Mechanistic studies of substrate-assisted inhibition of ubiquitin-activating enzyme by adenosine sulfamate analogues.

Ubiquitin-activating enzyme (UAE or E1) activates ubiquitin via an adenylate intermediate and catalyzes its transfer to a ubiquitin-conjugating enzyme (E2). MLN4924 is an adenosine sulfamate analogue that was identified as a selective, mechanism-based inhibitor of NEDD8-activating enzyme (NAE), another E1 enzyme, by forming a NEDD8-MLN4924 adduct that tightly binds at the active site of NAE, a novel mechanism termed substrate-assisted inhibition (Brownell, J. E., Sintchak, M. D., Gavin, J. M., Liao, H., Bruzzese, F. J., Bump, N. J., Soucy, T. A., Milhollen, M. A., Yang, X., Burkhardt, A. L., Ma, J., Loke, H. K., Lingaraj, T., Wu, D., Hamman, K. B., Spelman, J. J., Cullis, C. A., Langston, S. P., Vyskocil, S., Sells, T. B., Mallender, W. D., Visiers, I., Li, P., Claiborne, C. F., Rolfe, M., Bolen, J. B., and Dick, L. R. (2010) Mol. Cell 37, 102-111). In the present study, substrate-assisted inhibition of human UAE (Ube1) by another adenosine sulfamate analogue, 5'-O-sulfamoyl-N(6)-[(1S)-2,3-dihydro-1H-inden-1-yl]-adenosine (Compound I), a nonselective E1 inhibitor, was characterized. Compound I inhibited UAE-dependent ATP-PP(i) exchange activity, caused loss of UAE thioester, and inhibited E1-E2 transthiolation in a dose-dependent manner. Mechanistic studies on Compound I and its purified ubiquitin adduct demonstrate that the proposed substrate-assisted inhibition via covalent adduct formation is entirely consistent with the three-step ubiquitin activation process and that the adduct is formed via nucleophilic attack of UAE thioester by the sulfamate group of Compound I after completion of step 2. Kinetic and affinity analysis of Compound I, MLN4924, and their purified ubiquitin adducts suggest that both the rate of adduct formation and the affinity between the adduct and E1 contribute to the overall potency. Because all E1s are thought to use a similar mechanism to activate their cognate ubiquitin-like proteins, the substrate-assisted inhibition by adenosine sulfamate analogues represents a promising strategy to develop potent and selective E1 inhibitors that can modulate diverse biological pathways.

Development of a charcoal paper adenosine triphosphate:pyrophosphate exchange assay: kinetic characterization of NEDD8 activating enzyme.

Ubiquitin activating enzyme (UAE, UBE1, or E1) and seven known homologous "E1s" initiate the conjugation pathways for ubiquitin and 16 other ubiquitin-like modifiers (ULMs) found in humans. The initial step catalyzed by E1s uses adenosine triphosphate (ATP) to adenylate the C terminus of the appropriate ULM and results in the production of inorganic pyrophosphate (PPi). The mechanism of these enzymes can be studied with assays that measure the rate of ULM-dependent ATP:PPi exchange. The traditional method follows the initial velocity of [32P]PPi incorporation into ATP by capturing the nucleotide on activated charcoal powder to separate it from excess [32P]PPi and then measuring [32P]ATP in a scintillation counter. We have modified the method by using charcoal paper to capture the nucleotide and a phosphorimager to quantify the [32P]ATP. The significant increase in throughput that these modifications provide is accomplished without any sacrifice in sensitivity or accuracy compared with the traditional method. To demonstrate this, we reproduce and extend the characterization of the NEDD8 activating enzyme.

Comparison of biochemical and biological effects of ML858 (salinosporamide A) and bortezomib.

Strains within the genus Salinospora have been shown to produce complex natural products having antibiotic and antiproliferative activities. The biochemical basis for the cytotoxic effects of salinosporamide A has been linked to its ability to inhibit the proteasome. Synthetically accessible salinosporamide A (ML858) was used to determine its biochemical and biological activities and to compare its effects with those of bortezomib. ML858 and bortezomib show time- and concentration-dependent inhibition of the proteasome in vitro. However, unlike bortezomib, which is a reversible inhibitor, ML858 covalently binds to the proteasome, resulting in the irreversible inhibition of 20S proteasome activity. ML858 was equipotent to bortezomib in cell-based reporter stabilization assays, but due to intramolecular instability is less potent in long-term assays. ML858 failed to maintain levels of proteasome inhibition necessary to achieve efficacy in tumor models responsive to bortezomib. Our results show that ML858 and bortezomib exhibit different kinetic and pharmacologic profiles and suggest that additional characterization of ML858 is warranted before its therapeutic potential can be fully appreciated.

Correlation of serpin-protease expression by comparative analysis of real-time PCR profiling data.

Imbalanced protease activity has long been recognized in the progression of disease states such as cancer and inflammation. Serpins, the largest family of endogenous protease inhibitors, target a wide variety of serine and cysteine proteases and play a role in a number of physiological and pathological states. The expression profiles of 20 serpins and 105 serine and cysteine proteases were determined across a panel of normal and diseased human tissues. In general, expression of serpins was highly restricted in both normal and diseased tissues, suggesting defined physiological roles for these protease inhibitors. A high correlation in expression for a particular serpin-protease pair in healthy tissues was often predictive of a biological interaction. The most striking finding was the dramatic change observed in the regulation of expression between proteases and their cognate inhibitors in diseased tissues. The loss of regulated serpin-protease matched expression may underlie the imbalanced protease activity observed in pathological states.