Melanoma differentiation associated gene-7 (MDA-7), also known as interleukin (IL)-24, is an intriguing member of the class II/IL-10 cytokine family [1]. This novel tumour suppressor gene was initially identified from human melanoma cells [2, 3]. Mapped within the IL-10 family cytokine cluster to 1q32.2-q41, the gene encodes a protein consisting of 206 amino acids, secreted in mature form as a 35-40 kDa phosphorylated glycoprotein [4, 5]. MDA-7 is expressed by diverse cell types, including: B cells, Nk cells, dendritic cells, monocytes and melanocytes. Although its physiological role is poorly understood, forced expression of MDA-7 in cancer cells results in irreversible growth inhibition, reversal of the malignant phenotype and terminal differentiation [6]. Further in-vitro and in-vivo studies have demonstrated these attributes to be tumour-selective and applicable to numerous solid malignancies. Many human cancer derived cell lines, including: prostate, breast, cervical, lung, fibrosarcoma, colorectal, melanoma, and glioblastoma, undergo apoptosis when exposed to MDA-7 [7–12]. Interestingly, similar effects are not apparent following transduction into their non-malignant counterparts [13]. Specific anti-tumour activity has also been established in a range of human tumour xenograft models and recently in several early-phase clinical trials involving patients with advanced solid cancers [10, 11, 14, 15]. MDA-7 is emerging as a differentiation, growth and apoptosis associated gene with potential utility for the gene-based therapy of several human cancers [4].

However, the mechanisms through which MDA-7 expression exerts its anti-neoplastic activity, tumour-specificity and efficacy across a spectrum of human cancers have yet to be fully elucidated. Akin to other cytokines, secreted MDA-7 operates via its cell surface receptor complex, involving the IL-20R1/IL-20R2 or IL-22R1/IL20R2 hetero-dimers [16, 17]. Although receptor activation is associated with the Janus activated kinase (JAK)/signal transducers and activators of transcription (STAT) signalling, specific tumour suppressor function may not be entirely dependent upon these pathways [18]. Indeed, selective anti-tumour activity is believed to be exerted through both secretory (extra-cellular) and non-secretory (intra-cellular) pathways [19, 20]. Intra-cellular activity is thought to be mediated in a cell surface receptor-independent manner, involving endoplasmic reticulum (ER) signalling, as evidenced by localisation studies, interaction with resident ER chaperones and the induction of particular genes associated with the ER stress response [13, 21–23]

In this study, the biological impact of MDA-7 on the behavior of breast cancer (BC) cells is evaluated. Furthermore, mRNA expression of MDA-7 is assessed in a cohort of women with BC. Transcript levels are compared with normal background tissues and evaluated against established pathological parameters and clinical outcome over a 10 year follow-up period.

The present study adds to the literature in support of the tumour suppressor function of MDA-7 in solid human malignancies, with particular reference to BC. Furthermore, this study is the first to quantitatively evaluate MDA-7 mRNA expression in a large cohort of BC patients and provide correlation with conventional pathological parameters and clinical outcomes over an extended follow-up period. MDA-7 expression was found to be substantially reduced in malignant breast tissue and low transcript levels were significantly associated with unfavourable pathological parameters, including nodal positivity; and adverse clinical outcomes including poor prognosis and shorter disease free survival. Despite the inferences drawn, the mechanisms through which MDA-7 expression exerts its tumour-specificity, anti-neoplastic activity and efficacy across a range of human cancers have yet to be fully elucidated and will undoubtedly be necessary to optimise potential therapeutic applications.

Several in-vitro studies, including the current study, have employed rh-MDA-7 for exposure testing in various experiments, however MDA-7 delivery can also be achieved via a non-replicating adenovirus, referred to as adenoviral-mediated gene transfer (Ad-MDA-7) or adenoviral-induced expression. Ni et al. [27] have also investigated the effects of MDA-7 on the growth and apoptosis of human BC cell lines. In contrast to the results of the present study which found MDA-7 to have only a marginal impact on tumour cell growth, their in-vitro transfection studies demonstrated a clear dose and time-dependent inhibition of cell growth in addition to enhanced apoptosis [27]. It is possible that differences in the method of MDA-7 delivery, in-vitro assay and cell lines employed could explain some of the discrepancies in efficacy noted between studies. MDA-7 is believed to be associated with the induction of key molecules, altering the balance between pro- and anti-apoptotic proteins which mediate growth inhibition and apoptosis in several tumour types. Mechanistic studies undertaken by Sauane et al. [5] have confirmed the specificity of MDA-7 for transformed cells and demonstrate localisation to the ER with the induction of sustained stress as evidenced by expression of ER stress markers. Apoptosis was found to be mediated through JAK/STAT independent and p38-mitogen-activated protein kinase (MAPK)-dependent pathways [5]. In keeping with this, Sarkar et al. [23] found that Ad-MDA-7 acts in a tumour-specific manner, resulting in the induction of various growth arrest and DNA damage inducible (GADD) genes, via p38-MAPK activation, associated with the stress response and ER stress pathways [5, 23]. Gupta et al. [21] have also shown the importance of MDA-7 interaction with the ER resident chaperone BiP/GRP78 for induction of ER stress signals and subsequent activation of p38-MAPK and GADD gene expression which culminate in apoptosis. Lebedeva et al. [28] have demonstrated that these effects are not present in normal and non-malignant cells. Another interesting observation with mechanistic implications comes from Suh et al. [29], who have combined Ad-MDA-7 with a selective cyclo-oxygenase (COX)-2 inhibitor and identified synergistic in-vitro tumouricidal activity against BC cell lines. In addition to the aforementioned studies evaluating cell growth and survival, the present study demonstrated MDA-7 to have a profound inhibitory effect on the motility and migration of BC cells in-vitro. It would appear that MDA-7 has a regulatory role in cell motility with the ability to modulate and influence the pathways involved with implications for both local invasion and metastatic potential.

In-vivo, MDA-7 mediated tumour specific apoptosis has been demonstrated to be supplemented by an additional 'anti-tumour bystander' effect, with the former 'intra-cellular killing' being receptor-independent and the latter dependent on MDA-7 secretion and canonical IL-20/IL-22 receptor complexes on the surface of target cells [14, 15, 19, 22]. In-vivo studies involving the inoculation of nude mice with human BC cell lines have demonstrated significant growth inhibition following MDA-7 injection [27]. Sarkar et al. [30] have devised a dual cancer-specific targeting strategy by constructing a conditionally replication competent adenovirus which demonstrates tumour-specific virus replication, in addition to the expression of MDA-7. Their results are encouraging, with the complete eradication of both primary and distant disease in human BC xenografts in nude mice [30]. MDA-7 has also been shown to influence endothelial cells, exerting an potentially anti-angiogenic effect within the tumour vasculature [31]. Chada et al. [32] found Ad-MDA-7 to mediate p53-independent inhibition of tumour growth, cell cycle arrest and apoptosis, associated with down-regulation of Bcl-2 and Akt. In-vivo, growth inhibition was demonstrated in multiple xenograft models. Furthermore, Ad-MDA-7 was demonstrated to have an additive or synergistic effect in both cellular and animal studies when combined with chemotherapy, biologic therapies and radiotherapy. These effects were associated with decreased Bcl-2 expression and BAX up-regulation [32]. Bocangel et al. [33] evaluated the treatment of a panel of Her-2/neu over-expressing cell lines and nude mice tumours with a combination of Ad-MDA-7 gene transfer and Trastuzumab/Herceptin, the anti-human epidermal growth factor receptor-2 (Her-2) monoclonal antibody. The study demonstrated synergistic tumour suppression with increased cell death, cell cycle block and apoptosis [33]. Their study is supported by that of McKenzie et al. [34].

Hence, in addition to utility as a prognostic marker, MDA-7 appears to offer significant therapeutic potential, enticing translational researchers with the prospect of tumour-specificity and efficacy against a range of solid human cancers. Indeed, therapeutic potential has been recently evaluated. The safety and efficacy of Ad-MDA-7 has been demonstrated in phase-1 clinical trials of intra-tumoural injection into several solid cancers, including melanoma [10, 11, 14, 15]. Albeit premature, MDA-7 has already been referred to by some authors as the "magic bullet for cancer" and "cancer's Achilles' heel" [8, 10]. However, further mechanistic studies are imperative to fully unlock and optimise future therapeutic applications.

Limitations of the present study include the single BC cell line evaluated and single method of MDA-7 delivery employed. In addition to motility, migration and growth, in-vitro studies could be extended to include assays of adhesion, invasiveness and apoptosis. Immuno-fluorescence could also be employed to evaluate changes in cyto-skeletal arrangement after treatment with rh-MDA-7. The use of background parenchyma from BC patients to provide 'normal tissue' for comparison is also contentious. Ideally, such material should be derived from patients without BC in order to avoid any 'field change' which may exist within cancer bearing tissues. Although the sample size and follow-up period were substantial, it is possible that a larger cohort, particularly with regard to subgroup analysis, may have influenced several results which approached, but failed to reach, statistical significance. In addition to the measurement of mRNA transcript levels, quantitative analysis of protein expression should be undertaken to ensure concordance. Correlation with associated molecules and other markers of invasiveness and metastatic competence would also have been of value.

MDA-7 significantly inhibits the motility and migration of human BC cells in-vitro. MDA-7 expression is substantially reduced in malignant breast tissue and low transcript levels are significantly associated with unfavourable pathological parameters, including nodal positivity; and adverse clinical outcomes including poor prognosis and shorter disease free survival. In addition to its prognostic utility, further mechanistic studies are warranted to explore the potential for therapeutic manipulation in human breast cancer.

The authors declare that they have no competing interests.