DIVERSITY OF METALLOTHIONEIN CONTENT AND SUBCELLULAR-LOCALIZATION IN THE NATIONAL-CANCER-INSTITUTE TUMOR PANEL

Metallothioneins (MTs) are major thiol-containing intracellular protei ns that bind metals, are induced by stress, and have been implicated i n resistance to drugs and heavy metals. Pur pose: To examine the hypot hesis that the protective functionality of MT may be dictated by its s ubcellular localization. Methods: We analyzed the basal MT content in 53 adherent cell lines of the National Cancer Institute (NCI) tumor pa nel and quantified the nuclear/cytoplasmic distribution of MT using co nfocal laser scanning microscopy and a recently described immunofluore scence-based algorithm. Results: Among these cell types we found a 400 -fold range in the basal MT levels and a tenfold range in the ratio of the nuclear to cytoplasmic MT immunostaining that was independent of basal MT content. Total MT levels and nuclear/cytoplasmic distribution were independent of total glutathione content, suggesting autonomous regulation of these protective protein and nonprotein thiol pools. App roximately 50% (29/53) of the cell lines had a greater nuclear than cy toplasmic MT density and were defined as having a karyophilic phenotyp e. Tissue specificity of MT localization was seen with breast cancer c ell lines, which were cytoplasmophilic, whereas prostate-derived cells were karyophilic. Among the 25000 unrestricted compounds in the NCI d atabase, we detected a correlation between total basal MT levels and r esistance to CdCl2, four Pt- and two Cu-containing compounds. High nuc lear/cytoplasmic MT values correlated with resistance to six Cu-, six Pb-, and one Zn-containing compounds. Conclusions: These results demon strated significant diversity in MT content and subcellular localizati on in human tumor cells. Moreover, both basal MT levels and subcellula r distribution appeared to be determinants of cellular responsiveness to metal-containing compounds.