16 Genotoxic and other stressful stimuli not only induce an accumulation of p53, but also the phosphorylation of mdm2, thereby enhancing p53′s nuclear localization, ubiquitination and subsequent degradation.17 Of note, mdm2 is overexpressed in many tumors and effectively impairs p53 function by http://www.selleckchem.com/products/ink128.html binding

directly to p53; this promotes its ubiquitination and targeting to the 26S proteasome for protein degradation.17 Protein–protein interactions have long been considered challenging targets for therapeutic intervention, because their opposing surfaces are often too large or flat for effective disruption by small molecules. The more successful chemical antagonists take advantage of specific interactions

within well-defined pockets on the surfaces PF 01367338 of one or both protein partners.18 The discovery that p53-mdm2 binding was dependent on only three p53 amino acid residues interacting with a discrete mdm2 pocket stimulated efforts to identify potential small molecule inhibitors.19 The first potent and selective antagonists of p53-mdm2 were the Nutlins.20 They represent a class of cis-imidazole analogues that bind to the p53 pocket on the surface of mdm2 in an enantiomer-specific manner. The three reported Nutlins, -1, -2 and -3, show potency against p53-mdm2 binding in the 100–300 nmol range with 150- to 200-fold range in affinity between enantiomers. They inhibit the p53-mdm2 selleck kinase inhibitor axis by mimicking the interaction of the three critical amino acid residues with the hydrophobic activity of mdm2.20 In addition to their high potency in vitro, they penetrate cell membranes, activate p53 and inhibit cell proliferation at a range of 1–3 µmol. Released from its negative control in the presence of Nutlin (Fig. 1b), p53 is stabilized and accumulates in cells leading to the activation of target genes; for example, p21 and mdm2. This effect, however, is dependent on the presence of wt p53, because cells in

which p53 is deleted or mutated do not respond to Nutlin treatment.20 In addition to parenteral routes, Nutlins can also be given orally, which is highly desirable for their applicability in animal models and in the clinic.20 In this issue of the Journal, Wang et al.21 utilized Nutlin-3 against three human HCC cell lines with wt (HepG2), mutant (Hep3B) and null p53 (Huh7). This selective mdm2 antagonist induced growth arrest in all three cell types in vitro with significant abrogation of the pro-proliferative genes, cyclin D1/cdk4, cyclin E/cdk2 and E2F transcription factor. Cell cycle arrest was mediated by upregulation of p21 only in p53-intact HepG2 cells, whereas p27, another downstream target of p53, was expressed by all three tumor cell lines. Regardless of p53 status, Nutlin-3 treatment triggered increased apoptosis in all tumor cells, as well as increased expression of Bax, Noxa and PUMA.