The oncoproteins MDM2 and MDMX negatively regulate the activity and stability of the tumor suppressor protein p53-a cellular process initiated by MDM2 and/or MDMX binding to the Nterminal transactivation domain of p53. MDM2 and MDMX in many tumors confer p53 inactivation and tumor survival, and are important molecular targets for anticancer therapy. We screened a duodecimal peptide phage library against site-specifically biotinylated p53-binding domains of human MDM2 and MDMX chemically synthesized via native chemical ligation, and identified several peptide inhibitors of the p53-MDM2/MDMX interactions. The most potent inhibitor (TSFAEYWNLLSP), termed PMI, bound to MDM2 and MDMX at low nanomolar affinities-approximately 2 orders of magnitude stronger than the wild-type p53 peptide of the same length (ETFSDLWKLLPE). We solved the crystal structures of synthetic MDM2 and MDMX, both in complex with PMI, at 1.6 Å resolution. Comparative structural analysis identified an extensive, tightened intramolecular H-bonding network in bound PMI that contributed to its conformational stability, thus enhanced binding to the 2 oncogenic proteins. Importantly, the C-terminal residue Pro of PMI induced formation of a hydrophobic cleft in MDMX previously unseen in the structures of p53-bound MDM2 or MDMX. Our findings deciphered the structural basis for highaffinity peptide inhibition of p53 interactions with MDM2 and MDMX, shedding new light on structure-based rational design of different classes of p53 activators for potential therapeutic use.

The HIV-1 envelope glycoprotein (Env) undergoes conformational transitions consequent to CD4 binding and coreceptor engagement during viral entry. The physical steps in this process are becoming defined, but less is known about their significance as targets of antibodies potentially protective against HIV-1 infection. Here we probe the functional significance of transitional epitope exposure by characterizing 41 human mAbs specific for epitopes exposed on trimeric Env after CD4 engagement. These mAbs recognize three epitope clusters: cluster A, the gp120 face occluded by gp41 in trimeric Env; cluster B, a region proximal to the coreceptor-binding site (CoRBS) and involving the V1/V2 domain; and cluster C, the coreceptor-binding site. The mAbs were evaluated functionally by antibody-dependent, cell-mediated cytotoxicity (ADCC) and for neutralization of Tiers 1 and 2 pseudoviruses. All three clusters included mAbs mediating ADCC. However, there was a strong potency bias for cluster A, which harbors at least three potent ADCC epitopes whose cognate mAbs have electropositive paratopes. Cluster A epitopes are functional ADCC targets during viral entry in an assay format using virionsensitized target cells. In contrast, only cluster C contained epitopes that were recognized by neutralizing mAbs. There was significant diversity in breadth and potency that correlated with epitope fine specificity. In contrast, ADCC potency had no relationship with neutralization potency or breadth for any epitope cluster. Thus, Fcmediated effector function and neutralization coselect with specificity in anti-Env antibody responses, but the nature of selection is distinct for these two antiviral activities.

HIV-1-infected cells presenting envelope glycoproteins (Env) in the CD4-bound conformation on their surface are preferentially targeted by antibody-dependent cell-mediated cytotoxicity (ADCC). HIV-1 has evolved a sophisticated mechanism to avoid exposure of ADCC-mediating Env epitopes by down-regulating CD4 and by limiting the overall amount of Env at the cell surface. Here we report that small-molecule CD4-mimetic compounds induce the CD4-bound conformation of Env, and thereby sensitize cells infected with primary HIV-1 isolates to ADCC mediated by antibodies present in sera, cervicovaginal lavages, and breast milk from HIV-1-infected individuals. Importantly, we identified one CD4 mimetic with the capacity to sensitize endogenously infected ex vivo-amplified primary CD4 T cells to ADCC killing mediated by autologous sera and effector cells. Thus, CD4 mimetics hold the promise of therapeutic utility in preventing and controlling HIV-1 infection.

The RV144 vaccine trial implicated epitopes in the C1 region of gp120 (A32-like epitopes) as targets of potentially protective antibody-dependent cellular cytotoxicity (ADCC) responses. A32-like epitopes are highly immunogenic, as infected or vaccinated individuals frequently produce antibodies specific for these determinants. Antibody titers, as measured by enzyme-linked immunosorbent assay (ELISA) against these epitopes, however, do not consistently correlate with protection. Here, we report crystal structures of CD4-stabilized gp120 cores complexed with the Fab fragments of two nonneutralizing, A32-like monoclonal antibodies (MAbs), N5-i5 and 2.

i Previous studies have shown that sera from HIV-1-infected individuals contain antibodies able to mediate antibody-dependent cellular cytotoxicity (ADCC). These antibodies preferentially recognize envelope glycoprotein (Env) epitopes induced upon CD4 binding. Here, we show that a highly conserved tryptophan at position 69 of the gp120 inner domain is important for ADCC mediated by anti-cluster A antibodies and sera from HIV-1-infected individuals. Human immunodeficiency virus type 1 (HIV-1) infection elicits a potent B cell response resulting in the production of antibodies (Abs) against the envelope glycoproteins (Env), which are exposed at the surface of viral particles and infected cells (1). We recently reported that these antibodies have the potential to eliminate HIV-1-infected cells by mediating antibody-dependent cellular cytotoxicity (ADCC) (2, 3). We found that these nonneutralizing CD4-induced (CD4i) ADCC-mediating antibodies are present in sera (2, 4), breast milk (4), and cervicovaginal lavage fluid (3, 4) of HIV-1-infected individuals and preferentially target Env in its CD4-bound "open" conformation. However, in order to evade ADCC responses, HIV-1 has developed a highly sophisticated strategy to keep Env at the surface of infected cells in the unbound "closed" conformation. HIV-1 accomplishes this through its accessory proteins Nef and Vpu, which decrease the overall amount of Env (via Vpu-mediated BST-2 downregulation) and CD4 at the cell surface (2, 5-7). In addition, decreased amounts of Env at the cell surface due to efficient internalization also help the virus to avoid ADCC responses (8). In agreement with the necessity for HIV-1 to avoid exposing Env in the CD4-bound conformation, we recently showed that forcing Env to adopt this conformation with small CD4 mimetics (CD4mc) sensitizes HIV-1-infected cells to ADCC mediated by sera, breast milk, and cervicovaginal fluids from HIV-1-infected subjects (4).Previous studies showed that the human monoclonal antibody (MAb) A32 targets an ADCC epitope commonly detected by antibodies present in sera from HIV-1-infected individuals (2, 5, 9, 10). Accordingly, an A32 Fab fragment blocked the majority of ADCC-mediating antibody (Ab) activity in plasma from chronically HIV-1-infected individuals (9). A subsequent study showed that the majority of ADCC responses were targeted against the gp120 core but not its variable regions V1, V2, V3, and V5 (2). Here, we evaluated the ADCC-mediating capacity of a panel of human antibodies targeting several well-defined epitopes in gp120 and gp41 and sera from randomly selected chronically HIV-1 clade B-infected individuals (HIV ϩ sera).We infected CEM.NKr cells with a panel of HIV-1 NL4.3-green fluorescent protein (GFP) constructs containing the ADA-Env and either wild-type or defective nef and vpu genes, as described previously (2, 5). Furthermore, we examined a wellcharacterized infectious molecular HIV-1 clone constructed from a transmitted/founder (T/F) virus (CH77) (11-14) containing intact or defective nef an...

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The oncoproteins MDM2 and MDMX negatively regulate the activity and stability of the tumor suppressor protein p53, conferring tumor development and survival. Antagonists targeting the p53-binding domains of MDM2 and MDMX kill tumor cells both in vitro and in vivo by reactivating the p53 pathway, promising a class of antitumor agents for cancer therapy. Aided by native chemical ligation and mirror image phage display, we recently identified a D-peptide inhibitor of the p53-MDM2 interaction termed The tumor suppressor protein p53 is a transcription factor that transactivates, in response to cellular stresses, the expression of various target genes that mediate cell cycle arrest, senescence, or apoptosis (1). Dubbed the "guardian of the genome" (2), p53 is critical for maintaining genetic stability and preventing tumor development (3). Not surprisingly, loss of p53 activity resulting from point mutations in the TP53 gene is responsible for approximately 50% of human tumors. Although p53 retains WT status in many other tumors, its tumor suppressor activity and in vivo stability are abrogated by regulatory molecules such as the E3 ubiquitin ligase MDM2 and its homologue MDMX (4, 5). Amplified or over-expressed in a significant fraction of cancers without concomitant TP53 mutation, MDM2 and MDMX directly contribute to p53 inactivation and tumor survival.MDM2 itself is transcriptionally inducible by p53 in a negative feedback loop (6). MDM2 binds the N-terminal transactivation domain of p53 with high affinity to block p53 regulating responsive gene expression (7). More importantly, MDM2 controls p53 stability by targeting the tumor suppressor protein for ubiquitinmediated constitutive degradation (8-10). Although MDMX lacks E3 ubiquitin ligase activity, the MDM2 homologue acts as an effective transcriptional antagonist of p53, and nonredundantly impedes p53-induced growth inhibitory and apoptotic responses (4, 5). In addition, MDMX forms heterodimers with MDM2 through their C-terminal RING finger domains, stimulating MDM2-mediated ubiquitination and degradation of p53 and MDMX itself (11-13). The interplay between MDM2 and MDMX confers a robust p53 inactivation in tumorigenesis (14).Recent studies show that restoring endogenous p53 activity can halt the growth of cancerous tumors in mice through cell typedependent multiple mechanisms, including apoptosis, senescence, and senescence-triggered innate inflammatory responses (15-17). Thus, antagonists of MDM2 and MDMX that activate the p53 pathway can potentially be developed into a class of therapeutic agents for cancer treatment (14). Much of the current efforts have been focused on combinatorial library search for and structurebased rational design of low molecular weight antagonists of MDM2 (18). Successful examples include a cis-imidazoline analogue, termed nutlin-3, and, a spiro-oxindole-derived compound, termed 20). For optimal efficacy, however, dual specific inhibitors may be needed to target both MDM2 and MDMX (14).We previously reported the synthesis of the p53...

Human immunodeficiency virus type 1 (HIV-1) has evolved a sophisticated strategy to conceal conserved epitopes of its envelope glycoproteins (Env) recognized by antibody-dependent cellular cytotoxicity (ADCC)-mediating antibodies. These antibodies, which are present in the sera of most HIV-1-infected individuals, preferentially recognize Env in its CD4-bound conformation. Accordingly, recent studies showed that small CD4-mimetics (CD4mc) able to “push” Env into this conformation sensitize HIV-1-infected cells to ADCC mediated by HIV + sera. Here we test whether CD4mc also expose epitopes recognized by anti-cluster A monoclonal antibodies such as A32, thought to be responsible for the majority of ADCC activity present in HIV + sera and linked to decreased HIV-1 transmission in the RV144 trial. We made the surprising observation that CD4mc are unable to enhance recognition of HIV-1-infected cells by this family of antibodies in the absence of antibodies such as 17b, which binds a highly conserved CD4-induced epitope overlapping the co-receptor binding site (CoRBS). Our results indicate that CD4mc initially open the trimeric Env enough to allow the binding of CoRBS antibodies but not anti-cluster A antibodies. CoRBS antibody binding further opens the trimeric Env, allowing anti-cluster A antibody interaction and sensitization of infected cells to ADCC. Therefore, ADCC responses mediated by cluster A antibodies in HIV-positive sera involve a sequential opening of the Env trimer on the surface of HIV-1-infected cells. The understanding of the conformational changes required to expose these vulnerable Env epitopes might be important in the design of new strategies aimed at fighting HIV-1.