Targeted proteomics of serum IGF-I, -II, IGFBP-2, -3, -4, -5, -6 and ALS

OBJECTIVES: The insulin-like growth factors (IGFs) regulate growth in humans. IGF-I and IGF binding protein (IGFBP)-3 are biomarkers in children with growth disorders. We investigate a targeted proteomics method for absolute quantitation of eight IGF protein family members in human serum, including the peptide hormones IGF-I and -II, and the six binding proteins IGFBP-2, -3, -4, -5, -6 and acid labile subunit (ALS).

METHODS: Serum preparation was optimized for targeted proteomics of IGF related proteins on a clinical LC-MS/MS platform (UHPLC coupled with Triple-Q MS). We created quality controls, standards and internal standards and 289 serum samples from healthy children and adolescents were measured in ten batches over two months. The method was compared to WHO reference standards, clinical and research immunoassays, and relative proteomics profiling.

RESULTS: The sensitivity and reproducibility were sufficient for most but not all IGF protein family members. Targeted proteomics correlated well with clinical immunoassays for IGF-I (R2=0.88) and for IGFBP-3 (R2=0.46), (p<0.001). The correlation between targeted proteomics and non-clinical immunoassays for IGF-II, IGFBP-2, -4, -5, -6 and ALS varied between proteins.

CONCLUSIONS: We present a method for parallel quantification of IGF-I, IGFBP-3, 5 and ALS for clinical verification studies, whereas targeted proteomics of the five remaining IGF related proteins (IGF-II, IGFBP-2, -4, and -6) require further examination. The sensitivity of our new IGF-I method suggests a possible diagnostic role for targeted proteomics of IGF-I in the management of children with extremely low levels of circulating IGF-I.