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1.  Synthesis of Lithocholic Acid Derivatives as Proteasome Regulators 
ACS medicinal chemistry letters  2012;3(11):925-930.
Accumulation of aberrant protein aggregates, such as amyloid beta peptide (Aβ), due to decreased proteasome activities might contribute to the neurodegeneration in Alzheimer's disease. In this study, lithocholic acid derivatives 3α-O-pimeloyl-lithocholic acid methyl ester (2) and its isosteric isomer (6) were found to activate the chymotrypsin-like activity of the proteasome at an EC50 of 7.8 and 4.3 μM, respectively. Replacing the C24 methyl ester in 2 with methylamide resulted in a complete devoid of proteasome activating activity. Epimerizing the C3 substituent from an alpha to beta orientation transformed the activator into a proteasome inhibitor. Unlike the cellular proteasome activator PA28, proteasome activated by 2 was not inhibited by Aβ. Furthermore, 2 potently antagonized the inhibitory effect of Aβ on the proteasome. In summary, compound 2 represents a novel class of small molecules that not only activates the proteasome but also antagonizes the inhibitory effect of Aβ on the proteasome.
doi:10.1021/ml3001962
PMCID: PMC3544189  PMID: 23330053
proteasome activator; lithocholic acid; Alzheimer's disease; amyloid beta
2.  Synthesis of Lithocholic Acid Derivatives as Proteasome Regulators 
ACS Medicinal Chemistry Letters  2012;3(11):925-930.
Accumulation of aberrant protein aggregates, such as amyloid β peptide (Aβ), due to decreased proteasome activities, might contribute to the neurodegeneration in Alzheimer's disease. In this study, lithocholic acid derivatives 3α-O-pimeloyl-lithocholic acid methyl ester (2) and its isosteric isomer (6) were found to activate the chymotrypsin-like activity of the proteasome at an EC50 of 7.8 and 4.3 μM, respectively. Replacing the C24 methyl ester in 2 with methylamide resulted in a complete devoid of proteasome activating activity. Epimerizing the C3 substituent from an α to β orientation transformed the activator into a proteasome inhibitor. Unlike the cellular proteasome activator PA28, proteasome activated by 2 was not inhibited by Aβ. Furthermore, 2 potently antagonized the inhibitory effect of Aβ on the proteasome. In summary, compound 2 represents a novel class of small molecules that not only activates the proteasome but also antagonizes the inhibitory effect of Aβ on the proteasome.
doi:10.1021/ml3001962
PMCID: PMC3544189  PMID: 23330053
proteasome activator; lithocholic acid; Alzheimer's disease; amyloid β
3.  Lithocholate glucuronide is a cholestatic agent. 
Journal of Clinical Investigation  1984;73(6):1507-1514.
Lithocholic acid and its taurine, glycine, and sulfate derivatives are potent cholestatic agents. Lithocholate glucuronide is present in the plasma and urine of patients with cholestatic syndromes, but little is known of its metabolism, excretion, and cholestatic potential. [3 beta-3H]lithocholate 3-O-beta-D-glucuronide was synthesized, and chemical and radiochemical purity were established. The aqueous solubility of lithocholate glucuronide was determined and found to be greater than that of lithocholic acid or several of its derivatives. In the range of concentrations examined, calcium ions precipitated lithocholate glucuronide stoichiometrically. The material was administered to rats prepared with an external biliary fistula. When 17-25 micrograms quantities were administered, 89.1 +/- 4.5% (mean +/- SEM) of the radiolabel was secreted in bile within the first 20 h after administration, the major fraction being secreted in less than 20 min. Four-fifths of the radiolabeled material in bile was the administered unaltered parent compound, while a minor fraction consisted of a more polar derivative(s). We showed that increasing biliary concentrations of more polar derivatives were observed with milligram doses of [3H]lithocholate glucuronide, and with time after the administration of these loading doses. Milligram doses of [3H]lithocholate glucuronide resulted in partial or complete cholestasis. When induced cholestasis was partial, secretion in bile remained the primary excretory route (82.5-105.6% recovery in bile), while, when complete cholestasis was induced, wide tissue distribution of radiolabel was observed. Cholestasis developed rapidly during infusion of [3H]lithocholate glucuronide. Bile flow was diminished within 10-20 min of the start of an infusion of 0.05 mumol, 100 g-1 body weight, minute-1, administered concomitantly with an equimolar infusion of taurocholate. The results establish that lithocholate glucuronide exerts cholestatic effects comparable to those exerted by unconjugated lithocholic acid.
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PMCID: PMC437060  PMID: 6547150
4.  Amino acid conjugates of lithocholic acid as antagonists of the EphA2 receptor 
Journal of medicinal chemistry  2013;56(7):2936-2947.
The Eph receptor–ephrin system is an emerging target for the development of novel antiangiogenetic agents. We recently identified lithocholic acid (LCA) as a small molecule able to block EphA2-dependent signals in cancer cells, suggesting that its (5β)-cholan-24-oic acid scaffold can be used as a template to design a new generation of improved EphA2 antagonists. Here, we report the design and synthesis of an extended set of LCA derivatives obtained by conjugation of its carboxyl group with different α-amino acids. Structure-activity relationships indicate that the presence of a lipophilic amino acid side chain is fundamental to achieve good potencies. The L-Trp derivative (20, PCM126) was the most potent antagonist of the series disrupting EphA2-ephrinA1 interaction and blocking EphA2 phosphorylation in prostate cancer cells at low μM concentrations, thus being significantly more potent than LCA. Compound 20 is among the most potent small molecule antagonists of the EphA2 receptor.
doi:10.1021/jm301890k
PMCID: PMC3953198  PMID: 23489211
5.  Adsorption of Lithocholic Acid to Fusarium equiseti M41 as an Essential Process in Its Conversion to Ursodeoxycholic Acid 
Fusarium equiseti M41 converts lithocholic acid to ursodeoxycholic acid. Adsorption of lithocholic acid particles to mycelia of F. equiseti M41 is essential in the conversion of lithocholic acid to ursodeoxycholic acid. Production of ursodeoxycholic acid was negligible when particles of lithocholic acid were absent. As the concentration of lithocholic acid particles increased, both the amount of mycelium-bound lithocholic acid and the production of ursodeoxycholic acid increased hyperbolically (K1/2 = 1.9 g/liter and Kmapparent = 1.9 g/liter. A fluorescent lithocholic acid derivative was used to confirm that insoluble particles of lithocholic acid attached to the surface of the mycelia. The hydrophobic nature of this binding was estimated from the close relationship observed between the hydrophobicity of bile acids and their binding capacity to the mycelia. By repeated washing with 30% dimethyl sulfoxide, two binding modes of lithocholic acid were distinguished, i.e., surface binding (59% of bound lithocholic acid) and tight binding (41% of bound lithocholic acid). From the amount of tightly bound lithocholic acid, the intracellular concentration of lithocholic acid was calculated to be 1,433-fold higher than its saturating concentration in the reaction mixture, thus promoting effective conversion to ursodeoxycholic acid in the mycelia. Several lines of evidence indicated that glycoproteins of the cell wall participated in the binding of lithocholic acid.
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PMCID: PMC202523  PMID: 16347578
6.  α,β-Unsaturated Carbonyl System of Chalcone-Based Derivatives is Responsible for Broad Inhibition of Proteasomal Activity and Preferential Killing of Human Papilloma Virus (HPV)-Positive Cervical Cancer Cells 
Journal of medicinal chemistry  2010;54(2):449-456.
Proteasome inhibitors have potential for the treatment of cervical cancer. We describe the synthesis and biological characterization of a new series of 1,3-diphenylpropen-1-one (chalcone)-based derivatives lacking the boronic acid moieties of the previously reported chalcone-based proteasome inhibitor 3,5-bis-(4-boronic acid-benzylidene)-1-methyl-piperidin- 4-one and bearing a variety of amino acid substitutions on the amino-group of the 4-piperidone. Our lead compound 2 (RA-1) inhibits proteasomal activity and has improved dose-dependent anti-proliferative and pro-apoptotic properties in cervical cancer cells containing human papillomavirus. Further, it induces synergistic killing of cervical cancer cell lines when tested in combination with an FDA approved proteasome inhibitor. Exploration of the potential mechanism of proteasomal inhibition by our lead compound using in silico docking studies suggests that the carbonyl group of its oxopiperidine moiety is susceptible to nucleophilic attack by the γ-hydroxy threonine side chain within the catalytic sites of the proteasome.
doi:10.1021/jm100589p
PMCID: PMC3204583  PMID: 21186794
Cervical Cancer; Proteasome Inhibitors; Chalcones; Ubiquitin Proteasome; System (UPS); UPS-stress
7.  Activation and inhibition of proteasomes by betulinic acid and its derivatives 
FEBS letters  2007;581(25):4955-4959.
This study discovered that betulinic acid (BA) is a potent proteasome activator that preferentially activates the chymotrypsin-like activity of proteasomes. Chemical modifications can transform BA into proteasome inhibitors. Chemical modifications at the C-3 position of BA resulted in compounds, such as dimethylsuccinyl BA (DSB), with various inhibitory activities against human 20S proteasomes. Interestingly, the proteasomal activation by BA and the inhibitory activity of DSB could be abrogated by introducing a side chain at the C-28 position. In summary, this study discovered a class of small molecules that can either activate or inhibit human proteasome activity depending on side chain modifications.
doi:10.1016/j.febslet.2007.09.031
PMCID: PMC2083647  PMID: 17904555
Betulinic acid; proteasome inhibitor; proteasome activator
8.  Fluorescent choleretic and cholestatic bile salts take different paths across the hepatocyte: transcytosis of glycolithocholate leads to an extensive redistribution of annexin II 
The Journal of Cell Biology  1994;127(2):401-410.
We have used fluorescent derivatives of the choleretic bile salts cholate and chenodeoxycholate, the cholestatic salt lithocholate, and the therapeutic agent ursodeoxycholate to visualize distinct routes of transport across the hepatocyte and delivery to the canalicular vacuole of isolated hepatocyte couplets. The cholate and chenodeoxycholate derivatives produced homogeneous intracellular fluorescence and were rapidly transported to the vacuole, while the lithocholate analogue accumulated more slowly in the canalicular vacuole and gave rise to punctate fluorescence within the cell. Fluorescent ursodeoxycholate showed punctate intracellular fluorescence against a high uniform background indicating use of both pathways. Inhibition of vesicular transport by treatment with colchicine and Brefeldin A had no effect on the uptake of any of the compounds used, but it dramatically impaired delivery of both the lithocholate and the ursodeoxycholate derivatives to the canalicular vacuole. We conclude that while the chenodeoxycholate and cholate analogues traverse the hepatocyte by a cytoplasmic route, lithocholate and ursodeoxycholate analogues are transported by vesicle-mediated transcytosis. Treatment of couplets with glycine derivatives of lithocholate and ursodeoxycholate, but not cholate or chenodeoxycholate, led to a marked relocalization of annexin II, which initially became concentrated at the basolateral membrane, then moved to a perinuclear distribution and finally to the apical membrane as the incubation progressed. This suggests that lithocholate and ursodeoxycholate treatment leads to a rapid induction of transcytosis and that annexin II exchange occurs upon membrane fusion at all stages of the hepatocyte transcytotic pathway. These results indicate that isolated hepatocyte couplets may provide an inducible model system for the study of vesicle-mediated transcytosis.
PMCID: PMC2120198  PMID: 7929584
9.  Structure-activity relationships and mechanism of action of Eph-ephrin antagonists: interaction of cholanic acid with the EphA2 receptor 
ChemMedChem  2012;7(6):1071-1083.
The Eph–ephrin system, including the EphA2 receptor and the ephrin-A1 ligand, plays a critical role in tumor and vascular functions during carcinogenesis. We previously identified (3α,5β)-3-hydroxycholan-24-oic acid (lithocholic acid) as an Eph-ephrin antagonist able to inhibit EphA2 receptor activation and therefore potentially useful as a novel EphA2 receptor targeting agent. Here, we explore the structure-activity relationships of a focused set of lithocholic acid derivatives, based on molecular modelling investigation and displacement binding assays. Our exploration shows that while the 3-α-hydroxyl group of lithocholic acid has a negligible role in the recognition of the EphA2 receptor, its carboxylate group is critical for disrupting the binding of ephrin-A1 to the EphA2. As a result of our investigation, we identified (5β)-cholan-24-oic acid (cholanic acid) as a novel compound that competitively inhibits EphA2-ephrin-A1 interaction with higher potency than lithocholic acid. Surface plasmon resonance analysis indicates that cholanic acid binds specifically and reversibly to the ligand-binding domain of EphA2, with a steady-state dissociation constant (KD) in the low micromolar range. Furthermore, cholanic acid blocks the phosphorylation of EphA2 and cell retraction and rounding in PC3 prostate cancer cells, two effects that depend on EphA2 activation by the ephrin-A1 ligand. These findings suggest that cholanic acid can be used as a template structure to design effective EphA2 antagonists, with potential impact in the elucidation of the role played by this receptor in pathological conditions.
doi:10.1002/cmdc.201200102
PMCID: PMC3677030  PMID: 22529030
Protein-protein interactions; Structure-activity relationships; Surface plasmon resonance; Steroids; Drug design
10.  Structural Analysis of Spiro β-Lactone Proteasome Inhibitors 
Journal of the American Chemical Society  2008;130(45):14981-14983.
Spiro β-lactone-based proteasome inhibitors were discovered in the context of an asymmetric catalytic total synthesis of the natural product (+)-lactacystin (1). Lactone 4 was found to be a potent inhibitor of the 26S proteasome, while its C-6 epimer (5) displayed weak activity. Crystallographic studies of the two analogs covalently bound to the 20S proteasome permitted characterization of the important stabilizing interactions between each inhibitor and the proteasome's key catalytic N-terminal threonine residue. This structural data supports the hypothesis that the discrepancy in potency between 4 and 5 may be due to differences in the hydrolytic stabilities of the resulting acyl enzyme complexes.
doi:10.1021/ja806059t
PMCID: PMC2587002  PMID: 18928262
11.  Potentiation of Temozolomide Cytotoxicity by Polymerase β Inhibition Is Increased in the Absence of BRCA2 
Cancer research  2009;70(1):409-417.
Base excision repair (BER) plays a critical role in the repair of bases damaged by oxidative metabolism or alkylating agents, such as those commonly utilized in cancer therapy. Incomplete BER generates intermediates that require activation of homology-dependent DNA repair to resolve. We investigated the effects of lithocholic acid, an inhibitor of the key BER enzyme, DNA polymerase β, in cells deficient in expression of the homology-dependent repair factor, BRCA2. In vitro studies show that lithocholic acid suppresses the DNA polymerase and 5′dRP lyase activities of DNA polymerase β by preventing the formation of a stable pol β-DNA complex, reducing BER effectiveness. Cytotoxicity assays based on colony formation revealed that lithocholic acid exhibits synergism with the alkylating agent, temozolomide, which engages BER through DNA methylation, and that the degree of synergism is increased in cells lacking functional BRCA2. BRCA2-deficient cells also showed heightened susceptibility to both lithocholic acid and temozolomide individually. The potentiation of temozolomide cytotoxicity by lithocholic acid owes to the conversion of single-stranded DNA breaks generated through incomplete BER of methylated nucleotides into double-stranded breaks during DNA replication, as indicated by γH2AX immunofluorescence. Death appears to be induced in co-treated cells through an accumulation of persistent double-stranded DNA breaks. Mutations of the BRCA2 gene have been extensively characterized and are present in various cancers, implying that inhibition of BER may offer a means to augment tumor selectivity in the use of conventional cancer therapies.
doi:10.1158/0008-5472.CAN-09-1353
PMCID: PMC2943728  PMID: 20028873
12.  Discovery and Synthesis of Hydronaphthoquinones as Novel Proteasome Inhibitors 
Journal of medicinal chemistry  2012;55(5):1978-1998.
Screening efforts led to the identification of PI-8182 (1), an inhibitor of the chymotrypsin-like (CT-L) activity of the proteasome. Compound 1 contains a hydronaphthoquinone pharmacophore with a thioglycolic acid side chain at position 2 and thiophene sulfonamide at position 4. An efficient synthetic route to the hydronaphthoquinone sulfonamide scaffold was developed and compound 1 was synthesized in-house to confirm the structure and activity (IC50 = 3.0 ± 1.6 μM [n=25]). Novel hydronaphthoquinone derivatives of the hit 1 were designed, synthesized and evaluated as proteasome inhibitors. The structure activity relationship (SAR) guided synthesis of more than 170 derivatives revealed that the thioglycolic acid side chain is required and the carboxylic acid group of this side chain is critical to the CT-L inhibitory activity of compound 1. Furthermore, replacement of the carboxylic acid with carboxylic acid isosteres such as tetrazole or triazole greatly improves potency. Compounds with a thio-tetrazole or thio-triazole side chain in position 2, where the thiophene was replaced by hydrophobic aryl moieties were the most active compounds with up to 20-fold greater CT-L inhibitory than compound 1 (compounds 15e, 15f, 15h 15j, IC50 values around 200 nM and compound 29, IC50 = 150 nM). The synthetic iterations described here not only led to improving potency in vitro but also resulted in the identification of compounds that are more active such as 39 (IC50 = 0.44 to 1.01 μM) than 1 (IC50 = 3.54 to 7.22 μM) at inhibiting the proteasome CT-L activity in intact breast cancer cells. Treatment with 39 also resulted in the accumulation of ubiquitinated cellular proteins and inhibition of tumor cell proliferation of breast cancer cells. The hit 1 and its analog 39 inhibited proteasome CT-L activity irreversibly.
doi:10.1021/jm201118h
PMCID: PMC3530929  PMID: 22220566
13.  Proteasome regulators: activators and inhibitors 
Current medicinal chemistry  2009;16(8):931-939.
This mini review covers the drug discovery aspect of both proteasome activators and inhibitors. The proteasome is involved in many essential cellular functions, such as regulation of cell cycle, cell differentiation, signal transduction pathways, antigen processing for appropriate immune responses, stress signaling, inflammatory responses, and apoptosis. Due to the importance of the proteasome in cellular functions, inhibition or activation of the proteasome could become a useful therapeutic strategy for a variety of diseases. Many proteasome inhibitors have been identified and can be classified into two groups according to their source: chemically synthesized small molecules and compounds derived from natural products. A successful case of developing a proteasome inhibitor as a clinically useful drug is that the peptide boronate, PS341 (Bortezomib), was approved for the treatment of multiple myeloma. In contrast to proteasome inhibitors, small molecules that can activate or enhance proteasome activity are rare and are not well studied. The fact that over-expression of the cellular proteasome activator PA28 exhibited beneficial effects on the Huntington’s disease neuronal model cells raised the prospect that small molecule proteasome activators could become useful therapeutics. The beneficial effect of oleuropein, a small molecule proteasome activator, on senescence of human fibroblasts also suggested that proteasome activators might have the potential to be developed into anti-aging agents.
PMCID: PMC3608511  PMID: 19275603
Proteasome; activator; inhibitor; natural products; betulinic acids; peptides; anti-cancer; bortezomib
14.  Inhibitors Selective for Mycobacterial versus Human Proteasomes 
Nature  2009;461(7264):621-626.
Summary
Many anti-infectives inhibit the synthesis of bacterial proteins, but none selectively inhibits their degradation. Most anti-infectives kill replicating pathogens, but few preferentially kill pathogens that have been forced into a non-replicating state by conditions in the host. To explore these alternative approaches we sought selective inhibitors of the proteasome of Mycobacterium tuberculosis (Mtb). Given that proteasome structure is extensively conserved, it is not surprising that inhibitors of all chemical classes tested have blocked both eukaryotic and prokaryotic proteasomes, and no inhibitor has proved substantially more potent on proteasomes of pathogens than of their hosts. Here we show that certain oxathiazol-2-ones kill non-replicating Mtb and act as selective suicide-substrate inhibitors of the Mtb proteasome by cyclo-carbonylating its active site threonine. Major conformational changes protect the inhibitor-enzyme intermediate from hydrolysis, allowing formation of an oxazolidin-2-one and preventing regeneration of active protease. Residues outside the active site whose H-bonds stabilize the critical loop before and after it moves are extensively non-conserved. This may account for the ability of oxathiazol-2-ones to inhibit the mycobacterial proteasome potently and irreversibly while largely sparing the human homolog.
doi:10.1038/nature08357
PMCID: PMC3172082  PMID: 19759536
15.  Discovery of a potent and highly β1 specific proteasome inhibitor from a focused library of urea-containing peptide vinyl sulfones and peptide epoxyketones† 
Organic & biomolecular chemistry  2011;10(1):181-194.
Syringolins, a class of natural products, potently and selectively inhibit the proteasome and show promising antitumour activity. To gain insight in the mode of action of syringolins, the ureido structural element present in syringolins is incorporated in oligopeptide vinyl sulfones and peptide epoxyketones yielding a focused library of potent new proteasome inhibitors. The distance of the ureido linkage with respect to the electrophilic trap strongly influences subunit selectivity within the proteasome. Compounds 13 and 15 are β5 selective and their potency exceeds that of syringolin A. In contrast, 5 may well be the most potent β1 selective compound active in living cells reported to date.
doi:10.1039/c1ob06554h
PMCID: PMC3769973  PMID: 22105930
16.  Synthesis and Proteasome Inhibition of Glycyrrhetinic Acid Derivatives 
Bioorganic & medicinal chemistry  2008;16(14):6696-6701.
This study discovered that glycyrrhetinic acid inhibited the human 20S proteasome at 22.3 µM. Esterification of the C-3 hydroxyl group on glycyrrhetinic acid with various carboxylic acid reagents yielded a series of analogs with marked improved potency. Among the derivatives, glycyrrhetinic acid 3-O-isophthalate (17) was the most potent compound with IC50 of 0.22 µM, which was approximately 100-fold more potent than glycyrrhetinic acid.
doi:10.1016/j.bmc.2008.05.078
PMCID: PMC2579312  PMID: 18562200
Glycyrrhetinic acid; proteasome inhibitor; triterpene
17.  New Betulinic Acid Derivatives as Potent Proteasome Inhibitors 
In this study, 22 new betulinic acid (BA) derivatives were synthesized and tested for their inhibition of the chymotrypsin-like activity of 20S proteasome. From the SAR study, we concluded that the C-3 and C-30 positions are the pharmacophores for increasing the proteasome inhibition effects, and larger lipophilic or aromatic side chains are favored at these positions. Among the BA derivatives tested, compounds 13, 20, and 21 showed the best proteasome inhibition activity with IC50 values of 1.42, 1.56, and 1.80 µM, respectively, which are three- to four-fold more potent than the proteasome inhibition controls LLM-F and lactacystin.
doi:10.1016/j.bmcl.2011.07.072
PMCID: PMC3171619  PMID: 21856154
18.  LITHOCHOLIC ACID DECREASES EXPRESSION OF UGT2B7 IN CACO-2 CELLS: A POTENTIAL ROLE FOR A NEGATIVE FARNESOID X RECEPTOR RESPONSE ELEMENT 
Human UDP-glucuronosyltransferase (UGT) 2B7 is the major isoform catalyzing the glucuronidation of a variety of endogenous compounds including bile acids. To determine the role of bile acids in the regulation of UGT2B7 expression, Caco-2 cells were incubated with the natural human farnesoid X receptor (hFXR) ligand, chenodeoxycholic acid, as well as the secondary bile acid, lithocholic acid, derived from chenodeoxycholic acid. Incubation of Caco-2 cells with lithocholic acid in the absence of exogenous hFXR resulted in a dose-dependent down-regulation of UGT2B7 mRNA levels, with an IC50 of 13 μM. Similar down-regulation was also observed with chenodeoxycholic acid; however, much higher concentrations were required. Transient transfection of Caco-2 cells with hFXR suppressed UGT2B7 mRNA expression both in the absence and presence of ligand. UGT2B7 promoter transfection experiments and deletion/mutation analysis showed that lithocholic acid-activated hFXR decreased UGT2B7 promoter activity via a negative hFXR response element (NFRE) located between nucleotides −148 and −134. Cotransfection with hFXR and/or human retinoid X receptor further enhanced the repression. Electrophoretic mobility shift assays additionally confirmed the role of NFRE in UGT2B7 down-regulation by lithocholic acid. These findings suggest that lithocholic acid, an activator of nuclear hFXR, acts as a negative regulator of UGT2B7 expression, indicating that hFXR may play an essential role in lithocholic acid homeostasis through negative regulation of this UGT that is involved in lithocholic acid biotransformation. Therefore, it is postulated that lithocholic acid toxicity may be due to down-regulation of genes involved in its detoxification, including UGT2B7, leading to limited excretion of lithocholic acid from the body.
doi:10.1124/dmd.104.003061
PMCID: PMC2652669  PMID: 15821044
19.  Interaction of Native Bile Acids with Human Apical Sodium Dependent Bile Acid Transporter (hASBT): Influence of Steroidal Hydroxylation Pattern and C-24 Conjugation 
Pharmaceutical research  2006;23(7):1451-1459.
Purpose
The human Apical Sodium-dependent Bile Acid Transporter (hASBT) is a potential target for drug delivery, but an understanding of hASBT substrate requirements is lacking. The objective of this study was to characterize hASBT interaction with its native substrates, bile acids, including an evaluation of C-24 conjugation and steroidal hydroxylation on transport affinity and inhibition potency.
Methods
Transport and inhibition kinetics of 15 bile acids were evaluated (cholate, chenodeoxycholate, deoxycholate, ursodeoxycholate, and lithocholate, including their glycine and taurine conjugates) using an hASBT-Madin-Darby canine kidney (MDCK) monolayer assay. Samples were analyzed via LC-MS or LC-MS-MS.
Results
C-24 conjugation improved the inhibitory potency of all native bile acids. There was an inverse association between number of steroidal hydroxyl groups and inhibitory potency. Glycolithocholate and taurolithocholate were the most potent inhibitors. Results from transport studies followed trends from inhibition studies. Conjugated dihydroxy and monohydroxy bile acids exhibited the highest hASBT-mediated transport (i.e. lower Kt and higher Jmax). Across the 15 bile acids, Kt generally followed Ki. Additionally, Jmax correlated with Ki, where greater inhibition potency was associated with higher transport capacity.
Conclusion
C-24 conjugation and steroidal hydroxylation pattern modulated native bile acid interaction with hASBT, with C-24 effect dominating steroidal hydroxylation effect. Results indicate that bile acid binding to hASBT may be the rate limiting step in the apical transport of bile acids.
doi:10.1007/s11095-006-0219-4
PMCID: PMC2882938  PMID: 16783481
20.  A mitochondrially targeted compound delays aging in yeast through a mechanism linking mitochondrial membrane lipid metabolism to mitochondrial redox biology☆ 
Redox Biology  2014;2:305-307.
A recent study revealed a mechanism of delaying aging in yeast by a natural compound which specifically impacts mitochondrial redox processes. In this mechanism, exogenously added lithocholic bile acid enters yeast cells, accumulates mainly in the inner mitochondrial membrane, and elicits an age-related remodeling of phospholipid synthesis and movement within both mitochondrial membranes. Such remodeling of mitochondrial phospholipid dynamics progresses with the chronological age of a yeast cell and ultimately causes significant changes in mitochondrial membrane lipidome. These changes in the composition of membrane phospholipids alter mitochondrial abundance and morphology, thereby triggering changes in the age-related chronology of such longevity-defining redox processes as mitochondrial respiration, the maintenance of mitochondrial membrane potential, the preservation of cellular homeostasis of mitochondrially produced reactive oxygen species, and the coupling of electron transport to ATP synthesis.
Graphical abstract
Highlights
•Lithocholic bile acid (LCA) delays aging in yeast.•LCA enters yeast cells and accumulates mainly in the inner mitochondrial membrane.•LCA elicits major changes in the composition of mitochondrial membrane lipids.•These changes in membrane lipidome alter mitochondrial redox processes.•These alterations in mitochondrial redox biology delay cellular aging.
doi:10.1016/j.redox.2014.01.011
PMCID: PMC3926115  PMID: 24563847
CL, cardiolipin; IMM, inner mitochondrial membrane; LCA, lithocholic acid; MLCL, monolysocardiolipin; OMM, outer mitochondrial membrane; PA, phosphatidic acid; PC, phosphatidylcholine; PG, phosphatidylglycerol; PE, phosphatidylethanolamine; PS, phosphatidylserine; ROS, reactive oxygen species; Aging; Anti-aging natural compounds; Mitochondrial lipids; Mitochondrial redox biology; Mitochondrial respiration; Mitochondrial reactive oxygen species
21.  Fellutamide B is a potent inhibitor of the Mycobacterium tuberculosis proteasome 
Via high-throughput screening of a natural compound library, we have identified a lipopeptide aldehyde, fellutamide B (1), as the most potent inhibitor of the Mycobacterium tuberculosis (Mtb) proteasome tested to date. Kinetic studies reveal that 1 inhibits both Mtb and human proteasomes in a time-dependent manner under steady-state condition. Remarkably, 1 inhibits the Mtb proteasome in a single-step binding mechanism with Ki = 6.8 nM, whereas it inhibits the human proteasome β5 active site following a two-step mechanism with Ki = 11.5 nM and Ki* = 0.93 nM. Co-crystallization of 1 bound to the Mtb proteasome revealed a structural basis for the tight binding of 1 to the active sites of the Mtb proteasome. The hemiacetal group of 1 in the Mtb proteasome takes the (R) - configuration, whereas in the yeast proteasome it takes the (S) - configuration, indicating that the pre-chiral CHO group of 1 binds to the active site Thr1 in a different orientation. Re-examination of the structure of the yeast proteasome in complex with 1 showed significant conformational changes at the substrate-binding cleft along the active site. These structural differences are consistent with the different kinetic mechanisms of 1 against Mtb and human proteasomes.
doi:10.1016/j.abb.2010.06.009
PMCID: PMC2930046  PMID: 20558127
Mycobacterium tuberculosis; proteasome; slow-binding inhibition; peptide aldehyde; fellutamide B; enzyme conformational change
22.  Bile acids induce apoptosis selectively in androgen-dependent and -independent prostate cancer cells 
PeerJ  2013;1:e122.
Prostate cancer is a prevalent age-related disease in North America, accounting for about 15% of all diagnosed cancers. We have previously identified lithocholic acid (LCA) as a potential chemotherapeutic compound that selectively kills neuroblastoma cells while sparing normal human neurons. Now, we report that LCA inhibits the proliferation of androgen-dependent (AD) LNCaP prostate cancer cells and that LCA is the most potent bile acid with respect to inducing apoptosis in LNCaP as well as androgen-independent (AI) PC-3 cells, without killing RWPE-1 immortalized normal prostate epithelial cells. In LNCaP and PC-3 cells, LCA triggered the extrinsic pathway of apoptosis and cell death induced by LCA was partially dependent on the activation of caspase-8 and -3. Moreover, LCA increased cleavage of Bid and Bax, down-regulation of Bcl-2, permeabilization of the mitochondrial outer membrane and activation of caspase-9. The cytotoxic actions of LCA occurred despite the inability of this bile acid to enter the prostate cancer cells with about 98% of the nominal test concentrations present in the extracellular culture medium. With our findings, we provide evidence to support a mechanism of action underlying the broad anticancer activity of LCA in various human tissues.
doi:10.7717/peerj.122
PMCID: PMC3740138  PMID: 23940835
Bile acids; Lithocholic acid; Apoptosis; Prostate cancer; Chemotherapy; In vitro; Cell death; Cytotoxicity; Mitochondrial membrane permeability; Extracellular
23.  Mitochondrial membrane lipidome defines yeast longevity 
Aging (Albany NY)  2013;5(7):551-574.
Our studies revealed that lithocholic acid (LCA), a bile acid, is a potent anti-aging natural compound that in yeast cultured under longevity-extending caloric restriction (CR) conditions acts in synergy with CR to enable a significant further increase in chronological lifespan. Here, we investigate a mechanism underlying this robust longevity-extending effect of LCA under CR. We found that exogenously added LCA enters yeast cells, is sorted to mitochondria, resides mainly in the inner mitochondrial membrane, and also associates with the outer mitochondrial membrane. LCA elicits an age-related remodeling of glycerophospholipid synthesis and movement within both mitochondrial membranes, thereby causing substantial changes in mitochondrial membrane lipidome and triggering major changes in mitochondrial size, number and morphology. In synergy, these changes in the membrane lipidome and morphology of mitochondria alter the age-related chronology of mitochondrial respiration, membrane potential, ATP synthesis and reactive oxygen species homeostasis. The LCA-driven alterations in the age-related dynamics of these vital mitochondrial processes extend yeast longevity. In sum, our findings suggest a mechanism underlying the ability of LCA to delay chronological aging in yeast by accumulating in both mitochondrial membranes and altering their glycerophospholipid compositions. We concluded that mitochondrial membrane lipidome plays an essential role in defining yeast longevity.
PMCID: PMC3765583  PMID: 23924582
cellular aging; longevity; yeast; caloric restriction; anti-aging compounds; mitochondria; mitochondrial membrane lipids; membrane curvature; mitochondrial abundance and morphology
24.  A novel dithiocarbamate analogue with potentially decreased ALDH inhibition has copper-dependent proteasome-inhibitory and apoptosis-inducing activity in human breast cancer cells 
Cancer letters  2010;300(1):87-95.
Dithiocarbamates are a class of sulfur-based metal-chelating compounds with various applications in medicine. We reported previously that certain members of dithiocarbamates, such as diethyldithiocarbamate, disulfiram (DSF) and pyrrolidine dithiocarbamate (PDTC), were able to bind with tumor cellular copper to inhibit tumor growth through the inhibition of proteasome activity and induction of cancer cell apoptosis. Since the DSF is an irreversible inhibitor of aldehyde dehydrogenase (ALDH), its ALDH-inhibitory activity might potentially affect its usefulness as an anti-cancer drug. For the purpose of selecting potent anti-cancer compounds that are not ALDH inhibitors and mapping out preliminary structure–activity relationship trends for these novel compounds, we synthesized a series of PDTC analogues and chose three novel compounds to study their ALDH-inhibitory activity, proteasome-inhibitory activity as well as the cancer cell apoptosis-inducing activity. The results showed that compared to DSF, compound 9 has less ALDH inhibition activity, and the in vitro results also proved the positive effects of 9-Cu in proteasome inhibition and apoptosis induction in breast cancer cells, suggesting that 9 as a lead compound could be developed into a novel proteasome inhibitor anti-cancer drug.
doi:10.1016/j.canlet.2010.09.010
PMCID: PMC3671753  PMID: 21035945
ALDH; Dithiocarbamates; Cancer; Proteasome inhibitor; Disulfiram
25.  Optimum conditions for ursodeoxycholic acid production from lithocholic acid by Fusarium equiseti M41. 
Ursodeoxycholic acid dissolves cholesterol gallstones in humans. In the present study optimum conditions for ursodeoxycholic acid production by Fusarium equiseti M41 were studied. Resting mycelia of F. equiseti M41 showed maximum conversion at 28 degrees C, pH 8.0, and dissolved oxygen tension of higher than 60% saturation. Monovalent cations, such as Na+, K+, and Rb+, stimulated the conversion rate more than twofold. In the presence of 0.5 M KCl, the initial uptake rate and equilibrium concentration of lithocholic acid (substrate) were enhanced by 5.7- and 1.7-fold, respectively. We confirmed that enzyme activity catalyzing 7 beta-hydroxylation of lithocholic acid was induced by substrate lithocholic acid. The activity in the mycelium was controlled by dissolved oxygen tension during cultivation: with a dissolved oxygen tension of 15% and over, the activity peak appeared at 25 h of cultivation, whereas the peak was delayed to 34 and 50 h with 5 and 0% dissolved oxygen tension, respectively. After reaching the maximum, the 7 beta-hydroxylation activity in the mycelium declined rapidly at pH 7.0, but the decline was retarded by increasing the pH to 8.0. Several combinations of operations, such as pH shift (from pH 7 to 8), addition of 0.5 M KCl, and dissolved oxygen control, were applied to the production of ursodeoxycholic acid in a jar fermentor, and a much larger amount of ursodeoxycholic acid (1.2 g/liter) was produced within 96 h of cultivation.
PMCID: PMC238404  PMID: 3985610

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