Translation Efficiency and Degradation of ER-Associated mRNAs Modulated by ER-Anchored poly(A)-Specific Ribonuclease (PARN)

Mitogen-activated protein kinase-activated protein kinase-2 (MK2) and its role in cell survival, inflammatory signaling, and migration in promoting cancer

Cancer and the immune system share an intimate relationship. Chronic inflammation increases the risk of cancer occurrence and can also drive inflammatory mediators into the tumor microenvironment enhancing tumor growth and survival. The p38 MAPK pathway is activated both acutely and chronically by stress, inflammatory chemokines, chronic inflammatory conditions, and cancer. These properties have led to extensive efforts to find effective drugs targeting p38, which have been unsuccessful. The immediate downstream serine/threonine kinase and substrate of p38 MAPK, mitogen-activated-protein-kinase-activated-protein-kinase-2 (MK2) protects cells against stressors by regulating the DNA damage response, transcription, protein and messenger RNA stability, and motility. The phosphorylation of downstream substrates by MK2 increases inflammatory cytokine production, drives an immune response, and contributes to wound healing. By binding directly to p38 MAPK, MK2 is responsible for the export of p38 MAPK from the nucleus which gives MK2 properties that make it unique among the large number of p38 MAPK substrates. Many of the substrates of both p38 MAPK and MK2 are separated between the cytosol and nucleus and interfering with MK2 and altering this intracellular translocation has implications for the actions of both p38 MAPK and MK2. The inhibition of MK2 has shown promise in combination with both chemotherapy and radiotherapy as a method for controlling cancer growth and metastasis in a variety of cancers. Whereas the current data are encouraging the field requires the development of selective and well tolerated drugs to target MK2 and a better understanding of its effects for effective clinical use.

mRNPs: Structure and role in development

In eukaryotic cells, mRNA molecules are coated with numerous RNA-binding proteins and so exist in ribonucleoproteins (mRNPs). The proteins associated with the mRNA regulate the fate of mRNA, including its localization, translation and decay. Before activation of translation, the mRNA does not display any template functions-it is masked. The coordinated activity of certain RNA-binding proteins determines the future fate of each mRNA individually. In embryo development, the temporal and spatial regulation of translation can cause a situation when the mRNA and the encoded protein are localized in different compartments and so the differentiation of the cells can be determined. The fundamentals of regulation of the mRNAs fate and functioning in nerves are similar to those already described for oo- and embryogenesis. Disorders in the mRNA masking and demasking result in the emergence of various diseases, in particular cancers and neuro-degenerative diseases.

Human Ccr4 and Caf1 Deadenylases Regulate Proliferation and Tumorigenicity of Human Gastric Cancer Cells via Modulating Cell Cycle Progression

Simple Summary

Cancer cells generally reprogram their gene expression profiles to satisfy continuous growth, proliferation, and metastasis. Most eukaryotic mRNAs are degraded in a deadenylation-dependent pathway, in which deadenylases are the key enzymes. We found that human Ccr4 (hCcr4a/b) and Caf1 (hCaf1a/b), the dominant cytosolic deadenylases, were dysregulated in several types of cancers including stomach adenocarcinoma. Stably knocking down hCaf1a/b or hCcr4a/b blocks cell cycle progression by enhancing the levels of cell cycle inhibitors and by inhibiting the formation of processing bodies, which are cytosolic foci involved in mRNA metabolism. More importantly, depletion of hCaf1a/b or hCcr4a/b dramatically inhibits cell proliferation and tumorigenicity. Our results suggest that perturbating global RNA metabolism may provide a potential novel strategy for cancer treatment.

Abstract

Cancer cells generally have reprogrammed gene expression profiles to meet the requirements of survival, continuous division, and metastasis. An interesting question is whether the cancer cells will be affected by interfering their global RNA metabolism. In this research, we found that human Ccr4a/b (hCcr4a/b) and Caf1a/b (hCaf1a/b) deadenylases, the catalytic components of the Ccr4-Not complex, were dysregulated in several types of cancers including stomach adenocarcinoma. The impacts of the four deadenylases on cancer cell growth were studied by the establishment of four stable MKN28 cell lines with the knockdown of hCcr4a/b or hCaf1a/b or transient knockdown in several cell lines. Depletion of hCcr4a/b or hCaf1a/b significantly inhibited cell proliferation and tumorigenicity. Mechanistic studies indicated that the cells were arrested at the G2/M phase by knocking down hCaf1a, while arrested at the G0/G1 phase by depleting hCaf1b or hCcr4a/b. The four enzymes did not affect the levels of CDKs and cyclins but modulated the levels of CDK–cyclin inhibitors. We identified that hCcr4a/b, but not hCaf1a/b, targeted the p21 mRNA in the MKN28 cells. Furthermore, depletion of any one of the four deadenylases dramatically impaired processing-body formation in the MKN28 and HEK-293T cells. Our results highlight that perturbating global RNA metabolism may severely affect cancer cell proliferation, which provides a potential novel strategy for cancer treatment.

Diverse functions of deadenylases in DNA damage response and genomic integrity

DNA damage response (DDR) is a coordinated network of diverse cellular processes including the detection, signaling, and repair of DNA lesions, the adjustment of metabolic network and cell fate determination. To deal with the unavoidable DNA damage caused by either endogenous or exogenous stresses, the cells need to reshape the gene expression profile to allow efficient transcription and translation of DDR-responsive messenger RNAs (mRNAs) and to repress the nonessential mRNAs. A predominant method to adjust RNA fate is achieved by modulating the 3'-end oligo(A) or poly(A) length via the opposing actions of polyadenylation and deadenylation. Poly(A)-specific ribonuclease (PARN) and the carbon catabolite repressor 4 (CCR4)-Not complex, the major executors of deadenylation, are indispensable to DDR and genomic integrity in eukaryotic cells. PARN modulates cell cycle progression by regulating the stabilities of mRNAs and microRNA (miRNAs) involved in the p53 pathway and contributes to genomic stability by affecting the biogenesis of noncoding RNAs including miRNAs and telomeric RNA. The CCR4-Not complex is involved in diverse pathways of DDR including transcriptional regulation, signaling pathways, mRNA stabilities, translation regulation, and protein degradation. The RNA targets of deadenylases are tuned by the DDR signaling pathways, while in turn the deadenylases can regulate the levels of DNA damage-responsive proteins. The mutual feedback between deadenylases and the DDR signaling pathways allows the cells to precisely control DDR by dynamically adjusting the levels of sensors and effectors of the DDR signaling pathways. Here, the diverse functions of deadenylases in DDR are summarized and the underlying mechanisms are proposed according to recent findings. This article is categorized under: RNA Processing > 3' End Processing RNA in Disease and Development > RNA in Disease RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms.