Role of microRNAs in gastric cancer

Abstract

Although gastric cancer (GC) is one of the leading causes of cancer-related death, major therapeutic advances have not been made, and patients with GC still face poor outcomes. The prognosis of GC also remains poor because the molecular mechanisms of GC progression are incompletely understood. MicroRNAs (miRNAs) are noncoding RNAs that are associated with gastric carcinogenesis. Studies investigating the regulation of gene expression by miRNAs have made considerable progress in recent years, and abnormalities in miRNA expression have been shown to be associated with the occurrence and progression of GC. miRNAs contribute to gastric carcinogenesis by altering the expression of oncogenes and tumor suppressors, affecting cell proliferation, apoptosis, motility, and invasion. Moreover, a number of miRNAs have been shown to be associated with tumor type, tumor stage, and patient survival and therefore may be developed as novel diagnostic or prognostic markers. In this review, we discuss the involvement of miRNAs in GC and the mechanisms through which they regulate gene expression and biological functions. Then, we review recent research on the involvement of miRNAs in GC prognosis, their potential use in chemotherapy, and their effects on Helicobacter pylori infections in GC. A greater understanding of the roles of miRNAs in gastric carcinogenesis could provide insights into the mechanisms of tumor development and could help to identify novel therapeutic targets.

Key Words: MicroRNA, Gastric cancer, Reverse transcription-polymerase chain reaction, Chemosensitivity, Helicobacter pylori, Circulating MicroRNA

Core tip: In this review, we discuss the involvement of miRNAs in gastric cancer (GC) and the mechanisms through which they regulate gene expression and biological functions. Then, we review recent research on the involvement of miRNAs in GC prognosis, their potential use in chemotherapy, and their effects on Helicobacter pylori infections in GC. A greater understanding of the roles of miRNAs in gastric carcinogenesis could provide insights into the mechanisms of tumor development and could help to identify novel therapeutic targets.

INTRODUCTION

Despite the decreasing incidence of gastric cancer (GC) in developed countries, GC remains the second leading cause of cancer-related deaths worldwide[1,2], with 700000 deaths attributed to this malignancy annually[3]. Therefore, the development of novel therapies to improve the prognosis of patients with GC is critical.

MicroRNAs (miRNAs) are a subset of small noncoding RNA molecules, typically 21-23 nucleotides in length, that are believed to regulate the expression of several genes[4]. Mature miRNAs are cleaved from 70- to 100-nucleotide hairpin pre-miRNA precursors[4]. The precursor is cleaved by cytoplasmic RNase III Dicer into a miRNA duplex[5-7]. One strand of the short-lived duplex is degraded, whereas the other strand serves as the mature miRNA[8]. Mature miRNAs associate with a cellular complex that is similar to the RNA-induced silencing complex that participates in RNA interference[9,10]. Recent studies have reported miRNA-mediated regulation of cell growth and apoptosis[11,12]. Moreover, the measurement of miRNA expression has shown that certain miRNAs are specifically involved in cancer[12-15]. In the context of GC, the number of publications investigating the relationship between miRNAs and GC has been increasing each year. miRNAs have the unique ability to negatively regulate gene expression, thereby resulting in changes in cell development, proliferation, and apoptosis[16]. These biological properties of miRNAs may provide the ability to regulate a variety of human diseases, including cancer[17]. According to recent findings, miRNAs may play important roles in human cancer by acting as potential oncogenes or tumor-suppressor genes[18-20].

In this review, we introduce and discuss the newest knowledge on the relationship between GC and miRNAs.

Aberrant expression of miRNAs in GC

Although more than 1000 miRNAs are thought to exist, no comprehensive analysis of miRNA expression in GC has been performed to date. Each miRNA modulates the expression of hundreds of genes, and we speculate that miRNAs act in a network-type fashion to mediate the expression of genes.

While several reports have published comprehensive expression analysis of miRNAs[21,22], more data, including expression profile analysis by high-throughput real-time reverse transcription-polymerase chain reaction (RT-PCR) or miRNA microarrays, are required. A summary of reported miRNA expression abnormalities in GC is presented in Table 1.

Table 1 Aberrant expression of miRNAs in gastric cancer.

Up-regulated miRNAs

let-7a, miR-9, -10a, -10b, -17, -17-5p, -18a, -18b, -19a, -19b, -20a, -20b, -21, -23a, -23b

miR-25, -26b, -27, -29b-1, -30b, -31, -34a, -34b, -34c, -92, -98, -99a, -100, -103, -106a

miR-106b, -107, -125b, -126, -128a, -130b, -138, -142-3p, -146a, -147, -150, -151-5p

miR-155, 181a, -181a-2, -181b, -181c, -185, -191, -192, -194, -196a, -196b, -199a

miR-199a-3p, -200b, -210, -214, -215, -221, -222, -223, -296-5p

miR-301a, -302f, -337-3p, -340, -370, -421, -520c-3p, -575, -601, -616, -658, -1259

Down-regulated miRNAs

let-7a, -7f, miR-7, -9, -22, -29c, -30a-5p, -31, -34a, -34b, -34c

miR-101, -126, -128b, -129, -129-2, -129-3p, -130b, -133b, -135a, -137, -141, -145

miR-146a, -148, -148b, -149, -152, -155, -181b, -181c, -182, -193b, -195, -195-5p, -197

miR-200, -204, -206, -210, -212, -218, -219-2-3p, -302b, -331-3p, -375

miR-378, -408-3p, -429, -433, -486, -495, -551a, -574-3p, -610, -622, -638, -663, -874

Upregulated miRNAs (oncomirs): Aberrant expression of miRNAs has been observed in many cancers[19,23]. Oncomirs are oncogenic miRNAs that are up-regulated in cancer cells and have been shown to act as oncogenes in GC. One such oncomir is miR-21, which has been widely reported as an oncomir in GC[24-26].

Overexpression of miR-21 has been reported in various cancers, such as esophageal cancer[27], breast cancer[28], and glioblastoma[29]. In GC, the expression of miR-21 is upregulated compared with normal tissues[22,30,31]. Motoyama et al[31] and Cao et al[32] reported an inverse correlation between miR-21 and PDCD4 expression in GC. PDCD4, a direct target gene of miR-21, encodes a protein that inhibits cell growth and invasion[33,34]. Moreover, Zhang et al[26] reported that PTEN, a well-known tumor-suppressor gene, is a target of miR-21. Thus, these data support the idea that miR-21 acts as a key oncomir in GC by inhibiting the tumor-suppressor genes PDCD4 and PTEN.

In addition to miR-21, miR-106a expression is also upregulated in GC (Table 1), as well as in several other human tumors, compared with adjacent normal tissues[35]. miR-106a mimics the function of positive regulators of the G₁-to-S transition[35]. In several human hematopoietic cell lines, miR-106a has been shown to target interleukin (IL) 10[36], downregulating the expression of this critical cytokine by binding to the 3′ untranslated region[36]. As a further regulatory element, SP1 and EGR1 indirectly downregulate IL10 expression by inducing miR-106a expression[36].

Downregulated miRNAs (tumor-suppressor miRNAs): Down-regulated miRNAs in cancer tissue are referred to as tumor-suppressor miRNAs[37-39]. The important target gene of a tumor-suppressor miRNA is usually an oncogene. Therefore, decreased expression of the tumor-suppressive miRNA induces the expression of the oncogene. The genomic loss of miR-101 in cancer leads to overexpression of EZH2 and concomitant dysregulation of epigenetic pathways, resulting in cancer progression[40,41]. Because miR-101 targets cyclooxygenase (COX) 2 in GC, downregulation of miR-101 induces COX2 expression[42]. COX2 activates the arachidonic acid/prostaglandin E2 pathway following cell proliferation[42]. Oncogenic targets of miR-101 induce cell proliferation in GC. Therefore, miR-101 may be useful for gene therapy in GC.

Another miRNA that has been suggested to have a role in cancer is let-7. Expression of let-7 reduces the expression of 3 human RAS genes, HRAS, KRAS, and NRAS. Moreover, let-7 expression is lower in lung tumors than in normal lung tissue, whereas expression of the RAS proteins is significantly higher in lung tumors, suggesting a possible role of let-7 in cancer[43]. The expression of let-7 miRNA is also reduced in human lung cancer[44], breast cancer[45], and hepatocellular carcinomas[46]. In addition, overexpression of let-7 inhibits the growth of lung cancer cells in vitro[44]. In GC, RAB40C, a target of let-7a, has been reported to play an essential role in gastric tumorigenesis[47].

MiR-148a has been shown to act as a tumor suppressor in prostate cancer, and its expression is lower in prostate cancer cells compared with normal prostate epithelial cells[48]. In GC, miR-148a is inactivated by hypermethylation of the promoter region[49]. This may result in the upregulation of DNA methyltransferase, which is a target of miR-148a[49]. Moreover, miR-148a suppresses tumor cell invasion by downregulating ROCK1[50]. In a report from our laboratory, we found that miR-148a expression is downregulated in undifferentiated GC[51].

Roles of miRNAs as prognostic factors

Because many factors affect the prognosis of cancer patients, it is difficult to clarify how miRNAs are involved in the prognosis of patients with GC. However, advances in research on the potential role of miRNAs in patient prognosis may lead to the use of miRNAs as tools in medical treatment or diagnosis in the future. The detection of miRNAs involved in the prognosis of GC patients will not only be useful for predicting prognosis but helpful for developing therapeutic targets in the future. In Table 2, we have summarized the current knowledge on the relationship between miRNAs and prognosis. In particular, miR-21, -93, and -125 have been well studied in this context. While we mentioned the usefulness of miR-21 previously, overexpression of miR-93 in GC cells has been shown to reduce the cellular response to transforming growth factor (TGF)-β (TGFB1) by interfering with the synthesis of p21 (CDKN1A) and BIM (BCL2L11), the 2 most important downstream effectors of TGF-β-dependent cell cycle arrest and apoptosis, respectively[52]. High expression of miR-98 was found to predict poor survival[53,54]. Interestingly, low expression of miR-125 in GC has been shown to be an independent prognostic factor for survival[55,56]. One target gene of miR-125a-5p is ERBB2 (HER2), which is an important molecular target in chemotherapy[56].

Table 2 miRNAs associated with prognosis in gastric cancer patients.

let-7a ,-7i

miR-10b, -20a, -20b, -21, -22, -25, -27a, -30a-5p, -34a, -93

miR-103, -106a, -106b, -107, -125a-5p, -126, -130, -142-5p, -144, -146a, -150

miR-155, -181c, -195, -196a, -199a-3p, -200c, -206, -221, -222, -223

miR-335, -338, -372, -375, -451

Notably, many genes act as prognostic factors for patients with GC; miRNAs may regulate these genes, thereby affecting prognosis. Thus, further analysis of candidate miRNAs is necessary.

MiRNAs involved in chemosensitivity

Chemotherapy is an important tool for the treatment of GC. However, with currently available tools, it is impossible to predict whether GC patients will respond to chemotherapeutic approaches. The ability to predict the effects of chemotherapy may help reduce the unnecessary use of chemotherapeutics in GC. Current chemotherapeutic agents used in the treatment of GC include 5FU, CDDP, taxan, and irinotecan. Many studies have reported that miRNAs may affect the efficacy of chemotherapy. Wang et al[57] identified 9 upregulated miRNAs and 18 downregulated miRNAs involved in 5FU sensitivity by microarray and RT-PCR (Table 3). Moreover, miR-143, miR-145, and miR-144 have been reported to be involved in 5FU sensitivity[58,59]. In Table 3, we summarize the involvement of miRNAs in CPT, CDDP, and CF sensitivity. In the near future, we will be able to use miRNA expression as a predictor of chemotherapeutic efficacy. Additionally, gene therapy with miRNAs may be able to induce chemosensitivity in patients with GC.

Table 3 MiRNAs involved in chemosensitivity.

5FU sensitivity

let-7g

miR-10b, -22, -30c, -31, -32,

miR-133b, -143, -144, -145, -181b, -190, -197, -200c, -204, -210

miR-335, -501, -501-5p, -532, -615, -615-5p, -766, -877

miR-1224-3p, -1229, -3131, -3149, -3162-3p, -4763-3p

CPT sensitivity

let-7g

miR-7, -31, -98, -126, -196a, -200, -338

CDDP, CF sensitivity

let-7g

miR-1, -16, -21, -34, -181, -181b, -342, -497

MiRNAs involved in Helicobacter pylori infection

Helicobacter pylori (H. pylori) selectively colonize the gastric epithelium and typically persist for the lifetime of the host. Among colonized individuals, however, only a fraction develop gastric adenocarcinoma, emphasizing the importance of understanding the pathogenic mechanisms through which H. pylori promote chronic inflammation and the progression to GC[60]. miRNAs involved in H. pylori infections have been reported in several papers. Let-7 expression has been shown to be downregulated by Cag A after H. pylori infection, and Ras, a target of let-7, is overexpressed in GC[61]. Additionally, miR-17/92, the miR-106b-93-25 cluster, miR-21, miR-194, miR-196, miR155, miR-222, and miR-223 are upregulated in gastric mucosa infected by H. pylori[62-64]. Among these miRNAs, Li et al[64] revealed that miR-222 targets RECK, which inhibits the tumorigenicity of GC[64-66]. Further analysis of miRNAs and target genes may clarify the complicated mechanism of GC that occurs in the context of H. pylori infections.

Circulating miRNAs as biomarkers

Presently, circulating miRNAs found in the blood of patients constitute the most promising type of miRNA for clinical use, because these are concise for blood collecting. We summarized the circulating miRNAs in the blood of GC patients in Table 4.

Table 4 Circulating miRNAs as biomarkers.

Up-regulated miRNAs

miR-1, -17, -17-5p, -20a, -21, -27a, -31, -34, -103, -106a, -106b, -107,-194, -200c

miR-210, -221, -223, -370, -376a, -378, 421, 423-5p, 451, -486, 744

Down-regulated miRNAs

miR-218, -375

There are a number of reports describing the use of miR-106[67-69], miR-17[67,70], miR-21[67,71], and miR-221 as potential biomarkers[72]. The detection of miRNA in peripheral blood may be a novel tool for monitoring circulating tumor cells in patients with gastric cancers. Moreover, circulating miRNA may be a promising, non-invasive molecular marker for tracking pathological progression, predicting prognosis and monitoring chemotherapeutic effects in gastric cancer.

Finally, the precise mechanism of each miRNA is not well known. Further reports about miRNA are expected to help us better understand cancer mechanisms. This research will be useful for clinical diagnosis or treatment for GC patients.

CONCLUSION

In this review, we presented and discussed the newest knowledge on miRNAs in gastric cancer and their potential usefulness as future medical treatments and diagnostic tools. Although the molecular biology of GC has been well characterized, research on miRNAs in GC is still in its infancy. Thus, in the near future, we anticipate that advances in miRNA research in GC may help to develop novel medical treatments or diagnostic tools, thereby improving the prognosis of GC patients.