The splicing regulator SLU7 is required to preserve DNMT1 protein stability and DNA methylation

Caspases compromise SLU7 and UPF1 stability and NMD activity during hepatocarcinogenesis

Background & Aims

The homeostasis of the cellular transcriptome depends on transcription and splicing mechanisms. Moreover, the fidelity of gene expression, essential to preserve cellular identity and function is secured by different quality control mechanisms including nonsense-mediated RNA decay (NMD). In this context, alternative splicing is coupled to NMD, and several alterations in these mechanisms leading to the accumulation of aberrant gene isoforms are known to be involved in human disease including cancer.

Methods

RNA sequencing, western blotting, qPCR and co-immunoprecipitation were performed in multiple silenced culture cell lines (replicates n ≥4), primary hepatocytes and samples of animal models (Jo2, APAP, Mdr2 -/- mice, n ≥3).

Results

Here we show that in animal models of liver injury and in human HCC (TCGA, non-tumoral = 50 vs. HCC = 374), the process of NMD is inhibited. Moreover, we demonstrate that the splicing factor SLU7 interacts with and preserves the levels of the NMD effector UPF1, and that SLU7 is required for correct NMD. Our previous findings demonstrated that SLU7 expression is reduced in the diseased liver, contributing to hepatocellular dedifferentiation and genome instability during disease progression. Here we build on this by providing evidence that caspases activated during liver damage are responsible for the cleavage and degradation of SLU7.

Conclusions

Here we identify the downregulation of UPF1 and the inhibition of NMD as a new molecular pathway contributing to the malignant reshaping of the liver transcriptome. Moreover, and importantly, we uncover caspase activation as the mechanism responsible for the downregulation of SLU7 expression during liver disease progression, which is a new link between apoptosis and hepatocarcinogenesis.

Impact and implications

The mechanisms involved in reshaping the hepatocellular transcriptome and thereby driving the progressive loss of cell identity and function in liver disease are not completely understood. In this context, we provide evidence on the impairment of a key mRNA surveillance mechanism known as nonsense-mediated mRNA decay (NMD). Mechanistically, we uncover a novel role for the splicing factor SLU7 in the regulation of NMD, including its ability to interact and preserve the levels of the key NMD factor UPF1. Moreover, we demonstrate that the activation of caspases during liver damage mediates SLU7 and UPF1 protein degradation and NMD inhibition. Our findings identify potential new markers of liver disease progression, and SLU7 as a novel therapeutic target to prevent the functional decay of the chronically injured organ.

The Role and Mechanism of the Histone Methyltransferase G9a in Tumors: Update

Methylation-mediated gene silencing is closely related to the occurrence and development of human tumors. The euchromatic histone lysine methyltransferase 2 (EHMT2, also known as G9a) is highly expressed in many tumors and is generally considered to be an oncogene, which is associated with the poor outcome of many tumors. Combined immunotherapy and immune checkpoint blockade therapy also have good efficacy and certain safety. However, there are still many difficulties in the drugs targeting G9a, and the combined effect and safety of G9a with many drugs is still under study. This article aims to summarize the role and mechanism of G9a and its inhibitors in tumors in the past two years, and to understand the application prospect of G9a from the perspective of diagnosis and treatment.

SLU7: A New Hub of Gene Expression Regulation—From Epigenetics to Protein Stability in Health and Disease

SLU7 (Splicing factor synergistic lethal with U5 snRNA 7) was first identified as a splicing factor necessary for the correct selection of 3′ splice sites, strongly impacting on the diversity of gene transcripts in a cell. More recent studies have uncovered new and non-redundant roles of SLU7 as an integrative hub of different levels of gene expression regulation, including epigenetic DNA remodeling, modulation of transcription and protein stability. Here we review those findings, the multiple factors and mechanisms implicated as well as the cellular functions affected. For instance, SLU7 is essential to secure liver differentiation, genome integrity acting at different levels and a correct cell cycle progression. Accordingly, the aberrant expression of SLU7 could be associated with human diseases including cancer, although strikingly, it is an essential survival factor for cancer cells. Finally, we discuss the implications of SLU7 in pathophysiology, with particular emphasis on the progression of liver disease and its possible role as a therapeutic target in human cancer.