MiR-155 at the heart of oncogenic pathways

Abstract

MicroRNAs are increasingly being recognized as oncogenes and tumor suppressors in cancer. MicroRNA-155 (miR-155) is an established oncomiR in breast cancer and regulates several pro-oncogenic pathways. In light of this, Chiang’s group has discovered a novel pathway regulated by miR-155. MiR-155 directly targets the VHL tumor suppressor and, by doing so, promotes the activity of HIF transcription factors and angiogenesis. This pathway appears to be particularly relevant in triple-negative breast cancer.

MicroRNAs are small noncoding RNAs that post-transcriptionally regulate gene expression. Canonical miRNA biogenesis begins with transcription of large primary miRs (pri-miRNA), which are often encoded in intronic regions of larger genes. Pri-miRNA transcripts are then processed by the ribonuclease Drosha to 60–100-bp-long stem-loop structures called pre-miRNAs. These are exported to the cytoplasm where another ribonuclease, Dicer, cleaves the stem-loop structures to 21–25-nucleotide-long dsRNAs. The strand showing complementarity with miR-recognition elements of specific mRNAs is recognized by a member of the Argonaute family of proteins, and is thus incorporated into the RNA-induced silencing complex. There are also Dicer-independent mechanisms that generate functional miRs. The main function of miRs is to suppress the translation of target genes, but they can also process mRNAs for cellular decay. Mature miRs have multiple targets, often members of the same regulatory networks, and often operate in regulatory feedback loops. This positions them as fine-tuning modulators of set points in homeostatic processes in normal cells. There is ample evidence that discrete sets of miRs are induced and repressed in different cancers. Indeed, certain miR expression profiles are specific to particular diagnoses and progression patterns, and predictive of responses to treatment. In cancer, aberrations in the levels of specific miRs may have well-defined tumor-suppressing or oncogenic function. Moreover, as a given miR has multiple targets, multiple pro-oncogenic or tumor-suppressing pathways are affected, and these pathways, in turn, regulate the miRs’ expression in a feedback-loop mechanism. In this issue of Oncogene, Cheng and colleagues¹ identify another novel pro-oncogenic pathway through which miR-155 (miR-155), which has well-established oncogenic properties, promotes the growth of triple-negative breast cancer. They also show that upregulation of this microRNA is associated with metastasis and poor prognosis in triple-negative breast cancer.

MiR-155 is a well-established oncogene, or oncomiR, in several blood and solid cancers, including breast cancer. It is processed from its own gene, BIC, which does not encode a protein product. Currently, more than 100 genes are confirmed to be directly targeted by this miR. The expression of miR-155 is augmented in breast cancer and correlates positively with several clinicopathological markers, high tumor grade, advanced stage and metastases to lymph nodes, whereas it correlates inversely with overall and disease-free survival.² Through its targets, miR-155 acts in the middle of oncogenic loops that repress the activity of several tumor suppressors. In addition, its expression is stimulated by pro-oncogenic conditions in tumors such as hypoxia and inflammation (Figure 1). In NMuMG mouse cells, miR-155 expression is induced by transforming growth factor-β through the direct activity of Smad4 transcription factor. This increased miR-155 activity partially accounts for the transforming growth factor-β-induced epithelial–mesenchymal transition, cancer cell migration and invasion, and involves direct targeting of RhoA by miR-155.³ In many human breast cancer cell lines, expression of miR-155 is also induced by factors that promote tumor inflammation, such as interleukin 6 or interferon γ. In this pathway, miR-155 inhibits SOCS1 and, by doing so, stimulates activity of the JAK2/STAT3 pathway, thus promoting further tumor inflammation and growth.⁴ MiR-155 also contributes to the oncogenic metabolism by stimulating glycolysis through induction of a glycolytic enzyme, hexokinase 2. This induction is partially accomplished through induction of STAT3; however, an additional mechanism is also employed. That is, miR-155 directly targets C/EBPβ, a transcription activator of miR-143, which in turn directly inhibits translation of hexokinase 2.⁵ Another crucial target of miR-155 is the pro-apoptotic transcription factor FOXO3a.⁶ The significance of this regulation in breast cancer is underlined by a negative correlation between miR-155 and FOXO3a in multiple breast cancer cells lines and tumors. Another tumor suppressor repressed by miR-155 is TP53INP1.⁷

Figure 1.

Model of the regulatory signaling network upstream and downstream of miR-155 in breast cancer.

In this issue of Oncogene, Chiang’s group expands on their previous work by identifying a novel pathway that is regulated by miR-155, and by showing that this pathway promotes oncogenesis in triple-negative breast cancer.¹ They demonstrate that miR-155 is highly upregulated in triple-negative breast cancer, and that its levels correlate with poor survival. They have identified a novel direct target of miR-155, VHL. VHL is a tumor suppressor lost in early stages of clear cell renal cell carcinoma. VHL’s canonical function is to act as a substrate-recognition component of the E3 ligase complex, which ubiquitylates and targets alpha subunits of HIFs for proteasomal degradation. This targeting is dependent on the hydroxylation by proline hydroxylases (a specialized group of enzymes) of two prolines within the N-terminal, activating domain of hypoxia-inducible factor (HIF)-α. As this hydroxylation requires molecular oxygen, it is inhibited during hypoxia, resulting in accumulation of HIF-αs and induction of HIF activity. Loss of VHL, such as the genetic loss common in clear cell renal cell carcinoma, mimics the effects of hypoxia. Kong et al.¹ reveal a unique mode of regulation for VHL expression in which the direct activity of miR-155 suppresses its translation, and results in increased HIF activity. These data complement the previously published evidence that hypoxia and HIF-1 induce expression of miR-155.^8,9 MiR-155 has a hypoxia-responsive element and its transcription is specifically stimulated by hypoxia-inducible factor 1 (HIF-1), but not HIF-2. Hypoxic induction of miR-155 contributes to the resistance of tumors to radiation therapies.⁹ Interestingly, however, miR-155 directly targets HIF-1α, but not HIF-2α, and suppresses its expression.⁸ While on the surface, such activity may seem to contradict the pro-oncogenic functions of miR-155, it is important to note that of the two HIFs, HIF-2 is more oncogenic. Thus miR-155, by repressing HIF-1α, may increase the contribution of HIF-2α to the oncogenic process when both HIFs are expressed.

Significantly, Chiang’s group performed a detailed analysis of miR-155 expression in clinical breast cancer samples and found that its upregulation was highly correlated with metastasis, poor prognosis and triple-negative breast cancer. Despite recent advancements, breast cancer still remains one of the leading causes of death for women, with more than 1.3 million cases and 450 000 deaths each year worldwide.¹⁰ Breast cancer can be largely divided into four major subtypes: estrogen receptor (ER)-positive, comprising luminal A and luminal B subtypes; HER2 (ERBB2)-amplified; and triple-negative breast cancer.^10,11 In recent years, good strides have been made in the development of various targeted endocrine therapy approaches for treating ER + and HER2 + breast cancer through endocrine therapies (for example, selective estrogen receptor modulators (SERMs), selective estrogen receptor downregulators (SERDs), aromatase inhibitors (AIs)) and anti-HER2 therapy (for example, Herceptin), respectively. Unfortunately, however, for triple-negative breast cancer that lacks ER and PR expression and HER2 amplification, there is currently no available targeted therapy, leaving treatment by chemotherapy as the only option. Further characterization of the miRs involved in triple-negative breast cancer is critical from the point of view of potential therapeutic applications, such as the use of miR-155 ‘antagomirs’ to target key oncogenic processes including hypoxia and inflammation, to be used alone or in combination with existing chemotherapeutic regimens.

The most recent comprehensive genomic and proteomic analyses of primary breast cancers have revealed unexpected molecular similarity between triple-negative breast cancers and ovarian tumors.¹² These new findings suggest a similar etiology of these cancers, and the potential usage of shared treatment strategies that target common pathways. Interestingly, miR-155 has recently been found to be regulated epigenetically by BRCA1, a common risk factor for both ovarian cancer and triple-negative breast cancer.¹⁰ This discovery placing miR-155 at the heart of the oncogenic pathways could have important therapeutic implications for both of these cancers. Additionally, miR-155 has been reported to be overexpressed in some other cancer types, such as lung and pancreatic cancers. Therefore, it will be of interest to know whether the oncogenic mechanisms promoted by miR-155 discussed above are common to all of these cancers, which may in turn, lead to even broader applications of these findings.