LX-2 Stellate Cells Are a Model System for Investigating the Regulation of Hepatic Vitamin A Metabolism and Respond to Tumor Necrosis Factor and Interleukin 1.

Hepatic stellate cells (HSCs) are the major site of vitamin A (retinol) esterification and subsequent storage as retinyl esters within lipid droplets. However, retinyl esters become depleted in many pathophysiological states, including acute and chronic liver injuries. Recently, using a liver slice culture system as a model of acute liver injury and fibrogenesis, a time-dependent increase and decrease in the apparent formation of the bioactive retinoid all--retinoic acid (RA) and retinyl palmitate was measured, respectively.

Effect of isotretinoin on CYP2D6 and CYP3A activity in patients with severe acne.

Aims: Previously, retinoids have decreased CYP2D6 mRNA expression in vitro and induced CYP3A4 in vitro and in vivo. This study aimed to determine whether isotretinoin administration changes CYP2D6 and CYP3A activities in patients with severe acne.

Methods: Thirty-three patients (22 females and 11 males, 23.

CYP2D6 Activity Is Correlated with Changes in Plasma Concentrations of Taurocholic Acid during Pregnancy and Postpartum in CYP2D6 Extensive Metabolizers.

Cytochrome P450 2D6 (CYP2D6) is involved in the metabolism of >20% of marketed drugs. CYP2D6 expression and activity exhibit high interindividual variability and is induced during pregnancy. The farnesoid X receptor (FXR) is a transcriptional regulator of CYP2D6 that is activated by bile acids.

Impact of Pregnancy and Vitamin A Supplementation on CYP2D6 Activity.

The mechanism of cytochrome P450 2D6 (CYP2D6) induction during pregnancy has not been evaluated in humans. This study assessed the changes in CYP2D6 and CYP3A activities during pregnancy and postpartum, and the effect of vitamin A administration on CYP2D6 activity. Forty-seven pregnant CYP2D6 extensive metabolizers (with CYP2D6 activity scores of 1 to 2) received dextromethorphan (DM) 30 mg orally as a single dose during 3 study windows (at 25 to 28 weeks of gestation, study day 1; at 28 to 32 weeks of gestation, study day 2; and at ≥3 months postpartum, study day 3).

Gut commensals expand vitamin A metabolic capacity of the mammalian host.

Conversion of dietary vitamin A (VA) into retinoic acid (RA) is essential for many biological processes and thus far studied largely in mammalian cells. Using targeted metabolomics, we found that commensal bacteria in the mouse gut lumen produced a high concentration of the active retinoids, all-trans-retinoic acid (atRA) and 13-cis-retinoic acid (13cisRA), as well as the principal circulating retinoid, retinol. Ablation of anerobic bacteria significantly reduced retinol, atRA, and 13cisRA, whereas introducing these bacteria into germ-free mice significantly enhanced retinoids.

Plasma Retinoid Concentrations Are Altered in Pregnant Women.

Vitamin A is vital to maternal-fetal health and pregnancy outcomes. However, little is known about pregnancy associated changes in maternal vitamin A homeostasis and concentrations of circulating retinol metabolites. The goal of this study was to characterize retinoid concentrations in healthy women ( = 23) during two stages of pregnancy (25-28 weeks gestation and 28-32 weeks gestation) as compared to ≥3 months postpartum.

Evidence of depot-specific regulation of all-trans-retinoic acid biosynthesis in human adipose tissue.

The prevalence of obesity continues to rise, underscoring the need to better understand the pathways mediating adipose tissue (AT) expansion. All-trans-retinoic acid (atRA), a bioactive vitamin A metabolite, regulates adipogenesis and energy metabolism, and, in rodent studies, aberrant vitamin A metabolism appears a key facet of metabolic dysregulation. The relevance of these findings to human disease is unknown, as are the specific enzymes implicated in vitamin A metabolism within human AT.

Objective Identification of Cannabis Use Levels in Clinical Populations Is Critical for Detecting Pharmacological Outcomes.

Cannabis is widely used for recreational and medical purposes, but its therapeutic efficacy remains unresolved for many applications as data from retrospective studies show dramatic discrepancy. We hypothesized that false self-reporting of cannabis use and lack of differentiation of heavy users from light or occasional users contribute to the conflicting outcomes. The goal of this study was to develop an objective biomarker of cannabis use and test how application of such biomarker impacts clinical study outcomes and dose-response measures.

Pregnancy Has No Clinically Significant Effect on the Pharmacokinetics of Bupropion or Its Metabolites.

Background: Bupropion (BUP) is a chiral antidepressant and smoking cessation aide with benefits and side effects correlated with parent and active metabolite concentrations. BUP is metabolized by CYP2B6, CYP2C19, and CYP3A4 to hydroxy-BUP (OH-BUP) as well as by 11β-hydroxysteroid dehydrogenase-1 and aldo-keto reductases to threohydrobupropion (Threo) and erythrohydrobupropion (Erythro), respectively. As pregnancy alters the activity of drug-metabolizing enzymes, the authors hypothesized that BUP metabolism and BUP metabolite concentrations would be altered during pregnancy, potentially affecting the efficacy and safety of BUP in pregnant women.

Altered vitamin A metabolism in human liver slices corresponds to fibrogenesis.

All-trans-retinoic acid (atRA), the active metabolite of vitamin A, has antifibrogenic properties in vitro and in animal models. Liver vitamin A homeostasis is maintained by cell-specific enzymatic activities including storage in hepatic stellate cells (HSCs), secretion into circulation from hepatocytes, and formation and clearance of atRA. During chronic liver injury, HSC activation is associated with a decrease in liver retinyl esters and retinol concentrations.

S-acylation status of bile acid transporter hASBT regulates its function, metabolic stability, membrane expression, and phosphorylation state.

The human apical sodium-dependent bile acid transporter (hASBT, SLC10A2) is the rate-limiting step of intestinal bile acid absorption in the enterohepatic circulation system of bile acids. Therefore, the regulation and stability of hASBT is vital in maintaining bile acid and cholesterol homeostasis and may serve as a potential target for cholesterol-related disorders. We hypothesized that post-translational mechanisms that govern hASBT function and regulation will provide novel insight on intestinal bile acid transport and homeostasis.

Analysis of vitamin A and retinoids in biological matrices.

Vitamin A signaling pathways are predominantly driven by the cellular concentrations of all-trans-retinoic acid (atRA), as the main mechanism of retinoid signaling is via activation of retinoic acid receptors. atRA concentrations are in turn controlled by the storage of vitamin A and enzymatic processes that synthesize and clear atRA. This has resulted in the need for robust and highly specific analytical methods to accurately quantify retinoids in diverse biological matrices.

Mechanistic PBPK Modeling of Urine pH Effect on Renal and Systemic Disposition of Methamphetamine and Amphetamine.

The effect of urine pH on renal excretion and systemic disposition has been observed for many drugs and metabolites. When urine pH is altered, tubular ionization, passive reabsorption, renal clearance, and systemic exposure of drugs and metabolites may all change dramatically, raising clinically significant concerns. Surprisingly, the urine pH effect on drug disposition is not routinely explored in humans, and regulatory agencies have neither developed guidance on this issue nor required industry to conduct pertinent human trials.

Tyrosine Phosphorylation Regulates Plasma Membrane Expression and Stability of the Human Bile Acid Transporter ASBT (102).

The human apical sodium-dependent bile acid transporter (hASBT; 102) is responsible for the reclamation of bile acids from the intestinal lumen, providing a primary mechanism for bile acid and cholesterol homeostasis. However, the regulation of hASBT at the post-translational level is not well understood. In the present study, we investigated the role of Src family kinases (SFKs) and protein tyrosine phosphatases (PTPs) in the regulation of surface expression and function of hASBT.

The retinoic acid hydroxylase Cyp26a1 has minor effects on postnatal vitamin A homeostasis, but is required for exogenous RA clearance.

The all--retinoic acid (RA) hydroxylase Cyp26a1 is essential for embryonic development and may play a key role in regulating RA clearance also in adults. We hypothesized that loss of Cyp26a1 activity via inducible knockout in juvenile or adult mice would result in decreased RA clearance and increased tissue RA concentrations and RA-related adverse effects. To test these hypotheses, Cyp26a1 was knocked out in juvenile and adult male and female floxed mice using standard Cre-Lox technology and tamoxifen injections.

Post-translational modifications of transporters.

Drug transporter proteins are critical to the distribution of a wide range of endogenous compounds and xenobiotics such as hormones, bile acids, peptides, lipids, sugars, and drugs. There are two classes of drug transporters- the solute carrier (SLC) transporters and ATP-binding cassette (ABC) transporters -which predominantly differ in the energy source utilized to transport substrates across a membrane barrier. Despite their hydrophobic nature and residence in the membrane bilayer, drug transporters have dynamic structures and adopt many conformations during the translocation process.

Human bile acid transporter ASBT (SLC10A2) forms functional non-covalent homodimers and higher order oligomers.

The human apical sodium-dependent bile acid transporter, hASBT/SLC10A2, plays a central role in cholesterol homeostasis via the efficient reabsorption of bile acids from the distal ileum. hASBT has been shown to self-associate in higher order complexes, but while the functional role of endogenous cysteines has been reported, their implication in the oligomerization of hASBT remains unresolved. Here, we determined the self-association architecture of hASBT by site-directed mutagenesis combined with biochemical, immunological and functional approaches.