Proteins Regulating Cell Survival and Apoptosis
Programmed cell death (PGD) can be divided into three types, including apoptosis, necrosis and autophagy. The existence of these self-destruction mechanisms is very important for maintaining normal physiological activities of the organism, and can also monitor and prevent tumors and other diseases caused by abnormal cell proliferation. The occurrence of PGD is generally strictly regulated. A large number of proteins participate in related signaling pathways, and a complex regulatory network is constructed to ensure that cells can respond to the received signals in time and start the correct enzyme cascade.
Apoptosis
Apoptosis can eliminate damaged or infected cells that may interfere with normal function and maintain the stability of the internal environment. Its initiation process can be divided into internal and external pathways. The external pathway is mediated by a subset of the tumor necrosis factor receptor (TNFR) superfamily, including TNFR, Fas and TRAIL. Activation of these death receptors leads to recruitment and activation of caspase. Active caspase 3 is responsible for the cleavage of many so-called death substrates. Subsequent reactions triggered by these substrates will cause apoptotic cells to exhibit signature changes, including DNA fragmentation, nuclear fragmentation, membrane blistering, and other morphologies or physiological changes.
The cellular autonomic or intrinsic pathway is mainly regulated by mitochondria. The most classic form is that the stress-mediated release of mitochondrial cytochrome C leads to the formation of apoptosome. The apoptosome then activates initiator caspase, typically caspase 9, which leads to the activation of the executioner caspase 3. The subsequent result is the same apoptotic response as the external pathway. In the internal pathway, members of the Bcl-2 protein family (Bax and Bak) act on mitochondria in response to activation of apoptotic stimuli, releasing a series of apoptosis-regulating proteins including cytochromes, serine protease Omi / HtrA2, endonuclease G (EndoG) and apoptosis-inducing factor (AIF).
It should also be noted that activation of the mitochondrial pathway can also occur after activation of the exogenous pathway, which occurs through the caspase 8 cleavage of the pro-apoptotic Bcl-2 member - Bid to its activated tBid form. This phenomenon occurs between two distinctly different apoptotic pathways, and their mutual interference further illustrates the complex interrelationship of cellular PGD regulation, which is now commonly observed in different processes regulating cell death.
Figure 1. Regulation of cell survival and apoptosis. (Portt, 2011)
Regulatory Proteins in Induction of Apoptosis
Apoptosis of cells is generally caused by a variety of different stresses. These internal and external stimuli activate different signaling molecules. In general, Bax, ROS and ceramide are important mediators of cell death. There is considerable literature on the regulatory mechanisms of these molecules. For example, the source of stress-mediated increase in ROS may be mitochondria or NADPH oxygenase. Similarly, the stress-mediated activation of sphingomyelinase (SMase), which produces ceramide by sphingomyelin during stress, maybe the main source of ceramide increase during stress. Ceramide, ROS, and active Bax appear to be independent of each other, as an increase in any of these is sufficient to trigger cell death. However, they are also closely linked such that an increase in one of the three results in an increase in the level of the other or an increase in their function.
In some gene therapy projects based on apoptosis, the function of apoptosis-controlling proteins can be more easily determined by identifying gene sequences that regulate ROS, active Bax, or ceramide levels. Some genes encoding ROS scavengers (such as peroxidase, superoxide dismutase, and glutathione peroxidase), genes using ceramide encoded by sphingomyelin synthase 1 (SMS1), or Bax gene have been identified, and their overexpression can prevent stress-induced apoptosis in cells. Interestingly, increased levels of activated Bax, ROS, or ceramide can all activate apoptosis, and blocking any of them alone is sufficient to prevent apoptosis.
Negative Regulatory Proteins that Help Cells Continue to Survive
Healthy cells generally have very well-developed environmental monitoring mechanisms to make continuous decisions about whether they should survive. When facing potential apoptotic signals, cells should always avoid triggering premature or unwanted apoptosis, and if the signal strength is high, make sure to initiate apoptosis. This is achieved by balancing pro-apoptotic and anti-apoptotic mechanisms. The definition of anti-apoptotic gene is that the decrease or increase of its expression level will lead to the change of the sensitivity of cells when they are treated with different doses of apoptosis-inducing agents. Genes are redundant in different anti-apoptotic processes. Many genes can prevent apoptosis when overexpressed, and their loss will not enhance the apoptotic response.
In most studies of the caspase-dependent apoptotic pathway, three distinct families of anti-apoptotic proteins have been described, including FLICE inhibitory protein (FLIP), Bcl-2, and apoptosis inhibitory protein (IAP).
- The protein family containing the Bcl-2 homogeneous domain (BH) is very important in regulating apoptosis. This family contains both negative and positive regulators of apoptosis, of which Bax is the most studied pro-apoptotic member and Bcl-2 is the most studied anti-apoptotic member.
- FLIP represents the most commonly examined exogenous pathway inhibitor. It contains the same death effector domain (DED) as caspase 8, so it can compete with caspase 8 and prevent its activation by interfering with caspase 8 recruitment to activating receptors.
- IAPs bind and inhibit promoters and effector caspases, so they can regulate both forms of apoptosis.
The mechanisms responsible for controlling cell death in response to external stimuli and initiating apoptotic pathways have complex regulatory networks. Although our understanding of anti-apoptosis has improved tremendously in the past few years, there is still much to be clarified. Some recent studies have made us realize that our ability to manipulate endogenous anti-apoptotic processes can be applied to functional therapies targeting multiple diseases and pathophysiology. This protective self-destruct mechanism has played a role in many cancer therapies.
Reference
- Portt, L.; et al. (2011). Anti-apoptosis and cell survival: a review. Biochimica et Biophysica Acta (BBA)-Molecular Cell Research. 1813(1): 238-259.
