Chromatin-associated sphingomyelin: metabolism in relation to cell function.

After the first histochemical demonstration by Chayen and Gahan of the presence of phospholipids and especially of sphingomyelin in chromatin, this became the object of long debate and of contradictory results. The general conclusion was that the presence of phospholipids may due to contamination during the isolation of chromatin. More recently the existence of a phospholipid chromatin fraction was confirmed by demonstrating that isolated hepatocyte nuclei, labelled by saturated and unsaturated radioiodination method, showed the presence of radioactivity only in the membrane and not in the isolated chromatin. The phospholipid composition showed an enrichment in sphingomyelin which increased during hepatocyte maturation or erythroleukemic cell differentiation induced by DMSO. A decrease in sphingomyelin was observed at the beginning of the S-phase in regenerating liver or in cultured proliferating cells. These changes were due to the presence of sphingomyelinase and sphingomyelin synthase in the chromatin, the activity of which paralleled the variation in sphingomyelin content. The sphingomyelin was co-localized with RNA as shown by biochemical and electron microscopy methods. Using bromo-uridine it was demonstrated that labelled RNA and sphingomyelin were present in actively transcribing nuclear regions. Isolated nuclear complexes after DNase and RNase digestion contained not only protein, but also RNA and sphingomyelin. After hydrolysis of sphingomyelin the RNAse-resistant RNA becomes RNAse sensitive. It can therefore be concluded that sphingomyelin and the related enzymes are present in the chromatin; sphingomyelin may have a role in RNA transcription protecting RNA by RNAse digestion before its transfer to the cytoplasm.