Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are key factors in the control of somatic growth. Recent work revealed a critical role for the transcription factor STAT5b in GH stimulated IGF-I gene expression. In obesity, the normal regulation of the GH/IGF-I axis is disturbed, with normal levels of circulating IGF-I despite blunted GH secretion. We hypothesized that leptin or other hormonal regulators of energy homeostasis, which can activate Stat5b-regulated gene expression or exert an effect on GH secretion, might be able to substitute for GH in terms of IGF-I synthesis. Thus, the aim of this study was to identify potential regulators of IGF-I serum levels in obesity with a particular focus on the interaction of leptin and IGF-I. In a cross-sectional study, we measured hormonal and auxiological parameters in 99 obese children who were referred to our obesity outpatient clinic and analysed correlations between unadjusted hormone levels and between hormone concentrations expressed as SDS values, adjusted for sex, age, and/or puberty and BMI. Serum concentrations of IGF-I correlated highly significant with IGFBP-3, leptin, fasting ghrelin and insulin (p<0.001). However, when expressing hormone levels as SDS values, only leptin SDS and IGFBP-3 SDS correlated significantly with IGF-I SDS (p<0.01). This correlation between leptin SDS and IGF-I SDS was more pronounced in prepubertal and in male subjects, with increasing IGF-I SDS values paralleling an increase in leptin SDS tertiles in prepubertal subjects. In linear regression analyses, leptin SDS, IGFBP-3 SDS and BMI SDS contributed significantly to the variation of IGF-I SDS values, and explained 53.3% of the variability of IGF-I SDS levels in male subjects.
Summary and conclusions: In summary, we demonstrated a strong correlation of age and sex-adjusted standard deviation scores (SDS) for IGF-I and leptin in obese children, which is modulated by sex, pubertal stage and body weight. Whether this association results from a direct induction of hepatic IGF-I gene expression by leptin or reflects a more complex interrelationship between IGF-I and obesity-related factors should be subject of future research.