JNK activation induced by ribotoxic stress is initiated from 80S monosomes but not polysomes

USP36 stabilizes nucleolar Snail1 to promote ribosome biogenesis and cancer cell survival upon ribotoxic stress

Tumor growth requires elevated ribosome biogenesis. Targeting ribosomes is an important strategy for cancer therapy. The ribosome inhibitor, homoharringtonine (HHT), is used for the clinical treatment of leukemia, yet it is ineffective for the treatment of solid tumors, the reasons for which remain unclear. Here we show that Snail1, a key factor in the regulation of epithelial-to-mesenchymal transition, plays a pivotal role in cellular surveillance response upon ribotoxic stress. Mechanistically, ribotoxic stress activates the JNK-USP36 signaling to stabilize Snail1 in the nucleolus, which facilitates ribosome biogenesis and tumor cell survival. Furthermore, we show that HHT activates the JNK-USP36-Snail1 axis in solid tumor cells, but not in leukemia cells, resulting in solid tumor cell resistance to HHT. Importantly, a combination of HHT with the inhibition of the JNK-USP36-Snail1 axis synergistically inhibits solid tumor growth. Together, this study provides a rationale for targeting the JNK-USP36-Snail1 axis in ribosome inhibition-based solid tumor therapy.

Phosphorylation of rpS3 by Lyn increases translation of Multi-Drug Resistance (MDR1) gene

Lyn, a tyrosine kinase that is activated by double-stranded DNAdamaging agents, is involved in various signaling pathways, such as proliferation, apoptosis, and DNA repair. Ribosomal protein S3 (RpS3) is involved in protein biosynthesis as a component of the ribosome complex and possesses endonuclease activity to repair damaged DNA. Herein, we demonstrated that rpS3 and Lyn interact with each other, and the phosphorylation of rpS3 by Lyn, causing ribosome heterogeneity, upregulates the translation of p-glycoprotein, which is a gene product of multidrug resistance gene 1. In addition, we found that two different regions of the rpS3 protein are associated with the SH1 and SH3 domains of Lyn. An in vitro immunocomplex kinase assay indicated that the rpS3 protein acts as a substrate for Lyn, which phosphorylates the Y167 residue of rpS3. Furthermore, by adding various kinase inhibitors, we confirmed that the phosphorylation status of rpS3 was regulated by both Lyn and doxorubicin, and the phosphorylation of rpS3 by Lyn increased drug resistance in cells by upregulating p-glycoprotein translation. [BMB Reports 2023; 56(5): 302-307].

Ribosomes in poxvirus infection

Poxviruses are large double-stranded DNA viruses that encode their own DNA replication, transcription, and mRNA biogenesis machinery, which underlies their ability to replicate entirely in the cytoplasm. However, like all other viruses, poxviruses remain dependent on host ribosomes to translate their mRNAs into the viral proteins needed to complete their replication cycle. While earlier studies established a fundamental understanding of how poxviruses wrestle with their hosts for control of translation initiation and elongation factors that guide ribosome recruitment and mRNA decoding, recent work has begun to reveal the extent to which poxviruses directly target the ribosome itself. This review summarizes our current understanding of the regulation of ribosomes and translation in poxvirus infection.

RACK1 Regulates Poxvirus Protein Synthesis Independently of Its Role in Ribosome-Based Stress Signaling

Receptor for activated C kinase 1 (RACK1) is a small ribosomal subunit protein that is phosphorylated by vaccinia virus (VacV) to maximize translation of postreplicative (PR) mRNAs that harbor 5' polyA leaders. However, RACK1 is a multifunctional protein that both controls translation directly and acts as a scaffold for signaling to and from the ribosome. This includes stress signaling that is activated by ribosome-associated quality control (RQC) and ribotoxic stress response (RSR) pathways. As VacV infection activates RQC and stress signaling, whether RACK1 influences viral protein synthesis through its effects on translation, signaling, or both remains unclear. Examining the effects of genetic knockout of RACK1 on the phosphorylation of key mitogenic and stress-related kinases, we reveal that loss of RACK1 specifically blunts the activation of c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) at late stages of infection. However, RACK1 was not required for JNK recruitment to ribosomes, and unlike RACK1 knockout, JNK inhibitors had no effect on viral protein synthesis. Moreover, reduced JNK activity during infection in RACK1 knockout cells contrasted with the absolute requirement for RACK1 in RSR-induced JNK phosphorylation. Comparing the effects of RACK1 knockout alongside inhibitors of late stage replication, our data suggest that JNK activation is only indirectly affected by the absence of RACK1 due to reduced viral protein accumulation. Cumulatively, our findings in the context of infection add further support for a model whereby RACK1 plays a specific and direct role in controlling translation of PR viral mRNAs that is independent of its role in ribosome-based stress signaling. IMPORTANCE Receptor for activated C kinase 1 (RACK1) is a multifunctional ribosomal protein that regulates translation directly and mediates signaling to and from the ribosome. While recent work has shown that RACK1 is phosphorylated by vaccinia virus (VacV) to stimulate translation of postreplicative viral mRNAs, whether RACK1 also contributes to VacV replication through its roles in ribosome-based stress signaling remains unclear. Here, we characterize the role of RACK1 in infected cells. In doing so, we find that RACK1 is essential for stress signal activation by ribotoxic stress responses but not by VacV infection. Moreover, although the loss of RACK1 reduces the level of stress-associated JNK activation in infected cells, this is an indirect consequence of RACK1's specific requirement for the synthesis of postreplicative viral proteins, the accumulation of which determines the level of cellular stress. Our findings reveal both the specific role of RACK1 and the complex downstream effects of its control of viral protein synthesis in the context of infection.

Anticancer Effects of Taraxacum via Cell Cycle Arrest, Necrosis, Apoptosis, and Endoplasmic Reticulum Stress

Dandelion (Taraxacum species) is a wild plant with over 2500 species. Flavonoids, phenolic compounds, saponins, sesquiterpenes, and sugars have been detected in the organs of Taraxacum, and for centuries it has been used in traditional medicine for the relief and treatment of various diseases. However, details of its working mechanism remain unclear. Bioactive compounds in herbal extracts generally have low yields, which makes their isolation and purification intensive in terms of time and cost. Here, to assess their versatility and safety, we applied aqueous extracts of two species of Taraxacum, T. mongolicum and T. formosanum, including extracts of both fresh and dried T. formosanum, to compare their potential antitumor effects on HeLa human cervical cancer cells, three liver cancer cell lines, and one normal liver cell line. After being treated with a lower dose of Taraxacum, the upregulation of subG1 and S populations, as well as increased levels of p-eIF2[Formula: see text]-to-eIF2[Formula: see text] ratio, were observed in HeLa cells, whereas the downregulation of S population and the absence of mRNA expressions were detected in HeLa cells when being treated with a higher dose of Taraxacum. These results indicated that Taraxacumcould induce apoptosis and endoplasmic reticulum stress while suppressing proliferation, transcription, colony formation, migration, and invasion. What's more, we also found that the effects of fresh T. formosanum were much stronger than that of T. mongolicumin HeLa cells. Based on these results, we suggest that T. formosanum may contain specific compound(s) that are potentially useful for cancer therapy. However, much work remains to identify these effective compounds for the future application of Taraxacumto cancer therapy.

Negative charge in the RACK1 loop broadens the translational capacity of the human ribosome

Although the roles of initiation factors, RNA binding proteins, and RNA elements in regulating translation are well defined, how the ribosome functionally diversifies remains poorly understood. In their human hosts, poxviruses phosphorylate serine 278 (S²⁷⁸) at the tip of a loop domain in the small subunit ribosomal protein RACK1, thereby mimicking negatively charged residues in the RACK1 loops of dicot plants and protists to stimulate translation of transcripts with 5′ poly(A) leaders. However, how a negatively charged RACK1 loop affects ribosome structure and its broader translational output is not known. Here, we show that although ribotoxin-induced stress signaling and stalling on poly(A) sequences are unaffected, negative charge in the RACK1 loop alters the swivel motion of the 40S head domain in a manner similar to several internal ribosome entry sites (IRESs), confers resistance to various protein synthesis inhibitors, and broadly supports noncanonical modes of translation.

How ribosomes functionally diversify to selectively control translation is only beginning to be understood. Rollins et al. show that negative charge in a loop domain of the small subunit ribosomal protein RACK1 increases the swiveling motion of the 40S head and broadens the translational capacity of the human ribosome.

Development of Carbazole Derivatives Compounds against Candida albicans: Candidates to Prevent Hyphal Formation via the Ras1-MAPK Pathway

Morphogenesis contributes to the virulence of the opportunistic human fungal pathogen Candida albicans. Ras1-MAPK pathways play a critical role in the virulence of C. albicans by regulating cell growth, morphogenesis, and biofilm formation. Ume6 acts as a transcription factor, and Nrg1 is a transcriptional repressor for the expression of hyphal-specific genes in morphogenesis. Azoles or echinocandin drugs have been extensively prescribed for C. albicans infections, which has led to the development of drug-resistant strains. Therefore, it is necessary to develop new molecules to effectively treat fungal infections. Here, we showed that Molecule B and Molecule C, which contained a carbazole structure, attenuated the pathogenicity of C. albicans through inhibition of the Ras1/MAPK pathway. We found that Molecule B and Molecule C inhibit morphogenesis through repressing protein and RNA levels of Ras/MAPK-related genes, including UME6 and NRG1. Furthermore, we determined the antifungal effects of Molecule B and Molecule C in vivo using a candidiasis murine model. We anticipate our findings are that Molecule B and Molecule C, which inhibits the Ras1/MAPK pathway, are promising compounds for the development of new antifungal agents for the treatment of systemic candidiasis and possibly for other fungal diseases.

A potential role for a novel ZC3H5 complex in regulating mRNA translation in Trypanosoma brucei

In Trypanosoma brucei and related kinetoplastids, gene expression regulation occurs mostly posttranscriptionally. Consequently, RNA-binding proteins play a critical role in the regulation of mRNA and protein abundance. Yet, the roles of many RNA-binding proteins are not understood. Our previous research identified the RNA-binding protein ZC3H5 as possibly involved in gene repression, but its role in controlling gene expression was unknown. We here show that ZC3H5 is an essential cytoplasmic RNA-binding protein. RNAi targeting ZC3H5 causes accumulation of precytokinetic cells followed by rapid cell death. Affinity purification and pairwise yeast two-hybrid analysis suggest that ZC3H5 forms a complex with three other proteins, encoded by genes Tb927.11.4900, Tb927.8.1500, and Tb927.7.3040. RNA immunoprecipitation revealed that ZC3H5 is preferentially associated with poorly translated, low-stability mRNAs, the 5'-untranslated regions and coding regions of which are enriched in the motif (U/A)UAG(U/A). As previously found in high-throughput analyses, artificial tethering of ZC3H5 to a reporter mRNA or other complex components repressed reporter expression. However, depletion of ZC3H5 in vivo caused only very minor decreases in a few targets, marked increases in the abundances of very stable mRNAs, an increase in monosomes at the expense of large polysomes, and appearance of "halfmer" disomes containing two 80S subunits and one 40S subunit. We speculate that the ZC3H5 complex might be implicated in quality control during the translation of suboptimal open reading frames.

Ribosome Collisions Trigger General Stress Responses to Regulate Cell Fate

Problems arising during translation of mRNAs lead to ribosome stalling and collisions that trigger a series of quality control events. However, the global cellular response to ribosome collisions has not been explored. Here, we uncover a function for ribosome collisions in signal transduction. Using translation elongation inhibitors and general cellular stress conditions, including amino acid starvation and UV irradiation, we show that ribosome collisions activate the stress-activated protein kinase (SAPK) and GCN2-mediated stress response pathways. We show that the MAPKKK ZAK functions as the sentinel for ribosome collisions and is required for immediate early activation of both SAPK (p38/JNK) and GCN2 signaling pathways. Selective ribosome profiling and biochemistry demonstrate that although ZAK generally associates with elongating ribosomes on polysomal mRNAs, it specifically auto-phosphorylates on the minimal unit of colliding ribosomes, the disome. Together, these results provide molecular insights into how perturbation of translational homeostasis regulates cell fate.