Factor VII-Activating Protease Is a Regulator of Histone Cytotoxicity

Background: During sepsis, ischemia-reperfusion injury and sterile inflammation, levels of circulating histones and nucleosomes have been found to correlate with disease severity. Although the pathophysiological effects of circulating nucleosomes are not fully understood, free histones have been described to mediate cytotoxicity and contribute to disease severity. Several plasma proteins have been identified that modulate these cytotoxic effects  by neutralizing or degrading histones. We have previously described that the plasma serine protease Factor VII-activating protease (FSAP) is activated upon contact with late apoptotic or necrotic cells and induces release of nucleosomes from these cells. During this release, histone H1 is cleaved. In addition, it has been described that FSAP is (auto)activated upon contact with histones and subsequently degrades histone H3.

Aims: To investigate the regulation of histone-induced cytotoxicity by FSAP.  Moreover, we aimed to compare differences in the cytotoxicity of histones and nucleosomes (histone+DNA) and potential differences in their proteolysis by FSAP.

Methods: To determine FSAP activation and proteolysis of histones in serum we incubated normal donor or FSAP-deficient serum with 100ug/ml calf thymus histones. Upon its activation FSAP is rapidly bound by plasma inhibitors and we measured complexes of FSAP and its plasma inhibitor alpha 2-antiplasmin as an indirect readout for FSAP activation. To detect FSAP mediated proteolysis of histones we performed immunoblotting for  histone H3. Results were corroborated using plasma-purified or recombinant FSAP. To determine histone-mediated cytotoxicity, we measured LDH levels after overnight incubation of HEK293 or ECRF cells with 100ug/ml calf thymus histone that had undergone a  pre-incubation step in the presence or absence of healthy donor serum, FSAP-deficient serum or FSAP-depleted serum. Moreover, we tested the cytotoxicity of purified nucleosomes that were pre-incubated in the presence or absence of benzonase to degrade DNA.

Results: Endogenous FSAP in serum was activated upon contact with histones, and histone proteolysis was observed. Similarly, histones were proteolyzed upon incubation using active plasma-purified or pre-activated recombinant FSAP ,whereas no histone proteolysis was observed using recombinant inactive FSAP. These results indicate that FSAP is responsible for the observed histone degradation. FSAP-mediated histone degradation was able to protect HEK293 and ECRF cells against histone-induced cytotoxicity, showing that  the resultant histone breakdown products had lost their cytotoxic effects. Recombinant inactive FSAP did not affect histone-induced cytotoxicity, indicating that the catalytic activity of FSAP is required for the observed protective effect of FSAP on cytotoxicity. When FSAP-deficient or FSAP-depleted serum was used, this led to greatly diminished histone proteolysis and reduced histone-induced cytotoxicity. Using our assay, we found that nucleosomes did not show cytotoxic effects unless they had been pre-incubated with benzonase to release histones. Notably, whereas intact nucleosomes were resistant against degradation by FSAP, FSAP protected against the cytotoxicity induced by histones released from these nucleosomes.

Conclusion: FSAP in serum is activated by histones and subsequently proteolyses histones ,resulting in protection of cells from histone-induced cytotoxicity. Histones in the form of nucleosomes did not induce cytotoxicity, however after DNA removal the histones regained cytotoxicity. FSAP efficiently protected against the cytotoxicity of the unbound histones. We predict that the form in which histones are released into the blood is highly relevant in the context of human health and disease. We speculate that when endogenous DNases in serum degrade the DNA in nucleosomes, or alternatively, when highly cytotoxic histones are inadvertently released from dead cells in their free form rather than in the form of nucleosomes, FSAP mediates their efficient degradation to protect against the damaging effects of free histones. Neutralization of histones by FSAP may be an effective therapeutic approach to treat systemic inflammation.