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Oral Pathology

Author : Dr. K.Sudheer Kanth M.D.S,

Abstract:
Apoptosis is a regulated event of cell death and is a gene-directed program has had profound implications for our understanding of developmental biology and tissue homeostasis, for it implies that cell numbers can be regulated by factors that influence cell survival as well as those that control proliferation and differentiation. Studies revealed a high frequency of apoptosis in spontaneously regressing tumors and in tumors treated with cytotoxic anticancer agents . Together, these observations suggested that apoptosis contributed to the high rate of cell loss in malignant tumors and, moreover, could promote tumor progression. Nevertheless, the importance of apoptosis in cancer remained under-appreciated for many years.

Introduction:
Apoptosis is a regulated event of cell death (ie) morphologically and biochemically distinct mode of cell death that occurs during embryogenesis, carcinogenesis ,cancer treatment or immune reactions as well as in cell proliferation. It is an important regulated process that counter balances the cells produced due to cell division and compliments differentiation in the overall tissue homeostatic mechanisms.

Cell survival is conditioned by genes that promote and inhibit apoptosis. The accumulation of neoplastic cells may occur not only by the activation of oncogenes or inactivation of tumor suppressor genes, but also by mutations in the genes that regulate apoptosis. Apoptosis was originally defined by Kerr as the orderly and characteristic sequence of structural changes resulting in the programmed death of the cell. Apoptosis means “falling off leaves“ in Greek, it is thought to be responsible for numerous physiologic and pathologic events .
Defects in apoptotic pathways occur during cancer development and is the mechanism by which cancer cells survive in the presence of mutations and in the sites of the body that are distant from the primary tumor.

Types of Cell Death:
Occurs in two forms apoptosis and necrosis. Apoptosis is characterized by cytoskeletal and organelle disruption, cell shrinkage, membrane blebbing, chromatin condensation and fragmentation and formation of small membrane bound apoptotic bodies, which are phagocytosed by macrophages or neighbouring cells.
Necrosis involves large group of cells and provokes an inflammatory process .It is associated with rapid cellular swelling due to influx of ions leading to loss of integrity of membranes ,with random DNA degradation 4.

Regulation of Apoptosis

It is a genetically regulated evolutionary conserved process that removes damaged or senescent cells from a tissue or organ. Living cells possess inherent inhibitory mechanisms to keep the apoptotic process in check.
The process of apoptosis is thought to occur in three distinct stages:

 

  1. Initiation
  2. Effector
  3. Degradation

The initiation phase is induced by a variety of signals including DNA damage, hypoxia, lack of nutrients, growth factors and activation of death receptors 2.

Signaling Pathways of Apoptosis:

  1. Transmembrane death receptors mediated by TNF receptor family-Extrinsic pathway
  2. Mitochondrial pathway-Intrinsic pathway

The regulation of apoptosis involves bcl-2 gene family, the tumor suppressor genes, oncogenes, mitochondrial factors and caspases.7

Apoptosis and ORAL Tumorigenesis:
Cancer cells attain the ability to evade activation of their inherent apoptotic programme and to escape apoptosis induced by the immune system. Tumour cells may evade apoptosis by inactivation of apoptosis-inducing genes or by enhancement of the activity of antiapoptosis genes

Caspases :
Caspases are a group of enzymes that are involved in the regulation of apoptosis resulting in the classical apoptotic features. They belong to a family of cysteine proteases that specifically cleave their substrates after aspartic acid. The intrinsic pathway activated by cellular stress is mediated by release of cytochrome c from the mitochondria leading to activation of caspase -9, which leads to activation or inactivation of cellular protein targets by a process of limited proteolysis.

Caspases are responsible for induction of apoptosis .Mutations leading to improper activation of caspases will lead to evasion of apoptosis.1,18,19

Bcl-2 gene:
B-cell leukemia gene is deregulated causing overexpression .This family consists of two groups of proteins: antiapoptotic and pro-apoptotic. They are located in the outer mitochondrial membrane ,the nuclear envelope and the endoplasmic reticulum of cells. The relative concentration of pro- and antiapoptotic members will decide the outcome of a cell challenged with an apoptotic stimulus. Their expression is regulated by the tumor suppressor gene.16

Increased expression of bcl-2 is seen in dysplastic lesions and oral squamous cell carcinoma. The preferential expression of bcl-2 in human pre-malignant and malignant oral keratinocytes is thought to protect cells from apoptosis and render them susceptible to mutations and tumor progression. Upregulation of bcl-2 in endothelial cells in oral squamous cell carcinoma is shown to enhance tumour angiogenesis and accelerated tumor growth. Expression of bcl-2 in tumor cells around the vasculature of tumors was found to be an indicator of poor prognosis.14

The proapoptotic protein Bax was found in all layers of normal and dysplastic oral epithelia and in oral squamous cell carcinoma. Its level decreases with tumor differentiation and low Bax expression correlated with poor prognosis in oral squamous cell carcinomas.

Since protein-protein interactions among members of the Bcl-2 family regulate apoptosis ,the Bcl-2/Bax expression ratio could be a useful marker of the outcome of an apoptotic stimulus . A decreased Bcl-2/Bax ratio was associated with increased apoptosis and proliferation in oral epithelial dysplasia and tumor differentiation in oral squamous cell carcinoma. A low Bcl/BAx ratio was found to be an indicator of favorable prognosis in squamous cell carcinoma of the tongue and is also useful to assess the response of tumor cells to chemotherapy.

Other members of the Bcl-2 family like Bag-1 an antiapoptotic protein has been shown to predict metastatic potential and prognosis of oral squamous cell carcinoma. 12,13,18 Gene therapy by introduction of an antiBcl-2 ribozyme was able to induce apoptosis in oral cancer cells. 5
TNF receptor /Fas ligand family:

It is a group of type I membrane proteins characterized by a repeating extracellular cysteine rich motifs. Resistance to Fas induced apoptosis has been shown to promote tumorigenesis in T-lymphocytes. In some solid tumors like hepatocellular carcinoma FasL expression is believed to confer privilege from attack by the immune system. T cells that normally kill tumor cells undergo apoptotic cell death on encountering FasL expressing cells. Decreased Fas and increased FasL expression has been reported in other carcinomas including esophageal and oral carcinomas. The role of Cd 40 and loss of polarized CD 40L in oral squamous cell carcinoma suggests that they may have a role in tumor progression. 9,15.

p53:
Stimuli such as DNA damage, hypoxia, heat shock and metabolic changes activate the p53 protein to induce cell cycle arrest, senescence or apoptosis. Loss of p53 function occurs in a high proportion of human malignancy .Studies indicate the suprabasal expression of p53 protein in dysplasias and the detections of mutations in normal tissues adjacent to tumors may indicate an increased risk of developing cancer. p53 mutations have been detected in more than 60% of oral squamous cell carcinomas. p53 mutations occur more frequently in aggressive tumors and are often associated with poor survival.

p53 increases the transcription of pro-apoptotic genes such as BAX. Lack of p53 activity caused by mutations in p53 or alterations in INK4a and MDM2 decreases transcription of the proapoptotic gene BAX, reduces the apoptotic activity and reduces the response to chemotherapy. Bax expression is required for the p53 induced apoptotic response. BID another proapoptotic member of the BCL-2 family is also regulated by p53 and might enhance cell death in response to chemotherapy. 10,11

Inactivation of p53 function provides selective advantage for clonal expansion of preneoplastic and neoplastic cells. Therefore reactivation of p53 function in tumors has obvious therapeutic potential. Advances in molecular biology have allowed development of novel therapeutic strategies such as in vitro introduction of wild type p53 via an adenoviral vector into head and neck squamous cell carcinoma cells. 3,4,6

Conclusion :
Apoptosis plays an important role in the removal of aberrant cells that might otherwise cause the development of tumors. The relationship between cell growth and cell death in cancer will predict the growth rate of the tumor. In premalignant and malignant oral lesions apoptosis is increased and major cell regulatory pathways including FAs,CD40,Bcl-2,Bax,p16 and p53 have been found to be dysregulated and they serve to be valuable as tumor markers in oral cancer.

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