Pharmacophore model for novel inhibitors of ubiquitin isopeptidases that induce p53-independent cell death.

The tumor suppressor p53 is mutated in more than 50% of all cancers. Importantly, most clinically useful antineoplastic agents are less potent and efficacious in the context of mutant p53. This situation has prompted a search for agents that cause tumor cell death via molecular mechanisms independent of p53. Our recent investigations with electrophilic prostaglandins enabled us to devise a pharmacophore and mechanism of action hypothesis relevant to this problem: a cross-conjugated alpha,beta-unsaturated dienone with two sterically accessible electrophilic beta-carbons is a molecular determinant that confers activity among this class of ubiquitin isopeptidases inhibitors, and that inhibitors of ubiquitin isopeptidases cause cell death in vitro independently of p53. Here, we report the use of the National Cancer Institute's Developmental Therapeutics Database to identify compounds to test this hypothesis. Shikoccin (a diterpene), dibenzylideneacetone, and curcumin fit the pharmacophore hypothesis, inhibit cellular isopeptidases, and cause cell death independently of p53 in isogenic pairs of RKO and HCT 116 cells with differential p53 status. The sesquiterpene achillin and 2,6-diphenyl-4H-thiopyran-4-one, which have cross-conjugated dienones with sterically hindered electrophilic beta-carbons, do not inhibit isopeptidases or cause significant cell death. Furthermore, we show that a catalytic-site proteasome inhibitor causes cell death independently of p53. Combined, these data verify the p53-independence of cell death caused by inhibitors of the proteasome pathway and support the proposition that the ubiquitin-dependent proteasome pathway may contain molecular targets suitable for antineoplastic drug discovery.