Inositol pyrophosphates are recognized components of cellular processes that regulate vesicle trafficking, telomere length, and apoptosis. We observed that pancreatic β cells maintain high basal concentrations of the pyrophosphate diphosphoinositol pentakisphosphate (Ins P ₇ or IP ₇ ). Inositol hexakisphosphate kinases (IP6Ks) that can generate IP ₇ were overexpressed. This overexpression stimulated exocytosis of insulin-containing granules from the readily releasable pool. Exogenously applied IP ₇ dose-dependently enhanced exocytosis at physiological concentrations. We determined that IP6K1 and IP6K2 were present in β cells. RNA silencing of IP6K1, but not IP6K2, inhibited exocytosis, which suggests that IP6K1 is the critical endogenous kinase. Maintenance of high concentrations of IP ₇ in the pancreatic β cell may enhance the immediate exocytotic capacity and consequently allow rapid adjustment of insulin secretion in response to increased demand.

Inositide‐specific phospholipase Cβ1 (PLCβ1) signaling in cell proliferation has been investigated thoroughly in the G ₁ cell cycle phase. However, little is known about its involvement in G ₂ / M progression. We used murine erythroleukemia cells to investigate the role of PLCβ1 in G ₂ /M cell cycle progression and screened a number of candidate intermediate players, particularly mitogen‐activated protein kinase (MAPK) and protein kinase C (PKC), which can, potentially, transduce serum mitogenic stimulus and induce lamin B1 phosphorylation, leading to G ₂ /M progression. We report that PLCβ1 colocalizes and physically interacts with lamin B1. Studies of the effects of inhibitors and selective si‐RNA mediated silencing showed a role of JNK, PKCα, PKCβI, and the β1 isoform of PI‐PLC in cell accumulation in G ₂ / M [as observed by fluorescence‐activated cell sorter (FACS)]. To shed light on the mechanism, we considered that the final signaling target was lamin B1 phosphorylation. When JNK, PKCα,orPLCβ1 were silenced, lamin B1 exhibited a lower extent of phosphorylation, as compared to control. The salient features to emerge from these studies are a common pathway in which JNK is likely to represent a link between mitogenic stimulus and activation of PLCβ1, and, foremost, the finding that the PLCβ1‐mediated pathway represents a functional nuclear inositide signaling in the G ₂ /M transition.— Fiume, R., Ramazzotti, G., Teti, G., Chiarini, F., Faenza, I., Mazzotti, G., Billi, A. M., Cocco, L., Involvement of nuclear PLCβl in lamin B1 phosphorylation and G2/M cell cycle progression. FASEB J. 23, 957–966 (2009)

Polyphosphoinositides (PPIns) are a family of seven lipid messengers that regulate a vast array of signalling pathways to control cell proliferation, migration, survival and differentiation. PPIns are differentially present in various sub-cellular compartments and, through the recruitment and regulation of specific proteins, are key regulators of compartment identity and function. Phosphoinositides and the enzymes that synthesise and degrade them are also present in the nuclear membrane and in nuclear membraneless compartments such as nuclear speckles. Here we discuss how PPIns in the nucleus are modulated in response to external cues and how they function to control downstream signalling. Finally we suggest a role for nuclear PPIns in liquid phase separations that are involved in the formation of membraneless compartments within the nucleus.