Heme oxygenase-1 induction in islet cells results in protection from apoptosis and improved in vivo function after transplantation

The role of endogenous heme oxygenase in the initiation of liver injury following limb ischemia/reperfusion

BACKGROUND/AIMS

Heme oxygenase (HO) derived liver protection was tested in mice following 1 h bilateral hindlimb ischemia and either 1.5 or 3 h reperfusion.

METHODS

Groups consisted of limb ischemia/reperfusion (I/R), sham (no I/R), I/R+chromium mesoporphyrin (I/R+CrMP;40 micromol/kg, i.p.), or I/R+hemin (10 mg/kg, i.p.). The vital dye propidium iodide (PI), was used to measure hepatocellular death (#/0.1 mm(3)), while the number of sinusoids perfused by red blood cells (SP(RBC)) were measured from the periportal (Pp) and pericentral (Pc) zones of liver acini using intravital microscopy. Whole organ injury was estimated from serum alanine aminotransferase (ALT).

RESULTS

SP(RBC) reduced within 1.5 h with no further decline following 3 h. CrMP resulted in a dramatic loss of SP(RBC) following 3 h only. Hemin restored perfusion in both zones. Hepatocellular death and organ injury increased at 1.5 and 3 h. At 1.5 h, CrMP further increased cell death in the Pc zone, as well as whole organ injury, while hemin restored cell viability. Increased HO mRNA, protein and activity suggested induction within 3 h.

CONCLUSIONS

HO does not protect perfusion during the early stage (1.5 h), but becomes increasingly important in preserving liver perfusion and cell viability during the later stage (3 h) of liver injury.

Carbon monoxide protects pancreatic beta-cells from apoptosis and improves islet function/survival after transplantation

Pancreatic islets transplanted to treat autoimmune type 1 diabetes often fail to function (primary nonfunction), likely because of islet beta-cell apoptosis. We show that carbon monoxide (CO), a product of heme oxygenase activity, protects beta-cells from apoptosis. Protection is mediated through guanylate cyclase activation, generation of cyclic GMP (cGMP), and activation of cGMP-dependent protein kinases. This antiapoptotic effect is still observed when beta-cells are exposed to CO for 1 h before the apoptotic stimulus. In a similar manner, mouse islets exposed to CO for just 2 h function significantly better after transplantation than islets not exposed to CO. These findings suggest a potential therapeutic application for CO in improving islet function/survival after transplantation in humans.

Continuous measurement of oxygen consumption by pancreatic islets

The rate of oxygen consumption is an important measure of mitochondrial function in all aerobic cells. In pancreatic beta cells, it is linked to the transduction mechanism that mediates glucose-stimulated insulin secretion. However, measurement of oxygen consumption over long periods of time is technically difficult owing to the error resulting from baseline drift and the challenge of measuring small changes in oxygen tension. We have adapted an ultrastable oxygen sensor based on the detection of the decay of the phosphorescent emission from an oxygen-sensitive dye to a previously developed islet flow culture system. The drift of the sensor is approximately 0.3%/24 h, allowing for the continuous measurement of oxygen consumption by 300 islets (or about 6 x 10(5) cells) for hours or days. Rat islets placed in the perifusion chamber for 24 h were well maintained as reflected by membrane integrity, insulin secretion, and oxygen consumption. Both acute changes in oxygen consumption as induced by glucose and chronic changes as induced by sequential pulses of azide were resolved. The features of the flow culture system--aseptic conditions, fine temporal control of the composition of the media, and the collection of outflow fractions for measurement of insulin, and other products--facilitate a systematic approach to assessing metabolic and functional viability in responses to a variety of stimuli. Applications to the measurement of effects of hypoxia on insulin secretion, membrane integrity, and the redox state of cytochromes are demonstrated. The system has particular application to the field of human islet transplantation, where assessment and the study of islet viability have been hampered by a lack of experimental methods.

Heme oxygenase-1 protects pancreatic beta cells from apoptosis caused by various stimuli

BACKGROUND

Several problems can occur after allogeneic islet transplantation: primary nonfunction, rejection, and the recurrence of autoimmune disease, which involve attack by the recipient's cytokines, T cells, natural killer cells, and monocytes on the donor's beta cells, which leads to beta-cell destruction. Recent studies have revealed that loss of transplanted islets is caused mainly by apoptosis. Heme oxygenase-1 (HO-1) is one of the antiapoptotic genes up-regulated under stress conditions. The aim of this work was to investigate any mechanisms of HO-1-mediated protection of beta cells from apoptosis.

METHODS

Apoptosis was assessed by comparison of viable transfected cells with and without apoptotic stimuli, and with and without HO-1 overexpression. Activation and function of p38 mitogen-activated protein kinase were determined using the specific inhibitor SB203580.

RESULTS

We have shown that HO-1 mediates antiapoptotic effects in beta cells. The percentage of apoptotic cells after stimulation with tumor necrosis factor a decreased from 75% without HO-1 to 5% when HO-1 was overexpressed. Our data indicate that HO-1 acts as a signal terminator of tumor necrosis factor alpha-induced apoptosis by modulation of the p38 mitogen-activated protein kinase pathway.

CONCLUSIONS

Profound cell stress that occurs in islets after transplantation, as well as at the onset of diabetes, results in beta-cell loss through apoptosis. Protection of beta cells by HO-1 improves their survival in vitro after various proapoptotic stimuli, suggesting that HO-1 suppresses one or several signaling pathways leading to apoptosis. We hypothesize that our in vitro findings can be extrapolated to the in vivo situation, and we propose that expression of HO-1 in islets may illuminate a valuable new approach to improving diabetes treatment.