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Despite the implementation of multiple HER2-targeted therapies, patients with advanced HER2+ breast cancer ultimately develop drug resistance. Stromal fibroblasts represent an abundant cell type in the tumor microenvironment and have been linked to poor outcomes and drug resistance. Here, we show that fibroblasts counteract the cytotoxic effects of HER2 kinase-targeted therapy in a subset of HER2+ breast cancer cell lines and allow cancer cells to proliferate in the presence of the HER2 kinase inhibitor lapatinib. Fibroblasts from primary breast tumors, normal breast tissue, and lung tissue have similar protective effects on tumor cells via paracrine factors. This fibroblast-mediated reduction in drug sensitivity involves increased expression of antiapoptotic proteins and sustained activation of the PI3K/AKT/MTOR pathway, despite inhibition of the HER2 and the RAS-ERK pathways in tumor cells. HER2 therapy sensitivity is restored in the fibroblast cocultures by combination treatment with inhibitors of MTOR or the antiapoptotic proteins BCL-XL and MCL-1. Expression of activated AKT in tumor cells recapitulates the effects of fibroblasts resulting in sustained MTOR signaling and poor lapatinib response. Lapatinib sensitivity was not altered by fibroblasts in tumor cells that exhibited sustained MTOR signaling due to a strong gain-of-function PI3KCA mutation. These findings indicate that in addition to tumor cellintrinsic mechanisms that cause constitutive PI3K/AKT/MTOR pathway activation, secreted factors from fibroblasts can maintain this pathway in the context of HER2 inhibition. Our integrated proteomic- phenotypic approach presents a strategy for the discovery of protective mechanisms in fibroblast-rich tumors and the design of rational combination therapies to restore drug sensitivity. [ABSTRACT FROM AUTHOR]

Sirtuin 3 (SIRT3) is a NAD+-dependent deacetylase downregulated in aging and age-associated diseases such as cancer and neurodegeneration and in high-fat diet (HFD)-induced metabolic disorders. Here, we performed a small-molecule screen and identified an unexpected metabolic vulnerability associated with SIRT3 loss. Azaserine, a glutamine analog, was the top compound that inhibited growth and proliferation of cells lacking SIRT3. Using stable isotope tracing of glutamine, we observed its increased incorporation into de novo nucleotide synthesis in SIRT3 knockout (KO) cells. Furthermore, we found that SIRT3 KO cells upregulated the diversion of glutamine into de novo nucleotide synthesis through hyperactive mTORC1 signaling. Overex-pression of SIRT3 suppressed mTORC1 and growth in vivo in a xenograft tumor model of breast cancer. Thus, we have uncovered a metabolic vulnerability of cells with SIRT3 loss by using an unbiased small-molecule screen. [ABSTRACT FROM AUTHOR]

Death-associated protein kinase (DAP kinase) is a proapoptotic, calcium/calmodulin-dependent serine/threonine kinase. Here, we report that DAP kinase phosphorylates the regulatory light chain of myosin H (MLC) both in vitro and in vivo, and that this phosphorylation occurs preferentially at residue Ser19. In quiescent fibroblasts, DAP kinase stabilizes stress fibers through phosphorylation of MLC, but it is dispensable for the formation of peripheral microfilament bundles. This cytoskeletal effect of DAP kinase occurs before the onset of apoptosis and does not require an intact death domain. In addition, DAP kinase is required for serum-induced stress-fiber formation, which is associated with the upregulation of its catalytic activity. Despite being both sufficient and necessary for the assembly or maintenance of stress fibers, DAP kinase is incapable of stimulating the formation of focal adhesions in quiescent cells. Moreover, it promotes the disassembly of focal adhesions but not stress fibers in cells receiving serum factors. Together, our results identify a novel and unique function of DAP kinase in the uncoupling of stress fibers and focal adhesions. Such uncoupling would lead to a perturbation of the balance between contractile and adhesion forces and subsequent cell detachment, which might contribute to its pro-apoptotic activity. [ABSTRACT FROM AUTHOR]

Keratins 8 and 18 (collectively referred to as K8/K18) are the major components of intermediate filaments of simple epithelial cells. Recent studies have revealed the function of K8/K18 in apoptosis modulation. Here, we show that eIF3k, originally identified as the smallest subunit of eukaryotic translation initiation factor 3 (eIF3) complexes, also localizes to keratin intermediate filaments and physically associates with K18 in epithelial cells. Upon induction of apoptosis, eIF3k colocalizes with K8/K18 in the insoluble cytoplasmic inclusions. Depletion of endogenous eIF3k de-sensitizes simple epithelial cells to various types of apoptosis through a K8/K18-dependent mechanism and promotes the retention of active caspase 3 in cytoplasmic inclusions by increasing its binding to keratins. Consequently, the cleavage of caspase cytosolic and nuclear substrates, such as ICAD and PARP, respectively, is reduced in eIF3k-depleted cells. This study not only reveals the existence of eIF3k in a subcellular compartment other than the eIF3 complex, but also identifies an apoptosis-promoting function of eIF3k in simple epithelial cells by relieving the caspase-sequestration effect of K8/K18, thereby increasing the availability of caspases to their non-keratin-residing substrates. [ABSTRACT FROM AUTHOR]