Olfactory sensitivity to bile fluid and bile salts in the European eel (Anguilla anguilla), goldfish (Carassius auratus) and Mozambique tilapia (Oreochromis mossambicus) suggests a 'broad range' sensitivity not confined to those produced by conspecifics alone.

Teleosts have high olfactory sensitivity to bile salts. To assess whether this phenomenon is involved in intra-specific chemical communication alone, or is part of a more ;broad range' sensitivity to bile salts produced by heterospecifics, we investigated possible differences in the odour of bile between the sexes and among different species - the eel (Anguilla anguilla), goldfish (Carassius auratus) and Mozambique tilapia (Oreochromis mossambicus) - using the electro-olfactogram (EOG). We also identified the main bile constituents by liquid chromatography and mass spectrometry. There were marked differences in olfactory response of the eel to thin-layer chromatography fractions of bile from both sexes, and mature and immature conspecifics. Smaller differences were seen in the potency of fractions of bile from male and female goldfish and tilapia. Eels, goldfish and tilapia demonstrated similar olfactory sensitivity to bile from a range of different species, with no apparent correlation between the olfactory potency of bile and a phylogenetic closeness and/or similarity of diet of the donor to the receiver. The three species were able to detect odorants in thin-layer chromatography fractions of heterospecific bile even in the absence of activity in conspecific bile. Eels, goldfish and tilapia responded to both sulphated C(27) bile salts (5beta-scymnol-sulphate and 5alpha-cyprinol sulphate) and to taurine-conjugated C(24) bile salts (taurochenodeoxycholic acid, taurolithocholic acid and taurocholic acid), irrespective of whether these bile salts were present in conspecific bile. Together, these results suggest that teleosts have a broad-range olfactory sensitivity to bile salts, with potential roles in both intra-specific chemical communication and in inter-specific interactions.

Physicochemical and physiological properties of cholylsarcosine. A potential replacement detergent for bile acid deficiency states in the small intestine.

The properties of cholylsarcosine (the synthetic N-acyl conjugate of cholic acid with sarcosine [N-methylglycine]) were examined to determine its suitability as a bile acid replacement agent for conditions of bile acid deficiency in the small intestine, which causes fat malabsorption. Previous studies in rodents had shown that the compound was well transported by the liver and ileum and underwent neither deconjugation nor dehydroxylation during enterohepatic cycling. By 1H-nuclear magnetic resonance, cholylsarcosine was found to exist in dilute aqueous solution as an almost equimolar mixture of two geometric isomers--cis and trans (around the amide bond)--in contrast to cholylglycine, which was present entirely in the trans form. The critical micellization concentration was 11 mmol/liter, similar to that of cholylglycine (10 mmol/liter). By nonaqueous titrimetry, the pKa' of cholylsarcosine was 3.7, only slightly lower than that of cholylglycine (3.9). Cholylsarcosine was poorly soluble below pH 3.7, but highly soluble above pH 4. In vitro, cholylsarcosine behaved as cholylglycine with respect to promoting lipolysis by lipase/colipase. There was little difference between cholylsarcosine and cholylglycine in their solubilization of an equimolar mixture of oleic acid, oleate, and monoolein (designed to simulate digestive products of triglyceride) or in their solubilization of monooleyl-glycerol alone. When a [3H]triolein emulsion with either cholylsarcosine or cholyltaurine was infused intraduodenally in biliary fistula rats, recovery of 3H in lymph was 52 +/- 10% (mean +/- SD) for cholylsarcosine and 52 +/- 11% for cholyltaurine. When perfused into the colon of the anesthetized rabbit, cholylsarcosine (5 mmol/liter) did not influence water absorption or permeability to erythritol, in contrast to chenodeoxycholate, which induced vigorous water secretion and caused erythritol loss. We conclude that cholylsarcosine possesses the physicochemical and physiological properties required for a suitable bile acid replacement in deficiency states.

Colonic secretion of water and electrolytes induced by bile acids: perfusion studies in man.

Each of the three major bile acids of man was tested for its influence on electrolyte and water absorption in the human colon. Transport from isotonic solutions, with or without added bile acids, was compared in 35 studies on 20 healthy volunteers by colonic perfusions under steady-state conditions. Electrolytes and water were always absorbed from control solutions, but dihydroxy bile acid solutions induced continuous secretion or inhibition of sodium, potassium, and water absorption, which was reversible. Deoxycholic acid caused consistent secretion at 3 mm concentrations, whereas chenodeoxycholic acid did not induce secretion until the concentration was 5 mm. The trihydroxy bile acid (cholic acid) produced no significant change in absorption at 10 mm. Inhibition of absorption was also induced by mixtures of the glycine or taurine conjugated bile acids. Secretion of sodium and chloride, induced by bile acid perfusion, was linearly correlated with secretion of water; potassium secretion was relatively constant regardless of the volume of secretion. These results establish a striking influence of bile acids on colonic absorptive activity, provide an explanation in part for the diarrhea that frequently accompanies ileal disease or resection, and imply that diarrhea should occur in other disease states that produce elevated concentrations of dihydroxy bile acids in the colonic lumen.

Effect of glycine-conjugated bile acids with and without lecithin on water and glucose absorption in perfused human jejunum.

Perfusion studies were performed in healthy volunteers to test whether the secretory effect of conjugated bile acids, previously shown for the colon, was also present in the jejunum. A perfusion system with a proximal occlusive balloon (and continuous aspiration of duodenal secretions) was used; isotonic test solutions contained glycine-conjugated bile acids with or without lecithin. Fluid movement was measured by changes in the concentration of polyethylene glycol (PEG, mol wt 4,000). Conjugated dihydroxy bile acids inhibited electrolyte and fluid absorption and, at higher concentrations, evoked secretion of an isotonic fluid. Glucose absorption continued, despite fluid secretion, but its rate decreased. The secretory effects of bile acids were abolished by the addition of lecithin to the bile acid solutions. A trihydroxy bile acid (cholylglycine) had no effect on jejunal absorption. Small amounts (6-9%) of conjugated bile acids were absorbed in the jejunum; lecithin was well absorbed (72-90%). The results indicate that dihydroxy bile acids influence salt and water transport in the human jejunum but that this effect may be abolished when a polar lipid such as lecithin is present. We speculate that this effect of bile acids may modify fluid movement in the small intestine postprandially after fat absorption has occurred.

Metabolism of steroid and amino acid moieties of conjugated bile acids in man. I. Cholylglycine.

Cholyl-2,4-(3)H-glycine-1-(14)C was administered orally to eight healthy subjects with indwelling nasoduodenal tubes. The distribution of radioactivity among bile acids and the specific activity of cholylglycine were determined in bile collected at intervals for 7 days. (3)H and (14)C were measured in stool. (14)C in breath was calculated from interval (14)CO(2) specific activity determinations. The daily fractional turnover of the glycine moiety (mean +/-SE, 106+/-17%) was three times greater than that of the cholyl moiety (38+/-7%). On the basis of certain assumptions, it was calculated that about 18% of the cholylglycine pool was deconjugated per enterohepatic cycle. The extent of deconjugation appeared to be unrelated to the efficiency of absorption of the cholyl moiety, which averaged 90-95% per enterohepatic cycle. (14)C was recovered predominantly in breath (52+/-5% of administered dose), and 24 hr (14)CO(2) excretion correlated highly (r = 0.95) with daily fractional turnover of the glycine moiety. (3)H excretion occurred predominantly in feces, and the rate correlated highly (r = 0.92) with the daily fractional turnover of the cholyl moiety. Deoxycholylglycine became labeled with (3)H rapidly, indicating the occurrence of bacterial 7-dehydroxylation of the cholyl moiety and absorption of deoxycholic acid. This biotransformation occurred in all eight subjects but varied in degree and was unrelated to the degree of deconjugation. Since ingested glycine-1-(14)C was not incorporated into bile acid glycine, appearance of (14)C in deoxycholylglycine (observed in three of eight subjects) indicated that 7-dehydroxylation of cholylglycine can occur without deconjugation. Dehydroxylation was also observed in vitro when fecal homogenates were incubated with cholylglycine.

Metabolism of steroid and amino acid moieties of conjugated bile acids in man. II. Glycine-conjugated dihydroxy bile acids.

Chenodeoxycholyl-2,4-(3)H-glycine-1-(14)C and deoxycholyl-2,4-(3)H-glycine-1-(14)C were synthesized and administered orally to 10 healthy subjects. Distribution of radioactivity among bile acids and specific activity of steroid and amino acid moieties were determined in bile samples. (3)H and (14)C were measured in feces. (14)C in breath was calculated from interval (14)CO(2) specific activity determinations. The daily fractional turnover of the glycine moiety of chenodeoxycholyl and deoxycholylglycines was more than three times that of the steroid moiety. Pool size of chenodeoxycholylglycine was about twice that of deoxycholylglycine, but similar fractional turnover rates of steroid and amino acid moieties suggested that intestinal absorption of the two conjugated bile acids was equally efficient (about 95%). The amount of unlabeled deoxycholic acid (newly formed by bacterial 7alpha-dehydroxylation) absorbed from the intestine approximated 30% of the cholic acid that was lost. (3)H radioactivity remained predominantly in administered bile acid implying that, normally, secondary bile acids derived from chenodeoxycholic acid are not appreciably absorbed from the intestine and that deoxycholic acid is not hydroxylated by the liver. Approximately 25% of administered (14)C was recovered in the breath in the first 24 hr and less than 8% in the feces in 8 days; (14)CO(2) excretion correlated highly with fractional turnover of the glycine moiety. (3)H appeared predominantly in feces, and the rate of excretion correlated highly with the fractional turnover of the steroid moiety of bile acids. From the results in this paper plus previous measurements on the metabolism of cholylglycine, we calculated that about 6 mmoles/day of glycine is used for bile acid conjugation in health.

Pancreozymin bioassay in man based on pancreatic enzyme secretion: potency of specific amino acids and other digestive products.

The ability of products of digestion to stimulate pancreozymin secretion in man was investigated using a bioassay procedure, based on duodenal perfusion, which quantified the total outputs of pancreatic enzymes evoked by intraduodenal stimuli under steady-state conditions. Patterns of response resulting from physiologic intraduodenal concentrations of test material were basal output (with isotonic saline), washout of enzymes (with dextrose, micellar fatty acid, and amino acids), and sustained output of enzymes (with amino acids and micellar fatty acid). The sustained secretion of pancreatic enzymes found during the 2nd hr of perfusion and subsequently was characteristic of pancreozymin-induced secretion. The enzyme output in response to a mixture of essential and nonessential amino acids was significantly higher than that evoked by micellar fatty acid and was comparable with that resulting from the maximally tolerated dose of pancreozymin given by vein. Perfusion with essential amino acids caused enzyme outputs comparable to those induced by perfusion with the original amino acid mixture, whereas perfusion with nonessential amino acids had no effect. When the essential amino acids were tested individually, only phenylalanine, methionine, and valine caused significant increases in pancreatic enzyme output; the effect of tryptophan was indeterminate. However, the pancreatic enzyme output was less in response to these three essential amino acids than to mixtures containing all of them.

Absorption of cholesterol from a micellar solution: intestinal perfusion studies in man.

The absorption of cholesterol has been studied in man by perfusing the upper jejunum with a micellar solution of bile salt, 1-monoglyceride, and cholesterol-(14)C, with a triple lumen tube with collection sites 50 cm apart. The absorption of micellar components between the collection sites was calculated from their concentration changes relative to those of the watersoluble marker, polyethylene glycol. Control experiments were performed with cholesterol-free perfusions of saline or bile salt-monoglyceride solutions. Steady state conditions were obtained.Each of the components of the micelle was absorbed to a different extent during passage through the test segment of jejunum. Bile salt was not absorbed (mean, -3%), but micellar monoglyceride was rapidly hydrolyzed and absorbed almost completely (mean, 98%). Cholesterol radioactivity was absorbed to an intermediate extent (mean, 73%), and the absorption of chemically determined cholesterol (mean, 46%) indicated that much of the disappearance of radioactivity represented true absorption and not simple exchange. The specific activity of the perfused cholesterol fell during passage through the loop. This fall was interpreted as signifying the continuous addition of nonradioactive endogenous cholesterol by the test segment. However, the decrease in specific activity may also be considered to signify exchange, in that nonradioactive molecules entered the lumen as radioactive molecules were absorbed. Plant sterols appeared in the intestinal contents during the perfusion and must have been contributed by the perfused segment. The perfusate and samples taken from the upper and lower collection sites were examined by ultracentrifugation to define the physical state of cholesterol. It was found that cholesterol in the perfusate or upper collection site samples did not sediment, but that 23% of the cholesterol in the lower collection site samples was sedimentable (mean of three experiments); bile salt, as control, was not sedimentable. Solubility experiments in model systems showed that cholesterol possessed low solubility in bile salt solution; its solubility increased markedly and in linear proportion to the amount of fatty acid or monoglyceride or both that was added to the bile salt solution. These findings suggest that polar lipid such as fatty acid or monoglyceride as well as bile salt is essential for normal micellar solubilization of cholesterol in intestinal content. They suggest the necessity of considering an insoluble sedimentable phase of particulate sterol in intestinal content as well as an oil and micellar phase for a complete description of sterol absorption. The marked difference in the rates of absorption of individual micellar components suggests that micellar lipid is not absorbed as an intact aggregate and is consistent with the view that polar lipid such as fatty acid is absorbed in molecular form by diffusion from a micellar solution. The experiments confirm previous findings demonstrating that fat absorption without bile salt absorption occurs in the upper small intestine in man.

Description and simulation of a physiological pharmacokinetic model for the metabolism and enterohepatic circulation of bile acids in man. Cholic acid in healthy man.

A multicompartmental pharmacokinetic model based on physiological principles, experimental data, and the standard mathematical principles of compartmental analysis has been constructed that fully describes the metabolism and enterohepatic cycling in man of cholic acid, a major bile acid. The model features compartments and linear transfer coefficients. The compartments are aggregated into nine spaces based on physiological considerations (liver, gallbladder, bile ducts, jejunum, ileum, colon, portal blood sinusoidal blood, and general circulation). The transfer coefficients are also categorized according to function: flow, i.e., emptying of gallbladder or intestinal spaces, and circulation of the blood; biotransformation, i.e., conjugation, deconjugation, or dehydroxylation; and transport, i.e., active or passive transport. The model is made time dependent by introducing meals, which trigger discrete increases in gallbladder emptying and intestinal flow. Each space contains three compartments. For cholic acid, these are unconjugated cholic acid, cholylglycine, and cholyltaurine. The model was then used with all existing experimental data to simulate cholic acid metabolism in healthy man over a 24-h period. Satisfactory agreement was obtained between simulated and experimental results for serum bile acid levels, hepatic bile acid secretion, and bile acid secretion into the intestine. The model was also used to classify 16 clinical instances in which the enterohepatic circulation of bile acids is altered by drugs or disease. The model can be extended to describe completely the metabolism and enterohepatic circulation of any bile acids in man in health and digestive disease. The model should also be broadly applicable to the description of the pharmacokinetics of all other drugs whose metabolism is similar to that of bile acids, i.e., drugs for which there are tissue and bacterial biotransformations, enterohepatic cycling, and appreciable first-pass clearance.

Effect of oral chenodeoxycholic acid on bile acid kinetics and biliary lipid composition in women with cholelithiasis.

Bile acid kinetics and biliary lipid composition were characterized in six women with gallstones before and after 6 mo of oral therapy with chenodeoxycholic acid, an agent that induces dissolution of cholesterol gallstones in man. Over a dosage range of 1-4 g/day, absorption varied from 0.8 to 2.3 g/day. The chenodeoxycholic acid pool expanded two-to sixfold, and bile became composed predominantly (> 90%) of chenodeoxycholic acid conjugated chiefly with glycine. Cholic acid and deoxycholic acid pools decreased markedly, so that the total bile acid pool expanded much less, about twofold on the average. Cholic acid synthesis decreased in five of the six patients, consistent with negative feedback inhibition of cholic acid synthesis by chenodeoxycholic acid. In four patients whose bile was above or close to saturation with cholesterol, the bile became unsaturated; in two patients, whose bile was unsaturated, it remained so. In five patients with radiolucent gallstones, chenodeoxycholic acid therapy was continued after completion of kinetic and composition measurements; the stones decreased in size or dissolved entirely during the subsequent 6 to 18 mo. Similar measurements of bile acid kinetics and biliary lipid composition were made before and after a 6-mo period without medication in a control group of six healthy women; no changes occurred.

Metabolism of steroid and amino acid moieties of conjugated bile acids in man. 3. Cholyltaurine (taurocholic acid).

After oral administration of [2,4-(3)H]-cholyl[(35)S]taurine to eight healthy subjects with indwelling nasoduodenal tubes, the specific activity of the cholyl and taurine moieties and the distribution of radioactivity in biliary bile acid and urinary metabolites, as well as total urinary and fecal (35)S and (3)H, were measured at intervals for 4-8 days. Similar measurements were made after [(35)S]taurine was given orally or intravenously or instilled into the distal intestine. The daily fractional turnover rate of the taurine moiety of cholyltaurine was low and similar to that of the cholyl moiety, indicating that deconjugation occurring during enterohepatic cycling was less than half that previously observed for glycine-conjugated bile acids. Some of the cholyl moiety was absorbed but, since reconjugation occurred predominantly with glycine, little reincorporation into the cholyltaurine pool was observed. Some of the taurine moiety was also absorbed intact but entered large taurine pools, and little reincorporation into the cholyltaurine pool was seen. Oral administration of taurine expanded the cholyltaurine pool and induced a decrease in the fractional turnover rate of the cholyl moiety of cholyltaurine, interpreted to indicate a greater reincorporation of the cholyl moiety because of increased reconjugation with taurine. Taurine moiety not absorbed as taurine appeared to be absorbed largely as sulfate which, like taurine, entered large endogenous pools. Little fecal excretion of (35)S occurred. (35)S was excreted in urine as taurine and sulfate, and excretion in the first 24 h (as percentage of administered dose) correlated highly (r = 0.93) with the daily fractional turnover rate of the taurine moiety. When taurine was instilled into the distal intestine, it appeared as such in plasma, but the more distal the site of instillation, the greater the fraction of urinary (35)S present as sulfate. The [(35)S]sulfate appeared to have come from bacterial degradation of [(35)S]taurine because, when [(35)S]taurine was given intravenously, (35)S was excreted in urine chiefly as [(35)S]taurine with little SO(4)=(-)[(35)S] being present.

Mast cells and histamine contribute to bile acid-stimulated secretion in the mouse colon.

Certain dihydroxy bile acids cause secretory diarrhea when present in the colonic lumen at inappropriately high concentrations. However, the mechanism underlying the secretagogue activity has not been fully elucidated. Experiments were performed to test whether mast cells and one of their major mediators, histamine, might contribute to the secretory effect. Chenodeoxycholic acid, a secretory bile acid, and ursodeoxycholic acid, a nonsecretory, hydrophilic bile acid, were compared for their ability to induce chloride secretion across segments of mouse colon mounted in Ussing chambers. Chenodeoxycholic acid, but not ursodeoxycholic acid, induced dose-dependent, biphasic chloride secretion that was greater after serosal than mucosal addition and was greater in distal versus proximal colonic segments. The secretory effect of chenodeoxycholic acid was inhibited by H1 histamine receptor antagonists and modified by the cyclooxygenase inhibitor indomethacin. However, it was unaffected by an H2 histamine receptor antagonist or by atropine. Secretory effects of chenodeoxycholic acid were diminished in magnitude and delayed in colonic tissues from mice with a genetic deficiency of tissue mast cells. Concentrations of chenodeoxycholic acid inducing secretion also released histamine from tissue segments. These data indicate that mast cells and histamine-mediated processes contribute significantly to the secretory effects of dihydroxy bile acids in the murine colon.

Triketocholanoic (dehydrocholic) acid. Hepatic metabolism and effect on bile flow and biliary lipid secretion in man.

[24-(14)C]Dehydrocholic acid (triketo-5-beta-cholanoic acid) was synthesized from [24-(14)C]cholic acid, mixed with 200 mg of carrier, and administered intravenously to two patients with indwelling T tubes designed to permit bile sampling without interruption of the enterohepatic circulation. More than 80% of infused radioactivity was excreted rapidly in bile as glycine- and taurine-conjugated bile acids. Radioactive products were identified, after deconjugation, as partially or completely reduced derivatives of dehydrocholic acid. By mass spectrometry, as well as chromatography, the major metabolite (about 70%) was a dihydroxy monoketo bile acid (3alpha,7alpha-dihydroxy-12-keto-5beta-cholanoic acid); a second metabolite (about 20%) was a monohydroxy diketo acid (3alpha-hydroxy-7,12-di-keto-5beta-cholanoic acid); and about 10% of radioactivity was present as cholic acid. Reduction appeared to have been sequential (3 position, then 7 position, and then 12 position) and stereospecific (only alpha epimers were recovered). Bile flow, expressed as the ratio of bile flow to bile acid excretion, was increased after dehydrocholic acid administration. It was speculated that the hydroxy keto metabolites are hydrocholeretics. The proportion of cholesterol to lecithin and bile acids did not change significantly after dehydrocholic acid administration. In vitro studies showed that the hydroxy keto metabolites dispersed lecithin poorly compared to cholate; however, mixtures of cholate and either metabolite had dispersant properties similar to those of cholate alone, provided the ratio of metabolite to cholate remained below a value characteristic for each metabolite. These experiments disclose a new metabolic pathway in man, provide further insight into the hydrocholeresis induced by keto bile acids, and indicate the striking change in pharmacologic and physical properties caused by replacement of hydroxyl by a keto substituent in the bile acid molecule.

Modeling and Experimental Studies of Obeticholic Acid Exposure and the Impact of Cirrhosis Stage.

Obeticholic acid (OCA), a semisynthetic bile acid, is a selective and potent farnesoid X receptor (FXR) agonist in development for the treatment of chronic nonviral liver diseases. Physiologic pharmacokinetic models have been previously used to describe the absorption, distribution, metabolism, and excretion (ADME) of bile acids. OCA plasma levels were measured in healthy volunteers and cirrhotic subjects. A physiologic pharmacokinetic model was developed to quantitatively describe the ADME of OCA in patients with and without hepatic impairment. There was good agreement between predicted and observed increases in systemic OCA exposure in subjects with mild, moderate, and severe hepatic impairment, which were 1.4-, 8-, and 13-fold relative to healthy volunteers. Predicted liver exposure for subjects with mild, moderate, and severe hepatic impairment were increased only 1.1-, 1.5-, and 1.7-fold. In subjects with cirrhosis, OCA exposure in the liver, the primary site of pharmacological activity along with the intestine, is increased marginally (∼2-fold).

Chemical synthesis of bilirubin glucuronide conjugates.

In dimethylformamide, the two carboxyl groups of bilirubin react with the bifunctional coupling agent, carbonyldimidazole, to form bilirubin diimidazole, which was isolated and crystallised. The bilirubin diimidazole, termed "activated bilirubin", was shown to react spontaneously with primary alcohols to form diesters of bilirubin. After addition of the tetrabutyl ammonium salt of glucuronic acid, compounds with chromatographic mobilities similar to those of bilirubin mono- and diglucuronides from bile were formed. Bilirubin diglucuronides were isolated by barium precipitation followed by solvent extraction. The bilirubin diglucuronides were considered to be a mixture of alpha and beta glucuronides esterified at positions 1,2,3, or 4 of glucuronic acid because the compound(s) was resistant to hydrolysis with glucuronidase and gave multiple sponts by chromatography after diazotization with ethyl anthranilate. The model compounds lauryl glucuronides were synthesized similarly; the most polar product by chromatography and had identical chromatographic mobility to synthetic lauryl 1-D-glucuronide prepared by reductive debenzylation of lauryl (benzyl (2,3,4-tri-O-benzyl))-D-glucuronide. It is concluded that bilirubin-1-di-beta-D-glucuronide can be synthesized when suitable protecting groups for the 2, 3, and 4 hydroxyl groups of glucuronic acid become available.

The Aspergillus nidulans uvsB gene encodes an ATM-related kinase required for multiple facets of the DNA damage response.

In Aspergillus nidulans, uvsB and uvsD belong to the same epistasis group of DNA repair mutants. Recent observations suggest that these genes are likely to control cell cycle checkpoint responses to DNA damage and incomplete replication. Consistent with this notion, we show here that UVSB is a member of the conserved family of ATM-related kinases. Phenotypic characterization of uvsB mutants shows that they possess defects in additional aspects of the DNA damage response besides checkpoint control, including inhibition of septum formation, regulation of gene expression, and induced mutagenesis. The musN227 mutation partially suppresses the poor growth and DNA damage sensitivity of uvsB mutants. Although musN227 partially suppresses several uvsB defects, it does not restore checkpoint function to uvsB mutants. Notably, the failure of uvsB mutants to restrain septum formation in the presence of DNA damage is suppressed by the musN227 mutation. We propose that UVSB functions as the central regulator of the A. nidulans DNA damage response, whereas MUSN promotes recovery by modulating a subset of the response.

The Aspergillus nidulans musN gene encodes a RecQ helicase that interacts with the PI-3K-related kinase UVSB.

In Aspergillus nidulans, the uvsB gene encodes a member of the PI-3K-related kinase family of proteins. We have recently shown that UVSB is required for multiple aspects of the DNA damage response. Since the musN227 mutation is capable of partially suppressing defects caused by uvsB mutations, we sought to understand the mechanism underlying the suppression by cloning the musN gene. Here, we report that musN encodes a RecQ helicase with homology to S. pombe rqh1, S. cerevisiae sgs1, and human BLM and WRN. Phenotypic characterization of musN mutant alleles reveals that MUSN participates in the response to a variety of genotoxic agents. The slow growth and genotoxin sensitivity of a musN null mutant can be partially suppressed by a defect in homologous recombination caused by the uvsC114 mutation. In addition, we present evidence suggesting that MUSN may promote recovery from the DNA damage response. We suggest that a block to recovery caused by the musN227 mutation, coupled with the modest accumulation of recombination intermediates, can suppress defects caused by uvsB mutations. Finally, we report that another RecQ helicase, ORQA, performs a function that partially overlaps that of MUSN.

Identification and characterization of genes required for hyphal morphogenesis in the filamentous fungus Aspergillus nidulans.

In the filamentous fungus Aspergillus nidulans, germination of an asexual conidiospore results in the formation of a hyphal cell. A key feature of spore germination is the switch from isotropic spore expansion to polarized apical growth. Here, temperature-sensitive mutations are used to characterize the roles of five genes (sepA, hypA, podB-podD) in the establishment and maintenance of hyphal polarity. Evidence that suggests that the hypA, podB, and sepA genes are required for multiple aspects of hyphal morphogenesis is presented. Notably, podB and sepA are needed for organization of the cytoskeleton at sites of polarized growth. In contrast, podC and podD encode proteins that appear to be specifically required for the establishment of hyphal polarity during spore germination. The role of sepA and the pod genes in controlling the spatial pattern of polarized morphogenesis in germinating spores is also described. Results obtained from these experiments indicate that the normal pattern of germ-tube emergence is dependent upon the integrity of the actin cytoskeleton.

The continuing importance of bile acids in liver and intestinal disease.

Bile acids, the water-soluble, amphipathic end products of cholesterol metabolism, are involved in liver, biliary, and intestinal disease. Formed in the liver, bile acids are absorbed actively from the small intestine, with each molecule undergoing multiple enterohepatic circulations before being excreted. After their synthesis from cholesterol, bile acids are conjugated with glycine or taurine, a process that makes them impermeable to cell membranes and permits high concentrations to persist in bile and intestinal content. The relation between the chemical structure and the multiple physiological functions of bile acids is reviewed. Bile acids induce biliary lipid secretion and solubilize cholesterol in bile, promoting its elimination. In the small intestine, bile acids solubilize dietary lipids promoting their absorption. Bile acids are cytotoxic when present in abnormally high concentrations. This may occur intracellularly, as occurs in the hepatocyte in cholestasis, or extracellularly, as occurs in the colon in patients with bile acid malabsorption. Disturbances in bile acid metabolism can be caused by (1) defective biosynthesis from cholesterol or defective conjugation, (2) defective membrane transport in the hepatocyte or ileal enterocyte, (3) defective transport between organs or biliary diversion, and (4) increased bacterial degradation during enterohepatic cycling. Bile acid therapy involves bile acid replacement in deficiency states or bile acid displacement by ursodeoxycholic acid, a noncytotoxic bile acid. In cholestatic liver disease, administration of ursodeoxycholic acid decreases hepatocyte injury by retained bile acids, improving liver tests, and slowing disease progression. Bile acid malabsorption may lead to high concentrations of bile acids in the colon and impaired colonic mucosal function; bile acid sequestrants provide symptomatic benefit for diarrhea. A knowledge of bile acid physiology and the perturbations of bile acid metabolism in liver and digestive disease should be useful for the internist.

Desaturation of bile and cholesterol gallstone dissolution with chenodeoxycholic acid.

The feeding of one of the major biliary bile acids, chenodeoxycholic acid, at a dose of 10 to 15 mg/kg per day causes the circulating bile acid pool to become greatly enriched in this bile acid. When chenodeoxycholic acid composes more than 70% of the biliary bile acids, the amount of cholesterol secreted in bile falls, and bile becomes unsaturated in cholesterol. If cholesterol gallstones are present and are exposed to this unsaturated bile, they will dissolve in 4 to 24 months in the majority of patients. Extensive clinical experience indicates that such medical therapy is safe, despite unequivocal toxicity of chenodeoxycholic acid in several nonhuman primates. When therapy is stopped, bile resaturates, and stones may recur. Since cholecystecomy is a rapid, safe, effective, and usually permanent treatment for all gallstones, the value of medical therapy remains uncertain at present, except for patients in whom surgery is inadvisable. Nonetheless, the demonstration that chenodeoxycholic acid ingestion will desaturate bile and induce gallstone dissolution would appear to be an important pharmacological advance.