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Biomedical subjects

George Thomas

Publications and source records attributed to George Thomas.

52 records · Page 3Linked to original sources
PubMed2002

Evaluation of microsatellite instability and immunohistochemistry for the prediction of germ-line MSH2 and MLH1 mutations in hereditary nonpolyposis colon cancer families.

Forty-eight hereditary nonpolyposis colorectal carcinoma (HNPCC) families for which a tumor sample was available were evaluated for the presence of germ-line mutations in MSH2 and MLH1, tumor microsatellite instability (MSI), and where possible, expression of MSH2 and MLH1 in tumors by immunohistochemistry. Fourteen of 48 of the families had a germ-line mutation in either MSH2 or MLH1 that could be detected by genomic DNA sequencing, and 28 of 48 of the families had MSI-H tumors. Four additional families showed loss of expression of MSH2, and one additional family showed loss of expression of MLH1 but did not have germ-line mutations in MSH2 or MLH1 that could be detected by DNA sequencing. MSI-H, as defined using the National Cancer Institute recommended five-microsatellite panel, had a 100% sensitivity for identifying samples having MSH2 or MLH1 mutations or loss of expression. In contrast, loss of MSH2 and MLH1 expression did not identify all samples having germ-line mutations in MSH2 or MLH1, because in five cases, a mutant protein product was expressed that could be detected by IHC. A combination of the Bethesda criteria for HNPCC and an MSI-H phenotype defined the smallest number of cases having all of the germ-line MSH2 and MLH1 mutations that could be detected by DNA sequencing.

Adaptor Proteins, Signal Transducing↗
PubMed2002

Regulation of an activated S6 kinase 1 variant reveals a novel mammalian target of rapamycin phosphorylation site.

A critical step in S6 kinase 1 (S6K1) activation is Thr(229) phosphorylation in the activation loop by the phosphoinositide-dependent protein kinase (PDK1). Thr(229) phosphorylation requires prior phosphorylation of the Ser/Thr-Pro sites in the autoinhibitory domain and Thr(389) in the linker domain, consistent with PDK1 more effectively catalyzing Thr(229) phosphorylation in a variant harboring acidic residues in these positions (S6K1-E389D(3)E). S6K1-E389D(3)E has high basal activity and exhibits partial resistance to rapamycin and wortmannin, and its activity can be further augmented by mitogens, effects presumably mediated by Thr(229) phosphorylation. However, PDK1-induced Thr(229) phosphorylation is reported to be constitutive rather than phosphatidylinositide 3,4,5-trisphosphate-dependent, suggesting that S6K1-E389D(3)E activity is mediated through a distinct site. Here we use phosphospecific antibodies to show that Thr(229) is fully phosphorylated in S6K1-E389D(3)E in the absence of mitogens and that regulation of S6K1-E389D(3)E activity by mitogens, rapamycin, or wortmannin parallels Ser(371) phosphorylation. Consistent with this observation, a dominant interfering allele of the mammalian target of rapamycin, mTOR, inhibits mitogen-induced Ser(371) phosphorylation and activation of S6K1-E389D(3)E, whereas wild type mTOR stimulates both responses. Moreover, in vitro mTOR directly phosphorylates Ser(371), and this event modulates Thr(389) phosphorylation by mTOR, compatible with earlier in vivo findings.

Androstadienes↗
PubMed2002

Insulin regulation of insulin-like growth factor-binding protein-1 gene expression is dependent on the mammalian target of rapamycin, but independent of ribosomal S6 kinase activity.

Insulin inhibits the expression of the hepatic insulin-like growth factor-binding protein-1 (IGFBP-1) and glucose-6-phosphatase (G6Pase) genes. The signaling pathway that mediates these events requires the activation of phosphatidylinositol 3-kinase, whereas transfection studies have suggested an involvement of Akt (protein kinase B) and FKHR, a transcription factor regulated by Akt. We now demonstrate that insulin repression of endogenous IGFBP-1 gene transcription was blocked by rapamycin or by amino acid starvation. Rapamycin inhibited the mammalian target of rapamycin (mTOR) and the subsequent activation of p70/p85 S6 protein kinase-1 (S6K1) by insulin, whereas amino acid depletion prevented insulin induction of these signaling molecules. Importantly, we demonstrate that insulin regulation of the thymine-rich insulin response element of the IGFBP-1 promoter was also inhibited by rapamycin. However, sustained activation of S6K1 did not repress this promoter. In addition, rapamycin did not affect insulin regulation of G6Pase expression or Akt activation. We propose that these observations indicate that an mTOR-dependent, but S6K-independent mechanism regulates the suppression of IGFBP-1 (but not G6Pase) gene expression by insulin. Therefore, although the insulin-responsive sequence of the G6Pase gene promoter is related to that of the IGFBP-1 promoter, the signaling pathways that mediate suppression of these genes are distinct.

Adenoviridae↗
PubMed2002

Regulation of cell size in growth, development and human disease: PI3K, PKB and S6K.

It has generally been observed that cells grow to a certain size before they divide. In the last few years, the PI3K signal transduction pathway has emerged as one of the main signaling routes utilized by cells to control their increase in size. Here we focus on two components of this pathway, PKB and S6K, and briefly review the experiments that initially uncovered their roles in cell size control. In addition, we discuss a number of recent observations suggesting that the generic models used to describe this pathway to date may have been oversimplified. Indeed, recent observations in Drosophila and mouse support a more complex interaction between these signaling components in development. Finally, we have utilized two contemporary studies involving PKB- and S6K-deficient mice as a paradigm to underscore the importance of cell size and to accurately delineate the connections between signaling pathways for human disease, such as diabetes mellitus.

Animals↗
PubMed2002

Kidney pathology: current classification of renal cell carcinoma.

This report discusses the new cytogenetic classification of renal cell carcinoma (RCC) and its biological and clinical significance. It describes the four major types (clear cell, chromophil, chromophobe, and collecting duct) as well as rarer entities, such as small cell carcinoma, cystic RCC, rhabdoid variant of RCC, and unclassified RCC. Sarcomatoid carcinomas are a diverse group representing high-grade transformation of the other types. In addition to a description of the pathologic findings, there is a discussion of tumor markers and prognostic indicators. The clinical significance of the classification is discussed, with reference to recent reports.

Carcinoma, Renal Cell↗
PubMed2002

dS6K-regulated cell growth is dPKB/dPI(3)K-independent, but requires dPDK1.

Genetic studies in Drosophila melanogaster underscore the importance of the insulin-signalling pathway in controlling cell, organ and animal size. Effectors of this pathway include Chico (the insulin receptor substrate homologue), dPI(3)K, dPKB, dPTEN, and dS6K. Mutations in any of these components have a striking effect on cell size and number, with the exception of dS6K. Mutants in dS6K affect cell size but not cell number, seemingly consistent with arguments that dS6K is a distal effector in the signalling pathway, directly controlled by dTOR, a downstream effector of dPI(3)K and dPKB. Unexpectedly, recent studies showed that dS6K activity is unimpaired in chico-deficient larvae, suggesting that dS6K activation may be mediated through the dPI(3)K docking sites of the Drosophila insulin receptor. Here, we show genetically, pharmacologically and biochemically that dS6K resides on an insulin signalling pathway distinct from that of dPKB, and surprisingly also from that of dPI(3)K. More striking, despite dPKB-dPI(3)K-independence, dS6K activity is dependent on the Drosophila homologue of the phosphoinositide-dependent protein kinase 1, dPDK1, demonstrating that both dPDK1, as well as dTOR, mediated dS6K activation is phosphatidylinositide-3,4,5-trisphosphate (PIP3)-independent.

3-Phosphoinositide-Dependent Protein Kinases↗
PubMed2002

The S6 kinase signaling pathway in the control of development and growth.

The discussion will focus on the role of the ribosomal protein S6 kinase (S6K) signaling pathway in the regulation of cell growth and proliferation. Although 40S ribosomal protein S6 phosphorylation was first described 25 years ago (Gressner and Wool, 1974), it only recently has been implicated in the translational up-regulation of mRNAs coding for the components of protein synthetic apparatus (Fumagalli and Thomas, 2000). These mRNAs contain an oligopyrimidine tract at their 5' transcriptional start site, termed a 5'TOP, which has been shown to be essential for their regulation at the translational level (Meyuhas et al., 1996). In parallel, a great deal of information has accumulated concerning the identification of the signaling pathway and the regulatory phosphorylation sites involved in controlling S6K activation (Dufner and Thomas, 1999). Despite this knowledge we are only beginning to identify the direct upstream elements involved in growth factor-induced kinase activation (Dennis et al., 2001; Pullen et al., 1998). Use of the immunosuppressant rapamycin, a bacterial macrolide, in conjunction with dominant interfering and activated forms of S6K1 has helped to establish the role of this signaling cascade in the regulation of growth and proliferation (Dennis and Thomas, 2002). In addition, current studies employing the mouse as well as Drosophila melanogaster have provided new insights into physiological function of S6K in the animal (Montagne et al., 1999; Pende et al., 2000). Loss of dS6K function in Drosophila melanogaster demonstrated its paramount importance in development and growth control (Montagne et al., 1999), whereas deletion of the S6K1 gene in the mouse led to an animal of reduced size and the identification of the S6K1 homologue, S6K2 (Shima et al., 1998). Such mice are significantly smaller during fetal development (Shima et al., 1998) and hypoinsulinemic in the adult, conditions known to lead to type 2 diabetes (Pende et al., 2000).

Animals↗