Normal physiological function
BCL-2 is localized to the outer membrane of mitochondria, where it plays an important role in promoting cellular survival and inhibiting the actions of pro-apoptotic proteins. The pro-apoptotic proteins in the BCL-2 family, including Bax and Bak, normally act on the mitochondrial membrane to promote permeabilization and release of cytochrome c and ROS, that are important signals in the apoptosis cascade. These pro-apoptotic proteins are in turn activated by BH3-only proteins, and are inhibited by the function of BCL-2 and its relative BCL-Xl.[10]
There are additional non-canonical roles of BCL-2 that are being explored. BCL-2 is known to regulate mitochondrial dynamics, and is involved in the regulation of mitochondrial fusion and fission. Additionally, in pancreatic beta-cells, BCL-2 and BCL-Xl are known to be involved in controlling metabolic activity and insulin secretion, with inhibition of BCL-2/Xl showing increasing metabolic activity,[11] but also additional ROS production; this suggests it has a protective metabolic effect in conditions of high demand.[12]
Role in disease
Damage to the Bcl-2 gene has been identified as a cause of a number of cancers, including melanoma, breast, prostate, chronic lymphocytic leukemia, and lung cancer, and a possible cause of schizophrenia and autoimmunity. It is also a cause of resistance to cancer treatments.[13]
Cancer
Cancer can be seen as a disturbance in the homeostatic balance between cell growth and cell death. Over-expression of anti-apoptotic genes, and under-expression of pro-apoptotic genes, can result in the lack of cell death that is characteristic of cancer. An example can be seen in lymphomas. The over-expression of the anti-apoptotic Bcl-2 protein in lymphocytes alone does not cause cancer. But simultaneous over-expression of Bcl-2 and the proto-oncogene myc may produce aggressive B-cell malignancies including lymphoma.[14] In follicular lymphoma, a chromosomal translocation commonly occurs between the fourteenth and the eighteenth chromosomes – t(14;18) – which places the Bcl-2 gene from chromosome 18 next to the immunoglobulin heavy chain locus on chromosome 14. This fusion gene is deregulated, leading to the transcription of excessively high levels of Bcl-2.[15] This decreases the propensity of these cells for apoptosis. Bcl-2 expression is frequent in small cell lung cancer, accounting for 76% cases in one study.[16]
Auto-immune diseases
Apoptosis plays an active role in regulating the immune system. When it is functional, it can cause immune unresponsiveness to self-antigens via both central and peripheral tolerance. In the case of defective apoptosis, it may contribute to etiological aspects of autoimmune diseases.[17] The autoimmune disease type 1 diabetes can be caused by defective apoptosis, which leads to aberrant T cell AICD and defective peripheral tolerance. Due to the fact that dendritic cells are the immune system's most important antigen-presenting cells, their activity must be tightly regulated by mechanisms such as apoptosis. Researchers have found that mice containing dendritic cells that are Bim -/-, thus unable to induce effective apoptosis, have autoimmune diseases more so than those that have normal dendritic cells.[17] Other studies have shown that dendritic cell lifespan may be partly controlled by a timer dependent on anti-apoptotic Bcl-2.[17]
Other
Apoptosis plays an important role in regulating a variety of diseases. For example, schizophrenia is a psychiatric disorder in which an abnormal ratio of pro- and anti-apoptotic factors may contribute towards pathogenesis.[18] Some evidence suggests that this may result from abnormal expression of Bcl-2 and increased expression of caspase-3.[18]
Diagnostic use
Antibodies to Bcl-2 can be used with immunohistochemistry to identify cells containing the antigen. In healthy tissue, these antibodies react with B-cells in the mantle zone, as well as some T-cells. However, positive cells increase considerably in follicular lymphoma, as well as many other forms of cancer. In some cases, the presence or absence of Bcl-2 staining in biopsies may be significant for the patient's prognosis or likelihood of relapse.[19]
Targeted therapies
Targeted and selective Bcl-2 inhibitors that have been in development or are currently in the clinic include:
Oblimersen
An antisense oligonucleotide drug, oblimersen (G3139), was developed by Genta Incorporated to target Bcl-2. An antisense DNA or RNA strand is non-coding and complementary to the coding strand (which is the template for producing respectively RNA or protein). An antisense drug is a short sequence of RNA that hybridises with and inactivates mRNA, preventing the protein from being formed.
Human lymphoma cell proliferation (with t(14;18) translocation) could be inhibited by antisense RNA targeted at the start codon region of Bcl-2 mRNA. In vitro studies led to the identification of Genasense, which is complementary to the first 6 codons of Bcl-2 mRNA.[20]
These showed successful results in Phase I/II trials for lymphoma. A large Phase III trial was launched in 2004.[21] As of 2016, the drug had not been approved and its developer was out of business.[22]
ABT-737 and navitoclax (ABT-263)
In the mid-2000s, Abbott Laboratories developed a novel inhibitor of Bcl-2, Bcl-xL and Bcl-w, known as ABT-737. This compound is part of a group of BH3 mimetic small molecule inhibitors (SMI) that target these Bcl-2 family proteins, but not A1 or Mcl-1. ABT-737 is superior to previous BCL-2 inhibitors given its higher affinity for Bcl-2, Bcl-xL and Bcl-w. In vitro studies showed that primary cells from patients with B-cell malignancies are sensitive to ABT-737.[23] ABT-737 does not directly induce apoptosis; it enhances the effects of apoptotic signals and causes single-agent-mechanism-based killing of cells in small-cell lung carcinoma and lymphoma lines.[citation needed]
In animal models, it improves survival, causes tumor regression and cures a high percentage of mice.[24] In preclinical studies utilizing patient xenografts, ABT-737 showed efficacy for treating lymphoma and other blood cancers.[25] Because of its unfavorable pharmacologic properties ABT-737 is not appropriate for clinical trials, while its orally bioavailable derivative navitoclax (ABT-263) has similar activity on small cell lung cancer (SCLC) cell lines and has entered clinical trials.[26] While clinical responses with navitoclax were promising, mechanistic dose-limiting thrombocytopenia was observed in patients under treatment due to Bcl-xL inhibition in platelets.[27][28][29]
Venetoclax (ABT-199)
Due to dose-limiting thrombocytopenia of navitoclax as a result of Bcl-xL inhibition, Abbvie successfully developed the highly selective inhibitor venetoclax (ABT-199), which inhibits Bcl-2, but not Bcl-xL or Bcl-w.[30] Clinical trials studied the effects of venetoclax, a BH3-mimetic drug designed to block the function of the Bcl-2 protein, on patients with chronic lymphocytic leukemia (CLL).[31][32] Good responses have been reported and thrombocytopenia was no longer observed.[32][33] A phase 3 trial started in Dec 2015.[34]
It was approved by the US FDA in April 2016 as a second-line treatment for CLL associated with 17-p deletion.[35] This was the first FDA approval of a BCL-2 inhibitor.[35] In June 2018, the FDA broadened the approval for anyone with CLL or small lymphocytic lymphoma, with or without 17p deletion, still as a second-line treatment.[36]
