Lipid-dependent nuclear signalling: Morphological and functional features

Abstract

Enzymes involved in lipid metabolism exist within the nucleus and are responsive to external stimuli. In particular, the kinases which phosphorylate phosphatidylinositol and phosphatidylinositol-4-monophosphate have been demonstrated in nuclei of both undifferentiated and differentiated Friend cells and of quiescent Swiss 3T3 cells as well as of those exposed to insulin-like growth factor I. Besides the lipid kinases, also the phosphoinositidases C (PIC) are active inside the nucleus. In Swiss 3T3 cells the nuclear PIC β1 is activated and its activation by IGF-I temporally precedes the translocation to the nucleus of protein kinase C. In Friend cell nuclei, on the other hand, when erythroid differentiation is induced, the PIC β1 activity is reduced. Another aspect of the nuclear signalling transduction system which appears quite interesting is its actual localization at subcellular level. By using electron microscope immunogold labelling, the nuclear PIC isoforms (the β1 isoform in Swiss 3T3 cells, the β1 and γ1 in Friend cells) are localized mainly in the interchromatin domains. This localization has been further confirmed on in situ matrix preparations of 3T3 cells in which PIC β1 is associated with the inner nuclear matrix but not with the nuclear pore-lamina complex. Colocalization experiments indicate that nuclear PIC β1 is present in sites in which both nuclear phospholipids and PKC can be detected, while the cytoplasmic PIC γ1 can be identified in close association with cytoskeletal filaments identified by anti-actin antibodies. The precise localization of the different PIC isoforms strongly indicates that the signal transduction system operating at the nuclear level may be part of a cross-talk between the cytoplasm and the nucleus controlling either cell proliferation or differentiation.