Protectin (CD59) is a
glycosylphosphatidylinositol (GPI)-anchored cell membrane
glycoprotein, broadly expressed on melanocytic cells, that represents the main restriction factor of
complement (C)-mediated lysis of human
melanoma cells. Levels of CD59 expression may impair the clinical efficacy of C-activating
monoclonal antibodies (mAb); thus, we investigated the molecular mechanisms underlying the lack of CD59 expression in selected
melanoma cells. Serological and biochemical analyses showed that MeWo
melanoma cells expressed CD59 neither at cell surface nor at cytoplasmic levels; however, no critical mutations were identified in their CD59
mRNA. Consistently, MeWo CD59
cDNA (MeWo-CD59) was appropriately translated when transfected into the CD59-positive Mel 100
melanoma cells, and into the CD59-negative Nalm-6 pre-B
leukemia cells that acquired resistance to C. In contrast, transfection of MeWo cells with CD59
cDNA from Mel 275
melanoma cells did not induce CD59 expression; however, their transfection with the CD59-TM chimeric construct, obtained by replacing the GPI-anchoring signal of MeWo-CD59 with the transmembrane tail of the human
low-density lipoprotein receptor, induced the expression of a C-protective transmembrane form of CD59. These data, together with the absent expression of additional
GPI-anchored proteins (i.e., CD55), suggest that defects in the biosynthesis and/or processing of
GPI-anchored proteins underlie the lack of CD59 expression in MeWo cells. Further unveiling of the molecular mechanism that turns off CD59 expression in human
melanoma cells will help to set-up more effective therapeutic strategies utilizing C-activating mAb in
melanoma patients.