A novel pathway of cell growth regulation mediated by a PLA₂α‐derived phosphoinositide metabolite

Abstract

The phosphoinositides have well-defined roles in the control of cellular functions, including cytoskeleton dynamics, membrane trafficking, and cell signaling. However, the interplay among the phosphoinositides and their diffusible derivatives that originate through phospholipase A2 action (the lysophosphoinositides and glycerophosphoinositols) remains to be fully elucidated. Here we demonstrate that in PCCl3 rat thyroid cells, the intracellular levels of glycerophosphoinositol are finely modulated by ATP and norepinephrine through the P2Y metabotropic and alpha-adrenergic receptors, respectively. The enzyme involved here is phospholipase A2 IValpha (PLA2 IValpha), which in these cells specifically hydrolyzes phosphatidylinositol, forming lysophosphatidylinositol, glycerophosphoinositol, and arachidonic acid. This receptor-mediated activation of PLA2 IValpha leads to stimulation of PCCl3 cell growth. The involvement of a PLA2 IValpha-mediated pathway is demonstrated by inhibition of the increase in intracellular glycerophosphoinositol levels and cell proliferation by specific inhibitors, RNA interference, and overexpression of the dominant-negative PLA2 IValpha(1-522). Modulation of PCCl3 cell growth is not seen with inhibitors of arachidonic acid metabolism. In conclusion, these data characterize glycerophosphoinositol as a mediator of the purinergic and adrenergic regulation of PCCl3 cell proliferation, defining a novel regulatory cascade specifically involving this soluble phosphoinositide derivative and widening the involvement of the phosphoinositides in the regulation of cell function.

Phospholipase A2IVα Regulates Phagocytosis Independent of Its Enzymatic Activity

“…Previous studies from our laboratory characterized PLA 2 IV␣ as the specific PLA 2 isoform that mediates GroPIns production from the membrane phosphatidylinositol, through the transient formation of lysophosphatidylinositol (lyso-PtdIns) (24). Thus, the selective activation of PLA 2 IV␣ by FcR-mediated phagocytosis can be further monitored through the cellular levels of GroPIns, as quantified by HPLC analysis after equilibrium labeling with myo- [ 3 H]inositol (see under "Experimental Procedures").…”

“…Serine phosphorylation on PLA 2 IV␣ (Ser-505 or Ser-727) is mediated mainly by the mitogen-activated protein kinases (MAPKs) ERK1/2, and by the stress kinases p38 and JNK, and this has been shown to increase the intrinsic enzymatic activity of PLA 2 IV␣ (23,24).…”

“…Indeed, PLA 2 IV␣ has been shown to be the specific enzyme that produces glycerophosphoinositols from the membrane phosphoinositides upon hormonal stimulation and oncogenic transformation (24,27,28). The PLA 2 s have been reported to participate in immune cell functions also through the regulation of phagocytosis, as initiated by interactions of ligand-coated particles (particles opsonized with immunoglobulins or complement) with specific receptors expressed on the surface of phagocytic cells (Fc receptors (FcRs) or complement receptors) (29).…”

New Members of the Mammalian Glycerophosphodiester Phosphodiesterase Family

“…GPs are produced via phospholipase A 1 and phospholipase A 2 activities, and they are degraded by GP phosphodiesterases (GP-PDEs) (1)(2)(3)(4). Six mammalian GP-PDEs were previously isolated, and investigations have been carried out to explore their physiological significance (5,6).…”

“…Previous work showed that both GDE1 and GDE3 can hydrolyze GroPIns (11,12) and that the biological function of GDE3 in osteoblast proliferation and differentiation appears to be mediated by GroPIns. Indeed, GroPIns has been shown to regulate cell growth in thyroid cells (4), and induction of its hydrolysis through GDE3 expression results in reduced osteoblast proliferation and the appearance of markers of osteoblast differentiation (12). Thus, mammalian GP-PDEs have an intriguing feature; they show restricted substrate specificities, which prompted us to consider the possibility that mammalian GP-PDEs modulate GroPCho and/or GroPIns concentrations, because these intracellular GPs are increasingly recognized as bioactive molecules that are involved in a variety of cellular events (6).…”