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Objectives: To evaluate whether the up-regulation in sepsis-induced gut epithelial apoptosis is balanced by an increase in intestinal proliferation and to assess mechanisms affecting the gut's regenerative response to overwhelming infection.
Design: Prospective, randomized, controlled study.
Setting: Animal laboratory in a university medical center.
Interventions: Mice were subjected to intratracheal injection of Pseudomonas aeruginosa and killed between 1.5 and 24 hrs after induction of pneumonia-induced sepsis to assess for gut epithelial proliferation and cell division and for apoptosis. Animals were compared with sham-operation controls, septic transgenic mice that overexpress Bcl-2 throughout their small intestinal epithelium, and septic p53 mice.
Measurements and Main Results: Proliferation and cell division were assessed by measuring S-phase and M-phase cells in intestinal crypts. The number of S-phase cells showed a progressive decline at all time points measured, with a 5-fold decrease in proliferation between control animals and septic mice 24 hrs after intratracheal injection of pathogenic bacteria (p < .0001). In contrast, cells in M-phase remained constant for the first 12 hrs after the onset of sepsis, but increased nearly 50% at 24 hrs after instillation of P. aeruginosa (p < .005). Both the decrease in S-phase cells and the increase in M-phase cells were partially suppressible in Bcl-2 overexpressors, but cellular proliferation and division were similar between septic p53 and p53 mice. Crypt apoptosis was increased at all time points, with maximal death occurring between 12 and 24 hrs.
Conclusions: Sepsis from P. aeruginosa pneumonia induces a p53-independent decrease in gut epithelial proliferation. Despite an increase in sepsis-induced intestinal apoptosis, there is no compensatory increase in intestinal epithelial proliferation, and there is evidence of a cell cycle block with an accumulation of cells in M-phase. Decreasing gut apoptosis by overexpression of Bcl-2 is associated with a partial reversal of the effect of sepsis on the cell cycle.
Sepsis is the leading cause of intensive care unit mortality nationwide, accounting for >210,000 deaths in the United States annually. A recent epidemiologic analysis estimates that 750,000 people become septic each year at a hospital cost of $16.7 billion, and the death rate from overwhelming infection has increased >90% over the last 20 yrs. Multiple studies have shown the gut plays a major role in the development of critical illness, leading to its characterization as the "motor" of the systemic inflammatory response.
Under basal conditions, proliferation in intestinal crypts is balanced by cell loss via apoptosis or exfoliation of intact cells into the intestine's lumen. Both gut proliferation and cell death are altered in critical illness. Gut epithelial proliferation decreases in a model of 60% body surface area burn in rats when measured by S-phase cells labeled with 5-bromo-2'deoxyuridine (BrdU). In contrast, proliferation is increased in rats subjected to either cecal ligation and puncture or endotoxemia when measured by incorporation of hydrogen-3 thymidine.
Both animal models and a human autopsy study showed increased gut epithelial apoptosis in sepsis. Elevated intestinal apoptosis seems to be detrimental in sepsis. Decreasing gut cell death by overexpression of the anti-apoptotic protein Bcl-2 in the intestinal epithelium of transgenic mice is associated with a 10-fold improvement in survival from Pseudomonas aeruginosa pneumonia-induced sepsis and a 2-fold increase in survival from cecal ligation and puncture.
These studies demonstrate the potential functional importance of altered proliferation and death in sepsis. However, they do not address whether increased sepsis-induced gut apoptosis is balanced by increased regeneration in the crypt epithelium. Uncompensated changes in the continuum between cell proliferation and cell loss could lead to alterations in intestinal function caused by changes in morphology and permeability. We therefore examined both gut proliferation and apoptosis after induction of sepsis from P. aeruginosa pneumonia to see if alterations in death led to compensatory increases in proliferation and cell division.
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