Dephosphorylation of human fibrinogen, previously phosphorylated in vitro by protein kinase C, by whole blood or intestinal alkaline phosphatase. Effects on thrombin-induced gelation of in vitro dephosphorylated human fibrinogen

Effects of Post-Translational Modifications of Fibrinogen on Clot Formation, Clot Structure, and Fibrinolysis: A Systematic Review

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Post-translational modifications of fibrinogen influence the occurrence and progression of thrombotic diseases. In this systematic review, we assessed the current literature on post-translational modifications of fibrinogen and their effects on fibrin formation and clot characteristics.

Alterations in C3 Activation and Binding Caused by Phosphorylation by a Casein Kinase Released from Activated Human Platelets

A casein kinase released from activated human platelets phosphorylates a number of plasma proteins extracellularly, and that activation of platelets in systemic lupus erythematosus patients parallels an increase in the phosphate content of plasma proteins, including C3. The present study was undertaken to characterize this platelet protein kinase and to further elucidate the effect(s) on C3 function of phosphorylation by platelet casein kinase. The phosphate content of human plasma C3 was increased from 0.15 to 0.60 mol phosphate/mol of C3 after platelet activation in whole blood or platelet-rich plasma. The platelet casein kinase was distinct from other casein kinases in terms of its dependence on cations, inhibition by specific protein kinase inhibitors, and immunological reactivity. C3 that had been phosphorylated with platelet casein kinase was tested for its susceptibility to cleavage by trypsin or the classical and alternative pathway convertases and its binding to EAC and IgG. Phosphorylation did not affect the cleavage of C3 into C3a and C3b, but the binding of fragments from phosphorylated C3 to EAC14oxy2 cells and to IgG in purified systems and in serum was increased by 1.6–4.5 times over that of unphosphorylated C3. A covariation was seen between the enhanced binding of C3 fragments to IgG after phosphorylation and an increased ratio of glycerol/glycine binding, from 2.0 for unphosphorylated C3 to 4.9 for phosphorylated C3. The present study suggests that an overall effect of phosphorylation of C3 by platelet casein kinase is to enhance the opsonization of immune complexes.

Increased phosphate content in complement component C3, fibrinogen, vitronectin, and other plasma proteins in systemic lupus erythematosus: covariation with platelet activation and possible association with thrombosis

OBJECTIVE

To investigate whether extracellular phosphorylation of plasma proteins takes place in vivo in patients with systemic lupus erythematosus (SLE), to determine possible correlations between phosphate levels and clinical and/or laboratory parameters, and to identify individual phosphorylated plasma proteins.

METHODS

Sera from SLE patients were analyzed for total amounts of protein-bound phosphate by a colorimetric technique, and for levels of beta-thromboglobulin by radioimmunoassay. In addition, the ability of these sera to activate platelets, resulting in the release of protein kinase, was tested using an assay in which platelet-rich plasma from healthy blood donors was incubated with sera or immune complexes from SLE patients. In this assay, [gamma-32P]ATP was added, and 32P-labeled C3 was quantified. Phosphate in individual proteins was detected by Western blot analysis.

RESULTS

32P-labeled, activated platelets were able to phosphorylate exogenously added proteins, without the addition of ATP or cations. Platelet-rich plasma from healthy blood donors became activated by sera or by polyethylene glycol-precipitated immune complexes from patients with SLE, which led to the extracellular phosphorylation of plasma proteins, exemplified in the C3 assay. The phosphate content in plasma proteins was increased in SLE patients with previous thrombosis. The degree of phosphorylation increased up to 3-fold in serial samples obtained from 2 SLE patients during periods of disease exacerbation. Substantial phosphate increases were seen in C3 and fibrinogen. The changes were linked to platelet activation because of the observed covariation with the levels of beta-thromboglobulin.

CONCLUSION

In SLE patients, the phosphate content in plasma proteins (including C3 and fibrinogen) increases due to platelet activation.

Increased phosphate content of fibrinogen in vivo correlates with alteration in fibrinogen behaviour

Fibrinogen was purified from five patients admitted for hip-replacement surgery the day before (day 0), the day after (day 2) and and one week after the operation (day 8). The behaviour of each patient's three fibrinogens was compared in thrombin gelation assays and plasmin degradation experiments to investigate whether the reported increase in protein-bound phosphate at day 2 and day 8 had any effect on the functional behaviour of fibrinogen as has been demonstrated in vitro. It was found that the thickness of the fibrin fibres produced by thrombin increased markedly at day 2 and declined thereafter. Susceptibility to plasmin appeared to decrease post-operatively by 50% and remained at that level on day 8 despite the phosphate content returning to normal. This has also been shown for fibrinogen phosphorylated in vitro. We conclude, after testing the fibrinogens with and without alkaline phosphatase pretreatment, that our data most resemble the published findings for in vitro phosphorylation of fibrinogen by casein kinase II.

The effects of in vitro phosphorylation and dephosphorylation on the thrombin-induced gelation and plasmin degradation of fibrinogen

The alpha-chain of human fibrinogen was found to be phosphorylated in EDTA-anticoagulated whole blood when trace amounts of (gamma-32P)ATP and 7.5 mM Mg2+ ions were added. Fibrinogen was not phosphorylated if only the ATP was added. The thrombin-induced gelation of fibrinogen phosphorylated by protein kinase A, casein kinase I or II was studied spectrophotomerically. It was found that phosphorylation by protein kinase A caused the formation of thinner fibrin fibres, whereas phosphorylation by casein kinase II resulted in fibres slightly thicker than those of the control fibrinogen (equivalent to a 20% increase in the control fibrinogen concentration). Phosphorylation with casein kinase I did not significantly affect the fibrin fibre thickness. Dephosphorylation by alkaline phosphatase removed 50% of the 32P-labelled phosphate from protein kinase A-phosphorylated fibrinogen and over 90% from the casein kinase I or II-phosphorylated fibrinogens. This dephosphorylation resulted in a general increase in fibre thickness in the gelation assay in all samples, although the fibres of the phosphorylated fibrinogens remained substantially thinner than the dephosphorylated control fibrinogen. Plasmin digestion of the phosphorylated fibrinogens showed that they were more resistant to cleavage, being cleaved at only 30% to 70% of the rate of control fibrinogen and that this resistance was unaltered by dephosphorylation, in contrast to the thrombin gelation experiments.

Dephosporylation with alkaline phosphatase of histone and fibrinogen phosphorylated with protein kinase C in vitro

Alkaline phosphatase from calf intestinal mucosa dephosphorylated histone H1 and fibrinogen that had been phosphorylated with protein kinase C. The reaction velocity was dependent on the ionic strength of the buffer; decreasing with increasing concentration. The pH optimum was around 7, which is lower than pH-optima described for other kinds of substrates. (32P) phosphorylated fibrinogen was dephosphorylated about 20 times faster than (32P)phosphohistone on a weight basis and the reaction continued linearily with time for the longest time tested (3 hs) even at 37 degrees C. As alkaline phosphatase is present in the blood the possible physiological significance of the dephosphorylation of phosphofibrinogen is discussed.

Conformational changes in human fibrinogen after in vitro phosphorylation and their relation to fibrinogen behaviour

The far-ultraviolet circular dichroism spectra of fibrinogens phosphorylated by protein kinase C or casein kinase II indicated a conformational change corresponding to an increase in ordered secondary structure. The spectra of protein kinase A- or casein kinase I-phosphorylated fibrinogens did not differ substantially from the control. Fluorescence studies indicated changes in the tertiary structure around tryptophan residues for protein kinase A- or C-phosphorylated fibrinogens, but failed to show any such change for fibrinogen phosphorylated by either of the casein kinases. This latter result was also confirmed by circular dichroism measurements in the near-ultraviolet region. The apparent increase in ordered structure was proposed as an explanation for the slower rate of plasmin degradation seen in fibrinogens after phosphorylation by protein kinase C [6], and casein kinase II, especially as both spectral changes and plasmin degradation rate were unaffected by alkaline phosphatase.

Plasmin digestion of human fibrinogen previously phosphorylated by protein kinase C or dephosphorylated by alkaline phosphatase in vitro

Human fibrinogen, either untreated or previously phosphorylated by protein kinase C, was incubated with plasmin generated by streptokinase, urokinase or tissue plasminogen activator and the resulting fragments were separated by gel electrophoresis. Plasmin degradation resulted in the expected X, Y and D fragments, but the degradation rates differed. In vitro phosphorylation of fibrinogen was seen to inhibit the plasmin digestion. Treatment with alkaline phosphatase did not reverse the inhibition.