Age-related fluctuations in insulin-like growth factor-I dependent proteoglycan synthesis in rat intervertebral disc cells were investigated.
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.
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.
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.
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 growthfactor binding protein-1 was strongly expressed in 40-week cells in comparison withthe 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.
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.
From the Department of Orthopaedic Surgery (E3), Osaka University Medical School, Osaka, Japan.
Supported by a grant from the Organization for Pharmaceutical Safety and Research and a grant-in-aid for Developmental Science Research from the Ministry of Education, Science, and Culture of Japan (11671431).
Acknowledgment date: January 1, 2001.
Acceptance date: April 13, 2001.
Device status category: 1.
Conflict of interest category: 12.
Address reprint requests to
Shin’ya Okuda, MD
Department of Orthopaedic Surgery (E3)
Osaka University Medical School
Osaka, 565-0871, Japan