Insulin-like growth factor (IGF)-I, IGF binding protein-3, and breast cancer risk: eight years on.

Insulin-like growth factor (IGF)-I, and its main binding protein, IGFBP-3, are multi-functional regulatory peptides of cell growth and survival, attributes important for tumourigenesis. Following seminal work published in 1998, it has been hypothesised that circulating concentrations of these growth factors may be associated with cancer risk. Systematic reviews have reported that high normal range circulating levels of total IGF-I predict for pre- but not post-menopausal breast cancer. By contrast, associations with circulating IGFBP-3 have been inconsistent. A cumulative meta-analysis demonstrates that earlier reported positive associations between IGFBP-3 and pre-menopausal breast cancer risk now seem less clear as large-size cohorts are published. The reasons are complex and include differences in study design, lack of standardisation between assays, and variations in IGFBP-3 proteolytic activity--these are discussed in this commentary.