Selective inhibition of c-Myc/Max dimerization by a pyrazolo[1,5-a]pyrimidine

Targeting MgrA-mediated virulence regulation in Staphylococcus aureus

Increasing antibiotic resistance in human pathogens necessitates the development of new approaches against infections. Targeting virulence regulation at the transcriptional level represents a promising strategy yet to be explored. A global transcriptional regulator, MgrA in Staphylococcus aureus, was identified previously as a key virulence determinant. We have performed a fluorescence anisotropy (FA)-based high-throughput screen that identified 5, 5-methylenedisalicylic acid (MDSA), which blocks the DNA binding of MgrA. MDSA represses the expression of α-toxin that is up-regulated by MgrA and activates the transcription of protein A, a gene down-regulated by MgrA. MDSA alters bacterial antibiotic susceptibilities via an MgrA-dependent pathway. A mouse model of infection indicated that MDSA could attenuate S. aureus virulence. This work is a rare demonstration of utilizing small molecules to block protein-DNA interaction, thus tuning important biological regulation at the transcriptional level.

Therapeutic Targeting of Myc

Protein-protein interactions between members of the Myc transcription factor network are potential targets of small molecule inhibitors and stabilizers. Diverse screening strategies, including fluorescence resonance energy transfer, fluorescence polarization, two hybrid and protein complementation assays have identified several lead compounds that inhibit Myc-Max dimerization and one compound that stabilizes the Max homodimer. Representative compounds interfere with Myc-induced transcriptional activation, Myc-mediated oncogenic transformation, Myc-driven cellular replication and DNA binding of Myc. For the best characterized compounds, specific binding sites have been determined, and molecular mechanisms of action have been documented. This knowledge of small molecule - protein interaction is currently applied to highly targeted approaches that seek to identify novel compounds with improved potency.

Intrinsically disordered proteins are potential drug targets

Intrinsically disordered (ID) proteins that lack stable secondary and tertiary structure in substantial regions (or throughout) are prevalent in eukaryotes. They exist as ensembles of rapidly fluctuating structures and many undergo coupled folding and binding reactions. Because ID proteins are overrepresented in major disease pathways they are desirable targets for inhibition; however, the feasibility of targeting proteins without defined structures was unclear. Recently, small molecules have been found that bind to the disordered regions of c-Myc, Abeta, EWS-Fli1, and various peptides. As with structured targets, initial hits were further optimized to increase specificity and affinity. Given the number and biological importance of ID proteins, the ability to inhibit their interactions opens tremendous potential in chemical biology and drug discovery.

A complex task? Direct modulation of transcription factors with small molecules

Transcription factors with aberrant activity in disease are promising yet untested targets for therapeutic development, particularly in oncology. Directly inhibiting or activating the function of a transcription factor requires specific disruption or recruitment of protein-protein or protein-DNA interactions. The discovery or design of small molecules that specifically modulate these interactions has thus far proven to be a significant challenge and the protein class is often perceived to be 'undruggable.' This review will summarize recent progress in the development of small-molecule probes of transcription factors and provide evidence to challenge the notion that this important protein class is chemically intractable.

A therapeutic role for targeting c-Myc/Hif-1-dependent signaling pathways

Deregulated c-Myc occurs in approximately 30% of human cancers. Similarly, hypoxia-inducible factor (HIF) is commonly overexpressed in a variety of human malignancies. Under physiologic conditions, HIF inhibits c-Myc activity; however, when deregulated oncogenic c-Myc collaborates with HIF in inducing the expression of VEGF, PDK1 and hexokinase 2. Most of the knowledge of HIF derives from studies investigating a role of HIF under hypoxic conditions, however, HIF-1alpha stabilization is also found in normoxic conditions. Specifically, under hypoxic conditions HIF-1-mediated regulation of oncogenic c-Myc plays a pivotal role in conferring metabolic advantages to tumor cells as well as adaptation to the tumorigenic micromilieu. In addition, our own results show that under normoxic conditions oncogenic c-Myc is required for constitutive high HIF-1 protein levels and activity in Multiple Myeloma (MM) cells, thereby influencing VEGF secretion and angiogenic activity within the bone marrow microenvironment. Further studies are needed to delineate the functional relevance of HIF, MYC, and the HIF-MYC collaboration in MM and other malignancies, also integrating the tumor microenvironment and the cellular context. Importantly, early studies already demonstrate promising preclinical of novel agents, predominantly small molecules, which target c-Myc, HIF or both.

Small-molecule modulators of c-Myc/Max and Max/Max interactions

The transcription factor c-Myc is overexpressed in many tumors in human beings and has been identified as a highly promising target for cancer therapy. Most biological functions of c-Myc require heterodimerization with its activation partner Max. Inhibition of the protein-protein interactions between c-Myc and Max by small molecules has been shown to be a feasible and powerful approach toward the inhibition of c-Myc functions. More recently, stabilization of Max homodimers to reduce the amount of Max available for activating c-Myc has also been demonstrated to counteract Myc activity. This review summarizes our current knowledge on small organic molecules that inhibit c-Myc by modulating protein-protein interactions relevant for the biological function of this important oncoprotein.

Small molecule inhibitors of Myc/Max dimerization and Myc-induced cell transformation

The preparation and evaluation of a series of inhibitors of Myc/Max dimerization and Myc-induced cell transformation are described providing mycmycin-1 (3) and mycmycin-2 (4).

Multiple independent binding sites for small-molecule inhibitors on the oncoprotein c-Myc

Deregulation of the c-Myc transcription factor is involved in many types of cancer, making this oncoprotein an attractive target for drug discovery. One approach to its inhibition has been to disrupt the dimeric complex formed between its basic helix-loop-helix leucine zipper (bHLHZip) domain and a similar domain on its dimerization partner, Max. As monomers, bHLHZip proteins are intrinsically disordered (ID). Previously we showed that two c-Myc-Max inhibitors, 10058-F4 and 10074-G5, bound to distinct ID regions of the monomeric c-Myc bHLHZip domain. Here, we use circular dichroism, fluorescence polarization, and NMR to demonstrate the presence of an additional binding site located between those for 10058-F4 and 10074-G5. All seven of the originally identified Myc inhibitors are shown to bind to one of these three discrete sites within the 85-residue bHLHZip domain of c-Myc. These binding sites are composed of short contiguous stretches of amino acids that can selectively and independently bind small molecules. Inhibitor binding induces only local conformational changes, preserves the overall disorder of c-Myc, and inhibits dimerization with Max. NMR experiments further show that binding at one site on c-Myc affects neither the affinity nor the structural changes taking place upon binding to the other sites. Rather, binding can occur simultaneously and independently on the three identified sites. Our results suggest the widespread existence of peptide regions prone to small-molecule binding within ID domains. A rational and generic approach to the inhibition of protein-protein interactions involving ID proteins may therefore be possible through the targeting of ID sequence.

Targeting protein–protein interactions for therapeutic intervention: a challenge for the future

Background: Over the last two decades, an increasing research effort in academia and industry has focused on the modulation (both inhibition and stabilization) of protein–protein interactions (PPIs) in order to develop novel therapeutic approaches and target-selective agents in drug discovery. Discussion: The diversity and complexity of highly dynamic systems such as PPIs present many challenges for the identification of drug-like molecules with the ability to modulate the PPI with the necessary selectivity and potency. In this review, a number of these strategies will be presented along with a critical overview of the challenges and potential solutions relating to the exploitation of PPIs as molecular targets. Conclusions: Both traditional drug discovery approaches and some more recently developed innovative strategies have already provided valuable tools for the discovery of PPI modulators, and a number of successful examples have highlighted the potential of targeting PPIs for therapeutic intervention, especially in the oncology area.