INTERACTIONS OF CATHEPSIN-D AND INSULIN-LIKE GROWTH FACTOR-II (IGF-II) ON THE IGF-II MANNOSE-6-PHOSPHATE RECEPTOR IN HUMAN BREAST-CANCER CELLS AND POSSIBLE CONSEQUENCES ON MITOGENIC ACTIVITY OF IGF-II

The lysosomal enzyme cathepsin-D (cath-D) and insulin-like growth factor-ll (IGF-II), which share a common IGF-ll/mannose-6-phosphate (M6P) transmembrane receptor, are both synthesized and secreted by breast cancer cells, upon which they might exert an intracrine/autocrine control on proliferation. We have evaluated the binding of125I-immunopu- rified human cath-D in different breast cell membrane preparations. The concentration of high affinity M6P reversible binding sites (mean Kd, 0.85 nM) varied among the different breast cancer cells (00.82 pmol/mg membrane protein), but there was no correlation between the presence of steroid receptor and M6P-dependent binding. Cross-linking experiments with [125l]cath-D and [125l]IGF-II showed the formation of complexes with the 270,000 mol wt IGF- II/M6P receptor molecule which migrated, respectively, at 330,000 and 270,000 mol wt in 3-10% gradient sodium dodecyl sulfate-polyacrylamide gels. [125I]IGF-II cross-linking was increased by M6P (20% above control), whereas cath-D strongly inhibited IGF-II interaction by 80%. Conversely, IGF-II reduced [125l]cath-D cross-linking by 55%. Direct ligand binding on receptors transferred onto nitrocellulose sheets by Western blotting confirmed the interaction of both ligands on the same receptor molecule. By studying IGF-II’s growth-promoting activity in these cells in a wide range of concentrations, we show that IGF-II triggers its mitogenic response via. © 1990 by The Endocrine Society.