ABERRANT RETENTION OF TYROSINASE IN THE ENDOPLASMIC-RETICULUM MEDIATES ACCELERATED DEGRADATION OF THE ENZYME AND CONTRIBUTES TO THE DEDIFFERENTIATED PHENOTYPE OF AMELANOTIC MELANOMA-CELLS

The loss of tyrosinase, the key enzyme in melanin synthesis, has been implicated in the dedifferentiation of malignant melanocytes, The pres ence of tyrosinase transcripts and antigenic peptides in melanoma tumo rs prompted us to investigate whether the basis for the loss of the en zyme nas proteolytic degradation, Toward this aim, we followed the kin etics of synthesis, degradation, processing, chaperone binding, inhibi tor sensitivity, and subcellular localization of tyrosinase in normal and malignant melanocytes. We found that, in amelanotic melanoma cell lines, tyrosinase failed to reach the melanosome, the organelle for me lanin synthesis, because it was retained in the endoplasmic reticulum (ER) and then degraded, Tyrosinase appeared mostly as a 70-kDa core-gl ycosylated, endoglycosidase H-sensitive, immature form bound to the ER chaperone calnexin and had a life-span of only 25% of normal, Maturat ion and transit from the ER to the Golgi compartment was facilitated b y lowering the temperature of incubation to 31 degrees C, Several prot easome inhibitors caused the accumulation of an approximate to 60-kDa tyrosinase doublet that was more prominent in malignant than in normal melanocytes and promoted, to various degrees, the maturation of tyros inase in melanoma cells and the translocation of the enzyme to melanos omes, The appearance of ubiquitinated tyrosinase after treatment of no rmal melanocytes with N-acetyl-L-leucinyl-L-leucinal-L-norleucinal rei nforced our notion that some tyrosinase is normally degraded by protea somes, Proteolysis of tyrosinase by proteasomes is consistent with the production of antigenic tyrosinase peptides that are presented to the immune system by major histocompatibility complex class I molecules.