Exploiting the therapeutic potential of microRNAs in human cancer

Abstract

Dysregulation of microRNAs (miRNAs) has been widely shown to be associated with the development and progression of cancer. Recent studies discovered a handful of miRNAs with great potential to act as therapeutic targets in various human cancers. Inhibition or overexpression of these oncomirs may regulate the expressions of their associated genes, which in turn represses the proliferation or metastasis of different cancers. Some miRNAs can reverse the phenotype of epithelial–mesenchymal transition, while others can be utilized to sensitize cells to DNA-damaging drugs. Most of their anticancer abilities have been validated in preclinical animal models. A merit of miRNA-based therapy is that it can target plenty of genes in different signaling pathways, but this also comes with the drawback of many unknown off-target effects. In addition, successful delivery is also a major obstacle to effective miRNA-based therapeutics. Nevertheless, new findings from recent studies and the rapid advances in systemic drug delivery systems provide an optimistic perspective on the evolution of the field.

Keywords::

• cancer
• microRNA
• miRNA
• oncomir
• therapeutic target

1. Introduction

Dysregulation of microRNAs (miRNAs) has been widely shown to be associated with the development and progression of cancer. Many miRNAs have been identified to act as oncogenes, tumor suppressors, or even modulators of cancer stem cells and metastasis [1]. With increasing understanding of the miRNA target genes and the cellular behaviors influenced by them, modulating miRNA activities may provide exciting opportunities for cancer therapy [2].

2. MicroRNAs as potential therapeutic targets and tools (Table 1)

Table 1. A summary of miRNAs and their target genes in various cancer types.

miRNAs	Deregulation in cancer	Target genes	Molecular mechanisms	Ref.
let-7	Increase in hepatocellular CSCs	SOCS-1, CASP3	Inhibition of let-7 increases the chemosensitivity of hepatocellular CSCs to doxorubicin and sorafenib	[5]
miR-10b	Increase in metastatic breast cancer	CCN5	Suppressed by CCN5 through the inhibition of TWIST1 expression: this inhibition is mediated through the translational inhibition and modification of HIF-1α via impeding the JNK signaling pathway	[14]
miR-15b, miR-200b	Decrease in chemotherapy-resistant tongue cancer	BMI1	Effectively reverse the phenotype of EMT and sensitize cancer cells to chemotherapy	[24]
miR-24-2	Decrease in breast cancer	BCL-2	Induces apoptosis by modulating different apoptotic pathways	[17]
miR-34b/c	Decrease in methylated MPM cells	c-MET	Exhibits antiproliferation with G1 cell cycle arrest and suppression of migration, invasion and motility	[11]
miR-101	Decrease in IPMN of the pancreas and in PCa	EZH2, COX-2	Loss of miR-101 can be a trigger for the adenomacarcinoma sequence of IPMN by upregulation of EZH2. Exogenous miR-10 suppresses the proliferation and growth of PCa cells in vitro and in vivo	[7,8]
miR-125b	Decrease in HCC	PlGF	Decreases cell proliferation, anchorage-independent growth, cell migration, invasion and angiogenesis	[12]
miR-148a	Decrease in GC	ROCK1	Suppresses GC cell migration and invasion in vitro and lung metastasis formation in vivo	[21]
miR-155	Increase in BRCA1-deficient tumors	BRCA-1	BRCA1 epigenetically represses miR-155 expression via its association with HDAC2, which deacetylates histones H2A and H3 on the miR-155 promoter	[3]
miR-181	Increase in hepatocellular CSCs	RASSF1A, TIMP3, NLK	Silencing of miR-181 leads to a reduction in the motility and invasion of hepatocellular CSCs	[5]
miR-193a	Increase in p53-deficient tumors	p53	Chemotherapy causes p63/p73-dependent induction of miR-193a-5p, thereby limiting chemosensitivity due to miRNA-mediated feedback inhibition of p73	[4]
miR-199a/b	Decrease in HCC	PAK4	Suppresses HCC growth through inhibiting PAK4/Raf/MEK/ERK pathway both in vitro and in vivo	[15]
miR-200a	Decrease in pancreatic cancer	DCAMKL-1	siRNA-mediated knockdown of DCAMKL-1 in cancer cells induces miR-200a along with downregulation of EMT-associated transcription factors ZEB1, ZEB2, Snail, Slug and Twist	[23]
miR-340	Decrease in aggressive breast cancer	c-Met	Suppresses tumor cell migration and invasion through the regulation of MMP-2 and MMP-9 expression	[19]
miR-342	Decrease in CRC	DNMT1	Dramatically reduces DNMT1 expression, which in turn reactivates ADAM23, Hint1, RASSF1A and RECK genes via promoter demethylation	[10]
miR-488*	Decrease in PCa	AR	Downregulates the transcriptional activity of AR and inhibits the endogenous AR protein production	[16]
miR-499	Increase in highly invasive CRC	FOXO4, PDCD4	Promotes CRC cell migration and invasion in vitro as well as lung and liver metastasis in vivo	[18]
miR-516a	Decrease in highly metastatic GC	Sulfatase 1	Atelocollagen-mediated delivery of a miR-516a-3p expression vector into orthotopic highly metastatic tumors may have a therapeutic application in blocking metastatic dissemination of GCs	[22]
miR-520b	Decrease in breast cancer with high metastatic potential	HBXIP	Regulates cancer cell migration via a network in which HBXIP promotes migration by stimulating NF-κB-mediated IL-8 expression	[20]
miR-558	Repressed by MYCN in neuroblastoma	Host gene BIRC6	Markedly increases colony formation, proliferation and tumor growth in vivo	[6]
miR-591	Repressed by MYCN in neuroblastoma	Host gene SLC25A13	Exhibits potent tumor suppressive effects in orthotopic neuroblastoma xenografts	[6]
miR-708	Decrease in RCC	ZEB2, BMI1	Decreases cell growth, clonability, invasion and migration and elicits a dramatic increase in apoptosis	[9]

AR: Androgen receptor; CRC: Colorectal cancer; CSCs: Cancer stem cells; EMT: Epithelial-mesenchymal transition; GC: Gastric cancer; HCC: Hepatocellular carcinoma; IPMN: Intraductal papillary mucinous neoplasm; miRNA: microRNA; MPM: Malignant pleural mesothelioma; PCa: Prostate cancer; RCC: Renal cell carcinoma.

2.1 The role of miRNAs in cancer progression

BRCA1 is a tumor suppressor with an established role in the repair of damaged DNA and cell cycle regulation. A study found that BRCA1 epigenetically represses miR-155 expression via its association with HDAC2, which deacetylates histones H2A and H3 on the miR-155 promoter. Overexpression of miR-155 accelerates, but the knockdown of miR-155 attenuates, the growth of tumor cells in vivo. Their findings suggest that miR-155 is a potential therapeutic target for BRCA1-deficient tumors [3]. p53 is another well-known tumor suppressor, but the p53-related transcription factors p63 and p73 are both overexpressed in squamous cell carcinomas. A study discovered that chemotherapy causes p63/p73-dependent induction of miR-193a-5p, thereby limiting chemosensitivity due to miRNA-mediated feedback inhibition of p73. Inhibiting miR-193a interrupts this feedback, suppresses tumor cell viability and induces dramatic chemosensitivity both in vitro and in vivo. Thus miR-193a inhibition may provide a new therapeutic opportunity in p53-deficient tumors [4].

Cancer stem cells (CSCs) hold the extensive proliferative and self-renewal potential necessary to develop cancer. A study showed that let-7 and miR-181 are upregulated in hepatocellular CSCs. It has been demonstrated that let-7 targets SOCS-1 and CASP3, while miR-181 targets RASSF1A, TIMP3 and NLK. Inhibition of let-7 increases the chemosensitivity of hepatocellular CSCs to doxorubicin and sorafenib, whereas silencing of miR-181 leads to a reduction in the motility and invasion of hepatocellular CSCs [5]. MYCN is a major driver of neuroblastoma tumorigenesis. A genome-wide search for MYCN target miRNA promoters differentially repressed under high-MYCN conditions has been performed. This showed that miR-558 markedly increases colony formation, proliferation and tumor growth in vivo, whereas miR-591 exhibits potent tumor suppressive effects in orthotopic neuroblastoma xenografts. The data revealed host-gene-independent functions of MYCN-target miRNAs and demonstrated that MYCN represses both proproliferative and tumor suppressive miRNAs [6].

To investigate the mechanism of intraductal papillary mucinous neoplasm (IPMN) carcinogenesis, a study revealed a lower miR-101 expression and a higher EZH2 expression in malignant than benign IPMN of the pancreas. miR-101 targets EZH2 at the post-transcriptional level and loss of miR-101 can be a trigger for the adenomacarcinoma sequence of IPMN by upregulation of EZH2. This study suggests miR-101-EZH2 blockade as a potential therapeutic target in IPMN carcinogenesis [7]. There is an association of chronic inflammation with tumorigenesis and overexpression of COX-2 is often found in tumor tissues. Another study found that COX-2 is a direct target in miR-101 regulation posttranscription, exogenous miR-101 suppresses the proliferation and growth of prostate cancer (PCa) cells in vitro and in vivo. These data indicate that exogenous miR-101 may provide a new cancer therapy by directly inhibiting COX-2 expression [8].

Chronic stress weakens a person's immune system, which in turn may increase the risk of cancer. miR-708 has been identified as a tumor suppressor in renal cell carcinoma (RCC) that has been implicated in stress control. Restoration of miR-708 expression in RCC cells decreases cell growth, clonability, invasion and migration, as well as elicits a dramatic increase in apoptosis. Intratumoral delivery of miR-708 is able to trigger in vivo regression of tumors in xenografts. The E-cadherin regulators ZEB2 and BMI1 have been identified as likely miR-708 targets. These findings may offer a new target for therapeutic intervention in RCC [9]. In fact numerous oncogenes have been identified in human cancer, miRNA can potentially regulate the expression of these oncogenes posttranscriptionally. Overexpressed DNMT1 strongly contributes to tumor suppressor gene silencing in colorectal cancer (CRC). A study showed that restoration of downregulated miR-342 results in a dramatic reduction of DNMT1 expression in CRC cells, which in turn reactivates ADAM23, Hint1, RASSF1A and RECK genes via promoter demethylation. In contrast, overexpression of miR-342 significantly inhibits tumor growth and lung metastasis in nude mice. This newly identified miR-342–DNMT1 link provides a potential therapeutic target for the treatment of CRC [10].

Anomalous methylation of miRNAs has also been reported in a wide variety of human cancers. A study showed that aberrant methylation of miR-34b/c is present in malignant pleural mesothelioma (MPM) cells and the expression of miR-34b/c is reduced in the methylated cells. Forced overexpression of miR-34b/c exhibits antiproliferation with G1 cell cycle arrest and suppression of migration, invasion and motility. The expression of miR-34b/c can be restored with 5-aza-2′-deoxycytidine treatment, which suggests a potential therapeutic option for MPM [11]. Using miRNA profiling, another study showed a decreased miR-125b expression in hepatocellular carcinoma (HCC) cells and the expression of miR-125b can also be increased by methylation inhibitor 5-aza-2′-deoxycytidine. The introduction of miR-125b precursor can decrease cell proliferation, anchorage-independent growth, cell migration, invasion and angiogenesis. PlGF has been identified as a target of miR-125b, overexpression of miR-125b decreases PlGF expression and alters the angiogenesis index in HCC cells [12].

2.2 The potential clinical implications of miRNAs in core signaling pathways

Many of the complex cancer signaling pathways underlying tumor initiation and progression have been elucidated in recent years. An increased understanding of the signaling pathways is driving the development of a new generation of anticancer therapies targeted at specific molecular events [13]. In the past few years, some potential miRNA-based therapeutic agents have emerged for the inhibition of single- or multi-targets in various signaling pathways.

It has been reported that the expression of miR-10b is increased in metastatic breast cancer cells. A study found that miR-10b expression can be suppressed by CCN5 through the inhibition of TWIST1 expression and this inhibition is mediated through the translational inhibition and modification of HIF-1α via impeding JNK signaling pathway. Reactivation of CCN5 in miR-10b-positive metastatic breast cancers may provide an alternative strategy to existing breast cancer therapy [14]. An in-depth analysis of miRNomes in HCC found that miR-199a/b-3p consistently decreases in HCC. This miRNA can target tumor-promoting PAK4 to suppress HCC growth through inhibiting the PAK4/Raf/MEK/ERK pathway both in vitro and in vivo. Their study reveals miR-199a/b-3p as a therapeutic target for HCC [15].

Androgen receptor (AR) plays a significant role in prostate carcinogenesis. A study demonstrated that overexpression of miR-488* downregulates the transcriptional activity of AR and inhibits the endogenous AR protein production. miR-488* blocks proliferation and enhances apoptosis of PCa cells. These data indicate that miR-488* targets AR and it is a potential modulator of AR-mediated signaling which may be utilized as a novel therapeutic to target AR in PCa [16]. On the other hand, many miRNAs are implicated as playing a role in cell proliferation and apoptosis. A study observed that miR-24-2 is capable of inducing apoptosis by modulating different apoptotic pathways and targeting BCL-2 in breast cancer cells. The cells overexpressing miR-24-2 are hypersensitive to DNA-damaging drugs, combination therapy with miR-24-2 along with an anticancer drug such as cisplatin may open a new avenue in cancer therapy for patients with tumors resistant to drugs [17].

2.3 Therapeutic approaches based on metastasis-related miRNAs

Emerging evidence demonstrates that miRNAs play a critical role in cancer metastasis. A CRC study focused on miR-499-5p as a candidate prometastatic miRNA and confirmed increased miR-499-5p levels in highly invasive CRC cells. Enhancing the expression of miR-499-5p promotes CRC cell migration and invasion in vitro as well as lung and liver metastasis in vivo, whereas silencing its expression results in reduced migration and invasion. FOXO4 and PDCD4 have been identified as direct and functional targets of miR-499-5p. These findings indicate that miR-499-5p promotes metastasis of CRC cells and may be useful as a potential therapeutic target for CRC [18].

Endogenous miR-340 expression is downregulated in aggressive breast cancer cells. A study showed that the induction of miR-340 expression can suppress tumor cell migration and invasion, whereas the knockdown of miR-340 expression induces breast cancer cell migration and invasion. It identified c-Met as a direct miR-340 target to mediate cell migration and invasion through regulation of MMP-2 and MMP-9 expression, suggesting that miR-340 may act as a potential therapeutic target for breast cancer metastasis [19]. Another study found that miR-520b also suppresses the migration of breast cancer cells with high metastatic potential. miR-520b is involved in regulating breast cancer cell migration by targeting HBXIP and IL-8 via a network in which HBXIP promotes migration by stimulating NF-κB-mediated IL-8 expression. These findings point to HBXIP as a potential therapeutic target for metastatic breast cancer [20].

A gastric cancer (GC) study demonstrated that miR-148a also functions as a tumor metastasis suppressor. Overexpression of miR-148a suppresses GC cell migration and invasion in vitro and lung metastasis formation in vivo, and ROCK1 has been found to be involved in this miR-148a-induced suppression of GC cell migration and invasion. Their findings suggest that miR-148a may have a therapeutic potential to suppress GC metastasis [21]. There is also study focusing on miR-516a-3p as a candidate antimetastatic miRNA in GC. They confirmed attenuated expression of miR-516a-3p in highly metastatic cells and identified sulfatase 1 as a direct target of this miRNA. Through atelocollagen-mediated delivery of a miR-516a-3p expression vector into orthotopic highly metastatic tumors, the feasibility of using this miRNA as a treatment agent has been reported. These findings define miR-516-3p as an antimetastatic miRNA with potential therapeutic applications in blocking metastatic dissemination of GCs [22].

Epithelial–mesenchymal transition (EMT) plays a key role in cancer invasion and metastasis. A study reported that siRNA-mediated knockdown of DCAMKL-1 in human pancreatic cancer cells induces miR-200a along with downregulation of EMT-associated transcription factors ZEB1, ZEB2, Snail, Slug and Twist. These results rationalize DCAMKL-1 as a therapeutic target for regulating EMT in pancreatic cancer through a miR-200a-dependent mechanism [23]. On the other hand, chemotherapy has been reported to induce EMT in tumor cells. A study illustrated that miR-15b and miR-200b are downregulated in chemotherapy-resistant tongue cancer cells. Ectopic expression of these miRNAs effectively reverses the phenotype of EMT and sensitizes cancer cells to chemotherapy, while enforced miR-15b or miR-200b expression also suppresses metastasis of tongue cancer xenografts. BMI1 has been identified as a target for miR-15b and miR-200b and these miRNAs may serve as therapeutic targets to reverse chemotherapy resistance in tongue cancers [24].

3. Expert opinion

An improved understanding of the underlying molecular mechanisms of miRNAs may help to translate their application as therapeutic targets in cancer. One of the advantages of miRNA-based therapy is that it can target plenty of genes in different signaling pathways. However, this also comes with the disadvantage of many unknown off-target effects. Although successful in vivo studies support the use of miRNAs as an effective intervention to treat preclinical cancer models [25], some small physiological changes (such as changes in liver and spleen size, as well as changes in the concentrations of serum proteins and metabolites) indicate that challenges remain for non-toxic delivery of miRNA therapeutics in vivo [26].

Technically, even though miRNAs are easy to synthesize, overcoming the hurdles in systemic delivery still constitutes a major obstacle to effective miRNA-based therapy. Rapid advances in systemic drug delivery systems shed light on the emergence of successful miRNA-based therapeutics. A recent preclinical study has demonstrated a therapeutic formulation using chemically synthesized miR-34a and a lipid-based delivery vehicle that blocks tumor growth in mouse models of non-small cell lung cancer. This formulation is well tolerated and does not induce an immune response, this approach may facilitate a rapid route for miRNA replacement therapy into clinical settings [27].

Other preclinical researches improve our understanding of the functional roles of miRNAs in cancer biology, which also leads to the discovery of novel therapies for cancer [28]. Although there is still a long way to go before the advent of an effective and safe miRNA-based cancer therapy, new findings from recent studies provide an optimistic perspective on the evolution of the field.

Declaration of interest

The author states no conflict of interest and has received no payment in preparation of this manuscript.