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Villanova abstract inglese
Università Politecnica delle Marche
Dottorato di Ricerca In Biotecnologie Biomediche
VII Ciclo (A.A. 2005-2008)
Coordinatore: Prof. Giulio Magni
Effects of dexamethasone on the estrinsic and intrinsic induced apoptosis on osteoblasts and
Dottoranda: Dott.ssa Federica Villanova
Glucocorticoids (CG) are synthetic drugs widely used to treat anti-inflammatory and
autoimmune diseases, but severe osteporosis frequently occurs as side effect of long term GC therapy. GC extend osteoclast lifespan, inhibit osteoclastogenesis, induce osteoblast apoptosis and reduce bone formation thus altering the fine balance regulating bone homeostasis. A potential mechanism of GC action is the influence of these drugs on number and lifespan of bone cells through regulated apoptosis. Despite the GC pro-apoptotic effects on osteoblasts have been studied both in vitro
and in vivo
, the precise biochemical mechanisms involved in the apoptotic process are still unclear. Data reported in literature are controversial.
The current study investigates the effect of dexamethasone on Fas- and starvation-
induced apoptosis of osteoclasts and their precursors and the molecular mechanisms involved.
The human osteoblastic cell line hFOB1.19 has been used as a validated experimental
model to study osteoblast differentiation from mesenchymal progenitors, since they express pre-osteoblast features when cultivated at the permissive temperature of 35°C and a mature osteoblast phenotype when cultivated at the restrictive temperature of 39.5°C. Osteosarcoma cell line MG63 was also analyzed.
Dexamethasone enhances pre-osteoblast but not mature cell susceptibility to apoptosis
induced by both the estrinsic (Fas stimulation) and intrinsic (serum withdrawal) pathways. Mature osteoblasts and MG63 are protected from apoptosis when treated with dexamethasone. Apoptosis occurs in a caspase-dependent manner and involves mitochondria. Apoptosis sensitization and resistance by dexhametasone is due to the modulation of anti-apoptotic molecules. In particular, dexamethasone down-regulates survivin expression in precursor cells, whereas it up-regulates survivin, and to a lesser extent Bcl-2, in mature osteoblasts and MG63. FLIP expression is also increased in dexamethasone treated MG63. Such effects are inhibited by the glucocorticoid antagonist RU486, suggesting that the dexamethasone influence on apoptosis sensitivity and anti apoptotic protein expression is specific and dependent on glucocorticoid receptor. In addition, Bcl-2 and survivin have been shown to be essential for pre- and mature osteoblast survival since their single silencing is sufficient to induce their spontaneous apoptosis.
These results outline a new molecular mechanism of glucocorticoid-mediated bone loss
due to GC effect on the apoptosis of pre-osteoblasts. Such mechanism could be useful to design
new preventive and curative strategies to avoid bone loss in GC induced osteoporosis by targeting
osteoblast survival and apoptosis. Furthermore, the data partially elucidate dexamethasone-induced
resistance of osteosarcoma cells to Fas-and stress-induced apoptosis.
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