Autocrine/paracrine mechanism of insulin-like growth factor-1 secretion, and the effect of insulin-like growth factor-1 on proteoglycan synthesis in bovine intervertebral discs

The present study was undertaken to investigate the effect of insulin-like growth factor-1 on proteoglycan synthesis and the autocrine/paracrine mechanisms involving insulin-like growth factor-1 in the bovine coccygeal intervertebral disc. Insulin-like growth factor-1 stimulated proteoglycan synthesis in cultured cells of the nucleus pulposus of bovine intervertebral discs in a dose-dependent manner, and the effect was inhibited by an anti-insulin-like growth factor-1 monoclonal antibody. In situ hybridization histochemistry revealed the expression of insulin-like growth factor-1 mRNA in the cultured cell, and its production in these cells was demonstrated by radioimmunoassay. Insulin-like growth factor-1 receptor in the cultured cells was also demonstrated by radioimmunoassay. Insulin-like growth factor-1 receptor in the cultured cells was also demonstrated immunohistochemically. Scatchard analysis using an [I-125]insulin-like growth factor-1 binding assay showed that the cells cultured in monolayer had a single type of insulin-like growth factor-1 receptor, whose affinity and number were estimated to be 7.38 x 10(8)/M and 9.27 x 10(4)/cell, respectively. These results suggest that insulin-like growth factor-1 stimulates proteoglycan synthesis in cells of the nucleus pulposus and that these cells in culture have an insulin-like growth factor-1 autocrine/paracrine mechanism. The expressions of insulin-like growth factor-1 mRNA and insulin-like growth factor-1 receptor in disc tissue were greater in cells of the nucleus pulposus of fetal bovine intervertebral discs than in those of the adult discs. These findings suggest that the action of autocrine/paracrine insulin-like growth factor-1 is more active in cells of the young nucleus pulposus than in cells of mature subjects.