Insulin-like growth factor-I and insulin-like growth factor binding protein-3 serum levels in ankylosing spondylitis

Low bone mass, vertebral osteopenia and fractures have been described in patients with ankylosing spondylitis (AS), but the aetiology of this osteoporosis (OP) remains unknown. Insulin-like growth factor-I (IGF-I), a bone-promoting peptide, may be considered as reflecting osteoblast function as well as its main binding protein, insulin-like growth factor binding protein-3 (IGFBP-3). Both were found to be decreased in post-menopausal women and male patients with idiopathic OP. In this study, we aimed to measure the circulating IGF-I and IGFBP-3 in AS patients.

Thirty-three AS patients were compared to 23 healthy controls. Bone mineral density (dual X-ray absorptiometry) was measured at the spine and the femoral neck. We determined the serum levels of growth hormone (GH), insulin, glycaemia, and the IGF-I and IGFBP-3 serum concentrations.

A lowered lumbar spine bone mineral density was found in the AS group (AS: 0.946 g/cm2, controls: 1.02 g/cm2; P = 0.05). AS patients had a higher glycaemia than controls, but results were in the normal range. There were no significant differences in the mean values for GH and insulin. Mean IGF-I serum levels were 218.3 ng/ml (+/-72.4) in patients and 212.1 (+/-71.1) in controls (P = 0.75). The serum concentrations of IGFBP-3 were significantly lower in AS (3.29+/-0.6 microg/ml) than in healthy subjects (3.63+/-0.6 microg/ml; P = 0.05). There was a negative correlation between the serum IGFBP-3 concentration and erythrocyte sedimentation rate (r = -0.39; P = 0.025).

Since IGFBP-3 is an important cofactor for IGF-I and modulates its bioavailability and activity in bone, these data suggest that osteoblast cell function could be impaired in AS. Inflammation could play a role in this IGFBP-3/IGF-I axis involvement. However, further studies are warranted to determine the role of the other growth factors and their binding proteins in the OP of AS.