The above-mentioned investigations on miRNAs implicated in breast cancers substantially advance our knowledge about the roles of specific miRNAs in breast cancer development and metastasis. It is clear that each of these miRNAs inhibit the expression of many genes, suggesting that comprehensive regulation can be achieved by antagonizing or over-expressing a single miRNA. Moreover, concomitant deregulation of these miRNAs would consequently alter the expression of many genes, thereby inducing tumorigenesis.
To better understand how the functions of these experimentally validated breast cancer miRNAs and their gene targets might be integrated within the pathogenesis of breast cancer, we have performed a gene interaction network analysis (Figure ) similar to our previous study [70
]. To do this, we generated a list of 34 genes known to be altered by the 11 miRNAs discussed above. Among these genes, 19 formed a well-connected gene interaction network in which MYC, a target of miR-34a, was the central node (10 connections). Other highly interacting genes were BCL-2
, with seven, seven, seven and six interactions, respectively.
Figure 1 Gene interaction network analysis of miRNA targets in breast cancer. Determined using Ingenuity software, the direct interaction network of 34 published targets of 11 miRNAs implicated in breast cancer pathogenesis is shown: miR-206 (grey), miR-17-5p (more ...)
These results suggest that breast cancer is associated with changes in the expression of multiple miRNAs that, in turn, disrupt a network of genes that either activate or inhibit each other's transcription or interact directly via protein-protein interactions. For instance, downregulation of miR-206 would enhance ESR1 expression, consequently increasing MYC transcription. MYC protein will be further elevated since loss of miR-34a would enhance its translation. Since CCND1, E2F1 and E2F3 are known to be activated by MYC, increased MYC would in turn enhance their expression. Moreover, down-modulation of miR-17-5p (regulates CCND1 and E2F1) and miR-34a (regulates E2F3, CCND1 and CDK6) would further elevate levels of these proteins. Our findings suggest that decreased expression of multiple tumor suppressor miRNAs leads to enhanced transcription and translation of these oncogenes in breast cancer via direct or indirect mechanisms.
The role of oncogenic or metastasis-promoting miRNAs is also important. For example, increased levels of miR-21, miR-31 and miR-373/520c would ensure that the protein synthesis of TPM1 (regulated by miR-21), CD44 (regulated by miR-373/520c), and MMP16 (regulated by miR-31) is repressed in breast cancer cells. In most cases, the protein encoded by a gene is up- or downregulated due to increased/decreased transcription and translation. Thus, tumor suppressor or oncogenic miRNAs regulate the transcription of some of these genes by targeting transcription factors and also provide an additional layer of regulation by regulating their translation. Although many of these gene regulatory events will likely occur at different stages of tumorigenesis and metastasis, it is clear that loss of miRNA regulation leads to a cascade of events that alter gene interaction networks important for breast cancer progression.
In addition to the above mentioned miRNAs, several other miRNAs, including miR-7, miR-128a, miR-210, miR-27b, miR-335, miR-126, miR-145 and miR-27a, have also been implicated in breast cancer. Uncovering the genes under the control of these miRNAs and how they integrate into the breast cancer gene interaction network will further aid our understanding of the disease. Furthermore, miRNA profiling is emerging as a powerful diagnostic tool to characterize features of different tumor types. This has been particularly useful in breast cancer, as miRNA signatures can unequivocally distinguish normal and malignant breast tissue and discriminate between breast cancer subtypes [71