Insulin stimulates turnover of phosphatidylcholine in rat adipocytes

The present study investigated the effect of insulin on phosphatidylcholine turnover in rat adipocytes labelled to equilibrium with [¹⁴C]-choline. Insulin induced a rapid turnover of this major phospholipid that was maximal by 1 min and transient in nature. Following a 1 min stimulation of the cells with insulin at a maximally effective concentration (7 nM), a 4–6% decrease in the percentage of total cellular choline associated with this phospholipid was observed. This reflected a significant transient increase in the percentage of total cellular choline associated with phosphorylcholine, which together with diacylglycerol are the phospholipase C cleavage products of phosphatidylcholine. These effects were observed over a physiological range of insulin concentrations. No effect of insulin on any other choline phospholipid or metabolite (sphingomyelin, lysophophatidylcholine, glycerophosphocholine or choline) was seen. These results suggest that insulin stimulates a phospholipase C-mediated turnover of phosphatidylcholine in rat adipocytes. The rapid nature of this turnover suggests a potential role in signal transduction.     

Isolation of insulin-sensitive phosphatidylinositol-glycan from rat adipocytes. Its impaired breakdown in the streptozotocin-diabetic rat.

Although exogenous phosphatidic acid (PA) has been shown to promote insulin release, the effects of endogenous PA on endocrine function are largely unexplored. In order to generate PA in situ, intact adult-rat islets were treated with exogenous phospholipases of the D type (PLD), and their effects on phospholipid metabolism and on insulin release were studied in parallel. Chromatographically purified PLD from Streptomyces chromofuscus stimulated the accumulation of PA in [14C]arachidonate- or [14C]myristate-prelabelled islets, and also promoted insulin secretion over an identical concentration range. During 30 min incubations, insulin release correlated closely with the accumulation of [14C]arachidonate-labelled PA (r2 = 0.98; P less than 0.01) or [14C]myristate-labelled PA (r2 = 0.97; P less than 0.01). Similar effects were seen both in freshly isolated and in overnight-cultured intact islets. In contrast, PLDs (from cabbage or peanut) which do not support phospholipid hydrolysis at the pH of the extracellular medium also did not promote insulin release. The effects on secretion of the active PLD preparation were inhibited by modest cooling (to 30 degrees C); dantrolene or Co2+ also inhibited PLD-induced secretion without decreasing PLD-induced PA formation. Additionally, the removal of PLD left the subsequent islet responsiveness to glucose intact, further supporting an exocytotic non-toxic mechanism. PLD-induced insulin release did not appear to require influx of extracellular Ca2+, nor could the activation of protein kinase C clearly be implicated. During incubations of 30 min, PLD selectively generated PA; however, more prolonged incubations (60 min) also led to production of some diacyglycerol and free arachidonic acid concomitant with progressive insulin release. These data suggest that PLD activation has both rapid and direct effects (via PA) and more delayed, secondary, effects (via other effects of PA or the generation of other lipid signals). Taken in conjunction with our demonstration that pancreatic islets contain an endogenous PLD which generates PA [Dunlop & Metz (1989) Biochem. Biophys. Res. Commun. 163, 922-928], these studies provide evidence suggesting that PLD activation (and possibly other pathways leading to PA formation) could play a role in stimulus-secretion coupling in pancreatic islets.     

Phospholipase C mimics insulin action on pyruvate dehydrogenase and insulin mediator generation but not glucose transport or utilization

This study investigated the extent to which a purified phosphatidylinositol-specific and a commercial non-specific phospholipase C mimicked acute insulin action in rat adipocytes. The enzymes mimicked insulin stimulation of pyruvate dehydrogenase (PDH) and breakdown of a glycophospholipid proposed as a precursor for an intracellular mediator of insulin action, but were much less effective in stimulating glucose transport and utilization. These observations corroborate recent suggestions that insulin may activate a phospholipase C to generate a mediator that can account for insulin activation of PDH from a mediator precursor with a phosphatidylinositol anchor. This mediator precursor is probably an outer membrane component since effects were obtained with intact cells. It is unlikely that this mechanism accounts fully for insulin action since phosphatidylinositol-specific and commercial phospholipase C stimulation of glucose transport was significantly less than that elicited by insulin.     

The effect of selective phosphodiesterase inhibitors,alone and in combination,on a murine model of allergic asthma

BackgroundThe anti-inflammatory effects of the selective phosphodiesterase (PDE) inhibitors cilostazol (PDE 3), RO 20-1724 (PDE 4) and sildenafil (PDE 5) were examined in a murine model of allergic asthma. These compounds were used alone and in combination to determine any potential synergism, with dexamethasone included as a positive control.MethodsControl and ovalbumin sensitised Balb/C mice were administered orally with each of the possible combinations of drugs at a dose of 3 mg/Kg for 10 days.ResultsWhen used alone, RO 20-1724 significantly reduced eosinophil influx into lungs and lowered tumour necrosis factor-α, interleukin-4 and interleukin-5 levels in the bronchoalveolar lavage fluid when compared to untreated mice. Treatment with cilostazol or sildenafil did not significantly inhibit any markers of inflammation measured. Combining any of these PDE inhibitors produced no additive or synergistic effects. Indeed, the anti-inflammatory effects of RO 20-1724 were attenuated by co-administration of either cilostazol or sildenafil.ConclusionsThese results suggest that concurrent treatment with a PDE 3 and/or PDE 5 inhibitor will reduce the anti-inflammatory effectiveness of a PDE 4 inhibitor.     

T-cell activation without proliferation in juvenile idiopathic arthritis

A study was done to determine if the differentiation and activation phenotype of T cells in synovial fluid (SF) from patients with juvenile idiopathic arthritis (JIA) is associated with T-cell proliferation in situ. Mononuclear cells were isolated from 44 paired samples of peripheral blood and SF. Differentiation and activation markers were determined on CD4 and CD8 T cells by flow cytometry. Cell-cycle analysis was performed by propidium iodide staining, and surface-marker expression was also assessed after culture of the T cells under conditions similar to those found in the synovial compartment. The majority of the T cells in the SF were CD45RO⁺CD45RB^dull. There was greater expression of the activation markers CD69, HLA-DR, CD25 and CD71 on T cells from SF than on those from peripheral blood. Actively dividing cells accounted for less than 1% of the total T-cell population in SF. The presence or absence of IL-16 in T-cell cultures with SF or in a hypoxic environment did not affect the expression of markers of T-cell activation. T cells from the SF of patients with JIA were highly differentiated and expressed early and late markers of activation with little evidence of in situ proliferation. This observation refines and extends previous reports of the SF T-cell phenotype in JIA and may have important implications for our understanding of chronic inflammation.     

A signal, from human mtDNA, of postglacial recolonization in Europe

Mitochondrial HVS-I sequences from 10,365 subjects belonging to 56 populations/geographical regions of western Eurasia and northern Africa were first surveyed for the presence of the T-->C transition at nucleotide position 16298, a mutation which has previously been shown to characterize haplogroup V mtDNAs. All mtDNAs with this mutation were then screened for a number of diagnostic RFLP sites, revealing two major subsets of mtDNAs. One is haplogroup V proper, and the other has been termed "pre*V," since it predates V phylogenetically. The rather uncommon pre*V tends to be scattered throughout Europe (and northwestern Africa), whereas V attains two peaks of frequency: one situated in southwestern Europe and one in the Saami of northern Scandinavia. Geographical distributions and ages support the scenario that pre*V originated in Europe before the Last Glacial Maximum (LGM), whereas the more recently derived haplogroup V arose in a southwestern European refugium soon after the LGM. The arrival of V in eastern/central Europe, however, occurred much later, possibly with (post-)Neolithic contacts. The distribution of haplogroup V mtDNAs in modern European populations would thus, at least in part, reflect the pattern of postglacial human recolonization from that refugium, affecting even the Saami. Overall, the present study shows that the dissection of mtDNA variation into small and well-defined evolutionary units is an essential step in the identification of spatial frequency patterns. Mass screening of a few markers identified using complete mtDNA sequences promises to be an efficient strategy for inferring features of human prehistory.