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One outcome of activation of the phosphatidylinositol 3-kinase (PI3K) pathway is increased aerobic glycolysis, but the upstream signaling events that regulate the PI3K pathway, and thus the Warburg effect, are elusive. Increasing evidence suggests that Plk1, a cell cycle regulator, is also involved in cellular events in addition to mitosis. To test whether Plk1 contributes to activation of the PI3K pathway, and thus aerobic glycolysis, we examined potential targets of Plk1 and identified PTEN as a Plk1 substrate. We hypothesize that Plk1 phosphorylation of PTEN leads to its inactivation, activation of the PI3K pathway, and the Warburg effect. Our data show that overexpression of Plk1 leads to activation of the PI3K pathway and enhanced aerobic glycolysis. In contrast, inhibition of Plk1 causes markedly reduced glucose metabolism in mice. Mechanistically, we show that Plk1 phosphorylation of PTEN and Nedd4-1, an E3 ubiquitin ligase of PTEN, results in PTEN inactivation. Finally, we show that Plk1 phosphorylation of PTEN promotes tumorigenesis in both its phosphatase-dependent and -independent pathways, revealing potentially new drug targets to arrest tumor cell growth. [ABSTRACT FROM AUTHOR]

Skeletal muscle is a postmitotic tissue that repairs and regenerates through activation of a population of stem-cell-like satellite cells. However, signaling mechanisms governing adult skeletal muscle regeneration remain less understood. In the present study, we have investigated the role of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), an adaptor protein involved in receptor-mediated activation of multiple signaling pathways in regeneration of adult myofibers. Skeletal muscle-specific depletion of TRAF6 in mice (TRAF6mko) improved regeneration of myofibers upon injury with a concomitant increase in the number of satellite cells and activation of the Notch signaling pathway. Ex vivo cultures of TRAF6mko myofiber explants demonstrated an increase in the proliferative capacity of myofiber-associated satellite cells accompanied by an upregulation of Notch ligands. Deletion of TRAF6 also inhibited the activity of transcription factor NF-κB and the expression of inflammatory cytokines and augmented the M2c macrophage phenotype in injured muscle tissues. Collectively, our study demonstrates that specific inhibition of TRAF6 improves satellite cell activation and skeletal muscle regeneration through upregulation of Notch signaling and reducing the inflammatory repertoire. [ABSTRACT FROM AUTHOR]

Accurate chromosome segregation during cell division maintains genomic integrity and requires the proper establishment of kinetochore-microtubule attachment in mitosis. As a key regulator of mitosis, Polo-like kinase 1 (Plk1) is essential for this attachment process, but the molecular mechanism remains elusive. Here we identify Sgt1, a cochaperone for Hsp90, as a novel Plk1 substrate during mitosis. We show that Sgt1 dynamically localizes at the kinetochores, which lack microtubule attachments during prometaphase. Plk1 is required for the kinetochore localization of Sgt1 and phosphorylates serine 331 of Sgt1 at the kinetochores. This phosphorylation event enhances the association of the Hsp90-Sgt1 chaperone with the MIS12 complex to stabilize this complex at the kinetochores and thus coordinates the recruitment of the NDC80 complex to form efficient microtubule-binding sites. Disruption of Sgt1 phosphorylation reduces the MIS12 and NDC80 complexes at the kinetochores, impairs stable microtubule attachment, and eventually results in chromosome misalignment to delay the anaphase onset. Our results demonstrate a mechanism for Plk1 in promoting kinetochore-microtubule attachment to ensure chromosome stability. [ABSTRACT FROM AUTHOR]

Notch signaling is a conserved cell fate regulator during development and postnatal tissue regeneration. Using skeletal muscle satellite cells as a model and through myogenic cell lineage-specific NICDOE (overexpression of constitutively activated Notch 1 intracellular domain), here we investigate how Notch signaling regulates the cell fate choice of muscle stem cells. We show that in addition to inhibiting MyoD and myogenic differentiation, NICDOE upregulates Pax7 and promotes the self-renewal of satellite cell-derived primary myoblasts in culture. Using MyoD-/- myoblasts, we further show that NICDOE upregulates Pax7 independently of MyoD inhibition. In striking contrast to previous observations, NICDOE also inhibits S-phase entry and Ki67 expression and thus reduces the proliferation of primary myoblasts. Overexpression of canonical Notch target genes mimics the inhibitory effects of NICDOE on MyoD and Ki67 but not the stimulatory effect on Pax7. Instead, NICD regulates Pax7 through interaction with RBP-Jκ, which binds to two consensus sites upstream of the Pax7 gene. Importantly, satellite cell-specific NICDOE results in impaired regeneration of skeletal muscles along with increased Pax7+ mononuclear cells. Our results establish a role of Notch signaling in actively promoting the self-renewal of muscle stem cells through direct regulation of Pax7. [ABSTRACT FROM AUTHOR]