S-Denitrosylation counteracts local inflammation and improves survival in mice infected with K. pneumoniae

Sepsis and septic shock remain are significant causes of mortality in the world. The inflammatory response should be at the basis of all organ dysfunction such as cardiovascular dysfunction, characterized by severe hypotension refractory to volume replacement and vasoconstrictor therapy. Nitric oxide (NO) has been implicated as a key element in both inflammatory and cardiovascular components of sepsis. In addition to activating soluble guanylate cyclase and potassium channels, NO also modifies proteins post-translationally by reacting with protein thiol groups, yielding S-nitrosothiols (RS-NO), which can act as endogenous NO reservoirs. Besides its use in quantifying free sulfhydryl groups of proteins and non-protein thiols, DTNB [5,5'-dithiobis-(2-nitrobenzoic acid)] has also been used as a pharmacological tool due to its specificity for oxidizing reactive sulfhydryl groups. Here we aimed to investigate the effects of DTNB in the inflammatory aspects of a sepsis model and to verify whether its effects can be attributed to S-denitrosylation.

Anesthetized female Swiss mice were intratracheally injected with 1 × 10⁸ CFU of K. pneumoniae. Twelve hours after pneumonia-induced sepsis, the animals were injected with vehicle (sodium bicarbonate 5 %, s.c.) or DTNB (31.5, 63 and 126 μmol/kg, s.c.). Twenty-four hours post-sepsis induction, plasma, bronchoalveolar lavage (BAL), and lung tissues were collected for assays (protein, cell count, nitrite + nitrate levels (NOx), cytokine levels, and sulfhydryl groups). In addition, lung S-nitrosylated proteins were visualized by a modified tissue assay for S-nitrosothiols.

Sepsis induced a significant vascular leakage in the lungs and elevated NOx levels in BAL, both reduced by DTNB. BAL leukocytosis and elevated IL-1β induced by sepsis were also reduced by DTNB, whereas it did not affect bacterial dissemination to liver, heart and BAL. Sepsis reduced free sulfhydryl groups in BAL and lung and DTNB did not change it. On the other hand, DTNB substantially reduced protein S-nitrosylation levels in the lung parenchyma and halved sepsis-induced mortality in septic mice.

Our results show that the administration of DTNB 12 h after bacterial instillation reduced most of the local inflammatory parameters and, more importantly, decreased mortality. These beneficial effects may be due to S-denitrosylation of RS-NO pools carried out by DTNB. Since DTNB was effective in reducing the inflammatory process after its onset, this mechanism of action could serve as a valuable proof of concept for compounds that can be useful to interfere with sepsis outcome.