Nuclear lipid-dependent signal transduction in human osteosarcoma cells

Abstract

The enzymes and substrates involved in phosphoinositide signal transduction which have been detected in the nucleus of several cell types have been demonstrated to be responsive to agonists. The complexity of this aspect of inositide function has been previously analyzed in some cell models characterized by a mitogenic or differentiating response to specific factors. An interesting experimental model is represented by human derived osteosarcoma Saos-2 cells, characterized by the expression of high affinity receptors for interleukin 1α (IL-1α), which is one of the most potent stimulators of bone resorption. In particular, we investigated the earliest intracellular events following the binding of IL-1α to its receptor, involving the inositide signal transduction pathway. Saos-2 cells present a partitioning of the phosphoi-nositidase (PLC) isoforms; in fact, the nucleus contains both PLC β 1 and γ 1, while the cytoplasm contains almost exclusively the γ 1 isoform. IL-1α evokes a rapid and transient increase of the PLC β 1 activity in the nucleus, which causes the hydrolysis of phosphatidylinositol mono- and bis-phosphate. In response to IL-1α, not only the canonical inositol lipid pathway appears to be involved; also the 3′-phosphorylated lipids generated by phosphatidylinositol 3-kinase (PI 3-K), which may act as second messengers, appear to be affected. In fact, Saos-2 cells present a nuclear PI 3-K activity which can be enhanced by the IL-1α treatment. Among the possible targets of the second messengers released by the nuclear PLC β 1 activation, we found that some protein kinase C isoforms, namely the ϵ and ζ, which are present within the nucleus, are activated after IL-1α exposure. These activated PKC isoforms, in turn, could modulate the activity of the transcription factor NFkB, which, 5 min after IL-lα treatment, has already translocated to the nucleus and bound to DNA to promote gene activation. The actual role of the inositide pathway in the Saos-2 cell function has also been investigated by utilizing cell clones transfected with the mouse sequence of the PLC β 1.