Dual inhibition of BDNF/TrkB and autophagy: a promising therapeutic approach for colorectal cancer

Colorectal cancer (CRC) is the most common digestive cancer in the Western world. Despite effective therapies, resistance and/or recurrence frequently occur. The present study investigated the impact of two survival pathways—neurotrophic factors (TrkB/BDNF) and autophagy—on cell fate and tumour evolution. In vitro studies were performed on two CRC cell lines, SW480 (primary tumour) and SW620 (lymph node invasion), which were also used for subcutaneous xenografts on a nude mouse model. In addition, the presence of neurotrophic factors (NTs) and autophagy markers were assessed in tissue samples representative of different stages. On the basis of our previous study (which demonstrated that TrkB overexpression is associated with prosurvival signaling in CRC cells), we pharmacologically inhibited NTs pathways with K252a. As expected, an inactivation of the PI3K/AKT pathway was observed and CRC cells initiated autophagy. Conversely, blocking the autophagic flux with chloroquine or with ATG5‐siRNA overactivated TrkB/BDNF signaling. In vitro, dual inhibition improved the effectiveness of single treatment by significantly reducing metabolic activity and enhancing apoptotic cell death. These findings were accentuated in vivo, in which dual inhibition induced a spectacular reduction in tumour volume following long‐term treatment (21 days for K252a and 12 days for CQ). Finally, significant amounts of phospho‐TrkB and LC3II were found in the patients’ tissues, highlighting their relevance in CRC tumour biology. Taken together, our results show that targeting NTs and autophagy pathways potentially constitutes a new therapeutic approach for CRC.     

New frontiers in the treatment of colorectal cancer: Autophagy and the unfolded protein response as promising targets

Colorectal cancer (CRC), despite numerous therapeutic and screening attempts, still remains a major life-threatening malignancy. CRC etiology entails both genetic and environmental factors. Macroautophagy/autophagy and the unfolded protein response (UPR) are fundamental mechanisms involved in the regulation of cellular responses to environmental and genetic stresses. Both pathways are interconnected and regulate cellular responses to apoptotic stimuli. In this review, we address the epidemiology and risk factors of CRC, including genetic mutations leading to the occurrence of the disease. Next, we discuss mutations of genes related to autophagy and the UPR in CRC. Then, we discuss how autophagy and the UPR are involved in the regulation of CRC and how they associate with obesity and inflammatory responses in CRC. Finally, we provide perspectives for the modulation of autophagy and the UPR as new therapeutic options for CRC treatment.     

Autophagy is a therapeutic target in anticancer drug resistance

Autophagy is a type of cellular catabolic degradation response to nutrient starvation or metabolic stress. The main function of autophagy is to maintain intracellular metabolic homeostasis through degradation of unfolded or aggregated proteins and organelles. Although autophagic regulation is a complicated process, solid evidence demonstrates that the PI3K-Akt-mTOR, LKB1-AMPK-mTOR and p53 are the main upstream regulators of the autophagic pathway. Currently, there is a bulk of data indicating the important function of autophagy in cancer. It is noteworthy that autophagy facilitates the cancer cells' resistance to chemotherapy and radiation treatment. The abrogation of autophagy potentiates the re-sensitization of therapeutic resistant cancer cells to the anticancer treatment via autophagy inhibitors, such as 3-MA, CQ and BA, or knockdown of the autophagy related molecules. In this review, we summarize the accumulation of evidence for autophagy's involvement in mediating resistance of cancer cells to anticancer therapy and suggest that autophagy might be a potential therapeutic target in anticancer drug resistance in the future.     

EphA2: A promising therapeutic target in breast cancer

Ephrin type-A receptor 2(EphA2), a receptor tyrosine kinase, is overexpressed in human breast cancers often linked to poor patient prognosis. Accumulating evidence demonstrates that EphA2 plays important roles in several critical processes associated with malignant breast progression, such as proliferation,survival, migration, invasion, drug resistance, metastasis, and angiogenesis. As its inhibition through multiple approaches can inhibit the growth of breast cancer and restore drug sensitivity, EphA2 has become a promising therapeutic target for breast cancer treatment. Here, we summarize the expression,functions, mechanisms of action, and regulation of EphA2 in breast cancer. We also list the potential therapeutic strategies targeting EphA2. Furthermore, we discuss the future directions of studying EphA2 in breast cancer.     

Phospholipase D-mediated autophagic regulation is a potential target for cancer therapy

Autophagy is a catabolic process in which cell components are degraded to maintain cellular homeostasis by nutrient limitations. Defects of autophagy are involved in numerous diseases, including cancer. Here, we demonstrate a new role of phospholipase D (PLD) as a regulator of autophagy. PLD inhibition enhances autophagic flux via ATG1 (ULK1), ATG5 and ATG7, which are essential autophagy gene products critical for autophagosome formation. Moreover, PLD suppresses autophagy by differentially modulating phosphorylation of ULK1 mediated by mTOR and adenosine monophosphate-activated protein kinase (AMPK), and by suppressing the interaction of Beclin 1 with vacuolar-sorting protein 34 (Vps34), indicating that PLD coordinates major players of the autophagic pathway, AMPK-mTOR-ULK1 and Vps34/Beclin 1. Ultimately, PLD inhibition significantly sensitized in vitro and in vivo cancer regression via genetic and pharmacological inhibition of autophagy, providing rationale for a new therapeutic approach to enhancing the anticancer efficacy of PLD inhibition. Collectively, we show a novel role for PLD in the molecular machinery regulating autophagy.     

Ovarian cancer stem cell: A potential therapeutic target for overcoming multidrug resistance

The cancer stem cell (CSC) model encompasses an advantageous paradigm that in recent decades provides a better elucidation for many important biological aspects of cancer initiation, progression, metastasis, and, more important, development of multidrug resistance (MDR). Such several other hematological malignancies and solid tumors and the identification and isolation of ovarian cancer stem cells (OV-CSCs) show that ovarian cancer also follows this hierarchical model. Gaining a better insight into CSC-mediated resistance holds promise for improving current ovarian cancer therapies and prolonging the survival of recurrent ovarian cancer patients in the future. Therefore, in this review, we will discuss some important mechanisms by which CSCs can escape chemotherapy, and then review the recent and growing body of evidence that supports the contribution of CSCs to MDR in ovarian cancer.     

Epidermal growth factor receptor: a promising therapeutic target for colorectal cancer

The epidermal growth factor receptor is a 170,000-kd transmembrane glycoprotein involved in signaling pathways affecting cellular growth, differentiation, and proliferation. An abnormal expression of the epidermal growth factor receptor (EGFR) has been described in many human tumors and implicated in the development and prognosis of malignancies, thus representing not only a possible prognostic marker, but primarily a rational molecular target for a new class of anticancer agents. The aim of this analysis is to review the available data about the biology of the EGFR and its use as a target for a new class of anticancer agents for colorectal cancer. Several clinical trials have been reported with the use of EGFR-targeted monoclonal antibodies and tyrosine kinase inhibitors, mainly in combination with chemotherapy for advanced colorectal cancer patients. Results available so far demonstrated a manageable and acceptable toxicity profile and a promising level of activity. Many critical issues are yet unresolved, such as the optimal chemotherapy regimen to combine with anti-EGFR treatment and the most adequate patient setting. Moreover, the biological selection of colorectal tumors more likely to benefit from this treatment approach is still to be defined.     

XIAP: Apoptotic brake and promising therapeutic target

The X-linked Inhibitor of Apoptosis, XIAP, is a key member of the newly discovered family of intrinsic inhibitors of apoptosis (IAP) proteins. IAPs block cell death both in vitro and in vivo by virtue of inhibition of distinct caspases. Although other proteins have been identified which inhibit upstream caspases, only the IAPs have been demonstrated to be endogenous repressors of the terminal caspase cascade. In turn, the caspase inhibiting activity of XIAP is negatively regulated by at least two XIAP-interacting proteins, XAF1 and Smac/DIABLO. In addition to the inhibition of caspases, recent discoveries from several laboratories suggest that XIAP is also involved in a number of other biologically significant cellular activities including modulation of receptor-mediated signal transduction and protein ubiquitination. XIAP is also translated by a rare cap-independent mechanism mediated by a specific sequence called IRES (for Internal Ribosome Entry Site) which is found in the XIAP 5 UTR. XIAP protein is thus synthesized under various conditions of cellular stress such as serum starvation and low dose -irradiation induced apoptosis, conditions that lead to the inhibition of cellular protein synthesis. The multiple biological activities of XIAP, its unique translational and post-translational control and the centrality of the caspase cascade make the control of XIAP expression an exceptionally promising molecular target for modulating apoptosis. Therapeutic benefits can be derived from both the suppression of inappropriate cell death such as in neurodegenerative disorders and ischemic injury or in the activation of latent cell death pathways such as in autoimmune disease and cancer where apoptosis induction is the desired outcome.     

Progranulin: a promising therapeutic target for rheumatoid arthritis

Progranulin (PGRN) is an autocrine growth factor with multiple functions. This review provides updates about the interplays of PGRN with extracellular matrix proteins, proteolytic enzymes, inflammatory cytokines, and cell surface receptors in cartilage and arthritis, with a special focus on the interaction between PGRN and TNF receptors (TNFR) and its implications in inflammatory arthritis. The paper also highlights Atsttrin, an engineered protein composed of three PGRN fragments that prevents inflammation in several inflammatory arthritis models. Identification of PGRN as a ligand of TNFR and an antagonist of TNFα signaling, together with the discovery of Atsttrin, not only betters our understanding of the pathogenesis of arthritis, but also provides new therapeutic interventions for various TNFα-mediated pathologies and conditions, including rheumatoid arthritis.     

Autophagy,citrullination and cancer

A cell needs to maintain a balance between biosynthesis and degradation of cellular components to maintain homeostasis. There are 2 pathways, the proteasome, which degrades short-lived proteins, and the autophagy/lysosomal pathway, which degrades long-lived proteins and organelles. Both of these pathways are also involved in antigen presentation or the effective delivery of peptides to MHC molecules for presentation to T cells. Autophagy (macroautophagy) is a key player in providing substantial sources of citrullinated peptides for loading onto MHC-II molecules to stimulate CD4⁺ T cell responses. Stressful conditions in the tumor microenvironment induce autophagy in cancer cells as a mechanism to promote their survival. We therefore investigated if citrullinated peptides could stimulate CD4⁺ T cell responses that would recognize these modifications produced during autophagy within tumor cells. Focusing on the intermediate filament protein VIM (vimentin), we generated citrullinated VIM peptides for immunization experiments in mice. Immunization with these peptides induced CD4⁺ T cells in response to autophagic tumor targets. Remarkably, a single immunization with modified peptide, up to 14 d after tumor implant, resulted in long-term survival in 60% to 90% of animals with no associated toxicity. These results show how CD4⁺ cells can mediate potent antitumor responses against modified self-epitopes presented on tumor cells, and they illustrate for the first time how the citrullinated peptides produced during autophagy may offer especially attractive vaccine targets for cancer therapy.     

O6-methylguanine DNA methyltransferase as a promising target for the treatment of temozolomide-resistant gliomas

Temozolomide (TMZ) is an alkylating agent currently used as first-line therapy for gliomas treatment due to its DNA-damaging effect. However, drug resistance occurs, preventing multi-cycle use of this chemotherapeutic agent. One of the major mechanisms of cancer drug resistance is enhanced activity of a DNA repair enzyme, O⁶-methylguanine-DNA-methyltransferase (MGMT), which counteracts chemotherapy-induced DNA alkylation and is a key component of chemoresistance. MGMT repairs TMZ-induced DNA lesions, O⁶-meG, by transferring the alkyl group from guanine to a cysteine residue. This review provides an overview of recent advances in the field, with particular emphasis on the inhibitors of MGMT and underlying mechanisms. Literature search was performed through PubMed and all relevant articles were reviewed, with particular attention to MGMT, its role in TMZ-resistant gliomas, effects of MGMT inhibitors and the underlying mechanisms. Several strategies are currently being pursued to improve the therapeutic efficacy of TMZ via inhibition of MGMT to reduce chemoresistance and improve overall survival. MGMT may be a promising target for the treatment of TMZ-resistant gliomas.     

Prolactin and endocrine therapy resistance in breast cancer: The next potential hope for breast cancer treatment

Breast cancer, a hormone-dependent tumour, generally includes four molecular subtypes (luminal A, luminal B, HER2 enriched and triple-negative) based on oestrogen receptor, progesterone receptor and human epidermal growth factor receptor-2. Multiple hormones in the body regulate the development of breast cancer. Endocrine therapy is one of the primary treatments for hormone-receptor-positive breast cancer, but endocrine resistance is the primary clinical cause of treatment failure. Prolactin (PRL) is a protein hormone secreted by the pituitary gland, mainly promoting mammary gland growth, stimulating and maintaining lactation. Previous studies suggest that high PRL levels can increase the risk of invasive breast cancer in women. The expression levels of PRL and PRLR in breast cancer cells and breast cancer tissues are elevated in most ER⁺ and ER⁻ tumours. PRL activates downstream signalling pathways and affects endocrine therapy resistance by combining with prolactin receptor (PRLR). In this review, we illustrated and summarized the correlations between endocrine therapy resistance in breast cancer and PRL, as well as the pathophysiological mechanisms and clinical practices. The study on PRL and its receptor would help explore reversing endocrine therapy-resistance for breast cancer.     

Autophagy

Multidisciplinary approaches are increasingly being used to elucidate the role of autophagy in health and disease and to harness it for therapeutic purposes. The broad range of topics included in the program of the Vancouver Autophagy Symposium (VAS) 2013 illustrated this multidisciplinarity: structural biology of Atg proteins, mechanisms of selective autophagy, in silico drug design targeting ATG proteins, strategies for drug screening, autophagy-metabolism interplay, and therapeutic approaches to modulate autophagy. VAS 2013 took place at the British Columbia Cancer Research Centre, and was hosted by the CIHR Team in Investigating Autophagy Proteins as Molecular Targets for Cancer Treatment. The program was designed as a day of research exchanges, featuring two invited keynote speakers, internationally recognized for their groundbreaking contributions in autophagy, Dr Ana Maria Cuervo (Albert Einstein College of Medicine, Bronx, NY) and Dr Jayanta Debnath (University of California, San Francisco). By bringing together international and local experts in cell biology, drug discovery, and clinical translation, the symposium facilitated rich interdisciplinary discussions focused on multiple forms of autophagy and their regulation and modulation in the context of cancer.     

Autophagy promotes resistance to photodynamic therapy-induced apoptosis selectively in colorectal cancer stem-like cells

Recent studies have indicated that cancer stem-like cells (CSCs) exhibit a high resistance to current therapeutic strategies, including photodynamic therapy (PDT), leading to the recurrence and progression of colorectal cancer (CRC). In cancer, autophagy acts as both a tumor suppressor and a tumor promoter. However, the role of autophagy in the resistance of CSCs to PDT has not been reported. In this study, CSCs were isolated from colorectal cancer cells using PROM1/CD133 (prominin 1) expression, which is a surface marker commonly found on stem cells of various tissues. We demonstrated that PpIX-mediated PDT induced the formation of autophagosomes in PROM1/CD133⁺ cells, accompanied by the upregulation of autophagy-related proteins ATG3, ATG5, ATG7, and ATG12. The inhibition of PDT-induced autophagy by pharmacological inhibitors and silencing of the ATG5 gene substantially triggered apoptosis of PROM1/CD133⁺ cells and decreased the ability of colonosphere formation in vitro and tumorigenicity in vivo. In conclusion, our results revealed a protective role played by autophagy against PDT in CSCs and indicated that targeting autophagy could be used to elevate the PDT sensitivity of CSCs. These findings would aid in the development of novel therapeutic approaches for CSC treatment.     

Autophagy in cancer biology and therapy

The role of macroautophagy （hereafter autophagy） in cancer biology and response to clinical intervention is complex. It is clear that autophagy is dysregulated in a wide variety of tumor settings, both during tumor initiation and progression, and in response to therapy. However, the pleiotropic mechanistic roles of autophagy in controlling cell behavior make it difficult to predict in a given tumor setting what the role of autophagy, and, by extension, the therapeutic outcome of targeting autophagy, might be. In this review we summarize the evidence in the literature supporting pro- and anti-tumorigenic and -therapeutic roles of autophagy in cancer. This overview encompasses roles of autophagy in nutrient management, cell death, cell senescence, regulation of proteotoxic stress and cellular homeostasis, regulation of tumor-host interactions and participation in changes in metabolism. We also try to understand, where possible, the mechanistic bases of these roles for autophagy. We specifically expand on the emerging role of genetically- engineered mouse models of cancer in shedding light on these issues in vivo. We also consider how any or all of the above functions of autophagy proteins might be targetable by extant or future classes of pharmacologic agents. We conclude by briefly exploring non-canonical roles for subsets of the key autophagy proteins in cellular processes, and how these might impact upon cancer.     

Human colonic fibroblasts regulate stemness and chemotherapy resistance of colon cancer stem cells

There is increasing evidence that cancers are heterogeneous and contain a hierarchical organization consisting of cancer stem cells and their differentiated cell progeny. These cancer stem cells are at the core of the tumor as they represent the clonogenic cells within a tumor. Moreover, these cells are considered to contain selective therapy resistance, which suggests a pivotal role in therapy resistance and tumor relapse. Here we show that differentiated cells can re-acquire stemness through factors secreted from fibroblasts. This induced CSC state also coincides with re-acquisition of resistance to chemotherapy. Resistance induced in newly formed CSCs is mediated by the anti-apoptotic molecule BCLXL and inhibition of BCLXL with the BH3 mimetic ABT-737 sensitizes these cancer cells toward chemotherapy. These data point to an important interplay between tumor cells and their microenvironment in the regulation of stemness and therapy resistance.