Nuclear phospholipids in human lymphocytes activated by phytohemagglutinin

Using a specific ultracytochemical technique, the labelling with phospholipase A2-gold complex, we have followed nuclear phospholipids (PL) along the G1 phase in human lymphocytes activated by PHA. Our data point out two main results relating nuclear PL to the transcriptional activity, characteristic of the G1 phase, during which many different molecules necessary both for progression through G1 and for the start of S phase are synthesized. PL quantitative changes parallel those of hnRNPs and snRNPs, which are markers of the levels of transcriptional activity and processing. We found that nuclei of G0 lymphocytes, with a very low transcription level, are poor of PL as well as of RNPs. The amount of PL increases in activated lymphocytes, along all G1, until the beginning of S phase. At the same time, hnRNPs and snRNPs strongly increase and maintain higher levels than in control cells, till the beginning of S phase. PL are localized on nuclear structures where also RNPs involved in transcription and splicing, are located, i. e. perichromatin fibrils, interchromatin granules and the dense fibrillar component of the nucleolus. Since it is known that during S phase nuclear PL decrease, while both the enzyme activities related to their breakdown and their hydrolysis products increase, PL seem to be involved in the generation of signal molecules triggering DNA replication. We suggest that PL in the nucleus can be involved in multiple functions, depending on the phase of the cell cycle.