The BECN1 gene encodes the Beclin-1 protein, which is a well-established regulator of the autophagic pathway. It is a mammalian orthologue of the ATG6 gene in yeast and was one of the first identified mammalian autophagy-associated genes. Beclin-1 interacts with a number of binding partners in the cell which can lead to either activation (eg, via PI3KC3/Vps34, Ambra 1, UV radiation resistance-associated gene) or inhibition (eg, via Bcl-2, Rubicon) of the autophagic pathway. Apart from its role as a regulator of autophagy, it is also shown to effect important biological processes in the cell such as apoptosis and embryogenesis. Beclin-1 has also been implicated to play a critical role in the pathology of a variety of disease states including cancer, neurological disorders (eg, Alzheimer's disease, Parkinson's disease) and viral infections. Thus, understanding the functions of Beclin-1 and its interactions with other cellular components will aid in its development as an important therapeutic target for future drug development.
- TUMOUR BIOLOGY
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Structure and localisation of gene and protein
The BECN1 gene is composed of 11 introns and 12 exons and is localised to a 400-kb region on chromosome 17q21,1–7 which has been found to be deleted in a number of cancers.6 The gene translates into the mature 450 amino acid long, 60-kDa Beclin-1 protein.8 The BECN1 cDNA encodes a transcript 2098-bp in length and includes a 120-bp 5′-untranslated region (UTR), a 1353-bp coding region and a 625-bp 3′-UTR.6 It is highly conserved among different species, with orthologues being identified in diverse organisms such as yeasts, nematodes, rodents, insects and amphibians, indicating an important biological function.9 ,10 The Beclin-1 protein has been shown to be ubiquitously expressed in mammalian cells,11 and is mainly localised within the cytoplasm and organelles such as mitochondria, endoplasmic reticulum and trans-golgi network.2 ,8
The protein consists of three structural domains, including: (1) an N-terminal Bcl-2 homology 3 (BH3) domain (amino acids 108–127);12 (2) a coiled–coiled domain (CCD; amino acids 174–266);13 and (3) the C-terminal evolutionary conserved domain (ECD; amino acids 244–337)10 (figure 1). Beclin-1 also contains a poorly conserved leucine zipper, which is responsible for a nuclear export signal required for its cytoplasmic localisation.14 The BH3 domain is known to interact with antiapoptotic Bcl-2 family members.12 The CCD domain interacts and undergoes hetero-dimerisation with another coiled–coiled protein, UV radiation resistance-associated gene (UVRAG).15 The CCD domain is also known to interact with Atg14L/Barkor,13 which leads to activation of autophagy. The ECD is required for the regulation of autophagy by Beclin-1, through its interaction with PI3KC3/Vps34.16
Biological functions of Beclin-1
Role in autophagy
Autophagy is an evolutionary conserved catabolic pathway involved in the lysosome-mediated recycling of damaged organelles and other cellular components.17 It is a programmed cell survival pathway that is activated in response to stress. The autophagic pathway is controlled via well characterised and orchestrated cellular machinery that is initiated by the formation of a crescent shaped structure (known as the phagophore) alongside the damaged cargo (organelle/cellular constituent) in the cytoplasm.17 The phagophore then elongates and encapsulates the damaged cargo to form a double membrane lined vesicle known as the autophagosome.17 The autophagosome fuses with a lysosome to form the autolysosome, where acidic hydrolases from the lysosome catabolise the damaged cargo, leading to the recycling of nutrients back to the cytoplasm for reuse.17
Beclin-1 plays a pivotal role in the autophagic initiation process that involves nucleation of the autophagic vesicle (ie, formation of phagophore).17 To initiate the nucleation of the vesicle, Beclin-1 interacts with its binding partners such as Class III phosphatidylinositol 3-kinase (PI3KC3)/Vps34, Ambra1, Vps15 and Atg14L (figure 1).17 Upon complexation with Beclin-1, PI3KC3/Vps34 is activated to generate its sole product, namely, phosphatidylinositol 3-phosphate, which is responsible for induction of autophagy through a mechanism that is still poorly understood.17 Additionally, Ambra1 is required for the interaction of Beclin-1 with PI3KC3/Vps34.18 Itakura et al have demonstrated that silencing of Atg14L results in suppression of the autophagic flux, indicating a critical role of this binding partner in the activity of the Beclin-1–PI3KC3 complex.19 Beclin-1 has also been shown to interact with other binding partners such as UVRAG, Bif and Rubicon.15 ,20 ,21 Interaction of UVRAG with Beclin-1 leads to increased PI3KC3/Vps34 kinase activity, which results in increased autophagic initiation.15 UVRAG is also involved in the interaction of Bif with Beclin-1, which facilitates the activation of PI3KC3/Vps34.20 On the other hand, Rubicon has also been shown to negatively regulate autophagy.21
Thus, Beclin-1 acts as an important regulator of the pro-survival autophagic pathway via its interaction with a number of binding partners.
Role of Beclin-1 in crosstalk between autophagy and apoptosis
Beclin-1 plays a significant role in the crosstalk between two important pathways involved in cell survival and death, namely, autophagy and apoptosis, respectively (figure 2). Beclin-1 is known to bind with antiapoptotic Bcl-2 and Bcl-XL, which leads to the inhibition of Beclin-1-mediated autophagic initiation.22 Beclin-1 protein has a BH3-only domain which is required for its interaction with the BH3 receptor domain of Bcl-2 or Bcl-XL.12 In contrast, this interaction of Bcl-2 with BH3-only domain of Beclin-1 does not alter the antiapoptotic activity of Bcl-2.23 A mutation in either the BH3-only domain in Beclin-1 or the BH3 receptor domain in Bcl-2 or Bcl-XL leads to loss of interaction between Beclin-1 and Bcl-2/Bcl-XL resulting in stimulation of the autophagic pathway.24
When cells are under stress, c-Jun N-terminal protein kinase 1 phosphorylates Bcl-2 at multiple sites.25 This phosphorylated Bcl-2 molecule fails to interact with BH3 domain of Beclin-1 and this results in activation of Beclin-1-mediated autophagic initiation.25 Another mechanism through which the inhibitory effects of Bcl-2 on Beclin-1 activity can be achieved is through activation of death-associated protein kinase (DAPK).26 DAPK phosphorylates Beclin-1 at Thr 119 in BH3 domain, which leads to dissociation of Beclin-1 from Bcl-2 and thus activates autophagy.26 This inhibitory interaction between Beclin-1 and Bcl-2 can also be reversed via release of Beclin-1 from the complex by pro-apoptotic BH3 proteins (eg, BNIP3, Bad, Bik, Noxa, Puma and BimEL27). The dissociation of the Bcl-2 and Beclin-1 complex removes the inhibitory effect of Bcl-2 on Beclin-1, thereby initiating autophagy.
Recent studies by Wirawan et al demonstrated that caspases can lead to cleavage of Beclin-1 at two sites, that is, TDVD133 and DQLD149, leading to the generation of Beclin-1 fragments that lack the ability to induce autophagy.28 These latter investigators showed that only a C-terminal fragment (Beclin-1-C) was able to sensitise cells towards apoptosis, but not the N-terminal fragment containing the BH3 domain.28 Beclin-1-C was demonstrated to primarily localise on the mitochondrial membrane and was shown to induce cytochrome c release from mitochondria.28 These results indicate the potential mechanism through which Beclin-1-C sensitises cells towards apoptosis. Consequently, the balance between the promotion and inhibition of Beclin-1-mediated autophagic initiation via molecules of apoptotic machinery may depend upon the relative activities of caspases and different BH3 proteins interacting with Beclin-1.
Role in embryogenesis
Autophagy is shown to play an important role in embryonic development, with autophagic activity being detected as early as fertilisation of the oocyte.29 Of interest to this review, in vivo studies demonstrated a critical role of Beclin-1 in embryonic development. For example, Yue et al30 showed that Beclin-1−/− mutant mice die early during embroyonic development. This previous investigation along with others also demonstrated that heterozygous Beclin-1+/− mutant mice showed a high incidence of spontaneous tumour formation.30 ,31 These studies indicate the important function of this gene in physiological development.
Role of Beclin-1 in pathological conditions
Beclin-1 is implicated in the pathophysiology of a number of disorders, such as cancer and Alzheimer's disease (AD). This is mainly attributed to its ability to regulate important cellular processes such as autophagy and apoptosis.
Beclin-1 in cancer
Autophagy has been previously shown to have both pro-cancer and anticancer effects depending on the stage of cancer and its microenvironment.32 Autophagy protects rapidly growing cancer cells from nutrient deprived conditions and, thus, increases the survival of cancer cells leading to oncogenic effects.33 We have recently demonstrated that the metastasis suppressor protein N-myc downstream regulated gene-1 can suppress the stress-induced autophagic pathway, which renders cells more susceptible to apoptosis.34 This latter study suggests a potential role of the autophagic pathway in cancer cell metastasis. On the other hand, autophagy is also considered as a mechanism for cell death,35 which can explain its tumour suppressor effects. The cell death mechanisms attributed to the autophagy pathway can be explained due to excessive autophagic activity in the cell under stress conditions, leading to death.35
The BECN1 gene has been previously shown to be a haplo-insufficient tumour suppressor, as Beclin+/− mice which express wild-type Beclin-1 mRNA and protein were shown to develop spontaneous tumours.30 Beclin-1 has also been demonstrated to be an important prognostic marker for a variety of cancers in clinical settings. In fact, studies have shown that high Beclin-1 levels are associated with good overall survival rate in patients with non-Hodgkin's lymphomas,36 salivary gland37 or colon38 cancers. Similarly, decreased Beclin-1 levels resulted in reduced overall survival rates in patients with breast39 and gastric40 cancers. In contrast, in a study done by Giatromanolaki et al, high Beclin-1 expression led to poor prognosis in patients suffering from endometrial adenocarcinomas.41 A study in colorectal cancer patients observed that patients with excessively high or low Beclin-1 level had a significantly poorer overall survival rate compared with groups of patients with either normal or limited overexpression of Beclin-1.42 These results suggest that a balance is required between two opposing autophagy-mediated mechanisms in order to achieve better prognosis in cancer patients.
Interestingly, a study by Park et al43 showed that high Beclin-1 levels were linked to poor overall survival following adjuvant therapy with 5-fluororuacil in colon cancer patients. These data are consistent with the notion that autophagy protects cancer cells against stress induced by chemotherapeutic agents. Collectively, these studies show conflicting results regarding the role of autophagy and specifically Beclin-1 in cancer progression, and additional studies are required to better understand the fundamental mechanisms affected by this pathway.
Beclin-1 in neurological disorders
Autophagy is known to play an important role in the pathology of a number of neurological disorders. Importantly, it especially affects disorders which involve the formation of intra-cytoplasmic aggregates, such as AD (τ accumulation), Parkinson's disease (PD; formation of Lewy bodies) or Huntington's disease (accumulation of mutant Huntingtin protein).44–46 Considering that these aggregates act as good substrates for the autophagic pathway, autophagy can play a critical role in the progression of these disorders.
Recently, there has been growing evidence involving the role of Beclin-1 in these disorders, potentially via its ability to regulate the autophagic pathway. Studies have shown that there is reduced expression of BECN1 in regions of the brain affected by AD.44 ,47 Pickford et al44 showed that the genetic reduction of BECN1 in a transgenic mouse model of AD (expressing human amyloid precursor protein) resulted in an exacerbation of clinical symptoms of AD, namely, intraneuronal amyloid β (Aβ) accumulation, extracellular Aβ deposition and neurodegeneration. What is more, in the same study, exogenous Beclin-1 reduced the pathological symptoms of AD.44 These studies indicate that increased Beclin-1 levels can play an important role in better prognosis of AD patients, presumably via its ability to increase autophagic processing of pathogenic protein aggregates.
Moreover, Spencer et al46 demonstrated that Beclin-1 has beneficial effects in PD and Lewy body disease (LBD), mainly due to its ability to stimulate autophagy. LBD is an overlap of PD and AD and is characterised by accumulation of synaptic protein α-synuclein (α-syn) into a structure known as the Lewy body.46 Expression of Beclin-1 ameliorated the α-syn-associated neurological symptoms in both in vitro and in vivo assays.46 These positive effects of Beclin-1 on LBD/PD development were blocked by the autophagy inhibitor, Bafilomycin A1, suggesting an important role of autophagy in Beclin-1-mediated suppression of LBD/PD.46
HD is characterised by accumulation of mutant Huntingtin protein in neurons.45 ,48 An investigation by Shibata et al45 demonstrated that mutant Huntingtin protein can recruit Beclin-1 and render it inactive. This can result in impaired autophagic activity leading to an aggravated pathological condition.
Similar cyto-protective effects of Beclin-1 were observed in an inherited ataxia, namely, Machado–Joseph disease, where autophagy induced by Beclin-1 aided the clearance of mutant ataxin-3 aggregates.49 Clearance of these aggregates by Beclin-1-mediated autophagy prevented the neurodegenerative effects of the disease.49 Together, these studies indicate an important role of Beclin-1-mediated autophagy in the pathophysiology of neurodegenerative disorders involving aggregation of misfolded/mutant proteins.
Autophagy is also implicated to play a role in a variety of other diseases such as viral infections, cardiac disorders and lipid storage disorders. Orvedahl et al50 investigated the role of Beclin-1 in encephalitis caused by herpes simplex virus type 1 (HSV-1). These latter investigators demonstrated that neuro-virulence protein, ICP34.5, binds to Beclin-1 and inhibits its ability to induce autophagy.50 They also showed that mutant ICP34.5 protein lacking the ability to bind Beclin-1 was unable to inhibit neuronal autophagy and did not lead to the development of lethal encephalitis in mice.50 This study demonstrates a critical role of Beclin-1-mediated autophagy in neuro-virulence caused by HSV-1.
Some studies have demonstrated a cardioprotective role of Beclin-1-mediated autophagy in ischaemia-reperfusion (I/R) injury.51 ,52 One of these studies showed that cells overexpressing Beclin-1 have enhanced autophagic flux after I/R and decreased activation of the pro-apoptotic Bax protein.52 On the other hand, silencing of Beclin-1 increased I/R-induced Bax activation.52
Beclin-1 is also shown to have an important role in other disorders such as Niemann–Pick C disease53 and cystic fibrosis.54 Taken together, these studies demonstrate a critical role of BECN1 in the pathology of a wide variety of diseases and offer an attractive target for future drug development.
The BECN1 gene is a critical regulator of the autophagic pathway and also plays a vital role in the crosstalk of autophagy with apoptosis. It has been implicated in a number of aggregatory neurological disorders, such as AD and LBD, mainly due to its ability to stimulate the autophagic pathway, which in turn can clear the pathological aggregates in cells known to play an important role in these diseases. Beclin-1 also plays an important role in tumour pathology and has been shown to act as a haplo-insufficient tumour suppressor gene. Collectively, due to the important role of this gene in essential physiological and pathological pathways, it can be investigated as a putative drug target for the development of new therapies.
Take home messages
BECN1 gene encodes Beclin-1 which is an important regulator of the autophagic pathway.
Beclin-1 interacts with a number of binding partners which can either positively (PI3KC3/Vps34, Ambra-1, UV radiation resistance-associated gene, Bif) or negatively (Bcl-2, Bcl-XL, Rubicon) regulate autophagy.
Beclin-1 plays an important role in crosstalk between autophagy and apoptosis, mainly through its BH3-binding domain.
It is a haplo-insufficient tumour suppressor gene.
Beclin-1 plays a crucial role in the pathology of a number of disorders such as Alzheimer's disease, Lewy body disease, encephalitis and lipid storage disorders, mainly due to its ability to regulate autophagy.
PJJ thanks the Cancer Institute NSW and Prostate Cancer Foundation of Australia for an Early Career Fellowship. DRR is the recipient of a National Health and Medical Research Council Senior Principal Research Fellowship and Project Grants.
SS and DRR contributed equally to the work as co-corresponding and senior authors.
Contributors SS and DRR were involved in design of the article. All authors contributed equally to writing of the article.
Competing interests None.
Provenance and peer review Not commissioned; internally peer reviewed.
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