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. Whereas there is significant literature evidence for the substrate specificity and structure-function relationship for clinically relevant drug transporters proteins, there is less of an understanding in the regulatory mechanisms that contribute to the functional expression of these proteins. Post-translational modifications have been shown to modulate drug transporter functional expression via a wide range of molecular mechanisms. These modifications commonly occur through the addition of a functional group (e.g. phosphorylation), a small protein (e.g. ubiquitination), sugar chains (e.g. glycosylation), or lipids (e.g. palmitoylation) on solvent accessible amino acid residues. These covalent additions often occur as a result of a signaling cascade and may be reversible depending on the type of modification and the intended fate of the signaling event. Here, we review the significant role in which post-translational modifications contribute to the dynamic regulation and functional consequences of SLC and ABC drug transporters and highlight recent progress in understanding their roles in transporter structure, function, and regulation.

2 The abbreviations used are: atRA, all-trans-retinoic acid; CFU, colony-forming unit; RE, retinyl ester; ROL, retinol; ROL-d 8 , all-trans-retinol-d 8 ; RP-d 4 , all-trans-retinyl palmitate-d 4 ; ANOVA, analysis of variance; BM, bone marrow; MRM, multiple reaction monitoring; CRABP, cellular retinoic acidbinding protein; AUC, area under the curve; RP-d 4 , all-trans-retinyl palmitate-d 4 ; IS, internal standard; ACN, acetonitrile; CL, clearance; dpp, day post-partum; Lrat, lecithin:retinol acetyltransferase.

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AUC, area under the plasma drug concentration versus time curve B/P, blood-to-plasma ratio CL, total plasma clearance CLf, formation clearance of the metabolite CLint, intrinsic clearance for the drug CLint,m, intrinsic clearance for the metabolite CLr, renal clearance CYP2D6, Cytochrome P450 2D6 enzyme DDI, drug-drug interaction fe, fraction elimination of kidney fu,p, unbound fraction in plasma ka, absorption constant Kp, tissue-to-plasma partition coefficient for the drug Kp,m, tissue-to-plasma partition coefficient for the metabolite Meth, S(+)-methamphetamine MDCK, Madin-Darby canine kidney PBPK, physiologically-based pharmacokinetics PET, positron emission tomography PK, pharmacokinetics Vss, volume of distribution at steady state This article has not been copyedited and formatted. The final version may differ from this version.

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. atRA is synthesized through two enzymatic steps from retinol, but it is unknown if the loss of retinoid stores is associated with changes in atRA formation and which cell types contribute to the metabolic changes. The aim of this study was to determine if the vitamin A metabolic flux is perturbed in acute liver injury, and if changes in atRA concentrations are associated with HSC activation and collagen expression. At basal levels, HSC and Kupffer cells expressed key genes involved in vitamin A metabolism, whereas after acute liver injury, complex changes to the metabolic flux were observed in liver slices. These changes include a reproducible spike in atRA tissue concentrations, decreased retinyl ester and atRA formation rate, and time‐dependent changes to the expression of metabolizing enzymes. Kinetic simulations suggested that oxidoreductases are important in determining retinoid metabolic flux after liver injury. These early changes precede HSC activation and upregulation of profibrogenic gene expression, which were inversely correlated with atRA tissue concentrations, suggesting that HSC and Kupffer cells are key cells involved in changes to vitamin A metabolic flux and signaling after liver injury. WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC? Vitamin A is metabolized in the liver for storage as retinyl esters in hepatic stellate cell (HSCs) or to all‐trans‐retinoic acid (atRA), an active metabolite with antifibrogenic properties. Following chronic liver injury, vitamin A metabolic flux is perturbed, and HSC activation leads to diminished retinoid stores. WHAT QUESTION DID THIS STUDY ADDRESS? Do changes in the expression of vitamin A metabolizing enzymes explain changes in atRA concentrations and the regulation of fibrosis following acute liver injury? WHAT DOES THIS STUDY ADD TO OUR KNOWLEDGE? In healthy liver, both HSC and Kupffer cells may mediate vitamin A homeostasis. Following acute liver injury, complex changes in metabolizing enzyme expression/activity alter the metabolic flux of retinoids, resulting in a transient peak in atRA concentrations. The atRA concentrations are inversely correlated with profibrogenic gene expression, HSC activation, and collagen deposition. HOW MIGHT THIS CHANGE CLINICAL PHARMACOLOGY OR TRANSLATIONAL SCIENCE? Improved understanding of altered vitamin A metabolic flux in acute liver injury may provide insight into cell‐specific contributions to vitamin A loss and lead to novel interventions in liver fibrosis.