ERK is one of the first mammalian MAPK genes to be identified and cloned. The cDNAs of ERK1 and ERK2 were both cloned as early as the 1990s, and they share up to 83% of identical amino acids[14,15]. Moreover, there are other isoforms of ERK, including ERK3, ERK4, ERK5, and ERK7/8[16]. In this section, we will mainly discuss the most important members that play critical roles in cancer invasion and metastasis, ERK1/2.

The integral ERK/MAPK pathway can be roughly divided into three levels, which are summarized in Figure 1. Raf isoforms are the most well-studied kinases, constituting the highest level of the ERK/MAPK pathway, and are also known as MAPKKKs. Extracellular growth factors, insulin and G-proteins may activate the MAPKKKs by directly binding to the N-terminus of the Raf protein and transforming its structure through phosphorylation. Then, the activating signal is passed to the MAPKKs through the phosphorylation of two serine residues on the MEK1 or MEK2 protein. The signal is finally transmitted to ERK by MEK1/2 through the phosphorylation of tyrosine and threonine residues[17]. When the entire signaling pathway is completely activated in order, hundreds of ERK/MAPK pathway substrates are phosphorylated, and these events affect ERK-dependent cellular activities, including cell proliferation, differentiation, neuronal flexibility, cell viability, cellular stress response and apoptosis[2].

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Figure 1 The integral extracellular signal-regulated kinase/mitogen-activated protein kinase pathway. ERK: Extracellular signal-regulated kinase; MAPK: Mitogen-activated protein kinase.

The dysregulation of ERK/MAPK occurs in various human diseases, including neurodegenerative diseases, developmental disorders, metabolic diseases, and cancer[18-22]. In the last decade, scientists increasingly focused on the relationship between the ERK/MAPK pathway and tumor genesis and progression, because it was found that mutation or abnormal activation of the ERK/MAPK pathway exists in over half of human cancer types[23]. As an upstream binding kinase of the ERK/MAPK pathway, Ras was reported to mutate to an oncogenic form in more than 15% of human cancers. Additionally, B-RAF mutated in 66% of malignant melanomas. Point mutations of the Ras and B-RAF genes cause dysregulation of the ERK/MAPK pathway and abnormal cellular motility, which primarily lead to the migration and invasion of cancer cells[24].

Many studies have elucidated that the ERK/MAPK pathway plays an active role in the invasion and metastasis progression of some malignant tumors. The pathologic process of tumor invasion and metastasis requires cell motility. The alteration of cellular adhesiveness directly affects cell movement. The epidermal growth factor receptor (EGFR)-induced disassembly of focal adhesions is regulated by activating the ERK/MAPK pathways[25]. Another study of epithelial cells demonstrated that ERK in vitro was closely correlated with metastasis both in the TGF-beta and the RAS/MAPK pathways[26]. Metastasis induced by dysregulation of ERK was also demonstrated in animal models. All of the three domain mutations V12S35, V12G37, and V12C40 of Ras are able to induce tumor genesis, but only the V12S35 mutation, which affects the activation of the ERK/MAPK pathway, rather than the other two domain mutations of Ras, induced tumor metastasis in mouse models[27]. This study showed that Ras could induce tumor genesis independently of the ERK/MAPK pathway; however, the ERK/MAPK pathway is indispensable in Ras domain mutation-induced tumor metastasis.

Recently, an increasing number of studies have revealed that the ERK/MAPK pathway is involved in regulating cellular mobility in gastric malignant tumors and gastric cancer cell lines. ERK may mediate the activity of MMPs, which in turn influences gastric cancer cell migration and invasion[28-30]; conversely, many factors upstream of the ERK/MAPK pathways, such as interleukin-22 (IL-22), RASAL1, phophatase of regenerating liver 3 (PRL3), NAIF1, CCDC134, and ZIC1, may influence invasion and migration in gastric cancer cell lines through the ERK/MAPK pathway[30-35]. Currently, most studies focus on the role of the ERK/MAPK pathway in gastric cancer cell lines. Evidence in tissues and animal models is sparse, and further research is needed.

Introduction of p38: p38 alpha, beta, gamma and delta are the four well-known members of the p38 MAPK family, of which p38 alpha and p38 beta are expressed ubiquitously, whereas the p38 gamma and p38 delta have more restricted expression patterns[16].

The mammalian p38 MAPK pathway is affected by various environmental stressors, including oxidative stress, UV, hypoxia, ischemia, as well as inflammatory cytokines and transforming growth factor-α (TGF-α)[36]. MEKK1-3 (MEK kinase 1-3), MLK2/3 (mixed lineage kinase 3), ASK1 (apoptosis signal regulating kinase 1), Tpl2 (tumor progression loci 2), TAK1 and TAO1/2 (thousand and-one amino acid) are all MAPKKKs in the P38 MAPK pathway[37]. These MAPKKKs activate p38 by phosphorylating it at the Thr-Gly-Tyr motif through selective activation of MKK3/6. MKK6 phosphorylates all four members of the p38/MAPK family, whereas MKK3 phosphorylates p38 alpha, p38 gamma, and p38 delta but not p38 beta[37]. p38 is found in both the nucleus and the cytoplasm and translocates into the nucleus in response to certain types of stress. The P38 kinase affects certain types of downstream substrates after being activated, including cPLA2, MNK1/2, MK2/3, HuR, Bax, and Tau in the cytoplasm and ATF1/2/6, MEF2, Elk-1, GADD153, Ets1, P53 and MSK1/2 in the nucleus[37].

The p38 pathway has been implicated in a range of complex biological processes, such as cell proliferation, differentiation, migration, and apoptosis. Dysregulation of P38 in patients with solid tumors, such as prostate cancer, breast cancer, bladder cancer, liver cancer and lung cancer, is associated with advanced stages and low survival rates[38]. The p38 signaling pathway exhibits some anti-tumor effects in xenograft experiments[39,40]. In hepatocellular carcinoma, the activity of P38 is down-regulated in the cancer tissue compared with the adjacent normal tissue, and the tumor volumes are related to the p38 activity[41]. As a result, tumor cells must modulate p38 activity to achieve metastasis and invasion.

The epithelial-mesenchymal transition (EMT) process plays an important role during the initiation of metastasis. P38 signaling is involved in the regulation of EMT in several ways. For example, p38 participates in regulating the EMT activity in mammary epithelial cells and in primary ovarian tumors by separately regulating the phosphorylation of Twist1 and the expression of Snail[42,43]. P38 is also involved in ROS-triggered EMT, and this process may be reversed by the introduction of the p38 inhibitor SB203580[44,45]. The MMP protein family plays a critical role in remodeling the extracellular matrix during metastasis. There are many studies of the relationship between p38 and the expression of MMP family members in liver, prostate, breast and skin cancer cell lines. Following inhibition of p38 in these cell lines, cellular invasion decreases[46-48]. Many small molecules can regulate the metastasis and invasion of gastric cancer through regulation of the p38 signaling pathway. For example, S100A8 and S100A9, the low-molecular-weight members of the S100 family of calcium-binding proteins, induced gastric cancer cell migration and invasion involving p38 and NF-κB, whereas they did not affect cell proliferation and cell viability, which leads to an increase in MMP2 and MMP12 expression[49]. In addition, the widely used anti-tumor drug baicalein also inhibits gastric cancer cell invasion and metastasis by reducing cell motility and migration via suppression of the p38 signaling pathway[50].

The c-Jun N-terminal kinase (JNK), also known as stress-activated protein kinase (SAPK), was first identified and named as c-Jun transcription factor. The three isoforms of JNK, JNK1/2/3, were cloned in the mid-1990s. These three isoforms are encoded by three different genes, sharing more than 85% homology; these isoforms result from more than 10 splices and have molecular weights varying from 46 kD to 55 kD[51,52]. JNK1/2 is widely expressed in mammary tissues, whereas JNK3 is expressed mainly in nervous tissues, testis, and cardiac muscle[53]. All members of the JNK family are activated by various stimulating factors, such as heat shock, oxidative stress, DNA damage, UV, cellular factors, and serum[54]. The MAPKKKs in the MAPK/JNK signaling pathway include MEKK1-4, MLK1-3, Tpl-2, DLK, TAO1/2, TAK1, and ASK1/2. These MAPKKKs activate the MAPKKs, such as MKK4 and MKK7, through phosphorylation. Subsequently, activated MKK4/7 will activate JNK by phosphorylating its threonine and lysine residues[16].

JNK serves as an important kinase that regulates cellular activity; current research suggests that JNK plays opposite roles in cancer initiation and development[55]. Experiments demonstrated that a mouse model with a JNK2 deficiency shows a lower incidence of tumor initiation[56]; however, a mouse model with a JNK1 deficiency may generate tumors more frequently compared with control mice[57]. Therefore, small molecular inhibitors of JNK could not be easily used in cancer therapy.

Additionally, JNK is also involved in gastric cancer metastasis and invasion. Recombination in Erdr1 suppresses the ability of the gastric cancer cell line SNU-216 to metastasize by up-regulating E-cadherin through JNK pathway activation, and this event is reversed by treating the cells with the JNK inhibitor SP600125[58]. JNK is also involved in TGF-beta1-induced invasion and metastasis of gastric cancer cells. As an important mediator of the tumor response to TGF-beta, fascin1 functions through the TGF-beta1-JNK/MAPK pathway to regulate gastric cancer invasion and metastasis[59].