Structure‐based design and molecular profiling of Smac‐mimetics selective for cellular IAP s

ARTS and small-molecule ARTS mimetics upregulate p53 levels by promoting the degradation of XIAP

Mutations resulting in decreased activity of p53 tumor suppressor protein promote tumorigenesis. P53 protein levels are tightly regulated through the Ubiquitin Proteasome System (UPS). Several E3 ligases were shown to regulate p53 stability, including MDM2. Here we report that the ubiquitin E3 ligase XIAP (X-linked Inhibitors of Apoptosis) is a direct ligase for p53 and describe a novel approach for modulating the levels of p53 by targeting the XIAP pathway. Using in vivo (live-cell) and in vitro (cell-free reconstituted system) ubiquitylation assays, we show that the XIAP-antagonist ARTS regulates the levels of p53 by promoting the degradation of XIAP. XIAP directly binds and ubiquitylates p53. In apoptotic cells, ARTS inhibits the ubiquitylation of p53 by antagonizing XIAP. XIAP knockout MEFs express higher p53 protein levels compared to wild-type MEFs. Computational screen for small molecules with high affinity to the ARTS-binding site within XIAP identified a small-molecule ARTS-mimetic, B3. This compound stimulates apoptosis in a wide range of cancer cells but not normal PBMC (Peripheral Blood Mononuclear Cells). Like ARTS, the B3 compound binds to XIAP and promotes its degradation via the UPS. B3 binding to XIAP stabilizes p53 by disrupting its interaction with XIAP. These results reveal a novel mechanism by which ARTS and p53 regulate each other through an amplification loop to promote apoptosis. Finally, these data suggest that targeting the ARTS binding pocket in XIAP can be used to increase p53 levels as a new strategy for developing anti-cancer therapeutics.

SMAC Mimetics for the Treatment of Lung Carcinoma: Present Development and Future Prospects

BACKGROUND

Uncontrolled cell growth and proliferation, which originate from lung tissue often lead to lung carcinoma and are more likely due to smoking as well as inhaled environmental toxins. It is widely recognized that tumour cells evade the ability of natural programmed death (apoptosis) and facilitates tumour progression and metastasis. Therefore investigating and targeting the apoptosis pathway is being utilized as one of the best approaches for decades.

OBJECTIVE

This review describes the emergence of SMAC mimetic drugs as a treatment approach, its possibilities to synergize the response along with current limitations as well as future perspective therapy for lung cancer.

METHOD

Articles were analysed using search engines and databases namely Pubmed and Scopus.

RESULT

Under cancerous circumstances, the level of Inhibitor of Apoptosis Proteins (IAPs) gets elevated, which suppresses the pathway of programmed cell death, plus supports the proliferation of lung cancer. As it is a major apoptosis regulator, natural drugs that imitate the IAP antagonistic response like SMAC mimetic agents/Diablo have been identified to trigger cell death. SMAC i.e. second mitochondria activators of caspases is a molecule produced by mitochondria, stimulates apoptosis by neutralizing/inhibiting IAP and prevents its potential responsible for the activation of caspases. Various preclinical data have proven that these agents elicit the death of lung tumour cells. Apart from inducing apoptosis, these also sensitize the cancer cells toward other effective anticancer approaches like chemo, radio, or immunotherapies. There are many SMAC mimetic agents such as birinapant, BV-6, LCL161, and JP 1201, which have been identified for diagnosis as well as treatment purposes in lung cancer and are also under clinical investigation.

CONCLUSION

SMAC mimetics acts in a restorative way in the prevention of lung cancer.

Characterization of a Potent and Orally Bioavailable Lys-Covalent Inhibitor of Apoptosis Protein (IAP) Antagonist

We have recently reported on the use of aryl-fluorosulfates in designing water- and plasma-stable agents that covalently target Lys, Tyr, or His residues in the BIR3 domain of the inhibitor of the apoptosis protein (IAP) family. Here, we report further structural, cellular, and pharmacological characterizations of this agent, including the high-resolution structure of the complex between the Lys-covalent agent and its target, the BIR3 domain of X-linked IAP (XIAP). We also compared the cellular efficacy of the agent in two-dimensional (2D) and three-dimensional (3D) cell cultures, side by side with the clinical candidate reversible IAP inhibitor LCL161. Finally, in vivo pharmacokinetic studies indicated that the agent was long-lived and orally bioavailable. Collectively our data further corroborate that aryl-fluorosulfates, when incorporated correctly in a ligand, can result in Lys-covalent agents with pharmacodynamic and pharmacokinetic properties that warrant their use in the design of pharmacological probes or even therapeutics.

Killing by Degradation: Regulation of Apoptosis by the Ubiquitin-Proteasome-System

Apoptosis is a cell suicide process that is essential for development, tissue homeostasis and human health. Impaired apoptosis is associated with a variety of human diseases, including neurodegenerative disorders, autoimmunity and cancer. As the levels of pro- and anti-apoptotic proteins can determine the life or death of cells, tight regulation of these proteins is critical. The ubiquitin proteasome system (UPS) is essential for maintaining protein turnover, which can either trigger or inhibit apoptosis. In this review, we will describe the E3 ligases that regulate the levels of pro- and anti-apoptotic proteins and assisting proteins that regulate the levels of these E3 ligases. We will provide examples of apoptotic cell death modulations using the UPS, determined by positive and negative feedback loop reactions. Specifically, we will review how the stability of p53, Bcl-2 family members and IAPs (Inhibitor of Apoptosis proteins) are regulated upon initiation of apoptosis. As increased levels of oncogenes and decreased levels of tumor suppressor proteins can promote tumorigenesis, targeting these pathways offers opportunities to develop novel anti-cancer therapies, which act by recruiting the UPS for the effective and selective killing of cancer cells.

Ensemble-based virtual screening in discovering potent inhibitors targeting Von Hippel-Lindau (VHL) E3 ubiquitin ligase

BACKGROUND

The Von Hippel-Lindau (VHL) E3 ubiquitin ligase, which mediates its substrate hypoxia-inducible factor 1α (HIF-1α) for ubiquitination and subsequent degradation, is an attractive drug target in various diseases, such as anemia, inflammation, neurodegeneration and cancer. Proteolysis targeting chimeras (PROTACs) containing a VHL ligand that can hijack the E3 ligase activity to degrade the target protein has also been studied in academic and in industry areas recently.

METHODS

Herein, by developing and optimizing the Bayesian Model, we report ensemble-based virtual screening as an effective strategy to discover potential VHL inhibitors from Specs database.

RESULTS

The virtual screening protocol was developed, ten representative molecules were obtained and five compounds were selected for subsequent binding mode analysis to be potent VHL inhibitors.

Medicinal Chemistry of Inhibiting RING-Type E3 Ubiquitin Ligases

The ubiquitin proteasome system (UPS) presents many opportunities for pharmacological intervention. Key players in the UPS are E3 ubiquitin ligases, responsible for conjugation of ubiquitin to specific cognate substrates. Numbering more than 600 members, these ligases represent the most selective way to intervene within this physiologically important system. This Perspective highlights some of the dedicated medicinal chemistry efforts directed at inhibiting the function of specific single-protein and multicomponent RING-type E3 ubiquitin ligases. We present opportunities and challenges associated with targeting this important class of enzymes.

Targeting XIAP for Promoting Cancer Cell Death-The Story of ARTS and SMAC

Inhibitors of apoptosis (IAPs) are a family of proteins that regulate cell death and inflammation. XIAP (X-linked IAP) is the only family member that suppresses apoptosis by directly binding to and inhibiting caspases. On the other hand, cIAPs suppress the activation of the extrinsic apoptotic pathway by preventing the formation of pro-apoptotic signaling complexes. IAPs are negatively regulated by IAP-antagonist proteins such as Smac/Diablo and ARTS. ARTS can promote apoptosis by binding and degrading XIAP via the ubiquitin proteasome-system (UPS). Smac can induce the degradation of cIAPs but not XIAP. Many types of cancer overexpress IAPs, thus enabling tumor cells to evade apoptosis. Therefore, IAPs, and in particular XIAP, have become attractive targets for cancer therapy. In this review, we describe the differences in the mechanisms of action between Smac and ARTS, and we summarize efforts to develop cancer therapies based on mimicking Smac and ARTS. Several Smac-mimetic small molecules are currently under evaluation in clinical trials. Initial efforts to develop ARTS-mimetics resulted in a novel class of compounds, which bind and degrade XIAP but not cIAPs. Smac-mimetics can target tumors with high levels of cIAPs, whereas ARTS-mimetics are expected to be effective for cancers with high levels of XIAP.

SMAC mimetics and RIPK inhibitors as therapeutics for chronic inflammatory diseases

New therapeutic approaches for chronic inflammatory diseases such as inflammatory bowel disease, rheumatoid arthritis, and psoriasis are needed because current treatments are often suboptimal in terms of both efficacy and the risks of serious adverse events. Inhibitor of apoptosis proteins (IAPs) are E3 ubiquitin ligases that inhibit cell death pathways and are themselves inhibited by second mitochondria-derived activator of caspases (SMAC). SMAC mimetics (SMs), small-molecule antagonists of IAPs, are being evaluated as cancer therapies in clinical trials. IAPs are also crucial regulators of inflammatory pathways because they influence both the activation of inflammatory genes and the induction of cell death through the receptor-interacting serine-threonine protein kinases (RIPKs), nuclear factor κB (NF-κB)-inducing kinase, and mitogen-activated protein kinases (MAPKs). Furthermore, there is an increasing interest in specifically targeting the substrates of IAP-mediated ubiquitylation, especially RIPK1, RIPK2, and RIPK3, as druggable nodes in inflammation control. Several studies have revealed an anti-inflammatory potential of RIPK inhibitors that either block inflammatory signaling or block the form of inflammatory cell death known as necroptosis. Expanding research on innate immune signaling through pattern recognition receptors that stimulate proinflammatory NF-κB and MAPK signaling may further contribute to uncovering the complex molecular roles used by IAPs and downstream RIPKs in inflammatory signaling. This may benefit and guide the development of SMs or selective RIPK inhibitors as anti-inflammatory therapeutics for various chronic inflammatory conditions.

Aryl-fluorosulfate-based Lysine Covalent Pan-Inhibitors of Apoptosis Protein (IAP) Antagonists with Cellular Efficacy

We have recently investigated the reactivity of aryl-fluorosulfates as warheads to form covalent adducts with Lys, Tyr, and His residues. However, the rate of reaction of aryl-fluorosulfates seemed relatively slow, putting into question their effectiveness to form covalent adducts in cell. Unlike the previously reported agents that targeted a relatively remote Lys residue with respect to the target's binding site, the current agents were designed to more directly juxtapose an aryl-fluorosulfate with a Lys residue that is located within the binding pocket of the BIR3 domain of X-linked inhibitor of apoptosis protein (XIAP). We found that such new agents can effectively and rapidly form a covalent adduct with XIAP-BIR3 in vitro and in cell, approaching the rate of reaction, cellular permeability, and stability that are similar to what attained by acrylamides when targeting Cys residues. Our studies further validate aryl-fluorosulfates as valuable Lys-targeting electrophiles, for the design of inhibitors of both enzymes and protein-protein interactions.

Targeting the BIR Domains of Inhibitor of Apoptosis (IAP) Proteins in Cancer Treatment

Inhibitor of apoptosis (IAP) proteins are characterized by the presence of the conserved baculoviral IAP repeat (BIR) domain that is involved in protein-protein interactions. IAPs were initially thought to be mainly responsible for caspase inhibition, acting as negative regulators of apoptosis, but later works have shown that IAPs also control a plethora of other different cellular pathways. As X-linked IAP (XIAP), and other IAP, levels are often deregulated in cancer cells and have been shown to correlate with patients' prognosis, several approaches have been pursued to inhibit their activity in order to restore apoptosis. Many small molecules have been designed to target the BIR domains, the vast majority being inspired by the N-terminal tetrapeptide of Second Mitochondria-derived Activator of Caspases/Direct IAp Binding with Low pI (Smac/Diablo), which is the natural XIAP antagonist. These compounds are therefore usually referred to as Smac mimetics (SMs). Despite the fact that SMs were intended to specifically target XIAP, it has been shown that they also interact with cellular IAP-1 (cIAP1) and cIAP2, promoting their proteasome-dependent degradation. SMs have been tested in combination with several cytotoxic compounds and are now considered promising immune modulators which can be exploited in cancer therapy, especially in combination with immune checkpoint inhibitors. In this review, we give an overview of the structural hot-spots of BIRs, focusing on their fold and on the peculiar structural patches which characterize the diverse BIRs. These structures are exploited/exploitable for the development of specific and active IAP inhibitors.