[Abstract unavailable

Mutation in the TSC2 tumor suppressor causes tuberous sclerosis complex, a disease characterized by hamartoma formation in multiple tissues. TSC2 inhibits cell growth by acting as a GTPase-activating protein toward Rheb, thereby inhibiting mTOR, a central controller of cell growth. Here, we show that Wnt activates mTOR via inhibiting GSK3 without involving beta-catenin-dependent transcription. GSK3 inhibits the mTOR pathway by phosphorylating TSC2 in a manner dependent on AMPK-priming phosphorylation. Inhibition of mTOR by rapamycin blocks Wnt-induced cell growth and tumor development, suggesting a potential therapeutic value of rapamycin for cancers with activated Wnt signaling. Our results show that, in addition to transcriptional activation, Wnt stimulates translation and cell growth by activating the TSC-mTOR pathway. Furthermore, the sequential phosphorylation of TSC2 by AMPK and GSK3 reveals a molecular mechanism of signal integration in cell growth regulation.

Target of rapamycin (TOR) proteins are members of the phosphatidylinositol kinase-related kinase (PIKK) family and are highly conserved from yeast to mammals. TOR proteins integrate signals from growth factors, nutrients, stress, and cellular energy levels to control cell growth. The ribosomal S6 kinase 1 (S6K) and eukaryotic initiation factor 4E binding protein 1(4EBP1) are two cellular targets of TOR kinase activity and are known to mediate TOR function in translational control in mammalian cells. However, the precise molecular mechanism of TOR regulation is not completely understood. One of the recent breakthrough studies in TOR signaling resulted in the identification of the tuberous sclerosis complex gene products, TSC1 and TSC2, as negative regulators for TOR signaling. Furthermore, the discovery that the small GTPase Rheb is a direct downstream target of TSC1-TSC2 and a positive regulator of the TOR function has significantly advanced our understanding of the molecular mechanism of TOR activation. Here we review the current understanding of the regulation of TOR signaling and discuss its function as a signaling nexus to control cell growth during normal development and tumorigenesis

Overexpression of Wnt10b from the osteocalcin promoter in transgenic mice increases postnatal bone mass. Increases in osteoblast perimeter, mineralizing surface, and bone formation rate without detectable changes in pre-osteoblast proliferation, osteoblast apoptosis, or osteoclast number and activity suggest that, in this animal model, Wnt10b primarily increases bone mass by stimulating osteoblastogenesis.Introduction: Wnt signaling regulates many aspects of development including postnatal accrual of bone. Potential mechanisms for how Wnt signaling increases bone mass include regulation of osteoblast and/or osteoclast number and activity. To help differentiate between these possibilities, we studied mice in which Wnt10b is expressed specifically in osteoblast lineage cells or in mice devoid of Wnt10b. Materials and Methods: Transgenic mice, in which mouse Wnt10b is expressed from the human osteocalcin promoter (Oc-Wnt10b), were generated in C57BL/6 mice. Transgene expression was evaluated by RNase protection assay. Quantitative assessment of bone variables was done by radiography, CT, and static and dynamic histomorphometry. Mechanisms of bone homeostasis were evaluated with assays for BrdU, TUNEL, and TRACP5b activity, as well as serum levels of C-terminal telopeptide of type I collagen (CTX). The endogenous role of Wnt10b in bone was assessed by dynamic histomorphometry in Wnt10b −/− mice. Results: Oc-Wnt10b mice have increased mandibular bone and impaired eruption of incisors during postnatal development. Analyses of femoral distal metaphyses show significantly higher BMD, bone volume fraction, and trabecular number. Increased bone formation is caused by increases in number of osteoblasts per bone surface, rate of mineral apposition, and percent mineralizing surface. Although number of osteoclasts per bone surface is not altered, Oc-Wnt10b mice have increased total osteoclast activity because of higher bone mass. In Wnt10b −/− mice, changes in mineralizing variables and osteoblast perimeter in femoral distal metaphyses were not observed; however, bone formation rate is reduced because of decreased total bone volume and trabecular number. Conclusions: High bone mass in Oc-Wnt10b mice is primarily caused by increased osteoblastogenesis, with a minor contribution from elevated mineralizing activity of osteoblasts.

This approach for establishing cementoblasts in vitro provides a model to study cementogenesis as required to enhance our knowledge of the mechanisms controlling development, maintenance, and regeneration of periodontal tissues.

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Synopsis X-box-binding protein 1 (XBP1) is a key modulator of unfolded protein response (UPR), which is involved in a wide range of pathological and physiological processes. The active/spliced form of XBP1 (XBP1s) messenger RNA is generated from unspliced form by IRE1 during UPR. However, post-translational modulation of XBP1s remains largely unknown. Here, we demonstrate that XBP1s is a target of acetylation and deacetylation mediated by p300 and SIRT1 respectively. p300 increases acetylation and protein stability of XBP1s, and enhances the transcriptional activity of XBP1s. SIRT1 deacetylates XBP1s and inhibits the transcriptional activity of XBP1s. Deficiency of SIRT1 enhances the XBP1s-mediated luciferase reporter activity in HEK293 cells and the upregulation of XBP1s target gene expression under ER stress in mouse embryonic fibroblasts (MEFs). Consistent with XBP1s favoring cell survival under ER stress, Sirt1−/− MEFs display a greater resistance to the ER stress-induced apoptotic cell death compared with Sirt1+/+ MEFs. Taken together, these results suggest that acetylation/deacetylation constitutes an important post-translational mechanism in controlling protein levels as well as transcriptional activity of XBP1s. This study provides a novel insight into molecular mechanisms by which SIRT1 regulates UPR signaling.