The Role of microRNAs in Helicobacter pylori Pathogenesis and Gastric Carcinogenesis

Abstract

Gastric carcinogenesis is a multistep process orchestrated by aberrancies in the genetic and epigenetic regulation of oncogenes and tumor suppressor genes. Chronic infection with Helicobacter pylori is the strongest known risk factor for the development of gastric cancer. H. pylori expresses a spectrum of virulence factors that dysregulate host intracellular signaling pathways that lower the threshold for neoplastic transformation. In addition to bacterial determinants, numerous host and environmental factors increase the risk of gastric carcinogenesis. Recent discoveries have shed new light on the involvement of microRNAs (miRNAs) in gastric carcinogenesis. miRNAs represent an abundant class of small, non-coding RNAs involved in global post-transcriptional regulation and, consequently, play an integral role at multiple steps in carcinogenesis, including cell cycle progression, proliferation, apoptosis, invasion, and metastasis. Expression levels of miRNAs are frequently altered in malignancies, where they function as either oncogenic miRNAs or tumor suppressor miRNAs. This review focuses on miRNAs dysregulated by H. pylori and potential etiologic roles they play in H. pylori-mediated gastric carcinogenesis.

Introduction

Microbial infections are among the most significant causes of cancer worldwide with nearly one in five malignancies resulting from infectious agents (Parkin, 2006). Gastrointestinal cancers represent a major global health concern and account for nearly 20% of all cancer-related deaths (Ferlay et al., 2007). Despite the decreasing incidence of gastric cancer in developed countries, it remains the second leading cause of cancer-related death throughout the world, with ∼700,000 deaths attributed to this malignancy annually (Parkin et al., 2005). The major contributing factor to the development of gastric cancer is colonization and chronic infection by the bacterial pathogen, Helicobacter pylori. H. pylori selectively colonizes the gastric epithelium of over 50% of the world’s population and typically persists for the lifetime of the host. Among colonized individuals, however, only a fraction develop gastric adenocarcinoma, emphasizing the importance of understanding the pathogenic mechanisms by which H. pylori promotes chronic inflammation and the progression to gastric cancer.

Virulence Factors that Mediate Helicobacter pylori Pathogenesis

Chronic gastric inflammation induced by the bacterial pathogen, H. pylori, is the strongest known risk factor for the development of atrophic gastritis, metaplasia, dysplasia, and ultimately gastric adenocarcinoma (Figure 1). H. pylori is a Gram-negative, helical-shaped bacterium specifically adapted to persist within the human gastric niche. H. pylori possesses numerous elements to successfully colonize the gastric mucosa, establish chronic infection, and induce gastric pathology. In vivo, approximately 20% of H. pylori adhere to the gastric epithelium (Hessey et al., 1990). The large repertoire of adhesins expressed by H. pylori likely contribute to its specific adaptation to the gastric niche, allowing flexibility to target specific host cells and to exert a dynamic range of effector functions on host cells. H. pylori expresses a number of adhesins that have been linked to virulence. SabA (sialic acid-binding adhesin), which binds host sialyl-Lewis^x, contributes to H. pylori persistence and mediates chronic gastric inflammation and injury (Mahdavi et al., 2002). The presence of blood group antigen-binding adhesin (BabA), which binds the host Lewis^b blood group antigen, increases the risk of gastric cancer in a synergistic fashion with other virulence factors, such as CagA (Ilver et al., 1998; Gerhard et al., 1999).

Figure 1

Following adherence and colonization of the gastric mucosa, H. pylori induces chronic gastritis and gastric injury, which are characterized by both neutrophilic and lymphocytic inflammation (Marshall et al., 1985; Goodwin et al., 1986). H. pylori expresses a number of factors capable of modulating the host immune system and eliciting proinflammatory immune responses. Some of these virulence factors include vacuolating cytotoxin (VacA) and the cag (cytotoxin associated gene) pathogenicity island. VacA is coded by the gene vacA, which is present in all strains of H. pylori, and which exhibits vacuolating activity (Leunk et al., 1988; Cover and Blaser, 1992; Cover et al., 1994; Phadnis et al., 1994; Schmitt and Haas, 1994; Telford et al., 1994). Additionally, VacA can induce apoptosis of host cells (Kuck et al., 2001; Xia and Talley, 2001) and suppress proliferation of T and B lymphocytes (Boncristiano et al., 2003; Gebert et al., 2003; Sundrud et al., 2004), which may contribute to the persistence of H. pylori through dysregulation of the host immune response. The cag pathogenicity island is present in ∼60% of all H. pylori strains and its presence is strongly associated with an increased risk of severe gastritis, ulcer disease, and gastric cancer (Censini et al., 1996; Tomb et al., 1997; Akopyants et al., 1998; Alm et al., 1999). The cag island encodes a type 4 secretion system (T4SS), which injects effector molecules, such as CagA, into host cells. CagA is a 120- to 140-kD protein that contains a number of tyrosine phosphorylation motifs (Covacci et al., 1993; Tummuru et al., 1993). Following its injection into host cells, CagA exerts a wide range of phosphorylation-dependent and independent effects, such as cytoskeletal rearrangements, disruption of cell polarity, and mitogenic and proinflammatory responses (Polk and Peek, 2010). Cumulatively, these bacterial factors contribute to adherence, persistence, host immune modulation, and virulence of H. pylori within the gastric niche, ultimately resulting in H. pylori-mediated chronic inflammation and a series of pathological outcomes that facilitate the development of gastric cancer.

Host Factors that Contribute to Gastric Carcinogenesis

In addition to microbial factors that potentiate gastric disease, there are a number of host factors that contribute to chronic gastritis and the progression to gastric adenocarcinoma. Cyclooxygenases (COX) are key enzymes that catalyze prostaglandin synthesis. Of the three isoforms identified, COX-2 is upregulated in gastric epithelial cells upon co-culture with H. pylori (Romano et al., 1998; Juttner et al., 2003; Meyer et al., 2003; Wu et al., 2005) and within the gastric mucosa of H. pylori-infected individuals (Sawaoka et al., 1998; Fu et al., 1999; McCarthy et al., 1999). In vivo studies show that COX-2 is further upregulated in H. pylori-mediated adenocarcinoma (Ristimaki et al., 1997; Sung et al., 2000). COX-2 expression levels are considered an independent factor for poor prognosis and correlate with reduced patient survival, suggesting that H. pylori-induced COX-2 overexpression is a risk factor for the development of gastric cancer.

Other host factors that increase the propensity for chronic inflammation and gastric adenocarcinoma are polymorphisms within human IL-1β (interleukin-1), TNF-α (tumor necrosis factor), and IL-8 promoters (El-Omar et al., 2000, 2003; Machado et al., 2001; Furuta et al., 2002), which lead to increased expression of the proinflammatory cytokines IL-1β, TNF-α, and IL-8 (Figure 1). These polymorphisms in combination with H. pylori virulent genotypes increase the risk of gastric cancer up to 87-fold over baseline (Figueiredo et al., 2002), emphasizing the importance of microbial–host interactions in the development of gastric cancer. Collectively, data demonstrate that H. pylori virulence factors, host genetics, and environmental factors interact to induce and maintain the persistent inflammatory immune response that initiates the multistep process leading to gastric cancer.

microRNAs

Recent discoveries have shed new light on the involvement of host microRNAs (miRNAs) in gastric carcinogenesis. miRNAs are small, non-coding RNAs ∼20–25 nucleotides in length, which function as critical post-transcriptional regulators of gene expression (Bartel, 2009). miRNAs were first characterized in 1993 (Lee et al., 1993), but their distinct role in transcriptional regulation was not recognized until the early 2000s. Most miRNAs are found in intergenic regions and contain their own promoter and regulatory units. Processed miRNAs function by binding to the 3′ untranslated region (3′UTR) of messenger RNAs (mRNAs), typically resulting in mRNA degradation and gene silencing or translational repression (Bartel, 2009). It is estimated that the human genome encodes thousands of miRNAs, targeting ∼30–60% of all protein-coding genes (Lewis et al., 2003). miRNAs are involved in many biological processes, including development, differentiation, angiogenesis, cell cycle progression, proliferation, apoptosis, and signal transduction pathways (Ambros, 2004). Dysregulation of miRNA expression with subsequent disruption of these biological processes can result in disease states. There is an increasing body of evidence regarding the regulatory roles of miRNAs in immune and inflammatory disorders (Wu et al., 2008; Sonkoly and Pivarcsi, 2009), and aberrant expression of miRNAs is observed in many cancers (Lu et al., 2005; Volinia et al., 2006). Thus, recent studies have begun to dissect the mechanisms by which miRNAs function as either oncogenic miRNAs (oncomiRs) or tumor suppressors to promote or prevent tumorigenesis.

Dysregulation of miRNAs in H. pylori-induced Gastric Carcinogenesis

The number of studies analyzing miRNA expression profiles in gastric cancer is rapidly increasing and a comprehensive list of miRNAs dysregulated in gastric cancer, confirmed mRNA targets, and the biological processes affected is shown in Tables A1 and A2 in the Appendix. The first study to address miRNA expression profiles in various cancers, including gastric cancer, was performed in 2005 (Lu et al., 2005). Subsequent studies have not only focused on miRNA expression profiles in gastric cancer, but also those that are altered in response to H. pylori.

Matsushima et al. (2011) conducted a study to characterize miRNA expression signatures in H. pylori-infected human gastric mucosa. Using high throughput profiling analysis, 31 miRNAs were identified as being differentially expressed between H. pylori-infected and H. pylori-uninfected gastric mucosa. The relationship between miRNA expression levels and H. pylori-induced acute inflammation, characterized by neutrophil infiltration, and chronic inflammation, characterized by mononuclear cell infiltration were also determined. Expression levels of many miRNAs correlated with either the degree of acute or chronic inflammation and in some cases both (Tables 1 and 2). The relationship between miRNA expression and extent of glandular atrophy, and intestinal metaplasia was also assessed, but no significant correlations were found (Matsushima et al., 2011). A comprehensive list of miRNAs dysregulated by H. pylori, confirmed mRNA targets, and biological processes affected is shown in Tables 1 and 2. These data suggest that H. pylori infection affects global miRNA expression in human gastric mucosa, and this effect is, in part, linked to H. pylori-induced host inflammatory immune responses.

miRNAs that are dysregulated in response to H. pylori infection may not be the same miRNAs that are dysregulated in later stages of gastric disease. A comprehensive review of the literature, however, revealed that there is a select subset of miRNAs dysregulated both following H. pylori infection as well asin gastric cancer. These include downregulated miRNAs, let-7a, miR-31, miR-101, miR-141, miR-203, miR-210, miR-218, miR-375, and miR-449 as well as upregulated miRNAs, miR-17, miR-20a, miR-21, miR-146a, miR-155, and miR-223. These miRNAs may be more biologically relevant to H. pylori-induced gastric inflammation and carcinogenesis and represent fruitful targets for studies focused on cancer that develops in the context of H. pylori infection.

The next sections will discuss miRNA dysregulation in H. pylori-induced disease and how specific miRNAs control various biological processes related to (1) host inflammatory immune response, (2) cell cycle progression, and (3) apoptosis and proliferation.

Helicobacter pylori-induced miRNA Dysregulation to Control Host Inflammatory Responses

Host cells recognize invading pathogens and/or pathogen-associated molecular patterns (PAMPs) through membrane-associated or cytoplasmic pathogen recognition molecules known as Toll-like receptors (TLRs) and Nod-like receptors (NLRs), respectively. PAMPs activate adaptor proteins and transcription factors that mediate host innate immunity through activation of NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) signaling (Figure 2). Gastric epithelial cells are the initial host element encountered by H. pylori. The innate immune response induced in epithelial cells is characterized by NOD1-dependent activation of NF-κB in response to H. pylori peptidoglycan (PGN), which is injected into host cells via the cag T4SS (Viala et al., 2004). Activation of NF-κB by H. pylori leads to induction of the proinflammatory cytokine IL-8 and likely contributes to carcinogenesis through activation of downstream targets that mediate inflammation, cell cycle progression, proliferation, and apoptosis. Myeloid cells constitute a second line of defense and secrete proinflammatory cytokines such as IL-6, IL-1, and TNF-α to establish T and B lymphocyte-mediated adaptive immunity.

Figure 2

The involvement of miRNAs in modulating both the innate and adaptive immune responses is well established (Chen et al., 2004) and H. pylori can dysregulate miRNA expression to evade host defenses and successfully persist in the gastric niche. miR-146a and miR-155 are specifically involved in H. pylori-induced negative regulation of the proinflammatory immune response (Figure 2). Changes in miR-146a expression occur in the development of gastric cancer and in the negative regulation of the innate inflammatory immune response. Single-nucleotide polymorphisms (SNPs) in miR-146a are associated with an increased susceptibility to gastric cancer (Okubo et al., 2010) and H. pylori upregulates miR-146a in vitro and in vivo in a CagA-independent and an NF-κB-dependent manner (Liu et al., 2010; Li et al., 2011a). miR-146a targets the TLR-signaling adaptor molecules IRAK1 (interleukin-1 receptor-associated kinase) and TRAF6 (TNF receptor-associated factor), resulting in negative regulation of TLR and downstream proinflammatory signaling (Figure 2; Liu et al., 2010; Li et al., 2011a). As a result, miR-146a overexpression negatively regulates H. pylori-induced IL-8, TNF-α, IL-1β, GRO-α [CXCL1, chemokine (C-X-C motif) ligand], and MIP-3α (macrophage inflammatory protein) expression, all key components to the proinflammatory innate and adaptive immune responses (Liu et al., 2010; Li et al., 2011a).

The second miRNA involved in H. pylori-induced downregulation of the host inflammatory immune response, miR-155, plays a critical role in regulating lymphocyte homeostasis and tolerance (Thai et al., 2007). miR-155 is increased in many malignancies of B cell or myeloid origin (Volinia et al., 2006). In transgenic murine models of miR-155 overexpression, mice develop spontaneous B cell lymphomas (Costinean et al., 2006). miR-155 is induced during both bacterial and viral infections in myeloid cells through activation of TLR-signaling pathways. H. pylori upregulates miR-155 expression in vitro and in vivo, which occurs in an NF-κB-dependent manner, and ultimately results in decreased induction of the proinflammatory cytokines, IL-8, and GRO-α (Xiao et al., 2009b; Tang et al., 2010). miR-155 targets MyD88 (myeloid differentiation primary response gene), the universal adapter protein used by TLRs to activate NF-κB (Figure 2; Xiao et al., 2009b; Tang et al., 2010). Decreased levels of MyD88 subsequently result in decreased NF-κB activation and dampening of the host inflammatory response (Xiao et al., 2009b; Tang et al., 2010). Therefore, these data demonstrate that H. pylori dysregulates host miRNA expression to manipulate the host inflammatory immune response, which may promote bacterial survival and persistence within the gastric mucosa. Because these miRNAs have established roles in carcinogenesis as well as innate immunity, they could serve as an important link between H. pylori-induced inflammation and carcinogenesis.

Helicobacter pylori and miRNAs Regulate Cell Cycle Progression

Disruption of cell cycle progression and increased cellular proliferation are common features of malignancies. Cell cycle progression requires coordinated expression of cyclins, which results in sequential activation of cyclin-dependent kinases (CDKs). miRNA dysregulation promotes cell cycle progression by upregulating cyclin expression and/or downregulating expression of CDK inhibitors (p15, p16, p18, p19, p21, p27, p28, p57) in various cancers, including gastric cancer (Figure 3). miR-449, a miRNA downregulated in H. pylori-infected gastric tissue and in gastric cancer, targets GMNN (geminin) and CCNE2 (cyclin E2; Figure 3). Both geminin and cyclin E2 are overexpressed in numerous malignancies and promote M/G1 and G1/Scell cycle progression and cell proliferation (Bou Kheir et al., 2011; Lize et al., 2011). Consequently, downregulation of miR-449, as occurs following H. pylori infection, promotes cell cycle progression and proliferation through upregulation of geminin and cyclin E2.

Figure 3

p42.3, a recently identified protein significantly upregulated in gastric cancer, regulates G2/M cell cycle progression and proliferation in gastric cancer cells (Xu et al., 2007). miR-29a, a miRNA significantly downregulated in gastric cancer, targets p42.3 (Cui et al., 2011; Figure 3). Thus, the downregulation of miR-29a results in a reciprocal increase in p42.3 expression, promoting increased cell cycle progression and proliferation.

The retinoblastoma protein (RB1) is a tumor suppressor dysregulated in many cancers. RB1 functions to prevent excessive cell proliferation by inhibiting G1/S cell cycle progression. RB1 binds and inhibits transcription factors of the E2F family. When RB1 is bound to E2F the complex acts as a growth suppressor and prevents progression through cell cycle. A number of miRNAs target these factors. For instance, miR-106a is upregulated in gastric cancer and targets RB1 (Volinia et al., 2006), while miR-331-3p is downregulated in gastric cancer and targets E2F1 (Guo et al., 2010; Figure 3).

TGFβ suppresses gastric cancer cell proliferation via the transcriptional upregulation of the CDK inhibitor, p21 (Yoo et al., 1999). miR-93 and miR-106b directly target p21, resulting in its transcriptional silencing and impairment of the tumor-suppressing activity of TGFβ (Petrocca et al., 2008; Kan et al., 2009; Figure 3). In addition, miR-25 targets the CDK inhibitor, p57, while miR-221 and miR-222 target the CDK inhibitors, p27 and p57 (Kim et al., 2009; Figure 3). These oncogenic miRNA clusters are also significantly upregulated in gastric cancer (Volinia et al., 2006; Petrocca et al., 2008; Guo et al., 2009; Kim et al., 2009; Yao et al., 2009). Overexpression of most of these miRNAs results in activation of CDK2, thereby promoting G1/S phase progression. Since numerous reports have described the role of H. pylori in the modulation of cyclins, CDKs, and CDK inhibitors and their link to gastric carcinogenesis (Shirin et al., 2001), these data suggest that H. pylori modulates expression of cyclins, CDKs, and CDK inhibitors through dysregulation of host miRNAs, which may increase the propensity for gastric transformation.

Helicobacter pylori and miRNA Dysregulation Inhibit Apoptosis and Promote Cell Survival

Increased cellular proliferation and evasion of apoptosis are hallmarks of cellular transformation. Apoptosis can be classified as being dependent on either the intrinsic or extrinsic pathways. The intrinsic pathway is initiated within cells and hinges on the balance of activity between pro-apoptotic and anti-apoptotic members of the Bcl-2 (B cell lymphoma 2) superfamily of proteins, which act to regulate the permeability of the mitochondrial membrane. miRNAs regulate apoptosis by altering expression and balance of members of the pro-apoptotic (e.g., Bax, Bak, Bim, Bad, Bid, and BNIP3L) and anti-apoptotic (e.g., Bcl-2, Bcl-xL, and Mcl-1) Bcl-2 protein family (Figure 4).

Figure 4

Numerous miRNAs overexpressed in gastric cancer function as oncomiRs by targeting members of the pro-apoptotic Bcl-2 protein family. In addition to their role in regulating cell cycle progression, miR-25, miR-93, and miR-106b also inhibit apoptosis by preventing expression of the pro-apoptotic protein, Bim (Kan et al., 2009; Figure 4). Overexpression of miR-130b also contributes to suppression of Bim and apoptosis by targeting RUNX3 (runt-related transcription factor; Lai et al., 2010), a known tumor suppressor frequently silenced in gastric cancer (Li et al., 2002). miR-150 targets the EGR2 (early growth response protein; Wu et al., 2010), a tumor-suppressive transcription factor that induces apoptosis by direct transactivation of pro-apoptotic factors, Bak, and BNIP3L (Unoki and Nakamura, 2003).

Numerous tumor suppressor miRNAs target members of the anti-apoptotic Bcl-2 protein family and are consequently downregulated in gastric cancer. miR-15b, miR-16, miR-34, miR-181b, miR-181c, and miR-497 directly target anti-apoptotic BCL2 (Ji et al., 2008; Xia et al., 2008; Zhu et al., 2010b, 2011b; Figure 4). These miRNA clusters are downregulated in gastric cancer cells (Guo et al., 2009), leading to increased expression of Bcl-2 and inhibition of apoptosis. The miR-200bc/429 cluster is downregulated in gastric cells, and these miRNAs directly target BCL2 and XIAP (x-linked inhibitor of apoptosis), leading to reduced expression and increased apoptosis (Zhu et al., 2011a; Figure 4). miR-101 and miR-512-5p target another anti-apoptotic member of the Bcl-2 family, MCL1 (myeloid leukemia cell differentiation protein; Saito et al., 2009; Wang et al., 2010; Figure 4). Both miR-101 and miR-512-5p are downregulated in gastric cancer, leading to increased levels of Mcl-1 and an anti-apoptotic phenotype. In addition, miR-101 is downregulated by H. pylori (Matsushima et al., 2011). miR-449 is also likely involved in mediating the intrinsic pathway of apoptosis and has been classified as a potent inducer of cell cycle arrest and cell death. miR-449 expression is reduced in H. pylori-infected gastric tissue, and its expression is lost in gastric tumors (Bou Kheir et al., 2011; Lize et al., 2011). Conversely, overexpression of miR-449 inhibits cellular proliferation and induces significant levels of apoptosis, and since miR-449 belongs to the family of p53-responsive miRNAs, its overexpression also results in activation of p53 and apoptosis-specific marker, caspase 3.

In contrast to the intrinsic pathway, the extrinsic pathway of apoptosis is initiated on the cell surface through the activation of specific pro-apoptotic, death receptors. Specific pro-apoptotic ligands are known to activate the extrinsic pathway of apoptosis via specific receptor binding. Ligand binding induces receptor clustering and the recruitment of the adaptor protein Fas-associated death domain (FADD), leading to induction of caspases and ultimately cell death. In addition to its role in regulating the host immune response, miR-155 targets FADD (Figure 4), leading to decreased expression of this key adaptor molecule (Xiao et al., 2009b). Therefore, the upregulation of miR-155 by H. pylori and during carcinogenesis results in downregulation of FADD and inhibition of apoptosis.

In addition to targeting proteins directly involved in the intrinsic and extrinsic pathways of cell death, miRNAs target other factors that ultimately lead to inhibition of apoptosis and increased proliferation. miR-375 targets 14-3-3 zeta, an anti-apoptotic protein that mediates cell survival by binding the pro-apoptotic protein Bad and sequestering it to the cytosol (Tsukamoto et al., 2010; Figure 4). miR-375 also targets PDK1 (3-phosphoinositide dependent protein kinase), a kinase that directly phosphorylates Akt, thereby regulating the PI3K/Akt signaling pathway (Figure 4). Overexpression of miR-375 was shown to substantially reduce cell viability through induction of the caspase-dependent apoptotic pathway. miR-375 is one of the most highly downregulated miRNAs in gastric cancer (Tsukamoto et al., 2010), suggesting its role as a potent tumor suppressor that contributes to the development of gastric carcinoma.

In contrast, miR-21, a known oncomir that targets many known tumor suppressors, is consistently upregulated in various human cancers, including gastric cancer (Volinia et al., 2006; Chan et al., 2008; Petrocca et al., 2008; Zhang et al., 2008; Guo et al., 2009), and miR-21 expression is increased in H. pylori-infected gastric tissues (Zhang et al., 2008). Overexpression of miR-21 shifts the balance between proliferation and apoptosis, increasing cellular proliferation and inhibiting apoptosis. Specifically, miR-21 targets PTEN (phosphatase and tensin homolog), a tumor suppressor and negative regulator of the PI3K/Akt signaling pathway (Yamanaka et al., 2009), which is involved in both apoptotic and proliferative pathways (Figure 4). Mutations in PTEN are important in the progression of many cancers, including gastric carcinoma (Kang et al., 2002). miR-21 likely also contributes to apoptosis by targeting PDCD4 (programmed cell death protein 4), which is localized to the nucleus of proliferating cells; however, its direct role in apoptosis has not been elucidated (Lu et al., 2008; Motoyama et al., 2010).

Similar to PI3K/Akt signaling, the NF-κB signaling pathway is important in inhibition of apoptosis and cell survival. The NF-κB signaling cascade is activated during H. pylori-induced gastritis and is constitutively active in gastric cancer (Sasaki et al., 2001). miR-218 expression is reduced in numerous cancers, including gastric cancer. H. pylori infection also reduces the expression of miR-218 in vitro and in vivo. miR-218 induces apoptosis in gastric cancer cells through direct targeting of ECOP (epidermal growth factor receptor-co-amplified and overexpressed protein), a known positive regulator of NF-κB transcriptional activity. Downregulation of miR-218 leads to overexpression of ECOP, inhibition of NF-κB transcriptional activation, and transcription of a downstream target COX-2, ultimately inhibiting apoptosis, and inducing cell proliferation (Gao et al., 2010). Another miRNA important in regulating NF-κB signal transduction pathways is miR-9, which directly targets NF-κB1, thereby suppressing NF-κB transcriptional activity (Figure 4). miR-9 is downregulated in gastric cancer and in vitro studies have shown that restoration of miR-9 expression suppresses proliferation of gastric cancer cells (Luo et al., 2009; Wan et al., 2010). Cumulatively, these studies demonstrate that aberrant activation of NF-κB signaling as a result of H. pylori-induced miRNA dysregulation results in inhibition of apoptosis and increased proliferation, thereby sensitizing cells for subsequent mutagenesis.

miR-451, another downregulated miRNA in H. pylori-infected gastric mucosa and gastric cancer, targets MIF (macrophage migration inhibitory factor; Bandres et al., 2009), a lymphokine involved in cell-mediated immunity that is expressed in response to H. pylori infection and during gastric carcinogenesis (He et al., 2006). Overexpression of miR-451 results in targeted downregulation of MIF, which is accompanied by a decrease in cell proliferation and increased apoptosis (Figure 4). Furthermore, there is an inverse correlation between miR-451 and MIF expression in gastric cancer, suggesting that miR-451 functions as a tumor suppressor by silencing MIF expression, leading to a proliferative and anti-apoptotic phenotype (Bandres et al., 2009). miR-141, another miRNA significantly decreased in H. pylori-infected gastric tissue (Matsushima et al., 2011) as well as gastric carcinoma, targets FGFR2 (fibroblast growth factor receptor), and overexpression of miR-141 leads to decreased FGFR2 expression and inhibition of proliferation (Du et al., 2009; Figure 4). miR-23a functions as a growth-promoting and anti-apoptotic factor. It is significantly upregulated in gastric adenocarcinoma and targets IL-6R (interleukin-6 receptor), which promotes increased proliferation and decreased apoptosis in gastric adenocarcinoma cells (Zhu et al., 2010a; Figure 4).

Helicobacter pylori and miRNA Dysregulation Promotes Cell Invasion and Metastasis

Invasion and metastasis are hallmarks of cancer cells. Several intracellular signaling pathways, such as those mediated by TGFβ and hepatocyte growth factor/Met signaling, promote metastasis. In addition to its role in regulating cell cycle progression, the H. pylori downregulated miR-449 also targets Met, a known proto-oncogene that encodes the hepatocyte growth factor receptor. Aberrant activation of Met triggers oncogenic processes, such as proliferation, angiogenesis, invasion, and metastasis (Bou Kheir et al., 2011; Lize et al., 2011). Thus, the targeted downregulation of miR-449 by H. pylori and during gastric carcinogenesis results in upregulation of Met, increased cell proliferation, and likely other oncogenic processes.

The metastatic potential of cancer cells is also regulated by mechanisms that control cell survival, cytoskeletal changes, as well as the activity of extracellular matrix-degrading proteinases (MMPs). Many miRNAs known to regulate cell cycle progression, proliferation, and apoptosis pathways are also involved in metastasis. For example, overexpression of miR-21 has been shown to increase the invasiveness of gastric cancer cells. In addition to its known tumor suppressor targets, miR-21 also targets RECK (reversion-inducing-cysteine-rich protein with kazal motifs), a tumor and metastasis suppressor that inhibits tumor metastasis and angiogenesis through modulation of matrix metalloproteinases (MMPs; Zhang et al., 2008). H. pylori induces expression of MMPs, including MMP-1, MMP-2, MMP-3, MMP-7, MMP-9, which have been linked to H. pylori-induced disease and carcinogenesis (Elkington et al., 2005). These data suggest that H. pylori has the potential to modulate expression of MMPs through dysregulation of host miRNAs and these disruptions may increase the propensity for gastric transformation.

miR-106a is significantly upregulated in cancer cells (Volinia et al., 2006) and is known to correlate with increased lymphatic and distant metastasis (Xiao et al., 2009a). Conversely, miR-218, a tumor suppressor miRNA, is downregulated in gastric cancer (Volinia et al., 2006), which correlates with increased metastasis and invasion. This is thought to occur through direct targeting of ROBO1 (roundabout homolog), which leads to enhanced signaling through the ROBO1 receptor. The SLIT/ROBO signaling pathway has been implicated in many biological responses through regulation of cell migration (Tie et al., 2010). Thus, disruption of this signaling cascade can result in increased invasion and metastasis.

Conclusion

The discovery of miRNAs just over a decade ago has challenged the central dogma of genetic and epigenetic regulation. Although extensive work has been dedicated to identifying miRNAs, mRNA targets, and their contribution to accepted regulatory networks, we have only begun to scratch the surface. With thousands of miRNAs within the human genome, and the ability of each miRNA to target and regulate numerous protein-coding mRNAs, affected regulatory networks are likely to be modified by countless miRNA contributors and will continue to evolve.

Many questions arise when comparing miRNA expression profiles in different model systems in vitro and in vivo and when comparing miRNA expression profiles in H. pylori-infected gastric tissue and gastric cancer. For example, miR-106b, a known oncogenic miRNA, upregulated in various malignancies including gastric cancer, is decreased in H. pylori-infected gastric mucosa. Similarly, miR-34b, miR-34c, miR-103, miR-200a, miR-200b, miR-214, and miR-372 are all overexpressed in gastric cancer, while in H. pylori-infected gastric mucosa these miRNAs are significantly downregulated. In contrast, miR-146a is significantly decreased in gastric cancer, but upregulated in H. pylori-infected gastric tissue. However, a recent report has shown that miR-146a is upregulated in a subset of gastric cancers. Although examining miRNA expression signatures in gastric cancer is clearly important in understanding the disease, development of novel therapeutics requires greater insight into the miRNA profiles in precancerous gastric tissues. Since the majority of gastric cancers arise within the context of chronic inflammation, it will be particularly important to discriminate between preneoplastic and tumor-specific miRNA expression profiles. A more comprehensive understanding of the roles of miRNAs in normal biological processes and disease is needed to fully appreciate miRNA dysregulation by pathogens, such as H. pylori. Furthermore, the relationship between single miRNAs and their targets are important to consider, but many of these relationships are cell and context specific. Thus, it is critically important to dissect these intricate pathways and understand how host–pathogen interactions disrupt these encompassing regulatory pathways.

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