C-Jun Transcription Factor Oncogenic Activation in Oral Carcinoma

Abstract

Introduction:

Oral carcinogenetic is based on a variety of genomic imbalances (gross chromosome or specific gene alterations) that drive the normal oral mucosa to its neoplastic/dysplastic epithelial form and finally to a totally malignant tissue transformation. In this multi-step procedure, down-regulation of suppressor genes combined with overactivation of oncogenes are two crucial and partially early genetic events involved in the onset and progression of neoplastic/malignant epithelia transformation. More specifically, deregulation of strong transcription factors negatively affects the normal expression of a broad spectrum of genes that are involved in cell proliferation and signalling transduction to the nucleus.

Objective:

The purpose of the current molecular review was to explore the c-Jun (chromosome location: 1p32-p31) transcription factor transformation mechanisms to oncogene in oral squamous cell carcinoma (OSCC).

Material and method:

A systematic review of the literature was carried out by searching in PubMed international database. The year 2010 was set as a prominent time limit for the publication date of the articles in the majority of them, whereas specific references of great importance and historical value in the field of the c-Jun gene discovery and analysis were also included. The following keywords were used: c-Jun, oncogene, signaling pathway, oral, carcinoma, transcription. A pool of 45 important articles were selected for the present study at the basis of combining molecular knowledge with new targeted therapeutic strategies.

Results:

C-Jun – as a part of the c-Jun/c-Fos transcription factors’ complex –critically regulates the expression levels in a variety of genes inside the cellular microenvironment. A broad spectrum of malignancies, including OSCC, demonstrate c-Jun alterations driving the gene to its oncogenic phenotype. Interestingly, c-Jun oncogenic activation is mediated by high-risk human papilloma virus (HR-HPV) persistent infection in significant subsets of these malignancies.

Conclusions:

C-Jun was the first oncogene – acting as a strong transcription factor – that was discovered and cloned 35 years ago. C-Jun is the living history of oncogenes and its discovery marks a significant step in the evolution of molecular biology.

Keywords:c-Jun, oncogene, signaling pathway, oral, carcinoma, transcription.

INTRODUCTION

Head and neck squamous cell carcinomas (HNSCC) represent a superfamily of pathological entities characterized by specific etio-pathogenetic characteristics (1-4). Concerning oral cavity, squamous cell carcinoma (SCC) is the prominent malignant sub-type. In fact, oral squamous cell carcinoma (OSCC) de­monstrate an aggressive phenotype due to their increased tendency to develop local and distant metastases combined with lymph node metastases (5). Specific altered mechanisms that affect negatively signaling transduction pathways, such as Notch, are involved (6). Extensive molecular analyses have shown that gross chromosome instability (CI-polysomy/aneuploidy) and specific gene alterations (amplification, deletion, point mutations) or epigenetic (aberrant promoter methylation, microRNAs deregulation) are implicated in the development and progression of solid malignancies, including OSCC (7-9). Oral squamous cell carcinoma demonstrate increased rates in individuals who are exposed to chronic irritating factors, including tobacco and alcohol consumption and also viral mediated deregulation (10, 11). Concerning viral oncogenic activity, persistent human papilloma virus (HPV) infection is responsible for malignant transformation of the corresponding oral mucosa (12, 13). Leukoplakia, including the rare proliferative verrucous aspect (PVL), represents the main dysplastic pre-malignant lesion in the oral epithelial carcinogenesis process (14). In fact, PVL is characte­rized by focal – at early stages – or adherent multifocal white, verruciform proliferative epithelia.

Among the genes that are involved in OSCC onset, development and progression, overactivated proto-oncogenes play a significant role in the regulation of signal transduction pathways. They not only induce cell proliferation by modifying and altering nuclear microenvironment, but are also implicated in the metastatic process (epithelial-mesenchymal transition phenomenon-EMT) (15-17). In fact, deregulation of transcription factors negatively affects the normal expression of a broad spectrum of other genes (18).

C-Fos/c-Jungenes form an important protein complex that influences intracellular signal transduction to the nucleus and modifies the expression of other genes such as the RAS family (19).C-Junand C-Fos proteins are implicated in critical cell functions, including differentiation, proliferation, survival and also tissue homeostasis affected by hypoxia and angiogenesis.C-Junis a proto-oncogene representing the human homolog of the viral oncoprotein v-jun (gene locus: 1p32-p31). It was the first oncogene discovered 35 years ago which triggered an evolution in molecular biology (20, 21). The gene encodes for a 65 kDa protein (p39) and is forming a heterodimer with another strong transcription factor, C-Fos, which results in the formation of AP-1 (Activator Protein-1) complex (Figure 1). The JUN protein complex comprisesC-Jun, JUN B and JUN D, respectively (22-24). In fact,C-Junwas the first pure oncogenic transcription factor that was discovered in the series of them (25). It is the homolog of the viral oncoprotein v-jun. V-jun was initially discovered in the avian sarcoma virus 17 (26). In normal cells,C-Junprotein is implicated in important functions, including proliferation, apoptosis, survival and tissue morphogenesis. Furthermore, the protein interacts with signal transduction pathways (27). Interestingly, the gene region on chromosome 1 that hosts the gene is frequently the target of translocations and deletions (breakpoints) in solid malignancies, including OSCC, which are detected by implementing karyotype-based analysis (28).

C-JUNONCOGENIC ACTIVATIONIN OSCC

In the current review, we focused on the role ofC-Junin OSCC because in this type of carcinoma there are limited but also interesting new data. It is well known thatC-Junis the living history of oncogenes and its discovery maeks a si­g­nificant step in the evolution of molecular bio­logy. Recently published molecular studies detected an overactivation ofC-Jun/C-Fosgenes complex in invasive parts – compared to adjacent non-malignant epithelia – of HNSCC in different anatomic regions including OSCC. Referring also to HPV-mediated carcinogenesis in oral mucosa, some studies have already detected an overactivation of strong transcriptional factors (29). A combination of nuclear and peri-nuclear cytoplasmic diffuse immunostaining was observed especially in cases demonstrated lymph node metastasis implicating also CD44-dependent signal transduction pathway (30). In fact,C-Junprotein phosphorylation/nuclear translocation and CD44 overexpression are combined with co-localization of HPV 16 E7 oncoprotein in a specific SCC cell line analysis. The study group also reported that targeting HA/CD44-mediatedC-Junsignaling by applying HPV16 E6-specific small interfering RNAs positively affected cisplatin chemosensitivity in HPV16E6-positive OPSCC cells. Based on data extracted by protein expression analysis,C-Junin normal, dysplastic and neoplastic/malignant oral mucosa was found to be progressively overactivated (31). This is indirect evidence ofC-Junderegulation in early stages of oral carcinogenesis. Additionally, specific gene signatures in OSSC – includingC-Jun– seem to be associated to an aggressive phenotype in subgroups of patients. Another genetic analysis shows that the CCND1/C-Jun/SPP1 complex deregulation leads to an aggressive OSCC phenotype characterized by extended lymph node metastasis (32). Furthermore, HPV16 E6/E7 oncogenic activation drives NF-κB to abnormal expression in subgroups of OSCC combined with p50 overexpression (33). Besides HR-HPV persistent infection, tobacco systematic consumption is another genotoxic factor involved in OSCC development that induces AP-1 (C-Jun/C-Fossystem) and promotes neoplasm angiogenesis in para­l­­-lel (34).

C-JUNINHIBITION MOLECULESIN OSCC

In modern oncology, development and implementation of novel mono- or multi-agent-based regimens are depended on specific gene signatures detected in the corresponding patients (35). Concerning theC-Juntargeted inhibition strategies, disruption of mitogen-activated protein kinase (MAPK)-mediated signaling transduction seems to be of main importance for subgroups of OSCC patients with specific genetic signatures (36). Similarly, the mitochondrial and also JNK signaling pathways are activated by applying lycorine hydrochloride that induces the reactive oxygen species-dependent apoptosis (37). Moreover, JunB is another member of the Jun family of transcription factors that promotes transcription of IL-8 transcription in OSCC providing another critical interaction between Jun genes and inflammatory proteins such as cytokines (38). Additionally,C-Junprotein seems to overreact with another significant complex, the Wnt/β-Catenin signaling transduction pathway in OSCC leading to more aggressive phenotypes and biological behavior (39). Interestingly, significant metabolic disorders (i.e., diabetes) seem to enhance the activation of Ras/Raf/MAPK signal transduction pathway and partially the expression of transcriptional factors includingC-Jun(40). Based on a c-DNAs platform, a genetic study showed that specific molecules such as the janus kinase (JNK) inhibitor SP600125 could play a critical role in the inhibition ofC-Junaberrant expression in a variety of malignancies (41). This is a representative paradigm of a targeted-gene therapeutic strategy in modern molecular onco­logy. Similarly, alpha-linolenic acid (ALA) seems to demonstrate anti-cancer activity in oral carcinoma by enhancing JNK phosphorylation andC-Junnuclear overexpression. The latter is strongly accumulated in cancer cells and increa­ses the apoptotic rates of malignant cells in conjunction with the induction of FasL/caspase8/cas­pase3/caspase9 interactive pathways (42).

CONCLUSION

In conclusion,C-Junis the representative 35-year-old living history of oncogenes and its discovery is a significant step in the evolution of molecular biology (43, 44). Overactivation of theC-Junoncogene is a frequent and crucial genetic event in OSCC, which is involved not only in the initial development of malignancies but also in their progression. Combined with the up-regulation of other oncogenes and down-regulation of suppressor genes, activatedC-Junnegatively affects the biological behavior of neoplasm in patients with specific genetic signatures. Novel agents that reduce the corresponding oncoprotein production levels inhibiting their activity should be a very promising approach for ap­plying targeted therapeutic strategies in a selec­ted group of OSCC patients. In fact, evolution of novel genomics, proteomics and radio-genomics provide new improved tools for handing HNSSC patients, including OSCC ones, at the basis of their unique clinic-molecular characteristics (45).

Authors’ contributions: conception and design – NM and ET; drafting the article – ET, AC, VP, DR, AA, SD, AA, SM, LM, PF, PP; critical revision for important intellectual content – NM, SP; approval of the final version of the manuscript – VR, EK, PS.

Conflicts of interest: none declared.

Financial support: none declared.

C-JUNin Oral Carcinoma

FIGURE 1.

Ideogram of c-JUN/c-FOS complex. Note the loci of these genes – that encode for strong transcriptional factors – on chromosomes 1 and 14, respectively