Subcellular localization of metallothionein IIA in human bladder tumor cells using a novel epitope-specific antiserum

Human metallothioneins (hMTs) are a family of highly homologous proteins thought to be critical in cellular protection against toxins. The comparative function of individual isoforms is, however, obscure. Antibodies to individual MT isoforms might help clarify this issue but their generation has been challenging. We now describe a strategy that has successfully produced an epitope-specific antiserum to a major human isoform, hMT IIA. The immunogen, a conjugated tridecyl amino acid synthetic peptide unique to residues 8-20 in hMT IIA (AAGDSCTCAGSCK), yielded an antiserum (B2) that reacted specifically with hMT IIA in a concentration- and titer-dependent manner and showed no reactivity with human or rabbit liver MT I or horse MTs. This antiserum recognized rabbit liver MT II and, surprisingly, also reacted weakly with chicken MT. MT amino acid sequence comparisons and peptide blocking studies suggested aspartate-11 and threonine-14 are important antigenic determinants for B2. Using confocal immunofluorescence microscopy and B2 antiserum, we observed nuclear localization of hMT IIA with human bladder T24 tumor cells in exponential growth but more cytoplasmic localization at confluence. These results suggest the subcellular location of hMT IIA is a function of cell density in T24 bladder carcinoma cells. The general approach of epitope-specific antibodies may be useful for the generation of antibodies to other MT isoforms and for studying the role of individual MT isoforms in biology and toxicology.