Cooperation among transcription factors is central for their ability to execute specific transcriptional programmes. The AP1 complex exemplifies a network of transcription factors that function in unison under normal circumstances and during the course of tumour development and progression. This Perspective summarizes our current understanding of the changes in members of the AP1 complex and the role of ATF2 as part of this complex in tumorigenesis.Activator protein 1 (AP1) 1,2 functions in almost all areas of eukaryotic cellular behaviour, from cell cycle proliferation and development to stress response and apoptosis. Indeed, AP1 is activated in response to a plethora of extracellular signals from cytokines and growth factors to stress and inflammation 3,4 . The expansive transcriptional repertoire executed by AP1 complexes is propagated from the diverse compositional array of homodimeric or heterodimeric combinations formed by members of the Jun, Atf, Fos and Maf transcription factor families (BOX 1). The dimeric combinations and transcriptional activity observed in vivo are largely influenced by the tissue-specific expression patterns of the individual proteins, and importantly by their specific activating mechanisms and post-translational modifications that facilitate their individual ability to dimerize with other basic leucine zipper (bZIP) domain proteins. This inherently diverse composition of AP1 complexes and their central role in transcriptional regulation places AP1 complexes at a functional epicenter for pathological signal relay in disease, particularly in the context of malignant cellular transformation in which AP1 proteins are often deregulated by oncoprotein signalling [4][5][6] . This Perspective describes the function and cooperation of Jun, Fos and Atf family members in tumour cells, and the emerging function of ATF2 as part of the dynamic AP1 complex. Competing interests statement:The authors declare no competing financial interests. The Ap1 transcription factor complexThe mammalian AP1 proteins are homodimers and heterodimers composed of proteins from the Jun (JUN, JUNB and JUND) and Fos (FOS, FOSB, FRA1 and FRA2) families, and the closely related activating transcription factor (Atf and Creb) subfamily and the Maf subfamily 5 . AP1 constituent proteins are structurally distinguished by a basic leucine zipper (bZIP) domain that is composed of leucine zipper and basic domains. It is through these domains that AP1 proteins dimerize and bind to DNA. These proteins are typically activated through phosphorylation by the indicated upstream kinases. The different AP1 dimers bind to DNA with varying affinities and differ in their transactivation efficiencies 8,15 . Jun proteins can form stable dimers that bind to the AP1 DNA recognition element 5′-TGAC/ GTCA-3′ (also known as TPA response element (TRE)) based on their ability to mediate transcriptional induction in response to the phorbol ester tumour promoter TPA 2,15 . Atf proteins, conversely, form dimers that preferentially bind to c...

We have examined the role of PRC1, a midzone-associated, microtubule bundling, Cdk substrate protein, in regulating the spatiotemporal formation of the midzone in HeLa cells. Cdk-mediated phosphorylation of PRC1 in early mitosis holds PRC1 in an inactive monomeric state. During the metaphase-to-anaphase transition, PRC1 is dephosphorylated, promoting PRC1 oligomerization. Using time-lapse video microscopy, RNA interference, 3D immunofluorescence reconstruction imaging, and rescue experiments, we demonstrate that the dephosphorylated form of PRC1 is essential for bundling antiparallel, nonkinetochore, interdigitating microtubules to establish the midzone that is necessary for cytokinesis. Our results thus indicate that PRC1 is an essential factor in controlling the spatiotemporal formation of the midzone in human cells.Cdk phosphorylation ͉ mitosis͞cytokinesis ͉ microtubule-associated proteins ͉ microtubule bundling D uring the metaphase-to-anaphase transition, a conspicuous network of antiparallel nonkinetochore interdigitating microtubules (MTs) assembles between separating chromosomes. This unique structure is referred to as the spindle midzone. The midzone is believed to be required for the maintenance of overall spindle architecture, spindle elongation, and cleavage furrow positioning (1, 2). Previous studies indicated that the midzone-associated centralspindlin complex might play a crucial role in regulating midzone formation. The complex exists as a heterotetramer comprising the midzone-associated kinesin motor, MKLP1, and its binding protein, MgcRacGAP (a Rho-family GTPase activating protein) (3). The heterotetramer, but not the individual MKLP1 or MgcRacGAP proteins, has MT bundling activity (3). Cdk (Cdc2͞ cyclin B) phosphorylates MKLP1 and negatively regulates its motor and MT-bundling activities (4). Because inactivation of Cdk activity (through the destruction of cyclin B) is critical for the metaphaseto-anaphase transition, it was suggested that Cdk phosphorylation of MPLK1 controls the timing of midzone formation (4, 5). However, recent studies indicate that MKLP1 is not directly involved in the early stages of midzone formation in mammalian cells (6, 7). Immunofluorescence and time-lapse live cell imaging analyses revealed that inhibition of MKLP1 expression does not perturb the bundling of midzone interdigitating MTs. Instead, it inhibits midbody formation and the completion of cytokinesis. Thus, the centralspindlin complex is required for constricting the midzone and forming the midbody that is essential for completing cytokinesis in mammalian cells (7).PRC1 originally was identified as a Cdk substrate in an in vitro phosphorylation screen and was subsequently shown to be a midzone-associated protein required for cytokinesis (8). PRC1 forms oligomers in vivo and has 9). Cdk phosphorylation of PRC1 appears to be important for suppressing PRC1 MT-bundling activity in early mitosis, because a Cdk-nonphosphorylatable mutant of PRC1 causes extensive bundling of the metaphase spindle (9). Perturbing t...

Summary Many tumor cells are fueled by altered metabolism and increased glutamine (Gln) dependence. We identify regulation of the L-glutamine carrier proteins SLC1A5 and SLC38A2 (SLC1A5/38A2) by the ubiquitin ligase RNF5. Paclitaxel-induced ER stress to breast cancer (BCa) cells promotes RNF5 association, ubiquitination and degradation of SLC1A5/38A2. This decreases Gln uptake, levels of TCA cycle components, mTOR signaling and proliferation while increasing autophagy and cell death. Rnf5-deficient MMTV-PyMT mammary tumors were less differentiated and showed elevated SLC1A5 expression. Whereas RNF5 depletion in MDA-MB-231 cells promoted tumorigenesis and abolished paclitaxel responsiveness, SLC1A5/38A2 knockdown elicited opposing effects. Inverse RNF5HI/SLC1A5/38A2LO expression was associated with positive prognosis in BCa. Thus, RNF5 control of Gln uptake underlies BCa response to chemotherapies.

Summary The transcription factor ATF2 elicits oncogenic activities in melanoma and tumor suppressor activities in non-malignant skin cancer. Here we identify that ATF2 tumor suppressor function is determined by its ability to localize at the mitochondria, where it alters membrane permeability following genotoxic stress. The ability of ATF2 to reach the mitochondria is determined by PKCε, which directs ATF2 nuclear localization. Genotoxic stress attenuates PKCε effect on ATF2, enables ATF2 nuclear export and localization at the mitochondria, where it perturbs the HK1-VDAC1 complex, increases mitochondrial permeability and promotes apoptosis. Significantly, high levels of PKCε, as seen in melanoma cells, block ATF2 nuclear export and function at the mitochondria, thereby attenuating apoptosis following exposure to genotoxic stress. In melanoma tumor samples, high PKCε levels associates with poor prognosis. Overall, our findings provide the framework for understanding how subcellular localization enables ATF2 oncogenic or tumor suppressor functions.

SummaryAn increasing number of transcription factors have been shown to elicit oncogenic and tumor suppressor activities, depending on the tissue and cell context. Activating transcription factor 2 (ATF2; also known as cAMP-dependent transcription factor ATF-2) has oncogenic activities in melanoma and tumor suppressor activities in non-malignant skin tumors and breast cancer. Recent work has shown that the opposing functions of ATF2 are associated with its subcellular localization. In the nucleus, ATF2 contributes to global transcription and the DNA damage response, in addition to specific transcriptional activities that are related to cell development, proliferation and death. ATF2 can also translocate to the cytosol, primarily following exposure to severe genotoxic stress, where it impairs mitochondrial membrane potential and promotes mitochondrial-based cell death. Notably, phosphorylation of ATF2 by the epsilon isoform of protein kinase C (PKCe) is the master switch that controls its subcellular localization and function. Here, we summarize our current understanding of the regulation and function of ATF2 in both subcellular compartments. This mechanism of control of a non-genetically modified transcription factor represents a novel paradigm for 'oncogene addiction'.