Autophagy and chemotherapy resistance: a promising therapeutic target for cancer treatment

Induction of cell death and inhibition of cell survival are the main principles of cancer therapy. Resistance to chemotherapeutic agents is a major problem in oncology, which limits the effectiveness of anticancer drugs. A variety of factors contribute to drug resistance, including host factors, specific genetic or epigenetic alterations in the cancer cells and so on. Although various mechanisms by which cancer cells become resistant to anticancer drugs in the microenvironment have been well elucidated, how to circumvent this resistance to improve anticancer efficacy remains to be defined. Autophagy, an important homeostatic cellular recycling mechanism, is now emerging as a crucial player in response to metabolic and therapeutic stresses, which attempts to maintain/restore metabolic homeostasis through the catabolic lysis of excessive or unnecessary proteins and injured or aged organelles. Recently, several studies have shown that autophagy constitutes a potential target for cancer therapy and the induction of autophagy in response to therapeutics can be viewed as having a prodeath or a prosurvival role, which contributes to the anticancer efficacy of these drugs as well as drug resistance. Thus, understanding the novel function of autophagy may allow us to develop a promising therapeutic strategy to enhance the effects of chemotherapy and improve clinical outcomes in the treatment of cancer patients.     

PLCε1: A potential target of RNA interference therapy for gastric cancer

Phospholipase C epsilon 1 (PLCε1) has been recently identified as a novel potential biomarker for gastric cancer because of its critical role in inflammation and tumorigenesis. Until now, there are no further reports to investigate the feasibility of gene therapy by suppressing PLCε1 expression for gastric cancer. In this study, a small interfering RNA (shRNA) targeting PLCε1 was firstly transfected into gastric cancer cells in order to silence PLCε1 expression. Both mRNA and protein expression of PLCε1 in gastric cancer cells significantly reduced by RT-PCR and Western blotting analysis. Moreover, subsequent results revealed that PLCε1 shRNA depressed the in vitro and in vivo growth of gastric cancer cells by using MTT assay and tumor xenograft experiment. Furthermore, after PLCε1 shRNA transfection, the expression of proinflammatory molecules including tumor necrosis factor-α (TNF-α), cyclooxygenase 2 (COX-2), interleukin (IL)-6 and chemokine (C–X–C motif) ligand (CXCL)-1 were unaffected, but only chemokine (C–C motif) ligand (CCL)-2 expression decreased in the gastric cancer cells. It is implied that PLCε1 may inhibit the growth of gastric cancer cells via CCL-2 protein mediated pathway. These results suggest that PLCε1 might be an alternative molecular target for gastric cancer gene therapy.     

AKR1B10: A potential target for cancer therapy

Aldo-keto reductase family 1 B10 (AKR1B10, also designated aldose reductase-like-1, ARL-1) is a novel protein identified from human hepatocellular carcinoma (HCC). This protein belongs to aldo-keto reductase superfamily, a group of proteins implicated in intracellular detoxification, cell carcinogenesis, and cancer therapeutics. AKR1B10 is primarily expressed in the colon and small intestine with low levels in the liver, thymus, prostate, and testis but overexpressed in the liver and lung cancer, making it a potential cancer diagnostic and/or prognostic marker. AKR1B10 could reduce retinals to retinols eliminating intracellular retinoic acid, a signaling molecule regulating cell proliferation and differentiation. AKR1B10 may impact the carcinogenesis process through controlling retinoic acid signaling.     

BRG1 is a prognostic indicator and a potential therapeutic target for prostate cancer

Brahma-related gene 1 (BRG1) is one of two mutually exclusive ATPases that function as the catalytic subunit of human SWItch/Sucrose NonFermentable (SWI/SNF) chromatin remodeling enzymes. BRG1 has been identified as a tumor suppressor in some cancer types but has been shown to be expressed at elevated levels, relative to normal tissue, in other cancers. Using TCGA (The Cancer Genome Atlas) prostate cancer database, we determined that BRG1 mRNA and protein expression is elevated in prostate tumors relative to normal prostate tissue. Only 3 of 491 (0.6%) sequenced tumors showed amplification of the locus or mutation in the protein coding sequence, arguing against the idea that elevated expression due to amplification or expression of a mutant BRG1 protein is associated with prostate cancer. Kaplan-Meier survival curves showed that BRG1 expression in prostate tumors inversely correlated with survival. However, BRG1 expression did not correlate with Gleason score/International Society of Urological Pathology (ISUP) Grade Group, indicating it is an independent predictor of tumor progression/patient outcome. To experimentally assess BRG1 as a possible therapeutic target, we treated prostate cancer cells with a biologic inhibitor called ADAADi (active DNA-dependent ATPase A Domain inhibitor) that targets the activity of the SNF2 family of ATPases in biochemical assays but showed specificity for BRG1 in prior tissue culture experiments. The inhibitor decreased prostate cancer cell proliferation and induced apoptosis. When directly injected into xenografts established by injection of prostate cancer cells in mouse flanks, the inhibitor decreased tumor growth and increased survival. These results indicate the efficacy of pursuing BRG1 as both an indicator of patient outcome and as a therapeutic target.     

The androgen receptor: a potential target for therapy of prostate cancer

The androgen receptor plays a pivotal role in the prostate. Its primary function is to provide responsive gene products for differentiation and growth, but under abnormal conditions it contributes to the development of prostate cancer. The goal of this review is to elucidate the molecular functions of the androgen receptor and its role in prostate cancer. Initially the function of the androgen receptor will be described. Next, the clinical diagnosis, epidemiological impact, and treatments of androgen-dependent and -independent prostate cancer will be discussed. Finally we will examine how the mechanism of androgen action has played a role in the translation of new therapies and how this may influence future treatment modalities of prostate cancer.     

Protein kinase D as a potential new target for cancer therapy

Protein kinase D is a novel family of serine/threonine kinases and diacylglycerol receptors that belongs to the calcium/calmodulin-dependent kinase superfamily. Evidence has established that specific PKD isoforms are dysregulated in several cancer types, and PKD involvement has been documented in a variety of cellular processes important to cancer development, including cell growth, apoptosis, motility, and angiogenesis. In light of this, there has been a recent surge in the development of novel chemical inhibitors of PKD. This review focuses on the potential of PKD as a chemotherapeutic target in cancer treatment and highlights important recent advances in the development of PKD inhibitors.     

ATG4B promotes colorectal cancer growth independent of autophagic flux

Autophagy is reported to suppress tumor proliferation, whereas deficiency of autophagy is associated with tumorigenesis. ATG4B is a deubiquitin-like protease that plays dual roles in the core machinery of autophagy; however, little is known about the role of ATG4B on autophagy and proliferation in tumor cells. In this study, we found that ATG4B knockdown induced autophagic flux and reduced CCND1 expression to inhibit G₁/S phase transition of cell cycle in colorectal cancer cell lines, indicating functional dominance of ATG4B on autophagy inhibition and tumor proliferation in cancer cells. Interestingly, based on the genetic and pharmacological ablation of autophagy, the growth arrest induced by silencing ATG4B was independent of autophagic flux. Moreover, dephosphorylation of MTOR was involved in reduced CCND1 expression and G₁/S phase transition in both cells and xenograft tumors with depletion of ATG4B. Furthermore, ATG4B expression was significantly increased in tumor cells of colorectal cancer patients compared with adjacent normal cells. The elevated expression of ATG4B was highly correlated with CCND1 expression, consistently supporting the notion that ATG4B might contribute to MTOR-CCND1 signaling for G₁/S phase transition in colorectal cancer cells. Thus, we report that ATG4B independently plays a role as a positive regulator on tumor proliferation and a negative regulator on autophagy in colorectal cancer cells. These results suggest that ATG4B is a potential biomarker and drug target for cancer therapy.     

Phospholipase D1 promotes cervical cancer progression by activating the RAS pathway

This study aimed to further investigate the effect of PLD1 on the biological characteristics of human cervical cancer (CC) cell line, CASKI and the potential related molecular mechanism. CRISPR/Cas9 genome editing technology was used to knock out the PLD1 gene in CASKI cells. Cell function assays were performed to evaluate the effect of PLD1 on the biological function of CASKI cells in vivo and in vitro. A PLD1‐overexpression rescue experiment in these knockout cells was performed to further confirm its function. Two PLD1‐knockout CASKI cell lines (named PC‐11 and PC‐40, which carried the ins1/del4 mutation and del1/del2/ins1 mutation, respectively), were constructed by CRISPR/Cas9. PLD1 was overexpressed in these knockout cells (named PC11‐PLD1 and PC40‐PLD1 cells), which rescued the expression of PLD1 by approximately 71.33% and 74.54%, respectively. In vivo, the cell function assay results revealed that compared with wild‐type (WT)‐CASKI cells, the ability of PC‐11 and PC‐40 cells to proliferate, invade and migrate was significantly inhibited. The expression of H‐Ras and phosphorylation of Erk1/2 (p‐Erk1/2) was decreased in PC‐11 and PC‐40 cells compared with WT‐CASKI cells. PC‐11 and PC‐40 cells could sensitize CASKI cells to cisplatin. More importantly, the proliferation, migration and invasion of PC11‐PLD1 and PC40‐PLD1 cells with PLD1 overexpression were significantly improved compared with those of the two types of PLD1 knockout cells. The sensitivity to cisplatin was decreased in PC11‐PLD1 and PC40‐PLD1 cells compared with PC‐11 and PC‐40 cells. In vivo, in the PC‐11 and PC‐40 tumour groups, tumour growth was significantly inhibited and tumour weight (0.95 ± 0.27 g and 0.66 ± 0.43 g vs. 1.59 ± 0.67 g, p = 0.0313 and 0.0108) and volume (1069.41 ± 393.84 and 1077.72 mm³ ± 815.07 vs. 2142.94 ± 577.37 mm³, p = 0.0153 and 0.0128) were significantly reduced compared to those in the WT‐CASKI group. Tumour differentiation of the PC‐11 and PC40 cells was significantly better than that of the WT‐CASKI cells. The immunohistochemistry results confirmed that the expression of H‐Ras and p‐Erk1/2 was decreased in PC‐11 and PC‐40 tumour tissues compared with WT‐CASKI tumour tissues. PLD1 promotes CC progression by activating the RAS pathway. Inhibition of PLD1 may serve as an attractive therapeutic modality for CC.     

Selective inhibitors for JNK signalling: a potential targeted therapy in cancer

Abstract

c-Jun N-terminal kinase (JNK) signalling regulates both cancer cell apoptosis and survival. Emerging evidence show that JNK promoted tumour progression is involved in various cancers, that include human pancreatic-, lung-, and breast cancer. The pro-survival JNK oncoprotein functions in a cell context- and cell type-specific manner to affect signal pathways that modulate tumour initiation, proliferation, and migration. JNK is therefore considered a potential oncogenic target for cancer therapy. Currently, designing effective and specific JNK inhibitors is an active area in the cancer treatment. Some ATP-competitive inhibitors of JNK, such as SP600125 and AS601245, are widely used in vitro; however, this type of inhibitor lacks specificity as they indiscriminately inhibit phosphorylation of all JNK substrates. Moreover, JNK has at least three isoforms with different functions in cancer development and identifying specific selective inhibitors is crucial for the development of targeted therapy in cancer. Some selective inhibitors of JNK are identified; however, their clinical studies in cancer are relatively less conducted. In this review, we first summarised the function of JNK signalling in cancer progression; there is a focus on the discussion of the novel selective JNK inhibitors as potential targeting therapy in cancer. Finally, we have offered a future perspective of the selective JNK inhibitors in the context of cancer therapies. We hope this review will help to further understand the role of JNK in cancer progression and provide insight into the design of novel selective JNK inhibitors in cancer treatment.     

ATG4B promotes colorectal cancer growth independent of autophagic flux

Autophagy is reported to suppress tumor proliferation, whereas deficiency of autophagy is associated with tumorigenesis. ATG4B is a deubiquitin-like protease that plays dual roles in the core machinery of autophagy; however, little is known about the role of ATG4B on autophagy and proliferation in tumor cells. In this study, we found that ATG4B knockdown induced autophagic flux and reduced CCND1 expression to inhibit G₁/S phase transition of cell cycle in colorectal cancer cell lines, indicating functional dominance of ATG4B on autophagy inhibition and tumor proliferation in cancer cells. Interestingly, based on the genetic and pharmacological ablation of autophagy, the growth arrest induced by silencing ATG4B was independent of autophagic flux. Moreover, dephosphorylation of MTOR was involved in reduced CCND1 expression and G₁/S phase transition in both cells and xenograft tumors with depletion of ATG4B. Furthermore, ATG4B expression was significantly increased in tumor cells of colorectal cancer patients compared with adjacent normal cells. The elevated expression of ATG4B was highly correlated with CCND1 expression, consistently supporting the notion that ATG4B might contribute to MTOR-CCND1 signaling for G₁/S phase transition in colorectal cancer cells. Thus, we report that ATG4B independently plays a role as a positive regulator on tumor proliferation and a negative regulator on autophagy in colorectal cancer cells. These results suggest that ATG4B is a potential biomarker and drug target for cancer therapy.     

AURKA is a potential target for multiple myeloma therapy

Multiple myeloma (MM) is an incurable disease with the second most frequent hematopoietic malignancy. In this study, we found that the expression of Aurora kinase A (AURKA) was significantly increased in patients with high-risk multiple myeloma, especially in proliferation subgroups. MLN8237, a small molecule AURKA inhibitor, inhibited MM cell proliferation by inducing cell apoptosis and injury. Thus, we speculate MLN8237 is a potential therapeutic agent for MM and AURKA may be a potential target for MM treatment.     

A novel role for a glycolytic pathway kinase in regulating autophagy has implications in cancer therapy

When it comes to cancer initiation and progression, macroautophagy/autophagy seemingly acts in a contradictory fashion, serving either as a suppressive factor that functions to protect against tumor formation or as a support mechanism that sustains the disease itself through its cytoprotective functions. In tumor suppression, autophagy assists by restricting oxidative stress and curbing genomic instability that could possibly cause oncogenic mutations. However, in certain circumstances, autophagy can also promote cancer by providing nourishment and by limiting stress-response pathways, leading to cancer cell survival and rapid proliferation. Thus, autophagy's role in oncogenesis is highly context-dependent and varies from one cancer type to another. As a consequence, identifying the mechanisms that alter and rewire autophagic regulation and flux is extremely crucial to target autophagy as a possible avenue for anticancer treatment. In a recent study, Qian et al. endeavored to identify one such key regulatory pathway in hypoxia- and glutamine deprivation-induced autophagy in tumorigenic cells. In this pathway, phosphatidylinositol 3-phosphate (PtdIns3P) production by the class III phosphatidylinositol 3-kinase (PtdIns3K) complex is greatly improved through a cascade of posttranslational modifications that culminates in the phosphorylation of the scaffolding protein BECN1 by the glycolytic pathway kinase PGK1.     

Glycolysis in cancer: a potential target for therapy

Clinical imaging of primary and metastatic cancers with Fluoro deoxy-d-Glucose Positron Emission Tomography (FdG PET) has clearly demonstrated that increased glucose flux compared to normal tissue is a common trait of human malignancies (Gambhir, 2002) This is a consequence of a shift of glucose metabolism to less efficient glycolytic pathways in response to regional hypoxia and evolution of aerobic glycolysis in many cancer phenotypes. This distinctive metabolic profile presents an inviting target for cancer treatment and prevention. Here, we summarize the therapeutic strategies under investigation to exploit or interrupt tumor glycolytic metabolism. Although a number of approaches are under investigation, none has been sufficiently successful to warrant widespread clinical application. We point out that the environmental heterogeneity and evolutionary capacity of tumor cells that likely led to development of upregulated glycolysis could also promote adaptive strategies that confer resistance to therapies designed to inhibit glucose metabolism.     

Sulforaphane enhances proteasomal and autophagic activities in mice and is a potential therapeutic reagent for Huntington's disease

The ubiquitin proteasome system (UPS) is impaired in Huntington's disease, a devastating neurodegenerative disorder. Sulforaphane, a naturally occurring compound, has been shown to stimulate UPS activity in cell cultures. To test whether sulforaphane enhances UPS function in vivo, we treated UPS function reporter mice ubiquitously expressing the green fluorescence protein (GFP) fused to a constitutive degradation signal that promotes its rapid degradation in the conditions of a healthy UPS. The modified GFP is termed GFP UPS reporter (GFPu). We found that both GFPu and ubiquitinated protein levels were significantly reduced and the three peptidase activities of the proteasome were increased in the brain and peripheral tissues of the mice. Interestingly, sulforaphane treatment also enhanced autophagy activity in the brain and the liver. To further examine whether sulforaphane promotes mutant huntingtin (mHtt) degradation, we treated Huntington's disease cells with sulforaphane and found that sulforaphane not only enhanced mHtt degradation but also reduced mHtt cytotoxicity. Sulforaphane‐mediated mHtt degradation was mainly through the UPS pathway as the presence of a proteasome inhibitor abolished this effect. Taken together, these data indicate that sulforaphane activates protein degradation machineries in both the brain and peripheral tissues and may be a therapeutic reagent for Huntington's disease and other intractable disorders.

     

Dismantling the autophagic arsenal when it is time to die

Under stress conditions cells activate different response pathways which result in cell survival or apoptosis depending on: (1) the nature of the insults, (2) the type, if acute or chronic stress, and (3) how long the stress persists. Generally, autophagy is induced early to sustain cell survival and inhibit cell death. However, adverse conditions are able to overcome autophagy to promote cell death. Increasing evidence suggests that the inhibition of autophagy by the apoptotic machinery has been proposed as one of the crucial events responsible for the irreversible switch from survival to death. The mechanism seems to be related to the selective apoptotic protease-mediated degradation of key autophagic proteins. We recently found that AMBRA1, an important regulator of the autophagic process mediating the initial steps of autophagosome formation, is also irreversibly degraded by the combined activity of caspases and calpains. This phenomenon is not merely a consequence of apoptosis execution but represents a key step required to efficiently promote the autophagic vs apoptosis switch.