Intestinal uptake of bile acids: effect of external abdominal irradiation

Comprehensive assessment of host responses to ionizing radiation by nuclear factor-κB bioluminescence imaging-guided transcriptomic analysis

The aim of this study was to analyze the host responses to ionizing radiation by nuclear factor-κB (NF-κB) bioluminescence imaging-guided transcriptomic tool. Transgenic mice carrying the NF-κB-driven luciferase gene were exposed to a single dose of 8.5 Gy total-body irradiation. In vivo imaging showed that a maximal NF-κB-dependent bioluminescent intensity was observed at 3 h after irradiation and ex vivo imaging showed that liver, intestine, and brain displayed strong NF-κB activations. Microarray analysis of these organs showed that irradiation altered gene expression signatures in an organ-specific manner and several pathways associated with metabolism and immune system were significantly altered. Additionally, the upregulation of fatty acid binding protein 4, serum amyloid A2, and serum amyloid A3 genes, which participate in both inflammation and lipid metabolism, suggested that irradiation might affect the cross pathways of metabolism and inflammation. Moreover, the alteration of chemokine (CC-motif) ligand 5, chemokine (CC-motif) ligand 20, and Jagged 1 genes, which are involved in the inflammation and enterocyte proliferation, suggested that these genes might be involved in the radiation enteropathy. In conclusion, this report describes the comprehensive evaluation of host responses to ionizing radiation. Our findings provide the fundamental information about the in vivo NF-κB activity and transcriptomic pattern after irradiation. Moreover, novel targets involved in radiation injury are also suggested.

Microbial influences on the small intestinal response to radiation injury

PURPOSE OF REVIEW

Injury to the small bowel from ionizing radiation occurs commonly in patients undergoing cancer therapy and less commonly in instances of accidental radiation overexposure. Several lines of evidence now suggest that dynamic interactions between the host's enteric microbiota and innate immune system are important in modulating the intestinal response to radiation. Here, we will review recent developments in the area of acute radiation enteropathy and examine the current state of knowledge regarding the impact of host-microbial interactions in the process.

RECENT FINDINGS

There is promise in the development and testing of new clinical biomarkers including serum citrulline. Toll-like receptor agonists and innate immune system signaling pathways including nuclear factor-kappa B profoundly alter intestinal epithelial cell apoptosis and crypt survival after radiation exposure. Germ-free conditions, probiotics and antibiotics are each identified as modifiers of disease development and course. A human study suggested that luminal microbiota composition may influence the host's intestinal response to radiation and may change in those developing postradiation diarrhea.

SUMMARY

New knowledge implies that investigations aimed at deciphering the microbiome-host interactions before and after small bowl radiation injury may eventually allow prediction of disease course and offer opportunities for the development of novel therapeutic or prophylactic strategies.

Biomarkers for radiation-induced small bowel epithelial damage: An emerging role for plasma Citrulline

Reduction of cancer treatment-induced mucosal injury has been recognized as an important target for improving the therapeutic ratio as well as reducing the economic burden associated with these treatment related sequellae. Clinical studies addressing this issue are hampered by the fact that specific objective parameters, which enable monitoring of damage in routine clinical practice, are lacking. This review summarizes pros and cons of currently available endpoints for intestinal injury. The metabolic background and characteristics of plasma citrulline, a recently investigated biomarker specifically for small intestinal injury, are discussed in more detail.

Effects of ionizing radiation on the activity of the major hepatic enzymes implicated in bile acid biosynthesis in the rat

In the days following high-dose radiation exposure, damage to small intestinal mucosa is aggravated by changes in the bile acid pool reaching the gut. Intestinal bile acid malabsorption, as described classically, may be associated with altered hepatic bile acid biosynthesis, which was the objective of this work. The activity of the main rate-limiting enzymes implicated in the bile acid biosynthesis were evaluated in the days following an 8-Gy gamma(60)Co total body irradiation of rats, with concomitant determination of biliary bile acid profiles and intestinal bile acid content. Modifications of biliary bile acid profiles, observed as early as the first post-irradiation day, were most marked at the third and fourth day, and resulted in an increased hydrophobicity index. In parallel, the intestinal bile acids' content was enhanced and hepatic enzymatic activities leading to bile acids were changed. A marked increase of sterol 12 alpha-hydroxylase and decrease of oxysterol 7 alpha-hydroxylase activity was observed at day 3, whereas both cholesterol 7 alpha-hydroxylase and oxysterol 7 alpha-hydroxylase activities were decreased at day 4 after irradiation. These results show, for the first time, radiation-induced modifications of hepatic enzymatic activities implicated in bile acid biosynthesis and suggest that they are mainly a consequence of radiation-altered intestinal absorption, which induces a physiological response of the enterohepatic bile acid recirculation.

Plasma citrulline concentration: a surrogate end point for radiation-induced mucosal atrophy of the small bowel. A feasibility study in 23 patients

PURPOSE

Plasma citrulline, a nitrogen end product of glutamine metabolism in small-bowel enterocytes, was suggested as a marker of radiation-induced small-bowel epithelial cell loss in mice after single-dose whole-body irradiation. Our objective was to evaluate the feasibility of citrulline as a marker for radiation-induced small-intestinal mucosal atrophy in patients during and after abdominal fractionated radiotherapy.

METHODS AND MATERIALS

Twenty-three patients were studied weekly during treatment and at intervals of 2 weeks and 3 and 6 months after treatment by postabsorptive plasma citrulline concentration and clinical toxicity grading. The interrelationship between these variables and the correlation with small-bowel dose and volume parameters were investigated.

RESULTS

During fractionated radiotherapy, citrulline concentration significantly decreased as a function of the radiation dose (p < 0.001) and the volume of small bowel treated (p = 0.001). The plasma citrulline concentration correlated with clinical toxicity during the last 3 weeks of treatment. As a whole, citrulline concentration correlated better with radiation dose and volume parameters than clinical toxicity grading.

CONCLUSIONS

In patients treated with fractionated radiation therapy for abdominal or pelvic cancer sites, plasma citrulline concentration may be a simple objective marker for monitoring epithelial cell loss, a major event in acute radiation-induced small-bowel toxicity.

Alteration of the enterohepatic recirculation of bile acids in rats after exposure to ionizing radiation

The aim of this work was to study acute alterations of the enterohepatic recirculation (EHR) of bile acids 3 days after an 8-Gy radiation exposure in vivo in the rat by a washout technique. Using this technique in association with HPLC analysis, the EHR of the major individual bile acids was determined in control and irradiated animals. Ex vivo ileal taurocholate absorption was also studied in Ussing chambers. Major hepatic enzyme activities involved in bile acid synthesis were also measured. Measurements of bile acid intestinal content and intestinal absorption efficiency calculation from washout showed reduced intestinal absorption with significant differences from one bile acid to another: absorption of taurocholate and tauromuricholate was decreased, whereas absorption of the more hydrophobic taurochenodeoxycholate was increased, suggesting that intestinal passive diffusion was enhanced, whereas ileal active transport might be reduced. Basal hepatic secretion was increased only for taurocholate, in accordance with the marked increase of CYP8B1 activity in the liver. The results are clearly demonstrate that concomitantly with radiation-induced intestinal bile acid malabsorption, hepatic bile acid synthesis and secretion are also changed. A current working model for pathophysiological changes in enterohepatic recycling after irradiation is thus proposed.Key words: irradiation, bile acids, intestine, liver, enterohepatic recycling.

Citrulline: a physiologic marker enabling quantitation and monitoring of epithelial radiation-induced small bowel damage

PURPOSE

Small bowel irradiation results in epithelial cell loss and consequently impairs function and metabolism. We investigated whether citrulline, a metabolic end product of small bowel enterocytes, can be used for quantifying radiation-induced epithelial cell loss.

METHODS AND MATERIALS

NMRI mice were subjected to single-dose whole body irradiation (WBI). The time course of citrullinemia was assessed up to 11 days after WBI. A dose-response relationship was determined at 84 h after WBI. In addition, citrullinemia was correlated with morphologic parameters at this time point and used to calculate the dose-modifying factor (DMF) of glutamine and amifostine on acute small bowel radiation damage.

RESULTS

After WBI, a time- and dose-dependent decrease in plasma citrulline level was observed with a significant dose-response relationship at 84 h. At this time point, citrullinemia significantly correlated with jejunal crypt regeneration (p < 0.001) and epithelial surface lining (p = 0.001). A DMF of 1.0 and 1.5 was computed at the effective dose 50 (ED50) level for glutamine and amifostine, respectively.

CONCLUSIONS

Citrullinemia can be used to quantify acute small bowel epithelial radiation damage after single-dose WBI. Radiation-induced changes in citrullinemia are most pronounced at 3 1/2 to 4 days postirradiation. At this time point, citrullinemia correlates with morphologic endpoints for epithelial radiation damage.

Nuclear scintigraphic assessment of radiation-induced intestinal dysfunction

Radiation-induced damage to the intestine can be measured by abnormalities in the absorption of various nutrients. Changes in intestinal absorption occur after irradiation because of loss of the intestinal absorptive surface and a consequent decrease in active transport. In our study, the jejunal absorption of (99m)Tc-pertechnetate, an actively transported gamma-ray emitter, was assessed in C3H/Kam mice given total-body irradiation with doses of 4, 6, 8 and 12.5 Gy and correlated with morphological changes in the intestinal epithelium. The absorption of (99m)Tc-pertechnetate from the intestinal lumen into the circulation was studied with a dynamic gamma-ray-scintigraphy assay combined with a multichannel analyzer to record the radiometry data automatically in a time-dependent regimen. The resulting radioactivity-time curves obtained for irradiated animals were compared to those for control animals. A dose-dependent decrease in absorptive function was observed 3.5 days after irradiation. The mean absorption rate was reduced to 78.8 +/- 9.3% of control levels in response to 4 Gy total-body irradiation (mean +/- SEM tracer absorption lifetime was 237 +/- 23 s compared to 187 +/- 12 s in nonirradiated controls) and to 28.3 +/- 3.7% in response to 12.5 Gy (660 +/- 76 s). The decrease in absorption of (99m)Tc-pertechnetate at 3.5 days after irradiation correlated strongly (P < 0.001) with TBI dose, with the number of cells per villus, and with the percentage of cells in the crypt compartment that were apoptotic or mitotic. A jejunal microcolony assay showed no loss of crypts and hence no measured dose-response effects after 4, 6 or 8 Gy TBI. These results show that dynamic enteroscintigraphy with sodium (99m)Tc-pertechnetate is a sensitive functional assay for rapid evaluation of radiation-induced intestinal damage in the clinically relevant dose range and has a cellular basis.

Inhibition of lipid absorption as an approach to the treatment of obesity

A reduction in fat intake may be achieved by making educated choices to reduce total calorie intake, to consume a lower quantity of total fats, or to modify the ratio of saturated-to-polyunsaturated lipids. Leptin agonists or NPY or CCK antagonists may prove to be useful to diminish appetite and thereby reduce the total intake of food. But eating has such cultural, social, and hedonistic attributes that such a single-pronged approach is unlikely to be successful. The use of fat substitutes may prove to be popular to provide a wide range of snack food options, but these are likely to be of minimal use in weight reduction programs because of their distribution of additives in only a limited number of foods. The inhibitors of lipid digestion will be modestly successful in the short term; their long-term success will be influenced by gastrointestinal adverse effects and the need to consume fat-soluble vitamin supplements to prevent the development of fat-soluble vitamin deficiencies. The inhibition of lipid absorption is an attractive targeted approach for the treatment of obesity, since this would reduce the uptake of visible as well as invisible fats, which would potentially offer convenient dosing, and could also be a means to inhibit secondarily the uptake of carbohydrate calories.

Short-chain fatty acids in the treatment of radiation proctitis: a randomized, double-blind, placebo-controlled, cross-over pilot trial

PURPOSE

Treatment of chronic radiation proctitis remains unsatisfactory. Short-chain fatty acids are the preferred energy source for the colonic epithelium. We aimed to determine for the first time whether topical butyric acid enemas relieve symptoms and improve the macroscopic and microscopic findings in chronic radiation proctitis.

METHODS

A randomized, double-blind, placebo-controlled, cross-over pilot trial compared patients given two weeks of butyric acid enemas (40 mmol) twice per day with those given placebo, with a one-week washout period; 15 patients were randomized and 12 completed both arms of the trial. A total symptom score combined six symptom items per week (rectal pain, episodes of rectal bleeding, amount of blood passed, days with diarrhea, number of stools, and urgency). Symptom, endoscopic, and histologic scores were obtained at the beginning of the study and again at the last week of each treatment arm.

RESULTS

Total symptom score at baseline (median, 5.5) improved for those patients receiving active treatment (median, 3.5), but compared with placebo (median, 4.5), the change was not significant. Endoscopic appearances were largely unaltered by active treatment. Histology was abnormal in 82 percent of patients receiving placebo compared with 55 percent of those given butyric acid enemas (P = not significant).

CONCLUSION

Butyric acid enemas do not appear to be superior to placebo in the treatment of chronic radiation proctitis.

Development of active and passive transport of bile acids in rabbit intestine

Previous studies have indicated that saturable, Na+-bile acid coupled transport is absent in the ileum throughout most of the suckling period and that the maturation of ileal bile acid uptake which occurs during weaning results primarily through an increase in functional bile acid carriers within the ileal brush border membrane. This study was undertaken in weanling and adult rabbits to establish the effect of maturation on the active ileal and passive jejunal and colonic uptake of 8 bile acids. The in vitro uptake of a range of concentrations of cholic (C), taurocholic (TC), glycocholic (GC), chenodeoxycholic (CDC), taurochenodeoxycholic (TCDC), glycochenodeoxycholic (GCDC), deoxycholic (DC) and taurodeoxycholic (TDC) acid was determined. Active ileal uptake of C, TC, GC, DC and TDC was greater in adult than in weanling animals, whereas uptake of CDC, TCDC and GCDC was similar in both groups. The relative permeability for passive uptake of bile acids into the jejunum and colon was similar in young and adult rabbits. Jejunal mucosal surface area was similar in the two groups, but was greater in the ileum in the adult than in the weanling rabbits due to an increase in villus height, width, and number of villi per unit serosal length. However, the age-associated differences in active bile acid uptake were not explained simply on the basis of these differences in ileal mucosal surface area or the concentration of bile acids in the intestinal lumen. Thus, maturation of the rabbit from weanling to adulthood does not influence the relative passive permeability of the jejunum or colon to bile acids, but does increase active ileal uptake of conjugated and unconjugated cholic acid and deoxycholic acid, but not chenodeoxycholic acid.

Fractionated low doses of abdominal irradiation alters jejunal uptake of nutrients

Abdominal radiation is associated with changes in intestinal uptake of nutrients that begins within three days and persist for over 33 weeks. Clinically, fractionated doses of radiation (FDR) are used in an attempt to minimize the complications of this therapy, but the effects of fractionated doses of radiation on intestinal transport have not been defined. Accordingly, an in vitro technique was used to assess the jejunal and ileal uptake of varying concentrations of glucose and leucine, as well as the uptake of single concentrations of fatty acids and decanol in rats exposed 3, 7, and 14 days previously to a course of 200 cGy given on each of five consecutive days. FDR was associated with an increase in the uptake of decanol, and therefore a decrease in the effective resistance of the unstirred water layer. FDR had a variable effect on the uptake of glucose and leucine, with a decline in the value of the Michaelis constant (Km) and the passive permeability coefficient for glucose (Pd), whereas the Km for leucine was unchanged and the Pd for leucine was variably affected by FDR. The maximal transport rate (Jdm) for leucine progressively rose following FDR, whereas the Jdm for glucose initially rose, then fell. The uptake of galactose and medium chain-length fatty acids was unchanged by FDR, whereas the jejunal uptake of myristic acid rose, and the uptake of cholic acid declined, then returned to normal. FDR was associated with greater body weight gain and jejunal and ileal weight, but these changes did not adequately explain the variable alterations in the kinetics of uptake. The changes in nutrient uptake following FDR differed from the absorption changes occurring after a single dose of radiation. Thus, fractionated doses of abdominal radiation produce complex changes in the intestinal uptake of actively and passively transported nutrients, and these variable changes are influenced by the time following radiation exposure and by the solute studied.

Determination of kinetic parameters of a carrier-mediated transport in the perfused intestine by two-dimensional laminar flow model: effects of the unstirred water layer

The kinetic parameters of a carrier-mediated transport for D-glucose and for taurocholate were determined from rat in situ intestinal single perfusion experiments. The true parameters were obtained by the two-dimensional laminar flow model, in which the solute concentration at the aqueous-intestinal membrane interface can be calculated numerically without assuming the aqueous diffusion layer, discriminating the effects of the unstirred water layer. The true Michaelis constant was 4.5 mM for D-glucose and 1.5 mM for taurocholate. The true maximal transport velocity was 3.4 nmol/s per cm2 for D-glucose and 0.29 nmol/s per cm2 for taurocholate. The apparent Michaelis constant was raised by the factor of 6.6 for D-glucose and 3.6 for taurocholate due to the effects of the unstirred water layer. The maximal transport velocity was relatively unaffected by the unstirred water layer in both compounds. The values of the effective (operational) thickness of the unstirred water layer were compatible with those reported previously by employing various experimental methods. The kinetic parameters obtained in vitro everted sacs, for comparison, almost coincided with the true ones in situ. Therefore, the two-dimensional laminar flow model is shown to be valid not only for determining the kinetic parameters of a carrier-mediated transport in situ but also for predicting the absorption rate in situ from the uptake rate in vitro.