miR-23a, a Critical Regulator of “migR”ation and Metastasis in Colorectal Cancer

Human colorectal cancer is a highly aggressive malignant disease, which remains the fourth most common human cancer and the second leading cause of cancer-related mortality in the United States. It is estimated that approximately 103,170 new colorectal cancer cases will be diagnosed and 51,690 deaths will occur in 2012 (1). Despite recent therapeutic advances, patients are often diagnosed with colorectal cancers at late stages that have already metastasized either as micro- or macro-metastatic disease. Because metastasis is the leading cause of treatment failure and tumor recurrence, there is an urgent need for achieving earlier diagnosis and developing innovative treatment strategies for improving the overall treatment outcome of this deadly disease (1).

Although the molecular mechanisms for colorectal cancer metastasis are not fully elucidated, accumulating evidence suggests that microRNA (miRNA) could play a critical role. miRNAs bind to the 3′ untranslated region (UTR) of target mRNAs, subsequently leading to target mRNA translational repression or degradation (2). Intriguingly, certain miRNAs with deregulated expression display antitumor activities in human malignances, whereas others exhibit oncogenic activities. Furthermore, unlike the “one mRNA, one transcribed protein” theme, a single miRNA could regulate multiple mRNA targets, and one given gene could also be governed by several different miRNAs (2).

Recent studies have shown that several miRNAs (miR), including miR-23a and miR-27a, are involved in the development and progression of human cancers, although details of the underlying molecular mechanisms remain unclear and may be context-dependent. Numerous studies have shown that expression of miR-23a is suppressed by c-Myc in prostate cancer and lymphoma cells and is downregulated by the oncogenic promyelocytic leukemia protein–retinoic acid receptor-α(PML–RARA) fusion protein in acute promyelocytic leukemia (3). In contrast, other reports have documented upregulation of miR-23a in a variety of human cancers including bladder cancer, gastric cancer, glioblastoma, hepatocellular carcinoma, breast cancer, and pancreatic cancer (3). miR-27a has been shown to exert an oncogenic function in human tumorigenesis. For example, in breast cancer, miR-27a exhibits oncogenic activity through downregulating zinc finger ZBTB10 protein, a putative specificity protein (SP) repressor, leading to overexpression of SP and SP-dependent prosurvival and proangiogenic genes including survivin, VEGF, and VEGF receptor 1 (VEGFR1; ref. 4). Moreover, downregulation of miR-27a decreases the percentage of breast cancer cells in the S-phase. Consistent with this finding, downregulation of miR-27a inhibited the proliferation of gastric cancer cells which was, in part, mediated through inhibiting cyclin D1 and upregulating p21 expression (5). Interestingly, miR-27a was also found to be involved in drug resistance through regulation of the expression of the drug transporter MDR1 (P-glycoprotein), a protein implicated in paclitaxel resistance in a variety of human cancers. However, the roles of miR-23a and miR-27a in the development and progression of colorectal cancers have not been mechanistically investigated.

In this issue of Cancer Discovery, Jahid and colleagues (6) revealed the novel roles of miR-23a and miR-27a in colorectal cancer progression. To determine whether these two miRNAs are deregulated in human colorectal cancers, they measured their expression in colorectal cancer clinical tissue samples at different malignant stages. They found that miR-27a displayed higher expression in all tumor tissues, whereas miR-23a expression was more restricted to invasive colorectal cancers. Consistent with this, the authors also showed that both miR-23a and miR-27a expression levels are increased in the mouse model with intestinal invasive adenocarcinomas (6). Because colon cancer stem cells (CCSC) have been characterized to play a critical role in the invasion and metastasis processes, this group conducted locked nucleotide analogue (LNA) microarray–based miRNA profiling of CCSCs. As expected, both miR-23a and miR-27a were highly expressed in CCSCs as well as in the invasive non-CCSC colorectal cancer cell lines. Taken together, miR-23a and miR-27a are highly expressed in colorectal cancer cell lines and mouse intestinal tumor as well as in human colorectal cancer tumor tissue specimens.

To further determine the molecular mechanisms by which miR-27a exerts its oncogenic functions in colorectal cancers, the authors used several molecular approaches to reveal FBXW7 as a direct target of miR-27a (6). FBXW7 is a well-studied E3 ubiquitin ligase that is reported to target various oncogenic proteins for ubiquitination, including cyclin E, Notch, Mcl-1, c-Myc, and c-Jun (7), and is considered a tumor suppressor largely due to its negative regulation of the stability of these oncogenic proteins. Indeed, it has been shown that FBXW7 is frequently inactivated by mutation, deletion, or promoter hypermethylation in multiple neoplasms including colon cancer (7). In addition, FBXW7 mutation was found in 11% of colorectal cancer. Lerner and colleagues (8) found that miR-27a suppresses FBXW7 expression during specific cell-cycle phases and promotes the degradation of proteins regulating G₁ to S-phase transition, such as cyclin E. They also showed that overexpression of miR-27a induces DNA replication stress and causes cyclin E dysregulation, in part, through inhibition of the FBXW7 expression (8). Moreover, Wang and colleagues (9) also independently discovered that FBXW7 is a potential miR-27a target and that the regulation of FBXW7 substrates such as cyclin E, c-Jun, and Notch1 might account for the observed elevation of cell growth arising from specific inhibition of FBXW7 mediated by overexpressing miR-27a. Consistent with previous reports, Jahid and colleagues confirmed that the E3 ubiquitin ligase FBXW7 is a direct miR-27a target in colorectal cancers and showed that depletion of endogenous miR-27a leads to the downregulation of oncogenic FBXW7 substrates such as Myc, Jun, and Notch (6). Taken together, miR-27a was shown to function as an oncogene, which is, in part, mediated through regulating the expression of the FBXW7 tumor suppressor in colorectal cancers.

As for the physiologic role of miR-27a, knockdown by LNA or short hairpin RNA led to inhibited CCSC growth and suppressed clonogenicity in vitro. Conversely, miR-27a overexpression promoted cell growth and increased clonogenicity in colorectal cancers. Furthermore, miR-27a knockdown significantly retarded in vivo tumor growth in a mouse xenograft model.

Interestingly, miR-23a knockdown had modest and minimal effect on cell proliferation and clonogenicity, respectively. Consistent with this notion, miR-23a knockdown did not cause a reduction in tumor volume in vivo either. Taken together, these results suggest that miR-27a, but not miR-23a, promotes cell proliferation in colorectal cancers both in vivo and in vitro. Having excluded a role for miR-23a in cell proliferation, the authors continued to further dissect a role for miR-23a in CCSC cell migration and invasion (6). To this end, one recent study has shown that miR-23a promotes colon cancer cell growth, invasion, and metastasis through inhibiting metastasis suppressor (MTSS) gene expression (10). Furthermore, the upregulation of miR-23a expression was associated with an advanced clinical stage and the depth of invasion, as well as lymph node metastasis, indicating that miR-23a could be a biomarker for the prognosis of colorectal cancers. In line with this finding, Jahid and colleagues (6) found that miR-23a knockdown inhibited cell motility, cell migration, and invasion. Moreover, they showed that suppression of migration and invasion by miR-23a knockdown is primarily due to upregulation of its target MTSS1 and downstream inhibition of Src signaling pathway (6), indicating that miR-23a could play an important role in cell migration and invasion, leading to the development of invasive colorectal cancers.

To further determine the roles of miR-23a and miR-27a in metastasis, Jahid and colleagues (6) conducted an in vivo assay of metastasis by injection of cells into the tail vein of immunodeficient mice. They showed that both miR-23a and miR-27a knockdown led to fewer lung tumors. Moreover, knockdown of miR-23a or miR-27a increased overall survival of mice. Together, they obtained experimental evidence to support the notion that miR-23a primarily increases cell motility, whereas miR-27a mainly promotes cell proliferation (Fig. 1).

These interesting studies shed light on the roles of miR-23a and miR-27a in colorectal cancer progression; however, several questions still need to be addressed in follow-up studies. For example, why was miR-23a expression lower in the late stage of patients with colorectal cancers? Why is there high expression of miR-27a target genes even though miR-27a levels do not decrease in colorectal cancers? Why did expression of cyclin E, a FBXW7 target, not change after miR-27a knockdown? Are additional miRNAs involved in colorectal cancer progression? Could miR-27a and miR-23a be biomarkers of prognosis for colorectal cancers, or are they only restricted to invasive colorectal cancers? Without a doubt, further studies will be ignited and are warranted to determine the physiologic functions of miR-23a and miR-27a in the development and progression of colorectal cancers.

Although further investigation is required to fully answer the questions raised above, the results reported by Jahid and colleagues (6) open a new avenue to target miR-23a and miR-27a for clinical benefits. Intriguingly, one recent study has shown that natural compound derivatives could decrease the expression of miR-27a in colorectal cancer cells (11) and inhibit cell growth and induction of apoptosis, suggesting that miR-27a could be a useful therapeutic target for colorectal cancers. We anticipate that establishing oncogenic roles of miR-23a and miR-27a in colorectal cancer progression will thus open new research directions for clinical management of this deadly disease.

No potential conflicts of interests were disclosed.

Some component of the work has been supported by grants from the National Cancer Institute, NIH (5R01CA131151, 1R01CA132794, and 1R01CA154321 to F.H. Sarkar), and from the National Institute of General Medical Sciences, NIH (GM089763 and GM094777 to W. Wei). W. Wei is an American Cancer Society scholar. Z. Wang is supported by the NIH NRSA fellowship.