The search for inhibitors of the ubiquitin–proteasome system from natural resources for drug development

Abstract

The ubiquitin-proteasome proteolytic pathway plays a major role in selective protein degradation and regulates various cellular events, including cell cycle progression, transcription, DNA repair, signal transduction, and immune response. In this pathway, ubiquitin, a highly conserved small protein in eukaryotes, attaches to a target protein prior to degradation. The polyubiquitin chains are recognized by the 26S proteasome, and the protein portion is degraded by the proteolytic active sites in a cavity in the 26S proteasome. The potential of specific proteasome inhibitors, which act as anti-cancer agents, is now under intensive investigation, and bortezomib (PS-341, Velcade), a proteasome inhibitor, was approved by the FDA for multiple myeloma treatment in 2003. Since ubiquitination of proteins requires the sequential action of three enzymes-ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitinprotein ligase (E3)-and polyubiquitination is a prerequisite for proteasome-mediated protein degradation, inhibitors of E1, E2, and E3 are reasonably thought to be drug candidates for treatment of diseases related to ubiquitination. In the course of our search for new classes of inhibitors against the ubiquitin-proteasome proteolytic pathway from natural resources for drug development, we succeeded in isolating agosterols, secomycalolides A, and 1,3-diphenylpropane derivatives as inhibitors of chymotrypsin-like activity of the proteasome, himeic acid A as an E1 inhibitor, and (-)-hexylitaconic acid as an inhibitor of p53-HDM2interaction. In addition, we found that girolline, an anticancer compound, is the first agent inhibiting the recruitment of polyubiquitinated p53 to the proteasome.

The Search for Inhibitors of the Ubiquitin-proteasome System from Natural Sources by Cell-based Screening in Reporter-expressing Cells

“…The extract of a marine─ derived lamentous fungus (strain 14F240) collected in Indonesia induced GFP accumulation with an F n index of 5.24. Bioassay─ guided fractionation of the extract identi ed NG391 36 (7) and lucilactaene 37 (8) as active substances (Figure 5 and 7). 38 Next, proteolytic inhibition due to 7 was evaluated by reporter degradation assay.…”

“…16 Several NPs inhibiting the proteasomal proteolysis and ubiquitination steps have been discovered. 7 Salinosporamide A (Marizomib), a 20S proteasome inhibitor with β ─ lactone scaffold, was isolated from a marine actinomycete belonging to the genus Salinospora. It is currently in Phase III clinical trials for glioblastoma multiforme (Figure 2).…”

“…In view of this, we have been extensively searching for new NPs that inhibit the UPS. 7 In this paper, we describe our recent studies on the search for UPS inhibitors in an in─ house natural extract library by using cell─ based assays in reporter─ expressing HeLa cells.…”