LACTADHERIN (FORMERLY BA46), A MEMBRANE-ASSOCIATED GLYCOPROTEIN EXPRESSED IN HUMAN-MILK AND BREAST CARCINOMAS, PROMOTES ARG-GLY-ASP (RGD)-DEPENDENT CELL-ADHESION

Lactadherin, a major glycoprotein of the human milk fat globule membra ne, is abundant in human breast milk and expressed in human breast car cinomas, Previously, we have shown that the mature protein, formerly k nown as BA46, has three domains: an epidermal growth factor (EGF)-like domain containing an Arg-Gly-Asp (RGD) cell adhesion sequence and C1 and C2 domains similar to those found in coagulation factors V and VII I, An alignment of lactadherin with its bovine (MGP57/53) and murine ( MFG-E8) homologs shows that the RGD sequence has been conserved during evolution, suggesting that the RGD sequence is not fortuitous, We dem onstrate that lactadherin purified using Triton X-114 phase partitioni ng promotes RGD-dependent cell attachment of green monkey kidney cells (MA104), mouse fibroblast cells (3T3-L1), and breast carcinoma cells (ELL-G). A lactadherin-specific monoclonal antibody, Mc3, inhibits att achment to purified lactadherin, suggesting that contaminants in the p urification are not responsible for binding, In addition, the anti-int egrin alpha(v) beta(3) monoclonal antibody LM609 inhibits cell attachm ent of MA104 cells to lactadherin. These results demonstrate that lact adherin promotes RGD-dependent cell adhesion via integrins, Denaturati on of lactadherin with heat and reducing conditions diminished cell at tachment, suggesting that optimal cell attachment to RGD is dependent on the structural presentation of the sequence.