Vasopressin elevation of Na+/H+ exchange is inhibited by genistein in human blood platelets

TRPC4 expression determines sensitivity of the platelet-type capacitative Ca2+entry channel to intracellular alkalosis

The present study was designed to analyze the molecular basis of the intracellular pH-dependent capacitative Ca2+ entry (CCE) of human platelets and megakaryocytic cells, specifically to test the hypothesis that members of the classical transient receptor potential (TRPC) protein family are involved in the CCE pathway that is promoted by intracellular alkalosis. Human platelets as well as the tested megakaryocytic cell lines (CMK cells, MEG-01 cells) and HEK293 cells displayed thapsigargin-induced CCE and responded to monensin with comparable elevation in intracellular pH. Promotion of CCE by monensin-induced intracellular alkalosis, however, was profound in mature platelets, moderate in CMK cells and lacking in MEG-01 cells as well as in HEK293 cells. Analysis of the TRPC expression pattern by immunoblotting revealed that mature platelets and CMK cells express TRPC4 along with TRPC1 and TRPC3, while TRPC4 is lacking in MEG-01 cells. HEK293 cells displayed CCE characteristics as well as lack of TRPC4 expression similar to MEG-01 cells. Over-expression of TRPC4 in HEK293 cells was found to result in a gain of pH-sensitivity of CCE with clearly detectable promotion of CCE in response to monensin. These results suggest that platelet CCE channel complexes contain TRPC4 as a molecular component that determines sensitivity of CCE to intracellular alkalosis.

Freeze-dried rehydrated human blood platelets regulate intracellular pH

BACKGROUND

Long-term storage of platelets (PLTs) in the dry state would greatly improve options for PLT storage. Whether trehalose-loaded freeze-dried and rehydrated PLTs could regulate intracellular pH (pHi) was evaluated.

STUDY DESIGN AND METHODS

Previously it was shown that human PLTs can be successfully preserved by freeze-drying with trehalose. Trehalose-loaded freeze-dried rehydrated PLTs and fresh control PLTs were labeled with the pH dye BCECF-AM. pHi was measured in resting cells, cells acidified with nigericin, and cells treated with thrombin. The sodium-proton pump was blocked by treatment with 5-(N-methyl-N-isobutyl)amiloride (MIA).

RESULTS

The pHi of rehydrated PLTs is the same as that of fresh control PLTs, 7.27+/-0.03 (SD; n=5) and 7.27+/-0.02 (n=5), respectively. Nigericin treatment of cells showed that the recovery in pHi was Na+-dependent and followed Michaelis-Menten kinetics. The Vmax values (DeltapH/9 sec) were 0.21+/-0.039 (n=3) and 0.22+/-0.025 (n=3) for rehydrated and control PLTs, respectively. The exchange constants were 17.7+/-2.3 mmol per L (n=3) and 17.0+/-1.9 mmol per L (n=3) for rehydrated and control PLTs, respectively. Treatment of cells with MIA showed that NHE1 remained sensitive to the inhibitor after freeze-drying and rehydration.

CONCLUSION

The results show that the pHi regulation system is largely preserved during freeze-drying and rehydration of PLTs.

Genistein inhibits osmotic activation of Na(+) /H( +) exchange in human platelets

The osmotic shrinkage is an important activator of the Na(+)/H( *) exchanger. The intracellular signaling mechanisms by which shrinkage changes intracellular pH have not been fully elucidated. In human platelets, the removal of calcium did not prevent the osmotic activation of the exchanger. The increase of pH(i) after an hyperosmotic stress was reduced by W-7 (63 micromol l(-1)), and by ML-7 (25 micromol l(-1)), inhibitors of responses mediated by calmodulin or by myosin light chain kinase, but the high concentrations needed suggested that non-specific effects could be involved. Although the exchanger was quiescent during preincubation in hypertonic sodium free solutions, some steps of the signal transduction chain that links the shrinkage to the exchanger activation suffers a modification. Therefore, upon exposure to isotonic sodium-containing media, the rate of recovery from acid loads was increased. The presence of genistein (100 micromol l( -1)) during the preincubation inhibited this activation of Na(+)/H( +) exchanger. We propose that shrinkage induce activation of tyrosine kinases, which in turn leads to the activation of Na(+)/H(+) exchanger and contributes to the restoration of cell volume in human platelets.

Sodium-hydrogen exchange and platelet function

On stimulation of platelets with agonists, for example, thrombin, a rapid rise in intracellular pH is observed. This alkalinization is mediated by an increase in transport activity of the Na(+)/H(+) exchanger isoform NHE1. In addition to this Na(+)/H(+) exchange mechanism, platelets express bicarbonate/chloride exchangers, which also contribute to pH(i) homeostasis. The main functions of NHE1 in platelets include pH(i) control, volume regulation, and participation in cell signaling. The isoform NHE1 is highly sensitive toward inhibition by EIPA, Hoe694, and Hoe642. The regulation of NHE1 activity is complex and is not completely understood. It includes the MAP kinase cascade, the Ca/calmodulin system, several heterotrimeric G proteins (Galpha12, Galpha13, Galphaq, and Galphai), small G proteins (ras, cdc42, rhoA), and downstream kinases (e.g., p160ROCK). Volume challenges stimulate tyrosine phosphorylation of cytoplasmic proteins, which ultimately activate NHE1. Thrombin, thromboxane, platelet-activating factor, angiotensin II, endothelin, phorbol ester, and Ca(2+) ionophors stimulate NHE1 activity in platelets. Blockade of platelet NHE1 can inhibit platelet activation. With the development of highly specific NHE1 inhibitors, detailed investigation of the relationships between NHE1 activity and platelet activation now becomes feasible.

Phyto-oestrogens and Western diseases

Incidences of breast, colorectal and prostate cancer are high in the Western world compared to countries in Asia. We have postulated that the Western diet compared to the semivegetarian diet in some Asian countries may alter hormone production, metabolism or action at the cellular level by some biochemical mechanisms. Our interest has been focused on two groups of hormone-like diphenolic phyto-oestrogens of dietary origin, the lignans and isoflavonoids abundant in plasma of subjects living in areas with low cancer incidence. The precursors of the biologically active compounds detected in man are found in soybean products, whole-grain cereal food, seeds, and berries. The plant lignan and isoflavonoid glycosides are converted by intestinal bacteria to hormone-like compounds. The weakly oestrogenic diphenols formed influence sex-hormone production, metabolism and biological activity, intracellular enzymes, protein synthesis, growth factor action, malignant cell proliferation, differentiation, cell adhesion and angiogenesis in such a way as to make them strong candidates for a role as natural cancer-protective compounds. Their effect on some of the most important steroid biosynthetic enzymes may result in beneficial modulation of hormone concentrations and action in the cells preventing development of cancer. Owing to their oestrogenic activity they reduce hot flushes and vaginal dryness in postmenopausal women and may to some degree inhibit osteoporosis, but alone they may be insufficient for complete protection. Soy intake prevents oxidation of the low-density lipoproteins in vitro when isolated from soy-treated individuals and affect favourably plasma lipid concentrations. Animal experiments provide evidence suggesting that both lignans and isoflavonoids may prevent the development of cancer as well as atherosclerosis. However, in some of these experiments it has not been possible to separate the phyto-oestrogen effect from the effect of other components in the food. The isoflavonoids and lignans may play a significant inhibitory role in cancer development particularly in the promotional phase of the disease, but recent evidence points also to a role in the initiation stage of carcinogenesis. At present, however, no definite recommendations can be made as to the dietary amounts needed for prevention of disease. This review deals with all the above-mentioned aspects of phyto-oestrogens.

The effect of BCECF on intracellular pH of human platelets

2',7'-Bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) is frequently used for fluorometric determination of intracellular pH (pHi) and its metabolic changes. Studies of BCECF-loaded platelets have reported different pHi values in the range of 6.98 to 7.35, despite the use of the same probe. It is now shown that intracellular BCECF (BCECFi) content affects pHi, and that its over-loading, leads to significantly lower pHi. Different pHi values can be reproduced by changing BCECFi, as reflected by fluorescence intensity. The major loading factors are: the concentration of the probe parent compound, BCECF acetoxymethyl ester (AM), and whether this ester is partly hydrolyzed externally when applied in plasma. When least affected by BCECF, platelet pHi is 7.34. High BCECFi does not affect ATP content, buffer capacity, activation of Na+/H+ exchange by protein kinase C (PKC) and basal PKC activity. On the other hand high BCECFi does inhibit the Na+/H+ exchange rate by over 50%. Since the Na+/H+ exchange strongly affects platelets pHi, it is proposed that this inhibition accounts, at least partly, for the lowered pHi in BCECF over-loaded platelets.

Stimulation of mitogen-activated protein kinase and Na+/H+ exchanger in human platelets. Differential effect of phorbol ester and vasopressin

Treatment of human platelets with phorbol 12-myristate 13-acetate (PMA) and arginine vasopressin (AVP) increase the phosphorylation and activation of mitogen-activated protein kinase (MAPK). Electrophoretic retardation of MAPK mobility on SDS-polyacrylamide gels was used for determination of MAPK phosphorylation. The activity of MAPK was tested in myelin basic protein (MBP)-containing polyacrylamide gels. In this study we compared the PMA and AVP signal transduction pathways leading to the activation of MAPKs and Na+/H+ exchanger (NHE). Both agonists stimulate MAPK and NHE activities in a similar time frame and concentration dependence. The MAPK and NHE activities induced by PMA were inhibited by staurosporine, a potent inhibitor for protein kinase C (PKC), and by MAPK kinase (MEK) inhibitor, PD98059, but were not affected by the tyrosine kinase inhibitor genistein. In contrast, both AVP-induced MAPK and NHE activities were inhibited by genistein and MEK inhibitor but were not affected by staurosporine. Immunoprecipitation studies demonstrate that PMA, but not AVP, enhances the basal phosphorylation of the NHE-1. In this study, MAPKs are suggested to be a part of converging signaling leading to NHE activation by PKC-dependent and AVP-tyrosine kinase-dependent pathways. We propose that the MAPK activation of the NHE-1 does not involve phosphorylation of this exchanger protein. On the other hand, PKC can lead to phosphorylation and to additional activation of the NHE-1 through a MAPK-independent pathway.