Tissue Characterization of Hypoxia, Nitric Oxide and Apoptosis Pathways in a Baboon Model of E. coli Sepsis.

Sepsis - the most common cause of death in hospitalized patients -is characterized by severe inflammatory response and profound systemic vasculitic pathology accompanied by activation of blood coagulation. We observed previously that infusion of E. coli in baboons produced severe disruption of the microcirculation, such as capillary leakage, intravascular coagulation and tissue injury, which may lead to global tissue hypoxia, shock, organ failure, and death. Here, we report data on the temporal dynamics of tissue specific changes of hypoxia, nitric oxide (NO) and apoptosis pathways in response to sublethal or lethal E. coli challenge. Animals were administered sublethal doses of E. coli (LD_30–50; 10⁸ CFU/kg) and sacrificed after 2, 8 and 24 hours or lethal doses (LD₁₀₀; 9x10⁹ −1x10¹⁰ CFU/kg) for 8 hours. Both groups were compared to non challenged controls (n=3 per experimental condition). Six organs (lung, liver, heart, kidney, spleen and lymph nodes) were analyzed by immunofluorescence/confocal imaging for Hypoxia Inducible Factor 1 (HIF1α), NOS2, NOS3 nitrotyrosine (NT), inflammatory cell markers (myeloperoxidase and CD68 for neutrophils and macrophages, respectively), and apoptosis markers (active caspase-3, TUNEL, Bcl2, Bax). Tissue extracts were analyzed by ELISA (NT and caspase-3), dot and western blot (NT, Bcl2 and Bax) and RT-PCR and microarray assays for mRNA expression. The results show that HIF1α mRNA was increased by 2–3 fold, reaching maximum values at 2 hours in the lung and 24 hours in the liver. HIF1α nuclear translocation was detected by immunostaining at 8 hours in lung and 24 hours in the liver, especially in the LD₁₀₀ E. coli challenged baboons. HIF1α functions as a master transcriptional regulator of the adaptive response to hypoxia, and controls the expression of over 40 genes including NOS2, an inducible enzyme that induces NO production and thus decreases the vascular tone, and VEGF, a very potent permeability factor. NOS2 mRNA and protein were induced after 8 hours mainly in inflammatory cells, while endothelial NOS3 showed a transient decrease at 2–8 hours, and increased at 24 hours as compared to controls. The overall increase in NO production was reflected in transient high levels of nitrosylated proteins at 8 hours in sublethal animals and persistent high levels in lethal baboons. Ultrastructural analysis of lung and kidney performed by electron microscopy showed signs of defective permeability, such as accumulation of plasma proteins in the lung alveolae and in the subendothelium of the kidney peritubular capillaries, concomitant with intra- and/or extravascular fibrin deposition. Dose-dependent induction of apoptosis was detected by TUNEL and caspase-3 staining, especially in the proximal contort tubes of the kidney, hepatocytes, lymphoid and pericapsular cells of spleen and lymph nodes. Mitochondrial manganese superoxide dismutase (SOD2), a protective enzyme produced in reaction to oxidative stress had a latter response, reaching the maximum mRNA expression at 8 hour in lung and 24 hours in the liver. Taken together, these results place hypoxia as a key factor in the development of organ dysfunction and multiple organ failure in sepsis and suggest that targeting hypoxia controlled transcription factors, such as HIF1α, or upregulating the antioxidant mechanisms may represent valuable therapeutic approaches.