Apoptosis Bid Research Articles

Canonical and Noncanonical Functions of the BH3 Domain Protein Bid in Apoptosis, Oncogenesis, Cancer Therapeutics, and Aging.

Effective cancer therapy with limited adverse effects is a major challenge in the medical field. This is especially complicated by the development of acquired chemoresistance. Understanding the mechanisms that underlie these processes remains a major effort in cancer research. In this review, we focus on the dual role that Bid protein plays in apoptotic cell death via the mitochondrial pathway, in oncogenesis and in cancer therapeutics. The BH3 domain in Bid and the anti-apoptotic mitochondrial proteins (Bcl-2, Bcl-XL, mitochondrial ATR) it associates with at the outer mitochondrial membrane provides us with a viable target in cancer therapy. We will discuss the roles of Bid, mitochondrial ATR, and other anti-apoptotic proteins in intrinsic apoptosis, exploring how their interaction sustains cellular viability despite the initiation of upstream death signals. The unexpected upregulation of this Bid protein in cancer cells can also be instrumental in explaining the mechanisms behind acquired chemoresistance. The stable protein associations at the mitochondria between tBid and anti-apoptotic mitochondrial ATR play a crucial role in maintaining the viability of cancer cells, suggesting a novel mechanism to induce cancer cell apoptosis by freeing tBid from the ATR associations at mitochondria.

Multiple partners can kiss-and-run: Bax transfers between multiple membranes and permeabilizes those primed by tBid.

During apoptosis Bid and Bax are sufficient for mitochondrial outer membrane permeabilization, releasing pro-apoptotic proteins such as cytochrome c and Smac/Diablo into the cytoplasm. In most cells, both Bid and Bax are cytoplasmic but bind to mitochondrial outer membranes to exert pro-apoptotic functions. Binding to membranes is regulated by cleavage of Bid to truncated Bid (tBid), by conformation changes in tBid and Bax, and by interactions with other proteins. At least at the peripherally bound stage, binding is reversible. Therefore, regulation of apoptosis is closely linked with the interactions of tBid and Bax with mitochondria. Here we use fluorescence techniques and cell-free systems containing mitochondria or liposomes that faithfully mimic tBid/Bax-dependent membrane permeabilization to study the dynamic interactions of the proteins with membranes. We confirm that the binding of both proteins to the membrane is reversible by quantifying the binding affinity of proteins for the membrane. For Bax, both peripherally bound (inactive) and oligomerized (active) proteins migrate between membranes but much slower than and independent of tBid. When re-localized to a new membrane, Bax inserts into and permeabilizes it only if primed by an activator. In the case of tBid, the process of transfer is synergetic with Bax in the sense that tBid ‘runs' faster if it has been ‘kissed' by Bax. Furthermore, Mtch2 accelerates the re-localization of tBid at the mitochondria. In contrast, binding to Bcl-XL dramatically impedes tBid re-localization by lowering the off-rate threefold. Our results suggest that the transfer of activated tBid and Bax to different mitochondria is governed by dynamic equilibria and potentially contributes more than previously anticipated to the dissemination of the permeabilization signal within the cell.

TBid Undergoes Multiple Conformational Changes at the Membrane Required for Bax Activation

Bid is a Bcl-2 family protein that promotes apoptosis by activating Bax and eliciting mitochondrial outer membrane permeabilization (MOMP). Full-length Bid is cleaved in response to apoptotic stimuli into two fragments, p7 and tBid (p15), that are held together by strong hydrophobic interactions until the complex binds to membranes. The detailed mechanism(s) of fragment separation including tBid binding to membranes and release of the p7 fragment to the cytoplasm remain unclear. Using liposomes or isolated mitochondria with fluorescently labeled proteins at physiological concentrations as in vitro models, we report that the two components of the complex quickly separate upon interaction with a membrane. Once tBid binds to the membrane, it undergoes slow structural rearrangements that result in an equilibrium between two major tBid conformations on the membrane. The conformational change of tBid is a prerequisite for interaction with Bax and is, therefore, a novel step that can be modulated to promote or inhibit MOMP. Using automated high-throughput image analysis in cells, we show that down-regulation of Mtch2 causes a significant delay between tBid and Bax relocalization in cells. We propose that by promoting insertion of tBid via a conformational change at the mitochondrial outer membrane, Mtch2 accelerates tBid-mediated Bax activation and MOMP. Thus the interaction of Mtch2 and tBid is a potential target for therapeutic control of Bid initiated cell death.