A new M-r 55,000 surface protein implicated in melanoma progression: Association with a metastatic phenotype

Emergence of the invasive phenotype is a key event in the progression of hu man melanoma from benign proliferative lesions to malignant lesions. Recent ly we successfully selected in vivo from a poorly metastatic M(4)Beu. human melanoma cell line two variants (7GP and T1P26) that generate a higher fre quency of spontaneous metastases to the lungs into immune-suppressed neonat al rats. Both cell lines showed no significant differences in the integrin profile of the subunits analyzed except for beta (3), which was reduced to a background level in metastatic variants. To investigate how these variant sublines of human melanomas manage to sustain growth in the absence of alp ha (v)beta (3), a subtractive immunization approach was used to elicit host antibody response against cell surface proteins expressed on metastatic va riants. In this study, a new monoclonal antibody (MoAb), LY1, that is highl y specific for the 7GP and T1P26 variants, was isolated. LY1 identifies a m embrane protein of M-r 55,000 on melanoma variants with epitopes that were resistant to sugar-cleaving enzymes. Immunostaining cells from variants by LY1 showed that staining is distributed to the cell periphery with high lab eling intensity at the cell-to-cell contact points. This MoAb significantly inhibited invasion of metastatic variants through a reconstituted basement membrane (Matrigel) in vitro. Moreover, tumor growth of melanoma variants was dramatically affected in vivo with this MoAb. In vitro studies indicate that the LY1 MoAb does not inhibit chemotactic migration of the metastatic variants, the adhesion of tumor cells to vitronectin, collagen IV, fibrone ctin, and laminin, or cell proliferation. Expression of this antigen is hig h in human striated muscle, heart, spleen, brain, and lung and absent in ki dney, liver, and pancreas. Using 59 fixed, paraffin-embedded archival tissu es of human melanomas and nevi, LY1-reactive cells were not observed in mel anocytes, nevi, or radial growth phase primary melanomas. In sharp contrast , LY1 selectively stained melanocytes derived from the vertical growth phas e of many primary melanomas and metastatic melanomas. These results provide evidence that the BI, 55,000 protein expressed by selected variants with i ncreased metastatic properties in vivo plays a functionally important role in determining metastasis. This molecule may represent a new metastatic ris k marker in human melanoma and may be of biological importance in the ident ification of fatal metastatic subpopulations that have acquired competence for metastasis production.