Protein expression of eIF4E and integrin αvβ6 in colon cancer can predict clinical significance, reveal their correlation and imply possible mechanism of interaction

Colon cancer is one of the leading causes of cancer-related deaths worldwide. With the improvement of current living standards and environmental degradation, its morbidity has been on the rise year by year [1]. Both eIF4E and integrin αvβ6 can play important roles in the development and progression of colonic carcinoma [2–4].

Eukaryotic initiation factor 4E (eIF4E), a 25-kDa cap binding protein, delivers cellular mRNAs to the eIF4F translation initiation complex by binding the 5′-cap structure of these mRNAs [5]. Only about 10% of mRNAs with 5′UTRs of sufficient length (>200 nt) and three-dimensional steric hindrance require the assistance of the eIF4E for efficient translation [6]. eIF4E overexpression can selectively affect transport of specific transcripts and up-regulate the translation of a limited pool of mRNAs with long G-C rich 5′-UTRs and complex structure, encoding proteins involved in cellular growth, angiogenesis, survival, tumorigenesis, and metastasis,such as cyclin D1, c-myc, vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF-2), matrix metalloprotease 9 (MMP-9) [2, 7–9]. It is considered as a central regulator of malignancy. eIF4E overexpression has been demonstrated in carcinomas of breast, colon, prostate, bladder, cervix, lung, head and neck (reviewed in [9]).

Integrins are a family of heterodimeric cell-adhesion molecules composed of two non-covalently bound transmembrane subunits (18 α-subunits and 8 β-subunits reported) [10]. These proteins collectively mediate cellular adhesion to extracellular matrix (ECM) and modulate cell proliferation, migration, invasion, and survival by activating intracellular signaling pathways [11]. The β6 integrin subunit with αv, its sole binding partner, is an epithelial-cell-restricted antigen, which is reported to be expressed in epithelial cancers (lung, breast, pancreas, ovary, oropharynx and colon), as well as during tissue remodeling events (e.g., fibrosis, wound healing) [12, 13]. Integrin αvβ6 plays an pivotal role in various aspects of cancer progression [14].

Integrin-mediated signals have been known to contribute to malignancy by regulating transcription [15]. On the other side, recent studies indicated they could also selectively enhance translation initiation of protein synthesis relating to malignancy, through activation of eIF4E via signal pathway of PI-3 K/Akt/mTOR and Ras/ERK/MNK [16–18]. However, the correlation between eIF4E and integrin αvβ6 and their possible interaction at molecular level are still not clearly known. The aim of our present study was to explore the coexpression and clinical significance of eIF4E combined with integrin αvβ6 in colon cancer, which could provide a theoretical basis for further study of molecular mechanism in therapeutic intervention.

Colorectal cancer has a higher morbidity and mortality in developed countries [19]. Despite improvements made in screening and treatments during the past years, the clinical outcome of colon cancer remains unsatisfactory. Understanding the molecular mechanism of tumorigenesis would contribute to treatment of patients with this disease.

Over the past decades, the contribution of the mRNA cap-binding protein, eIF-4E, to malignant transformation and progression has been illuminated. eIF4E gene is reported to be recognized as an proto-oncogene in vivo,and the extent of eIF4E overexpression can predict cancer recurrence and outcome in head neck squamous cell carcinoma (HNSCC), breast cancer and lung cancer [9, 20]. In our present study, patients with high expression eIF4E tended to have a poorer prognosis than those with low expression. And the level of eIF4E expression had significant association with TNM stage, while P-Values of T stage, N stage and M stage respectively on eIF4E were all between 0.05 and 0.10. From this point, we could speculate that, to some extent, the expression of eIF4E was related to tumor’s metastasis and progression. In brief, the association between eIF4E expression and poor prognosis could be explained by the hypothesis of “strong and weak mRNAs”. Translation of strong mRNAs (e.g. those encoding housekeeping proteins such as beta-actin) will quickly reach a maximum in the presence of low levels of free eIF4E while weak mRNAs are poorly translated. With the increasing number of free activated eIF4E, the translation of these “weak mRNAs” relating to malignancy is disproportionately enhanced, which contributes to the malignant activities [21, 22].

The integrin αvβ6 occurs in many epithelial tumors and has been considered to play a prominent role in tumor invasion and metastasis [14]. Recently, it was reported as an independent unfavorable prognostic indicator in aggressive colonic and gastric carcinomas of humans [12, 13], which was confirmed in our study according to the Cox regression model. Similar to the study of Bates [12], our results also showed that the αvβ6 expression were significantly associated with clinical stage and the depth of invasion of tumors. These results could be explained logically by our previous studies, that αvβ6 serves to direct growth factor-activated ERK to downstream cytoplasmic targets involved in regulating cell growth, apoptosis and cytoskeletal reorganization [23, 24], and in promoting cellular migration by mediating MMP-9 secretion [25].

Integrin-dependent translational control in cancer progression has been widely investigated and recognized in recent years [16]. The mechanism involves the ability of integrins to activate the PI-3 K/Akt/mTOR and Ras-ERK-MNK pathway, which subsequently enhance eIF4E function through the inhibition of 4E-BP1 [16–18, 26] and phosphorylation of eIF4E at ser209 by MNK [27]. Released eIF4E, accompanied with eIF4G and eIF4A, is then able to engage and activate eIF4F complex, leading to translation initiation of genes associated with malignancy. The role of integrin on translation has been addressed in models requiring Integrin β3, Integrin β4, and Integrin β1 (reviewed in [16]).

In this study, we revealed that high expression of eIF4E combined with positive αvβ6 can predict a poorer prognosis. And their expressions were positively correlated in colon cancer. However, integrin αvβ6 dependent translational control hasn’t been investigated before. As a member of integrin families, integrin αvβ6 could trigger ERK pathway, leading to the activation of downstream signals [23, 24]. According to the theoretical basis mentioned above, we can logically speculate that integrin αvβ6 probably could also mediate the activation of eIF4E, leading to translation of “weak mRNA” consequently. On the other side, another bold but reasonable hypothesis is that the activated eIF4E could up-regulate the protein synthesis of Integrin β6, as the mRNA of Integrin β6 has long 5′-UTRs with rich GC, complex structure (Gene ID: 3694, from NCBI), meeting the criteria of “weak mRNAs”. Thus, a signal loop involving eIF4E andαvβ6 probably exists to play a key role in tumor cellular growth and migration. Of course, our further work is to demonstrate our hypotheses and determine whether it can be translated into real clinical benefit.

In summary, our findings indicated that the levels of eIF4E and αvβ6 expression are elevated in colonic carcinoma, which were associated with tumor progression and poor prognosis of patients with colon cancer. Additionally, the expression of eIF4E and integrin αvβ6 are moderate-poorly correlated, and with cautious speculation, they may interact with each other at molecular levels, with significant implications for therapeutic intervention.