Inter-isoform-dependent Regulation of the Drosophila Master Transcriptional Regulator SIN3

Analysis of the chromatin landscape and RNA polymerase II binding at SIN3-regulated genes

The chromatin environment has a significant impact on gene expression. Chromatin structure is highly regulated by histone modifications and RNA polymerase II binding dynamics. The SIN3 histone modifying complex regulates the chromatin environment leading to changes in gene expression. In Drosophila melanogaster, the Sin3A gene is alternatively spliced to produce different protein isoforms, two of which include SIN3 220 and SIN3 187. Both SIN3 isoforms are scaffolding proteins that interact with several other factors to regulate the chromatin landscape. The mechanism through which the SIN3 isoforms regulate chromatin is not well understood. Here, we analyze publicly available data sets to allow us to ask specific questions on how SIN3 isoforms regulate chromatin and gene activity. We determined that genes repressed by the SIN3 isoforms exhibited enrichment in histone H3K4me2, H3K4me3, H3K14ac and H3K27ac near the transcription start site. We observed an increase in the amount of paused RNA polymerase II on the promoter of genes repressed by the isoforms as compared to genes that require SIN3 for maximum activation. Furthermore, we analyzed a subset of genes regulated by SIN3 187 that suggest a mechanism in which SIN3 187 might exhibit hard regulation as well as soft regulation. Data presented here expand our knowledge of how the SIN3 isoforms regulate the chromatin environment and RNA polymerase II binding dynamics.

Coordination of cross-talk between metabolism and epigenetic regulation by the SIN3 complex

Post-translational modifications of histone proteins control the expression of genes. Metabolites from central and one-carbon metabolism act as donor moieties to modify histones and regulate gene expression. Thus, histone modification and gene regulation are connected to the metabolite status of the cell. Histone modifiers, such as the SIN3 complex, regulate genes involved in proliferation and metabolism. The SIN3 complex contains a histone deacetylase and a histone demethylase, which regulate the chromatin landscape and gene expression. In this chapter, we review the cross-talk between metabolic pathways that produce donor moieties, and epigenetic complexes regulating proliferation and metabolic genes. This cross-talk between gene regulation and metabolism is tightly controlled, and disruption of this cross-talk leads to metabolic diseases. We discuss promising therapeutics that directly regulate histone modifiers, and can affect the metabolic status of the cell, alleviating some metabolic diseases.

Isoforms of the transcriptional cofactor SIN3 differentially regulate genes necessary for energy metabolism and cell survival

The SIN3 scaffolding protein is a conserved transcriptional regulator known to fine-tune gene expression. In Drosophila, there are two major isoforms of SIN3, SIN3 220 and SIN3 187, which each assemble into multi-subunit histone modifying complexes. The isoforms have distinct developmental expression patterns and non-redundant functions. Gene regulatory network analyses indicate that both isoforms affect genes encoding proteins in pathways such as the cell cycle and cell morphogenesis. Interestingly, the SIN3 187 isoform uniquely regulates a subset of pathways including post-embryonic development, phosphate metabolism and apoptosis. Target genes in the phosphate metabolism pathway include nuclear-encoded mitochondrial genes coding for proteins responsible for oxidative phosphorylation. Here, we investigate the physiological effects of SIN3 isoforms on energy metabolism and cell survival. We find that ectopic expression of SIN3 187 represses expression of several nuclear-encoded mitochondrial genes affecting production of ATP and generation of reactive oxygen species (ROS). Forced expression of SIN3 187 also activates several pro-apoptotic and represses a few anti-apoptotic genes. In the SIN3 187 expressing cells, these gene expression patterns are accompanied with an increased sensitivity to paraquat-mediated oxidative stress. These findings indicate that SIN3 187 influences the regulation of mitochondrial function, apoptosis and oxidative stress response in ways that are dissimilar from SIN3 220. The data suggest that the distinct SIN3 histone modifying complexes are deployed in different cellular contexts to maintain cellular homeostasis.

Same agent, different messages: insight into transcriptional regulation by SIN3 isoforms

SIN3 is a global transcriptional coregulator that governs expression of a large repertoire of gene targets. It is an important player in gene regulation, which can repress or activate diverse gene targets in a context-dependent manner. SIN3 is required for several vital biological processes such as cell proliferation, energy metabolism, organ development, and cellular senescence. The functional flexibility of SIN3 arises from its ability to interact with a large variety of partners through protein interaction domains that are conserved across species, ranging from yeast to mammals. Several isoforms of SIN3 are present in these different species that can perform common and specialized functions through interactions with distinct enzymes and DNA-binding partners. Although SIN3 has been well studied due to its wide-ranging functions and highly conserved interaction domains, precise roles of individual SIN3 isoforms have received less attention. In this review, we discuss the differences in structure and function of distinct SIN3 isoforms and provide possible avenues to understand the complete picture of regulation by SIN3.