A New Approach of Abnormal Apoptosis Implicated in Malignancy and Autoimmunity

What is it about?

New cancer therapies In experimental models, disrupting the MDM2–p53 interaction restored p53 function and sensitized tumors to chemotherapy or radiotherapy. [8] This strategy could be particularly beneficial in treating cancers that do not harbor TP53 mutations. For example in hematologic malignancies, such as multiple myeloma, chronic lymphocytic leukemia (CLL), acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and Hodgkin's disease, the induction of p53 – using a small MDM2-inhibitor molecule, nutlin-3 – can induce the apoptosis of malignant cells. Nutlins are a group of cis-imidazoline analogs, first identified by Vassilev et al [8], which have a high binding potency and selectivity for MDM2. Crystallization data have shown that nutlin-3 mimics the three residues of the helical region of the trans-activation domain of p53 (Phe19, Trp23 and Leu26), which are conserved across species and critical for binding to MDM2 [9]. Nutlin-3 displaces p53 by competing for MDM2 binding. It has also been found that nutlin-3 potently induces apoptosis in cell lines derived from hematologic malignancies, including AML, myeloma, ALL, and B-cell CLL [10]. A large cohort study of primary CLL, done on over 100 patients, examined the samples from the patients for a response to MDM2 inhibition. The study found direct correlation between wild-type TP53 status and MDM2 inhibitor-induced (nutlin-3 and MI-219) cytotoxicity across various CLL subtypes. This response was not predicted by other biomarkers used clinically for CLL, including in B cells, expression of the zeta-chain-associated protein kinase 70 (ZAP70), un-mutated immunoglobulin variable genes, and mono-allelic ATM (ataxia telangectazica mutated gene) loss. The protein ZAP70 is a member of the protein-tyrosine kinase family. ZAP70 is normally expressed in T cells and natural killer cells, and has a critical role in the initiation of T-cell signaling. ZAP70 in B cells is used as a prognostic marker in identifying different forms of chronic lymphocytic leukemia (CLL). Some studies showed that patients with cancer make antibodies against p53 proteins, but the frequency and magnitude of this response is still under debate [12]. However, a large number of patients with cancer did produce p53-reactive T cells [15]. The results from these studies served as a good reason to attempt the vaccination of patients using p53-derived peptides, and a several clinical trials are currently in progress. The most advanced work used a long synthetic peptide mixture derived from p53 (p53-SLP; ISA Pharmaceuticals, Bilthoven, the Netherlands) [13, 14, 15]. The vaccine is delivered in the adjuvant setting and induces T helper type cells. However, the response may not be potent enough to result in clinical benefit as a mono-therapy: This indicated that these p53-specific T-helper responses are not polarized. Therefore, approaches are being investigated to promote a stronger and more correctly polarized response using both DNA-based and dendritic cell-delivered p53 vaccines [Figure 3]. FasL and Oxidative Stress in Apoptotic Events The main death receptor (DR), named CD95 (Fas), as well as CD 120a (TNF-R1), DR3, DR4, DR5, and DR6, are responsive to cytokines belonging to the tumor necrosis factors (TNF-α, lymphotoxin, Fas ligand (FasL}, Apo-13). The link between apoptosis and TNF activity shows why abnormal production of TNF plays an important role in several autoimmune diseases: rheumatoid arthritis (RA), multiple sclerosis (MS), diabetes mellitus, ulcerative colitis [16]. Fas ligand (FasL) is a type II membrane protein which belongs to the tumor necrosis factor (TNF) family. FasL induces apoptosis in target cells bearing the receptor Fas. The role of the Fas-FasL system has been best characterized in the immune system: interactions between Fas and FasL are functionally involved in tissue-specific regulation of various immune processes: for example, FasL expression has been detected in immune-privileged organs, such as the eye and the testis, which are protected from destructive immune responses by inducing the apoptosis of infiltrating Fas-bearing immune cells. Endothelial cells express Fas, but are normally resistant to Fas-mediated apoptosis [2, 31]. Oxidative stress has been shown to alter various aspects of endothelial functions: increase in endothelial adhesiveness to neutrophils via protein-kinase-C-activation-dependent pathways, increase in the production of platelet-activating factor, as well as in the expression of intracellular adhesion molecule-1. Thus, up-regulation of FasL expression on the endothelium may contribute to anti-inflammatory reactions by reducing leukocyte transmigration in tissues. Recent and previous studies have shown that increased oxidative stress induces FasL expression by T-lymphocytes, microglial cells, and intestinal epithelial cells, suggesting that oxidative stress is involved in the FasL-mediated apoptotic mechanism of Fas-bearing target cells [17, 18; 19, 31]. H2O2 is one of the most important oxidant agents derived from leukocytes and endothelial cells. It exerts a toxic effect on susceptible cells at high concentrations, but alters cell functions at low concentrations, by modulating signal transduction pathways in certain cells, including endothelial cells [19]. Cigarette smoke is an important source of oxidative agents, including H2O2, and is thought to be a significant risk factor for chronic endothelial damage leading to atherosclerosis [20, 21]. However, the mechanism for oxidative-stress-induced FasL expression is still unclear. A previous report documented an association between oxidative-stress-induced FasL expression and the NF-κB nuclear transcription factor. The functional role of NF-κB has not been fully demonstrated [22]. Two NF-κB binding sites are located at positions -537 to -521 and -57 to -47, respectively, relative to the transcription start site of the human CD95L promoter. [23]. High levels of soluble CD95 were found in rheumatoid arthritis (RA): these high levels contribute to the inhibition of apoptosis of synoviocytes and, inflammatory cells. An inadequate apoptosis due to defective CD95 may promote an extended survival of synoviocytes; additionally their responsiveness to CD95L is decreased by TGFβ, IL1-β, and TNF-α. Simultaneously, expression of CD95 and its ligand causes apoptotic cells death by paracrine or autocrine mechanism and during inflammation, IL1-β and interferon-1α induce massive CD up-regulation [24]. High rate of apoptosis can overload the phagocytic capacity and may trigger an autoimmune reaction, through the presentation of nucleosomes to the immune system. Apoptosis also plays a role in negative selection of B and T lymphocytes that escaped the self-reactive nature; again, in this case, apoptosis can be a source of auto-antibodies. Elevated activity of the receptor CD3 +, CD4 + or Th1 helper cells will be induced by high serum level of interleukins (ILs): IL1-β, IL2 and TNF [25].

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Why is it important?

Apoptosis has traditionally been thought of as non-inflammatory process which does not induce an immune response. However, recent studies indicate that apoptotic cells can be involved in autoimmune processes. In some cases the cells can display auto-reactive antigens on their surface blebs, can activate T cells and B cells, and can induce the formation of auto-antibodies. The most important cell-regulatory mechanisms of apoptosis in mammalian T cells and B cells are: death receptors, caspases, mitochondria, the Bcl-2 family proto-oncogene, tumor suppressor gene TP53, TNF, and nuclear translocation factor, NF-Κb and recently MicroRNAs (miRNAs) which are small non-coding RNAs that act at the posttranscriptional level to regulate protein expression.