Inflammatory stimuli activate ectodomain shedding of TNF-α, L-selectin and other transmembrane proteins. We show that p38 MAP kinase, which is activated in response to inflammatory or stress signals, directly activates TACE, a membrane-associated metalloprotease that effects shedding in response to growth factors and Erk MAP kinase activation. p38α MAP kinase interacts with the cytoplasmic domain of TACE, and phosphorylates it on Thr735, which is required for TACE-mediated ectodomain shedding. Activation of TACE by p38 MAP kinase results in the release of TGF-α family ligands, which activate EGF receptor signaling leading to enhanced cell proliferation. Conversely, depletion of p38α MAP kinase activity suppresses EGF receptor signaling and downstream Erk MAP kinase signaling, as well as autocrine EGF receptor-dependent proliferation. Autocrine EGF receptor activation through TACE-mediated ectodomain shedding intimately links inflammation and cancer progression, and may play a role in stress and conditions that relate to p38 MAP kinase activation.

The Hippo pathway senses cellular conditions and regulates YAP/TAZ to control cellular and tissue homeostasis, while TBK1 is central for cytosolic nucleic acid sensing and antiviral defense. The correlation between cellular nutrient/physical status and host antiviral defense is interesting but not well understood. Here we find that YAP/TAZ act as natural inhibitors of TBK1 and are vital for antiviral physiology. Independent of transcriptional regulation and through transactivation domain, YAP/TAZ associate directly with TBK1 and abolish virus-induced TBK1 activation, by preventing TBK1 K63-linked ubiquitination and adaptors/substrates binding. Accordingly, YAP/TAZ deletion/depletion or cellular conditions inactivating YAP/TAZ through Lats1/2 kinases relieve TBK1 suppression and boost antiviral responses, whereas expression of the transcriptionally inactive YAP dampens cytosolic RNA/DNA sensing and weakens the antiviral defense in cells and zebrafish. Thus, we describe a function of YAP/TAZ and the Hippo pathway in innate immunity, by linking cellular nutrient/physical status to antiviral host defense.

TGF-β family signaling through Smads is conceptually a simple and linear signaling pathway, driven by sequential phosphorylation, with type II receptors activating type I receptors, which in turn activate R-Smads. Nevertheless, TGF-β family proteins induce highly complex programs of gene expression responses that are extensively regulated, and depend on the physiological context of the cells. Regulation of TGF-β signaling occurs at multiple levels, including TGF-β activation, formation, activation and destruction of functional TGF-β receptor complexes, activation and degradation of Smads, and formation of Smad transcription complexes at regulatory gene sequences that cooperate with a diverse set of DNA binding transcription factors and coregulators. Here we discuss recent insights into the roles of post-translational modifications and molecular interaction networks in the functions of receptors and Smads in TGF-β signal responses. These layers of regulation demonstrate how a simple signaling system can be coopted to exert exquisitely regulated, complex responses.

Summary Regulating TGF-β receptor presentation provides an avenue to alter a cell’s responsiveness to TGF-β. We report that activation of the Erk MAP kinase pathway decreases the TGF-β-induced Smad3 activation, due to decreased cell surface levels of the type I receptor TβRI, but not the type II receptor. Inhibition of TACE activity or expression enhanced the cell surface TβRI levels and TGF-β-induced Smad3 and Akt activation. Accordingly, silencing TACE expression in cancer cells enhanced the TβRI presentation and TGF-β responsiveness, including the antiproliferative effect of TGF-β, and epithelial to mesenchymal transition. These results establish a mechanism for downregulating TGF-β signaling through TACE activation by the Erk MAP kinase pathway, and a strategy for evasion of tumor suppression and modulation of epithelial to mesenchymal transition, during cancer progression. The decreased growth inhibition by TGF-β, due to elevated TACE activity, complements the growth stimulation resulting from increased release of TGF-α family ligands.

Ectodomain shedding mediated by tumor necrosis factor–α (TNF-α)–converting enzyme [TACE; also known as ADAM17 (a disintegrin and metalloproteinase 17)] provides an important switch in regulating cell proliferation, inflammation, and cancer progression. TACE-mediated ectodomain cleavage is activated by signaling of the mitogen-activated protein kinases (MAPKs) p38 and ERK (extracellular signal– regulated kinase). Here, we found that under basal conditions, TACE was predominantly present as dimers at the cell surface, which required its cytoplasmic domain and enabled efficient association with tissue inhibitor of metalloproteinase-3 (TIMP3) and silencing of TACE activity. Upon activation of the ERK or p38 MAPK pathway, the balance shifted from TACE dimers to monomers, and this shift was associated with increased cell surface presentation of TACE and decreased TIMP3 association, which relieved the inhibition of TACE by TIMP3 and increased TACE-mediated proteolysis of transforming growth factor–α. Thus, cell signaling altered the dimer-monomer equilibrium and inhibitor association to promote activation of TACE-mediated ectodomain shedding, a regulatory mechanism that may extend to other ADAM proteases.

No abstract

Transforming growth factor β (TGF-β) family signaling dictates highly complex programs of gene expression responses, which are extensively regulated at multiple levels and vary depending on the physiological context. The formation, activation, and destruction of two major functional complexes in the TGF-β signaling pathway (i.e., the TGF-β receptor complexes and the Smad complexes that act as central mediators of TGF-β signaling) are direct targets for posttranslational regulation. Dysfunction of these complexes often leads or contributes to pathogenesis in cancer and fibrosis and in cardiovascular, and autoimmune diseases. Here we discuss recent insights into the roles of posttranslational modifications in the functions of the receptor-activated Smads in the common Smad4 and inhibitory Smads, and in the control of the physiological responses to TGF-β. It is now evident that these modifications act as decisive factors in defining the intensity and versatility of TGF-β responsiveness. Thus, the characterization of posttranslational modifications of Smads not only sheds light on how TGF-β controls physiological and pathological processes but may also guide us to manipulate the TGF-β responses for therapeutic benefits.

The epithelial-to-mesenchymal transition (EMT) is a process by which differentiated epithelial cells reprogram gene expression, lose their junctions and polarity, reorganize their cytoskeleton, increase cell motility and assume a mesenchymal morphology. Despite the critical functions of the microtubule (MT) in cytoskeletal organization, how it participates in EMT induction and maintenance remains poorly understood. Here we report that acetylated ␣-tubulin, which plays an important role in microtubule (MT) stabilization and cell morphology, can serve as a novel regulator and marker of EMT. A high level of acetylated ␣-tubulin was correlated with epithelial morphology and it profoundly decreased during TGF-␤-induced EMT. We found that TGF-␤ increased the activity of HDAC6, a major deacetylase of ␣-tubulin, without affecting its expression levels. Treatment with HDAC6 inhibitor tubacin or TGF-␤ type I receptor inhibitor SB431542 restored the level of acetylated ␣-tubulin and consequently blocked EMT. Our results demonstrate that acetylated ␣-tubulin can serve as a marker of EMT and that HDAC6 represents an important regulator during EMT process.