No abstract

Summary Viral protein homeostasis depends entirely on the machinery of the infected cell. Accordingly, viruses can illuminate the interplay between cellular proteostasis components and their distinct substrates. Here we define how the Hsp70 chaperone network mediates the dengue virus life cycle. Cytosolic Hsp70 isoforms are required at distinct steps of the viral cycle, including entry, RNA replication and virion biogenesis. Hsp70 function at each step is specified by nine distinct DNAJ cofactors. Of these, DnaJB11 relocalizes to virus-induced replication complexes to promote RNA synthesis, while DnaJB6 associates with capsid protein and facilitates virion biogenesis. Importantly, an allosteric Hsp70 inhibitor, JG40, potently blocks infection of different dengue serotypes in human primary blood cells without eliciting viral resistance or exerting toxicity to the host cells. JG40 also blocks replication of other medically-important flaviviruses including yellow fever, West Nile and Japanese encephalitis viruses. Thus, targeting host Hsp70 subnetworks provides a path for broad-spectrum antivirals.

Bag3, a nucleotide exchange factor of the heat shock protein Hsp70, has been implicated in cell signaling. Here we report that Bag3 interacts with the SH3 domain of Src, thereby mediating the effects of Hsp70 on Src signaling. Using several complementary approaches, we established that the Hsp70-Bag3 module is a broad-acting regulator of cancer cell signaling, including by modulating the activity of the transcription factors NF-kB, FoxM1 and Hif1α, the translation regulator HuR and the cell cycle regulators p21 and survivin. We also identified a small molecule inhibitor, YM-1, that disrupts Hsp70-Bag3 interaction. YM-1 mirrored the effects of Hsp70 depletion on these signaling pathways, and in vivo administration of this drug was sufficient to suppress tumor growth in mice. Overall, our results defined Bag3 as a critical factor in Hsp70-modulated signaling and offered a preclinical proof-of-concept that the Hsp70-Bag3 complex may offer an appealing anti-cancer target.

We sought novel strategies to reduce levels of the polyglutamine androgen receptor (polyQ AR) and achieve therapeutic benefits in models of spinobulbar muscular atrophy (SBMA), a protein aggregation neurodegenerative disorder. Proteostasis of the polyQ AR is controlled by the Hsp90/Hsp70-based chaperone machinery, but mechanisms regulating the protein’s turnover are incompletely understood. We demonstrate that overexpression of Hip, a co-chaperone that enhances binding of Hsp70 to its substrates, promotes client protein ubiquitination and polyQ AR clearance. Furthermore, we identify a small molecule that acts similarly to Hip by allosterically promoting Hsp70 binding to unfolded substrates. Like Hip, this synthetic co-chaperone enhances client protein ubiquitination and polyQ AR degradation. Both genetic and pharmacologic approaches targeting Hsp70 alleviate toxicity in a Drosophila model of SBMA. These findings highlight the therapeutic potential of allosteric regulators of Hsp70, and provide new insights into the role of the chaperone machinery in protein quality control.

The rhodacyanine, MKT-077, has anti-proliferative activity against cancer cell lines through its ability to inhibit members of the heat shock protein 70 (Hsp70) family of molecular chaperones. However, MKT-077 is rapidly metabolized, which limits its use as either a chemical probe or potential therapeutic. We report the synthesis and characterization of MKT-077 analogs designed for greater stability. The most potent molecules, such as 30 (JG-98), were at least 3-fold more active than MKT-077 against the breast cancer cell lines MDA-MB-231 and MCF-7 (EC50 values of 0.4 ± 0.03 μM and 0.7 ± 0.2 μM, respectively). The analogs modestly destabilized the chaperone “clients”, Akt1 and Raf1, and induced apoptosis in these cells. Further, the microsomal half-life of JG-98 was improved at least 7-fold (t1/2 = 37 min) compared to MKT-077 (t1/2 < 5 min). Finally, NMR titration experiments suggested that these analogs bind an allosteric site that is known to accommodate MKT-077. These studies advance MKT-077 analogs as chemical probes for studying Hsp70’s roles in cancer.