Involvement of Insulin-Like Growth Factor I in Development of Ossification of the Posterior Longitudinal Ligament of the Spine

In order to investigate the pathogenesis of ossification of the posterior longitudinal ligament (OPLL) of the spine, we examined the distribution of insulin-like growth factor I (IGF-I) in the posterior longitudinal ligaments of OPLL patients, and analyzed the effects of IGF-I on the cultured spinal ligament cells. For that purpose we established eight varieties of OPLL and non-OPLL cell lines obtained from spinal ligaments of corresponding patients, respectively. In contrast to non-OPLL cases, all the OPLL cases were histologically shown to contain round-shaped cartilage-like cells in the transitional region from preossifying to ossifying ligaments, and these cells were strongly stained with an antibody for IGF-I. In the vicinity of preossifying cartilaginous tissues, ligament cells also had a rod-like appearance and were positive for IGF-I immunohistochemically. The effects of IGF-I on cultured spinal ligament cells were assayed by alkaline phosphatase (AP) activity, DNA synthesis, and the amounts of collagen produced. The number of OPLL cell lines that increased AP activity, responding to IGF-I irrespective of 1,25(OH)2D3, was significantly larger than that of non-OPLL cell lines, although IGF-I stimulated DNA and procollagen type I carboxyl-terminal peptide synthesis in most of both OPLL and non-OPLL cell lines. These data demonstrate the dominant expression of IGF-I in the posterior longitudinal ligaments of OPLL patients, and suggest that IGF-I preferentially induces osteogenic differentiation in OPLL cells rather than in non-OPLL cells. IGF-I, therefore, may be involved in the local ossification process of spinal ligaments observed in OPLL patients.