Inhibition of nitric oxide synthase gene expression in vivo and in vitro by repeated doses of endotoxin

Abstract

We have examined the effects of repeated endotoxin administration in vivo and in vitro on the induction of nitric oxide synthase (NOS). In vivo, hepatic NOS activity and mRNA were increased markedly by the administration of Escherichia coli lipopolysaccharide (LPS). The change in hepatic NOS activity coincided with a marked accumulation of hepatic citrulline. Both enzyme activity and citrulline concentration returned to normal by 12 h after LPS administration. At this time, a subsequent administration of endotoxin caused no change in either NOS mRNA, NOS activity, or citrulline concentration, and thus an endotoxin-refractory state for nitric oxide (NO) synthesis was established. Normal sensitivity was reestablished by 24 h after the initial dose. In vitro studies using both a macrophage cell line (HD11) and primary macrophages indicated that LPS pretreatment caused cells in culture to become completely refractory to subsequent stimulation by LPS. Finally, we tested the hypothesis that NO may be involved in the development of the refractory state. Various inhibitors blocked the initial synthesis of NO by > 90% but failed to influence the development of the refractory state. Our study demonstrates both in vivo and in vitro that NO synthesis is completely blocked after repeated exposure to endotoxin by a mechanism that appears to be pretranslational. This model of early endotoxin tolerance may provide insight into the molecular mechanisms that regulate expression of the NOS gene.

Broiler pulmonary hypertensive responses during lipopolysaccharide-induced tolerance and cyclooxygenase inhibition

“…Plasma NO concentrations do not remain elevated for more than 8 h after an i.v. LPS injection (Chapman and Wideman, 2006a;, which coincides with the onset of tolerance within 12 h and inability to restimulate NOS expression in hepatic tissues and macrophages for up to 24 h after chickens initially are exposed to LPS (Chang et al, 1996). In experiment 1, l-NAME increased PAP by only 4 mmHg to a nonhypertensive average of 24 mmHg in the 24-h tolerant + LPS group, providing no evidence that ongoing NO synthesis was aggressively modulating or masking a major PH response to LPS.…”

“…Broilers emerging from the interval of tolerance after 120 h exhibit interindividual variation in their PH responses to LPS, ranging from complete unresponsiveness (fully tolerant individuals) to high-amplitude PAP responses (post-tolerant individuals). Tolerance has previously been demonstrated at the level of isolated leukocytes and likely involves mechanisms similar to those responsible for tolerance in mammals, including downregulation or desensitization of components of the LPS receptor-mediated signaling cascade resulting in reduced production of key proinflammatory cytokines and altered synthesis of eicosanoid modulators (Geisel et al, 1994;Chang et al, 1996;Medvedev et al, 2000;Sato et al, 2000;Wang et al, 2002a). Tolerance might be an important compensatory or protective mechanism for broilers whose pulmonary vascular capacity is marginal under optimal conditions and whose respiratory systems are chronically challenged with LPS in commercial production facilities.…”

“…that 50 μg of LPS/kg of BW is the threshold dose for reliably inducing tolerance based on the refractoriness of hepatic NOS expression for up to 24 h in chickens (Chang et al, 1996), and 20 μg of LPS/kg of BW is the threshold dose for reliably inducing PH in broilers (Chapman et al, 2005).…”

“…Tolerance has been described for a range of pathophysiological responses to LPS that become attenuated or completely suppressed after an initial exposure to gram-negative bacteria or LPS. Tolerance elicited by an initial exposure to LPS desensitizes animals and is believed to confer resistance to the detrimental effects of an ongoing LPS challenge (Geisel et al, 1994;Chang et al, 1996;Makhlouf et al, 1998;Zeisberger and Roth, 1998;Sato et al, 2000;Wang et al, 2002aWang et al, , 2003O'Brien et al, 2005;Broad et al, 2006). Within the context of PH triggered by LPS, tolerance should reduce the likelihood of broilers developing PHS by reducing the amplitude and duration of pulmonary hemodynamic challenges triggered by inevitably recurring environmental exposure to LPS.…”

Cellular immune response of pigeons in the conditions of endotoxin fever and pyrogenic tolerance

“…The first phase of endotoxin tolerance, known as early-phase pyrogenic tolerance, is characterized by the reduction of synthesis and release of proinflammatory cytokines as a consequence of the changes in the infected cell. Suppression of tumor necrosis factor synthesis (Matsuura et al 1994) and inhibition of interleukin-6 (Roth et al 1994) and nitric oxide (Chang et al 1996) production belong to the main effects of the early-phase pyrogenic tolerance. Additionally, cellular immunity (Soszyński 2000) is stimulated, as a result of increased hepatic macrophage activation, responsible for LPS detoxication (Dinarello et al 1968).…”