Molecular chaperones, especially members of the heat shock protein 90 (Hsp90) family, are thought to promote tumor cell survival, but this function is not well understood. Here, we show that mitochondria of tumor cells, but not most normal tissues, contain Hsp90 and its related molecule, TRAP-1. These chaperones interact with Cyclophilin D, an immunophilin that induces mitochondrial cell death, and antagonize its function via protein folding/refolding mechanisms. Disabling this pathway using novel Hsp90 ATPase antagonists directed to mitochondria causes sudden collapse of mitochondrial function and selective tumor cell death. Therefore, Hsp90-directed chaperones are regulators of mitochondrial integrity, and their organelle-specific antagonists may provide a previously undescribed class of potent anticancer agents.

Although therapeutically targeting a single signaling pathway that drives tumor development and/or progression has been effective for a number of cancers, in many cases this approach has not been successful. Targeting networks of signaling pathways, instead of isolated pathways, may overcome this problem, which is probably due to the extreme heterogeneity of human tumors. However, the possibility that such networks may be spatially arranged in specialized subcellular compartments is not often considered in pathway-oriented drug discovery and may influence the design of new agents. Hsp90 is a chaperone protein that controls the folding of proteins in multiple signaling networks that drive tumor development and progression. Here, we report the synthesis and properties of Gamitrinibs, a class of small molecules designed to selectively target Hsp90 in human tumor mitochondria. Gamitrinibs were shown to accumulate in the mitochondria of human tumor cell lines and to inhibit Hsp90 activity by acting as ATPase antagonists. Unlike Hsp90 antagonists not targeted to mitochondria, Gamitrinibs exhibited a "mitochondriotoxic" mechanism of action, causing rapid tumor cell death and inhibiting the growth of xenografted human tumor cell lines in mice. Importantly, Gamitrinibs were not toxic to normal cells or tissues and did not affect Hsp90 homeostasis in cellular compartments other than mitochondria. Therefore, combinatorial drug design, whereby inhibitors of signaling networks are targeted to specific subcellular compartments, may generate effective anticancer drugs with novel mechanisms of action.

Molecular chaperones may promote cell survival, but how this process is regulated, especially in cancer, is not well understood. Using high throughput proteomics screening, we identified the cell cycle regulator and apoptosis inhibitor survivin as a novel protein associated with the molecular chaperone Hsp60. Acute ablation of Hsp60 by small interfering RNA destabilizes the mitochondrial pool of survivin, induces mitochondrial dysfunction, and activates caspase-dependent apoptosis. This response involves disruption of an Hsp60-p53 complex, which results in p53 stabilization, increased expression of pro-apoptotic Bax, and Bax-dependent apoptosis. In vivo, Hsp60 is abundantly expressed in primary human tumors, as compared with matched normal tissues, and small interfering RNA ablation of Hsp60 in normal cells is well tolerated and does not cause apoptosis. Therefore, Hsp60 orchestrates a broad cell survival program centered on stabilization of mitochondrial survivin and restraining of p53 function, and this process is selectively exploited in cancer. Hsp60 inhibitors may function as attractive anticancer agents by differentially inducing apoptosis in tumor cells.

The mitochondrial pool of Hsp90 and its mitochondrial paralogue, TRAP1, suppresses cell death and reprograms energy metabolism in cancer cells; therefore, Hsp90 and TRAP1 have been suggested as target proteins for anticancer drug development. Here, we report that the actual target protein in cancer cell mitochondria is TRAP1, and current Hsp90 inhibitors cannot effectively inactivate TRAP1 because of their insufficient accumulation in the mitochondria. To develop mitochondrial TRAP1 inhibitors, we determined the crystal structures of human TRAP1 complexed with Hsp90 inhibitors. The isopropyl amine of the Hsp90 inhibitor PU-H71 was replaced with the mitochondria-targeting moiety triphenylphosphonium to produce SMTIN-P01. SMTIN-P01 showed a different mode of action from the nontargeted PU-H71, as well as much improved cytotoxicity to cancer cells. In addition, we determined the structure of a TRAP1-adenylyl-imidodiphosphate (AMP-PNP) complex. On the basis of comparative analysis of TRAP1 structures, we propose a molecular mechanism of ATP hydrolysis that is crucial for chaperone function.

Glioblastoma (GBM) cancer stem cells (CSC) are primarily responsible for metastatic dissemination, resistance to therapy, and relapse of GBM, the most common and aggressive brain tumor. Development and maintenance of CSCs require orchestrated metabolic rewiring and metabolic adaptation to a changing microenvironment. Here, we show that cooperative interplay between the mitochondrial chaperone TRAP1 and the major mitochondria deacetylase sirtuin-3 (SIRT3) in glioma stem cells (GSC) increases mitochondrial respiratory capacity and reduces production of reactive oxygen species. This metabolic regulation endowed GSCs with metabolic plasticity, facilitated adaptation to stress (particularly reduced nutrient supply), and maintained "stemness." Inactivation of TRAP1 or SIRT3 compromised their interdependent regulatory mechanisms, leading to metabolic alterations, loss of stemness, and suppression of tumor formation by GSC in vivo. Thus, targeting the metabolic mechanisms regulating interplay between TRAP1 and SIRT3 may provide a novel therapeutic option for intractable patients with GBM.Significance: Discovery and functional analysis of a TRAP1-SIRT3 complex in glioma stem cells identify potential target proteins for glioblastoma treatment.