Doubly truncated FosB isoform (Delta2DeltaFosB) induces osteosclerosis in transgenic mice and modulates expression and phosphorylation of Smads in osteoblasts independent of intrinsic AP-1 activity.
BACKGROUND
Activator protein (AP)-1 family members play important roles in the development and maintenance of the adult skeleton. Transgenic mice that overexpress the naturally occurring DeltaFosB splice variant of FosB develop severe osteosclerosis. Translation of Deltafosb mRNA produces both DeltaFosB and a further truncated isoform (Delta2DeltaFosB) that lacks known transactivation domains but, like DeltaFosB, induces increased expression of osteoblast marker genes.


METHODS
To test Delta2DeltaFosB's ability to induce bone formation in vivo, we generated transgenic mice that overexpress only Delta2DeltaFosB using the enolase 2 (ENO2) promoter-driven bitransgenic Tet-Off system.


RESULTS
Despite Delta2DeltaFosB's failure to induce transcription of an AP-1 reporter gene, the transgenic mice exhibited both the bone and the fat phenotypes seen in the ENO2-DeltaFosB mice. Both DeltaFosB and Delta2DeltaFosB activated the BMP-responsive Xvent-luc reporter gene and increased Smad1 expression. Delta2DeltaFosB enhanced BMP-induced Smad1 phosphorylation and the translocation of phospho-Smad1 (pSmad1) to the nucleus more efficiently than DeltaFosB and showed a reduced induction of inhibitory Smad6 expression.


CONCLUSIONS
DeltaFosB's AP-1 transactivating function is not needed to induce increased bone formation, and Delta2DeltaFosB may act, at least in part, by increasing Smad1 expression, phosphorylation, and translocation to the nucleus.
Sabatakos
George
G
Department of Orthopaedics, Yale University School of Medicine, New Haven, Connecticut, USA.
Rowe
Glenn C
GC
Kveiborg
Marie
M
Wu
Meilin
M
Neff
Lynn
L
Chiusaroli
Riccardo
R
Philbrick
William M
WM
Baron
Roland
R
D017398:Alternative Splicing; D000818:Animals; D001483:Base Sequence; D002478:Cells, Cultured; D017931:DNA Primers; D006801:Humans; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D010010:Osteoclasts / Q000378:metabolism*; D010026:Osteosclerosis / Q000235:genetics*; D010766:Phosphorylation; D020033:Protein Isoforms / Q000235:genetics / Q000502:physiology*; D016760:Proto-Oncogene Proteins c-fos / Q000235:genetics / Q000502:physiology*; D012333:RNA, Messenger / Q000235:genetics; D051898:Smad1 Protein / Q000378:metabolism*; D018808:Transcription Factor AP-1 / Q000502:physiology*
D017931:DNA Primers; C483819:Fosb protein, mouse; D020033:Protein Isoforms; D016760:Proto-Oncogene Proteins c-fos; D012333:RNA, Messenger; D051898:Smad1 Protein; C497401:Smad1 protein, mouse; D018808:Transcription Factor AP-1