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Open Access Publications from the University of California

Scholarly Works (10 results)

  • Article
  • Peer Reviewed
UC San Diego Previously Published Works (2015)
The Hippo signaling pathway is important for controlling organ size and tissue homeostasis. Originally identified in Drosophila melanogaster, the core components of the Hippo pathway are highly conserved in mammals. The Hippo pathway can be modulated by a wide range of stimuli, including G protein-coupled receptor (GPCR) signaling, changes in the actin cytoskeleton, cell-cell contact, and cell polarity. When activated, the Hippo pathway functions as a tumor suppressor to limit cell growth. However, dysregulation by genetic inactivation of core pathway components or amplification or gene fusion of its downstream effectors results in increased cell proliferation and decreased apoptosis and differentiation. Unsurprisingly, this can lead to tissue overgrowth, tumorigenesis, and many other diseases.
    Cover page: Disease implications of the Hippo/YAP pathway
    • Thesis
    • Peer Reviewed
    UC San Diego Electronic Theses and Dissertations (2019)

    The recent TCGA study identified the Hippo pathway as one of the nine major signaling pathways that are frequently altered in human cancer. The Hippo pathway is a master regulator of cell proliferation, cell death, and cell differentiation. Its dysregulation has been implicated intensively in human diseases especially cancer. Exploring the regulatory network of the Hippo pathway answers basic biological questions such as how organ size is controlled during development as well as contributes to cancer target therapy. My research focuses on elucidating how Hippo pathway is regulated by cellular stress, more specifically osmotic stress. Physiological effect of the Hippo pathway is carried through transcription co-activator YAP. We found that osmotic stress induces YAP nuclear translocation even though it is phosphorylated at Ser 127 site, where its phosphorylation has been implicated to cause YAP cytoplasmic retention. We identified a novel phosphorylation site on YAP and a novel upstream regulator NLK that lead to YAP activation and subsequent cell proliferation. This finding alters the well-established dogma that YAP is inhibited by phosphorylation.

    The mechanisms by which osmotic stress-activated the Hippo pathway remained unclear. We discovered that NF2 lipid binding is essential for osmotic stress-induced activation of the Hippo pathway. We showed that osmotic stress induces ARF6 and PIP5K family interaction, leading to PIP5K activation and enhanced PI(4,5)P2 membrane distribution. Membrane-associated PI(4,5)P2 interacts with NF2, which further induces downstream Hippo pathway activation. This work completes the missing piece in the field on how NF2 is involved in Hippo pathway activation. Completion of these projects not only address fundamental questions in Hippo regulation but also provide valuable leads for therapeutic intervention for cancer treatment.

      Cover page: Identifying Novel Regulators and Regulation of the Hippo-YAP Pathway
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2020)
      The Hippo pathway is a master regulator of tissue homeostasis and organ size. NF2 is a well-established tumor suppressor, and loss of NF2 severely compromises Hippo pathway activity. However, the precise mechanism of how NF2 mediates upstream signals to regulate the Hippo pathway is not clear. Here we report that, in mammalian cells, NF2's lipid-binding ability is critical for its function in activating the Hippo pathway in response to osmotic stress. Mechanistically, osmotic stress induces PI(4,5)P2 plasma membrane enrichment by activating the PIP5K family, allowing for NF2 plasma membrane recruitment and subsequent downstream Hippo pathway activation. An NF2 mutant deficient in lipid binding is unable to activate the Hippo pathway in response to osmotic stress, as measured by LATS and YAP phosphorylation. Our findings identify the PIP5K family as novel regulators upstream of Hippo signaling, and uncover the importance of phosphoinositide dynamics, specifically PI(4,5)P2, in Hippo pathway regulation.
        Cover page: Critical roles of phosphoinositides and NF2 in Hippo pathway regulation
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2016)
        The Hippo pathway is important for regulating tissue homeostasis, and its dysregulation has been implicated in human cancer. However, it is not well understood how the Hippo pathway becomes dysregulated because few mutations in core Hippo pathway components have been identified. Therefore, much work in the Hippo field has focused on identifying upstream regulators, and a complex Hippo interactome has been identified. Nevertheless, it is not always clear which components are the most physiologically relevant in regulating YAP/TAZ. To provide an overview of important Hippo pathway components, we created knockout cell lines for many of these components and compared their relative contributions to YAP/TAZ regulation in response to a wide range of physiological signals. By this approach, we provide an overview of the functional importance of many Hippo pathway components and demonstrate NF2 and RHOA as important regulators of YAP/TAZ and TAOK1/3 as direct kinases for LATS1/2.
          Cover page: Characterization of Hippo Pathway Components by Gene Inactivation
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2015)
            Cover page: Opposing roles of conventional and novel PKC isoforms in Hippo-YAP pathway regulation
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2017)
            YAP is the major downstream effector of the Hippo pathway, which controls cell growth, tissue homeostasis, and organ size. Aberrant YAP activation, resulting from dysregulation of the Hippo pathway, is frequently observed in human cancers. YAP is a transcription co-activator, and the key mechanism of YAP regulation is its nuclear and cytoplasmic translocation. The Hippo pathway component, LATS, inhibits YAP by phosphorylating YAP at Ser127, leading to 14-3-3 binding and cytoplasmic retention of YAP Here, we report that osmotic stress stimulates transient YAP nuclear localization and increases YAP activity even when YAP Ser127 is phosphorylated. Osmotic stress acts via the NLK kinase to induce YAP Ser128 phosphorylation. Phosphorylation of YAP at Ser128 interferes with its ability to bind to 14-3-3, resulting in YAP nuclear accumulation and induction of downstream target gene expression. This osmotic stress-induced YAP activation enhances cellular stress adaptation. Our findings reveal a critical role for NLK-mediated Ser128 phosphorylation in YAP regulation and a crosstalk between osmotic stress and the Hippo pathway.
              Cover page: Osmotic stress‐induced phosphorylation by NLK at Ser128 activates YAP
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2019)
              Coordination of growth and genomic stability is critical for normal cell physiology. Although the E3 ubiquitin ligase BRCA1 is a key player in maintenance of genomic stability, its role in growth signaling remains elusive. Here, we show that BRCA1 facilitates stabilization of YAP1 protein and turning "off" the Hippo pathway through ubiquitination of NF2. In BRCA1-deficient cells Hippo pathway is "turned On." Phosphorylation of YAP1 is crucial for this signaling process because a YAP1 mutant harboring alanine substitutions (Mt-YAP5SA) in LATS1 kinase recognition sites not only resists degradation but also rescues YAP1 transcriptional activity in BRCA1-deficient cells. Furthermore, an ectopic expression of the active Mt-YAP5SA, but not inactive Mt-YAP6SA, promotes EGF-independent proliferation and tumorigenesis in BRCA1-/- mammary epithelial cells. These findings establish an important role of BRCA1 in regulating stability of YAP1 protein that correlates positively with cell proliferation.
                Cover page: BRCA1/BARD1-dependent ubiquitination of NF2 regulates Hippo-YAP1 signaling
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2019)
                The mammalian target of rapamycin complex 1 (mTORC1) regulates cell growth, metabolism, and autophagy. Extensive research has focused on pathways that activate mTORC1 like growth factors and amino acids; however, much less is known about signaling cues that directly inhibit mTORC1 activity. Here, we report that G-protein coupled receptors (GPCRs) paired to Gαs proteins increase cyclic adenosine 3'5' monophosphate (cAMP) to activate protein kinase A (PKA) and inhibit mTORC1. Mechanistically, PKA phosphorylates the mTORC1 component Raptor on Ser 791, leading to decreased mTORC1 activity. Consistently, in cells where Raptor Ser 791 is mutated to Ala, mTORC1 activity is partially rescued even after PKA activation. Gαs-coupled GPCRs stimulation leads to inhibition of mTORC1 in multiple cell lines and mouse tissues. Our results uncover a signaling pathway that directly inhibits mTORC1, and suggest that GPCRs paired to Gαs proteins may be potential therapeutic targets for human diseases with hyperactivated mTORC1.
                  Cover page: GPCR signaling inhibits mTORC1 via PKA phosphorylation of Raptor
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2015)
                  The Hippo pathway plays a central role in tissue homoeostasis, and its dysregulation contributes to tumorigenesis. Core components of the Hippo pathway include a kinase cascade of MST1/2 and LATS1/2 and the transcription co-activators YAP/TAZ. In response to stimulation, LATS1/2 phosphorylate and inhibit YAP/TAZ, the main effectors of the Hippo pathway. Accumulating evidence suggests that MST1/2 are not required for the regulation of YAP/TAZ. Here we show that deletion of LATS1/2 but not MST1/2 abolishes YAP/TAZ phosphorylation. We have identified MAP4K family members--Drosophila Happyhour homologues MAP4K1/2/3 and Misshapen homologues MAP4K4/6/7-as direct LATS1/2-activating kinases. Combined deletion of MAP4Ks and MST1/2, but neither alone, suppresses phosphorylation of LATS1/2 and YAP/TAZ in response to a wide range of signals. Our results demonstrate that MAP4Ks act in parallel to and are partially redundant with MST1/2 in the regulation of LATS1/2 and YAP/TAZ, and establish MAP4Ks as components of the expanded Hippo pathway.
                    Cover page: MAP4K family kinases act in parallel to MST1/2 to activate LATS1/2 in the Hippo pathway
                    • Article
                    • Peer Reviewed
                    UC San Diego Previously Published Works (2018)
                    Mammalian cells are surrounded by neighbouring cells and extracellular matrix (ECM), which provide cells with structural support and mechanical cues that influence diverse biological processes1. The Hippo pathway effectors YAP (also known as YAP1) and TAZ (also known as WWTR1) are regulated by mechanical cues and mediate cellular responses to ECM stiffness2,3. Here we identified the Ras-related GTPase RAP2 as a key intracellular signal transducer that relays ECM rigidity signals to control mechanosensitive cellular activities through YAP and TAZ. RAP2 is activated by low ECM stiffness, and deletion of RAP2 blocks the regulation of YAP and TAZ by stiffness signals and promotes aberrant cell growth. Mechanistically, matrix stiffness acts through phospholipase Cγ1 (PLCγ1) to influence levels of phosphatidylinositol 4,5-bisphosphate and phosphatidic acid, which activates RAP2 through PDZGEF1 and PDZGEF2 (also known as RAPGEF2 and RAPGEF6). At low stiffness, active RAP2 binds to and stimulates MAP4K4, MAP4K6, MAP4K7 and ARHGAP29, resulting in activation of LATS1 and LATS2 and inhibition of YAP and TAZ. RAP2, YAP and TAZ have pivotal roles in mechanoregulated transcription, as deletion of YAP and TAZ abolishes the ECM stiffness-responsive transcriptome. Our findings show that RAP2 is a molecular switch in mechanotransduction, thereby defining a mechanosignalling pathway from ECM stiffness to the nucleus.
                      Cover page: RAP2 mediates mechanoresponses of the Hippo pathway
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