Factors involved in the cell density-dependent regulation of nuclear/cytoplasmic distribution of the 11.5-kDa Zn2+-binding protein (parathymosin-alpha) in rat hepatocytes

Although the 11.5 kDa Zn2+-binding protein (ZnBP, parathymosin-alpha) posse sses a functional bipartite nuclear localization signal it was found in mos t tissues in the cytoplasm, The cultivation of freshly isolated rat hepatoc ytes for 24 hours under standard conditions was associated with an almost c omplete translocation of ZnBP from the cytoplasm to the nuclei. Here we dem onstrate, that this translocation is negatively correlated with cell densit y. The translocation of ZnBP to the nucleus can be inhibited or abolished b y inhibitors of protein synthesis (cycloheximide) or transcription (actinom ycin D), Moreover, cycloheximide can induce a relocation of ZnBP to the cyt oplasm when applied after the appearance of ZnBP in the nuclei. DMSO, an in hibitor of dedifferentiation of cultured hepatocytes, abolishes also the tr anslocation of ZnBP into the nucleus. Thinly seeded cells keep their ZnBP i n the cytoplasm if they are co-cultured with plasma membranes from Morris M H7777 hepatoma cells or antibodies against E-cadherin indicating the involv ement of cell adhesion proteins. We have enriched a protein from the cytoso l of fresh hepatocytes which inhibits the translocation of ZnBP, but not th at of albumin-NLS into the nucleus in a permeabilized cell system. Such an activity could not be found in the cytoplasm of permanent cell lines which harbor ZnBP only in the nucleus. A model for the regulation of the nuclear import of ZnBP is proposed.