Reduced free IGF-I and increased IGFBP-3 proteolysis in Turner syndrome: modulation by female sex steroids

Abstract

The bioactivity of the growth hormone-insulin-like growth factor (IGF) system is reduced in Turner syndrome and may explain the reduction seen in final height. We compared levels of free and total IGF-I, immunoreactive and Western ligand blot IGF-binding protein (IGFBP)-3, and IGFBP-3 proteolysis in women with Turner syndrome ( n = 23) before (TB) and during 6 mo treatment with 17β-estradiol and norethisterone. An age-matched group of controls ( n = 24) was included. Total IGF-I and immunoreactive levels of IGFBP-3 were comparable in TB and controls, whereas free IGF-I ( P = 0.02) in TB was less than in controls. Western ligand blotting (WLB)-IGFBP-3 was significantly lower in TB than in controls ( P = 0.0005). Accordingly, IGFBP-3 proteolysis was greater in Turner syndrome ( P = 0.001). Female sex steroid treatment increased WLB-IGFBP-3 ( P = 0.0005), whereas immunoreactive IGFBP-3 and IGFBP-3 proteolysis were normalized ( P = 0.004). Free IGF-I remained unchanged ( P = 0.8), with a tendency toward a decrease in total IGF-I ( P = 0.1). In conclusion, despite normal total IGF-I and immunoreactive IGFBP-3, free serum IGF-I is less and IGFBP-3 proteolysis is greater in Turner syndrome than in controls. During sex steroid treatment, IGFBP-3 proteolysis normalized, without any change in free IGF-I.