Factors involved in the cell density-dependent regulation of nuclear/cytoplasmic distribution of the 11.5-kDa Zn(2+)-binding protein (parathymosin-alpha) in rat hepatocytes

Abstract

Although the 11.5 kDa Zn(2+)-binding protein (ZnBP, parathymosin-alpha) possesses a functional bipartite nuclear localization signal it was found in most tissues in the cytoplasm. The cultivation of freshly isolated rat hepatocytes for 24 hours under standard conditions was associated with an almost complete translocation of ZnBP from the cytoplasm to the nuclei. Here we demonstrate, that this translocation is negatively correlated with cell density. The translocation of ZnBP to the nucleus can be inhibited or abolished by inhibitors of protein synthesis (cycloheximide) or transcription (actinomycin D). Moreover, cycloheximide can induce a relocation of ZnBP to the cytoplasm when applied after the appearance of ZnBP in the nuclei. DMSO, an inhibitor of dedifferentiation of cultured hepatocytes, abolishes also the translocation of ZnBP into the nucleus. Thinly seeded cells keep their ZnBP in the cytoplasm if they are co-cultured with plasma membranes from Morris MH7777 hepatoma cells or antibodies against E-cadherin indicating the involvement of cell adhesion proteins. We have enriched a protein from the cytosol of fresh hepatocytes which inhibits the translocation of ZnBP, but not that 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.