Overview of Stem Cell Apoptosis and Signal Transduction
Apoptosis is a form of programmed cell death which happens in multicellular organisms. Once a cell is induced to apoptosis, several characteristic cell morphological changes follow which include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, chromosomal DNA fragmentation, and global mRNA decay. To some extent, the self-renewal and population of stem cell is controlled by apoptosis. It is apoptosis that keeps stem cell number in balance between the lost due to differentiation or apoptosis and the gained due to the proliferation. Therefore, apoptosis plays an important role in the regulation of cell number and growth and elimination of abnormal and seriously damaged cells. Owing to such significance, the process of apoptosis is accurately regulated. Cell apoptosis signaling can be initiated either at the cell surface through the receptor-induced signaling pathway, or from the cell itself through the release of proapoptotic factors such as cytochrome c from the triggered mitochondria. Otherwise, stressed caused by other organelles, such as the ER (Endoplasmic reticulum), nucleus and lysosomes, is also responsible to initiate the apoptotic pathways. The proteases of the caspase family are the main executioners of the apoptotic pathways which function in a strictly regulated proteolytic cascade resulting in the disintegration of the cell.
Figure 1. Apoptotic pathways.
When Fas-ligand or agonistic antibodies are bound to the homotrimeric Fas receptors, the death domains aggregate and thus the FADD (Fas-associating protein with a novel death domain) is recruited. Then recruitment of pro-caspase-8 to FADD by its DED (Death Effector Domain) domain leads to the formation of a protein complex called DISC(death inducing signaling complex). Other DED-containing proteins including caspase-10, Flice inhibitory protein (FLIP) and Daxx can also be recruited to the DISC. Finally, the Fas receptor-ligand complex is internalized, directing toward an endosomal pathway.
Following this pathway, there will be two different possible scenarios. In the ‘type I’ pathway, active caspase-8 is released from the DISC within seconds of triggering, allowing direct activation of downstream executioner caspases such as caspase-3. While the ‘type II’ pathway results in the mitochondrial events which amplify the apoptotic signal, among which the central event is the cleavage of the BH3 domain-containing proapoptotic Bcl-2 family member Bid by caspase-8. The truncated Bid will translocate to the mitochondria activating the mitochondrial apoptotic pathway.
The first step of TNF signal transduction is the high affinity binding site for TRADD created by clustering and trimerization of TNF-R1 receptors. The following pathway requires two distinct signaling complexes. The TNF-R-bound TRADD recruits the DD-containing Ser/Thr Kinase RIP1 and forms the basis of the first complex called complex I . Within one to a few hours after formation, the complex I will be internalized by endocytosis. While the complex I is degraded, the remaining complex dissociates and is released into the cytoplasm. The DDs of TRADD or RIPs are free again to bind to the FADD, and subsequently pro-caspase-8 and/or -10, leading to the formation of complex II. Pro-caspase-8 undergoes conformational changes to activate the caspase-8 heterotetramer. RIP1 will be cleaved by activated caspase-8 in the complex and is released to initiate both type I and type II apoptotic pathways.
Two receptors: DR4 (TRAIL-R1) and DR5 (TRAIL-R2/TRICK2) can mediate TRAIL-induced apoptosis. TRAIL forms a homotrimer that binds three receptors, and the TRAIL DISC appears to be similar to the FAS, as FADD is recruited. Then caspase-8 is activated. Caspase-10 will interact with FADD by the homotypic association with its DED, and then to be activated. Caspase-10, independently of caspase-8, can initiate TRAIL receptor-mediated apoptosis.
The release of cytochrome c and other proapoptotic factors from the mitochondria is mediated by the Bcl-2 family proteins through inducing or preventing the permeabilization of the outer mitochondrial membrane. Bcl-2 family consists of both pro- and antiapoptotic members which can be divided into three classes according to the Bcl-2 homology (BH) domains they contain. Antiapoptotic members contain three or four BH domains, such as Bcl-2 and Bcl-xL, while proapoptotic family members contain either only BH3 domains, such as Bad and Bid, or two or three BH domains, such as Bax and Bak. The exact mechanism of which mitochondrial proteins are released is still not investigated clearly. Mostly likely, the BH3-only proteins cooperate with multidomain proapoptotic Bcl-2 family members. Bax can translocate from the cytosol to the mitochondria, while Bak is exactly the integral proteins of the mitochondrial membrane. Interaction with the BH3 domains of the BH3-only proteins such as tBid and Bim leads to the conformational change of Bak and Bax, which makes them to multimerize to form a pore in the outer mitochondrial membrane (OMM). Mitochondrial proteins will be released from the pore, such as cytochrome c and Smac. Another mechanism is that BH3-only proteins and /or Bax can also interact with the permeability transition pore (PTP). Once the cytochrome c is released to the cytosol, it will participate in the formation of a high molecular weight proapoptotic complex named apoptosome and activate the caspase-9.
Radiation, UV light and other similar factors may cause DNA damage. Cell has several sensors for the DNA damage, such as Ataxia-Telangiectasia mutated (ATM), ATM- and Rad3-related (ATR) and DNA-dependent protein kinase (DNA-PK), which will phosphorylate a large number of substrates, such as the c-Abl and checkpoint protein Chk2. This kinase cascade will result in the activation of transcription factors which will regulate the gene expression involved in the DNA repair, cell cycle arrest and apoptosis. Besides, Chk2 can also induce apoptosis through the phosphorylation of promyelocytic leukemia (PML) protein and the transcription factor and tumor suppressor protein P53. In addition to the activation of DNA damage sensors, radiation can also induce the generation of ceramide. Ceramide then function as a second messenger and initiate the apoptotic response.
Endoplasmic reticulum (ER) can also sense and transduce apoptotic signals. There are several transducer proteins which control both the survival and death pathways, which contain PERK, ATF6 and IRE1. When ER stress continues, the cell will reach a point of no return and die. The PERK/ATF4 pathway can induce the expression of the proapoptotic-CHOP/GADD153 transcription factor, which will lead to the translocation from cytosol to mitochondria of BAX. Another apoptotic pathway is the activation of the pro-caspase-12, a caspase regarded as an ER-associated proximal effector of apoptosis. Besides the propagation of death-inducing stress signals, ER also contributes to the apoptosis caused by surface death receptors and to the pathways because of DNA damage.
A variety of stress signals can result in the lysosomal destabilization and rupture. When the lysosomal membrane becomes destabilized, the lysosomal pH gradient and lysosomal cathepsins are released, inducing the activation of Bax and/or Bak and mitochondrial membrane permeabilization (M-MP).
Cytoskeletal perturbation such as the loss of cell attachment to the ECM will lead to cell apoptosis. The central regulators of the actin cytoskeleton are integrins, which mediate anchorage to the ECM and the cell survival signaling. The release of the integrin contacts will result in the depolymerization of actin filaments and subsequent apoptosis.
Functions and Relations with Diseases
The apoptosis of stem cell plays an important role in many ways, such as it can keep the balance of the cell number, and it functions in the tissue generation. Also, it is a crucial event in maintaining the normal cell turnover in various renewing tissues. In the haematopoietic system, if the apoptosis is prevented by the over-expression of the oncogene Bcl-2, the number of HSCs (hemopoietic stem cells) will increase, which suggests that apoptosis is involved in regulating the homeostasis of HSCs. In the epidermis, a proper regulation of keratinocyte growth and differentiation is indispensible for maintaining normal function of skin components. In several dermatoses, the balance between the cell proliferation and differentiation has been disturbed. Otherwise, in the gut epithelium, apoptotic is an altruistic mechanism which leads to elimination of the ultimate stem cells that incur DNA template errors and lets the intestinal crypt survive, maintaining the high self-renewal capability.
Apoptosis is also a major component of an organism’s defence against cancer. In the haematopoietic system, a changed balance between proliferation and apoptosis may result in the myeloid compartment expansion observed in myeloid leukaemias. Some aggressive cancer pathologies like glioblastoma may result from the apoptotic resistance of stem cells and the inhibition of apoptosis, contributing to the development and progression of cancer. And in solid tumors, cells which are adjacent to the cancer cells initiate a suicidal program in order to protect the other parts from the spread of disease.
Recently, the cancer stem cell become more and more to be investigated, which may play a crucial role in the initiation and diffusion of tumor. On the other side, researchers may target the apoptotic pathway of cancer stem cell to cure the cancer. For example, JNKs (c-jun–NH2–kinases) are critical kinases together with other key signaling molecules or transcription factors that govern the development and fate of stem cells and CSCs. On the other side, JNKs are also a set of key stress-responsive kinases that mediate apoptosis. So there is likely to be some JNK-based strategies that target CSC to cure cancer.