CONTROL PATHWAYS OF THE 67 KDA LAMININ-BINDING PROTEIN - SURFACE EXPRESSION AND ACTIVITY OF A NEW LIGAND-BINDING DOMAIN

A number of papers have been published on the clinical correlation of the expression of the 67 kDa laminin binding protein (LBP) with the me tastatic potential of solid tumors. Both mRNA and protein expression l evels have been reported, but both the relationship between them and t he molecular nature of the 67 kDa surface product remain unclear. We h ave utilized a homotypic overexpression system to investigate the cell surface presentation of the 67 kDa LBP and the contribution of this p rotein to the invasive phenotype of cultured cell lines. We report her e that the cellular mRNA levels do not directly reflect the levels of the 67 kDa LBP observed on the cell surface in this overexpression sys tem. Methotrexate amplification of transfected plasmids expressing the 67 kDa LBP leads to an initial elevation of both the LBP mRNA and sur face protein levels. This is accompanied by an altered, more flattened , cell morphology. Later, apparent adaptation of the cells to methotre xate is accompanied by a down-regulation of the surface expression of the protein, mRNA levels, however, remain elevated. A nine amino acid sequence, CDPGYIGSR (peptide 11), within the beta chain of laminin 1 h as been identified as a probable binding domain for the 67 kDa LBP. Pr evious studies have identified a region of the 67 kDa LBP which may be involved in laminin interaction, although not necessarily via the pep tide 11 domain. We have identified a second site within the amino acid coding sequence of the 67 kDa LBP which also shows biological activit y both in vitro and in vivo. A peptide with this sequence, LBP residue s 205-229, binds laminin-1 in a peptide 11 inhibitable manner. The rec eptor-derived peptide modulates invasion of basement membrane matrix i n vitro and inhibits experimental lung colony formation when injected along with B16BL6 mouse melanoma cells. However, pretreatment of the m elanoma cells with the peptide enhances lung colony formation. Thus, t he interaction of the 67 kDa LBP with basement membrane matrix appears to involve a complex series of events including multiple adhesive sit es and tight regulation of cell surface expression.