Hydroxylation, sulfation, and conjugation of bile acids in rat hepatoma and hepatocyte cultures under the influence of glucocorticoids

Identification and characterization of a novel PPARα-regulated and 7α-hydroxyl bile acid-preferring cytosolic sulfotransferase mL-STL (Sult2a8)

PPARα has been known to play a pivotal role in orchestrating lipid, glucose, and amino acid metabolism via transcriptional regulation of its target gene expression during energy deprivation. Recent evidence has also suggested that PPARα is involved in bile acid metabolism, but how PPARα modulates the homeostasis of bile acids during fasting is still not clear. In a mechanistic study aiming to dissect the spectrum of PPARα target genes involved in metabolic response to fasting, we identified a novel mouse gene (herein named mL-STL for mouse liver-sulfotransferase-like) that shared extensive homology with the Sult2a subfamily of a superfamily of cytosolic sulfotransferases, implying its potential function in sulfonation. The mL-STL gene expressed predominantly in liver in fed state, but PPARα was required to sustain its expression during fasting, suggesting a critical role of PPARα in regulating the mL-STL-mediated sulfonation during fasting. Functional studies using recombinant His-tagged mL-STL protein revealed its narrow sulfonating activities toward 7α-hydroxyl primary bile acids, including cholic acid, chenodeoxycholic acid, and α-muricholic acid, and thus suggesting that mL-STL may be the major hepatic bile acid sulfonating enzyme in mice. Together, these studies identified a novel PPARα-dependent gene and uncovered a new role of PPARα as being an essential regulator in bile acid biotransformation via sulfonation during fasting.

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.

Differential effects of 12-O-tetradecanoylphorbol 13-acetate on cytochrome P-450-dependent monooxygenase activities in rat hepatoma cells: induction of P-450I and suppression of P-450II

We have studied the effects of the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) on cytochrome P-450-dependent monooxygenase activities in several differentiated and dedifferentiated Reuber rat hepatoma cell lines using aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH), ethoxyresorufin O-deethylase (EROD), and aldrin epoxidase (AE) as test systems. The following results were obtained: (1) Exposure of cultures to 400 nM TPA for 18-24 h increased AHH activities in the differentiated lines 2sFou, H41IEC3/G- and Fao as well as in the dedifferentiated line 5L, 1.5-2.5-fold. The phorbol ester did not affect AHH activity in the dedifferentiated line H5. (2) EROD, a marker for P-450I, was induced by the phorbol ester to a similar degree as AHH. (3) A monoclonal antibody directed against P-450I strongly inhibited the AHH activity induced by TPA. (4) The onset of AHH or EROD induction by TPA was much later than that elicited by benz[a]anthracene. (6) In contrast to the induction of AHH and EROD, TPA decreased AE activity, a marker for P-450II, by about 50% in all the cell lines containing this monooxygenase activity. (7) The half-maximum-effect concentration of TPA for inducing or suppressing AHH and AE, respectively, was approximately 20 nM. (8) TPA did not interfere with AHH induction by benz[a]anthracene. However, the phorbol ester moderately decreased AHH induction and markedly suppressed AE induction by dexamethasone. The results indicate that TPA simultaneously induces P-450I and suppresses P-450II forms in rat hepatoma cells. P-450I induction by TPA in these cells did not appear to depend on their status of differentiation. Furthermore, the results suggest that the mechanism of P-450I induction by TPA differs from that elicited by polycyclic aromatic hydrocarbons or glucocorticoids.

Taurine and glycine conjugation and sulfation of lithocholate in primary hepatocyte cultures

Rat primary liver cells were used to study taurine and glycine conjugation and sulfation of lithocholate. After addition of [14C]lithocholate to the tissue culture medium, synthesis and excretion of amidated and/or sulfated products were investigated for up to 24 h. After incubation for 1 h, more than 83% of the labeled bile salt was amidated but not sulfated and between 5 and 11% was sulfated, with more than 80% of the sulfated bile salts being also amidated. After 24 h, the proportion of sulfated lithocholate had increased to about 23% and more than 99% of the lithocholate sulfate was additionally conjugated with glycine or taurine. Both sulfates and non-sulfates were preferably amidated with taurine. We conclude that in primary rat hepatocytes, (1) lithocholate is rapidly and almost completely conjugated with glycine or taurine (amidated), whereas sulfation of lithocholate (and its amidates) proceeds slowly and even after 24 h represents only a small proportion of the total lithocholate metabolites, and (2) sulfated and unsulfated bile salts are both preferably amidated with taurine.

Enhancing effect of a phorbol ester and of retinoic acid on glucocorticoid induction of chenodeoxycholate hydroxylation in hepatoma cultures

In cultures of the differentiated clones Faza 967, Fao and HF, derived from Reuber hepatoma, physiological doses of glucocorticoid induce chenodeoxycholate 6 beta-hydroxylation, a microsomal cytochrome-P-450-mediated activity (enhanced in liver by phenobarbital and not by benzo[a]anthracene). Whereas 12-O-tetradecanoylphorbol 13-acetate (TPA) alone has no effect the tumor promoter, when added to dexamethasone, enhances this induction. This enhancement, half-maximum with 10 ng/ml TPA, is a function of the dose between 1 ng/ml and 50 ng/ml; 50 ng/ml (80 nM) increase 4-7-fold the induction rate (as measured in cultures by the amount of bile acid hydroxylated per 10(6) cells in 24 h, and in homogenates from treated cells) and 2.5-fold the maximum activity attained by the third day of induction. When added to cultures of the dedifferentiated clone H5, treated with benzo[a]anthracene, TPA does not influence benzo[a]pyrene hydroxylase induction, as shown by the total and relative amounts of the various hydrosoluble benzo[a]pyrene metabolites. TPA does not affect tyrosine aminotransferase induction in dexamethasone-treated Fao cultures. The enhancement is not suppressed by indomethacin, an inhibitor of prostaglandin synthesis. After dexamethasone removal from induced Faza 967 cultures, addition of TPA to the medium does not affect the decay rate of the chenodeoxycholate-hydroxylating activity. Retinoic acid similarly enhances the induction by dexamethasone of chenodeoxycholate hydroxylation, both in treated Faza 967 cultures and in homogenates from treated cultures. The effects of TPA and retinoic acid are additive. These results suggest a possible cooperation at the transcriptional level between transactive factors, involving TPA-mediated alterations, retinoic acid and glucocorticoid receptors. The system described might provide a convenient experimental approach in the study of its mechanism.

Hydroxylation, conjugation and sulfation of bile acids in primary monolayer cultures of rat hepatocytes

Hydroxylation of lithocholic, chenodeoxycholic, deoxycholic and cholic acids was studied in monolayers of rat hepatocytes cultured for 76 h. The majority of added lithocholic and chenodeoxycholic acids was metabolized to beta-muricholic acid (56-76%). A small part of these bile acids (9%), however, and a considerable amount of deoxycholic and cholic acids (21%) were converted into metabolites more polar than cholic acid in the first culture period. Formation of these compounds decreased during the last day of culture. Bile acids synthesized after addition of [4-14C]-cholesterol were almost entirely (97%) sulfated and/or conjugated, predominantly with taurine (54-66%), during culture. Sulfated bile acids were mainly composed of free bile acids. The ability of hepatocytes to sulfurylate bile acids declined with culture age. Thus, rat hepatocytes in primary monolayer culture are capable to sulfurylate bile acids and to hydroxylate trihydroxylated bile acids, suggesting formation of polyhydroxylated metabolites.

Human liver microsomal steroid metabolism: identification of the major microsomal steroid hormone 6 beta-hydroxylase cytochrome P-450 enzyme

Cytochrome P-450-dependent steroid hormone metabolism was studied in isolated human liver microsomal fractions. 6 beta hydroxylation was shown to be the major route of NADPH-dependent oxidative metabolism (greater than or equal to 75% of total hydroxylated metabolites) with each of three steroid substrates, testosterone, androstenedione, and progesterone. With testosterone, 2 beta and 15 beta hydroxylation also occurred, proceeding at approximately 10% and 3-4% the rate of microsomal 6 beta hydroxylation, respectively, in each of the liver samples examined. Rates for the three steroid 6 beta-hydroxylase activities were highly correlated with each other (r = 0.95-0.97 for 25 individual microsomal preparations), suggesting that a single human liver P-450 enzyme is the principal microsomal 6 beta-hydroxylase catalyst with all three steroid substrates. Steroid 6 beta-hydroxylase rates correlated well with the specific content of human P-450NF (r = 0.69-0.83) and with its associated nifedipine oxidase activity (r = 0.80), but not with the rates for debrisoquine 4-hydroxylase, phenacetin O-deethylase, or S-mephenytoin 4-hydroxylase activities or the specific contents of their respective associated P-450 forms in these same liver microsomes (r less than 0.2). These correlative observations were supported by the selective inhibition of human liver microsomal 6 beta hydroxylation by antibody raised to either human P-450NF or a rat homolog, P-450 PB-2a. Anti-P-450NF also inhibited human microsomal testosterone 2 beta and 15 beta hydroxylation in parallel to the 6 beta-hydroxylation reaction. This antibody also inhibited rat P-450 2a-dependent steroid hormone 6 beta hydroxylation in uninduced adult male rat liver microsomes but not the steroid 2 alpha, 16 alpha, or 7 alpha hydroxylation reactions catalyzed by other rat P-450 forms. Finally, steroid 6 beta hydroxylation catalyzed by either human or rat liver microsomes was selectively inhibited by NADPH-dependent complexation of the macrolide antibiotic triacetyloleandomycin, a reaction that is characteristic of members of the P-450NF gene subfamily (P-450 IIIA subfamily). These observations establish that P-450NF or a closely related enzyme is the major catalyst of steroid hormone 6 beta hydroxylation in human liver microsomes, and furthermore suggest that steroid 6 beta hydroxylation may provide a useful, noninvasive monitor for the monooxygenase activity of this hepatic P-450 form.

Effects of ursodeoxycholic acid, analogues of ursodeoxycholic acid and combination of bile acids on bile acid synthesis in cultured rat hepatocytes

The effect of individual 7 beta-hydroxy bile acids (ursodeoxycholic and ursocholic acid), bile acid analogues of ursodeoxycholic acid, combination of bile acids (taurochenodeoxycholate and taurocholate), and mixtures of bile acids, phospholipids and cholesterol in proportions found in rat bile, on bile acids synthesis was studied in cultured rat hepatocytes. Individual steroids tested included ursodeoxycholate (UDCA), ursocholate (UCA), glycoursodeoxycholate (GUDCA) and tauroursodeoxycholate (TUDCA). Analogues of UDCA (7-methylursodeoxycholate, sarcosylursodeoxycholate and ursooxazoline) and allochenodeoxycholate, a representative of 5 alpha-cholanoic bile acid were also tested in order to determine the specificity of the bile acid biofeedback. Each individual steroid was added to the culture media at concentrations ranging from 10 to 200 microM. Mixtures of taurochenodeoxycholate (TDCA) and taurocholate in concentrations ranging from 150 to 600 microM alone and in combination with phosphatidylcholine (10-125 microM) and cholesterol (3-13 microM) were also tested for their effects on bile acid synthesis. Rates of bile acid synthesis were determined as the conversion of added lipoprotein [4-14C]cholesterol or [2-14C]mevalonate into 14C-labeled bile acids and by GLC quantitation of bile acids secreted into the culture media. Individual bile acids, bile acid analogues, combination of bile acids and mixture of bile acids with phosphatidylcholine and cholesterol failed to inhibit bile acid synthesis in cultured hepatocytes. The addition of UDCA or UCA to the culture medium resulted in a marked increase in the intracellular level of both bile acids, and in the case of UDCA there was a 4-fold increase in beta-muricholate. These results demonstrate effective uptake and metabolism of these bile acids by the rat hepatocytes. UDCA, UCA, TUDCA and GUDCA also failed to inhibit cholesterol-7 alpha-hydroxylase activity in microsomes prepared from cholestyramine-fed rats. The current data confirm and extend our previous observations that, under conditions employed, neither single bile acid nor a mixture of bile acids with or without phosphatidylcholine and cholesterol inhibits bile acid synthesis in primary rat hepatocyte cultures. We postulate that mechanisms other than a direct effect of bile acids on cholesterol-7 alpha-hydroxylase might play a role in the regulation of bile acid synthesis.

Effect of dexamethasone on cytochrome P-450 mediated metabolism of 2-acetylaminofluorene in cultured rat hepatocytes

The metabolism of 2-acetylaminofluorene (AAF) to its six oxidative metabolites has been used to investigate the effect of dexamethasone on cytochrome P-450 activity in cultured rat hepatocytes. In control hepatocytes the metabolism of AAF to its 1-, 5-, 7-, 9- and N-hydroxylated metabolites rapidly declined in culture over the first 24 hr while 3-hydroxylation remained relatively constant. These activities either remained unchanged or increased slightly during the next 48 hr in culture. The addition of dexamethasone (100 nM) to the culture medium had little effect in arresting the initial decline but by 72 hr the 7-, 5- and 3-hydroxylations increased to values 2.5, 16 and 21 times the respective 24-hr values. The inductive effect of dexamethasone on the 3- and 5-hydroxylations of AAF was maximal at 100 nM whereas the 7-hydroxylation increased linearly as a function of the dexamethasone concentration up to 1 microM. Cortisol and corticosterone and the non-glucocorticoids fluoxymesterone and methyltestosterone induced a pattern of AAF metabolism resembling that in dexamethasone-treated cultures, suggesting that a range of steroids not restricted to glucocorticoids may induce multiple cytochrome P-450 isozymes via related mechanisms. Pregnenolone 16 alpha-carbonitrile induced only the 7-hydroxylation of AAF probably reflecting induction of cytochrome P-450p. While dexamethasone was a strong inducer of the 3- and 5-hydroxylations of AAF in hepatocyte culture, assay of these activities in freshly isolated cells after in vivo treatment with dexamethasone showed a strong induction of 7-hydroxylation but only small effects on 3- and 5-hydroxylations. Indeed the profile of AAF metabolism induced in culture by dexamethasone resembles more closely the profile induced by 3-methylcholanthrene in vivo. These data suggest that factors yet to be identified strongly influence the steroid-induced pattern of cytochrome P-450 gene expression.

Cholesterol metabolism and sterol carrier protein-2 (non-specific lipid transfer protein)

Hepatic sterol carrier protein-2 significantly enhances the microsomal conversion of cholesterol to 7 alpha-hydroxy-cholesterol. In the present work we have attempted to correlate the hepatic content of sterol carrier protein-2 with bile acid formation. We have determined the amount of this protein in a variety of physiological and experimental conditions, in which the rate of bile acid synthesis varies over a wide range, viz. during fetal development, in inbred strains of rats with different rates of bile acid synthesis, and in rats fed diets containing drugs which modify the rate of bile acid synthesis. The outcome of these experiments does not support the idea that sterol carrier protein-2 has any association with bile acid synthesis. From our data we further conclude that hepatic sterol carrier protein-2 is an adaptable protein because its level increases during development from the fetal to the post-weaning stage of the rat and since it can be modulated by oral administration of certain drugs. Furthermore, it is demonstrated that the level of sterol carrier protein-2 varies between six inbred strains of rats.

Synergistic induction of monooxygenase activity by glucocorticoids and polycyclic aromatic hydrocarbons in human fetal hepatocytes in primary monolayer culture

The ability of polycyclic aromatic hydrocarbons and glucocorticoids to regulate monooxygenase activity of human fetal liver has been studied using hepatocytes prepared by collagenase digestion of liver samples from human abortuses of 13 to 19 weeks of gestational age, and maintained in primary monolayer culture for periods up to 5 days. Addition of 1,2-benzanthracene to the cells caused an increase in monooxygenase activity (3-hydroxylation of benzo[a]pyrene and O-deethylation of 7-ethoxycoumarin) in a time-and concentration-dependent fashion. The concentration of 1,2-benzanthracene required to achieve half-maximal induction was 5 microM. The inductive effect of the polycyclic hydrocarbon was potentiated approximately 2.5-fold when dexamethasone (250 nM) or other glucocorticoids were included in the culture medium. Dexamethasone alone had little or no effect on the induction of monooxygenase activity. The concentration of dexamethasone required for half-maximal stimulation of monooxygenase activity in the presence of 1,2-benzanthracene was 5-10 nM, and the action of dexamethasone was reversed by the addition of cortisol 21-mesylate, consistent with the concept that the action of dexamethasone was mediated by binding to a glucocorticoid receptor. These results are suggestive that glucocorticoids, which are produced by the fetal adrenal and have an important role in the regulation of fetal development, act synergistically with polycyclic aromatic hydrocarbons to induce the activity of liver monooxygenases in the human fetus.

Rat liver epithelial cell cultures in a serum-free medium: primary cultures and derived cell lines expressing differentiated functions

Rat liver epithelial cells explanted in a serum-free medium (SFM) composed of Ham's F10 basal medium plus free fatty acids adsorbed on bovine albumin gave successful rise to primary cultures and then to long-term cell lines that expressed liver functions; induction of L-tyrosine aminotransferase by glucocorticoids, hepatic pattern of progesterone metabolism, and biosynthesis of murine primary bile acids; chenodeoxycholic and cholic acid common to higher vertebrates and alpha-muricholic acid specific of the rat bile.

Induction of cytochrome(s) P450-dependent drug metabolism in cultured MH1C1 hepatoma cells

A cell line derived from a Morris hepatoma, MH1C1, was examined for its in vitro expression of monooxygenases. These cells were found to contain different forms of cytochrome P450, as shown by the response to inducers, namely phenobarbital (PB), 3-methylcholanthrene (MC) and metyrapone (MP). MH1C1 cell monolayers exposed to PB or MC showed an increase in the concentration of two spectrally distinct forms of cytochrome P450. The PB and MC treatments elicited enzyme activities towards the substrates aminopyrine and benzo(a)pyrene, respectively. The cell treatment with metyrapone led to a simultaneous stimulation of aminopyrine demethylase and benzo(a)pyrene hydroxylase activities, so underlining the peculiar features of this inducer.

Glucocorticoid hormones increase the activity of gamma-glutamyltransferase in a highly differentiated hepatoma cell line

Gamma-Glutamyltransferase activity was detected in the plasma membrane of the highly differentiated hepatoma cell line Fao, (0.93 mU/mg cell protein). Dexamethasone (1 microM) provoked a 2-3-fold increase in the activity of the enzyme in the presence of fetal calf serum. Maximal induction occurred 48-72 h after addition of the glucocorticoid to the cell culture medium. The hormonal specificity was demonstrated by the relative potencies of several glucocorticoids and sex steroids: hydrocortisone and corticosterone increased gamma-glutamyltransferase activity while tetrahydrocorticosterone and all sex steroids tested were ineffective. The effect of dexamethasone on gamma-glutamyltransferase activity wa specific since the activities of several other plasma membrane enzymes were not modified. The mechanism of the dexamethasone-induced increase in gamma-glutamyltransferase activity was neither by modification of the affinity of the enzyme for its substrates nor by alteration of the subcellular distribution of the enzyme. This increase was prevented by cycloheximide and actinomycin D. The data presented are consistent with a specific glucocorticoid receptor-mediated induction of gamma-glutamyltransferase activity in Fao cells. The kinetic parameters of the induction process by glucocorticoids are very similar to those found in adult rat liver. These results suggest that the Fao cell line is a very convenient system for the study of the molecular mechanisms of glucocorticoid effects on differentiated cells.