Keywords

1 MicroRNAs: Biogenesis, Processing and Mode of Action

MicroRNAs (miRNAs or miRs) are a class of non-coding small RNAs of ~22 nucleotides that regulate gene expression by targeting specific mRNAs bearing partially complementary target sequences for degradation and/or translational repression (Liu et al. 2008; Babashah and Soleimani 2011). The first discovery of a small non-coding RNA dates back to 1993, when Victor Ambros and collaborators identified lin-4 in Caenorhabditis elegans (Lee et al. 1993). Lin-4 was believed to be a unique species until year 2000 when another small non-coding RNA, let-7, was reported in C. elegans (Reinhart et al. 2000) and in a variety of other organisms (Pasquinelli et al. 2000). Since then, hundreds of small non-coding RNA sequences (now known to be miRNAs) have been identified in a wide range of organisms from nematodes to vertebrates, plants and human. Currently, the official miRNA database miRBase lists 1,872 human miRNA gene loci, generating 2,578 mature miRNA sequences (http://www.mirbase.org, Release 20.0, June 2013). Precise attribution of miRNA effects on gene expression can be complicated by the fact that often each miRNA may control several hundred target genes directly or indirectly, whereas a single protein coding gene target could be regulated by more than one miRNA. In fact, miRNAs are predicted to target up to one-third of human transcripts (Zhong et al. 2012; Friedman et al. 2009).

The biogenesis of miRNAs begins in the nucleus with the synthesis of a relatively long double-stranded RNA molecule, known as primary (pri)-miRNA, by RNA polymerase II or III. The resultant pri-miRNA transcript is often more longer than 1 kb in length and includes a stable stem-loop hairpin structure that contains the sequence for the mature miRNA. The hairpin structure is excised in the nucleus from pri-miRNA as a ~70-nucleotide long precursor (pre)-miRNA by the nuclear RNase III endonuclease Drosha and DGCR8 (the “microprocessor complex”) (Lee et al. 2003; Denli et al. 2004; Gregory et al. 2004). DGCR8 is essential as a molecular anchor for Drosha’s activity on pri-mRNAs, as it recognizes the pri-miRNA at double-stranded RNA – single-stranded RNA junction and directs Drosha to cleave approximately 11 nucleotides from the base of the stem to free the hairpin from the primary transcript (Han et al. 2006). Members of the microprocessor complex have additional cellular functions, as Drosha is also involved in the processing of ribosomal RNA (Wu et al. 2000) and DGCR8 also acts as a heme-binding protein (Faller et al. 2007). The resultant pre-miRNA contains a 5′ phosphate and a distinctive 3′ two-nucleotide overhang which is signal to transport into the cytoplasm by a protein complex consisting of Exportin-5 and Ran-GTPase (Yi et al. 2003; Lund et al. 2004; Bohnsack et al. 2004) (Fig. 1.1). In cytoplasm, further processing facilitated by the second RNase III endonuclease Dicer, cuts off the terminal loop and generates an imperfect double-stranded RNA with about 17-26-nucleotide in length. This duplex molecule contains the mature miRNA (often designated miR) and its complementary miRNA*. The duplex binds to one of four proteins of the Argonaute (Ago) family, which are part of the RNA-induced silencing complex (RISC). After unwinding the double-stranded RNA and discarding and degrading the passenger strand (miRNA*), the mature miRNA is loaded onto the RISC and interacts with the complementary sequences that are mostly located in the 3′ untranslated region (3′ UTR) of the targeted mRNAs (Cullen 2004; Liu et al. 2008; Ikeda et al. 2007). Subsequent mechanisms by which miRNAs regulate gene expression depend on the degree of complementarity between mRNA target sites and the nucleotide sequence from position 2–8 at the 5′ end of miRNAs (the seed region). The rare occasion of perfect (or near perfect) Watson-Crick complementarity leads to Ago-catalysed cleavage of the targeted mRNA. More commonly, imperfect complementarity leads to translational inhibition, although the precise mechanisms and the players involved are still under debate (reviewed in Fabian and Sonenberg (2012); Pasquinelli (2012)) (Fig. 1.1).

Fig. 1.1
figure 1

Schematic representation of biogenesis, processing and function of microRNA. The biogenesis of miRNAs begins in the nucleus and is completed in the cytoplasm. For more details, see the text. Pri-miRNA primary miRNA, Pre-miRNA precursor miRNA, Drosha RNase III endonuclease, DGCR8 DiGeorge syndrome critical region 8, Dicer RNase III endonuclease, RISC RNA-induced silencing complex

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Owing to the imperfect complementarity between miRNAs and their target mRNAs almost observed in mammals, direct prediction of relevant downstream targets of a miRNA is particularly difficult. Several bioinformatic approaches and various algorithms have been developed to predict miRNA-controlled target mRNAs in silico (Lewis et al. 2003; Krek et al. 2005; Paraskevopoulou et al. 2013). A list of computational tools for miRNA target prediction is summarized in Table 1.1. However, as the bioinformatic approach focuses on identifying conserved targets in the 3′-UTR of an mRNA, many non-conserved targets are missed. In addition, there are several lines of evidence indicating that miRNAs can also regulate gene expression through binding to “seedless” 3′-UTR miRNA recognition elements (Lal et al. 2009) or to sites located within the coding regions of transcript (Lee et al. 2009). Therefore, the efficacy of such a bioinformatic approach needs to be validated by in vitro or in vivo experiments.

Table 1.1 MicroRNA databases
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MiRNAs are involved in the control of a variety of biological processes, including cellular proliferation, tissue differentiation, organ development, maintenance of stem cell potency and apoptosis (Babashah and Soleimani 2011; Cheng et al. 2005; Chen et al. 2004; Ambros 2004). Given this wide variety of functions, it is not surprising that miRNAs are affected in many diseases such as cancer. In fact, dysregulation of miRNAs has been widely observed in different types and stages of cancer and mounting evidence points to their important roles in the development of a variety of human cancers (Bandyopadhyay et al. 2010; Esquela-Kerscher and Slack 2006; Lu et al. 2005; Volinia et al. 2006).

2 The Oncogenic and Tumor Suppressive Roles of MicroRNAs in Cancer

Aberrant expression of miRNAs has been frequently noted in almost all types of cancer (Croce 2009; Farazi et al. 2011). Functional analysis of these aberrantly expressed miRNAs indicates that they might function as either oncogenes or tumor suppressors. The oncogenic miRNAs, called as “oncomiRs”, are up-regulated in cancer and usually promote tumor development by inhibiting tumor suppressor genes and/or genes that control cell differentiation or apoptosis. On the contrary, there are many down-regulated miRNAs which may be considered as tumor suppressors in cancer. These miRNAs are called as “TSmiRs” and may function by inhibiting oncogenes and/or genes that inhibit cell differentiation or apoptosis (Bandyopadhyay et al. 2010; Esquela-Kerscher and Slack 2006; Lu et al. 2005; Babashah and Soleimani 2011). Deregulation of miRNA expression frequently results from genetic mutations and/or epigenetic alterations, represented by deletions, amplifications, point mutations and aberrant DNA methylation events. Indeed, about half of the cancer-related miRNA genes are located at fragile sites of the genome as well as in minimal regions with loss of heterozygosity, minimal regions of amplification or common breakpoint regions (Calin et al. 2002, 2004b).

The first evidence for the involvement of miRNAs in tumorigenesis was reported by Calin et al. (2002) in describing a chromosome region containing the miR-15a/miR-16-1 cluster, which is frequently lost or down-regulated in B-cell chronic lymphocytic leukemia (B-CLL). Down-regulation of the miR-15a/miR-16-1 cluster in CLL and several solid tumors raised the question whether they might function as tumor suppressors (Calin et al. 2002). Cimmino et al. (2005) demonstrated that both miR-15a and miR-16-1 promote the normal apoptotic response by directly targeting the anti-apoptotic gene BCL-2, indicating the possible tumor suppressive role of these two miRNAs in tumorigenesis.

A common tumor suppressive role for the let-7 family of miRNAs has been described in different types of human tissues, particularly in lung. It has been shown that let-7 is able to negatively regulate the expression of various oncogenes such as RAS and MYC as well as other cell cycle progression genes (Johnson et al. 2005; Bhat-Nakshatri et al. 2009). Reduced expression of Let-7 has been observed in different types of cancers, including lung, breast and prostate cancers. It has been shown that down-regulation of let-7 correlates with increased lymph node metastasis and proliferation capacity, suggesting a potential tumor suppressive role for this family of miRNAs in cancer progression (Lynam-Lennon et al. 2009; Liu et al. 2012). Although it has been demonstrated that induction of let-7 reduces tumor growth in a murine model of lung cancer (Esquela-Kerscher et al. 2008; Kumar et al. 2008), the regulation of individual let-7 targets on tumorigenesis needs to be further investigated in more in vivo models of human cancers.

The miR-17-92 cluster (containing seven homologous miRNAs: miR-17-3p, miR-17-5p, miR-18a, miR-20a, miR-19a, miR-19b-1 and miR-92a-1; with genomic positions on chromosomes X, 7 and 13) is the first and well-studied miRNA cluster with oncogenic activity. He et al. (2005) investigated the potential oncogenic role of the miR-17-92 cluster. They demonstrated that over-expression of the miR-17-92 cluster in the hematopoietic system acted with c-myc expression to accelerate tumor development and progression in a transgenic mouse model of B-cell lymphoma. Importantly, tumors resulting from combined c-Myc and miR-17-92 expression were able to evade from normal apoptotic responses that were otherwise prevalent in tumors lacking the cluster. O’Donnell et al. (2005) found that c-Myc activates expression of a set of six miRNAs on human chromosome 13 that was tied to the development of human lymphoma. They also found that expression of E2F1 was negatively regulated by two miRNAs in this cluster, miR-17-5p and miR-20a. These findings reveal a mechanism through which the c-Myc simultaneously promotes E2F1 transcription and represses following translation, indicating a tightly controlled proliferative signal. Woods et al. (2007) proposed a model in which the miR-17-92 cluster promotes cell proliferation by shifting the E2F transcriptional balance away from the pro-apoptotic E2F1 and toward the proliferative E2F3 transcriptional network. The miR-17-92 cluster might also inhibit apoptosis by negatively regulating the tumor suppressor PTEN and the pro-apoptotic protein Bim (Xiao et al. 2008; Mendell 2008). Bim is induced by Myc in B-cells and is able to antagonize anti-apoptotic proteins such as Bcl-2. Therefore, down-regulation of Bim by the miR-17-92 cluster may contribute to the ability of these miRNAs to exacerbate disease progression in a mouse model of B-cell leukemia (Egle et al. 2004).

As stated above, miRNAs can function either as oncogenes or tumor suppressors. However, it has been demonstrated that a miRNA can exploit both functions according to the cellular context of their target genes. For instance, there is a body of evidence pointing to the tumor suppressive activity of the miR-17-92 cluster, which contrasts with the hypothesized oncogenic role observed in other cancers (Yu et al. 2008). This implies that the tissue- and developmental-stage-specific expression decisively controls appropriate function of a miRNA.

3 MicroRNAs and Tumor Metastasis

Tumor invasion and metastasis are major characteristics of aggressive phenotypes observed in human cancers (Steeg 2003). During the “invasion-metastasis cascade”, cancer cells (a) are detached and migrate out of the primary tumor site; (b) invade the basement membrane to enter the circulatory system (intravasation); (c) are translocated through the vasculature; (d) exit circulatory vessels at the metastatic site (extravasation); (e) survive within the foreign microenvironment; and finally (f) re-initiate their proliferative machinery to establish macroscopic secondary tumors (colonization) (Fig. 1.2) (Harquail et al. 2012; Fidler 2003). Despite the clinical significance of metastasis for determining disease outcome in human cancers, our current understanding on how cancer cells actually migrate out of primary tumors, adapt to distant tissues and organs, and form a secondary tumor are still not completely understood (Gupta and Massague 2006).

Fig. 1.2
figure 2

Schematic representation of multistep metastatic process by which primary tumor cells are detached from the primary tumor site, consequently adapt into distant tissues and organs, and form a secondary tumor

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MiRNAs have recently been more widely investigated due to their potential role as critical regulators of tumor metastasis in cancer development. The link between altered expression levels of miRNAs and cancer development and metastasis can be observed either through the loss of tumor suppressor miRNAs or the over-expression of oncogenic miRNAs in different cancer cells. Some miRNAs involved in metastasis are summarized in Table 1.2, most of which will be discussed in more detail in the sections below.

Table 1.2 Some microRNAs involved in the regulation of the multistep metastatic process
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3.1 Pro-metastatic miRNAs

Multiple lines of evidence highlight the contribution of certain miRNAs to promoting tumor metastasis. MiR-10b is the first miRNA identified to positively regulate the metastatic potential of human cancer cell. Ma et al (2007) showed that miR-10b over-expression endowed otherwise non-metastatic breast cancer cells with the capacity to acquire invasive and metastatic behavior. MiR-10b is able to induce migration and invasion capacities in breast cancer cells through direct targeting of homeobox D10 (HOXD10), a receptor of genes involved in cell migration and extracellular matrix remodeling. Notably, systemic treatment of breast tumor-bearing mice with miR-10b antagomirs decreased the metastatic tumor burden, providing promising evidence that antagomirs can be efficiently delivered to rapidly growing tumor cells in vivo, preventing metastasis (Ma et al. 2010). To identify miRNAs that have the capacity to promote metastasis, Huang et al. (2008) set up a genetic screen involving over-expression of approximately 450 miRNAs in non-metastatic, human breast tumor cell line. They found that miR-373 and miR-520c (both belonging to a miRNA family that shares similar seed sequence) can induce tumor cell migration and invasion in vitro and in vivo, and that the migratory phenotype of certain cancer cell lines depends on endogenous miR-373 expression. They proposed that suppression of cell migration by an anti-miR-373 oligonucleotide may be a potential strategy for developing efficient therapies against tumor metastasis. After that, two independent studies indicated that apart from the oncogenic role of miR-21 in tumorigenesis, this miRNA also plays a critical role in invasion and metastasis of human breast and colorectal carcinoma cells (Asangani et al. 2008; Zhu et al. 2008). These studies suggest that suppression of miR-21 might offer another promising therapeutic approach against advanced cancers (Table 1.2).

3.2 Anti-metastatic miRNAs

Multiple lines of evidence highlight the contribution of certain miRNAs to suppressing tumor metastasis. MiR-31 expression levels correlate inversely with metastasis in human breast cancer patients. By deploying gain- and loss-of-function strategies, Valastyan et al. (2009) demonstrated that miR-31 is capable of suppressing the metastatic potential of human breast tumor cells. They also successfully showed that miR-31 is involved during the multiple step metastatic process in vivo, including local invasion, extravasation or initial survival at a distant site, and metastatic colonization. MiR-126 and miR-335 have been identified as human breast cancer metastasis suppressor miRNAs that exert their unique effects on distinct steps of the invasion-metastasis cascade. By performing array-based miRNA profiling, Tavazoie et al. (2008) revealed that the expression of both miRNAs is lost in the majority of primary breast tumors with metastatic relapse, and the loss of expression of either miRNA is associated with poor distal metastasis-free survival. Importantly, in vivo experiments showed that miR-126 restoration reduced overall tumor growth and proliferation (at both primary site and distant organs), whereas miR-335 caused a significant reduction in cell motility and invasive capacity. The strong association of the loss of miR-335 and miR-126 expression with clinical metastatic relapse suggests the potential for the use of these miRNAs in prognostic assessment of breast cancer patients in addition to conventional clinical and pathological staging markers. Moreover, another study identified that miR-193b significantly inhibited the growth and dissemination of xenograft breast tumors in an immunodeficient mouse model. This study showed that the loss of miR-193b confers the metastatic colonization ability to the cells. As the loss of miR-193b expression is strongly correlated with metastasis, the use of this miRNA in addition to conventional clinical and pathological staging markers could be an attractive option for the prognostic stratification of patients with breast cancer (Li et al. 2009) (Table 1.2).

3.3 MiRNAs and Epithelial to Mesenchymal Transition

Epithelial to mesenchymal transition (EMT), in which polarized epithelial cells are converted into motile cells, plays an important role in tumor invasion and metastasis (Thiery 2002; Yang and Weinberg 2008; Togawa et al. 2011). The effect of miR-125b on metastatic activities of breast cancer cells was studied by Tang et al. They reported that miR-125b significantly up-regulates the expression of two EMT markers (i.e. vimentin and α-SMA expression) but another EMT marker (E-Cadherin) shows no significant change. Elevating vimentin and α-SMA expression results in a high metastasis potentiality and some mesenchymal cell characteristics in breast cancer cells (Tang et al. 2012). A large body of evidence indicates that the miR-200 family inhibits EMT and cancer cell migration by enhancing E-cadherin expression through direct targeting of the EMT-promoting transcription factors Zeb1 and Zeb2 (Korpal et al. 2008; Gregory et al. 2008; Park et al. 2008; Burk et al. 2008; Bracken et al. 2008). However, a study reported that over-expressing miR-200 in Murine breast cancer cell line 4TO7 enhances the ability of these cells to metastasize to lung and liver. This study reported that miR-200 expression leads to promote a mesenchymal to epithelial cell transition (MET) by suppressing Zeb2 expression. This finding contrasts with the EMT hypothesis of cancer metastasis that implies that the induction of epithelial characteristics would inhibit the formation of metastasis. This apparent contradiction could be explained on the basis that for some tumors, a reversion of the mesenchymal phenotype of malignant cells may facilitate tumor colonization at metastatic sites. This suggests that the epithelial nature of a tumor does not predict metastatic outcome. Moreover, these results imply that the cellular context of miRNA expression decisively controls the function of a miRNA (Dykxhoorn et al. 2009).

4 MicroRNAs and Tumor Angiogenesis

Angiogenesis is characterized by growth of new blood vessels from pre-existing vasculature in response to physiological or pathophysiological stimuli. This process, which involves proliferation, migration, and maturation of endothelial cells, plays an important role during tumor growth and metastasis (Urbich et al. 2008; Chung et al. 2010).

Evidence for the significance of miRNAs as regulators of angiogenesis comes from observations that Dicer is a critical component for embryonic angiogenesis. It has been shown that blood vessel formation/maintenance in Dicer-deficient mice embryos and their yolk sacs was severely compromised, suggesting a possible role for Dicer in angiogenesis through its function in the processing of miRNAs (Yang et al. 2005). Consistent with this observation, another studies showed that genetic silencing of Dicer in endothelial cells leads to down-regulation of several key positive regulators of the angiogenic phenotype and impairs tube formation activity in vitro and in vivo (Suarez et al. 2007; Kuehbacher et al. 2007). Mounting studies suggest that a number of angiogenesis-related miRNAs affect cancerous phenotype of malignant cells. MiRNAs can modulate angiogenesis by targeting positive or negative regulators in angiogenic signaling pathways (Hong et al. 2013; Landskroner-Eiger et al. 2013). Some miRNAs involved in tumor angiogenesis are summarized in Table 1.3, most of which will be discussed in more detail in the sections below.

Table 1.3 Some microRNAs involved in the regulation of tumor angiogenesis
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4.1 Pro-angiogenic miRNAs

Up-regulation of pro-angiogenic growth factor receptors (such as platelet-derived growth factor receptor, “PDGFR” and vascular endothelial growth factor receptor, “VEGFR”) on endothelial cells is a common feature of angiogenesis (Batchelor et al. 2007; Shih and Holland 2006). Wurdinger et al. (2008) showed that glioma- or growth factor-mediated induction of miR-296 in endothelial cells leads to increased levels of pro-angiogenic growth factor receptors VEGFR2 and PDGFR-β. Possible role of miR-296 in promoting angiogenesis in tumor was further supported when inhibition of miR-296 with antagomirs reduced angiogenesis in tumor xenografts in vivo.

Some other miRNAs, such as miR-378 and miR-17-92 cluster, have been also implicated in tumor angiogenesis. MiR-378 functions as an oncogene by enhancing tumor cell survival, blood vessel expansion, and tumor growth by targeting two tumor suppressors, SuFu (suppressor of fused) and Fus-1 (Lee et al. 2007a). The miR-17-92 cluster not only augments angiogenesis in endothelial cells during normal development (Suarez et al. 2008), but also its upregulation in cancer cells can serve to promote angiogenesis during tumor growth in a xenograft model (Dews et al. 2006). Importantly, this angiogenic effect is exerted through down-regulation of anti-angiogenic thrombospondin-1 (TSP-1) and related proteins, such as connective tissue growth factor (CTGF) (Dews et al. 2006).

One study showed that many miRNAs derived from tumor cells are packaged into microvesicles and then directly delivered to their microenvironment. These tumor-secreted microvesicles are then capable of interacting with proximal endothelial cells to transport miRNAs in endothelial cells. Among these miRNAs, it was shown that tumor-secreted miR-9 promotes endothelial cell migration and tumor angiogenesis by activating JAK-STAT pathway, one of the major oncogenic signaling pathways activated in a variety of human malignancies. Importantly, administration of miR-9 antagomiRs (anti-miR-9) or JAK inhibitors impaired microvesicles-induced cell migration in vitro and decreased tumor burden in vivo. Taken together, these observations support a novel intercellular communication in which tumor-secreted miRNAs function as pro-angiogenic mediators during tumorigenesis (Zhuang et al. 2012) (Table 1.3).

4.2 Anti-angiogenic miRNAs

The miR-34 family of miRNAs (miR-34a, b and c) as direct, conserved p53 target genes presumably induces apoptosis, cell cycle arrest and senescence (Bommer et al. 2007; Chang et al. 2007). MiR-34a functions as a tumor suppressor that is frequently down-regulated in various tumor types. Kumar et al. (2012) demonstrated that miR-34a expression is significantly down-regulated in head and neck squamous cell carcinoma tumors and cell lines. Ectopic expression of miR-34a reduced head and neck tumor cell proliferation, colony formation and migration and also significantly inhibited tumor growth and tumor angiogenesis in a SCID mouse xenograft model. This in vivo tumor growth study revealed that miR-34a inhibits tumor angiogenesis by down-regulating VEGF, a key angiogenic factor.

Siragam et al. (2012) defined a regulatory role for miR-98 in tumor angiogenesis and invasion using a highly aggressive breast cancer model in vitro. They showed that miR-98 inhibits tumor angiogenesis and invasion by repressing activin receptor-like kinase-4 (ALK4) and matrix metalloproteinase-11 (MMP11) expression.

Another study showed that transient induction of miR-125b inhibits in vitro tube formation of endothelial cells through suppression of vascular endothelial (VE)-cadherin. Importantly, induction of miR-125b induced non-functional blood vessels, resulting in inhibition of tumor growth. It seems that prolonged over-expression of miR-125b could be an option in cancer therapy by causing collapse of the lumen of endothelial cells (Muramatsu et al. 2013) (Table 1.3).

5 MicroRNA Profiling by High-Throughput Technologies

Considering the fact that current cancer detection tests have their own limitations, the use of miRNAs as promising biomarkers for diagnosis and prognosis of cancer has aroused intense research interests. Additionally, distinctive pattern of miRNA expression also serves as markers of important histopathologic features such as tumor stage, proliferative capacity and vascular invasion (Lynam-Lennon et al. 2009).

Many expression profiling studies of miRNA genes have been performed on different types of cancer. However, the results of analyses of the same type of cancer by different groups are not always consistent. The disparity in these results might attribute to the different platforms for miRNA profiling in each case and the use of different sample storage methods (Calin and Croce 2009).

Currently, the most widely used methods for miRNA profiling are based on sequencing, microarray, and real-time quantitative PCR. Microarray platforms have been used for miRNA profiling, but suffer from background and cross-hybridization problems and are generally restricted to identifying the relative abundance of previously discovered miRNAs (Calin et al. 2004a; Chen et al. 2009). Sequencing-based applications for identifying and profiling miRNAs have been hindered by laborious cloning techniques and the expense of capillary DNA sequencing (Pfeffer et al. 2005; Cummins et al. 2006). High-throughput sequencing-based approaches to generate miRNA profiles, hugely enabled by next-generation technologies, provide several advantages over probe-based methodologies, including the ability to discover novel miRNAs and the potential to detect variations in the mature miRNA length and miRNA editing (Morozova and Marra 2008). Next-generation sequencing technologies are able to identify low abundance miRNAs or those exhibiting modest expression differences among samples, which may not be detected by hybridization-based methods. Real-time quantitative PCR, another highly sensitive technique for miRNA quantification, is capable of distinguishing mature and precursor miRNA, and produces fewer false-positives and reduced bias when compared with microarray or sequencing approaches (Chen et al. 2009; Fuller et al. 2009; Petriv et al. 2010). Real-time PCR may be used to validate the expression of miRNAs discovered during high throughput arrays and study the expression of individual miRNAs. This method provides several important advantages for miRNA profiling studies including low cost, superior detection of low-abundance species and high throughput (Schmittgen et al. 2008). The emergence of novel high-throughput technologies will allow more sensitive and efficient miRNA detection in patient samples, and identification of novel miRNAs. However, standardization of these novel methods is necessary to overcome the variability observed when different miRNA-expression detection platforms are used.

6 Potential Use of MicroRNAs in Cancer Therapy

Dysregulation of miRNA has been widely observed in different types of human cancers (Table 1.4), and there is mounting evidence demonstrating their important roles during cancer development and progression. Uncovering the possible mechanisms underlying the importance of miRNAs in the pathogenesis of human cancers may lead to the development of miRNA-based therapeutic strategies or diagnostic/prognostic biomarkers.

Table 1.4 Some microRNAs aberrantly expressed in different cancers
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Since cancer cells often have a distinctive expression pattern of oncogenic and tumor suppressive miRNAs (Babashah et al. 2012; Babashah and Soleimani 2011; Calin and Croce 2006), approaches that manipulate miRNA expression levels, either alone or in combination with currently used therapies, may prove to be therapeutically beneficial. Sequence-specific knockdown of oncogenic miRNAs by chemically engineered oligonucleotides termed “antagomirs” or locked nucleic acid (LNA)-modified oligonucleotides is a plausible therapeutic approach for inhibiting expression levels of oncogenic miRNAs in cancer (Orom et al. 2006; Krutzfeldt et al. 2005). In contrast, elevating the expression level of tumor suppressive miRNAs that could be achieved by viral or liposomal delivery of mimic miRNAs represents a potential therapeutic strategy against cancer (Calin and Croce 2006; Meng et al. 2006). However, many concerns need to be addressed before consideration of conducting miRNA-based therapy including dosage, safety, specificity, stability, efficacy, and problems of delivery to the target (Chen et al. 2010; Cho 2010b; Tong and Nemunaitis 2008; Wu et al. 2007).

7 Conclusions and Perspectives

  • As miRNAs can regulate various target genes, precise attribution of their functions on gene expression is very complicated. However, the critical involvement of miRNAs in many aspect of cancer biology is irrefutable.

  • Although miRNAs are postulated to function as either oncogenes or tumor suppressors in human cancers, further studies establishing such roles for miRNAs using in vivo experimental models are needed to elucidate precise mechanisms of miRNAs functions in cancer.

  • MiRNA expression profiling of human cancers has identified diagnostic and prognostic signatures. Additionally, miRNA signatures could be used for cancer classification and prediction of therapeutic efficacy.

  • The association of miRNA dysregulation with oncogenesis demonstrates the feasibility of manipulating miRNA levels as a potential strategy for therapeutic purposes.

  • Given the potential involvement of candidate miRNAs in the pathogenesis of human cancers, it seems that pharmacological modulation of miRNA expression will have a brilliant future and become a promising option in cancer therapy.