Relevance of Apoptosis in Oral Benign and Malignant Tumours - A Systemic Review

Introduction

The removal of cells due to genetic factors is an acknowledged and accepted aspect of apoptosis, the process of ‘programmed’ cell death. ‘Apoptosis’, which means ‘falling off’ in Greek, is the term used to describe the seasonal loss of leaves on trees. Cell death, especially apoptosis, is without a doubt one of the topics that cell biologists have studied the most.¹ Apoptosis, a process of programmed cell death, is how multicellular creatures achieve and preserve tissue homeostasis. Knowing how apoptosis functions in disease contexts is essential since it can provide insight into the aetiology of the illness and advise on recommended therapy. It happens naturally during development and ageing and functions as a homeostatic process to maintain stable populations of tissue cells.²

As a normal homeostatic mechanism, tissues go through apoptosis to maintain cell populations during growth and ageing. Furthermore, when cells are injured by a disease, exposed to toxic substances, or react to an immunological stimuli, they may undergo apoptosis as a protective strategy.³ Apoptosis can be triggered by a wide range of normal and pathological conditions

and stimuli, although not all cells will certainly die in response to a given stimulus.⁴

Apoptosis can eliminate infected cells

Should a cell survive, the body might potentially be in jeopardy. Cells that may show this tendency include those that have DNA damage, are precancerous, or are infected with a virus. If these cells die, then the hazard to the organism as a whole-like the progression of cancer or a viral infection-is removed. When a cell experiences damage to its DNA, it often detects the issue and tries to repair it. The cell will frequently go through apoptosis if the damage is irreversible in order to prevent the damaged DNA from being carried on. Research indicates that cells with damaged DNA that have not completed apoptosis may be on the verge of developing cancer (Figure 1).⁵

Apoptosis is key to immune function

A strong immune system is established and maintained in part by the process of apoptosis. The development of B and T cells, two types of immune cells, is observed to see if they react to any of the body's own "self" molecules. These cells bind specific substances. Apoptosis, an immediate process of cell death, occurs in these cells. In the event that this mechanism fails, the body can unleash immune-system cells that target bodily tissues and cause autoimmune disorders.⁵

Morphologic features

The following morphologic changes in apoptosis were found by employing histologic and electron microscopic research.

• One or a few aberrant cells among regular cells.
• Apoptotic cells, often called "mummified" cells, are reduced to round or oval lumps of highly eosinophilic cytoplasm.
• Nuclear chromatin that has undergone pyknosis, karyorrehexis, or both.
• The surface convolutions or projections of the cell membrane.
• Apoptosis bodies are membrane-bound structures that resemble spherical forms with compacted organelles; they can form on or around the cell.
• Ageing causes an immediate inflammatory reaction, but macrophages phagocytose apoptotic particles at different rates. There may be quick phagocytosis, loosely floating apoptotic cells, or considerable cell loss in the tissue without a noticeable change in the overall structure of the tissue when individual cells lose contact with the basement membrane and with one other.⁶

Major inducers of apoptosis

Long-term DNA damage, abnormalities in the cell cycle, variations in the metabolism of cells, and the elimination of chemicals from cells that either stop or extend cell life Death molecules include the entry of calcium and the elimination of the growth factor and Fas ligand. Treatments include radiation therapy, corticosteroids, thermal stress, free radicals, and chemotherapy drugs.⁷

Biochemical changes in apoptosis

Apoptosis frequently results in three primary biochemical changes: membrane modifications, protein and DNA breakage, caspase activation, and phagocytic cell detection.

The outer layers of the cell membrane express phosphotidylserine (PS), which has "flipped out" from the inner layers during the early stages of apoptosis. Phagocytosis stops the release of inflammation causing cellular components by enabling macrophages to identify dead cells early on. The following step is the standard fragmentation of DNA, which results in big fragments that range in size from 50 to 300 kilobases. Endonucleases then cleave multiples of 180-200 base pairs of internucleosomal DNA to form oligonucleosomes.⁸

Is apoptosis an irreversible process?

The killing and engulfment phases of programmed cell death have been shown to have molecular underpinnings that are evolutionarily conserved, and these processes are regulated by a multitude of genes genes.⁹ For a long time, it was thought that apoptosis was an irreversible process because engulfment genes eliminated the dead cells and caspase activation triggered cell death. But in order for macrophages to effectively absorb and destroy apoptotic cells, they might have to do more than just clear away cell debris. According to Hoeppner et al. (2001), when cells in C. elegans embryos are subjected to modest pro-apoptotic signals, inhibiting engulfment genes promotes cell survival.¹⁰

Apoptosis execution pathway in pathologies

In cancer tissue, the apoptotic execution route is completely disturbed in advanced stages of the disease. Since apoptotic persons are unable to activate caspases, they are often employed as prognostic indications in colorectal cancer. The expression patterns of Smac, XIAP, APAF-1, and Caspases-3/9 differ between colorectal cancer cells and normal tissue. Additionally, higher expression of Smac and APAF-111 and decreased expression of XIAP have been linked to prolonged patient lives. The findings imply that focusing on Caspase-3 activation may be a useful strategy for cancer treatment. It is important to keep in mind that the relative abundance and interactions of multiple important apoptotic proteins, rather than the presence or lack of specific proteins, decide whether a cell will undergo apoptosis or survive. To ascertain the objectives of therapeutic interventions and forecast patient prognosis, it may be more beneficial to analyse the system as a whole as opposed to concentrating on specific markers.¹¹

Diseases associated with increased apoptosis

• AIDS
• Neurodegenerative conditions like Parkinson's and Alzheimer's; Aplastic anaemia and other myelodysplastic syndromes. Ischemic illnesses like myocardial infarction and stroke.
• Alcohol-induced liver disease caused by toxins.¹²

Apoptosis and reactive lesions

A benign vascular tumour that mostly affects the skin and mucous membranes but can also form subcutaneously or intravenously is called a lobular capillary hemangioma, usually referred to as a pyrogenic granuloma (PG). All three of the capillary anomalies, injuries, and pyrogenic granulomas can occur on their own. Many factors, including hormone fluctuations, trauma, or low-grade local irritation, can result in pyrogenic granulomas (PGs), inflammatory hyperplasia of the oral cavity.¹³ PG displayed increased Bcl-2/Bax expression and lower ISTR (in situ 3-tailing reaction) labelling indices when compared to granulation tissue and capillary hemangioma. These findings suggest that PG's apparent rapid growth is directly related to its low apoptotic rate. In addition to augmenting the angiogenic factor synthesis in pregnant tumours, female sex hormones have been demonstrated to prolong the angiogenic effect by a reduction in granuloma cell death. Microvasculature could be protected against apoptosis by Vascular endothelial growth factor (VEGF) alone or in conjunction with Ang-2, while Ang-2 by itself had no impact. VEGF insufficiency is therefore associated with endothelial cell death and granuloma regression.¹⁴

Apoptosis and giant cell granuloma

The central giant cell granuloma (CGCG) is a rare, histologically benign, but locally aggressive and destructive osteolytic lesion of osteoclastic origin in the craniofacial region, particularly in the jaw bones. A multilocular radiolucency with scalloped borders and a honeycomb or soap bubble look can be seen on radiographs of the same lesion.¹⁵ The enormous, multi-nucleated cells that resemble osteoclasts on a mononuclear background are characteristic of the central giant cell lesion (CGCL) and peripheral giant cell lesion (PGCL) of the jaws. Large cells in soft tissue and bone tumours also typically show 18 times more apoptotic nuclei than mononuclear cells, indicating that these cells are reactive rather than malignant. CGCL and PGCL had greater amounts of Bax and Bcl-2 mRNA expression. The TUNEL study indicates that the PGCL has a greater apoptotic index (ratio Bax/Bcl-2) than the CGCL, which may account for the difference in clinical behaviour between the two groups.¹⁴

Apoptosis and diseases of periodontium

The destruction of the periodontal ligaments and the degeneration of the alveolar bone surrounding the teeth are hallmarks of the advanced stages of periodontal disease, a chronic inflammatory illness. It is one of the two primary threats to dental health and a major factor in tooth loss. The oral cavity contains eight hundred distinct types of microorganisms. Periodontal disease is thought to be caused by a complicated interaction between human defence mechanisms and bacterial infection, which can be impacted by lifestyle factors like smoking.¹⁶ Compared to those with gingivitis, patients with periodontitis showed a higher proportion of apoptotic keratinocytes than proliferative keratinocytes, but only in the most apical part of the sulcus. Because Actinomycetemcomitans leukotoxin depends on caspase 1-activation, it is more likely to trigger monocyte lysis. Furthermore, it has been discovered that a decrease in apoptosis and an increase in fibroblast proliferation are the causes of gingival overgrowth.¹⁴

Apoptosis and oral lichen planus

Lichen planus is a persistent inflammatory skin and mucous membrane condition. The mucosal equivalent of cutaneous lichen planus, oral lichen planus (OLP), affects women 1.4 times more often than men. OLP is more prevalent in the 40s and 50s. 1% to 2% of people are afflicted by the illness. Clinically, it manifests as bullous, erosive, reticular, papular, atrophic, or plaque like lesions. The tongue, gingiva, and buccal mucosa are the intraorally most typically afflicted locations, while other sites may also be impacted sometimes. Oral mucosal lesions can occur by themselves or in combination with skin lesions. Usually, the face is not affected, but violaceous flat-topped papules appear on the genitalia, wrists, and ankles as skin lesions.¹⁶

The breakdown of the epithelium's base layer (LP) is a distinctive histopathologic trait of lichen planus. Colloid, hyaline, cytoid, or Civatte bodies are other names for eosinophilic globular forms, which are also seen in the basal layer of LP and the upper connective tissue. These entities were formerly thought to be degenerative keratinocytes. Apoptosis appears to be the initial step in basal layer degradation.¹⁴

Apoptosis and erythema multiforme

Erythema multiforme (EM), a cutaneous and occasionally mucocutaneous condition, can be brought on by the most common viral infection, herpes simplex virus (HSV), as well as several medications.¹⁴

Erythema multiforme is a hypersensitive dermatological condition that mostly affects the skin or mucous membranes, but can eventually affect both. Occasionally, they may comprise the more severe visceral categories. There is a broad spectrum of clinical patterns associated with it, such as toxic epidermal necrolysis, Stevens Johnson syndrome, EM minor, and EM severe. The stimulation of apoptosis in EM may be linked to changes in the expression of apoptosis-regulating proteins, such as members of the Bcl-2 family and p53, as well as the activation of the Fas/Fas-ligand system.¹⁴

Apoptosis and lupus erythematosus

One connection between lupus and immune system abnormalities is multisystemic inflammation. Either periods without overt symptoms or indicators, or episodes of varying degrees of intensity, are experienced by patients. The four forms of lupus (SLE) include, drug-induced lupus (DIL), discoid lupus erythematosus (DLE), neonatal and paediatric lupus erythematosus (NLE), and systemic lupus erythematosus (SLE). It is thought that maternal autoantibodies that cross the placenta cause neonatals with the uncommon kind of lupus known as NLE. The percentage of paediatric patients with NLE whose mothers had positive autoantibody testing, however, is just 1% to 2%. It is thought that maternal autoantibodies that cross the placenta cause neonatals with the uncommon kind of lupus known as NLE. Only 1% to 2% of paediatric patients whose mothers had positive autoantibody testing are affected by NLE, nevertheless. Typical clinical indicators include those pertaining to the skin, heart, and liver. Nonetheless, most NLE patients with organ involvement (skin, liver, or blood) experience a spontaneous remission of their symptoms within 4-6 months. Along with cardiac symptoms, significant morbidity and mortality have also been documented.¹⁴

Apoptosis and pemphigus vulgaris

Rare autoimmune bullous disorders that affect the skin and mucous membranes are known as phenophigus diseases. In Central Europe, the annual number of new cases per million people is estimated to be two. They persist in exhibiting evident morbidity and death along with a noticeable reduction in life quality. Pathogenic autoantibodies against several desmosome proteins primarily of the IgG type-produce desmogleins. These autoantibodies bind desmosome components, diminishing intraepidermal adhesion and resulting in erosions, blisters, and vesicles on the skin and/or mucous membranes.¹⁶

Clinical blisters arise as a result of keratinocyte cell adhesion loss, which is a hallmark of the autoimmune blistering condition, pemphigus. Pemphigus causes apoptosis or the production of proapoptotic proteins. IgG may originate directly from acantholysis caused by sera and IgG, or it may originate from both acantholysis and adhesion loss (anoikis). The Fas/Fas ligand cell death pathway is thought to be a major factor in anoikis in pemphigus illness since sera from untreated individuals contain significantly greater amounts of the ligand.¹⁴

Apoptosis and virus associated diseases

The most frequently associated viruses with the aetiology of various oral lesions (HIV) are the herpes simplex virus (HSV), HIV, and human papillomaviruses (HPV). CTLs or specific cytokines that trigger apoptosis frequently identify and eliminate virus-infected cells. Several instances include the activation of the phosphoinositol 3-kinase-Akt signalling pathway, the suppression of p53 and other apoptotic proteins, and the prevention of apoptosis by viral BCL-2 (vBCL-2) homologues. Apoptosis is the result of HSV infection in infected cells. However, proteins that are later produced by infected cells stop apoptotic cell death. The herpes simplex virus is brought on by this control of apoptosis. The downregulation of the cellular FLICE-inhibitory protein (c-FLIP), which is not dependent on proteases, occurs in conjunction with HSV-1-induced apoptosis in immature dendritic cells. Adults infected with HSV-1 develop encephalitis as a result of the virus's destruction of CNS neurons. HSV-1 causes apoptosis by expressing Fas and its ligand on the surfaces of immature neutrophils. The high-risk HPV oncoproteins E6, E7, and E5 can influence host-mediated apoptosis and the survival of infected cells by blocking death receptor signalling.¹⁴

Apoptosis is inhibited by HPV E6 proteins via both p53- dependent and p53-independent pathways. When HPV18 E6 prevents Bak-induced apoptosis in differentiated keratinocytes, HPV replicates. Toxins or environmental stress cause HIV to enter apoptosis when growth hormones are not present; one of the death-inducing ligands, such as TNF, FasL, or TRAIL58, may then mediate this apoptosis. It has been discovered that a number of HIV-encoded proteins are either pro- or antiapoptotic. Gp120, a glycoprotein found on the surface of HIV, crosses CD4 and primes cells for destruction. HIV-1 can also cause internal apoptosis by expressing particular viral genes, including tat, nef, vpu, and vpr.¹⁴

Apoptosis and ameloblastomas

Ameloblastoma is the most prevalent benign jaw tumour, accounting for 1% of all jaw cysts and tumours and 10% of odontogenic tumours. Malassez was the first to describe ameloblastoma in 1885 and stated that the condition develops from the epithelial remnants of the expanding root sheath. The actual cause of ameloblastoma is still unknown, though. It is unclear if dental cysts cause ameloblastoma to develop or if ameloblastoma develops first as a cyst because there are no established diagnostic criteria for either illness. Though they are typically believed to be benign, locally aggressive tumours, ameloblastomas can occasionally migrate to neighbouring lymph nodes and even farther afield.¹⁵ Ameloblastoma is the most common cancer generated from odontogenic epithelium. It is distinguished by a high risk of recurrence and by benign but locally aggressive activity. Using Reverse Transcription Polymerase Chain Reaction (RT-PCR)and immunohistochemistry (IHC), it was possible to identify the expression of cytochrome c, APAF-1, caspase-9, and AIF in tooth germs as well as benign and malignant ameloblastomas. This shows that the mitochondria-mediated apoptotic pathway, which may be implicated in oncogenesis, cytodifferentiation, and malignant transformation, causes apoptotic cell death in both healthy and malignant odontogenic epithelium.¹⁴

Apoptosis and oral leukoplakia

The most common potentially cancerous disorder affecting the oral mucosa is oral leukoplakia (OL). Despite being noticed in clinical evaluations since 1969, OL was initially described by the World Health Organisation in 1978 as a white patch or plaque that cannot be clinically or pathologically classified as any other disease. Since then, the concept of oral leukoplakia hasn't really evolved. Following an international workshop in Uppsala, Sweden, in 1994, the inclusion of the information that oral leukoplakia can proceed to cancer and is not caused by any physical or chemical variables other than smoking was made to the criteria.¹⁵

Prior to invasion, apoptosis may alter in a premalignant lesion of the human oropharynx and oral cavity. From normal to carcinoma in situ (CIS), the apoptotic index (AI) grew gradually; it then began to drop in SCC, the cancer form with the highest AI:MI ratio of all. However, they were distributed freely via CIS and SCC. Both normal and dysplastic epithelia had apoptotic entities restricted to their lowest layers. Leukoplakia cases with delayed keratinocyte maturation exhibited increased expression of MDM2 and Bcl-2, while wt-p53 and p21 were more expressed in OLP cases.

Despite the different outcomes of apoptotic signalling proteins, there was no significant difference in the quantity of apoptotic epithelial cells between leukoplakia and OLP instances. The prognosis is bad for leucoplakia patients with elevated levels of the mitotic, apoptotic, and Ki-67 indices. If there is less cell proliferation in oral leukoplakia due to the presence of p27, premalignant cancer cells will go through apoptosis and the tumour will stop growing. It is thought that the antiapoptotic action of the Epstein-Barr virus (EBV) causes altered expression of apoptosis-associated proteins in oral hairy leukoplakia.¹⁴

Apoptosis and salivary gland tumours

The complex human salivary glands (SGs) are made of a web of ducts and acini that form throughout embryonic development (Cutler, 1990; Harunaga et al., 2011). All facets of growth, proliferation, differentiation, migration, and cell death are included in their branching morphogenesis. These glands are developing at the following stages: prebud, first bud, pseudoglandular, canalicular, and terminal bud (Tucker, 2007; Teshima et al., 2016 a,b; de Paula et al., 2017). Epithelial cells bud and internalise to generate branched solid cords from the primitive mouth cavity (Melnick & Jaskoll, 2000; Patel et al. 2006, 2011).¹⁴ Salivary gland tumours (SGTs) are uncommon tumours that display various biologic and histopathologic traits. Given that 70% of pleomorphic adenomas (PA) tested negative and all malignant salivary gland tumours tested positive, P53/bcl-2 immunostaining reactivity could be useful in elucidating the invasion and developmental behaviour of SGTs.¹⁴

Apoptosis and epidermolysis bullosa

Epidermolysis bullosa (EB) is a rare, clinically and genetically diverse genodermatosis that affects an estimated 500,000 people worldwide. Mechanobullous dermatoses are characterised by moderate to moderate epithelial tissue fragility, which results in the characteristic blistering or erosions that occur after mild stress. Mutations in at least 20 genes, which encode elements involved in cell junctions like desmosomes and hemidesmosomes, intraepidermal adhesion in the skin and mucous membranes, and segments of the cytoskeletal keratin intermediate filaments, are the cause of epidermolysis bullosa.¹²

Skin blistering brought on by basal keratinocyte fragility and oral mucosal blistering are signs of the hereditary vesiculobullous disease, epidermolysis bullosa. Using an EBS cellular model, an analysis revealed that caspases 3 and 8 activated apoptosis-prone HaCaT cells, but not caspases 9 or 12. Furthermore, protein aggregation is connected to cell death in this keratinocyte EBS biological model by TNF-release and subsequent autocrine/paracrine TNF- receptor activation. The K14 mutation impedes the K14-TRADD interaction, another cytoprotective mechanism that affects keratinocyte sensitivity to caspase-8-mediated death.¹⁴

Conclusion

It is critical to comprehend the role apoptosis plays in health and illness. Apoptosis has grown in significance as a biological mechanism over time, contributing to both normal physiology and the aetiology of numerous diseases. A comprehensive knowledge of apoptosis and its significant role in the aetiology of various oral disorders may enable therapeutic intervention at particular checkpoints. It is crucial to understand apoptotic cell death since it impacts an organism's life from conception to death and is involved in certain diseases. The different functions of apoptosis in dental development can be assessed by understanding the temporally and geographically restricted distribution patterns of apoptotic cells. This study aimed to give an overview of the knowledge surrounding the molecular basis of apoptotic cell death, which has aided in the development of therapeutic methods for many diseases, in addition to highlighting the role of apoptosis in oral disorders. The molecular mechanisms behind disorders of the oral cavity are still not entirely understood. Research is needed to identify the conditions that permit selective regulation of this process, which is necessary to target apoptosis for therapeutic intervention in a safe and effective manner.

Conflict of Interest

None

Financial assistance

None