Properties of membranous phospholipase C from WRK1 cell: sensitivity to guanylnucleotides and bacterial toxins

Evidence for a direct negative coupling between dopamine-D2 receptors and PLC by heterotrimeric Gi1/2 proteins in rat anterior pituitary cell membranes

Dopamine (DA) is known to inhibit basal and hormone TRH- or angiotensin II (AngII)-stimulated PRL secretion and inositol phosphate accumulation in rat pituitary cells in primary culture. This inhibition persists when cells are incubated in a calcium-free medium (a condition in which DA could not inhibit PLC activities by blocking calcium influx) and is abolished by a Pertussis toxin treatment. These data suggest that DA receptor could be negatively coupled to PLC by a direct mechanism involving a Pertussis toxin-sensitive G protein. To demonstrate this hypothesis, we measured PLC activities on crude plasma membranes obtained from rat pituitary cells in primary culture grown in the presence of tritiated myo-inositol. We showed that 1) DA and quinpirole or RU24926 (specific D2 agonists) inhibited both basal and TRH- or AngII-stimulated membrane PLC activities. 2) Such inhibitions were completely prevented by sulpiride (specific D2 antagonist). 3) Heterotrimeric Gi1/2 proteins coupled the DA receptors to PLC because DA inhibitions were completely reversed by preincubation either with Pertussis toxin or with a specific G(alpha)i1/(alpha)i2 antibody. Such data are in favor of the existence of a direct negative coupling between DA-D2 receptor and PLC on a native physiological plasma membrane model.

Annexin I modulates cell functions by controlling intracellular calcium release

Annexin I is an intracellular protein in search of a function. Ex vivo it has calcium- and phospholipid-binding properties. To evaluate its role in vivo, MCF-7 cells were stably transfected with annexin I in sense or antisense orientations. In cells overexpressing annexin I, calcium release was abrogated on stimulation of purinergic or bradykinin receptors, whereas non-transfected cells or cells with down-regulated annexin I released calcium within seconds. Basal calcium and calcium stores were not affected. The impaired calcium release was paralleled by a down-regulation of the activities of phospholipase C, group II phospholipase A2, and E-cadherin with altered adhesion and enhanced tumor growth on soft agar. Significantly smaller tumors, with the histologically most differentiated cells, were observed in nude mice inoculated with cells transfected with the antisense rather than with the sense plasmid. These observations indicate that annexin I modulates cell functions by controlling intracellular calcium release. Frey, B. M., Reber, B. F. X., Vishwanath, B. S., Escher, G., Frey, F. J. Annexin I modulates cell functions by controlling intracellular calcium release.

Altered levels of phosphatidylinositol phospholipase C activity in rat liver cells in response to insulin, epinephrine, acetylcholine and bacterial phospholipase C

Rat liver cells were homogenized and subsequently fractionated by a simplified method based on microfiltration, which proved to give a high recovery of membrane-bound phosphatidylinositol phospholipase C (PI-PLC) activity. The effects of insulin, acetylcholine (AC), epinephrine (EN) and bacterial phospholipase C (bPLC) on the PI-PLC activity were studied after in vitro treatment of isolated membranes or after in situ application in rat liver. A dose-dependent increase of membranous PI-PLC (up to 3-fold) and corresponding 36 to 72% decline of the cytosolic activity were established upon treatment with supraphysiological doses of insulin or with bPLC, respectively. AC induced a biphasic response with a maximal stimulation in the micromolar range. On the other hand EN promoted a slight but significant dose-dependent inhibition of PI-PLC in both cytosol and membranes. Sodium fluoride was also a potent inhibitor of the membrane-associated PI-PLC with an EC50 value of about 5 mM. The combined assay with NaF and EN revealed no additivity between their inhibitory effects, suggesting a common step in the mechanism(s) of inhibition caused by the two agents. The stimulatory effects of insulin and AC were partially reduced by soluble cytosolic factors, which still remain to be identified. When insulin and AC were applied in combination in the presence of cytosol, this resulted in a 56% inhibition of PI-PLC below the control level.

Cytosolic Ca2+ oscillations in REF52 fibroblasts: Ca(2+)-stimulated IP3 production or voltage-dependent Ca2+ channels as key positive feedback elements

Oscillations in cytosolic free calcium concentrations ([Ca2+]i) can be elicited in REF52 fibroblasts by three different modes of stimulation. We have previously demonstrated that [Ca2+]i oscillations result when these cells are simultaneously depolarized and stimulated with a hormone linked to phosphoinositide breakdown. Further evidence is now presented that such oscillations are linked to fluctuations in the concentration of IP3 and the Ca2+ content of an IP3-sensitive Ca2+ store. [Ca2+]i oscillations can also be generated in REF52 cells either by direct stimulation of G-proteins with GTP gamma S or AlF4- or by destabilizing the membrane potential and opening voltage-dependent calcium channels. This report compares the different types of oscillations and their mechanisms.

Dual effects of ATP on phosphatidylinositol breakdown in rat hepatocyte membranes

The mechanisms whereby adenosine-5'-triphosphate (ATP) regulates the inositol phospholipid-signalling system were studied in rat hepatocytes. Intact hepatocytes respond to extracellular ATP, adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S), ADP and weakly to guanosine-5'-triphosphate (GTP), but not to other purine nucleotides (GDP or AMP). This is consistent with the idea that a P2 purinergic receptor is coupled to the phosphatidylinositol metabolism in these cells. Partially purified plasma membranes prepared from myo-[3H]inositol prelabelled hepatocytes exhibit a phosphatidylinositol-4,5-bisphosphate phospholipase C activity sensitive to ATP, ATP gamma S and guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S). Moreover the GTP gamma S effect is greatly enhanced by ATP and ATP gamma S. These potentiating effects differ according to the adenylnucleotide considered. ATP produces (1) an increase in the GTP gamma S-PLC sensitivity, (2) a potentiation of the phospholipase C (PLC) response induced by maximal dose of GTP gamma S, and (3) an increase in the inositol lipids pools. At variance, ATP gamma S, a nonhydrolysable analogue of ATP, only increases the PLC-sensitivity towards GTP gamma S. These results may signify that ATP stimulates inositol phosphate accumulation via at least two distinct mechanisms (i) a direct activation of a P2 purinergic receptor coupled to a PLC via a GTP binding protein and (ii) a stimulation of the phosphatidylinositol (PI) and phosphatidylinositol-4-phosphate (PIP) kinases which increased the pool of phospholipase C substrates.