Characterization of phosphotyrosine phosphatase activity in sheep platelets: amphiphilic and hydrophilic forms

Comparison of Changes in Erythrocyte and Platelet Fatty Acid Composition and Protein Oxidation in Advanced Non-Small Cell Lung Cancer

The formation of free radicals and lipid peroxidation products is linked both to carcinogenesis and tumor behavior. Blood samples from 50 patients with advanced (Stages III-IV) non-small cell lung cancer (NSCLC), and from 50 healthy volunteers were used for plasma beta-thromboglobulin (beta-TG) measurements, red blood cell (RBC) and platelet lipid analyses, and lipid fluorescence determinations. Samples from 15 randomly selected patients and 15 controls also were used for analysis of the expression of oxidized proteins. We observed: (a) higher levels of plasma beta-TG in patients, (b) alterations in membrane fatty acids. The RBC fatty acid profile changed especially in the 18-carbon species (increases in stearic and oleic and a decrease in linoleic fatty acids), and in arachidonic acid, which also decreased significantly. The platelet fatty acid profile mainly showed a decrease in arachidonic acid and a parallel increase in palmitic fatty acid; (c) the loss of polyunsaturated fatty acids (PUFA) in RBC and platelets could be correlated with changes in lipid extract fluorescence only for platelets; (d) protein oxidation levels were increased also only in the case of platelets. The changes detected point to platelet activation and lipid peroxidation processes associated with NSCLC. The oxidative stress affected RBC and platelets differently, although changes in PUFA might still have important physiological consequences in both types of cells.

Comparison of changes in erythrocyte and platelet phospholipid and fatty acid composition and protein oxidation in chronic obstructive pulmonary disease and asthma

OBJECTIVE

To analyse and compare the phospholipid and fatty acid composition of total lipids and the occurrence of lipid peroxidation and protein oxidation directly in erythrocytes or platelets from chronic obstructive pulmonary disease (COPD) and asthma patients.

PATIENTS

Fifteen consecutive outpatients with COPD (all smokers) and asthma (non-smokers) recruited during a moderate-to-severe (COPD) or moderate (asthma) exacerbation. Fifteen subjects with smoking habits similar to those of COPD patients were studied as a control group.

METHODS

Phospholipid and total fatty acid compositions were analysed by two-dimensional thin layer chromatography or gas chromatography-mass spectrometry, respectively. The lipid fluorescence of lipid extracts was measured by spectrofluorimetry. Protein carbonyl contents and profiles were measured by immunoblot detection.

RESULTS

No differences were found either in erythrocyte or platelet cholesterol or phospholipid levels. Only a decrease in the content of phosphatidylserine + phosphatidylinositol (P<0.003) was detected in platelets from the asthma patients. In erythrocytes, the fatty acid profile changed in both lung pathologies, especially as regards polyunsaturated fatty acids (decreases in arachidonic and 22:4 fatty acid contents). Other observed changes were: COPD, an increase in palmitic fatty acid; asthma, an increase in oleic and decreases in eicosapentaenoic and 22:6 + 24:1 fatty acids. In platelets, the fatty acid profiles revealed many differences between both lung pathologies: COPD, a decrease in 18:1 and increases in 20:5 and 22:5 + 24:0; asthma, a decrease in 20:4 and increase in 22:6 + 24:1. In COPD vs. asthma patients, fatty acid changes were mainly detected in platelets, especially in 18-carbon species, with decreases in stearic and 18:1 fatty acids in the COPD patients. Protein oxidation levels were increased in both lung pathologies in both erythrocytes and platelets.

CONCLUSIONS

COPD and asthma are associated with common or specific changes in the lipid composition of erythrocytes and/or platelets. The data point to lipid peroxidation and protein oxidation phenomena in both types of blood cell, although platelets would be more susceptible to stress.

Structural characteristics of a lipid peroxidation product, trans-2-nonenal, that favour inhibition of membrane-associated phosphotyrosine phosphatase activity

Protein-tyrosine phosphatases (PTPs) are very susceptible to oxidation by reactive oxygen species (ROS), which induce the oxidation of catalytic cysteines, thereby inactivating these PTPs. PTPs are also inactivated by treatment with different aldehydes (such as trans-2-nonenal), produced after tissue damage by ROS. However, the molecular mechanisms behind such aldehyde-due inactivation remain unknown. Using commercially available compounds, we examined the structural characteristics of trans-2-nonenal that allow the inhibition of platelet membrane-associated PTP activity, as well as how these compounds affect the dynamics of SH-, CO- and NH2- protein groups on the membranes. PTP was effectively inhibited by physiological amounts of trans-2-nonenal (1-10 microM). Incubation with trans-2-nonene (10 microM) also decreased PTP activity, although to a lower extent. Treatment with nonyl aldehyde almost eliminated PTP inhibition. Decreases in protein thiols were visible after trans-2-nonenal and trans-2-nonene treatments. Both the latter compounds also increased protein carbonyls (although trans-2-nonenal was more effective) and decreased protein amino groups to an equal extent. Collectively, our data indicate that alpha,beta unsaturation (and not a double bond in another position) is the most important structural determinant for PTP inhibition, the alkenal with 9-carbon atoms being the most effective in eliciting such inhibition. The data allow us to predict the modification of sulfhydryls and/or the formation of addition products with lysyl or histidyl residues, and hence the kind of specific antibodies that it would be necessary to generate in order to test such modifications directly.

Loss of phosphotyrosine phosphatase activity and changes in the tyrosine phosphorylation state of proteins after storage of sheep platelets in plasma or Seto solution at 4 degrees C

BACKGROUND AND OBJECTIVES

During platelet storage an array of deleterious changes occur, through mechanisms not fully understood, which impair platelet haemostasis. Transfused platelets should maintain the integrated networks of signalling pathways that regulate platelet activation and functionality. We hypothesized that protein phosphorylation and dephosphorylation, which play a fundamental role in these pathways, might be affected by platelet storage. We therefore investigated whether the activity of phosphotyrosine phosphatase (PTP), which belongs to an oxidant-susceptible group of enzymes involved in the platelet signal-transduction pathways that ensure platelet functionality, is affected by platelet storage.

MATERIALS AND METHODS

Using sheep platelet species as a model system, we conducted serial studies on the membranes of platelets and microparticles shed during platelet storage, in their own plasma or in a synthetic medium called Seto, for up to 5 days at 4 degrees C.

RESULTS

A progressive decrease in both total and specific membrane-associated PTP activities from whole platelets (but not from microparticles) located within each platelet storage bag was observed from day 1 onwards in both types of storage media. These decreases could be partly avoided by the addition of vitamin E. Additionally, the observed decrease in PTP activity was accompanied with increases in the tyrosine phosphorylation of proteins from whole platelets or crude platelet membranes, the tyrosine phosphorylation state of proteins from microparticles remaining basically unchanged.

CONCLUSION

Our findings suggest that alterations of at least the tyrosine phosphorylation balance might be one of the reasons for the decrease in the haemostatic function of stored platelets.

Amphiphilic and hydrophilic nature of sheep and human platelet phosphotyrosine phosphatase forms

To date, although at least 75 different PTPases (protein-tyrosine-phosphate-phosphohydrolase, EC 3.1.3.48) have been identified, those detected in platelets are rather scarce. Based on previous results from our laboratory, we investigated the existence of new PTPases in platelets. Triton X-114 phase partitioning of Triton X-100-solubilized human and sheep platelet membranes allowed PTPase to be recovered in the detergent-rich (40-35%, respectively) and -poor phases (60-65%, respectively). Sedimentation analyses of both phases from the sheep species revealed hydrophilic 6S and 3.7S, and amphiphilic 7.5S and 10.3S PTPase forms. Sedimentation analyses of human platelet membrane-associated or cytosolic PTPase revealed hydrophilic 6.7S and 4.3S, and amphiphilic 5.5S and 10.8S forms, or hydrophilic 4S, 5.9S and 6.9S forms, respectively. Western blot analysis using monoclonal antibodies (MoAb) against human PTP1B, PTP1C, PTP1D and RPTPalpha (mouse anti-human PTPase MoAbs) showed that RPTPalpha was not present in platelets and that the PTP1C type and PTP1D type (but probably not the PTP1B type) were expressed in sheep species. Immunoblots also revealed that all PTPases detected were mainly membrane-associated, with similar percentages of cellular distribution in both species. All PTPases were mainly recovered in the detergent-poor phases from the Triton X-114 phase partitioning, although PTP1D from human species was also significantly present (30%) in the detergent-rich phase. Additionally, all PTPases sedimented within the same PTPase peak in sucrose gradients (sedimentation coefficients around 4S). These findings indicate that amphiphilic and hydrophilic PTPases different from PTP1B, PTP1C, PTP1D or RPTPalpha, with higher sedimentation coefficients and with higher activity when O-phosphotyrosine or a synthetic peptide phosphorylated on tyrosine were used as substrates, are present in platelets.

Oxidative inactivation of human and sheep platelet membrane-associated phosphotyrosine phosphatase activity

Incubation of human or sheep platelet crude membranes with xanthine oxidase/hypoxanthine in the presence of Fe2+/ADP inactivated phosphotyrosine phosphatase (PTPase, protein-tyrosine-phosphate-phosphohydrolase, EC 3.1.3.48) activity in a time-dependent manner, this inhibition being significant within 5 min of treatment. The dynamics of protein thiols differed depending on the platelet species, but in any case decreases in protein thiols were only visible 20-45 min after the start of the treatment. The inhibition of PTPase activity in general showed good a correlation with the production of thiobarbituric acid-reactive substances (TBARS). The results with several antioxidants suggest that the inhibition of PTPase activity is related to the generation of alkoxyl and/or peroxyl radicals. Furthermore, the formation of fluorescent products and changes in amino groups were observed only after long incubation times with the oxidizing agents, these fluorescent products and the residual enzyme activity remaining in the membrane fraction. Treatment of platelet membranes with trans-2-nonenal and n-heptaldehyde, but not with acetaldehyde, also inhibited membrane-associated PTPase activity. However, the amount of protein thiols was reduced only by treatment with trans-2-nonenal. Fluorescence product formation was always higher with trans-2-nonenal, these products being mainly located in the protein fraction. The results with aldehydes suggest that secondary degraded products of lipid hydroperoxides affect PTPase activity. Kinetic studies of PTPase activity indicated that with all treatments enzyme inhibition is mainly due to a decrease in the Vmax value. The results of fluorescence anisotropy measurements of labeled platelet membranes did not support the notion of a contribution of the lipid organization to peroxidation-mediated PTPase inhibition. All the above results indicate that platelet membrane-associated PTPase inhibition due to treatment with xanthine oxidase/ hypoxanthine in the presence of Fe2+/ADP is a very complex, time-dependent process, and that it is probably related, at least after long periods of peroxidation, to changes in protein thiols and amino groups. We predict that the sensitivity of PTPase to lipid peroxidation must be physiologically relevant because of the increasing importance of tyrosine phosphorylation in signal transduction, in general, and in platelet activation and aggregation in particular.

Amphiphilic and hydrophilic forms of acetylcholinesterase from sheep platelets

Acetylcholinesterase (AChE, EC 3.1.1.7) was extracted from sheep platelets by successive homogenizations, yielding low-salt soluble (LSS), high-salt soluble (HSS) and detergent-soluble (DS) fractions. These accounted, respectively, for about 30%, 7% and 60% of total AChE activity. Applications of hydrophobic chromatography on phenyl-agarose to three solubilized fractions revealed that hydrophilic forms were almost exclusively located in the LSS fraction ( approximately 27% of total AChE), whereas most amphiphilic forms were present in DS extracts ( approximately 59% of total AChE), the remaining forms being distributed among aqueous soluble fractions. Enzyme molecular forms in the solubilized extracts were identified by centrifugation in 5-20% sucrose gradients containing Triton X-100 or Brij 97 to differentiate between hydrophilic or amphiphilic species. A predominance of hydrophilic dimeric forms ( approximately 22%), with small amounts of hydrophilic monomers (5%) and amphiphilic dimers and monomers (3%), was found in soluble AChE (LSS fraction). Amphiphilic AChE forms extracted in the HSS and DS fractions had a single peak in the sedimentation profiles with sedimentation coefficients of about 6S in gradients with Triton X-100; these were slightly shifted in the presence of Brij 97. After treatment with dithiothreitol, this molecular form solubilized in DS was converted to another molecular form with a lower sedimentation coefficient. Our results show that amphiphilic globular dimers are the dominant molecular form in sheep platelet AChE, suggesting a partial conversion of this membrane-bound form into soluble dimers and monomers, mainly with a hydrophilic character, through the action of either endogenous proteases and phospholipases or residual endogenous reducing agents.