Tissue-specific expression of ALA synthase-1 and heme oxygenase-1 and their expression in livers of rats chronically exposed to ethanol

Carbon monoxide: a critical quantitative analysis and review of the extent and limitations of its second messenger function

Endogenously produced carbon monoxide (CO) is commonly believed to be a ubiquitous second messenger involved in a wide range of physiological and pathological responses. The major evidence supporting this concept is that CO is produced endogenously via heme oxygenase-catalyzed breakdown of heme and that experimental exposure to CO alters tissue function. However, it remains to be conclusively demonstrated that there are specific receptors for CO and that endogenous CO production is sufficient to alter tissue function. Unlike other signaling molecules, CO is not significantly metabolized, and it is removed from cells solely via rapid diffusion into blood, which serves as a near infinite sink. This non-metabolizable nature of CO renders the physiology of this gas uniquely susceptible to quantitative modeling. This review analyzes each of the steps involved in CO signaling: 1) the background CO partial pressure (PCO) and the blood and tissue CO binding; 2) the affinity of the putative CO receptors; 3) the rate of endogenous tissue CO production; and 4) the tissue PCO that results from the balance between this endogenous CO production and diffusion to the blood sink. Because existing data demonstrate that virtually all endogenous CO production results from the routine "housekeeping" turnover of heme, only a small fraction can play a signaling role. The novel aspect of the present report is to demonstrate via physiological modeling that this small fraction of CO production is seemingly insufficient to raise intracellular PCO to the levels required for the conventional, specific messenger receptor activation. It is concluded that the many physiological alterations observed with exogenous CO administration are probably produced by the non-specific CO inhibition of cytochrome C oxidase activity, with release of reactive oxygen species (ROS) and that this ROS signaling pathway is a potential effector mechanism for endogenously produced CO.

Protective role of HO-1 and carbon monoxide in ethanol-induced hepatocyte cell death and liver injury in mice

BACKGROUND & AIMS

Alcoholic liver disease is associated with inflammation and cell death. Heme oxygenase-1 (HO-1) is a stress-inducible enzyme with anti-apoptotic and anti-inflammatory properties. Here we tested the hypothesis that induction of HO-1 or treatment with a carbon monoxide releasing molecule (CORM) during chronic ethanol exposure protects and/or reverses ethanol-induced liver injury.

METHODS

Female C57BL/6J mice were allowed free access to a complete liquid diet containing ethanol or to pair-fed control diets for 25days. Mice were treated with cobalt protoporphyrin (CoPP) to induce HO-1 expression during ethanol feeding or once liver injury had been established. Mice were also treated with CORM-A1, a CO-releasing molecule (CORM), after ethanol-induced liver injury was established. The impact of HO-1 induction on ethanol-induced cell death was investigated in primary cultures of hepatocytes.

RESULTS

Induction of HO-1 during or after ethanol feeding, as well as treatment with CORM-A1, ameliorated ethanol-induced increases in AST and expression of mRNAs for inflammatory cytokines. Treatment with CoPP or CORM-A1 also reduced hepatocyte cell death, indicated by decreased accumulation of CK18 cleavage products and reduced RIP3 expression in hepatocytes. Exposure of primary hepatocyte cultures to ethanol increased their sensitivity to TNFα-induced cell death; this response was attenuated by necrostatin-1, an inhibitor of necroptosis, but not by caspase inhibitors. Induction of HO-1 with CoPP or CORM-3 treatment normalized the sensitivity of hepatocytes to TNFα-induced cell death after ethanol exposure.

CONCLUSIONS

Therapeutic strategies to increase HO-1 and/or modulate CO availability ameliorated chronic ethanol-induced liver injury in mice, at least in part by decreasing hepatocellular death.

Regulation of heme oxygenase expression by alcohol, hypoxia and oxidative stress

AIM: To study the effect of both acute and chronic alcohol exposure on heme oxygenases (HOs) in the brain, liver and duodenum.

METHODS: Wild-type C57BL/6 mice, heterozygous Sod2 knockout mice, which exhibit attenuated manganese superoxide dismutase activity, and liver-specific ARNT knockout mice were used to investigate the role of alcohol-induced oxidative stress and hypoxia. For acute alcohol exposure, ethanol was administered in the drinking water for 1 wk. Mice were pair-fed with regular or ethanol-containing Lieber De Carli liquid diets for 4 wk for chronic alcohol studies. HO expression was analyzed by real-time quantitative polymerase chain reaction and Western blotting.

RESULTS: Chronic alcohol exposure downregulated HO-1 expression in the brain but upregulated it in the duodenum of wild-type mice. It did not alter liver HO-1 expression, nor HO-2 expression in the brain, liver or duodenum. In contrast, acute alcohol exposure decreased both liver HO-1 and HO-2 expression, and HO-2 expression in the duodenum of wild-type mice. The decrease in liver HO-1 expression was abolished in ARNT^+/- mice. Sod2^+/- mice with acute alcohol exposure did not exhibit any changes in liver HO-1 and HO-2 expression or in brain HO-2 expression. However, alcohol inhibited brain HO-1 and duodenal HO-2 but increased duodenal HO-1 expression in Sod2^+/- mice. Collectively, these findings indicate that acute and chronic alcohol exposure regulates HO expression in a tissue-specific manner. Chronic alcohol exposure alters brain and duodenal, but not liver HO expression. However, acute alcohol exposure inhibits liver HO-1 and HO-2, and also duodenal HO-2 expression.

CONCLUSION: The inhibition of liver HO expression by acute alcohol-induced hypoxia may play a role in the early phases of alcoholic liver disease progression.

The anti-inflammatory effects of adiponectin are mediated via a heme oxygenase-1-dependent pathway in rat Kupffer cells

Altered expression and activity of immunomodulatory cytokines plays a major role in the pathogenesis of alcoholic liver disease. Chronic ethanol feeding increases the sensitivity of Kupffer cells, the resident hepatic macrophage, to lipopolysaccharide (LPS), leading to increased tumor necrosis factor alpha (TNF-alpha) expression. This sensitization is normalized by treatment of primary cultures of Kupffer cells with adiponectin, an anti-inflammatory adipokine. Here we tested the hypothesis that adiponectin-mediated suppression of LPS signaling in Kupffer cells is mediated via an interleukin-10 (IL-10)/heme oxygenase-1 (HO-1) pathway after chronic ethanol feeding. Knockdown of IL-10 expression in primary cultures of Kupffer cells with small interfering RNA (siRNA) prevented the inhibitory effect of globular adiponectin (gAcrp) on LPS-stimulated TNF-alpha expression. gAcrp increased IL-10 mRNA and protein expression, as well as expression of the IL-10 inducible gene, HO-1; expression was higher in Kupffer cells from ethanol-fed rats compared with pair-fed controls. Although IL-10 receptor surface expression on Kupffer cells was not affected by ethanol feeding, IL-10-mediated phosphorylation of STAT3 and expression of HO-1 was higher in Kupffer cells after ethanol feeding. Inhibition of HO-1 activity, either by treatment with the HO-1 inhibitor zinc protoporphyrin or by siRNA knockdown of HO-1, prevented the inhibitory effect of gAcrp on LPS-stimulated TNF-alpha expression in Kupffer cells. LPS-stimulated TNF-alpha expression in liver was increased in mice after chronic ethanol exposure. When mice were treated with cobalt protoporphyrin to induce HO-1 expression, ethanol-induced sensitivity to LPS was ameliorated.

CONCLUSION

gAcrp prevents LPS-stimulated TNF-alpha expression in Kupffer cells through the activation of the IL-10/STAT3/HO-1 pathway. Kupffer cells from ethanol-fed rats are highly sensitive to the anti-inflammatory effects of gAcrp; this sensitivity is associated with both increased expression and sensitivity to IL-10.

Anti-inflammatory pathways and alcoholic liver disease: Role of an adiponectin/interleukin-10/heme oxygenase-1 pathway

The development of alcoholic liver disease (ALD) is a complex process involving both the parenchymal and non-parenchymal cells in the liver. Enhanced inflammation in the liver during ethanol exposure is an important contributor to injury. Kupffer cells, the resident macrophages in liver, are particularly critical to the onset of ethanol-induced liver injury. Chronic ethanol exposure sensitizes Kupffer cells to activation by lipopolysaccharide via Toll-like receptor 4. This sensitization enhances production of inflammatory mediators, such as tumor necrosis factor-α and reactive oxygen species, that contribute to hepatocyte dysfunction, necrosis, apoptosis, and fibrosis. Impaired resolution of the inflammatory process probably also contributes to ALD. The resolution of inflammation is an active, highly coordinated response that can potentially be manipulated via therapeutic interventions to treat chronic inflammatory diseases. Recent studies have identified an adiponectin/interleukin-10/heme oxygenase-1 (HO-1) pathway that is profoundly effective in dampening the enhanced activation of innate immune responses in primary cultures of Kupffer cells, as well as in an in vivo mouse model of chronic ethanol feeding. Importantly, induction of HO-1 also reduces ethanol-induced hepatocellular apoptosis in this in vivo model. Based on these data, we hypothesize that the development of therapeutic agents to regulate HO-1 and its downstream targets could be useful in enhancing the resolution of inflammation during ALD and preventing progression of early stages of liver injury.

B2 SINE retrotransposon causes polymorphic expression of mouse 5-aminolevulinic acid synthase 1 gene

5-Aminolevulinic acid synthase 1 (ALAS1) is the key enzyme in the homeostasis of nonerythroid heme and of fundamental importance in respiration, the metabolism of drugs, chemicals and steroids and cell signalling. The regulation of ALAS1 in response to stimuli occurs at transcriptional, translational and post-translational levels which could depend on inter-individual variation in basal expression. A genetic difference in hepatic ALAS1 mRNA levels between C57BL/6J and DBA/2 mice was detected by microarray and was >5-fold in whole liver or hepatocytes when estimated by qRT-PCR. Analysis of the ALAS1 promoter showed a 210 nt insert in the DBA/2 containing a B2 SINE retrotransposon causing a marked repression of expression by intracellular reporter systems. Deletions across the B2 SINE demonstrated that the full sequence was required for transcriptional inhibition. The findings show that a B2 SINE can contribute to the regulation of ALAS1 and SINEs in 5'-UTR regions contribute to inter-individual differences in gene expression.