First identified in 1974, m6A RNA methylation, which serves as a predominant internal modification of RNA in higher eukaryotes, has gained prodigious interest in recent years. Modifications of m6A are dynamic and reversible in mammalian cells, which have been proposed as another layer of epigenetic regulation similar to DNA and histone modifications. m6A RNA methylation is involved in all stages in the life cycle of RNA, ranging from RNA processing, through nuclear export, translation modulation to RNA degradation, which suggests its potential of influencing a plurality of aspects of RNA metabolism. All of the recent studies have pointed to a complicated regulation network of m6A modification in different tissues, cell lines, and space–time models. m6A methylation has been found to have an impact on tumor initiation and progression through various mechanisms. Furthermore, m6A RNA methylation has provided new opportunities for early stage diagnosis and treatment of cancers. Herein, we review the chemical basis of m6A RNA methylation, its multiple functions and potential significance in cancer.

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Multiple functions of m 6 A RNA methylation in cancer

Recent studies show that YTH domain family 2 (YTHDF2) preferentially binds to m6A-containing mRNA regulates localization and stability of the bound mRNA. However, the role of YTHDF2 in pancreatic cancers remains to be elucidated. Here, we find that YTHDF2 expression is up-regulated in pancreatic cancer tissues compared with normal tissues at both mRNA and protein levels, and is higher in clinical patients with later stages of pancreatic cancer, indicating that YTHDF2 possesses potential clinical significance for diagnosis and prognosis of pancreatic cancers. Furthermore, we find that YTHDF2 orchestrates two cellular processes: promotes proliferation and inhibits migration and invasion in pancreatic cancer cells, a phenomenon called "migration-proliferation dichotomy", as well as epithelial-mesenchymal transition (EMT) in pancreatic cancer cells. Furthermore, YTHDF2 knockdown significantly increases the total YAP expression, but inhibits TGF-β/Smad signaling, indicating that YTHDF2 regulates EMT probably via YAP signaling. In summary, all these findings suggest that YTHDF2 may be a new predictive biomarker of development of pancreatic cancer, but a serious consideration is needed to treat YTHDF2 as a target for pancreatic cancer.

https://www.tandfonline.com/doi/pdf/10.1080/15384101.2017.1380125

YTH domain family 2 orchestrates epithelial-mesenchymal transition/proliferation dichotomy in pancreatic cancer cells.

Wnt signaling is essential for tissue homeostasis and its dysregulation causes cancer. Wnt ligands trigger signaling by activating Frizzled receptors (FZDRs), which belong to the G-protein coupled receptor superfamily. However, the mechanisms of G protein activation in Wnt signaling remain controversial. In this study, we demonstrate that FZDRs activate G proteins and trigger non-canonical Wnt signaling via the Dishevelled-binding protein, Daple. Daple contains a Gα-binding and activating (GBA) motif, which activates Gαi proteins and an adjacent domain that directly binds FZDRs, thereby linking Wnt stimulation to G protein activation. This triggers non-canonical Wnt responses, that is, suppresses the β-catenin/TCF/LEF pathway and tumorigenesis, but enhances PI3K-Akt and Rac1 signals and tumor cell invasiveness. In colorectal cancers, Daple is suppressed during adenoma-to-carcinoma transformation and expressed later in metastasized tumor cells. Thus, Daple activates Gαi and enhances non-canonical Wnt signaling by FZDRs, and its dysregulation can impact both tumor initiation and progression to metastasis.

DOI: http://dx.doi.org/10.7554/eLife.07091.001

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Daple is a novel non-receptor GEF required for trimeric G protein activation in Wnt signaling

Autophagy is the major catabolic process responsible for the removal of aggre - gated proteins and damaged organelles. Autophagy is regulated by both G proteins and growth factors, but the underlying mechanism of how they are coordinated during initiation and reversal of autophagy is unknown. Using protein-protein interaction assays, G protein enzymology, and morphological analysis, we demonstrate here that G α-interacting, vesicle- associated protein (GIV, a.k.a. Girdin), a nonreceptor guanine nucleotide exchange factor for Gαi3, plays a key role in regulating autophagy and that dynamic interplay between Gi3, ac- tivator of G-protein signaling 3 (AGS3, its guanine nucleotide dissociation inhibitor), and GIV determines whether autophagy is promoted or inhibited. We found that AGS3 directly binds light chain 3 (LC3), recruits Gαi3 to LC3-positive membranes upon starvation, and promotes autophagy by inhibiting the G protein. Upon growth factor stimulation, GIV disrupts the Gαi3-AGS3 complex, releases Gαi3 from LC3-positive membranes, enhances anti-autophagic signaling pathways, and inhibits autophagy by activating the G protein. These results provide mechanistic insights into how reversible modulation of Gαi3 activity by AGS3 and GIV main- tains the delicate equilibrium between promotion and inhibition of autophagy.

A GDI (AGS3) and a GEF (GIV) regulate autophagy by balancing G protein activity and growth factor signals

Background. The dichotomy between glioblastoma cell migration and proliferation is regulated by various parameters including oxygen tension. In glioblastoma stem-like cells, hypoxia induces downregulation of pentose phosphate pathway (PPP) enzymes and a flux shift towards glycolysis. We investigated whether the 2 parallel glucose metabolic pathways are intrinsically linked with cell function and whether these pathways are mechanistically involved in regulating functional programs. Methods. Enzyme expression, migration, and proliferation under hypoxia were studied in multiple cell types. Rapidly and slowly dividing or migrating glioblastoma cells were separated, and enzyme profiles were compared. Glucose-6-phosphate dehydrogenase (G6PD) and Aldolase C (ALDOC), the most strongly inversely regulated PPP and glycolysis enzymes, were knocked down by short hairpin RNA. Results. Hypoxia caused downregulation of PPP enzymes and upregulation of glycolysis enzymes in a broad spectrum of cancer and nonneoplastic cells and consistently stimulated migration while reducing proliferation. PPP enzyme expression was increased in rapidly dividing glioblastoma cells, whereas glycolysis enzymes were decreased. Conversely, glycolysis enzymes were elevated in migrating cells, whereas PPP enzymes were diminished. Knockdown of G6PD reduced glioblastoma cell proliferation, whereas ALDOC knockdown decreased migration. Enzyme inhibitors had similar effects. G6PD knockdown in a highly proliferative but non-invasive glioblastoma cell line resulted in prolonged survival of mice with intracerebral xenografts, whereas ALDOC knockdown shortened survival. In a highly invasive glioblastoma xenograft model, tumor burden was unchanged by either knockdown. Conclusions. Cell function and metabolic state are coupled independently of hypoxia, and glucose metabolic pathways are causatively involved in regulating "go or grow" cellular programs.

Glycolysis and the pentose phosphate pathway are differentially associated with the dichotomous regulation of glioblastoma cell migration versus proliferation.

Cells respond to growth factors by either migrating or proliferating, but not both at the same time, a phenomenon termed migration-proliferation dichotomy. The underlying mechanism of this phenomenon has remained unknown. We demonstrate here that Gαi protein and GIV, its nonreceptor guanine nucleotide exchange factor (GEF), program EGF receptor (EGFR) signaling and orchestrate this dichotomy. GIV directly interacts with EGFR, and when its GEF function is intact, a Gαi–GIV–EGFR signaling complex assembles, EGFR autophosphorylation is enhanced, and the receptor's association with the plasma membrane (PM) is prolonged. Accordingly, PM-based motogenic signals (PI3-kinase-Akt and PLCγ1) are amplified, and cell migration is triggered. In cells expressing a GEF-deficient mutant, the Gαi–GIV-EGFR signaling complex is not assembled, EGFR autophosphorylation is reduced, the receptor's association with endosomes is prolonged, mitogenic signals (ERK 1/2, Src, and STAT5) are amplified, and cell proliferation is triggered. In rapidly growing, poorly motile breast and colon cancer cells and in noninvasive colorectal carcinomas in situ in which EGFR signaling favors mitosis over motility, a GEF-deficient splice variant of GIV was identified. In slow growing, highly motile cancer cells and late invasive carcinomas, GIV is highly expressed and has an intact GEF motif. Thus, inclusion or exclusion of GIV's GEF motif, which activates Gαi, modulates EGFR signaling, generates migration-proliferation dichotomy, and most likely influences cancer progression.

Erin P. Forry

Papers

A Gαi–GIV Molecular Complex Binds Epidermal Growth Factor Receptor and Determines Whether Cells Migrate or Proliferate