Background Platelets contain a high amount of potentially active A subunit dimer of coagulation factor XIII (cellular FXIII; cFXIII). It is of cytoplasmic localization, not secreted, but becomes translocated to the surface of platelets activated by convulxin and thrombin (CVX+Thr). Objective To explore the difference in cFXIII translocation between receptor mediated and non-receptor mediated platelet activation and if translocation can also be detected on platelet-derived microparticles. Our aim was also to shed some light on the mechanism of cFXIII translocation. Methods Gel-filtered platelets were activated by CVX+Thr or Ca²⁺-ionophore (calcimycin). The translocation of cFXIII and phosphatidylserine (PS) to the surface of activated platelets and platelet-derived microparticles was investigated by flow cytometry, immunofluorescence and immune electron microscopy. Fluo-4-AM fluorescence was used for the measurement of intracellular Ca²⁺ concentration. Results Receptor mediated activation by CVX+Thr exposed cFXIII to the surface of over 60% of platelets. Electron microscopy revealed microparticles with preserved membrane structure and microparticles devoid of labeling for membrane glycoprotein CD41a. cFXIII was observed on both types of microparticles but was more abundant in the absence of CD41a. Rhosin, a RhoA inhibitor significantly decreased cFXIII translocation. Non-receptor mediated activation of platelets by calcimycin elevated intracellular Ca²⁺ concentration, induced the translocation of PS to the surface of platelets and microparticles, but failed to expose cFXIII. Conclusions The elevation of intracellular Ca²⁺ concentration is sufficient for the translocation of PS from the internal layer of the membrane, while the translocation of cFXIII from the platelet cytoplasm requires additional receptor mediated mechanism(s).

Macrophages express the A subunit of coagulation factor XIII (FXIII-A), a transglutaminase which cross-links proteins through Nε-(γ-L-glutamyl)-L-lysyl iso-peptide bonds. Macrophages are major cellular constituents of the atherosclerotic plaque; they may stabilize the plaque by cross-linking structural proteins and they may become transformed into foam cells by accumulating oxidized LDL (oxLDL). The combination of oxLDL staining by Oil Red O and immunofluorescent staining for FXIII-A demonstrated that FXIII-A is retained during the transformation of cultured human macrophages into foam cells. ELISA and Western blotting techniques revealed that the transformation of macrophages into foam cells elevated the intracellular FXIII-A content. This phenomenon seems specific for macrophage-derived foam cells; the transformation of vascular smooth muscle cells into foam cells fails to induce a similar effect. FXIII-A containing macrophages are abundant in the atherosclerotic plaque and FXIII-A is also present in the extracellular compartment. The protein cross-linking activity of FXIII-A in the plaque was demonstrated using an antibody labeling the iso-peptide bonds. Cells showing combined staining for FXIII-A and oxLDL in tissue sections demonstrated that FXIII-A-containing macrophages within the atherosclerotic plaque are also transformed into foam cells. Such cells may contribute to the formation of lipid core and the plaque structurization.

The potentially active A subunit of coagulation factor XIII (FXIII-A) is an intracellular transglutaminase expressed in various cell types including platelets and monocytes/macrophages. It is involved in stabilizing protein structures by cross-linking through Nε-(?-L-glutamyl)-L-lysyl iso-peptide bonds. Macrophages are major cellular constituents of the atherosclerotic plaque and are important in determining its structural/functional features. Two of their important functions are the accumulation of oxidized LDL in the lipid core, and by cross-linking structural proteins they may stabilize the plaque and protect the thrombi of atherogenic origin against fibrinolytic degradation. It is important to know whether these functions operate in parallel utilizing the same cellular compartments. First, we showed that monocyte-derived human macrophages significantly increase their FXIII-A content when up-taking oxidized LDL. This phenomenon is very likely independent of the process of transformation into foam cells, as the transformation of vascular smooth muscle cells into foam cells fails to result in the expression of FXIII-A. FXIII containing macrophage-like cells are abundant in the plaque and FXIII-A is also present in the extracellular core. Several cells co-stained for FXIII-A and for Oil Red O suggest that expression of FXIII-A and lipid up-take are common features of macrophages present in the atherosclerotic plaque.