in green leaves and green teas in the blood can neutralize bortezomib by directly interaction between two chemicals. Here we summarize why dietary flavonoids should be avoided in patients who take bortezomib as chemotherapeutic drug.

Canonical and new generation anticancer drugs also target energy metabolism

Significant efforts have been made for the development of new anticancer drugs (protein kinase or proteasome inhibitors, monoclonal humanized antibodies) with presumably low or negligible side effects and high specificity. However, an in-depth analysis of the side effects of several currently used canonical (platin-based drugs, taxanes, anthracyclines, etoposides, antimetabolites) and new generation anticancer drugs as the first line of clinical treatment reveals significant perturbation of glycolysis and oxidative phosphorylation. Canonical and new generation drug side effects include decreased (1) intracellular ATP levels, (2) glycolytic/mitochondrial enzyme/transporter activities and/or (3) mitochondrial electrical membrane potentials. Furthermore, the anti-proliferative effects of these drugs are markedly attenuated in tumor rho (0) cells, in which functional mitochondria are absent; in addition, several anticancer drugs directly interact with isolated mitochondria affecting their functions. Therefore, several anticancer drugs also target the energy metabolism, and hence, the documented inhibitory effect of anticancer drugs on cancer growth should also be linked to the blocking of ATP supply pathways. These often overlooked effects of canonical and new generation anticancer drugs emphasize the role of energy metabolism in maintaining cancer cells viable and its targeting as a complementary and successful strategy for cancer treatment.

Ubiquitination by SAG regulates macrophage survival/death and immune response during infection

The checkpoint between the life and death of macrophages is crucial for the host's frontline immune defense during acute phase infection. However, the mechanism as to how the immune cell equilibrates between apoptosis and immune response is unclear. Using in vitro and ex vivo approaches, we showed that macrophage survival is synchronized by SAG (sensitive to apoptosis gene), which is a key member of the ubiquitin–proteasome system (UPS). When challenged by pathogen-associated molecular patterns (PAMPs), we observed a reciprocal expression profile of pro- and antiapoptotic factors in macrophages. However, SAG knockdown disrupted this balance. Further analysis revealed that ubiquitination of Bax and SARM (sterile α- and HEAT/armadillo-motif-containing protein) by SAG-UPS confers survival advantage to infected macrophages. SAG knockdown caused the accumulation of proapoptotic Bax and SARM, imbalance of Bcl-2/Bax in the mitochondria, induction of cytosolic cytochrome c and activation of caspase-9 and -3, all of which led to disequilibrium between life and death of macrophages. In contrast, SAG-overexpressing macrophages challenged with PAMPs exhibited upregulation of protumorigenic cytokines (IL-1β, IL-6 and TNF-α), and downregulation of antitumorigenic cytokine (IL-12p40) and anti-inflammatory cytokine (IL-10). This suggests that SAG-dependent UPS is a key switch between immune defense and apoptosis or immune overactivation and tumorigenesis. Altogether, our results indicate that SAG-UPS facilitates a timely and appropriate level of immune response, prompting future development of potential immunomodulators of SAG-UPS.

The DeISGylase USP18 limits TRAIL-induced apoptosis through the regulation of TRAIL levels: Cellular levels of TRAIL influences responsiveness to TRAIL-induced apoptosis

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a promising molecule for anti-cancer therapies. Unfortunately, cancer cells frequently acquire resistance to rhTRAIL. Various co-treatments have been proposed to overcome apoptosis resistance to TRAIL. Here we show that downregulation of the deISGylase USP18 sensitizes cancer cells to rhTRAIL, whereas, elevate levels of USP18 inhibit TRAIL-induced apoptosis, in a deISGylase-independent manner. USP18 influences TRAIL signaling through the control of the IFN autocrine loop. In fact, cells with downregulated USP18 expression augment the expression of cellular TRAIL. Downregulation of cellular TRAIL abrogates the synergism between TRAIL and USP18 siRNA and also limits cell death induced by rhTRAIL. By comparing the apoptotic responsiveness to TRAIL in a panel of cancer cell lines, we have discovered a correlation between TRAIL levels and the apoptotic susceptibility to rhTRAIL, In cells expressing high levels of TRAIL-R2 susceptibility to rhTRAIL correlates with TRAIL expression. In conclusion, we propose that cellular TRAIL is an additional factor that can influence the apoptotic response to rhTRAIL.

Evading apoptosis in cancer

Carcinogenesis is a mechanistically complex and variable process with a plethora of underlying genetic causes. Cancer development comprises a multitude of steps that occur progressively starting with initial driver mutations leading to tumorigenesis and, ultimately, metastasis. During these transitions, cancer cells accumulate a series of genetic alterations that confer on the cells an unwarranted survival and proliferative advantage. During the course of development, however, cancer cells also encounter a physiologically ubiquitous cellular program that aims to eliminate damaged or abnormal cells: apoptosis. Thus, it is essential that cancer cells acquire instruments to circumvent programmed cell death. Here we discuss emerging evidence indicating how cancer cells adopt various strategies to override apoptosis, including amplifying the antiapoptotic machinery, downregulating the proapoptotic program, or both.

Targeting Oncogene-Induced Autophagy: A New Approach in Cancer Therapy?

Autophagy is a tightly controlled self-degradation process utilised by cells to sustain cellular homeostasis and to support cell survival in response to metabolic stress and starvation. Thus, autophagy plays a critical role in promoting cell integrity and maintaining proper function of cellular processes. Defects in autophagy, however, can have drastic implications in human health and diseases, including cancer. Described as a double-edged sword in the context of cancer, autophagy can act as both suppressor and facilitator of tumorigenesis. As such, defining the precise role of autophagy in a multistep event like cancer progression can be complex. Recent findings have implicated a role for components of the autophagy pathway in oncogene-mediated cell transformation, tumour growth, and survival. Notably, aggressive cancers driven by Ras oncoproteins rely on autophagy to sustain a reprogrammed mitochondrial metabolic signature and evade cell death. In this review, we summarize our current understanding of the role of oncogene-induced autophagy in cancer progression and discuss how modulators of autophagic responses can bring about therapeutic benefit and eradication of a subset of cancers that are addicted to this ancient recycling machinery.

Targeted therapy of the XIAP/proteasome pathway overcomes TRAIL-resistance in carcinoma by switching apoptosis signaling to a Bax/Bak-independent 'type I' mode

TRAIL is a promising anticancer agent, capable of inducing apoptosis in a wide range of treatment-resistant tumor cells. In ‘type II' cells, the death signal triggered by TRAIL requires amplification via the mitochondrial apoptosis pathway. Consequently, deregulation of the intrinsic apoptosis-signaling pathway, for example, by loss of Bax and Bak, confers TRAIL-resistance and limits its application. Here, we show that despite resistance of Bax/Bak double-deficient cells, TRAIL-treatment resulted in caspase-8 activation and complete processing of the caspase-3 proenzymes. However, active caspase-3 was degraded by the proteasome and not detectable unless the XIAP/proteasome pathway was inhibited. Direct or indirect inhibition of XIAP by RNAi, Mithramycin A or by the SMAC mimetic LBW-242 as well as inhibition of the proteasome by Bortezomib overcomes TRAIL-resistance of Bax/Bak double-deficient tumor cells. Moreover, activation and stabilization of caspase-3 becomes independent of mitochondrial death signaling, demonstrating that inhibition of the XIAP/proteasome pathway overcomes resistance by converting ‘type II' to ‘type I' cells. Our results further demonstrate that the E3 ubiquitin ligase XIAP is a gatekeeper critical for the ‘type II' phenotype. Pharmacological manipulation of XIAP therefore is a promising strategy to sensitize cells for TRAIL and to overcome TRAIL-resistance in case of central defects in the intrinsic apoptosis-signaling pathway.

Human cytomegalovirus inhibits apoptosis by proteasome-mediated degradation of Bax at endoplasmic reticulum-mitochondrion contacts

Human cytomegalovirus (HCMV) encodes the UL37 exon 1 protein (pUL37x1), which is the potent viral mitochondrion-localized inhibitor of apoptosis (vMIA), to increase survival of infected cells. HCMV vMIA traffics from the endoplasmic reticulum (ER) to ER subdomains, which are physically linked to mitochondria known as mitochondrion-associated membranes (MAM), and to mitochondria. The antiapoptotic function of vMIA is thought to primarily result from its ability to inhibit Bax-mediated permeabilization of the outer mitochondrial membrane (OMM). Here, we establish that vMIA retargets Bax to the MAM as well as to the OMM from immediate early through late times of infection. However, MAM localization of Bax results in its increased ubiquitination and proteasome-mediated degradation. Surprisingly, HCMV infection does not increase OMM-associated degradation (OMMAD) of Bax, even though the ER and mitochondria are physically connected at the MAM. It was recently found that lipid rafts at the plasma membrane can connect extrinsic and intrinsic apoptotic pathways and can serve as sites of apoptosome assembly. In transfected permissive human fibroblasts, vMIA mediates, through its cholesterol affinity, association of Bax and apoptosome components with MAM lipid rafts. While Bax association with MAM lipid rafts was detected in HCMV-infected cells, association of apoptosome components was not. These results establish that Bax recruitment to the MAM and its MAM-associated degradation (MAMAD) are a newly described antiapoptotic mechanism used by HCMV infection to increase cell survival for its growth.

Single-agent Smac-mimetic compounds induce apoptosis in B chronic lymphocytic leukaemia (B-CLL)

Defective apoptosis is a hallmark of the progression of B chronic lymphocytic leukaemia (B-CLL). Smac-mimetics have been shown to induce apoptosis in several tumours. We describe the in vitro pro-apoptotic activity and regulation of the molecular pathway induced by new Smac-mimetics in B-CLL. The cytotoxic effect was significantly higher in B-CLL samples than in healthy controls. No significant synergistic effect was observed in combined treatment. In conclusion one of our compounds (Smac66), used as monotherapy and not in combination, is highly active against B-CLL cells thus suggesting a promising therapeutic potential as a new class of antileukemic drugs in haematology.

BaxΔ2 is a novel bax isoform unique to microsatellite unstable tumors

The pro-death Bcl-2 family protein and tumor suppressor Bax is frequently mutated in tumors with microsatellite instability (MSI). The mutation often results in a "Bax negative" phenotype and therefore is generally thought to be beneficial to the development of the tumor. Here, we report the identification of a novel Bax isoform, BaxΔ2, which is unique to microsatellite unstable tumors. BaxΔ2 is generated by a unique combination of a microsatellite deletion in Bax exon 3 and alternative splicing of Bax exon 2. Consistently, BaxΔ2 is only detected in MSI cell lines and primary tumors. BaxΔ2 is a potent cell death inducer but does not directly target mitochondria. In addition, BaxΔ2 sensitizes certain MSI tumor cells to a subset of chemotherapeutic agents, such as adriamycin. Thus, our data provide evidence that mutation and alternative splicing of tumor suppressors such as Bax are not always beneficial to tumor development but can be detrimental instead.

Bortezomib and TRAIL: a perfect match for apoptotic elimination of tumour cells?

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine that selectively eradicates tumour cells via specific cell surface receptors and is intensively explored for use as a novel anticancer approach. To enhance the efficacy of TRAIL receptor agonists the proteasome inhibitor bortezomib is one of the most potent sensitizers. Here we review the main mechanisms underlying bortezomib-dependent TRAIL sensitization, including stimulation of apoptosis by increasing expression of TRAIL receptors, reduction of cFLIP and enhancement of caspase 8 activation, and modulation of Bcl-2 family proteins and inhibitor of apoptosis proteins (IAPs). Concomitantly, pro-survival signals are suppressed such as elicited by NF-κB and Akt. The different preclinical tumour models explored with this combination, including primary tumour (stem) cells, stroma co-culture and mice models, are discussed, as well as possible hurdles for clinical activity. Collectively, anticipating a solid rationale for bortezomib-TRAIL combination and very promising preclinical results, its clinical activity remains to be demonstrated.

ZGDHu-1 promotes apoptosis of chronic lymphocytic leukemia cells

Chronic lymphocytic leukemia (CLL) is a low-grade lymphoid malignancy incurable with conventional modalities of chemotherapy. We aimed to examine the proapoptotic effects of a novel proteasome inhibitor, N,N'-di-(m-methylphenyi)-3,6- dimethyl-1,4-dihydro-1,2,4,5-tetrazine-1,4-dicarboamide (ZGDHu-1), on CLL cells. B lymphocytes were isolated from CLL patients and normal healthy controls, and treated with various concentrations of ZGDHu-1 for different days. CLL cell viability was detected by MTT assay. The apoptosis, mitochondrial membrane potential (∆ψm) and reactive oxidative species (ROS) were examined by flow cytometry. The expression of caspase-3 and Bcl-2/Bax ratio was detected by western blotting. ZGDHu-1 significantly reduced the viability of CLL cells and induced apoptosis in comparison to the control cells (both P<0.05). Normal peripheral B cells were resistant to the apoptosis-inducing effects of ZGDHu-1. Apoptosis induced by ZGDHu-1 was accompanied with generation of ROS, loss of ∆ψm, downregulation of Bcl-2 and increase of caspase-3 cleavage. Results of this study indicate that ZGDHu-1 is a promising specific treatment for CLL in the clinic.

Blocking autophagy prevents bortezomib-induced NF-κB activation by reducing I-κBα degradation in lymphoma cells

Here we show that bortezomib induces effective proteasome inhibition and accumulation of poly-ubiquitinated proteins in diffuse large B-cell lymphoma (DLBCL) cells. This leads to induction of endoplasmic reticulum (ER) stress as demonstrated by accumulation of the protein CHOP, as well as autophagy, as demonstrated by accumulation of LC3-II proteins. Our data suggest that recruitment of both ubiquitinated proteins and LC3-II by p62 directs ubiquitinated proteins, including I-κBα, to the autophagosome. Degradation of I-κBα results in increased NF-κB nuclear translocation and transcription activity. Since bortezomib treatment promoted I-κBα phosphorylation, ubiquitination and degradation, this suggests that the route of I-κBα degradation was not via the ubiquitin-proteasome degradation system. The autophagy inhibitor chloroquine (CQ) significantly inhibited bortezomib-induced I-κBα degradation, increased complex formation with NF-κB and reduced NF-κB nuclear translocation and DNA binding activity. Importantly, the combination of proteasome and autophagy inhibitors showed synergy in killing DLBCL cells. In summary, bortezomib-induced autophagy confers relative DLBCL cell drug resistance by eliminating I-κBα. Inhibition of both autophagy and the proteasome has great potential to kill apoptosis-resistant lymphoma cells.

WITHDRAWN: Evidence for the critical roles of NF-κB p65 and specificity proteins in the apoptosis-inducing activity of proteasome inhibitors in leukemia cells

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Mitochondria as therapeutic targets for the treatment of malignant disease

SIGNIFICANCE

Mitochondria exert vital functions during normal physiology and are also centrally involved in the regulation of various modes of cell death. Thus, engaging the mitochondrial apoptosis pathway presents an attractive possibility to activate lethal effectors in cancer cells.

RECENT ADVANCES

Compounds that directly target mitochondria offer the advantage to initiate mitochondrial outer membrane permeabilization independently of upstream signal transduction elements that are frequently impaired in human cancers. As a consequence, mitochondrion-targeted agents may bypass some forms of drug resistance.

CRITICAL ISSUES

An ever-increasing number of compounds, including natural compounds and rationally designed drugs, has been shown to directly act on mitochondria.

FUTURE DIRECTIONS

Forthcoming insights into the fine regulation of mitochondrial apoptosis will likely open future perspectives for cancer drug development.

Novel insights into the synergistic interaction of Bortezomib and TRAIL: tBid provides the link

The proteasome inhibitor Bortezomib has been identified as a potent enhancer of TRAIL-induced apoptosis in several human cancers. However, the identification of the underlying molecular mechanisms of this synergistic cell death induction has been ongoing over the last years. A recent study identifies a new mechanism of action for the synergism of TRAIL and Bortezomib.

Regulating mitochondrial outer membrane proteins by ubiquitination and proteasomal degradation

Mitochondrial outer membrane proteins have been found to be ubiquitinated and degraded by the proteasome. This process shares at least one component of the ERAD pathway of ER membrane protein degradation, the AAA ATPase cdc48/p97/VCP, thought to extract integral membrane proteins from the lipid bilayer and chaperone them to the proteasome. Proteasomal degradation of the outer mitochondrial membrane (OMM) protein Mcl1 regulates apoptosis whereas Parkin-mediated ubiquitination and degradation of Mitofusins can inhibit mitochondrial fusion and promote mitophagy. The breadth of OMM ubiquitin/proteasome substrates and the physiological relevance of their turnover are only beginning to be understood.

A novel tumor antigen derived from enhanced degradation of bax protein in human cancers

Cancer cells frequently exhibit defects in apoptosis, which contribute to increased survival and chemotherapeutic resistance. For example, genetic mutations or abnormal proteasomal degradation can reduce expression of Bax which limits apoptosis. In cancers where abnormal proteasomal degradation of Bax occurs, we hypothesized that Bax peptides that bind to human leukocyte antigen (HLA) class I molecules would be generated for presentation to CD8(+) T cells. To test this hypothesis, we generated T cells against pooled Bax peptides, using the blood of healthy human donors. Although T-cell responses were of low frequency (0.15%), a CD8(+) T-cell clone (KSIVB17) was isolated that optimally recognized Bax(136-144) peptide (IMGWTLDFL) presented by HLA-A*0201. KSIVB17 was able to recognize and kill a variety of HLA-matched cancer cells including primary tumor cells from chronic lymphocytic leukemia (CLL). No reactivity was seen against HLA-matched, nontransformed cells such as PHA blasts and skin fibroblasts. Furthermore, KSIVB17 reactivity corresponded with the proteasomal degradation patterns of Bax protein observed in cancer cells. Taken together, our findings suggest a new concept for tumor antigens based on regulatory proteins that are ubiquitously expressed in normal cells, but that have abnormally enhanced degradation in cancer cells. Bax degradation products offer candidate immune antigens in cancers such as CLL in which increased Bax degradation correlates with poor clinical prognosis.

Aberrantly activated anti-apoptotic signalling mechanisms in chronic lymphocytic leukaemia cells: clues to the identification of novel therapeutic targets

Chronic lymphocytic leukaemia (CLL) is the commonest haematological malignancy in the western world and is incurable by cytotoxic therapy. Considerable research effort has identified the signal transduction pathways in CLL cells that contribute to anti-apoptotic signalling. Some pathways are constitutively activated in CLL cells but upregulated in normal cells only when protein tyrosine kinases (PTKs) are activated by ligands. This review describes which PTKs are aberrantly activated in CLL cells and are potential targets for inhibition. Additional potential targets within pathways downstream of these PTKs include Mek/Erk, mTorc1, protein kinase C, PI-3 kinase/Akt, nuclear factor-κB and cyclin-dependent protein kinase. Numerous studies have identified chemical agents and antibodies that selectively kill CLL cells, irrespective of their genetic resistance to conventional chemotherapeutic agents, and which can overcome cytoprotective microenvironmental signalling. These studies have resulted in identification of novel therapies, some of which are currently undergoing clinical trials. In vitro and animal model studies and clinical trials could determine which inhibitors of which targets are the likely to be most effective and least toxic either singly or in combination.