Mortalin (HSPA9) facilitates BRAF -mutant tumor cell survival by suppressing ANT3-mediated mitochondrial membrane permeability

Abstract

Mortalin [also known as heat shock protein family A (HSP70) member 9 (HSPA9) or glucose-regulated protein 75 (GRP75)] is a mitochondrial molecular chaperone that is often up-regulated and mislocalized in tumors with abnormal activation of the kinases MEK and ERK. Here, we found that mortalin depletion was selectively lethal to tumor and immortalized normal cells expressing the mutant kinase B-RafV600E or the chimeric protein ΔRaf-1:ER and that MEK-ERK–sensitive regulation of the peptide-binding domain in mortalin was critical to cell survival or death. Proteomics screening identified adenine nucleotide translocase 3 (ANT3) as a previously unknown mortalin substrate and cell survival/death effector. Mechanistically, increased MEK-ERK signaling activity and mortalin function converged opposingly on the regulation of mitochondrial permeability. Specifically, whereas MEK-ERK activity increased mitochondrial permeability by promoting the interaction between ANT3 and the peptidyl-prolyl isomerase cyclophilin D (CypD), mortalin decreased mitochondrial permeability by inhibiting this interaction. Hence, mortalin depletion increased mitochondrial permeability in MEK-ERK–deregulated cells to an extent that triggered cell death. HSP70 inhibitor derivatives that effectively inhibited mortalin suppressed the proliferation of B-RafV600E tumor cells in culture and in vivo, including their B-Raf inhibitor–resistant progenies. These findings suggest that targeting mortalin has potential as a selective therapeutic strategy in B-Raf–mutant or MEK-ERK–driven tumors.

Mortalin/HSPA9 targeting selectively induces KRAS tumor cell death by perturbing mitochondrial membrane permeability

“…Our data suggest that this conditional lethality is driven by K-Ras mut –induced MEK/ERK activity and is mediated by ANT and CypD. These findings are consistent with our recent observation that B-Raf V600E tumor cells undergo ANT/CypD-mediated cell death in the absence of mortalin in a MEK/ERK-dependent manner 61 . Therefore, mortalin appears to have potential as a therapeutic target selective to not only B-Raf mut – but also K-Ras mut –driven tumors, wherein deregulated MEK/ERK activity is the common denominator.…”

“…Moreover, JG-231 treatment increased MEK1/2 phosphorylation in tumors ( Fig. 7E ), which is consistent with its effects in BRAF -mutant tumor xenografts in mice 61 . These data strongly support the feasibility of exploiting mortalin to selectively suppress K-Ras mut tumors.…”

“…Molecular chaperones can facilitate tumorigenesis by altering the stability or activity of client proteins involved in diverse cellular processes 26 . We recently identified ANT3 as a novel client for mortalin and demonstrated that a highly coordinated regulation of ANT3 by mortalin and MEK/ERK is critical for the survival of BRAF mutant tumors 61 . The present study extends this finding to KRAS tumor cells and provides consistent evidence that mortalin has pivotal role in suppressing the lethal potential of ANT3 in MEK/ERK-dependent tumors.…”

“…We previously demonstrated that mortalin facilitates tumor cell proliferation and survival by modulating MEK/ERK activity 23 , 59 , 60 and mitochondrial bioenergetics 48 . Moreover, we recently reported that deregulated MEK/ERK activity in BRAF -mutated tumor cells increase mitochondrial permeability whereas mortalin counteracts this increase by limiting the physical interaction between ANT3 and CypD and, hence, mortalin depletion in these cells results in perturbed mitochondrial permeability, which causes robust tumor cell death 61 . Based upon these findings, we hypothesized that mortalin is a candidate target exploitable to selectively induce mitochondrial cell death in MEK/ERK-dependent tumors and predicted that mortalin targeting would selectively suppress KRAS -mutated tumors because these tumors often exhibit deregulated MEK/ERK activity.…”

Growth Inhibitory Signaling of the Raf/MEK/ERK Pathway

“…Currently, it is not known whether this mechanism is also involved in p21 CIP1 regulation upon mortalin depletion in BRAF tumor cells. Because mortalin depletion or inhibition can induce lethality associated with altered mitochondrial permeability and bioenergetics in various tumor cells [100][101][102][103][104], it may be possible that mortalin has a role in coordinating oncogenic MEK/ERK activity and mitochondrial metabolism to facilitate tumor cell survival and proliferation.…”