Nuclear co-localization of prolactin and the prolactin receptor in rat Nb2 node lymphoma cells

Previous studies have indicated that prolactin (PRL) interacts with specific, high affinity, immunoreactive binding sites within isolated rat hepatocyte nuclei. Moreover, endogenous PRL appears to be bound to this site. However, it remained important to demonstrate nuclear PRL receptors and hormonal translocation in an intact cell system. Therefore, we sought nuclear translocation of PRL and its receptor in the nucleus of PRL-dependent Nb2 node lymphoma cells. Utilizing immunofluorescence (IF) microscopy, growth-arrested cells were found to constitutively express the PRL receptor in the nucleus and in the membrane/cytosol compartments. Addition of PRL stimulated rapid hormone internalization followed by translocation to the nucleus within 6-12 hrs. The translocation of PRL was found to be reversible and dependent upon ATP. These results indicate that an early event coupled to the mitogenic action of PRL in Nb2 cells is hormone transport to the nucleus during the G1 and S phases of cell cycle. Once in the nucleus, PRL bound to its receptor may directly influence gene transcription.