High‐fat diet increases IGF‐I activity in skeletal growth plates before the onset of obesity

Childhood obesity can be one of the greatest perils to the growing skeleton because its effects are often hidden until a child is disabled by pain. Obesity is characterized by accelerated linear growth and skeletal maturation, which often leads to slipped epiphyses and fractures. It is unclear what causes this growth acceleration. IGF‐I, the major growth‐promoting hormone, is paradoxically normal in obesity. Equally puzzling, our lab found that a high‐fat diet accelerates bone lengthening in mice before they become obese. Our goal here was to determine whether a high‐fat diet alters local IGF‐I signaling and structure of the growth plate. We tested the HYPOTHESIS: that a high‐fat diet increases IGF‐I activity in growth plates before the onset of obesity. If diet‐induced changes in IGF‐I signaling underlie the growth acceleration, we expected to find dysregulated IGF‐I activity and concomitant changes in growth plate structure without changes in body mass. METHODS 3‐week old C57BL/6 mice (N=8/diet/group) were fed control (10% kcal fat) or high‐fat (60% kcal fat) diets for 14 days. Immunostaining was performed on proximal tibial histology sections using antibodies against the phosphorylated (activated) IGF‐I receptor and IGFBP‐4. Growth plate height (total and individual zones), protein expression (percent cells positively stained) and columnar orientation (angle of a line drawn through a column of at least 5 proliferative cells relative to long axis of the bone) were quantified in ImageJ. Statistical significance (p<0.05) was determined in SPSS using t‐tests. RESULTS Tibial elongation rate was over 10% greater in the high‐fat diet group (t=2.1, p<0.05) with no difference in body mass (t=0.5, p=0.62). Growth plate height was also greater in high‐fat diet mice (t=8.6, p<0.001), with marked increases in height of hypertrophic (t=7.3, p<0.001) and proliferative (t=6.5, p<0.001) zones. Columnar orientation significantly differed between diets (t=7.1, p<0.001). High‐fat diet mice exhibited columns that deviated nearly 8 degrees from the longitudinal axis of the bone, while columns in the control mice aligned within 2 degrees of the long axis. Immunostaining revealed a significant increase in IGF‐I activity in high‐fat diet relative to control mice, assessed by the percentage of chondrocytes expressing the phosphorylated IGF‐I receptor (t=2.4, p<0.05), but a decrease in IGFBP‐4 expression (t=5.9, p<0.001). DISCUSSION Our results support the hypothesis that a high‐fat diet increases IGF‐I activity in growth plates before the onset of obesity. Key findings that suggest IGF‐I signaling is dysregulated by a high‐fat diet include: increased IGF‐I receptor activation, decreased inhibitory binding proteins, increased growth plate height and decreased columnar organization. SIGNIFICANCE These results highlight the fundamental role of diet in the earliest stages of developing obesity‐related skeletal complications and are relevant for establishing early interventions to prevent childhood obesity and its damaging effects. Support or Funding Information Supported by the National Institute of General Medical Sciences (1P20GM121299‐01) and National Institute of Arthritis and Musculoskeletal and Skin Diseases of the NIH (1R15AR067451‐01).