The Importance of Apoptosis in Cancer Development and Treatment

Apoptosis is one of the most acceptable mechanisms for anticancer activity in cancer cells. The regulatory mechanisms of apoptosis are very complex. Reactive oxygen species, caspase activation, tumor necrosing factor (TNF), protein kinase, and mitochondrial pathway form the basis of apoptosis.[]

What Is Cancer?
Cancer has been a common problem in humans and animals throughout the known history. The earliest known records about cancer date back to 3000 BC. The word cancer is derived from the words "canker" or "carcinos," which means crab in Latin. The term tumor was first used by Hippocrates in the 3^rd century BC because it compared the swollen veins around the tumor to the legs of a crab, and the term "oncos," which means swelling, was used.[]

Cancer is a complex disease caused by uncontrolled division and proliferation of cells and under the influence of genetic and environmental conditions. Cancer is also a personal disease, although there are more than 100 known types of cancer and standard approaches have been developed for certain types of cancers. Cancer is the uncontrolled division, proliferation, and accumulation of cells in an organism. It can affect a single organ as well as spread to distant organs and show its effect.[] Cancer is a disease associated with a decrease in apoptosis.[]

What Is Apoptosis?
Apoptosis is a word formed by the combination of ancient Greek apo (separate) and ptosis (fall) and used by Homeres to describe leaf fall in autumn. For this reason, it is the type of cell death that occurs when some cells dry up like autumn leaves and leave the body and make room for the cells coming from behind. It was named "apoptosis" with the suggestion of the classical Greek historian James Cormack.[]

Apoptosis; it is programmed cell death, which is an important cell growth control component that ensures the elimination of cells that are not needed, whose functions are impaired, that have completed their biological task or are damaged after exposure to a certain stimulus.[]

There are two pathways in apoptosis: Extrinsic pathway and intrinsic pathway.[,]

Extrinsic Pathway (Receptor Pathway)
This pathway is stimulated by the tumor necrosis factor-alpha (TNF-alpha) family, which binds to the CD95 ligand (Fas ligand=CD95L) by extracellular signals. This binding causes the formation of deathinducing signal complex (DISC) by causing conformational changes independent of ATP at the receptor. With the CD95 adapter molecule Fas receptor-associated measurement unit (FADD), FADD also combines with procaspase-8 and procaspase-10 to form the DISC complex. Separation of small and large subunits is not required for activation. The next step is activation of procaspase-8, causing proteolysis from caspase-3 to separate from the small subunit, thereby activating the enzyme. This pathway combines with the mitochondrial pathway through the caspase-3 activation, thereby strengthening the apoptosis signal. Cellular FLICE-inhibiting proteins (cFLIP, cFLIPL, cFLIPs) can combine with DISC to inhibit the activation of caspase-8 and 10, preventing apoptosis.[,]

Instrinsic Pathway (Mitochondrial Pathway)
Unlike this pathway death receptor pathway, it can be induced by both extracellular signals (growth factor or hormone deficiency, ultraviolet beam, and various cytokines) and intracellular signals that cause DNA damage. While the intracellular signal stimulates the proapoptotic members of the Bcl-2 family, it migrates to the mitochondria, while the proapoptotic members settle on the outer mitochondrial membrane and form pores (mitochondrial passages) here. These pores allow cytochrome-c separation from mitochondria. Cytochrome c; Combining with apaf-1, ATP, dATP, and procaspase-9, the apoptosome activates caspase-3. Although their beginnings are different, they both result in caspase-3 activation in both pathways. Furthermore, apoptosis-inducing factor (AIF) from mitochondria is released in molecules such as G, Smac/DIABLO, and Omi/HtrA2. Endonuclease G and AIF, while inducing DNA fragmentation, Smac/DIABLO, and Omi/HtrA2 neutralize apoptosis-inhibiting protein (ınhibitor of apoptosis proteins [IAPs]). Molecules such as Smac/ DIABLO and Omi/HtrA2 are located in the intermembrane region of the mitochondria, such as cytochrome c. Smac/DIABLO and Omi/HtrA2 have many similar aspects. However, while Smac/DIABLO is not found in the heart and detected in the brain, on the other hand, there is a widespread distribution of Omi/HtrA2.[,]

Apoptosis and Cancer Relationship
In cancer, an important disease associated with apoptosis, the balance between cell proliferation and cell death is disrupted, and cells are unable to receive death signals to cause apoptosis. This can cause problems at any step of the apoptotic pathways. Better understanding of the molecular events regulating apoptosis mechanisms enables the creation or development of new treatment options for the activation or inhibition of the target molecules in these pathways.[]

The irregularity of apoptotic pathways not only promotes tumor formation but can also make the cancer cell resistant to treatment. Therefore, irregularity of apoptosis is an important cancer symptom.[]

As with B cell lymphoma, decreased apoptosis in some tumors can cause tumor development. In general, an increase in apoptosis is observed due to the increase in proliferation in tumor tissue.[]

Disorders in programmed cell death (apoptosis) mechanisms play important roles in tumor pathogenesis. Apoptosis allows neoplastic cells to survive over their intended lifespan, reduces the need for exogenous survival factors, and provides protection from oxidative stress and hypoxia as the tumor mass expands.[] It has been found that anti-apoptotic Bcl-2 subgroup is induced in many types of cancer. And it has been determined that the levels of inhibitors are induced in tumor cell lines. Tumor by inducing the molecule level can cause deproliferation or apoptosis. For example, IL-3 induces the Bcl-xL level to improve survival of the myeloid sequence and interleukin IL-5 and IL-15 protect eosinophils and mast cells from apoptosis by inducing the Bcl-xL level.[]

In chronic lymphocytic leukemia, IL-4 levels increase, excessive production of Bcl-2 occurs, and the life span of malignant B cells is prolonged. Two antiapoptotic cytokines IL-4 and IL-10 are released in thyroid carcinoma cells and contribute to cancer development.[] There are kinds of molecular mechanisms that tumor cells use to suppress apoptosis. Tumor cells can become resistant to apoptosis by the expression of antiapoptotic proteins such as Bcl-2 or by down-regulation or mutation of proapoptotic proteins such as BAX.[] Some forms of human B cell lymphoma have Bcl-2 overexpression. This example represents the first and strongest evidence that the absence of apoptosis contributes to cancer.[]

p53 and Cancer
The first gene therapy product used in cancer treatment is Gendicine, which is specifically designed to express p53 (rAd-p53). In a study conducted on patients with oral cancer, it has been shown that administration of rAd-p53 infusion together with chemotherapy significantly increases the survival rate in patients. Apart from that, it has been stated that it is used effectively in hepatocellular carcinoma, tongue cancer, and some other types of cancer.[-]

Regulation of apoptosis in humans is a process that starts with p53 and continues up to caspases. Cell life is prolonged when p53, which works as a tumor suppressor gene, is mutated or absent. Cell damage caused by genotoxic events activates a transcription regulator gene, p53. After the p53 protein product binds directly to DNA and recognizes the damage, it either induces cell cycle arrest in G1, gaining time for repair, or directs apoptosis if the damage is greater. In addition, p53 is thought to regulate the ratios of Bax/Bax, Bax/Bcl-2, and Bcl-2/Bcl-2 groups.[]

Caspases and Cancer
Although the mechanism of apoptosis is not fully explained, the most important event associated with apoptosis is the activation of caspases.[,]

Initiator caspases transmit death signals initiated by apoptotic stimulation to effector caspases. Effector caspases, on the other hand, break down related proteins and cause the formation of apoptotic cell morphology. Defects in the caspase can contribute to autoimmune diseases, cancer, and the formation of some neurological disorders.[,]

IAPs and Cancer
IAP (inhibitors of apoptosis), a family of caspase inhibitors, selectively inhibit caspases. Thus, they stop the apoptotic mechanism. These inhibitors are expressed by many malignant cells. IAPs can also stop apoptosis by affecting the cell cycle.[]

IAPs are a group of structurally and functionally similar proteins that regulate apoptosis, cytokinesis, and signal transduction. They are characterized by the presence of a baculovirus IAP repeat (BIR) protein domain.[]

Irregular IAP expression has been reported in many cancers. For example, Lopes et al.,[] it showed that the abnormal expression of the IAP family in pancreatic cancer cells and this abnormal expression is also responsible for resistance to chemotherapy. Among the IAPs tested, the study concluded that drug resistance was the most significant correlation with cIAP-2 expression in pancreatic cells.

Survivin, another IAP, has been reported to be overexpressed in various cancers. Small et al.,[] transgenic mice overexpressing Survivin in hematopoietic cells are high in terms of hematological malignancy risk and observed that hematopoietic cells designed to overexpress Survivin are less susceptible to apoptosis.

Survivin has been found to be overexpressed in non-small cell lung carcinomas (NSCLC) in conjunction with XIAP. It has been concluded that Survivin overexpression in most NSCLCs is accompanied by excess or upregulated expression of XIAP.[]

Carcinogenesis by Intrinsic Apoptosis Pathway Mutations
The intrinsic apoptosis pathway is one of the most important pathways for apoptosis induction. Therefore, disrupting this pathway is an effective way of preventing apoptosis. Bcl-2 family proteins (c=24 in humans) are central regulators of the intrinsic pathway, either suppressing or promoting changes in mitochondrial membrane permeability required for the release of cyt-c and other apoptogenic proteins.[,]

Antiapoptotic proteins block the death signal by partially antagonizing the effects of Bax/Bak through known mechanisms. Furthermore, antiapoptotic proteins prevent Bax/Bak activation by sequestrating/inhibiting BH3-only proteins "activator" and/or directly inhibiting Bax/Bak activation.[]

Antiapoptotic proteins (e.g., Bcl-2, Bcl-xL, Bcl-W, Mcl-1, and Bfl-1/A1, which exhibit sequence homology in all BH1-BH4 domains) increase cell survival, while proapoptotic proteins are receptor, endoplasmic reticulum (ER), or mitochondria. ER mediates stressinduced apoptosis. The next group contains proteins that contain the multi-domain or BH3- only. The first consists of Bax and Bak, which are necessary for apoptosis.[]

The central role played by Bax/Bak in apoptosis is supported by studies that BH3-only proteins do not induce apoptosis in cells with Bax/Bak deficiency.[,]

Carcinogenesis by Extrinsic Apoptosis Pathway Mutations
It is activated in vivo by TNF family ligands that bind DD-containing receptors, which means that DED-containing caspases lead to its activation. "Death ligands" are expressed on CTLs, NK cells, and other immunerelated cells (activated monocytes/macrophages and nitrite cells) and are used as weapons for destroying transformed cells.[]

Another molecule, c-FLIP, which regulates caspase- 8 activation, is c-FLIP (L), which are long IL-1Β converting enzyme inhibitor proteins similar to cellular FADD. Another molecule, c-FLIP, which regulates caspase-8 activation, is c-FLIP (L), which are long interleukin- 1 beta converting enzyme inhibitor proteins similar to cellular FADD.[,]

Melanoma, hepatocellular carcinoma, non-small cell lung carcinoma[] and endometrial,[] colon,[] and prostate cancer,[] including increased expression of c-FLIP has been detected in many human malignancies. Its overexpression is associated with cancer progression and/or poor prognosis in BL, HCC, and ovarian, endometrial, colon, and prostate cancer.[,,]

The high expression of C-FLIP blocks caspase-8 and makes cells resistant to cell receptor-mediated apoptosis.[] Overexpression of C-FLIP has been reported to bind proteins involved in these signaling pathways, including TRAF1, TRAF2, RIP, and RAF1, thereby promoting the activation of NF-kB and ERK as downstream molecules.[]

Mice-based studies on the use of neutralizing antibodies and Fc-fusion proteins as well as genetic changes in genes encoding death ligands or their receptors, found that death ligands and the genes encoding them play important roles in tumor suppression by cellular immune mechanisms. aFas ligand (FasL) is important for CTL-mediated killing of some tumor targets, and TRAIL (Apo2 ligand) is critical for NK-mediated tumor suppression. Some tumor cells resist the death receptor pathway"s response to FasL produced by T cells to avoid immune damage. Many tumor cells show intrinsic resistance to TRAIL. This directly sequesters Fas of ligand or expression Fas ligand on the surface of tumor cells. This is downregulation of the Fas receptor, a dysfunctional Fas receptor, and it creates the secretion of high levels of Fas receptor.[] Some tumor cells may undertake a Fas ligand-mediated "counterattack" causing apoptotic depletion of active tumor-infiltrating lymphocytes.[]

Apoptosis and Cancer Treatment
Cancer-related defects in apoptosis play a role in resistance to treatment with traditional treatments such as chemotherapy and radiotherapy, increasing the cell death threshold, thus requiring higher doses of tumoricidal agents.[]

Apoptosis and Cancer Theratment targets apoptosis via receptor-chondrial-mediated. Many drugs are used in cancer treatment today to kill cells. Disruption of mitochondrial membrane potential, cytochrome c release, and activation of different caspases have been identified following treatment of cells with different chemotherapeutic agents (Table 1).[,]

Table 1: Drugs and strategies targeting apoptosis in cancer treatment[]

Table 1: Cont.

Tumor selective expression of proapoptotic Bax by adenoviral gene transfer leads to selective toxicity in tumor cells.[]

Bortezomib has a sensitive effect on apoptosis induced by NF-κB suppression by downregulated Bcl-2. [] On the other hand, it has been reported that BH3 proteins Bik and Bim levels, which are an important mediator of antitumor activity, increase in various cell lines through bortezomib.[] Heat shock proteins are also pharmacological targets. Geldanamycin, an Hsp 90 inhibitor, shows a clear antitumor effect.[] The drug known as eurycoma has proven excellent antiproliferative and anticancer effectiveness against a variety of human cancers in most in vitro and in vivo studies. Eurycoma mechanism of action; it is the induction of apoptosis and down-regulation of expression by up-regulation of p53 (tumor suppressor protein) and proapoptotic protein (Bax) expression and down-regulation of expression.[]