Muscarinic acetylcholine receptors mediate eIF4B phosphorylation in SNU-407 colon cancer cells

Long non-coding RNAs in gastrointestinal cancers: implications for protein phosphorylation

Phosphorylation of proteins is one of the most extensively investigated post-translational protein modifications. Threonine, serine and tyrosine in proteins are the most commonly phosphorylated amino acids. Dysregulated cancer-related signaling pathways due to aberrant phosphorylation status of the key protein(s) in these pathways exist in most malignancies. Intensive studies in the recent decade have implicated long non-coding RNAs (lncRNAs) in the precise regulation of protein phosphorylation in cancers. In this review, we systematically delve into recent advance that underlines the multidimensional role of lncRNAs in modulating protein phosphorylation, regulating cancerous signaling and impacting prognosis of gastrointestinal (GI) cancers including hepatocellular carcinoma, colorectal cancer, gastric cancer, esophageal cancer, and pancreatic cancer. LncRNAs regulate protein phosphorylation via directly binding to the target protein(s), interacting with the partner protein(s) of the target protein(s) or lncRNAs-encoded small peptides. Although there are still extensive studies on disclosing the intricate interactions between lncRNAs and proteins and their impacts on protein phosphorylation, we believe that targeting lncRNAs controlling phosphorylation of key protein(s) in cancerous signaling pathways might provide novel paths for precision therapeutics of GI cancers in the future.

Long noncoding RNA GMAN promotes hepatocellular carcinoma progression by interacting with eIF4B

Gastric cancer metastasis associated long noncoding RNA (GMAN), a long noncoding RNA, is associated with metastasis in gastric cancer. However, its underlying mechanisms in hepatocellular carcinoma (HCC) are unclear. We found that lncRNA-GMAN was significantly overexpressed in HCC tissues. GMAN expression is associated with vascular invasion, histological grade, tumor, node, metastasis (TNM) stage, short overall survival, and disease-free survival. Knockdown of GMAN induced apoptosis and suppressed invasive and migration potential in vitro and vivo, whereas ectopic GMAN expression produced the opposite effect. We also found that the inhibition of apoptosis, rather than promotion of proliferation, was responsible for GMAN-enhanced cellular viability. Mechanistic analyses indicated that GMAN directly combined with eukaryotic translation initiation factor 4B (eIF4B) and promoted its phosphorylation at serine-422 by preventing eIF4B binding and dephosphorization of the protein phosphatase 2A subunit B. The results demonstrated the stability of p-eIF4B and the elevation of mRNA translation and anti-apoptosis-related protein expression, which further induced proliferation and metastasis of HCC. The current study demonstrates that GMAN regulates the progression of HCC by inhibiting apoptosis and promoting the survival of cancer cells.

Muscarinic acetylcholine receptors regulate the dephosphorylation of eukaryotic translation elongation factor 2 in SNU-407 colon cancer cells

Previously, we showed that muscarinic acetylcholine receptors (mAChRs) promote global protein biosynthesis in SNU-407 colon cancer cells. However, the molecular mechanisms underlying this event are poorly understood. Here, we asked whether mAChRs modulate the activity of eukaryotic translation elongation factor 2 (eEF2), which controls ribosomal translocation during the peptide elongation step. When SNU-407 cells were treated with the cholinergic agonist carbachol, eEF2 phosphorylation at T56 was decreased in a dose- and time-dependent manner. The muscarinic antagonist atropine almost completely blocked this effect of carbachol, demonstrating that mAChRs specifically regulate eEF2 dephosphorylation. We also investigated the signaling pathways that connect mAChR stimulation to eEF2 dephosphorylation using chemical inhibitors. Treating cells with U0126, a potent MEK1/2 inhibitor, decreased carbachol-stimulated eEF2 dephosphorylation. In contrast, the mTORC1 inhibitor rapamycin did not have a significant effect on eEF2 dephosphorylation. We also found that the protein kinase C (PKC) inhibitor GF109203X substantially reduced eEF2 dephosphorylation. Together, our experimental data indicate that the MEK1/2-ERK1/2 pathway and the PKC pathway, but not the mTORC1-S6K1 pathway, are involved in mAChR-mediated eEF2 dephosphorylation.