Mechanisms of age-related decline in insulin-like growth factor-I dependent proteoglycan synthesis in rat intervertebral disc cells.
METHODS
Age-related fluctuations in insulin-like growth factor-I dependent proteoglycan synthesis in rat intervertebral disc cells were investigated.


OBJECTIVE
The purpose of this study was to determine whether synthetic responses to insulin-like growth factor-I decline with age and to explore the possibility that an age-related increase in the expression of insulin-like growth factor binding proteins suppresses matrix synthesis in intervertebral disc cells.


BACKGROUND
Several studies have reported that the responsiveness of chondrocytes to insulin-like growth factor-I decreases with age and furthermore that this phenomenon may be related to increased expression of insulin-like growth factor binding proteins by chondrocytes.


METHODS
Nucleus pulposus tissue and cells were obtained from the coccygeal vertebrae of 8-week-old, 40-week-old, and 120-week-old rats. Age-related changes in the expression of insulin-like growth factor-I and its receptor were assessed together with insulin-like growth factor-I dependent proteoglycan synthesis by the cultured nucleus pulposus cells. Also, western blot analysis of insulin-like growth factor binding protein-1 was carried out, and further examination was performed of insulin-like growth factor-I signal transduction through tyrosine phosphorylation of insulin receptor substrate-1, which is a signal transducer of insulin-like growth factor-I.


RESULTS
Semiquantitative reverse transcription polymerase chain reaction analysis indicated that the expression of insulin-like growth factor-I receptor in 120-week cells decreased clearly in comparison with the cells of younger animals. By contrast, insulin-like growth factor-I dependent proteoglycan synthesis decreased with age, and the sharpest decline of synthesis was found between 8-week and 40-week cells, although the level of insulin-like growth factor-I/insulin-like growth factor-I receptor gene expression was maintained in 40-week-old animals. Consistent with the results of proteoglycan synthesis, the expression of phosphorylated insulin receptor substrate-1 decreased with age. Thus, the expression of insulin-like growth factor binding protein-1 and proteoglycan synthesis was investigated by use of Long R3 insulin-like growth factor-I, which was not influenced by insulin-like growth factor binding proteins. Insulin-like growth factor binding protein-1 was strongly expressed in 40-week cells in comparison with the expression in 8-week cells. Furthermore, proteoglycan synthesis in 40-week cells supplemented with Long R3 insulin-like growth factor-I was upregulated in comparison with that in 40-week cells supplemented with insulin-like growth factor-I.


CONCLUSIONS
The present findings indicate that the age-related decline in insulin-like growth factor-I dependent proteoglycan synthesis in nucleus pulposus is caused, at least in part, by the increase in insulin-like growth factor binding proteins at the early stages of aging, and further suggest that a loss of proteoglycan synthesis during the late stages of aging is caused by the downregulation of insulin-like growth factor-I receptor in addition to an increase in insulin-like growth factor binding proteins.
Okuda
S
S
Department of Orthopaedic Surgery (E3), Osaka University Medical School, Suita, Osaka, Japan. okuda@orh.go.jp
Myoui
A
A
Ariga
K
K
Nakase
T
T
Yonenobu
K
K
Yoshikawa
H
H
D000375:Aging / Q000502:physiology*; D000818:Animals; D002478:Cells, Cultured; D024043:Collagen Type II / Q000235:genetics; D024161:Collagen Type XI / Q000235:genetics; D015870:Gene Expression; D055504:Insulin Receptor Substrate Proteins; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000031:analogs & derivatives* / Q000096:biosynthesis / Q000235:genetics / Q000494:pharmacology / Q000502:physiology*; D007403:Intervertebral Disc / Q000166:cytology / Q000378:metabolism*; D008297:Male; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011509:Proteoglycans / Q000096:biosynthesis*; D051381:Rats; D017208:Rats, Wistar; D017526:Receptor, IGF Type 1 / Q000235:genetics; D012445:Sacrococcygeal Region; D014443:Tyrosine / Q000378:metabolism
D024043:Collagen Type II; D024161:Collagen Type XI; D055504:Insulin Receptor Substrate Proteins; D018970:Insulin-Like Growth Factor Binding Protein 1; C526296:Irs1 protein, rat; D010750:Phosphoproteins; D011509:Proteoglycans; C085700:long R(3)-insulin-like growth factor-I; D014443:Tyrosine; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1