title: Effects of c-Myc overexpression in osteoblasts on bone homeostasis, sarcoma formation and dormant hematopoietic stem cell maintenance
creator: Brückmann, Ines
subject: 570
subject: 570 Life sciences
description: The transcription factor c-Myc is an important proto-oncogene and its expression is induced by a variety of mitogenic signals promoting cell proliferation, cell growth and stem cell maintenance while concomitantely inhibiting terminal differentiation. In order to elucidate the role of c-Myc in the HSC niche, a doxycycline-inducible double transgenic ColtTA;TreMyc mouse model was used to conditionally overexpress human c-Myc in endosteal niche osteoblasts (mutant; mt). These mice display developmental bone disorders with signs of osteopetrosis. The animals developed thicker bones with concomitantly increased amounts of osteoblasts and osteoclasts. In addition the structure of the compact bone compartment was interrupted with cavernal structures, filled with ectopic bone marrow. qPCR analysis revealed that the expression of the cell cycle inhibitor p57KIP2 was significantly lower in mutant osteoblasts, suggesting that c-Myc dependent down-regulation of the cell cylce inhibitor induced elevated cell proliferation of mutant osteoblasts resulting in a higher bone-turnover. Most strikingly, the elevated bone remodeling caused a fast reduction of dormant hematopoietic stem cells (dHSCs), known to harbor the most potent HSCs. BrdU-label retaining assays revealed similar turnover kinetics for dormant and homeostatic HSCs, both dividing every 58 days in mutant animals. These data suggest that dHSC are not maintained in mutant animals. Besides decreased numbers in dHSCs a simultaneous decrease of long- and short-term HSCs (LT-, ST-HSCs) was observed, while the number of more differentiated progenitor and mature cells did not differ between mutant and control mice. Interestingly, dHSC loss appeared to be compensated by elevated Bmi1 expression and down-regulated  p16INK4a cell cycle inhibitor in cycling LT-HSCs in order to prevent their premature depletion. Interestingly, HSC differentiation capacity in mutant mice was not impaired and the bone marrow reconstitution ability of mutant HSCs after bone marrow transplantation was also normal. Notably, the proportion of LT- and ST-HSCs was significantly decreased in recipients of primary and secondary bone marrow transplantations, indicating that only the number but not self-renewal and differentiation capacities of HSCs were reduced in c-Myc overexpressing mice. Interestingly, unlike wild type HSCs, HSCs from double transgenic animals could not be efficiently activated by the HSC activators dsRNA PolyI:C or lipopolysaccharide (LPS).     Strikingly, 26% of transgenic ColtTA;TreMyc animals developed osteosarcoma often associated with simultaneous lung and liver metastasis. Analysis of osteoblasts from tumor bearing mice revealed high similarities to human osteosarcoma, with the tumor osteoblasts displaying an immature osteoblastic phenotype with deregulated Wnt signaling and high expression of the tyrosine kinase receptor c-kit. Interestingly it was recently shown that c-kit is expressed in human osteosarcomas and that its expression negatively correlates with patient survival. Thus this mouse model system might be an interesting tool for further pre-clinical studies, as it is the first osteosarcoma model with spontaneous tumor development closely resembling human osteoblastic osteosarcoma. Finally, our model highlights the important cross-talk between endosteal osteoblasts, hematopoietic stem cells and tumorigenesis, which are also found deregulated in a variety of human bone marrow diseases. The mouse model generated and characterized in this work may facilitate to elucidate the complex molecular and cellular mechanisms operative in the diseased organism and may be used to the development of novel intervention strategies.
date: 2012-11-22
type: Dissertation
type: info:eu-repo/semantics/doctoralThesis
type: NonPeerReviewed
format: application/pdf
identifier: https://archiv.ub.uni-heidelberg.de/volltextserverhttps://archiv.ub.uni-heidelberg.de/volltextserver/14273/1/2012_09_25%20Thesis_Ines_2.pdf
identifier: DOI:10.11588/heidok.00014273
identifier: urn:nbn:de:bsz:16-heidok-142736
identifier:   Brückmann, Ines  (2012) Effects of c-Myc overexpression in osteoblasts on bone homeostasis, sarcoma formation and dormant hematopoietic stem cell maintenance.  [Dissertation]     
relation: https://archiv.ub.uni-heidelberg.de/volltextserver/14273/
rights: info:eu-repo/semantics/openAccess
rights: http://archiv.ub.uni-heidelberg.de/volltextserver/help/license_urhg.html
language: eng