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Exposure to heat stress triggers a well‐defined acute response marked by HSF1‐dependent transcriptional upregulation of heat shock proteins. Cells allowed to recover acquire thermotolerance, but this adaptation is poorly understood. By quantitative proteomics, we discovered selective upregulation of HSP70‐family chaperone HSPA1 and its co‐factors, HSPH1 and DNAJB1, in MCF7 breast cancer cells acquiring thermotolerance. HSPA1 was found to have dual function during heat stress response: (i) During acute stress, it promotes the recruitment of the 26S proteasome to translating ribosomes, thus poising cells for rapid protein degradation and resumption of protein synthesis upon recovery; (ii) during thermotolerance, HSPA1 together with HSPH1 maintains ubiquitylated nascent/newly synthesized proteins in a soluble state required for their efficient proteasomal clearance. Consistently, deletion of HSPH1 impedes thermotolerance and esophageal tumor growth in mice, thus providing a potential explanation for the poor prognosis of digestive tract cancers with high HSPH1 and nominating HSPH1 as a cancer drug target. We propose dual roles of HSPA1 either alone or in complex with HSPH1 and DNAJB1 in promoting quality control of nascent/newly synthesized proteins and cellular thermotolerance. Synopsis: How cells exposed to heat stress acquire thermotolerance is poorly understood. Here, quantitative proteomics identifies upregulation of HSP70‐family chaperone HSPA1, which fulfills dual function in quality control of nascent proteins at translating ribosomes. Proteotoxic stress causes translation arrest, ubiquitylation of nascent/newly synthesized proteins, and proteasome recruitment to ribosomes.Proteasome recruitment to ribosomes is augmented by HSP70 activity independently of nascent chain ubiquitylation.HSPA1 in complex with HSPH1‐DNAJB1 cofactors maintains solubility and proteasomal clearance of ubiquitylated proteins.Low HSPH1 expression impedes nascent‐protein quality control and tumor growth, correlating with favorable outcome. [ABSTRACT FROM AUTHOR]

SMIP004-7 is a small molecule inhibitor of mitochondrial respiration with selective in vivo anti-cancer activity through an as-yet unknown molecular target. We demonstrate here that SMIP004-7 targets drug-resistant cancer cells with stem-like features by inhibiting mitochondrial respiration complex I (NADH:ubiquinone oxidoreductase, complex I [CI]). Instead of affecting the quinone-binding site targeted by most CI inhibitors, SMIP004-7 and its cytochrome P450-dependent activated metabolite(s) have an uncompetitive mechanism of inhibition involving a distinct N-terminal region of catalytic subunit NDUFS2 that leads to rapid disassembly of CI. SMIP004-7 and an improved chemical analog selectively engage NDUFS2 in vivo to inhibit the growth of triple-negative breast cancer transplants, a response mediated at least in part by boosting CD4+ and CD8+ T cell-mediated immune surveillance. Thus, SMIP004-7 defines an emerging class of ubiquinone uncompetitive CI inhibitors for cell autonomous and microenvironmental metabolic targeting of mitochondrial respiration in cancer. [Display omitted] • SMIP004 compounds inhibit NADH:ubiquinone oxidoreductase (complex I) • Unique ubiquinone uncompetitive mechanism of inhibition • In vivo target engagement involves complex I subunit NDUFS2 • Complex I inhibition reverses hypoxia and promotes cancer immune surveillance Mitochondrial complex I is identified as the target of cancer selective cytotoxic agent SMIP004-7. Jain et al. show that anti-tumor activity involves ubiquinone uncompetitive inhibition of complex I, leading to a reversal of tumor hypoxia and promotion of tumor immune surveillance. [ABSTRACT FROM AUTHOR]