Table of Contents  
REVIEW ARTICLE
Year : 2017  |  Volume : 9  |  Issue : 2  |  Page : 71-74

The expression of Bcl-2 in oral squamous cell carcinoma—A review


Department of Oral Pathology and Microbiology, Thai Moogambigai Dental College and Hospital, Chennai, India

Date of Web Publication8-Jan-2018

Correspondence Address:
Dr. Jayakumar Arumugam
Jeyam Dental Clinic, 45 B6, Salem Main Road, Gandhinagar, Edappadi-637105
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jofs.jofs_88_16

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  Abstract 


Currently, oral cancer is one of the most alarming health problems facing mankind. More than 90% of all oral cancers are oral squamous cell carcinoma (OSCC). BCL-2 proteins are one of the most prominent anti-apoptotic proteins expressed in OSCC. They contribute to cancer development and mediate resistance to current anticancer treatments. Various studies have been conducted to evaluate the expression of Bcl-2 in OSCC. This article emphasizes the importance of Bcl-2 expression as a prognostic indicator in OSCC.

Keywords: apoptosis, Bcl-2, carcinogenesis, OSCC


How to cite this article:
Arumugam J, Jeddy N, Ramamurthy A, Thangavelu R. The expression of Bcl-2 in oral squamous cell carcinoma—A review. J Orofac Sci 2017;9:71-4

How to cite this URL:
Arumugam J, Jeddy N, Ramamurthy A, Thangavelu R. The expression of Bcl-2 in oral squamous cell carcinoma—A review. J Orofac Sci [serial online] 2017 [cited 2018 Oct 23];9:71-4. Available from: http://www.jofs.in/text.asp?2017/9/2/71/222391




  Introduction Top


Currently, oral cancer is one of the most alarming health problems facing mankind in terms of morbidity and mortality. It is the sixth most common type of cancer worldwide. Oral cancer accounts for 4% of all malignancies in men and 2% in women.[1] More than 90% of all oral cancers are oral squamous cell carcinoma (OSCC),[2] which is often preceded by a premalignant lesion. According to the World Health Organization, OSCC is the eighth most common type of cancer worldwide, with geographical variations.[3] The major etiological factors in the genesis of oral carcinoma constitute tobacco chewing/smoking, alcohol consumption, and human papilloma viruses.[1] A diet low in fresh fruits and vegetables has also recently been implicated in the etiopathogenesis of OSCC.[2] Overall incidence and mortality attributed to OSCC is increasing, with the current estimates of age-standardized incidence and mortality being 6.6/100,000 and 3.1/100,000 in men and 2.9/100,000 and 1.4/100,000 in women, respectively.[4]

Cancer is caused by the accumulation of genetic and epigenetic mutations in the genes that normally play a role in the regulation of cell proliferation, thus leading to uncontrolled cell growth. The genes involved in tumorigenesis include those whose products (1) directly regulate cell proliferation (either promoting or inhibiting), (2) control programmed cell death or apoptosis, and (3) are involved in the repair of damaged DNA. Depending on how they affect each process, these genes are named as tumor suppressor genes (growth inhibitory), proto-oncogenes (growth promoting), or anti-apoptotic genes (inhibits apoptosis).[1]

B-cell lymphoma/leukemia-2 (Bcl-2) is an anti-apoptotic protein that interacts with and is regulated by p53. It is a part of the regulatory system that controls the cell cycle and the induction of apoptosis. Bcl-2 gene family and related proteins form the core of the apoptotic program and the major effector arm of the cell death program. Bcl-2 gene was first discovered in follicular non-Hodgkin’s B-cell lymphoma. In this translocation, the bcl-2 gene is moved from its normal chromosomal location at 18q21 in to proximity with powerful enhanced elements in the immunoglobulin heavy chain (IgH) locus at 14q32. This results in the deregulation of the translocated bcl-2 gene and the overproduction of bcl-2 mRNAs and their encoded proteins.[5] The Bcl-2 gene family consists of the following two opposing groups of proteins: proapoptotic (death agonists) and anti-apoptotic proteins (death antagonists), which are listed in [Table 1].[6]
Table 1: List of proapoptotic and anti-apoptotic genes

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The level of Bcl-2 protects the cells from undergoing apoptosis, and its level should be higher than the level of the death inducer, bax. If, on the other hand, bax is in excess, the opposite effect occurs, and the cells become susceptible to apoptosis. The BH3 domain of death agonists such as bax and Bak is required for these proteins to heterodimerize with bcl-2 and bcl-xL to promote apoptosis. The overexpression of those proteins containing only the BH3 domain is sufficient for interacting with and inhibiting the function of Bcl-2. Thus, this class of proteins functions as the dominant negative inhibitors of cell death.[7]

Mechanism of Action of the Bcl-2 Protein

The mechanism by which Bcl-2 inhibits apoptosis is still uncertain. Bcl-2 has been localized to the areas of contact between the outer and the inner mitochondrial membranes, suggesting that bcl-2 may protect the cell from apoptosis by altering mitochondrial function. bcl-xL and bcl-x′ were identified in humans. Bcl-xL, such as Bcl-2, inhibits apoptosis. Bcl-x′ is a dominant negative repressor of bcl-2 and enhances apoptotic signals in the cells that express bcl-2. The mitochondrial permeability transition (the depolarization of the transmembrane potential) can cause the outflow of mitochondrial proteins (e.g., apoptosis-inducing factor and cytochrome c) into the cytosol and activate specific apoptogenic proteases such as caspase-3. The activation of this caspase is dependent on the presence of cytochrome c in the cytosol and results in subsequent DNA fragmentation. Therefore, both bcl-2 and bcl-xL may prevent apoptosis by inhibiting cytochrome c translocation and, thus, preventing caspase activation and the subsequent downstream events in the apoptotic process.[8],[9],[10]

Bcl2 and Cancer Development

Unlike most other oncogenes, which promote cell cycle progression, the overexpression of the bcl-2 oncogene was found to protect the cells from undergoing apoptosis in response to a number of stimuli. The overexpression of the death agonist bax or related genes in the cancer cells is able to counteract the survival effect of bcl-2 and induce apoptosis by eliminating the effects of the survival proteins.[11] Bcl-2 protein expression has been observed in several epithelial stem cells but not in more differentiated cells. This suggests that Bcl-2 may also have a protective function in the stem cells. Bcl-x is more highly and widely expressed in the malignant keratinocytes of the skin and oral mucosa. An increased expression of Bcl-2 has also been observed in precancerous lesions.[12] Two proteins regulated by P53 are bcl-2 and bax. Deficiency in p53 has been found to result in the elevated levels of bcl-2 protein and the reduced levels of bax in several tissues. This may permit a population of the genetically damaged cells to escape the normal process of apoptotic deletion. The loss of p53 function may predispose the cells to go for further genomic instability and lead to gene amplification.[13] The activation of bCl-2 through inactivation or the loss of p53 might also enhance the growth of the metastatic tumor cells. The expression levels of bcl-2 and bcl-xL are often markedly elevated in invading and metastatic oral squamous carcinomas. The earliest step in the carcinogenic process involves the emergence of the "initiated" cell populations that are genetically altered. The enhanced expression of bcl-2 and/or bcl-xL in the human premalignant and malignant oral keratinocytes suggests the development of oral cancer.[8]

Bcl-2 in Oral Squamous Cell Carcinoma

Bcl-2 expression appears to be altered to varying degrees in oral mucosal dysplasia. The recent retrospective cross-sectional studies have suggested that the upregulation of bcl-2 may be an early event in epithelial carcinogenesis.[8] The carcinogenesis of OSCC is a multistage process involving the activation of oncogenes and the inactivation of tumor suppressor genes with an imbalance of cell death and growth. Although considerable interest has recently been focused on the identification of the regulator of apoptosis, that may be a potential and key influence on the balance of cell death and cell growth in cancer.[14] Various studies have been conducted on the oral tissues to study the expression of bcl2 in oral dysplastic lesions.

Coutinho-Camillo et al. characterized the expression of proteins that inhibit (Bcl-2, Bcl-x, Bcl-xL, Bcl-2-related protein A1, and BAG-1) or promote (Bak, Bax, Bim/Bod, Bim-Long, Bad, Bid, and PUMA) apoptosis and determine possible correlations between the expression of these proteins and the clinicopathological features of OSCC using immunohistochemistry. Their results demonstrated that the increased expression of the anti-apoptotic proteins Bcl-xL and Bcl-2-related protein was associated with the absence of vascular invasion (P = 0.0003 and P = 0.0010, respectively). The increased expression of Bcl-2-related protein was associated with the tumor occurring in the floor of the mouth (P = 0.0152, when compared with other sites and not with oral tongue). The increased expression of Bcl-2-related protein was also associated with early-stage tumors (stages I and II) (P = 0.0509). As their study showed the involvement of the Bcl-2 family of proteins in OSCC tumorigenesis, they suggested that the expression of apoptotic molecules might be used as a prognostic indicator for OSCC.[3] Sulkowska et al. estimated possible correlations between the Bcl-2 protein expression and some clinicopathological features of OSCC. The results of their study showed statistically significant positive correlations between Bcl-2 expression in the tumor cells and higher tumor grading (P < 0.005), higher tumor mitotic index (P < 0.005), a higher index of atypical mitoses (P < 0.001), as well as a microfocal pattern of tumor invasive margin (P < 0.001). Therefore, they concluded that positive Bcl-2 expression may be a valuable factor supplementing the established unfavorable histopathological features of OSCC.[15] Popović et al. estimated the level of overexpression of bcl-2 proteins in OSCC using immunohistochemistry. The results showed a low percentage of positively stained Bcl-2 cells and concluded that the level of Bcl-2 expression could be a valuable predictor of tumor behavior and disease outcome.[16]

Juneja et al. did an immunohistochemical (IHC) study to evaluate and compare the expression of Bcl‑2 protein in oral epithelial dysplasia and OSCC using a monoclonal antibody against anti-human Bcl‑2 oncoprotein. For Bcl-2, 26.7% positivity was observed in oral epithelial dysplasia, and 30% in OSCC. They concluded that the altered expression of Bcl‑2 may be an early molecular event, which leads to prolonged cell survival, an increased chance of accumulation of genetic alterations, and subsequent increase in malignant transformation potential.[14] A study by Arul et al. evaluated correlated the expressions of anti-apoptotic marker Bcl-2 and proliferative marker MIB-1 in varying grades of OSCC using immunohistochemistry. Anti-apoptosis was found dominant in well-differentiated lesions than in moderately and poorly differentiated lesions. On the other hand, cell proliferation dominated in poorly differentiated lesions than in moderately and well-differentiated lesions. They also found a significant correlation between Bcl-2 and MIB-1 in poorly differentiated OSCC.[17] In contrast, Rahmani et al. examined the impact of PTEN (phosphatase and tensin homolog deleted on chromosome TEN—a tumor suppressor gene mutated in a variety of human cancers including the prostate, breast, and brain) and Bcl2 in the genesis of OSCC by the use of anti-PTEN and anti-bcl2 antibody through immunohistochemistry. The expression of bcl2 was found to be restricted to the tumor cells in well- and moderately differentiated tumors. The intense expression of Bcl2 was observed throughout the tumor cell in poorly differentiated tumors. The overexpression of Bcl2 and the loss of PTEN expression were correlated to poor differentiation, lymph node involvement, and late stages. Thus, they have concluded that the alteration of PTEN and Bcl2 is likely an important molecular event in the pathogenesis and carcinogenesis of oral carcinoma.[1] Thomas and Sethupathy investigated the expression of the apoptosis-related proteins Bax, Bcl-2, and p53 in OSCC to explore the possible relationship among these apoptotic markers in oral carcinogenesis. They have observed the overexpression of p53 and bcl2 and the decreased expression of bax in patients with OSCC. They concluded that apoptotic mechanism can be accounted for oral carcinogenesis.[18]

In contrast to the aforementioned studies, the role of Bcl-2 in Oral potentially malignant disorders (oral leukoplakia, OSMF and OLP) and in OSCC was studied by Sudha and Hemavathy using IHC staining. The results of positive topographic expression between OSCC and potentially malignant disorders were statistically significant (P value—0.0144). As a definite number of cases showed bcl-2 overexpression in their study, they concluded that the role of Bcl-2 in the development and progression of oral neoplasia needs further investigation along with other oncogenes.[19] A study by Suri evaluated the expression, as well as quantified and determined the intensity and the pattern of Bcl-2 in various histological grades of OSCC using immunohistochemistry. The number of cells expressing Bcl-2 increased from well-differentiated to poorly differentiated OSCCs, showing an inverse relationship with the degree of differentiation. Thus, a conclusion was made that further correlative studies using markers for other members of the Bcl-2 family are necessary to elucidate the specific molecular defects critical to the life of this tumor, which will have an impact on its diagnosis and treatment.[20]

Loro et al. investigated whether the loss of bcl2 in oral epithelial dysplasia and OSCC, and of BAX in poorly differentiated OSCC, could be attributed to bcl-2 and bax mutations using immunohistochemistry and in-situ hybridization. There were no mutations found that could explain the loss of bcl2 during oral dysplasia and carcinoma. Thus, they concluded that the downregulation of Bcl2 in OED and OSCC may be the result of transcriptional regulation.[21] Gibson et al. examined the effect of anti-Bcl-2 ribozyme on the phenotype of oral cancer cells, and it was found that anti-Bcl-2 activity was effective in the treatment of oral cancer, because the expression of Bcl-2 oncogene is necessary for tumor growth.[22]


  Conclusion Top


BCL-2 proteins are one of the most prominent anti-apoptotic proteins expressed in OSCC. Most of the studies suggest that the expression of Bcl-2 might be used as a prognostic indicator for OSCC. They contribute to cancer development and mediate resistance to current anticancer treatments. Several promising inhibitors of BCL-2 proteins have been developed in recent years, and it is very important to utilize these compounds in the treatment of OSCC. Therefore, in future, BCL-2 inhibitors should be included in personalized cancer treatments for those tumors that express addiction to BCL-2 proteins.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Sulkowska M, Famulski W, Sulkowski S, Reszeć J, Koda M, Baltaziak M et al. Correlation between Bcl-2 protein expression and some clinicopathological features of oral squamous cell carcinoma. Pol J Pathol 2003;54:49-52.  Back to cited text no. 15
    
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Popović B, Jekić B, Novaković I, Luković LJ, Tepavcević Z, Jurisić V et al. Bcl-2 expression in oral squamous cell carcinoma. Ann N Y Acad Sci 2007;1095:19-25.  Back to cited text no. 16
    
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Arul AS, Solomon RD, Arul AS, Santhil VS. Immunohistochemical evaluation of Bcl-2 and Ki-67 in varying grades of oral squamous cell carcinoma. J Sci Ind Res 2011;70:923-8.  Back to cited text no. 17
    
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