Time course of nitric oxide synthases, nitrosative stress, and poly(ADP ribosylation) in an ovine sepsis model

Severe sepsis and septic shock continue to be major causes of morbidity and mortality of ICU patients [1]. Among the sources of nosocomial infections, ICU-acquired pneumonia represents the leading cause of death [2, 3]; and Pseudomonas was the second most frequently identified bacteria species causing sepsis among ICU patients in a recent multi-center, observational study [4].

Previous studies revealed the important roles of the different isoforms of nitric oxide (NO) synthases (NOS), peroxynitrite (ONOO^-), and poly-ADP ribose (PAR) in the pathophysiology of cardiopulmonary derangements induced by acute lung injury (ALI) and sepsis, thereby offering potentially new treatment options such as inhibition of NOS [5], decomposition catalyzation of ONOO^- [6], or inhibition of PAR polymerase (PARP) [7].

When considering possible treatment strategies of patients with sepsis, however, it may be crucial to identify the time changes of the expression of the above mentioned pathogenic factors. The present study was therefore conducted to determine the time course of endothelial NOS (eNOS), neuronal NOS (nNOS), inducible NOS (iNOS), 3-nitrotyrosine (3-NT), an index of protein nitration and ONOO^-, as well as PAR in lung tissue using an established ovine model of sepsis induced by ALI and instillation of live Pseudomonas bacteria into the lungs [8].

This study was approved by the Animal Care and Use Committee of the University of Texas Medical Branch and conducted in compliance with the guidelines of the National Institutes of Health and the American Physiological Society for the care and use of laboratory animals.

In the present study, induction of sepsis following ALI contributed to an early and severe deterioration of pulmonary function, which was associated with early over-expression of eNOS and iNOS, enhanced NOS activity, and increased expression of markers of nitrosative stress and DNA damage in lung tissue.

The pulmonary response to ALI and sepsis in sheep has been comprehensively studied in previous experiments [5, 8, 9]. It has been demonstrated that excessively produced NO may exert cytotoxic effects by reacting with superoxide radicals from activated neutrophils, thereby yielding reactive oxygen and nitrogen species such as ONOO^-. ONOO^- in turn may induce cell damage by oxidizing and nitrating/nitrosating proteins and lipids [11, 12]. Furthermore, ONOO^- can induce excessive activation of the nuclear repair enzyme PARP [13, 14], which may cause ATP depletion and cell damage [14, 15]. Together, these changes can induce endothelial damage, pulmonary capillary hyperpermeability, and pulmonary edema [9], resulting in severe deterioration of the pulmonary gas exchange.

Increased knowledge of these pathomechanisms provides novel therapeutical options for patients with ALI and sepsis such as inhibition of NOS [5, 16] and PARP [7] or decomposition catalyzation of ONOO^- [6]. In this regard, extensive research has been conducted to identify the roles of the three different isoforms of NOS. It is commonly believed that NO produced by constitutively expressed isoforms (nNOS and eNOS) is implicated in important physiologic processes, whereas excessively produced NO by iNOS is suspected to be critically involved in the pathophysiology of various diseases including sepsis and ALI [17, 18]. Increasing evidence suggests that not only is iNOS-derived NO, in part, responsible for the cardiopulmonary derangements following ALI or sepsis, but so is NO from constitutively expressed nNOS and eNOS [19‐22]. In this regard, the results from previous studies suggested beneficial effects of selective NOS inhibition in ALI and sepsis [23‐25] at the time of their maximum activity. In contrast, non-selective inhibition of NOS [16] or selective inhibition of different NOS isoforms at the wrong time points may be ineffective or even detrimental [10, 26]. Likewise, inhibition of PARP in septic sheep only partially attenuated the sepsis-related cardiopulmonary derangements [7]. The wrong timing of interventions may provide an explanation for these failures in treatment, and thus examination of the pulmonary tissue response at different time points may deliver valuable information for treatment strategies in future experiments.

Excessive NO production may not only be attributed to over-expression of NOS, but also to enhanced activity of constitutively expressed enzymes. Therefore, to profoundly understand the roles of different NOS isoforms in ALI and sepsis, we measured mRNA, protein expression, and enzyme activity in lung tissue, as well as plasma levels of stable NO metabolites in the present study. Although neither mRNA nor protein expression of nNOS was increased at any of the evaluated time points, both eNOS and iNOS protein expressions started to increase early after the injury. Albeit the changes in mRNA of eNOS and iNOS were not statistically significant, they tended to be elevated prior to the increase in protein expression of the respective isoenzyme. Subsequent to enhanced transcription and expression of eNOS and iNOS, both total NOS activity and plasma NOS levels were increased from 12 to 24 hours after the double-hit injury. Interestingly, total NOS, but not iNOS, activity was significantly increased by the injury, suggesting that constitutively expressed NOS contributed substantially to the increases in NOS activity. With the applied methods, it cannot be differentiated between nNOS and eNOS activity. It is therefore conceivable that both isoforms were involved.

Increased expression of 3-NT, a marker of inflammation-related processes and ONOO^-, and PAR, an index of DNA damage, in lung tissue were early events after the injury, and protein expression returned to values of control animals at 18 and 12 hours, respectively. This was possibly due to decreased ONOO^- and PARP activity in the later course of the injury or simply to the fact that a majority of cells with high ONOO^- production and PARP activation had already died. The latter assumption is supported by the early peak of p65 protein expression, a subunit of the nuclear factor-kappaB, in lung tissue. Regardless, it is obvious that pharmacologic intervention such as ONOO^- decomposition catalyzation or PARP inhibition starting later than 12 hours after injury must become less effective in this model. On the other hand, if pharmacologic interventions have started early, more cells may be vital at later time points and thus treatment may still be efficient later than 12 hours post-injury.

The up-regulations of NOS, ONOO^-, and PAR protein were followed by a transient increase in protein contents of the pro-inflammatory IL-8 in the lung. For technical reasons, it was not feasible to measure the plasma concentrations of inflammatory cytokines in sheep, but it is conceivable that the increase in IL-8 protein in the lung was secondary to an elevation in blood concentrations.

The current study design does not allow detection of causative mechanisms. It can be discussed, however, that the deterioration in pulmonary function was probably not solely due to the excessive increase in plasma NOx concentrations caused by increased iNOS expression, because these were later events and the pulmonary oxygenation index was already markedly reduced as early as four hours after the injury. More likely, the earlier occurring up-regulation of eNOS may have contributed to increased ONOO^- production and PARP activation which, in turn, may have induced endothelial cell damage in the lung. For this purpose, small amounts of eNOS-derived NO appear to be enough, because plasma NOx levels were not increased at this early time point. Alternatively, the increase in constitutive NOS-derived NO may have been missed due to the absence of data on lung tissue NOS activity at earlier time points than four hours. When studying Figure 1, however, it becomes apparent that both oxygenation index and shunt fraction markedly worsened between 12 and 18 hours post-injury. It can be speculated that these secondary deteriorations were now due to the up-regulation of iNOS and excessively produced NO, which increased pulmonary shunting phenomena thereby further impairing pulmonary oxygenation.

There are some limitations of the study we want to acknowledge. First, the study was designed to monitor the sepsis-related pulmonary tissue response for 24 hours after the injury and, unfortunately, we were not able to include more time points for tissue harvesting in this large animal model. It is thus conceivable that we missed the respective time point of peak protein expression and/or activity of nNOS. In this context, experimental evidence revealed that increased activity and expression of nNOS may occur earlier than four hours in the paraventricular nucleus of rats subjected to lipopolysaccharide injection [27]. Second, it may be unexpected that eNOS expression was increased in the present study because eNOS is supposed to be a constitutive enzyme, which cannot be increasingly expressed. However, it needs to be considered that the present investigation evaluated protein expressions and enzyme activities in whole lung homogenates, but not in single cells. In this regard, previous studies demonstrated that constitutive NOS can be expressed by circulating cells, such as neutrophils [28, 29]. Consequently, an injury-related increase in inflammatory cells may account for increased protein expression of constitutive NOS in the current study. The discussed issues may be addressed in future studies utilizing genetically modified mice (e.g. nNOS or eNOS deficient). This approach may allow for elimination of possible interactions between NOS isoforms and inclusion of numerous time points due to reduced costs of a small animal model. It further needs to be regarded as a limitation of the current study that the time changes in some parameters may have missed statistical significance due to the relatively low number of animals per group. In addition, the present study investigated female subjects only, and thus gender-specific differences in time changes of NOS, 3-NT, and PARP could not be detected. In this context, it has previously been reported that inhibition of PARP showed protective effects only in male rodents subjected to ischemic stroke or endotoxin-induced inflammation [30‐32]. Female gender per se provided protection against these injuries. However, pharmacologic inhibition of PARP also had protective effects in female subjects of different species [33].

The current study describes the time course of NOS isoform expression and NOS activity as well as important markers of ONOO^- and PARP activation in lung tissue of sheep subjected to ALI and sepsis. This detailed information may greatly enhance the understanding of pathophysiologic alterations in our ovine model. The identification of the time changes of the described pathogenetic factors may ameliorate the timing of treatment strategies in future studies.