Partition of insulin-like growth factor (IGF)-binding sites between the IGF-I and IGF-II receptors and IGF-binding proteins in the human kidney

Quantitative ligand binding autoradiography and in situ hybridization were employed to analyze [¹²⁵I]insulin-like growth factor-I ([¹²⁵I]IGF-I) and [¹²⁵I]IGF-II-binding sites in human kidney sections. Binding sites for both ligands were concentrated in the inner medulla and glomeruli, with low levels present in the tubulo-interstitial cortex. Competition with cold IGF- I, IGF-II, and insulin was used to determine nonspecific binding and differentiate binding of ligands to the IGF-I and IGF-II receptors and IGF- binding proteins (IGFBPs). Nonspecific binding was less than 20% of the total for both ligands. Insulin (10^-5 mol/L), which binds to the IGF-I receptor, but not to the IGF-II receptor or IGFBPs, displaced 39 ± 8% of [¹²⁵I]IGF- I binding in glomeruli, 60 ± 7% in the tubulo-interstitial cortex, and 32 ± 7% in the medulla. Insulin produced no detectable decrease in [¹²⁵I]IGF- II binding in any region. IGF-I (10^-8 mol/L), which binds strongly to IGFBPs, but not appreciably to the IGF-II receptor, produced reductions of 46 ± 9%, 35 ± 8%, and 39 ± 12% in [¹²⁵I]IGF-II binding in glomeruli, tubulo-interstitial cortex, and medulla, respectively. In situ hybridization showed that IGFBP-1-5 mRNAs were all expressed in glomeruli. IGFBP-2 mRNA was abundant in medullary collecting duct epithelium, whereas IGFBP-3, -4, and - 5 mRNAs were localized in interstitial and vascular cells throughout the kidney. IGF-I and -II receptor mRNAs were widely distributed in renal epithelium. The abundance of local IGFBP gene expression was positively correlated with insulin-nondisplaceable IGF binding in specific kidney regions. In summary, [¹²⁵I]IGF-I binding appears to be partitioned largely to IGFBPs in glomeruli and largely to the IGF-I receptor in the tubulo- interstitial cortex, with binding in the medulla more evenly divided. The proportion and regional distribution of [¹²⁵I]IGF-II binding to IGFBPs are similar, but the balance appears to be primarily associated with the IGF-II, rather than the IGF-I, receptor. Finally, this study shows that [¹²⁵I]IGF binding autoradiography combined with in situ hybridization can be used to localize and potentially quantitate expression of IGFBPs in tissue sections.