Mode of growth hormone action in osteoblasts.
Growth hormone (GH) affects bone size and mass in part through stimulating insulin-like growth factor type 1 (IGF-1) production in liver and bone. Whether GH acts independent of IGF-1 in bone remains unclear. To define the mode of GH action in bone, we have used a Cre/loxP system in which the type 1 IGF-1 receptor (Igf1r) has been disrupted specifically in osteoblasts in vitro and in vivo. Calvarial osteoblasts from mice homozygous for the floxed IGF-1R allele (IGF-1R(flox/flox)) were infected with adenoviral vectors expressing Cre. Disruption of IGF-1R mRNA (>90%) was accompanied by near elimination of IGF-1R protein but retention of GHR protein. GH-induced STAT5 activation was consistently greater in osteoblasts with an intact IGF-1R. Osteoblasts lacking IGF-1R retained GH-induced ERK and Akt phosphorylation and GH-stimulated IGF-1 and IGFBP-3 mRNA expression. GH-induced osteoblast proliferation was abolished by Cre-mediated disruption of the IGF-1R or co-incubation of cells with an IGF-1-neutralizing antibody. By contrast, GH inhibited apoptosis in osteoblasts lacking the IGF-1R. To examine the effects of GH on osteoblasts in vivo, mice wild type for the IGF-1R treated with GH subcutaneously for 7 days showed a doubling in the number of osteoblasts lining trabecular bone, whereas osteoblast numbers in similarly treated mice lacking the IGF-1R in osteoblasts were not significantly affected. These results indicate that although direct IGF-1R-independent actions of GH on osteoblast apoptosis can be demonstrated in vitro, IGF-1R is required for anabolic effects of GH in osteoblasts in vivo.
DiGirolamo
Douglas J
DJ
Department of Pathology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0019, USA.
Mukherjee
Aditi
A
Fulzele
Keertik
K
Gan
Yujun
Y
Cao
Xuemei
X
Frank
Stuart J
SJ
Clemens
Thomas L
TL
D000256:Adenoviridae / Q000235:genetics; D000483:Alleles; D000818:Animals; D000831:Animals, Newborn; D017209:Apoptosis / Q000187:drug effects; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D005260:Female; D005822:Genetic Vectors; D049452:Green Fluorescent Proteins / Q000378:metabolism; D013006:Growth Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D007279:Injections, Subcutaneous; D051379:Mice; D008822:Mice, Transgenic; D008954:Models, Biological; D010006:Osteoblasts / Q000187:drug effects*; D012333:RNA, Messenger / Q000378:metabolism; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D012886:Skull / Q000166:cytology; D013997:Time Factors
D012333:RNA, Messenger; D049452:Green Fluorescent Proteins; D013006:Growth Hormone; D017526:Receptor, IGF Type 1