INTRODUCTION: Cartilage Intermediate Layer Protein (CILP) has been implicated in cartilage diseases. Genetic analysis showed an association between a functional single‐nucleotide polymorphism (SNP) of the CILP gene and patients with lumbar disc herniation and sciatica requiring surgery. CILP is regulated by TGF‐β, but the effect of BMP‐2, age, and experimentally induced disc degeneration is not known. Excessive binding of CILP to growth factors has been suggested as a pathomechanism of disc disease.
METHODS: CILP expression levels were evaluated in rabbits of different ages (6 month old vs 5 years old) and different degrees of disc degeneration (induced by needle puncture). The expression of CILP, proteoglycan, and collagen II were measured by Western blot and Real‐time PCR. To study the effect of BMP‐2 on CILP expression, primary disc cells from young and old rabbits were treated with rhBMP‐2 with or without inhibitors of BMP‐2 mediators (siRNA to Smad 1, 5, and 8), and the gene expression of CILP was analyzed by Western blot and Real‐time PCR. The activity of the CILP promoter was measured by using the Dual Luciferase Reporter Assay System.
RESULTS: Our study demonstrates that the intervertebral disc expresses significant levels of CILP in young rabbit discs, and that the expression of CILP increases substantially with increasing age and disc degeneration. In contrast, the expression of proteoglycan and collagen II decreases with increasing age and disc degeneration. BMP‐2 induces the expression of CILP protein and stimulates the activity of the CILP promoter in the rabbit primary disc cells. The induction of CILP by BMP‐2 can be augmented with age.
DISCUSSION: Our data demonstrate that disc degeneration, age, and BMP‐2 upregulate CILP. As excessive CILP maybe harmful our findings have particular clinical significance and suggest targets for how CILP expression can be modified.