Oncogenesis and tumor progression are the multistep and multifactorial process involving different genes, which is accompanied by alterations in a variety of gene expression patterns that in turn affect cancer cell survival, growth, cycle, apoptosis, migration and invasion regulated by these genes . In recent years, accumulating evidence has certified that MECP2, as a key epigenetic regulator, plays an important oncogene role in several cancer types [18, 28]. For examle, MECP2 expression is upregulated and promotes tumor progression in breast cancer, lung cancer, cervical cancer and uterine cancer . It enhances oral squamous cell carcinoma and colorectal cancer growth, and facilitates oncogenesis and development of osteosarcoma and neuroblastoma [29, 30]. Silencing MECP2 reduces human prostate transformed cell proliferation . Recent research has found that MECP2 regulates cancer cell migration and invasion in glioma and breast cancer [32, 33]. Our previous studies have proved that MECP2 promotes cell proliferation and cell cycle G1-S transition, and restrains cell apoptosis in liver cancer, gastric cancer and breast cancer [19–22]. By expanding sample, the present study aims to further identify the effect and the molecular mechanism of MECP2 on GC cell migration and invasion. Our results again demonstrate that MECP2 expression is upregulated in primary GC and the high expression is closely related with tumor histology and T stage. The findings suggest that MECP2 may play a key role in GC progression.
Our results demonstrated that high expression of MECP2 in GC significantly promoted cancer cell migration and invasion, while silencing MECP2 remarkably suppressed cell migration and invasion by upregulating the expressions of matrix metalloproteinase-2 (MMP-2) and MMP-9. The process of cancer metastasis is a multi-step biochemical reaction involving many molecular events. An essential step of tumor invasion and metastasis is the degradation of matrix proteins, while the matrix metalloproteinases (MMPs) are the most crucial proteolytic enzymes involved in the process. MMPs facilitate cancer cells accessing vasculature and then the target organs, forming tumor metastasis by degrading the basement membrane and extracellular matrix (ECM) . MMPs can also stimulate cancer cell proliferation and movement to develop metastasis by enhancing the release of growth factors . MMP-2 and MMP-9 can degrade type IV collagen, the major component of basement membranes separating the epithelial cells from the stroma [36, 37], and they have been reported to promote GC cell migration and invasion [38, 39]. Taking these together with the present study, we suggest that MECP2 facilitates GC cell migration and invasion by upregulating MMP-2/9 expression.
Our study identifies FBXW7 as a MECP2-targeting gene. MECP2 binds to the methylated CpG site in the promoter region of FBXW7, resulting in inhibition of FBXW7 transcription. This finding is consistent with that of some previous studies which show that MECP2 functions as a transcriptional repressor by binding to the methylated CpG sites of gene promoter regions and recruiting corepressors (e.g. histone deacetylases and Sin3A), to restrain the expression of some genes (e.g., MYOD1, FOXF1, BDNF, Cdkl5, RPL11 and RPL5) [17, 20, 21]. FBXW7, a member of the F-box family of proteins, has been characterized as a tumor suppressor gene that plays an important function in cancer cell survival, proliferation, cycle, apoptosis, differentiation, metabolism, tumor metastasis and drug resistance [40, 41]. FBXW7 expression is frequently downregulated in multiple human cancers, such as lung cancer, breast cancer, colorectal cancer, liver cancer, gastric cancer, pancreatic cancer, cervical cancer, prostate cancer, and esophagus cancer . Loss-of-function mutations of FBXW7 are frequently discovered in human cancers, and the total mutation rate is approximately 6% . Recent studies have shown that FBXW7 suppresses cholangiocarcinoma and colorectal cancer cell migration and invasion [44, 45]. The present study further demonstrates that FBXW7 is frequently downregulated in human GC tissues and FBXW7 exression is correlated with the clinicopathologic features of GC. Overexpression of FBXW7 suppresses GC cell migration and invasion, while silencing FBXW7 promotes cell migration and invasion. Co-transfection with MECP2 siRNA and FBXW7 siRNA rescues the effect of MECP2 knockdown on cell migration and invasion. These findings suggest that MECP2 promotes GC cell migration and invasion via suppressing FBXW7 transcription by binding to the methylated CpG islands of the promoter region of FBXW7.
FBXW7 is a subunit of a SCF-type ubiquitin ligase complex that induces the ubiquitination and proteasomal degradation of oncoproteins, including SREBP1, Cyclin E, c-Jun, c-Myc, mTOR, Notch1, Notch4, MCL-1, KLF5 and MCL-1 [46–49]. This study showed that MECP2 increased the protein expressions of c-Myc, mTOR, and Notch1 by inhibiting FBXW7 transcription and then preventing ubiquitination degradation of oncoproteins. c-Myc is a member of Myc gene family involved in multiple biological processes, such as embryonic development, cell proliferation, cell cycle, apoptosis, differentiation, and protein synthesis , and is frequently amplified in many human cancers and promotes cell proliferation, migration and invasion . mTOR acts as a serine/threonine protein kinase that modulates cell proliferation, motility, survival, protein synthesis, autophagy as well as cell cycle progression, and activation of the mTOR pathway promotes cancer cell proliferation, migration and invasion . The Notch signaling pathways regulate cellular differentiation, proliferation and apoptotic events . Notch1-4 are trans-membrane proteins that interact with ligands of the Delta-like and Jagged family. Binding of ligand to its receptor leads to the cleavage of Notch receptor  and the Notch1 intracellular domain (NICD1) is generated. NICD1 enters into the nucleus and promotes Hes1, MMP-2 and MMP-9 expressions in some cancers [55–57]. Previous studies have confirmed that Notch1 is upregulated in gastric cancer, liver cancer and ovarian cancer [58–60]. The present study reveals that, by regulating the FBXW7/Notch1 signaling pathway, MECP2 upregulates Hes1, MMP-2 and MMP-9 expressions. MECP2 promotes migration and invasion of GC cells by regulating the Notch1/c-Myc/mTOR signaling pathways through inhibiting FBXW7 transcription (Fig. 8).