Substrate-assisted inhibition of ubiquitin-like protein-activating enzymes: the NEDD8 E1 inhibitor MLN4924 forms a NEDD8-AMP mimetic in situ.

The NEDD8-activating enzyme (NAE) initiates a protein homeostatic pathway essential for cancer cell growth and survival. MLN4924 is a selective inhibitor of NAE currently in clinical trials for the treatment of cancer. Here, we show that MLN4924 is a mechanism-based inhibitor of NAE and creates a covalent NEDD8-MLN4924 adduct catalyzed by the enzyme. The NEDD8-MLN4924 adduct resembles NEDD8 adenylate, the first intermediate in the NAE reaction cycle, but cannot be further utilized in subsequent intraenzyme reactions. The stability of the NEDD8-MLN4924 adduct within the NAE active site blocks enzyme activity, thereby accounting for the potent inhibition of the NEDD8 pathway by MLN4924. Importantly, we have determined that compounds resembling MLN4924 demonstrate the ability to form analogous adducts with other ubiquitin-like proteins (UBLs) catalyzed by their cognate-activating enzymes. These findings reveal insights into the mechanism of E1s and suggest a general strategy for selective inhibition of UBL conjugation pathways.

An inhibitor of NEDD8-activating enzyme as a new approach to treat cancer.

The clinical development of an inhibitor of cellular proteasome function suggests that compounds targeting other components of the ubiquitin-proteasome system might prove useful for the treatment of human malignancies. NEDD8-activating enzyme (NAE) is an essential component of the NEDD8 conjugation pathway that controls the activity of the cullin-RING subtype of ubiquitin ligases, thereby regulating the turnover of a subset of proteins upstream of the proteasome. Substrates of cullin-RING ligases have important roles in cellular processes associated with cancer cell growth and survival pathways. Here we describe MLN4924, a potent and selective inhibitor of NAE. MLN4924 disrupts cullin-RING ligase-mediated protein turnover leading to apoptotic death in human tumour cells by a new mechanism of action, the deregulation of S-phase DNA synthesis. MLN4924 suppressed the growth of human tumour xenografts in mice at compound exposures that were well tolerated. Our data suggest that NAE inhibitors may hold promise for the treatment of cancer.

MLN4924, a NEDD8-activating enzyme inhibitor, is active in diffuse large B-cell lymphoma models: rationale for treatment of NF-{kappa}B-dependent lymphoma.

MLN4924 is a potent and selective small molecule NEDD8-activating enzyme (NAE) inhibitor. In most cancer cells tested, inhibition of NAE leads to induction of DNA rereplication, resulting in DNA damage and cell death. However, in preclinical models of activated B cell-like (ABC) diffuse large B-cell lymphoma (DLBCL), we show that MLN4924 induces an alternative mechanism of action. Treatment of ABC DLBCL cells with MLN4924 resulted in rapid accumulation of pIkappaBalpha, decrease in nuclear p65 content, reduction of nuclear factor-kappaB (NF-kappaB) transcriptional activity, and G(1) arrest, ultimately resulting in apoptosis induction, events consistent with potent NF-kappaB pathway inhibition. Treatment of germinal-center B cell-like (GCB) DLBCL cells resulted in an increase in cellular Cdt-1 and accumulation of cells in S-phase, consistent with cells undergoing DNA rereplication. In vivo administration of MLN4924 to mice bearing human xenograft tumors of ABC- and GCB-DLBCL blocked NAE pathway biomarkers and resulted in complete tumor growth inhibition. In primary human tumor models of ABC-DLBCL, MLN4924 treatment resulted in NF-kappaB pathway inhibition accompanied by tumor regressions. This work describes a novel mechanism of targeted NF-kappaB pathway modulation in DLBCL and provides strong rationale for clinical development of MLN4924 against NF-kappaB-dependent lymphomas.

Characterization of a new series of non-covalent proteasome inhibitors with exquisite potency and selectivity for the 20S beta5-subunit.

The mammalian 26S proteasome is a 2500 kDa multi-catalytic complex involved in intracellular protein degradation. We describe the synthesis and properties of a novel series of non-covalent di-peptide inhibitors of the proteasome based [corrected] on a capped tri-peptide that was first identified by high-throughput screening of a library of approx. 350000 compounds for inhibitors of the ubiquitin-proteasome system in cells. We show that these compounds are entirely selective for the beta5 (chymotrypsin-like) site over the beta1 (caspase-like) and beta2 (trypsin-like) sites of the 20S core particle of the proteasome, and over a panel of less closely related proteases. Compound optimization, guided by X-ray crystallography of the liganded 20S core particle, confirmed their non-covalent binding mode and provided a structural basis for their enhanced in vitro and cellular potencies. We demonstrate that such compounds show low nanomolar IC50 values for the human 20S beta5 site in vitro, and that pharmacological inhibition of this site in cells is sufficient to potently inhibit the degradation of a tetra-ubiquitin-luciferase reporter, activation of NFkappaB (nuclear factor kappaB) in response to TNF-alpha (tumour necrosis factor-alpha) and the proliferation of cancer cells. Finally, we identified capped di-peptides that show differential selectivity for the beta5 site of the constitutively expressed proteasome and immunoproteasome in vitro and in B-cell lymphomas. Collectively, these studies describe the synthesis, activity and binding mode of a new series of non-covalent proteasome inhibitors with unprecedented potency and selectivity for the beta5 site, and which can discriminate between the constitutive proteasome and immunoproteasome in vitro and in cells.

Absolute quantification of E1, ubiquitin-like proteins and Nedd8-MLN4924 adduct by mass spectrometry.

Ubiquitin (Ub) and ubiquitin-like (Ubl) proteins regulate a variety of important cellular processes by forming covalent conjugates with target proteins or lipids. Ubl conjugation is catalyzed by a cascade of proteins including activating enzymes (E1), conjugating enzymes (E2), and in many cases ligation enzymes (E3). The discovery of MLN4924 (Brownell et al., Mol Cell 37: 102-111, 1), an investigational small molecule that is a mechanism-based inhibitor of NEDD8-activating enzyme (NAE), reveals a promising strategy of targeting E1/Ubl pathway for therapeutic purposes. In order to better understand, the biochemical dynamics of Ubl conjugation in cells and tissues, we have developed a mass spectrometry-based method to quantify E1 and Ubls using isotope-labeled proteins as internal standards. Furthermore, we have used the described method to quantify levels of the covalent Nedd8-inhibitor adduct formed in MLN4924 treated cells and tissues. The Nedd8-MLN4924 adduct is a tight-binding inhibitor of NAE, and its cellular concentration represents an indirect pharmacodynamic readout of NAE/Nedd8 pathway inhibition.

Inhibition of NEDD8-activating enzyme induces rereplication and apoptosis in human tumor cells consistent with deregulating CDT1 turnover.

Loss of NEDD8-activating enzyme (NAE) function by siRNA knockdown or inhibition by the small molecule NAE inhibitor MLN4924 leads to increased steady-state levels of direct Cullin-RING ligase (CRL) substrates by preventing their ubiquitination and proteasome-dependent degradation. Many of these CRL substrates are involved in cell cycle progression, including a critical DNA replication licensing factor CDT1. Cell cycle analysis of asynchronous and synchronous cultures after NAE inhibition revealed effects on cell cycle distribution and activation of DNA break repair signaling pathways similar to that reported for CDT1 overexpression. The siRNA knockdown of cullins critical for the turnover of CDT1 recapitulated the aberrant rereplication phenotype while CDT1 knockdown was suppressing. Although NAE inhibition leads to deregulation of many CRL substrates, these data demonstrate that CDT1 accumulation mediates the DNA rereplication phenotype resulting from loss of NAE function. DNA rereplication is an unrecoverable cellular insult and the small molecule inhibitor MLN4924, currently in phase I trials, represents an unprecedented opportunity to explore this mechanism of cytotoxicity for the treatment of cancer.

The NEDD8 Conjugation Pathway and Its Relevance in Cancer Biology and Therapy.

Cancer cells depend on signals that promote cell cycle progression and prevent programmed cell death that would otherwise result from cumulative, aberrant stress. These activities require the temporally controlled destruction of specific intracellular proteins by the ubiquitin-proteasome system (UPS). To a large extent, the control points in this process include a family of E3 ubiquitin ligases called cullin-RING ligases (CRLs). The ligase activity of these multicomponent complexes requires modification of the cullin protein situated at their core with a ubiquitin-like protein called NEDD8. Neddylation results in conformational rearrangements within the CRL, which are necessary for ubiquitin transfer to a substrate. The NEDD8 pathway thus has a critical role in mediating the ubiquitination of numerous CRL substrate proteins involved in cell cycle progression and survival including the DNA replication licensing factor Cdt-1, the NF-κB transcription factor inhibitor pIκBα, and the cell cycle regulators cyclin E and p27. The initial step required for attachment of NEDD8 to a cullin is catalyzed by the E1, NEDD8-activating enzyme (NAE). The first-in-class inhibitor of NAE, MLN4924, has been shown to block the activity of NAE and prevent the subsequent neddylation of cullins. Preclinical studies have demonstrated antitumor activity in various solid tumors and hematological malignancies, and preliminary clinical data have shown the anticipated pharmacodynamic effects in humans. Here, we review the NEDD8 pathway, its importance in cancer, and the therapeutic potential of NAE inhibition.

Targeting NEDD8-activated cullin-RING ligases for the treatment of cancer.

E3 ubiquitin ligases regulate many dynamic cellular processes important for cancer cell survival. Together with ubiquitin-activating enzyme (E1) and ubiquitin-conjugating enzymes (E2s), E3s catalyze the ubiquitination of numerous protein substrates that are subsequently targeted to the 26S proteasome for degradation. The clinical success of the proteasome inhibitor bortezomib has encouraged the evaluation of other components of the ubiquitin proteasome system for pharmaceutical intervention. Targeting specific E3s is particularly attractive because there is the potential to selectively block the degradation of certain cellular proteins and possibly avoid unwanted effects on other proteins. The cullin-RING ubiquitin E3 ligases (CRLs) represent the largest subfamily of E3s. The requirement that CRLs be activated by NEDD8 modification on the cullin protein offers an "achilles heel" for modulating this entire subfamily. NEDD8-activating enzyme (NAE) catalyzes the first step in the NEDD8 pathway and as such controls the activity of CRLs. In this article, we describe the role of the NEDD8 pathway in activating CRLs and discuss the preclinical findings with a first-in-class NAE inhibitor that is currently in phase I clinical trials for both solid tumor and hematological malignancies. In addition, we speculate where NAE inhibitors may find clinical utility.