Distinct plasma-membrane PtdIns(4)P and PtdIns(4,5)P2 dynamics in secretagogue-stimulated beta-cells.

Phosphoinositides regulate numerous processes in various subcellular compartments. Whereas many stimuli trigger changes in the plasma-membrane PtdIns(4,5)P(2) concentration, little is known about its precursor, PtdIns(4)P, in particular whether there are stimulus-induced alterations independent of those of PtdIns(4,5)P(2). We investigated plasma-membrane PtdIns(4)P and PtdIns(4,5)P(2) dynamics in insulin-secreting MIN6 cells using fluorescent translocation biosensors and total internal reflection microscopy. Loss of PtdIns(4,5)P(2) induced by phospholipase C (PLC)-activating receptor agonists or stimulatory glucose concentrations was paralleled by increased PtdIns(4)P levels. In addition, glucose-stimulated cells regularly showed anti-synchronous oscillations of the two lipids. Whereas glucose-induced PtdIns(4)P elevation required voltage-gated Ca(2+) entry and was mimicked by membrane-depolarizing stimuli, the receptor-induced response was Ca(2+) independent, but sensitive to protein kinase C (PKC) inhibition and mimicked by phorbol ester stimulation. We conclude that glucose and PLC-activating receptor stimuli trigger Ca(2+)- and PKC-dependent changes in the plasma-membrane PtdIns(4)P concentration that are independent of the effects on PtdIns(4,5)P(2). These findings indicate that enhanced formation of PtdIns(4)P, apart from ensuring efficient replenishment of the PtdIns(4,5)P(2) pool, might serve an independent signalling function by regulating the association of PtdIns(4)P-binding proteins with the plasma membrane.