Mechanisms of MEK/ERK growth arrest signaling

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The Raf/MEK/ERK pathway is frequently deregulated in cancer, thus being a key target for therapy. Although recent development of advanced inhibitors targeting B-Raf and MEK1/2 has improved therapy of B-RafV600E,K melanoma, the ability of these tumors to develop drug resistance and the intrinsic resistance of many other MEK/ERK-deregulated tumor types to these inhibitors demand additional therapeutic strategies. Because tumor cells have the ability to develop drug resistances by reactivating MEK/ERK via genetic alterations or feedback rearrangement of the associated signaling network, it is predicted that current therapeutic strategies aiming MEK/ERK inhibition will be continuously challenged. Given that, one promising strategy would be to exploit a weakness(s) of cancer cells, which is inevitably associated with their aberrant MEK/ERK activity. The goal of this project is to determine whether mortalin (GRP75/HSPA9), a molecular chaperone in the HSP70 family, can be targeted to selectively induce mitochondria-driven death mechanisms in MEK/ERK-deregulated tumor cells. In the previous funding period, we discovered that mortalin upregulation in cancer is a key mechanism that facilitates MEK/ERK-mediated tumor cell proliferative signaling by suppressing the pathway’s potential to trigger growth arrest. In an effort to further elaborate mortalin functions in cancer, we discovered that mortalin can also facilitate tumor cell survival by regulating mitochondrial bioenergetics. This latter function of mortalin needs to be thoroughly evaluated in the context of MEK/ERK signaling because the data from our preliminary studies strongly suggest that mortalin deprivation can selectively induce robust cell death in tumor cells exhibiting aberrant MEK/ERK activity, including the B-RafV600E tumor cells that have acquired resistance to B-Raf inhibitors. Moreover, our data suggest that mortalin depletion induces cell death in MEK/ERK-deregulated tumor cells by regulating adenine nucleotide translocases (ANT) and voltage-dependent anion channel (VDAC), the mitochondrial channels that are critical for cellular bioenergetics but can also turn into powerful death machinery when stressed. We therefore hypothesize that a key function of mortalin is to protect tumor cells from mitochondrial bioenergetics stress caused by aberrant MEK/ERK activity. By extension, we predict that mortalin provides a unique target to selectively trigger mitochondrial death mechanisms in MEK/ERK- deregulated tumor cells. To test these hypotheses, Aim 1 will determine whether mortalin regulates cell death/survival via ANT/VDAC-associated mitochondrial permeability transition and Ca2+ flux from ER to mitochondria. Aim 2 will determine whether mortalin suppresses mitochondrial cell death caused by bioenergetics stress in MEK/ERK-deregulated tumor cells. Aim 3 will determine whether mortalin targeting can effectively suppress MEK/ERK-deregulated tumors in vivo. The obtained results will help define how mortain promotes survival and proliferation of MEK/ERK-deregulated cancers.