Non-receptor mediated, post-transcriptional regulation of insulin-like growth factor binding protein (IGFBP)-3 in Hs578T human breast cancer cells

Hs578T human breast cancer cells secrete insulin-like growth factor binding protein 3 (IGFBP-3) as the major BP species. In addition, cell surface-associated IGFBP-3 is demonstrable by the use of cell monolayer affinity cross-linking or immunoperoxidase staining of the cell surface with a specific polyclonal anti-human IGFBP-3 antibody (αIGFBP-3γ1). In this study, we have demonstrated that regulation of Hs578T IGFBP-3 by IGF peptides is specific, non-receptor mediated, and post-translational by showing: 1) dose-dependent increase of IGFBP-3 in conditioned media (CM) following addition of IGF-I and -II (maximum 13 fold increase at 100 ng/ml), but not by insulin up to 1 mg/ml; 2) no change in CM IGFBP-3 level by [Gln³,Ala⁴,Tyr¹⁵,Leu¹⁶]IGF-I, which has decreased affinity for IGFBPs; 3) no change in IGFBP-3 mRNA following addition of IGFs; 4) release of cell surface-associated IGFBP-3 into CM by the addition of IGFs, but not by [Gln³,Ala⁴,Tyr¹⁵,Leu¹⁶]IGF-I. These studies demonstrate that IGF peptides regulate CM concentrations of IGFBP-3 through non-receptor mediated dissociation of cell surface-associated IGFBP-3.