Disruption of the insulin-like growth factor type 1 receptor in osteoblasts enhances insulin signaling and action.
Defective bone formation is common in patients with diabetes, suggesting that insulin normally exerts anabolic actions in bone. However, because insulin can cross-activate the insulin-like growth factor type 1 receptor (IGF-1R), which also functions in bone, it has been difficult to establish the direct (IGF-1-independent) actions of insulin in osteoblasts. To overcome this problem, we examined insulin signaling and action in primary osteoblasts engineered for conditional disruption of the IGF-1 receptor (DeltaIGF-1R). Calvarial osteoblasts from mice carrying floxed IGF-1R alleles were infected with adenoviral vectors expressing the Cre recombinase (Ad-Cre) or green fluorescent protein (Ad-GFP) as control. Disruption of IGF-1R mRNA (>90%) eliminated IGF-1R without affecting insulin receptor (IR) mRNA and protein expression and eliminated IGF-1R/IR hybrids. In DeltaIGF-1R osteoblasts, insulin signaling was markedly increased as evidenced by increased phosphorylation of insulin receptor substrate 1/2 and enhanced ERK/Akt activation. Microarray analysis of RNA samples from insulin-treated, DeltaIGF-1R osteoblasts revealed striking changes in several genes known to be downstream of ERK including Glut-1 and c-fos. Treatment of osteoblasts with insulin induced Glut-1 mRNA, increased 2-[1,2-(3)H]-deoxy-d-glucose uptake, and enhanced proliferation. Moreover, insulin treatment rescued the defective differentiation and mineralization of DeltaIGF-1R osteoblasts, suggesting that IR signaling can compensate, at least in part, for loss of IGF-1R signaling. We conclude that insulin exerts direct anabolic actions in osteoblasts by activation of its cognate receptor and that the strength of insulin-generated signals is tempered through interactions with the IGF-1R.
Fulzele
Keertik
K
Division of Molecular and Cellular Pathology, Department of Pathology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0019, USA.
DiGirolamo
Douglas J
DJ
Liu
Zhongyu
Z
Xu
Jie
J
Messina
Joseph L
JL
Clemens
Thomas L
TL
D000256:Adenoviridae; D000818:Animals; D000963:Antimetabolites / Q000494:pharmacology; D049109:Cell Proliferation / Q000187:drug effects; D003847:Deoxyglucose / Q000494:pharmacology; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics; D051272:Glucose Transporter Type 1; D007004:Hypoglycemic Agents / Q000378:metabolism / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism* / Q000494:pharmacology; D055504:Insulin Receptor Substrate Proteins; D019426:Integrases / Q000235:genetics / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D020411:Oligonucleotide Array Sequence Analysis; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism*; D010012:Osteogenesis / Q000187:drug effects / Q000502:physiology*; D010750:Phosphoproteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D012333:RNA, Messenger / Q000096:biosynthesis; D017526:Receptor, IGF Type 1 / Q000172:deficiency / Q000378:metabolism*; D011972:Receptor, Insulin / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D012886:Skull / Q000166:cytology / Q000378:metabolism*
D000963:Antimetabolites; D051272:Glucose Transporter Type 1; D007004:Hypoglycemic Agents; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; C526295:Irs1 protein, mouse; C526298:Irs2 protein, mouse; D010750:Phosphoproteins; D012333:RNA, Messenger; C495678:Slc2a1 protein, mouse; D003847:Deoxyglucose; D017526:Receptor, IGF Type 1; D011972:Receptor, Insulin; D051057:Proto-Oncogene Proteins c-akt; D048049:Extracellular Signal-Regulated MAP Kinases; C045073:Cre recombinase; D019426:Integrases