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      • Record: found
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      Akting and Cycling: A Tale of the Pituitary

      ,

      Hormone Research in Paediatrics

      S. Karger AG

      Tumour, Cell cycle, Pituitary, Adenoma

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          Abstract

          Pituitary tumours are characterized by a series of phenotypic abnormalities, but the molecular nature of the underlying defects has proved peculiarly intractable. Oncogenes and tumour suppressor genes involved in other tumours do not appear to play a major role in the pathogenesis of pituitary tumours. In addition, germline genetic disorders in which pituitary tumours are a common feature have not shed much light on the more common sporadic tumour. A number of defects in specific feedback regulation in the secretory tumours have been identified, but it is presently unclear as to what extent these are a consequence of the tumour, possibly enhancing its growth or survival, rather than the cause. However, recent studies on the cell cycle have demonstrated significant abnormalities that have been traced to a cytoplasmic kinase which appears to be abnormally expressed in the majority of pituitary adenomas, and we are beginning to see a possible unifying abnormality.

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          Most cited references 41

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          A receptor in pituitary and hypothalamus that functions in growth hormone release.

          Small synthetic molecules termed growth hormone secretagogues (GHSs) act on the pituitary gland and the hypothalamus to stimulate and amplify pulsatile growth hormone (GH) release. A heterotrimeric GTP-binding protein (G protein)-coupled receptor (GPC-R) of the pituitary and arcuate ventro-medial and infundibular hypothalamus of swine and humans was cloned and was shown to be the target of the GHSs. On the basis of its pharmacological and molecular characterization, this GPC-R defines a neuroendocrine pathway for the control of pulsatile GH release and supports the notion that the GHSs mimic an undiscovered hormone.
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            Phosphorylation-dependent ubiquitination of cyclin E by the SCFFbw7 ubiquitin ligase.

            Cyclin E binds and activates the cyclin-dependent kinase Cdk2 and catalyzes the transition from the G1 phase to the S phase of the cell cycle. The amount of cyclin E protein present in the cell is tightly controlled by ubiquitin-mediated proteolysis. Here we identify the ubiquitin ligase responsible for cyclin E ubiquitination as SCFFbw7 and demonstrate that it is functionally conserved in yeast, flies, and mammals. Fbw7 associates specifically with phosphorylated cyclin E, and SCFFbw7 catalyzes cyclin E ubiquitination in vitro. Depletion of Fbw7 leads to accumulation and stabilization of cyclin E in vivo in human and Drosophila melanogaster cells. Multiple F-box proteins contribute to cyclin E stability in yeast, suggesting an overlap in SCF E3 ligase specificity that allows combinatorial control of cyclin E degradation.
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              PKB/Akt phosphorylates p27, impairs nuclear import of p27 and opposes p27-mediated G1 arrest.

              Mechanisms linking mitogenic and growth inhibitory cytokine signaling and the cell cycle have not been fully elucidated in either cancer or in normal cells. Here we show that activation of protein kinase B (PKB)/Akt, contributes to resistance to antiproliferative signals and breast cancer progression in part by impairing the nuclear import and action of p27. Akt transfection caused cytoplasmic p27 accumulation and resistance to cytokine-mediated G1 arrest. The nuclear localization signal of p27 contains an Akt consensus site at threonine 157, and p27 phosphorylation by Akt impaired its nuclear import in vitro. Akt phosphorylated wild-type p27 but not p27T157A. In cells transfected with constitutively active Akt(T308DS473D)(PKB(DD)), p27WT mislocalized to the cytoplasm, but p27T157A was nuclear. In cells with activated Akt, p27WT failed to cause G1 arrest, while the antiproliferative effect of p27T157A was not impaired. Cytoplasmic p27 was seen in 41% (52 of 128) of primary human breast cancers in conjunction with Akt activation and was correlated with a poor patient prognosis. Thus, we show a novel mechanism whereby Akt impairs p27 function that is associated with an aggressive phenotype in human breast cancer.
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                Author and article information

                Journal
                HRE
                Horm Res Paediatr
                10.1159/issn.1663-2818
                Hormone Research in Paediatrics
                S. Karger AG
                978-3-8055-7833-2
                978-3-318-01154-8
                1663-2818
                1663-2826
                2004
                October 2004
                17 November 2004
                : 62
                : Suppl 3
                : 117-123
                Affiliations
                Department of Endocrinology, St. Bartholomew’s Hospital, London, UK
                Article
                80512 Horm Res 2004;62(suppl 3):117–123
                10.1159/000080512
                15539812
                © 2004 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                References: 57, Pages: 7
                Categories
                ESPE Research Fellowship Grant Lecture andPlenary Lecture

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