The intranuclear amount of phospholipase C β₁ decreases following cell differentiation in Friend cells, whereas γ₁ 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 β₁ 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 β₁ and γ₁ isoforms are both present within the nucleus, whereas mainly the γ₁ isoform is detected in the cytoplasm. The small amount of cytoplasmic PLC β₁ 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 β₁ amount in the nucleus and with an almost complete disappearance of that isoform in the cytoplasm, whereas the PLC γ₁ isoform is unaffected. The two PLC isoforms, moreover, appear to be differently associated with the nuclear components, PLC β₁ being steadily bound to the inner nuclear matrix, whereas PLC γ₁ is almost completely soluble.