Search Results (32)

Branched-chain amino acids (BCAAs) are essential amino acids in humans, with reported anti-proliferative effects on HepG2 hepatoma cells and the potential to reduce hepatocellular carcinoma (HCC) development in cirrhotic patients. PDCD4, a tumor suppressor that is downregulated in many cancers, is also suppressed by serum, EGF, or TPA treatment. This study examined the effect BCAA has on PDCD4 expression and related cellular pathways in Huh7 hepatoma cells. Cells were treated with different concentrations of BCAA, and analyzed by Western blotting, qRT-PCR, and immunofluorescence staining. Treatment with BCAA upregulated the protein levels of PDCD4, while downregulating its mRNA levels. BCAA enhanced the phosphorylation of mTORC1 substrate 4E-BP1, p70S6K1, and p70S6K1 substrate S6 ribosomal protein. BCAA also elevated the protein levels of autophagy factors p62 and ATG5 while reducing LC3-II particle formation, thus indicating impaired autophagy. ULK1 knockdown also upregulated the protein levels of PDCD4 and p62. Additionally, BCAA upregulated the phosphorylation of ULK1 at serine 757, which was inhibited by rapamycin. These findings suggest that BCAA inhibits autophagy through the mTORC1-mediated phosphorylation of ULK1 at serine 757, thereby impairing autophagosome formation and upregulating the PDCD4 protein levels by inhibiting its degradation via autophagy. Furthermore, FACS analysis showed that BCAA inhibited the proliferation of Huh7 cells. BCAA may have a preventive effect against tumor development through the modulation of autophagy and the tumor suppressor pathways. Full article

Extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitors show therapeutic potential in triple-negative breast cancer (TNBC), but resistance through compensatory signaling limits their efficacy. We previously identified the secretory carrier membrane protein 3 (SCAMP3) as a regulator of TNBC progression and ERK1/2 activation. Here, we investigated the role of SCAMP3 in ERK1/2 signaling and therapeutic response using TMT-based LC-MS/MS phosphoproteomics of wild-type (WT) and SCAMP3 knockout (SC3KO) SUM-149 cells under basal conditions, after epidermal growth factor (EGF) stimulation, and during ERK1/2 inhibition with MK-8353. A total of 4408 phosphosites were quantified, with 1093 significantly changed. SC3KO abolished residual ERK activity under MK-8353 and affected the compensatory activation of oncogenic pathways observed in WT cells. SC3KO reduced the phosphorylation of ERK feedback regulators RAF proto-oncogene serine/threonine-protein kinase Raf-1 (S43) and the dual-specificity mitogen-activated protein kinase kinase 2 (MEK2) (T394), affected other ERK targets, including nucleoporins, transcription factors, and metabolic enzymes triosephosphate isomerase (TPI1) (S21) and ATP-citrate lyase (ACLY) (S455). SCAMP3 loss also impaired the mammalian target of rapamycin complex I (mTORC1) signaling and disrupted autophagic flux, evidenced by elevated sequestosome-1 (SQSTM1/p62) and microtubule-associated protein light chain 3 (LC3B-II) with reduced levels of the autophagosome lysosome maturation marker, Rab7A. Beyond ERK substrates, SC3KO affected phosphorylation events mediated by other kinases. These findings position SCAMP3 as a central coordinator of ERK signaling and autophagy. Our results support SCAMP3 as a potential therapeutic target to enhance ERK1/2 inhibitor clinical efficacy and overcome adaptive resistance mechanisms in TNBC. Full article

The Alzheimer’s disease (AD)-affected brain is known to be deficient in the utilization of glucose, its main energy substrate, and systemic diabetes is a significant risk factor for AD. In the course of biochemical and molecular investigations into this puzzling relationship, it has been shown that resistance to insulin action is a prominent feature of early stages of AD in the brain, thereby contributing to an energy failure state and a decline in synaptic function. In one AD-like cellular model, we found that β-amyloid (Aβ) accumulation inhibited insulin signaling and cell viability through an alteration of the PI3K/PDK-1/Akt signal pathway, an effect overcome by mTORC2 stimulation. A PDK-1 allosteric agonist, PS48, as well as newly synthesized analogs, were also found to reverse the metabolic defects caused by intracellular Aβ42 accumulation. In vivo, we previously showed that oral dosing of PS48 significantly improves learning and memory in APP/PS1 transgenic mice. Herein, we present evidence using unbiased immunohistological quantification and Western blot analyses demonstrating that ingested PS48 crosses into brain tissue where it targeted Akt and GSK3-β activities. Beneficial effects on neuronal number and Tau phosphorylation were found. Not unexpectedly, Aβ levels remained unchanged. These results support a path toward a future therapeutic trial of this untested strategy and agent in humans. Full article

The mammalian or mechanistic target of rapamycin complex 1 (mTORC1) is activated on the surface of lysosomes and phosphorylates substrates at various subcellular locations, including the lysosome, cytosol, and nucleus. However, the signaling and biological functions of nuclear mTORC1 (nmTORC1) are not well understood, primarily due to limited tools for monitoring mTORC1 activity in the nucleus. In this study, we developed a genetically encoded nmTORC1 sensor, termed nTORSEL, based on the phosphorylation of the eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4EBP1) by mTORC1 within the nucleus. nTORSEL, like its predecessor TORSEL, exhibits a fluorescent punctate pattern in the nucleus through multivalent protein–protein interactions between oligomerized 4EBP1 and eIF4E when nmTORC1 activity is low. We validated nTORSEL using biochemical analyses and imaging techniques across representative cell lines with varying levels of nmTORC1 activity. Notably, nTORSEL specifically detects physiological, pharmacological, and genetic inhibition of nmTORC1 in mouse embryonic fibroblast (MEF) cells but not in HEK293T cells. Therefore, nTORSEL is an effective tool for investigating nuclear mTORC1 signaling in cell lines. Full article

RAPTOR (regulatory-associated protein of mTOR) is a pivotal component of the mammalian target of rapamycin complex 1 (mTORC1), playing a central role in regulating cell growth, metabolism and stress responses. As a scaffold protein, RAPTOR recruits key substrates such as eukaryotic initiation factor 4E-binding protein-1 (4E-BP1) and ribosomal protein S6 kinase (S6K), facilitating their phosphorylation by mTORC1, which in turn drives protein synthesis, lipid metabolism and cellular proliferation. Its regulatory function becomes especially crucial under conditions of nutrient deprivation or stress, where it enhances the stability of the mTORC1 complex, allowing cells to adapt to fluctuating environmental cues. The hyperactivation of mTORC1, largely mediated by RAPTOR, is frequently observed in various cancers, contributing to uncontrolled cell proliferation and tumorigenesis. Moreover, RAPTOR’s modulation of immune responses and metabolic pathways extends its influence beyond oncogenesis, impacting inflammatory diseases and metabolic disorders. This review meticulously elucidates RAPTOR’s structure, post-translational modifications as well as its indispensable role within the mTORC1 complex, emphasizing its regulatory functions in cellular growth, metabolic adaptation, immune response and disease pathology including oncogenesis. Furthermore, it explores emergent therapeutic avenues targeting RAPTOR-mediated mTORC1 signaling, underscoring their potential to revolutionize cancer treatment and the management of related pathophysiological conditions. Full article

The evolutionarily conserved target of rapamycin (TOR) serine/threonine kinase controls eukaryotic cell growth, metabolism and survival by integrating signals from the nutritional status and growth factors. TOR is the catalytic subunit of two distinct functional multiprotein complexes termed mTORC1 (mechanistic target of rapamycin complex 1) and mTORC2, which phosphorylate a different set of substrates and display different physiological functions. Dysregulation of TOR signaling has been involved in the development and progression of several disease states including cancer and diabetes. Here, we highlight how genetic and biochemical studies in the model system Drosophila melanogaster have been crucial to identify the mTORC1 and mTORC2 signaling components and to dissect their function in cellular growth, in strict coordination with insulin signaling. In addition, we review new findings that involve Drosophila Golgi phosphoprotein 3 in regulating organ growth via Rheb-mediated activation of mTORC1 in line with an emerging role for the Golgi as a major hub for mTORC1 signaling. Full article

Metabolic stress in skeletal muscle cells causes sustained metabolic changes, but the mechanisms of the prolonged effects are not fully known. In this study, we tested C2C12 cells with the AMP-activated protein kinase (AMPK) stimulator AICAR and measured the changes in the metabolic pathways and signaling kinases. AICAR caused an acute increase in the phosphorylation of the AMPK target ULK1, the mTORC1 substrate S6K, and the mTORC2 target Akt. Intriguingly, prior exposure to AICAR only decreased glucose-6 phosphate dehydrogenase activity when it underwent three-hour recovery after exposure to AICAR in a bicarbonate buffer containing glucose (KHB) instead of Dulbecco’s Minimum Essential Medium (DMEM). The phosphorylation of the mTORC1 target S6K was increased after recovery in DMEM but not KHB, although this appeared to be specific to S6K, as the phosphorylation of the mTORC1 target site on ULK1 was not altered when the cells recovered in DMEM. The phosphorylation of mTORC2 target sites was also heterogenous under these conditions, with Akt increasing at serine 473 while other targets (SGK1 and PKCα) were unaffected. The exposure of cells to rapamycin (an mTORC1 inhibitor) and PP242 (an inhibitor of both mTOR complexes) revealed the differential phosphorylation of mTORC2 substrates. Taken together, the data suggest that prior exposure to AICAR causes the selective phosphorylation of mTOR substrates, even after prolonged recovery in a nutrient-replete medium. Full article

A serine/threonine-specific protein kinase B (PKB), also known as Akt, is a key factor in the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway that regulates cell survival, metabolism and proliferation. Akt phosphorylates many downstream specific substrates, which subsequently control the nuclear envelope breakdown (NEBD), centrosome maturation, spindle assembly, chromosome segregation, and cytokinesis. In vertebrates, Akt is also an important player during oogenesis and preimplantation development. In the signaling pathways regulating mRNA translation, Akt is involved in the control of mammalian target of rapamycin complex 1 (mTORC1) and thereby regulates the activity of a translational repressor, the eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1). In this review, we summarize the functions of Akt in mitosis, meiosis and early embryonic development. Additionally, the role of Akt in the regulation of mRNA translation is addressed with respect to the significance of this process during early development. Full article

mTORC1 regulates mammalian cell metabolism and growth in response to diverse environmental stimuli. Nutrient signals control the localization of mTORC1 onto lysosome surface scaffolds that are critically implicated in its amino acid-dependent activation. Arginine, leucine and S-adenosyl-methionine (SAM) can serve as major mTORC1-signaling activators, with SAM binding to SAMTOR (SAM + TOR), a fundamental SAM sensor, preventing the protein’s (SAMTOR’s) inhibitory action(s) against mTORC1, thereby triggering its (mTORC1) kinase activity. Given the lack of knowledge regarding the role of SAMTOR in invertebrates, we have identified the Drosophila SAMTOR homologue (dSAMTOR) in silico and have, herein, genetically targeted it through the utilization of the GAL4/UAS transgenic tool. Survival profiles and negative geotaxis patterns were examined in both control and dSAMTOR-downregulated adult flies during aging. One of the two gene-targeted schemes resulted in lethal phenotypes, whereas the other one caused rather moderate pathologies in most tissues. The screening of head-specific kinase activities, via PamGene technology application, unveiled the significant upregulation of several kinases, including the dTORC1 characteristic substrate dp70S6K, in dSAMTOR-downregulated flies, thus strongly supporting the inhibitory dSAMTOR action(s) upon the dTORC1/dp70S6K signaling axis in Drosophila brain settings. Importantly, genetic targeting of the Drosophila BHMT bioinformatics counterpart (dBHMT), an enzyme that catabolizes betaine to produce methionine (the SAM precursor), led to severe compromises in terms of fly longevity, with glia-, motor neuron- and muscle-specific dBHMT downregulations exhibiting the strongest effects. Abnormalities in wing vein architectures were also detected in dBHMT-targeted flies, thereby justifying their notably reduced negative geotaxis capacities herein observed mainly in the brain–(mid)gut axis. In vivo adult fly exposure to clinically relevant doses of methionine revealed the mechanistic synergism of decreased dSAMTOR and increased methionine levels in pathogenic longevity, thus rendering (d)SAMTOR an important component in methionine-associated disorders, including homocystinuria(s). Full article

Through a comprehensive analysis of the gene expression and dependency in HCC patients and cell lines, LAT1 was identified as the top amino acid transporter candidate supporting HCC tumorigenesis. To assess the suitability of LAT1 as a HCC therapeutic target, we used CRISPR/Cas9 to knockout (KO) LAT1 in the epithelial HCC cell line, Huh7. Knockout of LAT1 diminished its branched chain amino acid (BCAA) transport activity and significantly reduced cell proliferation in Huh7. Consistent with in vitro studies, LAT1 ablation led to suppression of tumor growth in a xenograft model. To elucidate the mechanism underlying the observed inhibition of cell proliferation upon LAT1 KO, we performed RNA-sequencing analysis and investigated the changes in the mTORC1 signaling pathway. LAT1 ablation resulted in a notable reduction in phosphorylation of p70S6K, a downstream target of mTORC1, as well as its substrate S6RP. This reduced cell proliferation and mTORC1 activity were rescued when LAT1 was overexpressed. These findings imply an essential role of LAT1 for maintenance of tumor cell growth and additional therapeutic angles against liver cancer. Full article

mTOR is constitutively activated in acute myeloid leukemia (AML) cells, as indicated by the phosphorylation of its substrates, 4EBP1 and P70S6K. Here, we found that quercetin (Q) and rapamycin (Rap) inhibited P70S6K phosphorylation, partially dephosphorylated 4EBP1, and activated ERK1/2 in U937 and THP1, two leukemia cell lines. ERK1/2 inhibition by U0126 induced a stronger dephosphorylation of mTORC1 substrates and activated AKT. The concomitant inhibition of ERK1/2 and AKT further dephosphorylated 4EBP1 and further increased Q- or Rap-mediated cytotoxicity, compared to the single ERK1/2 or AKT inhibition in cells undergoing Q- or Rap-treatments. Moreover, quercetin or rapamycin reduced autophagy, particularly when used in combination with the ERK1/2 inhibitor, U0126. This effect was not dependent on TFEB localization in nuclei or cytoplasm or on the transcription of different autophagy genes, but did correlate with the reduction in protein translation due to a strong eIF2α-Ser51 phosphorylation. Thus, ERK1/2, by limiting 4EBP1 de-phosphorylation and eIF2α phosphorylation, behaves as a paladin of protein synthesis. Based on these findings, the combined inhibition of mTORC1, ERK1/2, and AKT should be considered in treatment of AML. Full article

Regulatory T (Treg) cells play an important role in immune homeostasis by inhibiting cells within the innate and adaptive immune systems; therefore, the stability and immunosuppressive function of Treg cells need to be maintained. In this study, we found that the expression of insulin receptor substrate 1 (IRS1) by Treg cells was lower than that by conventional CD4 T cells. IRS1-overexpressing Treg cells showed the downregulated expression of FOXP3, as well as Treg signature markers CD25 and CTLA4. IRS1-overexpressing Treg cells also showed diminished immunosuppressive functions in an in vitro suppression assay. Moreover, IRS1-overexpressing Treg cells were unable to suppress the pathogenic effects of conventional T cells in a transfer-induced colitis model. IRS1 activated the mTORC1 signaling pathway, a negative regulator of Treg cells. Moreover, IRS1 destabilized Treg cells by upregulating the expression of IFN-γ and Glut1. Thus, IRS1 acts as a negative regulator of Treg cells by downregulating the expression of FOXP3 and disrupting stability. Full article

Hypoxia is a critical condition that governs survival, self-renewal, quiescence, metabolic shift and refractoriness to leukemic stem cell (LSC) therapy. The present study aims to investigate the hypoxia-driven regulation of the mammalian Target of the Rapamycin-2 (mTORC2) complex to unravel it as a novel potential target in chronic myeloid leukemia (CML) therapeutic strategies. After inducing hypoxia in a CML cell line model, we investigated the activities of mTORC1 and mTORC2. Surprisingly, we detected a significant activation of mTORC2 at the expense of mTORC1, accompanied by the nuclear localization of the main substrate phospho-Akt (Ser473). Moreover, the Gene Ontology analysis of CML patients’ CD34+ cells showed enrichment in the mTORC2 signature, further strengthening our data. The deregulation of mTOR complexes highlights how hypoxia could be crucial in CML development. In conclusion, we propose a mechanism by which CML cells residing under a low-oxygen tension, i.e., in the leukemia quiescent LSCs, singularly regulate the mTORC2 and its downstream effectors. Full article

Insulin signaling plays a key role in glucose uptake, glycogen synthesis, and protein and lipid synthesis. In insulin-resistant states like obesity and type 2 diabetes mellitus, these processes are dysregulated. Regular physical exercise is a potential therapeutic strategy against insulin resistance, as an acute bout of exercise increases glucose disposal during the activity and for hours into recovery. Chronic exercise increases the activation of proteins involved in insulin signaling and increases glucose transport, even in insulin resistant states. Here, we will focus on the effect of acute exercise on insulin signaling and protein kinase B (Akt) pathways. Activation of proximal proteins involved in insulin signaling (insulin receptor, insulin receptor substrate-1 (IRS-1), phosphoinoside-3 kinase (PI3K)) are unchanged in response to acute exercise/contraction, while activation of Akt and of its substrates, TBC1 domain family 1 (TBC1D1), and TBC domain family 4 (TBC1D4) increases in response to such exercise/contraction. A wide array of Akt substrates is also regulated by exercise. Additionally, AMP-activated protein kinase (AMPK) seems to be a main mediator of the benefits of exercise on skeletal muscle. Questions persist on how mTORC1 and AMPK, two opposing regulators, are both upregulated after an acute bout of exercise. Full article

The inactivation of von Hippel–Lindau (VHL) is critical for clear cell renal cell carcinoma (ccRCC) and VHL syndrome. VHL loss leads to the stabilization of hypoxia-inducible factor α (HIFα) and other substrate proteins, which, together, drive various tumor-promoting pathways. There is inadequate molecular characterization of VHL restoration in VHL-defective ccRCC cells. The identities of HIF-independent VHL substrates remain elusive. We reinstalled VHL expression in 786-O and performed transcriptome, proteome and ubiquitome profiling to assess the molecular impact. The transcriptome and proteome analysis revealed that VHL restoration caused the downregulation of hypoxia signaling, glycolysis, E2F targets, and mTORC1 signaling, and the upregulation of fatty acid metabolism. Proteome and ubiquitome co-analysis, together with the ccRCC CPTAC data, enlisted 57 proteins that were ubiquitinated and downregulated by VHL restoration and upregulated in human ccRCC. Among them, we confirmed the reduction of TGFBI (ubiquitinated at K676) and NFKB2 (ubiquitinated at K72 and K741) by VHL re-expression in 786-O. Immunoprecipitation assay showed the physical interaction between VHL and NFKB2. K72 of NFKB2 affected NFKB2 stability in a VHL-dependent manner. Taken together, our study generates a comprehensive molecular catalog of a VHL-restored 786-O model and provides a list of putative VHL-dependent ubiquitination substrates, including TGFBI and NFKB2, for future investigation. Full article