Katherine Chang

Hyperglycemic Pseudohypoxia and Diabetic Complications

Vasodilation and increased blood flow are characteristic early vascular responses to acute hyperglycemia and tissue hypoxia. In hypoxic tissues these vascular changes are linked to metabolic imbalances associated with impaired oxidation of NADH to NAD+ and the resulting increased ratio of NADH/NAD+. In hyperglycemic tissues these vascular changes also are linked to an increased ratio of NADH/NAD+, in this case because of an increased rate of reduction of NAD+ to NADH. Several lines of evidence support the likelihood that the increased cytosolic ratio of free NADH/NAD+ caused by hyperglycemia, referred to as pseudohypoxia because tissue partial pressure oxygen is normal, is a characteristic feature of poorly controlled diabetes that mimics the effects of true hypoxia on vascular and neural function and plays an important role in the pathogenesis of diabetic complications. These effects of hypoxia and hyperglycemia-induced pseudohypoxia on vascular and neural function are mediated by a branching cascade of imbalances in lipid metabolism, increased production of superoxide anion, and possibly increased nitric oxide formation.

Sepsis-Induced Apoptosis Causes Progressive Profound Depletion of B and CD4+ T Lymphocytes in Humans

Patients with sepsis have impaired host defenses that contribute to the lethality of the disorder. Recent work implicates lymphocyte apoptosis as a potential factor in the immunosuppression of sepsis. If lymphocyte apoptosis is an important mechanism, specific subsets of lymphocytes may be more vulnerable. A prospective study of lymphocyte cell typing and apoptosis was conducted in spleens from 27 patients with sepsis and 25 patients with trauma. Spleens from 16 critically ill nonseptic (3 prospective and 13 retrospective) patients were also evaluated. Immunohistochemical staining showed a caspase-9-mediated profound progressive loss of B and CD4 T helper cells in sepsis. Interestingly, sepsis did not decrease CD8 T or NK cells. Although there was no overall effect on lymphocytes from critically ill nonseptic patients (considered as a group), certain individual patients did exhibit significant loss of B and CD4 T cells. The loss of B and CD4 T cells in sepsis is especially significant because it occurs during life-threatening infection, a state in which massive lymphocyte clonal expansion should exist. Mitochondria-dependent lymphocyte apoptosis may contribute to the immunosuppression in sepsis by decreasing the number of immune effector cells. Similar loss of lymphocytes may be occurring in critically ill patients with other disorders.

Caspase inhibitors improve survival in sepsis: a critical role of thelymphocyte

Sepsis induces lymphocyte apoptosis and prevention of lymphocyte death may improve the chances of surviving this disorder. We compared the efficacy of a selective caspase-3 inhibitor to a polycaspase inhibitor and to caspase-3−/− mice. Both inhibitors prevented lymphocyte apoptosis and improved survival. Caspase-3−/− mice shared a decreased, but not total, block of apoptosis. The polycaspase inhibitor caused a very substantial decrease in bacteremia. Caspase inhibitors did not benefit RAG-1−/− mice, which had a >tenfold increase in bacteremia compared to controls. Adoptive transfer of T cells that overexpressed the anti-apoptotic protein Bcl-2 increased survival. T cells stimulated with anti-CD3 and anti-CD28 produced increased interleukin 2 and interferon γ by 6 h. Thus, caspase inhibitors enhance immunity by preventing lymphocyte apoptosis and lymphocytes act rapidly, within 24 h, to control infection.

Prevention of Diabetic Vascular Dysfunction by Guanidines: Inhibition of Nitric Oxide Synthase Versus Advanced Glycation End-Product Formation

This study was undertaken to compare the ability of two guanidine compounds (aminoguanidine and methylguanidine), with different in vitro effects on NO synthase activity and AGE formation, to inhibit diabetic vascular dysfunction developing early after the onset of diabetes. In rats with STZ-induced diabetes of 5-wk duration, regional vascular [125I]albumin permeation was increased about two- to threefold in ocular tissues, sciatic nerve, and aorta; in general, both guanidine compounds normalized albumin permeation in diabetic rats without affecting it in controls. Methylguanidine was only ∼7% as effective as aminoguanidine as an inhibitor of AGE formation from L-lysine and G6P; both compounds were poor inhibitors of AR. Methylguanidine was ∼1–5% as potent as aminoguanidine and L-NMMA as an inhibitor of the cytokine- and endotoxin-inducible isoform of NO synthase. In contrast, the potency of methylguanidine as an inhibitor of the constitutive isoform of NO synthase was comparable to that of aminoguanidine, and both guanidine compounds were much less effective than L-NMMA. These observations suggest a role for a relative or absolute increase in NO production in the pathogenesis of early diabetic vascular dysfunction and raise the possibility that inhibition of diabetic vascular functional changes by aminoguanidine may reflect inhibition of NO synthase activity rather than, or in addition to, prevention of AGE formation.

Accelerated Lymphocyte Death in Sepsis Occurs by both the Death Receptor and Mitochondrial Pathways

Patients with sepsis are immune compromised, as evidenced by their failure to clear their primary infection and their propensity to develop secondary infections with pathogens that are often not particularly virulent in normal healthy individuals. A potential mechanism for immunosuppression in sepsis is lymphocyte apoptosis, which may occur by either a death receptor or a mitochondrial-mediated pathway. A prospective study of blood samples from 71 patients with sepsis, 55 nonseptic patients, and 6 healthy volunteers was undertaken to quantitate lymphocyte apoptosis and determine cell death pathways and mechanisms of apoptosis. Apoptosis was evaluated by flow cytometry and Western blotting. Lymphocyte apoptosis was increased in CD4 and CD8 T cells, B cells (CD20), and NK cells (CD56) in septic vs nonseptic patients. Samples taken sequentially from 10 patients with sepsis showed that the degree of CD3 T cell apoptosis correlated with the activity of his/her sepsis. In septic patients, apoptotic lymphocytes were positive for active caspases 8 and 9, consistent with death occurring by both mitochondrial-mediated and receptor-mediated pathways. In support of the concept that both death pathways were operative, lymphocyte apoptosis occurred in cells with markedly decreased Bcl-2 (an inhibitor of mitochondrial-mediated apoptosis) as well as cells with normal concentrations of Bcl-2. In conclusion, apoptosis occurs in a broad range of lymphocyte subsets in patients with sepsis and correlates with the activity of the disease. Lymphocyte loss occurs by both death receptor and mitochondrial-mediated apoptosis, suggesting that there may be multiple triggers for lymphocyte apoptosis.

Identification of the PGRMC1 protein complex as the putative sigma-2 receptor binding site

The sigma-2 receptor, whose gene remains to be cloned, has been validated as a biomarker for tumor cell proliferation. Here we report the use of a novel photoaffinity probe, WC-21, to identify the sigma-2 receptor binding site. WC-21, a sigma-2 ligand containing both a photoactive moiety azide and a fluorescein isothiocyanate group, irreversibly labels sigma-2 receptors in rat liver; the membrane-bound protein was then identified as PGRMC1 (progesterone receptor membrane component-1). Immunocytochemistry reveals that both PGRMC1 and SW120, a fluorescent sigma-2 receptor ligand, colocalizes with molecular markers of the endoplasmic reticulum and mitochondria in HeLa cells. Overexpression and knockdown of the PGRMC1 protein results in an increase and a decrease in binding of a sigma-2 selective radioligand, respectively. The identification of the putative sigma-2 receptor binding site as PGRMC1 should stimulate the development of unique imaging agents and cancer therapeutics that target the sigma-2 receptor/PGRMC1 complex.

Prevention of Hemodynamic and Vascular Albumin Filtration Changes in Diabetic Rats by Aldose Reductase Inhibitors

This study investigated hemodynamic changes in diabetic rats and their relationship to changes in vascular albumin permeation and increased metabolism of glucose to sorbitol. The effects of 6 wk of streptozocin-induced diabetes and three structurally different inhibitors of aldose reductase were examined on 1) regional blood flow (assessed with 15-μm 85Sr-labeled microspheres) and vascular permeation by 125I-Iabeled bovine serum albumin (BSA) and 2) glomerular filtration rate (assessed by plasma clearance of 57Co-labeled EDTA) and urinary albumin excretion (determined by radial immunodiffusion assay). In diabetic rats, blood flow was significantly increased in ocular tissues (anterior uvea, posterior uvea, retina, and optic nerve), sciatic nerve, kidney, new granulation tissue, cecum, and brain. 125I-BSA permeation was increased in all of these tissues except brain. Glomerular filtration rate and 24-h urinary albumin excretion were increased 2- and 29-fold, respectively, in diabetic rats. All three aldose reductase inhibitors completely prevented or markedly reduced these hemodynamic and vascular filtration changes and increases in tissue sorbitol levels in the anterior uvea, posterior uvea, retina, sciatic nerve, and granulation tissue. These observations indicate that early diabetes-induced hemodynamic changes and increased vascular albumin permeation and urinary albumin excretion are aldose reductase-linked phenomena. Discordant effects of aldose reductase inhibitors on blood flow and vascular albumin permeation in some tissues suggest that increased vascular albumin permeation is not entirely attributable to hemodynamic changes. We hypothesize that 1) increases in blood flow may reflect impaired contractile function of smooth muscle cells in resistance arterioles and 2) increases in vascular 125I-BSA permeation and urinary albumin excretion reflect impaired vascular barrier functional integrity in addition to increased hydraulic conductance secondary to microvascular hypertension associated with decreased vascular resistance.

IL-15 Prevents Apoptosis, Reverses Innate and Adaptive Immune Dysfunction, and Improves Survival in Sepsis

IL-15 is a pluripotent antiapoptotic cytokine that signals to cells of both the innate and adaptive immune system and is regarded as a highly promising immunomodulatory agent in cancer therapy. Sepsis is a lethal condition in which apoptosis-induced depletion of immune cells and subsequent immunosuppression are thought to contribute to morbidity and mortality. This study tested the ability of IL-15 to block apoptosis, prevent immunosuppression, and improve survival in sepsis. Mice were made septic using cecal ligation and puncture or Pseudomonas aeruginosa pneumonia. The experiments comprised a 2 × 2 full factorial design with surgical sepsis versus sham and IL-15 versus vehicle. In addition to survival studies, splenic cellularity, canonical markers of activation and proliferation, intracellular pro- and antiapoptotic Bcl-2 family protein expression, and markers of immune cell apoptosis were evaluated by flow cytometry. Cytokine production was examined both in plasma of treated mice and splenocytes that were stimulated ex vivo. IL-15 blocked sepsis-induced apoptosis of NK cells, dendritic cells, and CD8 T cells. IL-15 also decreased sepsis-induced gut epithelial apoptosis. IL-15 therapy increased the abundance of antiapoptotic Bcl-2 while decreasing proapoptotic Bim and PUMA. IL-15 increased both circulating IFN-γ, as well as the percentage of NK cells that produced IFN-γ. Finally, IL-15 increased survival in both cecal ligation and puncture and P. aeruginosa pneumonia. In conclusion, IL-15 prevents two immunopathologic hallmarks of sepsis, namely, apoptosis and immunosuppression, and improves survival in two different models of sepsis. IL-15 represents a potentially novel therapy of this highly lethal disorder.

Adoptive transfer of apoptotic splenocytes worsens survival, whereas adoptive transfer of necrotic splenocytes improves survival in sepsis

In sepsis, both necrotic and apoptotic cell death can occur. Apoptotic cells induce anergy that could impair the host response, whereas necrotic cells cause immune activation that might result in enhanced antimicrobial defenses. We determined whether adoptive transfer of apoptotic or necrotic cells impacted survival in a clinically relevant sepsis model. We also evaluated the effects of adoptive transfer of apoptotic or necrotic cells on the prototypical TH1 and TH2 cytokines IFN-γ and IL-4, respectively. C57BL6/J mice had adoptive transfer of apoptotic (irradiated) or necrotic (freeze thaw) splenocytes. Controls received saline. Apoptotic cells greatly increased mortality, whereas necrotic splenocytes markedly improved survival, P ≤ 0.05. The contrasting effects that apoptotic or necrotic cells exerted on survival were mirrored by opposite effects on splenocyte IFN-γ production with greatly decreased and increased production, respectively. Importantly, either administration of anti-IFN-γ antibodies or use of IFN-γ knockout mice prevented the survival benefit occurring with necrotic cells. This study demonstrates that the type of cell death impacts survival in a clinically relevant model and identifies a mechanism for the immune suppression that is a hallmark of sepsis. Necrotic cells (and likely apoptotic cells) exert their effects via modulation of IFN-γ

Delayed administration of anti-PD-1 antibody reverses immune dysfunction and improves survival during sepsis

There is increasing recognition that a major pathophysiologic event in sepsis is the progression to an immunosuppressive state in which the host is unable to eradicate invading pathogens. Although there are likely numerous causes for the immunosuppression, expression of negative costimulatory molecules on immune effector cells is a likely contributing factor. PD‐1 is a recently described, negative costimulatory molecule that has potent effects to inhibit T cell activation, cytokine production, and cytotoxic functions. PD‐1 plays a critical role in the host response to specific pathogens, but relatively little work has been done on the possible effects of PD‐1 in sepsis. We hypothesized that the anti‐PD‐1 antibody would improve survival in sepsis. Mice underwent CLP, and PD‐1 expression was quantitated. Additionally, the effects of anti‐PD‐1 antibody on lymphocyte apoptosis, cytokine production, host immunity, and survival were determined. PD‐1 expression increased beginning 48 h after sepsis, and >20% of CD4 and CD8 T cells were positive by 7 days. Anti‐PD‐1 antibody administered 24 h after sepsis prevented sepsis‐induced depletion of lymphocytes and DCs, increased Bcl‐xL, blocked apoptosis, and improved survival. Anti‐PD‐1 also prevented the loss in DTH, a key indicator of immunocompetence in sepsis. Thus, delayed administration of anti‐PD‐1 antibody, an important therapeutic advantage, was effective in sepsis. Furthermore, these results add to the growing body of evidence that modulation of the positive and negative costimulatory pathways on immune cells represents a viable therapeutic approach in reversing immunosuppression and improving sepsis survival.