Effect of perturbing diacylglycerol metabolism on cytosolic free Ca2+ oscillations induced in single hepatocytes

1,2-sn-Diacylglycerol in plant cells: Product, substrate and regulator

1,2-sn-Diacylglycerol (DAG) is a family of lipidic molecular species varying in the lengths and desaturation levels of acyl groups esterified at positions sn-1 and sn-2 of the glycerol backbone. In plant cells, DAG originating from plastid and from extraplastidial membranes have distinct molecular signatures, C18/C16 and C18/C18 structures, respectively. Under normal conditions, DAG is consumed nearly as fast as it is produced and is therefore a transient compound in the cell. In plants, DAG proved to be the most basic ingredient for cell membrane biogenesis and fat storage, but we still lack formal evidence to assert that DAG is also an intracellular messenger, as demonstrated for animals. From the biochemical and molecular comparisons of the best known DAG-manipulating proteins of prokaryotic and eukaryotic cells (phosphatidate phosphatases, diacylglycerol kinases, MGDG synthase, protein kinase C, etc.) this review aims to identify general rules driving DAG metabolism, and emphasizes its unique features in plant cells. DAG metabolism is an intricate network of local productions and utilizations: many isoenzymes can catalyse similar DAG modifications in distinct cell compartments or physiological processes. The enzymatic- or binding-specificity for DAG molecular species demonstrates that discrete DAG molecular subspecies fluxes are finely controlled (particularly for C18/C16 and C18/C18 structures in plastid membrane biogenesis). Eventually, this review stresses the diversity of structures and functioning of DAG-manipulating proteins. As a consequence, because DAG metabolism in plants is unique, the deciphering of genomic information cannot rely on homology searches using known prokaryotic, animal or yeast sequences, but requires sustained efforts in biochemical and molecular characterizations of plant DAG-manipulating proteins.

Agonist-specific behaviour of the intracellular Ca2+ response in rat hepatocytes

A variety of agonists stimulate in hepatocytes a response that takes the shape of repetitive cytosolic free Ca2+ transients called Ca2+ oscillations. The shape of spikes and the pattern of oscillations in a given cell differ depending on the agonist of the phosphoinositide pathway that is applied. In this study, the response of individual rat hepatocytes to maximal stimulation by arginine vasopressin (AVP), phenylephrine and ADP was investigated by fluorescence microscopy and flash photolysis. Hepatocytes loaded with Ca2+-sensitive probes were stimulated with a first agonist to evoke a maximal response, and then a second agonist was added. When phenylephrine or ADP was used as the first agonist, AVP applied subsequently could elicit an additional response, which did not happen when AVP was first applied and phenylephrine or ADP was applied later. Cells microinjected with caged myo-inositol 1,4,5-trisphosphate (IP3) were challenged with the different agonists and, when a maximal response was obtained, photorelease of IP3 was triggered. Cells maximally stimulated with AVP did not respond to IP3 photorelease, whereas those stimulated with phenylephrine or ADP responded with a fast Ca2+ spike above the elevated steady-state level, which was followed by an undershoot. In contrast, with all three agonists, IP3 photorelease triggered at the top of an oscillatory Ca2+ transient was able to mobilize additional Ca2+. These experiments indicate that the differential response of cells to agonists is found not only during Ca2+ oscillations but also during maximal agonist stimulation and that potency and efficacy differences exist among agonists.

Suppression of agonist induced Ca2+ oscillations in cultured hepatocytes by nafenopin: possible involvement of protein kinase C

The alpha 1-agonist phenylephrine (5 microM) induces an increase in the free cytosolic Ca2+ concentration, followed by repetitive transients of the cytoplasmic Ca2+ concentration, in single Fura-2 loaded hepatocytes. The tumor promoting, hypolipidemic drug nafenopin suppressed the cellular Ca2+ response to phenylephrine. The effect of nafenopin on the Ca2+ increase and Ca2+ oscillations was largely prevented by the specific protein kinase C inhibitor Gö 6976. This finding suggests involvement of protein kinase C in the action of nafenopin on phenylephrine induced Ca2+ mobilization.

Both activators and inhibitors of protein kinase C promote the inhibition of phenylephrine-induced [Ca2+]i oscillations in single intact rat hepatocytes

In single isolated rat hepatocytes Ca(2+)-mobilising hormones induce oscillations in cytosolic free Ca2+ ([Ca2+]i) in which the frequency of spiking depends on agonist dose, but the time course of individual spikes depends on the hormone species, rather than agonist concentration. We have previously presented data using sphingosine and staurosporine as evidence of a negative feedback role for protein kinase C (PKC) in the elongation of the falling phase of [Ca2+]i spikes. We show here that the principal effect of three specific PKC inhibitors, namely the bis-indolylmaleimide GF 109203X, the tetracyclic aromatic alkaloid chelerythrine, and a myristoylated PKC pseudosubstrate peptide, that act at different sites on the PKC molecule, is a reduction in, or a complete suppression of, the phenylephrine-induced [Ca2+]i oscillation frequency. These results resemble the effects of activators of PKC and modulators of diacylglycerol (DAG) metabolism. Furthermore, following phorbol ester-induced inhibition of the hepatocyte [Ca2+]i oscillator, the addition of all three of these PKC inhibitors further reduces the [Ca2+]i oscillation frequency, with high concentrations of chelerythrine being the only agent that overcomes this inhibition by phorbol ester. These paradoxical results point to the need for caution in interpreting the effects of protocols involving PKC activators and inhibitors in assessing the feedback control from PKC on cellular [Ca2+]i oscillations.