The intranuclear amount of phospholipase C beta 1 decreases following cell differentiation in Friend cells, whereas gamma 1 isoform is not affected.

The existence of a signal transduction system in the nucleus, based on polyphosphoinositide breakdown mediated by specific phosphoinositidases (PLC), has been widely documented. In different cell systems, nuclear PLCs can be modulated, in response to agonists, either by enhancing or by down-regulating their activity, thus leading to DNA replication or to cell differentiation. Friend cells, induced to erythroid differentiation by dimethyl sulfoxide (DMSO), show a down-regulation of PLC beta 1 isoform, as indicated by the reduction of the transcription of its mRNA and of the in vitro synthesis of its translation product. The intracellular localization and the amount of different PLC isoforms have been evaluated by electron microscope immunocytochemistry. In untreated Friend cells, PLC beta 1 and gamma 1 isoforms are both present within the nucleus, whereas mainly the gamma 1 isoform is detected in the cytoplasm. The small amount of cytoplasmic PLC beta 1 is probably representative only of the newly synthesized enzyme. Quantitative immunolabeling analyses demonstrate that erythroid differentiation is associated with a significant decrease of the PLC beta 1 amount in the nucleus and with an almost complete disappearance of that isoform in the cytoplasm, whereas the PLC gamma 1 isoform is unaffected. The two PLC isoforms, moreover, appear to be differently associated with the nuclear components, PLC beta 1 being steadily bound to the inner nuclear matrix, whereas PLC gamma 1 is almost completely soluble.