Development of Agonist-Based PROTACs Targeting Liver X Receptor

Direct degradation and stabilization of proteins: New horizons in treatment of nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is featured with excessive hepatic lipid accumulation and its global prevalence is soaring. Nonalcoholic steatohepatitis (NASH), the severe systemic inflammatory subtype of NAFLD, is tightly associated with metabolic comorbidities, and the hepatocytes manifest severe inflammation and ballooning. Currently the therapeutic options for treating NASH are limited. Potent small molecules specifically intervene with the signaling pathways that promote pathogenesis of NASH. Nevertheless they have obvious adverse effects and show long-term ineffectiveness in clinical trials. It poses the fundamental question to efficiently and safely inhibit the pathogenic processes. Targeted protein degradation (TPD) belongs to the direct degradation strategies and is a burgeoning strategy. It utilizes the small molecules to bind to the target proteins and recruit the endogenous proteasome, lysosome and autophagosome-mediated degradation machineries. They effectively and specifically degrade the target proteins. It has exhibited promising therapeutic effects in treatment of cancer, neurodegenerative diseases and other diseases in a catalytic manner at low doses. We critically discuss the principles of multiple direct degradation strategies, especially PROTAC and ATTEC. We extensively analyze their emerging application in degradation of excessive pathogenic proteins and lipid droplets, which promote the progression of NASH. Moreover, we discuss the opposite strategy that utilizes the small molecules to recruit deubiquinases to stabilize the NASH/MASH-suppressing proteins. Their advantages, limitations, as well as the solutions to address the limitations have been analyzed. In summary, the innovative direct degradation strategies provide new insights into design of next-generation therapeutics to combat NASH with optimal safety paradigm and efficiency.

Advancing targeted protein degradation for metabolic diseases therapy

The development and application of traditional drugs represented by small molecule chemical drugs and biological agents, especially inhibitors, have become the mainstream drug development. In recent years, targeted protein degradation (TPD) technology has become one of the most promising methods to remove specific disease-related proteins using cell self-destruction mechanisms. Many different TPD strategies are emerging based on the ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway (ALP), including but not limited to proteolysis-targeting chimeras (PROTAC), molecular glues (MG), lysosome targeting chimeras (LYTAC), chaperone-mediated autophagy (CMA)-targeting chimeras, autophagy-targeting chimera (AUTAC), autophagosome-tethering compound (ATTEC), and autophagy-targeting chimera (AUTOTAC). The advent of targeted degradation technology can change most protein targets in human cells from undruggable to druggable, greatly expanding the therapeutic prospect of refractory diseases such as metabolic syndrome. Here, we summarize the latest progress of major TPD technologies, especially in metabolic syndrome and look forward to providing new insights for drug discovery.

Endocytosis of LXRs: Signaling in liver and disease

Nuclear receptors are among one of the major transcriptional factors that induces gene regulation in the nucleus. Liver X receptor (LXR) is a transcription factor which regulates essential lipid homeostasis in the body including fatty acid, cholesterol and phospholipid synthesis. Liver X receptor-retinoid X receptor (LXR-RXR) heterodimer is activated by either of the ligand binding on LXR or RXR. The promoter region of the gene which is targeted by LXR is bound to the response element of LXR. The activators bind to the heterodimer once the corepressor is dissociated. The cellular process such as endocytosis aids in intracellular trafficking and endosomal formation in transportation of molecules for essential signaling within the cell. LXR isotypes play a crucial role in maintaining lipid homeostasis by regulating the level of cholesterol. In the liver, the deficiency of LXRα can alter the normal physiological conditions depicting the symptoms of various cardiovascular and liver diseases. LXR can degrade low density lipoprotein receptors (LDLR) by the signaling of LXR-IDOL through endocytic trafficking in lipoprotein uptake. Various gene expressions associated with cholesterol level and lipid synthesis are regulated by LXR transcription factor. With its known diversified ligand binding, LXR is capable of regulating expression of various specific genes responsible for the progression of autoimmune diseases. The agonists and antagonists of LXR stand to be an important factor in transcription of the ABC family, essential for high density lipoprotein (HDL) formation. Endocytosis and signaling mechanism of the LXR family is broad and complex despite their involvement in cellular growth and proliferation. Here in this chapter, we aimed to emphasize the master regulation of LXR activation, regulators, and their implications in various metabolic activities especially in lipid homeostasis. Furthermore, we also briefed the significant role of LXR endocytosis in T cell immune regulation and a variety of human diseases including cardiovascular and neuroadaptive.

Interactions governing transcriptional activity of nuclear receptors

The key players in transcriptional regulation are transcription factors (TFs), proteins that bind specific DNA sequences. Several mechanisms exist to turn TFs ‘on’ and ‘off’, including ligand binding which induces conformational changes within TFs, subsequently influencing multiple inter- and intramolecular interactions to drive transcriptional responses. Nuclear receptors are a specific family of ligand-regulated TFs whose activity relies on interactions with DNA, coregulator proteins and other receptors. These multidomain proteins also undergo interdomain interactions on multiple levels, further modulating transcriptional outputs. Cooperation between these distinct interactions is critical for appropriate transcription and remains an intense area of investigation. In this review, we report and summarize recent findings that continue to advance our mechanistic understanding of how interactions between nuclear receptors and diverse partners influence transcription.

PROTACs: great opportunities for academia and industry (an update from 2020 to 2021)

PROteolysis TArgeting Chimeras (PROTACs) technology is a new protein-degradation strategy that has emerged in recent years. It uses bifunctional small molecules to induce the ubiquitination and degradation of target proteins through the ubiquitin–proteasome system. PROTACs can not only be used as potential clinical treatments for diseases such as cancer, immune disorders, viral infections, and neurodegenerative diseases, but also provide unique chemical knockdown tools for biological research in a catalytic, reversible, and rapid manner. In 2019, our group published a review article “PROTACs: great opportunities for academia and industry” in the journal, summarizing the representative compounds of PROTACs reported before the end of 2019. In the past 2 years, the entire field of protein degradation has experienced rapid development, including not only a large increase in the number of research papers on protein-degradation technology but also a rapid increase in the number of small-molecule degraders that have entered the clinical and will enter the clinical stage. In addition to PROTAC and molecular glue technology, other new degradation technologies are also developing rapidly. In this article, we mainly summarize and review the representative PROTACs of related targets published in 2020–2021 to present to researchers the exciting developments in the field of protein degradation. The problems that need to be solved in this field will also be briefly introduced.

Discovery of the First-in-Class Agonist-Based SOS1 PROTACs Effective in Human Cancer Cells Harboring Various KRAS Mutations

Regulating SOS1 functions may result in targeted pan-KRAS therapies. Small-molecule SOS1 inhibitors showed promising anticancer potential, and the most advanced inhibitor BI 1701963 is currently under phase I clinical studies. SOS1 agonists provide new opportunities to treat cancer; however, the underlying mechanisms still warrant investigation. We here report the discovery of the first SOS1 PROTACs designed uniquely by connecting a VHL ligand to the reported SOS1 agonist, ensuring that the observed inhibitory activity results from degraders. The best compound 9d induced SOS1 degradation in various KRAS-driven cancer cells and displayed superior antiproliferation activity compared to the agonist itself. Tumor xenograft study clearly showed the promising antitumor potency of 9d against human lung cancer. This study provides good evidence of using agonists to design SOS1 PROTACs and demonstrates that targeted SOS1 degradation represents an effective therapeutic strategy for overcoming KRAS-driven cancers.

VHL-based PROTACs as potential therapeutic agents: Recent progress and perspectives

Proteolysis targeting chimeras (PROTACs), which hijack proteins of interest (POIs) and recruit E3 ligases for target degradation via the ubiquitin-proteasome pathway, are a novel drug discovery paradigm that has been widely used as biological tools and medicinal molecules with the potential of clinical application value. To date, a wide variety of small molecule PROTACs have been developed. Importantly, VHL-based PROTACs have emerged to be a promising approach for proteins, including those non-druggable ones, such as transcriptional factors and scaffold proteins. VHL-based PRTOACs have been developed for the treatment of diseases that are difficult to be dealt with by conventional methods, such as radiotherapy, chemotherapy, and small molecule inhibitors. In this review, the recent advances of VHL-based PRTOACs were summarized, and the chances and challenges associated with this area were also highlighted.