Guanine nucleotides stimulate hydrolysis of phosphatidylinositol and polyphosphoinositides in permeabilized Swiss 3T3 cells

Inositol triphosphate receptors in sea urchin sperm

Inositol 1,4,5-triphosphate (Ins(1,4,5)P3) is a second messenger that regulates Ca2+ channels in many important cell signalling pathways. In sea urchin sperm the outer investment of the egg triggers the acrosome reaction (AR) that involves Ins(1,4,5)P3 production and the opening of two Ca2+ channels. Here we have sought to identify a high-affinity Ins(1,4,5)P3 receptor in Strongylocentrotus purpuratus sperm. An Ins(1,4,5)P3 binding component was affinity-purified 12-fold from sperm extracts. It displayed similar characteristics to the Ins(1,4,5)P3 receptor from other sources: pH-dependent high affinity for Ins(1,4,5)P3 (KD = 261 nM), a tau1/2 of association and dissociation of 50 and 40 s, respectively, specificity (IC50 > 5 microM for Ins(1)P1, Ins(1,4)P2 and Ins(1,3,4,5)P4), and pharmacological sensitivity (10 and 100 microg heparin/ml inhibited 75% and 100% binding respectively). An antibody against the carboxy-terminal of the type I Ins(1,4,5)P3 receptor of somatic cells recognised a plasma membrane component in the sperm head and less intensely in the flagella. This antibody also recognised a 240 kDa band from isolated head plasma membranes, and weakly in flagellar membrane. This IP3 receptor-like protein may mediate the sustained uptake of Ca2+ through the second Ca2+ channel opened during the AR.

PDGF and neomycin induce similar changes in the actin cytoskeleton in human fibroblasts

The addition of platelet-derived growth factor (PDGF) to serum-starved fibroblasts induces increased motility, formation of lamellipodia, increased ruffling activity, and actin ring structures associated with dorsal ruffles. Involvement of the phosphatidylinositol cycle (PI-cycle) in these morphological changes was investigated by observing the effects of neomycin, an inhibitor of the PI-cycle, on cultured human foreskin fibroblasts. The role of actin in the changes was investigated by using cytochalasin D (CD). Actin in detergent-extracted cells was labelled with TRITC-phalloidin and examined with fluorescence microscopy. Using PDGF and neomycin simultaneously potentiated lamellipodia formation, ruffling activity, as well as the number of cells with actin rings. Furthermore, neomycin by itself induced morphological changes similar to those induced by PDGF. Quantitation of actin rings showed dose and time dependency for PDGF and neomycin respectively, with a maximal number of cells containing rings after 15 min of exposure to either 3.5 mM neomycin or 10 ng PDGF/ml. Comparing the two substances, PDGF induced ring formation in a greater number of cells. These processes were inhibited by the presence of CD. PDGF- and neomycin-induced changes in the actin cytoskeleton were also observed in human embryonic lung fibroblasts, human glial cells, and embryonic mouse fibroblasts, all of which are known to express PDGF-receptors. In conclusion, the present study indicates that an increased turnover of the PI-cycle is not essential for the changes in actin organization induced by PDGF.

Evidence for phosphatidylinositol hydrolysis in pancreatic islets stimulated with carbamoylcholine. Kinetic analysis of inositol polyphosphate metabolism

Anion-exchange h.p.l.c. was used initially to analyse the products formed after addition of either [3H]Ins(1,3,4,5)P4 or [3H]Ins(1,4,5)P3 to homogenates of pancreatic islets. Metabolic routes similar to those of other tissues were established: dephosphorylation of Ins(1,4,5)P3 to Ins(1,4)P2 and then Ins4P; and sequential degradation of Ins(1,3,4,5)P4 to Ins(1,3,4)P3, Ins(3,4)P2 and Ins(3 or 1)P. In addition, there was a limited conversion of Ins(1,3,4)P3 into Ins(1,3)P2. After stimulation of [3H]inositol-prelabelled islets with the muscarinic-receptor agonist carbamoylcholine (carbachol), there was a rapid (10 s) increase in Ins(1,4,5)P3, Ins(1,3,4)P3, Ins(1,4)P2 and Ins4P. In the presence of 10 mM-LiCl, Ins1P was also significantly increased (P less than 0.05) by 5 s, before any increase in Ins4P (10 s), Ins(1,3)P2 (60 s) or Ins(3,4)P2. When carbachol was displaced with atropine, after 1 h pre-stimulation, the maximal decreases in Ins(1,4,5)P3 and Ins1P from the stimulated steady state (5 s) clearly preceded those of the other metabolites. These declines were used to calculate the turnover times and rate of metabolic flux through the various inositol phosphates. These experiments confirmed the relatively minor importance of the Ins(1,3)P2 pathway (less than 10% of the total flux) and demonstrated that Ins(1,4,5)P3 removal was evenly distributed through the Ins(1,4)P2 and Ins(1,3,4,5)P4 routes. They also established that flux through Ins1P was 8-fold greater than that through Ins(1,4,5)P3, indicating that the former could not have been derived from PtdInsP2 hydrolysis. Similarly, in islets pretreated with neomycin, which binds to PtdInsP2 with greater affinity than to PtdIns, the increase in Ins1P caused by 1 min stimulation with carbachol was not affected, despite virtual abolition of the increase in Ins4P, and an overall inhibition of PtdInsP2 hydrolysis by 67%. The results indicate that, in addition to PtdInsP2 breakdown, carbachol also promotes a rapid PtdIns hydrolysis which becomes increasingly predominant with prolonged stimulation.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

The control of cytoskeletal actin and exocytosis in intact and permeabilized adrenal chromaffin cells: role of calcium and protein kinase C

The control of cytoskeletal actin and exocytosis was examined in intact and digitonin-permeabilized chromaffin cells. Cytoskeletal actin was assayed by determining the actin content of Triton-insoluble cytoskeletons. The secretagogues nicotine, high K+ and Ba2+ resulted in a rapid reduction in the amount of actin associated with the cytoskeleton. The effect of nicotine but not high K+ on cytoskeletal actin was independent of external Ca2+ and the reduction in cytoskeletal actin was mimicked by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate suggesting a role for protein kinase C. In digitonin-permeabilized cells micromolar calcium produced both catecholamine secretion and a reduction in cytoskeletal actin. The reduction in cytoskeletal actin was transient. Secretion was enhanced by the GTP analogue guanosine 5'-(3-O-thio)triphosphate and the analogue also reduced cytoskeletal actin at low calcium levels. The effects of guanosine 5'-(3-O-thio)triphosphate were inhibited by the phospholipase C inhibitor neomycin and were mimicked by 12-O-tetradecanoylphorbol-13-acetate. An additional GTP analogue, guanyl-5'-yl imidodiphosphate, had no effect on cytoskeletal actin. These results provide further evidence for a requirement for reorganisation of cortical actin in the secretory processes and suggest that the reduction in actin associated with the cytoskeleton may be mediated by protein kinase C and/or calcium in intact and permeabilized chromaffin cells.