TAC-tics for Leveraging Proximity Biology in Drug Discovery.

Chemically induced proximity (CIP) refers to co-opting naturally occurring biological pathways using synthetic molecules to recruit neosubstrates that are not normally encountered or to enhance the affinity of naturally occurring interactions. Leveraging proximity biology through CIPs has become a rapidly evolving field and has garnered considerable interest in basic research and drug discovery. PROteolysis TArgeting Chimera (PROTAC) is a well-established CIP modality that induces the proximity between a target protein and an E3 ubiquitin ligase, causing target protein degradation via the ubiquitin-proteasome system. Inspired by PROTACs, several other induced proximity modalities have emerged to modulate both proteins and RNA over recent years. In this review, we summarize the critical advances and opportunities in the field, focusing on protein degraders, RNA degraders and non-degrader modalities such as post-translational modification (PTM) and protein-protein interaction (PPI) modulators. We envision that these emerging proximity-based drug modalities will be valuable resources for both biological research and therapeutic discovery in the future.

Hijacking Methyl Reader Proteins for Nuclear-Specific Protein Degradation.

Targeted protein degradation (TPD) by PROTACs is a promising strategy to control disease-causing protein levels within the cell. While TPD is emerging as an innovative drug discovery paradigm, there are currently only a limited number of E3 ligase:ligand pairs that are employed to induce protein degradation. Herein, we report a novel approach to induce protein degradation by hijacking a methyl reader:E3 ligase complex. L3MBTL3 is a methyl-lysine reader protein that binds to the Cul4DCAF5 E3 ligase complex and targets methylated proteins for proteasomal degradation. By co-opting this natural mechanism, we report the design and biological evaluation of L3MBTL3-recruiting PROTACs and demonstrate nuclear-specific degradation of FKBP12 and BRD2. We envision this as a generalizable approach to utilize other reader protein-associated E3 ligase complexes in PROTAC design to expand the E3 ligase toolbox and explore the full potential of TPD.

Targeted degradation of transcription factors by TRAFTACs: TRAnscription Factor TArgeting Chimeras.

Many diseases, including cancer, stem from aberrant activation or overexpression of oncoproteins that are associated with multiple signaling pathways. Although proteins with catalytic activity can be successfully drugged, the majority of other protein families, such as transcription factors, remain intractable due to their lack of ligandable sites. In this study, we report the development of TRAnscription Factor TArgeting Chimeras (TRAFTACs) as a generalizable strategy for targeted transcription factor degradation. We show that TRAFTACs, which consist of a chimeric oligonucleotide that simultaneously binds to the transcription factor of interest (TOI) and to HaloTag-fused dCas9 protein, can induce degradation of the former via the proteasomal pathway. Application of TRAFTACs to two oncogenic TOIs, NF-κB and brachyury, suggests that TRAFTACs can be successfully employed for the targeted degradation of other DNA-binding proteins. Thus, TRAFTAC technology is potentially a generalizable strategy to induce degradation of other transcription factors both in vitro and in vivo.

Differential profiles of HDAC1 substrates and associated proteins in breast cancer cells revealed by trapping.

Histone deacetylase (HDAC) proteins, which regulate the acetylation state of proteins, are the targets of multiple clinical drugs for cancer treatment. Due to the heterogeneity of tumors, HDAC proteins play different roles in the progression of various cancer types. For example, MDA-MB-468 and MDA-MB-231 cells are both triple negative breast cancer cells but belong to different subtypes that display different response to HDAC inhibitor drugs. To investigate the role of HDAC proteins in breast cancer, the substrate and associated proteins of HDAC1 in MDA-MB-231, MDA-MB-468, and a normal breast epithelial cell line, MCF10A, were analyzed using substrate trapping mutants and proteomics-based mass spectrometry. All three cell lines demonstrated nonoverlapping substrate protein profiles. While both normal MCF10A and cancerous MDA-MB-468 cell lines contained similar HDAC1 associated proteins, including proteins associated with epigenetic and RNA processing mechanisms, the HDAC1 associated protein profile of MDA-MB-231 cells was devoid of expected epigenetic proteins. The variable associated protein profiles of MDA-MB-231 and MDA-MB-468 suggest that HDAC1 plays distinct roles in breast cancer cell biology, which might affect cancer aggressiveness and HDAC inhibitor sensitivity.

Double-Barreled CRISPR Technology as a Novel Treatment Strategy For COVID-19.

Coronavirus is one of the causative agents for multiple human respiratory illnesses. A novel coronavirus, similar to the one that caused severe acute respiratory syndrome (SARS) in 2003, was identified as the cause of the current pandemic of coronavirus disease (COVID-19), which was first reported in late December 2019 in Wuhan, China. Since then, this novel coronavirus has spread across the globe, with most identified COVID-19 cases and fatalities occurring in the United States. In this Perspective, we discuss coronavirus pathogenicity, conventional antiviral therapies, prophylactic strategies, and novel treatment strategies for COVID-19. We highlight the application of CRISPR technology as an emerging pan-antiviral therapy. We also discuss the challenges of in vivo delivery of CRISPR components and propose novel approaches to achieve selective delivery exclusively into SARS-CoV-2-infected cells with high efficiency by hijacking the surface proteins of SARS-CoV-2.

Targeted Degradation of Oncogenic KRASG12C by VHL-Recruiting PROTACs.

KRAS is mutated in ∼20% of human cancers and is one of the most sought-after targets for pharmacological modulation, despite having historically been considered "undruggable." The discovery of potent covalent inhibitors of the KRASG12C mutant in recent years has sparked a new wave of interest in small molecules targeting KRAS. While these inhibitors have shown promise in the clinic, we wanted to explore PROTAC-mediated degradation as a complementary strategy to modulate mutant KRAS. Herein, we report the development of LC-2, the first PROTAC capable of degrading endogenous KRASG12C. LC-2 covalently binds KRASG12C with a MRTX849 warhead and recruits the E3 ligase VHL, inducing rapid and sustained KRASG12C degradation leading to suppression of MAPK signaling in both homozygous and heterozygous KRASG12C cell lines. LC-2 demonstrates that PROTAC-mediated degradation is a viable option for attenuating oncogenic KRAS levels and downstream signaling in cancer cells.

PROTACs: An Emerging Therapeutic Modality in Precision Medicine.

Targeted protein degradation (TPD) has emerged as an exciting new era in chemical biology and drug discovery. PROteolysis TArgeting Chimera (PROTAC) technology targets cellular proteins for degradation by co-opting the ubiquitin-proteasome system. Over the last 5 years, numerous studies have expanded our understanding of the unique mode of action and advantages of PROTACs, which has in turn spurred interest in both academia and industry to explore PROTACs as a novel therapeutic strategy. In this review, we first highlight the key advantages of PROTACs and then discuss the spatiotemporal regulation of protein degradation. Next, we explore current chemically tractable E3 ligases focusing on expanding the existing repertoire with novel E3 ligases to uncover the full potential of TPD. Collectively, these studies are guiding the development of the PROTAC technology as it emerges as a new modality in precision medicine.

Retraction of "Targeted Protein Internalization and Degradation by ENDosome TArgeting Chimeras (ENDTACs)".

[This retracts the article DOI: 10.1021/acscentsci.9b00224.].

Targeted Protein Internalization and Degradation by ENDosome TArgeting Chimeras (ENDTACs).

Targeted protein degradation has generated excitement in chemical biology and drug discovery throughout academia and industry. By hijacking the machinery responsible for protein degradation via the ubiquitin proteasome system (UPS), various cellular targets have been selectively degraded. However, since the tools used, often termed PROteolysis TArgeting Chimeras (PROTACs), hijack the intracellular quality control machinery, this technology can only access targets within the cell. Extracellular targets such as growth factors, cytokines, and chemokines bind to cell surface receptors, often initiating aberrant signaling in multiple diseases such as cancer and inflammation. However, efforts to develop small molecule inhibitors for these extracellular target proteins have been challenging. Herein, we developed a proof-of-concept approach to evaluate if extracellular proteins can be internalized and degraded via the receptor-mediated endolysosomal pathway. Using a heterodimeric molecule, termed "ENDosome TArgeting Chimera" (ENDTAC), internalization and degradation of an extracellular recombinant eGFP-HT7 fusion protein was achieved by hijacking the decoy GPCR receptor, CXCR7. This proof-of-concept study suggests that using ENDTACs to co-opt the endosomal-lysosomal degradation pathway, in contrast to PROTACs using the UPS, may provide an avenue for degrading extracellular targets such as cytokines. Overall, the technology described herein provides a novel expansion to the field of targeted protein degradation.

HDAC1 Substrate Profiling Using Proteomics-Based Substrate Trapping.

Histone deacetylase (HDAC) proteins are overexpressed in multiple diseases, including cancer, and have emerged as anticancer drug targets. HDAC proteins regulate cellular processes, such as the cell cycle, apoptosis, and cell proliferation, by deacetylating histone and non-histone substrates. Although a plethora of acetylated proteins have been identified using large-scale proteomic approaches, the HDAC proteins responsible for their dynamic deacetylation have been poorly studied. For example, few substrates of HDAC1 have been identified, which is mainly due to the scarcity of substrate identification tools. We recently developed a mutant trapping strategy to identify novel substrates of HDAC1. Herein, we introduce an improved version of the trapping method that uses mass spectrometry (MS)-based proteomics to identify multiple substrates simultaneously. Among the substrate hits, CDK1, AIFM1, MSH6, and RuvB-like 1 were identified as likely HDAC1 substrates. These newly discovered HDAC1 substrates are involved in various biological processes, suggesting novel functions of HDAC1 apart from epigenetics. Substrate trapping combined with MS-based proteomics provides an efficient approach to HDAC1 substrate identification and contributes to the full characterization of HDAC function in normal and disease states.

HDAC Inhibitor-Induced Mitotic Arrest Is Mediated by Eg5/KIF11 Acetylation.

Histone deacetylase 1 (HDAC1) is an epigenetic enzyme that regulates key cellular processes, such as cell proliferation, apoptosis, and cell survival, by deacetylating histone substrates. Aberrant expression of HDAC1 is implicated in multiple diseases, including cancer. As a consequence, HDAC inhibitors have emerged as effective anti-cancer drugs. HDAC inhibitor-induced G0/G1 cell-cycle arrest has been attributed to epigenetic transcriptional changes mediated by histone acetylation. However, the mechanism of G2/M arrest remains poorly understood. Here, we identified mitosis-related protein Eg5 (KIF11) as an HDAC1 substrate using a trapping mutant strategy. HDAC1 colocalized with Eg5 during mitosis and influenced the ATPase activity of Eg5. Importantly, an HDAC1- and HDAC2-selective inhibitor caused mitotic arrest and monopolar spindle formation, consistent with a model in which Eg5 deacetylation by HDAC1 is critical for mitotic progression. These findings revealed a previously unknown mechanism of action of HDAC inhibitors involving Eg5 acetylation, and provide a compelling mechanistic hypothesis for HDAC inhibitor-mediated G2/M arrest.

LSD1 Substrate Binding and Gene Expression Are Affected by HDAC1-Mediated Deacetylation.

Lysine Specific Demethylase 1 (LSD1) catalyzes the demethylation of histone 3 to regulate gene expression. With a fundamental role in gene regulation, LSD1 is involved in multiple cellular processes, including embryonic development, cell proliferation, and metastasis. Significantly, LSD1 is overexpressed in multiple cancers and has emerged as a potential anticancer drug target. LSD1 is typically found in association with another epigenetic enzyme, histone deacetylase (HDAC). HDAC and LSD1 inhibitor compounds have been tested as combination anticancer agents. However, the functional link between LSD1 and HDAC has yet to be understood in detail. Here, we used a substrate trapping strategy to identify cellular substrates of HDAC1. Using inactive HDAC1 mutants, we identified LSD1 as an HDAC1 substrate. HDAC1 mediated deacetylation of LSD1 at K374 in the substrate binding lobe, which affected the histone 3 binding and gene expression activity of LSD1. The mechanistic link between HDAC1 and LSD1 established here suggests that HDAC inhibitors influence LSD1 activity, which will ultimately guide drug design targeting epigenetic enzymes.

Mutagenesis studies of the 14 Å internal cavity of histone deacetylase 1: insights toward the acetate-escape hypothesis and selective inhibitor design.

Histone deacetylase (HDAC) proteins are promising targets for cancer treatment, as shown by the approval of two HDAC inhibitors for the treatment of cutaneous T-cell lymphoma. HDAC1 in particular has been linked to cell growth and cell cycle regulation and is therefore an attractive target for anticancer drugs. The HDAC1 active site contains a hydrophobic 11 Å active-site channel, with a 14 Å internal cavity at the bottom of the active site. Several computational and biochemical studies have proposed an acetate-escape hypothesis where the acetate byproduct of the deacetylation reaction escapes via the 14 Å internal cavity. Selective HDAC inhibitors that bind to the 14 Å cavity have also been created. To understand the influence of amino acids lining the HDAC1 14 Å cavity in acetate escape and inhibitor binding, we used mutagenesis coupled with acetate competition assays. The results indicate that amino acids lining the 14 Å cavity are critical for catalytic activity and acetate competition, confirming the role of the cavity in acetate escape. In addition, these mutagenesis studies will aid in HDAC1-inhibitor design that exploits the 14 Å cavity.