Insulin-like growth factor-I, IGF binding protein-3, and breast cancer in young women: a comparison of risk estimates using different peptide assays.

Circulating insulin-like growth factor-I (IGF-I) and its major binding protein IGF binding protein-3 (IGFBP-3) have been associated with increased risk of premenopausal breast cancer, although risk estimates varied broadly. An extension of a case-control study (138 cases, 259 matched controls) on IGF-I and breast cancer in premenopausal women nested in the New York University Women's Health Study cohort offered the opportunity to address the hypothesis that such variability may have been the result of variations in the ability of different IGFBP-3 assays to specifically measure intact/functional forms of the protein. IGF-I and IGFBP-3 had originally been measured using in-house RIAs. These measurements were repeated using commercially available ELISAs [Diagnostic System Laboratories (DSL), Webster, Texas], and a third ELISA with greater specificity for active forms for IGFBP-3. Pearson's correlations between IGF-I concentrations in the original study and DSL ELISA were very high [r = 0.92; 95% CI, 0.90-0.94]. Correlations with DSL ELISA were much lower for IGFBP-3 (r = 0.58; 0.49-0.66) and even lower still with the assay for functional IGFBP-3 (r = 0.33; 0.20-0.44). IGF-I and IGFBP-3 measurements by the DSL ELISA methods showed statistically significant relationships with risk. The odds ratios (OR) for top versus bottom quartiles were 1.93 (1.00-3.72; P = 0.02) and 2.03 (1.09-3.76; P = 0.02), respectively, in agreement with the original observations. In contrast, measurements of functional IGFBP-3 tended to be unrelated to risk [ORs for the top versus bottom quartile, 0.97 (0.44-2.11)]. The association with IGF-I became substantially weaker and lost statistical significance after adjustment for IGFBP-3 using DSL ELISA, but became considerably stronger when adjusting for the functional IGFBP-3 measurements [OR = 2.43 (1.21-4.90); P = 0.005], or when considering the molar ratio of IGF-I to IGFBP-3 [OR = 2.37 (1.13-5.00); P = 0.02]. These results are consistent with an association of breast cancer risk in young women with elevated IGF-I and IGFBP-3, and show that for IGFBP-3, the strength of such an association could vary substantially depending on the assay used.