Regulation of bile acid synthesis

Vlahčevič, Z. Reno; Heuman, Douglas M.; Hylemon, Phillip B.

doi:10.1002/hep.1840130331

Cited by 179 publications

(70 citation statements)

References 91 publications

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“…Other factors, such as hormones, may also be of importance in its regulation. 1,[8][9][10] During the last decade, primary cultures of hepatocytes have been employed to investigate bile acid and cholesterol metabolism. The major bile acids formed by cultured rat hepatocytes are ␤-muricholic acid and cholic acid.…”

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Bile acid synthesis in primary cultures of rat and human hepatocytes

et al. 1998

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The regulation of hepatic bile acid formation is incompletely understood. Primary cultures of mammalian hepatocytes offer an opportunity to examine putative regulatory factors in relative isolation. Using rat and human hepatocytes in primary culture, we examined bile acid composition and the expression of the rate-limiting enzyme of formation, cholesterol 7␣-hydroxylase. Control rat hepatocytes showed a declining bile acid production over 4 days, from 156 ؎ 24 ng/mL (67% cholic acid) on day 1 to 55 ؎ 11 ng/mL (55% cholic acid) on day 4. In addition to cholic acid, chenodeoxycholic acid, ␣-muricholic acid, and ␤-muricholic acid were formed. Treatment with triidothyronine (T 3 ) or dexamethasone alone had no significant effect on bile acid production. A combination of T 3 and dexamethasone significantly increased the total bile acid production on day 4 (224 ؎ 54 ng/mL) and resulted in a marked change in composition to 23% cholic acid and 77% non-12␣-hydroxylated bile acids. Control rat hepatocytes had a cholesterol 7␣-hydroxylase activity of 3.3 ؎ 0.6 pmol/mg protein/min after 4 days in culture. Cells treated with the combination of dexamethasone and T 3 had an activity of 16.4 ؎ 3.6 pmol/mg protein/min. The cholesterol 7␣-hydroxylase messenger RNA (mRNA) levels, determined by solution hybridization after 4 days of culture, showed results similar to those for the activity data; control cells had 5.3 ؎ 0.9 cpm/ g total nucleic acids (tNAs). T 3-or dexamethasone-treated cells did not differ from control cells, whereas the combination of T 3 and dexamethasone increased the mRNA levels to 20.6 ؎ 2.8 cpm/ g tNAs. In human hepatocytes, isolated from donor liver, bile acid formation increased from 206 ؎ 79 ng/mL on day 2 to 1490 ؎ 594 ng/mL on day 6 and then declined slightly. Cholic acid and chenodeoxycholic acid were formed, constituting about 80% and 20%, respectively. The combined addition of T 3 and dexamethasone had a tendency to decrease rather than increase bile acid formation. Also, mRNA levels of the cholesterol 7␣-hydroxylase increased severalfold in the human hepatocytes from day 2 to day 4 and then declined. The addition of T 3 or dexamethasone did not effect the mRNA levels in any consistent way. It is noteworthy that the capacity of the cultured human hepatocytes to produce bile acids was higher than that of cultured rat hepatocytes, in spite of the fact that the production of bile acids in rat liver is 3-to 5-fold higher than that in human liver in vivo. It is also evident that while hormonal factors appear to regulate bile acid synthesis in the rat, no evidence for this was found in human hepatocytes. As the composition of bile acids secreted by human hepatocytes in primary culture closely resembles that found in vivo, this represents a useful model for further studies of the synthesis and regulation of bile acids. (HEPATOLOGY 1998;27: 615-620.)Bile acid synthesis is localized within the liver and represents an important pathway for cholesterol elimination from the body. The major bile acids, primarily f...

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Bile acid synthesis in primary cultures of rat and human hepatocytes

et al. 1998

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“…The hepatospecific enzyme cholesterol 7␣-hydroxylase (C7␣H; EC 1.14.13.17; CYP7A) catalyzes a rate-limiting step in this important process. [2][3][4] Its activity is tightly regulated in vivo by a number of different effectors acting predominantly at the transcriptional level. C7␣H is modulated by hormones, including thyroxin, glucocorticoids, and insulin [5][6][7] ; and by dietary factors.…”

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Cholesterol 7α-hydroxylase (CYP7A): Patterns of messenger RNA expression during rat liver development

et al. 1998

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Cholesterol 7␣-hydroxylase is a rate-limiting enzyme in bile acid synthesis, a major pathway for cholesterol catabolism. It plays a crucial role in postnatal development and survival. In an adult liver, its activity and messenger RNA (mRNA) are heterogeneously distributed with concentration in the pericentral area. We defined how the pattern of cholesterol 7␣-hydroxylase mRNA evolves during rat liver development, correlated this with its total liver mRNA levels, and determined when its heterogeneous pattern of expression is established. Cholesterol 7␣-hydroxylase mRNA was undetectable in 18-day-old fetal livers by Northern blot. It was increased markedly in newborns with a homogeneous liver lobular distribution as determined by in situ hybridization. At postnatal day four, mRNA levels were markedly decreased with concomitant appearance of a lobular gradient: mRNA was detected only in a few hepatocytes located around efferent venules. At 22 days, the time of highest mRNA expression, a marked extension of the gradient towards the periportal area was observed, indicating that the increase in total liver cholesterol 7␣-hydroxylase mRNA level was a result of recruitment of hepatocytes upstream from the central vein area. By 28 days, the adult pattern was observed. Thus, expression of cholesterol 7␣-hydroxylase mRNA is tightly regulated during rat liver development, both temporally and spatially supporting its critical role in normal postnatal development.(HEPATOLOGY 1998;28:1064-1072.)Bile acids play a key role in absorption of lipids, including fat-soluble vitamins. Bile acid synthesis represents the major regulated pathway for catabolism and excretion of cholesterol in mammals. The hepatospecific enzyme cholesterol 7␣-hydroxylase (C7␣H; EC 1.14.13.17; CYP7A) catalyzes a rate-limiting step in this important process. [2][3][4] Its activity is tightly regulated in vivo by a number of different effectors acting predominantly at the transcriptional level. C7␣H is modulated by hormones, including thyroxin, glucocorticoids, and insulin 5-7 ; and by dietary factors. [8][9][10] It is regulated in a negative feedback manner by hydrophobic bile acids returning to the liver via the enterohepatic circulation. 11-13 It exhibits a diurnal rhythm, with maximum activity in the dark phase and minimum during the light phase in rats. [13][14][15] The expression of C7␣H in adult rats is tightly regulated spatially along the liver cell plate; the enzymatic activity 16 and mRNA level 17,18 are not distributed uniformly among parenchymal cells; rather, C7␣H is predominantly found in a subgroup of hepatocytes localized around the central vein.Liver cell heterogeneity is considered to be a major determinant for proper execution and regulation of most liver functions. 19,20 It reflects differentiation of hepatocytes within the liver plate, which is established during development. In most cases, it is regulated at the transcriptional level. The role of liver cell heterogeneity in the developmental regulation of bile acid synthesis is unknown....

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“…1 and 2). Bile acids are synthesized exclusively in the liver via a multistep transformation of cholesterol precursor molecules (3,4). Once synthesized and amidated, bile acids are transported across the canalicular membrane of the hepatocyte to participate in the intraluminal digestion and absorption of a variety of hydrophobic compounds.…”

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Multiple Factors Regulate the Rat Liver Basolateral Sodium-dependent Bile Acid Cotransporter Gene Promoter

Karpen

Sun

Kudish

et al. 1996

Journal of Biological Chemistry

120

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The hepatic uptake of bile acids from the portal circulation is primarily dependent upon a sodium-dependent basolateral membrane transporter. In order to begin to investigate the factors controlling rat liver sodiumdependent bile acid cotransporter (ntcp) gene expression, we isolated Ϸ30 kilobase pairs of rat genomic DNA in three overlapping phage clones. The rat ntcp gene is distributed over 16.5 kilobase pairs as five exons. Primer extension analysis revealed two closely spaced transcription initiation sites, 27 and 41 nucleotides downstream of a TATA sequence. Regulation of transcription was investigated first by transfection of primary rat hepatocytes by a series of 5-deleted rat ntcp promoterdriven luciferase constructs (from Ϸ ؊6 kilobase pairs to ؊59 base pairs of upstream sequences, terminating at nucleotide ؉47), identifying a minimal promoter element: nucleotide ؊158 to ؉47. This minimal promoter was active in transfected HepG2, but inactive in NIH3T3, Caco-2, and Madin-Darby canine kidney cells, indicating that the determinants of hepatocyte-specific expression reside within this region. The individual elements within the minimal promoter were investigated via transfection of HepG2 cells by a series of 20 mutant plasmids, each containing a 10-base pair sequential block mutation. Eight mutant constructs profoundly suppressed promoter activity; encompassing sequences from ؊66 to ؉4 nt, and ؉15 to ؉24 nucleotides, while no other 10-base pair mutation significantly interfered with minimal promoter activity. Deoxyribonuclease I footprint analysis of the minimal promoter revealed three bound regions; ؊92 to ؊74 (footprint C), ؊50 to ؊37 (footprint B), and ؊17 to ؉12 (footprint A). Gel mobility shift assays provided evidence for hepatocyte nuclear factor 1 binding within footprint A and a liverenriched factor(s) that binds within a novel palindrome in footprint B. These studies indicate that three elements direct the basal and tissue-restricted expression of the rat ntcp promoter; a TATA element, the liverenriched transcription factor hepatocyte nuclear factor 1, and an unknown liver-enriched factor that binds within a novel palindrome in footprint B.

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Regulation of bile acid synthesis

Cited by 179 publications

References 91 publications

Bile acid synthesis in primary cultures of rat and human hepatocytes

Bile acid synthesis in primary cultures of rat and human hepatocytes

Cholesterol 7α-hydroxylase (CYP7A): Patterns of messenger RNA expression during rat liver development

Multiple Factors Regulate the Rat Liver Basolateral Sodium-dependent Bile Acid Cotransporter Gene Promoter

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