Blood growth hormone-binding protein levels in premenopausal and postmenopausal women: roles of body weight and estrogen levels

Abstract

A substantial proportion of GH circulates bound to high affinity GH-binding protein (GHBP), which corresponds to the extracellular domain of the GH receptor. Current evidence indicates that nutritional status has an important role in regulating plasma GHBP levels in humans. In the present study the relationship among plasma GHBP levels, body composition [by bioelectrical impedance analysis (BIA) and dual energy x-ray absorptiometry (DEXA)] and serum estradiol (E(2)) was evaluated in premenopausal (n = 92) and postmenopausal (n = 118) healthy women with different body weight [three groups according to body mass index (BMI): normal, 18.5-24.99; overweight, 25-29.99; obese, 30-39.99 kg/m(2)]. Plasma GHBP levels were measured by high pressure liquid chromatography gel filtration. GH and insulin-like growth factor I levels were determined by immunoradiometric assay and RIA, respectively. GHBP levels were significantly higher in premenopausal women with BMI above 25 kg/m(2) (overweight, 3.789 +/- 0.306 nmol/L; obese, 4.372 +/- 0.431 nmol/L) than those observed in postmenopausal women (overweight, 1.425 +/- 0.09 nmol/L; obese, 1.506 +/- 0.177 nmol/L). No significant differences were found between normal weight premenopausal (1.741 +/- 0.104 nmol/L) and postmenopausal (1.524 +/- 0.202 nmol/L) women. In premenopausal women GHBP levels correlated positively with BMI (r = 0.675; P < 0.001), fat mass (FM; r = 0.782; P < 0.001; by BIA; r = 0.776; P < 0.001; by DEXA), truncal fat (TF; r = 0.682; P < 0.001), waist to hip circumference ratio (WHR; r = 0.551; P < 0.001), and E(2) (r = 0.298; P < 0.05), whereas no significant correlation was found in postmenopausal women between GHBP levels and BMI, FM, TF, WHR, or E(2). In normal weight pre- and postmenopausal women GHBP levels did not change between the ages of 20 and 69 yr. No statistically significant correlation was found between GHBP and age for all groups studied. Moreover, in two distinct subgroups of pre- and postmenopausal women, aged 40-49 yr, the direct relationship between GHBP levels and all indexes of adiposity were only observed in premenopausal women [BMI: r = 0.836; P < 0.001; FM: r = 0.745 (BIA) and r = 0.832 (DEXA); P < 0.001; TF: r = 0.782; P < 0.001; WHR: r = 0.551; P < 0.05], but not in postmenopausal women. In conclusion, the present data indicate a strong direct correlation between GHBP and body fat in premenopausal, but not in postmenopausal women, whereas they failed to detect a relationship between GHBP and age. Therefore, these results suggest that endogenous estrogen status may be an important determinant of the changes in GHBP levels in women with different body weights.