The role of Gi and the membrane-fluidizing agent benzyl alcohol in modulating the hysteretic activation of human platelet adenylate cyclase by guanylyl 5'-imidodiphosphate

A functional role for interleukin (IL)-4-driven cyclic amp accumulation in human b lymphocytes

Interleukin-4 (IL-4) regulates the expression of the 55-kDa alpha-subunit (CD25) of the IL-2 receptor complex in human B lymphocytes. This report suggests that the cAMP/protein kinase A (PKA) component of the IL-4 receptor signalling programme in human tonsillar B cells has a functionally important role in regulating expression of the CD25 gene by attenuating activity of a protein binding to a potent negative regulatory element (NRE) in the CD25 promoter; this effect can be mimicked by agents that elevate cAMP and blocked by inhibitors of PKA but not protein kinase C (PKC). In a B-cell line that fails to elevate cAMP, attenuate NRE-binding protein (NRE-BP) activity or express CD25 following IL-4 treatment, stimulation of cAMP accumulation by forskolin facilitates IL-4-mediated induction of both the endogenous gene and an exogenous reporter gene under the control of a minimal promoter/enhancer fragment of the CD25 gene.

Release of calcium and P-selectin from intraplatelet granules is hampered by procaine

The inhibition of platelets by some local anaesthetics has been related to the modulation of platelet membrane lipid fluidity, and one of these compounds, procaine, has been proven to be particularly effective inhibitor. In the present study, we examined the effect of procaine on the mobilization of intracellular granule contents in isolated washed platelets. We revealed that the presence of 10 mg/ml procaine significantly hampered platelet release reaction, as demonstrated by the significant reduction in the expression of platelet P-selectin (CD62) on one hand, and significantly enhanced expression of GPIb alpha (CD42b) antigen on the other, following either 1 hour incubation of washed platelets at room temperature (%CD62: 37.1+/-6.8% of control incubated without procaine, p<<0.0001; %CD42b: 116.2+/-6.3% of control, p<0.0001) or activation of whole blood platelets with ADP, TRAP, or thrombin. Procaine, which acted as a rigidizer, significantly decreased platelet membrane fluidity (ESR h(+1)/h0 ratio of 5-DOXYL-Ste reduced down to 93.1+/-3.7% of control, p<0.001). In washed Fura-2-loaded platelets procaine not only brought about the significantly reduced Ca2+ release from intraplatelet storage pools after platelet stimulation with 15 micromol/l ADP (25.3+/-12.5% of control, p<0.001), but also it significantly increased the reduction in Ca2+ concentration upon the addition of Ca2+ chelator, EDTAK2 (48.9+/-13.5% vs. 40.9+/-12.1% of initial [Ca2+]i concentration, p(1,alpha)<0.025). Overall, procaine considerably reduced calcium mobilization from intraplatelet storage pools and Ca2+ efflux across platelet membrane. Based on these data, we suggest that the preventive effects of procaine on platelet release reaction and calcium mobilization might relate to the changes in the organization of membrane components embedded into a lipid bilayer, which are crucial in triggering of platelet release reaction. Procaine-mediated dislocations of some membrane components and/or distortion of lipid-protein interactions could generate a steric hindrance, which might interfere with platelet signal transduction, thus leading to impaired mobilization of Ca2+ and other components from intraplatelet storage pools.

Effect of membrane lipid alteration on the growth, phospholipase C activity and G protein of HT-29 tumor cells

The objective of the present study was to examine the effect of modifying the fatty acid composition of membranes on cell growth and phosphoinositide specific phospholipase C (PLC) activity in HT-29 colon cancer cells. Cells were seeded at a density of 12 x 10(3) cells/cm2 and supplemented with 30 microM of either 18:0, 18:2 (n6) or 18:3 (n3) complexed to bovine serum albumin (BSA) in DMEM medium. Cell growth was followed for 12 days. The 18:0 supplemented cells (control) reached maximum growth at day nine which was greater than either 18:2 (n6) or 18:3 (n3) supplemented cells. There was no difference between the latter two groups in their growth. To investigate the fatty acid incorporation of the supplemented fatty acid and how they may influence composition in the cell membrane, we examined the fatty acid composition of each phospholipid (PL) species. Both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were significantly influenced by the type of fatty acid supplemented. Supplementation with 18:0 resulted in HT-29 cell membranes having more monounsaturated fatty acids than the cells grown in the other fatty acids. Polyunsaturated fatty acid (PUFA) supplementation (both 18:2 and 18:3) resulted in the enrichment of PUFA in the PL fractions. Cells supplemented with 18:3 (n3) had the highest unsaturation index in membrane PE as compared to the other phospholipid species. PLC activity of the membranes was measured using PIP2 as a substrate in the presence of 15 micrograms alamethicin and 42 microM free calcium. The contribution of G protein to the activity of the enzyme was assessed using GTP gamma(S). PLC activity of HT-29 cells was 16% higher in the presence of GTP gamma(S) response. GTP gamma(S)-activated PLC activity of 18:3 (n3) supplemented cells was 81% of those supplemented with either 18:0 or 18:2 (n6) cells. It is concluded that the decrease in cell proliferation with supplementation with 18:3 (n3) may be mediated through its inhibitory effect on PLC, which provides the second messengers for protein kinase C (PKC) activation. PLC may be influenced by an increased unsaturation index of the PE fraction of the HT-29 tumor cell membranes.

ML-7 and W-7 facilitate thromboxane A2-mediated Ca2+ mobilization in rabbit platelets

The effects of 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-7), a myosin light chain kinase inhibitor, and (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), a calmodulin antagonist, on thromboxane A2 receptor-mediated signal transduction were examined in rabbit washed platelets. ML-7 and W-7 at 10-30 microM slightly potentiated the aggregation induced by a thromboxane A2 receptor agonist, 9,11-dideoxy-9 alpha,11 alpha- epoxymethanoprostaglandin F2 alpha (U46619), in spite of their known inhibitory actions. ML-7 and W-7 concentration-dependently enhanced U46619-induced phosphoinositide hydrolysis and the increase in internal free Ca2+ concentration in the presence or absence of external Ca2+. While ML-7 and W-7 inhibited basal GTPase activity, they augmented U46619-induced activation of GTPase in a concentration-dependent manner. The present results suggest that ML-7 and W-7 enhance thromboxane A2 receptor-mediated signal transduction at the receptor/G protein coupling, leading to the enhancement of phosphoinositide hydrolysis and Ca2+ mobilization, independently of the inhibition of myosin light chain kinase or calmodulin.