BH3
‐Only Proteins
Authors: Tuzlak Selma , Villunger Andreas 
Published: November 2016

The deregulation of programmed cell death, apoptosis, is a major contributor to the development of diseases, including cancer or autoimmunity, and can impair the response of transformed cells to therapy. The Bcl‐2 family of proteins includes critical regulators of the intrinsic apoptosis pathway. BH3 (Bcl‐2 homologous 3)‐only proteins are proapoptotic members that share with each other and the wider Bcl‐2 family only the ‘BH3 domain’. This short stretch of about 16 amino acids is necessary for protein–protein interaction that is critical for their apoptosis‐inducing capacity. BH3‐only proteins promote cell death by regulating activation of Bax and Bak, in a cell‐type and stimulus‐specific manner. Studies of gene‐targeted mice that lack two or more BH3‐only proteins have helped to unravel the overlapping functions of these apoptosis initiators. With the dawn of CRISPR/Cas9 gene‐editing technologies, this strategy was taken even further, opening up allegedly answered questions again, at least at the cellular level. BH3 mimetics were recently approved for the treatment of patients suffering from cancer and it can be anticipated that these drugs make second use to treat autoimmunity and ageing‐related disorders in the future.

Key Concepts

The ‘Bcl‐2‐regulated’ (also called ‘intrinsic’ or ‘mitochondrial’) apoptotic pathway is triggered by developmental cues or a broad range of cell stressors (e.g. growth factor deprivation and γ‐irradiation) and is regulated by the interplay of the pro‐ and antiapoptotic members of the Bcl‐2 family of proteins controlling integrity of the outer mitochondrial membrane (OMM).

The survival versus death fate of a cell is decided not only by the relative levels of pro‐ and antiapoptotic Bcl‐2 family members, but also depends on their ‘activation status’, frequently imposed by different posttranslational modifications on these proteins.

Abnormal cell survival during establishment and maintenance of immune‐(self)‐tolerance as well as defects in apoptosis signalling at the end of an immune response may lead to neoplastic transformation and tumourigenesis.

The Bcl‐2 family consists of three subgroups of proteins that can be differentiated based on amino acid sequence, 3D structure and function. Bcl‐2, Bcl‐xL, Bcl‐w, Bcl‐B, Mcl‐1 and A1 are essential for cell survival, with cell‐type‐specific expression, while Bcl‐B is found active only in humans. The two other, proapoptotic, subfamilies encompass the multi‐BH domain Bcl‐2 family members Bax, Bak and Bok as well as the BH3‐only protein subfamily members Bad, Bid, Bik/Nbk/Blk, Hrk/DP5, Bim/Bod/Bcl2L11, Noxa/Pmaip, Bmf and Puma/Bbc3. The BH3‐only proteins are critical activators of the effector phase of the intrinsic death pathway, while Bax and Bak act as executers of apoptosis by initiating mitochondrial outer membrane permeabilisation (MOMP). The role of Bok in this group remains to be clarified in full.

Both Puma and Noxa, apoptosis initiators that can be transcriptionally activated by p53 in response to DNA damage or oncogenic stress, can act as tumour suppressors in their own right, particularly in the context of an oncogenic lesion that subverts cell cycle control. Yet, in most circumstances tested, Puma appears more effective, suggesting roles for Noxa outside the p53 response.

Bid functions as the link between the ‘death receptor’ and the ‘Bcl‐2‐regulated’ apoptotic pathways by causing an amplification of the caspase cascade that leads to cell destruction. Remarkably, Bid is critical for Fas ‘death receptor’‐induced apoptosis in certain cell types, such as hepatocytes, but dispensable in others, including lymphoid cells. Additional roles of Bid, downstream of other proteases, have been suggested.

Bim, the most studied BH3‐only protein, is critical for many physiologic and pathologic cell death processes and is a principal regulator of homeostasis in the lymphoid and myeloid compartment. Bim is crucial for the negative selection of autoreactive immature T‐ and B‐lymphoid cells and for growth factor deprivation‐induced apoptosis of many cell types. Loss of BIM in certain human cancers substantiates its role as a tumour suppressor and ample data supports a key role in anticancer therapy.

Additional BH3‐only proteins such as Bmf, Bad, Hrk and Bik are less well studied and poorly understood. Available data suggests that they support the key effectors Bim, Puma or Bid in potentiating cell killing in a cell‐type and stimulus‐dependent manner. Functions outside canonical apoptosis signalling remain plausible.

BH3‐only proteins have a crucial function in chemotherapeutic drug‐induced killing of tumour cells and their loss is frequently associated with resistance to anticancer therapy. Some proteins are equally important in killing autoreactive immune cells or can become aberrantly activated in chronic inflammatory or degenerative disorders, contributing to diverse pathologies.

Mimicking BH3‐only proteins represents a promising strategy for enhancing the effects of conventional anticancer therapy and for treating autoimmune diseases, whereas the blockade of these proteins may be beneficial in the management of certain degenerative diseases that are characterised by abnormal killing of cells that should be kept alive.