Key Points Calreticulin mutants responsible for myeloproliferative neoplasms specifically activate the thrombopoietin receptor and in turn JAK2. Activation of the thrombopoietin receptor requires the glycan binding site and a novel C-terminal tail of the mutant calreticulin.

By using a phage display derived peptide as an initial template, compounds have been developed that are highly specific against Mdm2/Mdm4. These compounds exhibit greater potency in p53 activation and protein-protein interaction assays than a compound derived from the p53 wild-type sequence. Unlike Nutlin, a small molecule inhibitor of Mdm2/Mdm4, the phage derived compounds can arrest cells resistant to p53 induced apoptosis over a wide concentration range without cellular toxicity, suggesting they are highly suitable for cyclotherapy.

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The activation of p53 has been proposed as a novel anti-cancer treatment in two distinct contexts. In the first activation of p53 in tumor cells can promote apoptosis and senescence and enhance the anti-tumor activity of cytotoxic chemotherapeutic drugs. In the second application activation of p53 in normal tissues can cause a reversible cell cycle arrest that can be used to protect normal cells from the action of anti-mitotics. In this cyclotherapy role p53 mutant tumor cells are not arrested and remain sensitive to anti-mitotics. The advent of specific p53 activating molecules such as nutlin-3 has encouraged both approaches. We have sought for a clinically approved drug that can mimic nutlin-3. We show here that low doses of actinomycin D mimic nutlin-3 in the highly specific activation of p53 dependant transcription, in the induction of a reversible protective growth arrest in normal cells and in the enhancement of the activity of chemotherapeutic drug induced killing of p53 positive human tumor cells. While high doses of actinomycin D reveal its more non-specific activities, low doses of the drug will allow exploration of the value of p53 activation in preclinical and clinical models before nutlin-3 like drugs are approved.

To identify novel inhibitors of Mycobacterium tuberculosis cell envelope biosynthesis, we employed a two-step approach. First, we screened the diverse synthetic small molecule 71,544-compound Enamine library for growth inhibitors using the non-pathogenic surrogate Mycobacterium bovis BCG as screening strain and turbidity as readout. Second, 16 confirmed hits were tested for their ability to induce the cell envelope stress responsive promoter piniBAC controlling expression of red fluorescent protein in an M. bovis BCG reporter strain. Using a fluorescence readout, the acetamide E11 was identified. Resistant mutant selection and whole genome sequencing revealed the mycolic acid transporter Mmpl3 as a candidate target of E11. Biochemical analysis using mycobacterial spheroplasts and various membrane assays suggest that E11 indirectly inhibits MmpL3-facilitated translocation of trehalose monomycolates by proton motive force disruption. E11 showed potent bactericidal activity against growing and non-growing M. tuberculosis, low cytotoxic, and hemolytic activity and a dynamic structure activity relationship. In addition to activity against M. tuberculosis, E11 was active against the non-tuberculous mycobacterium M. abscessus, an emerging opportunistic pathogen. In conclusion, we identified a novel bactericidal anti-mycobacterial lead compound targeting MmpL3 providing an attractive starting point for optimization.