Aging impairs IGF-I receptor activation and induces skeletal resistance to IGF-I.
IGF-I plays an important anabolic role in stimulating bone formation and maintaining bone mass. We show that the pro-proliferative, anti-apoptotic, and functional responses to IGF-I in bone and BMSCs decrease with aging. These changes are associated with impaired receptor activation and signal transduction through the MAPK and PI3K pathways.


BACKGROUND
IGF-I is a potent anabolic agent having effects across diverse tissues and cell types. With aging, bone becomes resistant to the anabolic actions of IGF-I. To examine the effects of aging on bone responsiveness to IGF-I, we measured the pro-proliferative, anti-apoptotic, and functional responses of bone and bone marrow stromal cells (BMSCs) to IGF-I and evaluated IGF-I signal transduction in young, adult, and old mice.


METHODS
Male C57BL/6 mice 6 wk (young), 6 mo (adult), and 24 mo (old) were treated with IGF-I for 2 wk using osmotic minipumps, and osteoblast proliferation (BrdU labeling) in vivo, and osteoprogenitor number (BMSC culture and calcium nodule formation) were measured. Proliferation, apoptosis, and expression of key osteoblast factors (alkaline phosphatase, collagen, osteocalcin, RANKL, osteoprotegerin (OPG), macrophage-colony stimulating factor [M-CSF]) and IGF-I signaling elements and their activation in IGF-I-treated cells were studied using QRT-PCR and Western blot analysis. Data were analyzed using ANOVA.


RESULTS
Aging decreased the basal and IGF-I-stimulated number of BrdU-labeled osteoblasts and reduced the ability of IGF-I to stimulate osteoprogenitor formation (calcium nodule number) by 50%. The pro-proliferative and anti-apoptotic actions of IGF-I were blunted in cells from old animals. These changes were accompanied by age-related alterations in the ability of IGF-I to regulate alkaline phosphatase, collagen, osteocalcin, RANKL, OPG, and M-CSF expression. IGF-I binding was normal, but IGF-I receptor mRNA and protein expression was increased in aged animals by 2- and 10-fold, respectively. The age-related changes in proliferation, apoptosis, and function were accompanied by loss of IGF-I-induced signaling at the receptor level and at key regulatory sites along the MAPK (ERK1/2) and PI3K (AKT) pathways.


CONCLUSIONS
Our data show that aging is accompanied by loss of bone and BMSC/osteoblast responsiveness to IGF-I and that these changes are associated with resistance to IGF-I signaling that involve receptor activation and downstream signaling events.
Cao
Jay J
JJ
Division of Endocrinology, Veterans Affairs Medical Center, and Department of Medicine, University of California, San Francisco, California 94121, USA.
Kurimoto
Pamela
P
Boudignon
Benjamin
B
Rosen
Clifford
C
Lima
Florence
F
Halloran
Bernard P
BP
D000375:Aging / Q000502:physiology*; D000469:Alkaline Phosphatase / Q000235:genetics; D000818:Animals; D001842:Bone and Bones / Q000187:drug effects* / Q000201:enzymology*; D002478:Cells, Cultured; D003094:Collagen / Q000235:genetics; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008297:Male; D051379:Mice; D015675:Osteocalcin / Q000235:genetics; D010010:Osteoclasts / Q000187:drug effects / Q000201:enzymology; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism*
D012333:RNA, Messenger; D015675:Osteocalcin; D007334:Insulin-Like Growth Factor I; D003094:Collagen; D017526:Receptor, IGF Type 1; D051057:Proto-Oncogene Proteins c-akt; D048049:Extracellular Signal-Regulated MAP Kinases; D000469:Alkaline Phosphatase