Bovine lactoferrin binds to insulin-like growth factor-binding protein-3

Area of Science:

• Biochemistry
• Cell Biology
• Endocrinology

Background:

• Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) exhibits IGF-independent actions mediated by cell membrane-bound proteins.
• Identifying these binding proteins is crucial for understanding IGFBP-3's diverse cellular functions.

Purpose of the Study:

• To identify mammary membrane proteins that bind IGFBP-3.
• To characterize the interaction between IGFBP-3 and bovine lactoferrin (bLf).

Main Methods:

• Western ligand blotting to detect proteins binding 125I-labeled rhIGFBP-3.
• Immunoprecipitation to identify the >70 kDa binding protein.
• Dot blot assays to analyze competitive binding interactions.
• In vitro studies using bovine primary mammary epithelial cell culture.

Main Results:

• Western ligand blotting identified several mammary membrane proteins that bind 125I-labeled rhIGFBP-3.
• Immunoprecipitation identified bovine lactoferrin (bLf) as the >70 kDa protein binding IGFBP-3.
• Specific binding was observed between rhIGFBP-3 and bLf (both apo- and holo-forms), but not bovine transferrin (bTf).
• bLf competed with [125I]rhIGF-II for binding to rhIGFBP-3 and displaced bound [125I]rhIGF-II from rhIGFBP-3.
• All-trans-retinoic acid stimulated the secretion of IGFBP-3 and bLf in mammary epithelial cells.

Conclusions:

• Bovine lactoferrin (bLf) is a novel binding protein for IGFBP-3 on mammary membranes.
• bLf may play a role in modulating IGFBP-3:IGF interactions, potentially dissociating IGF from IGFBP-3 at high concentrations.
• This interaction reveals a new function for bLf in the mammary gland context.