Background The molecular pathways involved in the process of new bone formation in ankylosing spondylitis (AS) are not entirely known. However, data suggests that this process is linked to the reactivation of developmental pathways. It was recently shown that the Hedgehog pathway (HH) is involved in osteophyte formation in osteoarthritis. Moreover, it appears to be the main controller of endochondral ossification, a process already known to participate in new bone formation in AS. The ligand that activates the HH pathway in the skeleton is Indian Hedgehog (IHH).
Objectives To assess i) serum levels of IHH in patients with AS compared to healthy subjects and patients with rheumatoid arthritis (RA) ii) the effect of anti-TNFα treatment on IHH levels in patients with AS and RA and iii) the effect of serum of patients with AS on HH pathway activation
Methods Serum samples were obtained from 59 patients with AS (36 on anti-TNFα treatment), 70 patients with RA (30 on anti-TNFα treatment) and 53 healthy subjects. IHH levels were measured using an established solid phase immunoassay. The effect of serum from patients with AS on HH pathway activation was evaluated using an experimental model based on osteoblast-like, Saos-2 cells. On day 0, 2X106 Saos-2 cells were cultured in EMEM/10%FBS; on day 2, 10% of serum was added and on day 3 cells were lysed and mRNA was extracted. RT-PCR was used to assess the expression of two HH pathway target genes (Ptch-1 and glypican 3).
Results IHH levels were significantly increased in AS patients not receiving anti TNF- treatment compared to healthy subjects (mean±SEM of OD: 0.37±0.02 vs 0.28±0.02, respectively, p=0.03). Patients with AS on anti-TNF treatment had significantly lower IHH levels compared to AS patients not on such treatment (mean±SEM: 0.28±0.02 vs 0.37±0.02, respectively, p=0.02). Interestingly, the exact opposite was true for patients with RA; patients on anti-TNF treatment had higher levels of IHH compared to patients not on such treatment (mean±SEM: 0.38±0.03 vs 0.27±0.02, respectively, p=0.01). In order to explore whether the differences in IHH levels found in AS patients have a functional effect, we assessed HH pathway activation in Saos2 cells following incubation with serum obtained form 2 AS patients prior to and 3 months following anti-TNF treatment. We found that the expression of both target genes (Ptch-1 and glypican 3) declined following anti-TNF treatment.
Conclusions IHH levels are increased in patients with AS and decrease following anti-TNF treatment; taking into account the critical role of this pathway in the bone forming process this finding may have pathogenic and clinical implications.
Disclosure of Interest None Declared