The essential role of the transporter ABCG2 in the pathophysiology of erythropoietic protoporphyria

Erythropoietic protoporphyrias: updates and advances

Protoporphyrias are caused by pathogenic variants in genes encoding enzymes involved in heme biosynthesis. They induce the accumulation of a hydrophobic phototoxic compound, protoporphyrin (PPIX), in red blood cells (RBCs). PPIX is responsible for painful cutaneous photosensitivity, which severely impairs quality of life. Hepatic elimination of PPIX increases the risk of cholestatic liver disease, requiring lifelong monitoring. Treatment options are scarce and mainly limited to supportive care such as protection from visible light. Here, we review the pathophysiology of protoporphyrias, their diagnosis, and current recommendations for medical care. We discuss new therapeutic strategies, some of which are currently undergoing clinical trials and are likely to radically alter the severity of the disease in the years to come.

Exploring current and emerging therapies for porphyrias

Porphyrias are rare, mostly inherited disorders resulting from altered activity of specific enzymes in the haem synthesis pathway that lead to accumulation of pathway intermediates. Photocutaneous symptoms occur when excess amounts of photoreactive porphyrins circulate in the blood to the skin, whereas increases in potentially neurotoxic porphyrin precursors are associated with neurovisceral symptoms. Current therapies are suboptimal and their mechanisms are not well established. As described here, emerging therapies address underlying disease mechanisms by introducing a gene, RNA or other specific molecule with the potential to cure or slow progression of the disease. Recent progress in nanotechnology and nanoscience, particularly regarding particle design and formulation, is expanding disease targets. More secure and efficient drug delivery systems have extended our toolbox for transferring specific molecules, especially into hepatocytes, and led to proof-of-concept studies in animal models. Repurposing existing drugs as molecular chaperones or haem synthesis inhibitors is also promising. This review summarizes key examples of these emerging therapeutic approaches and their application for hepatic and erythropoietic porphyrias.

Update on the Porphyrias

The porphyrias are a group of rare diseases, each resulting from a defect in a different enzymatic step of the heme biosynthetic pathway. They can be broadly divided into two categories, hepatic and erythropoietic porphyrias, depending on the primary site of accumulation of heme intermediates. These disorders are multisystemic with variable symptoms that can be encountered by physicians in any specialty. Here, we review the porphyrias and describe their clinical presentation, diagnosis, and management. We discuss novel therapies that are approved or in development. Early diagnosis is key for the appropriate management and prevention of long-term complications in these rare disorders.

Metabolism-guided development of Ko143 analogs as ABCG2 inhibitors

ATP-binding cassette subfamily G member 2 (ABCG2), an efflux transporter, is involved in multiple pathological processes. Ko143 is a potent ABCG2 inhibitor; however, it is quickly metabolized through carboxylesterase 1-mediated hydrolysis of its t-butyl ester moiety. The current work aimed to develop more metabolically stable ABCG2 inhibitors. Novel Ko143 analogs were designed and synthesized by replacing the unstable t-butyl ester moiety in Ko143 with an amide group. The synthesized Ko143 analogs were evaluated for their ABCG2 inhibitory activity, binding mode with ABCG2, cytotoxicity, and metabolic stability. We found that the amide modification of Ko143 led to metabolically stable ABCG2 inhibitors. Among these Ko143 analogs, K2 and K34 are promising candidates with favorable oral pharmacokinetic profiles in mice. In summary, we synthesized novel Ko143 analogs with improved metabolic stability, which can potentially be used as lead compounds for the future development of ABCG2 inhibitors.

Cannabidiol and Cannabigerol Modify the Composition and Physicochemical Properties of Keratinocyte Membranes Exposed to UVA

The action of UVA radiation (both that derived from solar radiation and that used in the treatment of skin diseases) modifies the function and composition of keratinocyte membranes. Therefore, this study aimed to assess the effects of phytocannabinoids (CBD and CBG), used singly and in combination, on the contents of phospholipids, ceramides, lipid rafts and sialic acid in keratinocyte membranes exposed to UVA radiation, together with their structure and functionality. The phytocannabinoids, especially in combination (CBD+CBG), partially prevented increased levels of phosphatidylinositols and sialic acid from occurring and sphingomyelinase activity after the UVA exposure of keratinocytes. This was accompanied by a reduction in the formation of lipid rafts and malondialdehyde, which correlated with the parameters responsible for the integrity and functionality of the keratinocyte membrane (membrane fluidity and permeability and the activity of transmembrane transporters), compared to UVA-irradiated cells. This suggests that the simultaneous use of two phytocannabinoids may have a protective effect on healthy cells, without significantly reducing the therapeutic effect of UV radiation used to treat skin diseases such as psoriasis.

Protoporphyrin IX-induced phototoxicity: Mechanisms and therapeutics

Protoporphyrin IX (PPIX) is an intermediate in the heme biosynthesis pathway. Abnormal accumulation of PPIX due to certain pathological conditions such as erythropoietic protoporphyria and X-linked protoporphyria causes painful phototoxic reactions of the skin, which can significantly impact daily life. Endothelial cells in the skin have been proposed as the primary target for PPIX-induced phototoxicity through light-triggered generation of reactive oxygen species. Current approaches for the management of PPIX-induced phototoxicity include opaque clothing, sunscreens, phototherapy, blood therapy, antioxidants, bone marrow transplantation, and drugs that increase skin pigmentation. In this review, we discuss the present understanding of PPIX-induced phototoxicity including PPIX production and disposition, conditions that lead to PPIX accumulation, symptoms and individual differences, mechanisms, and therapeutics.

How I treat erythropoietic protoporphyria and X-linked protoporphyria

Erythropoietic protoporphyria (EPP) is an inherited cutaneous porphyria caused by reduced expression of ferrochelatase, the enzyme that catalyzes the final step in heme biosynthesis. The resultant accumulation of protoporphyrin IX leads to severe, painful cutaneous photosensitivity, as well as potentially life-threatening liver disease in a small percentage of patients. X-linked protoporphyria (XLP) is clinically similar to EPP but results from increased activity of δ-aminolevulinic acid synthase 2, the first step in heme biosynthesis in the bone marrow, and also causes protoporphyrin accumulation. Although historically the management of EPP and XLP (collectively termed protoporphyria) centered around avoidance of sunlight, novel therapies have recently been approved or are in development, which will alter the therapeutic landscape for these conditions. We present 3 patient cases, highlighting key treatment considerations in patients with protoporphyria, including (1) approach to photosensitivity, (2) managing iron deficiency in protoporphyria, and (3) understanding hepatic failure in protoporphyria.

The Role of ABC Transporters in Skin Cells Exposed to UV Radiation

ABC transporters are expressed in skin cells to protect them against harmful xenobiotics. Moreover, these transmembrane proteins have a number of additional functions that ensure skin homeostasis. This review summarizes the current knowledge about the role of specific ABC proteins in the skin, including multi-drug resistance transporters (MDR1/3), the transporter associated with antigen processing 1/2 (TAP1/2), the cystic fibrosis transmembrane conductance regulator (CFTR), sulfonylurea receptors (SUR1/2), and the breast cancer resistance protein (BCRP). Additionally, the effect of UV radiation on ABC transporters is shown. The exposure of skin cells to UV radiation often leads to increased activity of ABC transporters-as has been observed in the case of MDRs, TAPs, CFTR, and BCRP. A different effect of oxidative stress has been observed in the case of mitochondrial SURs. However, the limited data in the literature-as indicated in this article-highlights the limited number of experimental studies dealing with the role of ABC transporters in the physiology and pathophysiology of skin cells and the skin as a whole. At the same time, the importance of such knowledge in relation to the possibility of daily exposure to UV radiation and xenobiotics, used for both skin care and the treatment of its diseases, is emphasized.

Synthetic biology-instructed transdermal microneedle patch for traceable photodynamic therapy

5-Aminolevulinic acid-based photodynamic therapy heavily depends on the biological transformation efficiency of 5-aminolevulinic acid to protoporphyrin IX, while the lack of an effective delivery system and imaging navigation are major hurdles in improving the accumulation of protoporphyrin IX and optimizing therapeutic parameters. Herein, we leverage a synthetic biology approach to construct a transdermal theranostic microneedle patch integrated with 5-aminolevulinic acid and catalase co-loaded tumor acidity-responsive copper-doped calcium phosphate nanoparticles for efficient 5-aminolevulinic acid-based photodynamic therapy by maximizing the enrichment of intratumoral protoporphyrin IX. We show that continuous oxygen generation by catalase in vivo reverses tumor hypoxia, enhances protoporphyrin IX accumulation by blocking protoporphyrin IX efflux (downregulating hypoxia-inducible factor-1α and ferrochelatase) and upregulates protoporphyrin IX biosynthesis (providing exogenous 5-aminolevulinic acid and upregulating ALA-synthetase). In vivo fluorescence/photoacoustic duplex imaging can monitor intratumoral oxygen saturation and protoporphyrin IX metabolic kinetics simultaneously. This approach thus facilitates the optimization of therapeutic parameters for different cancers to realize Ca²⁺/Cu²⁺-interferences-enhanced repeatable photodynamic therapy, making this theranostic patch promising for clinical practice.

Bi-allelic hydroxymethylbilane synthase inactivation defines a homogenous clinico-molecular subtype of hepatocellular carcinoma

BACKGROUND & AIMS

Acute intermittent porphyria (AIP), caused by heterozygous germline mutations of the heme synthesis pathway enzyme HMBS (hydroxymethylbilane synthase), confers a high risk of hepatocellular carcinoma (HCC) development. Yet, the role of HMBS in liver tumorigenesis remains unclear.

METHODS

Herein, we explore HMBS alterations in a large series of 758 HCC cases, including 4 patients with AIP. We quantify the impact of HMBS mutations on heme biosynthesis pathway intermediates and we investigate the molecular and clinical features of HMBS-mutated tumors.

RESULTS

We identify recurrent bi-allelic HMBS inactivation, both in patients with AIP acquiring a second somatic HMBS mutation and in sporadic HCC with 2 somatic hits. HMBS alterations are enriched in truncating mutations, in particular in splice regions, leading to abnormal transcript structures. Bi-allelic HMBS inactivation results in a massive accumulation of its toxic substrate porphobilinogen and synergizes with CTNNB1-activating mutations, leading to the development of well-differentiated tumors with a transcriptomic signature of Wnt/β-catenin pathway activation and a DNA methylation signature related to ageing. HMBS-inactivated HCC mostly affects females, in the absence of fibrosis and classical HCC risk factors.

CONCLUSIONS

These data identify HMBS as a tumor suppressor gene whose bi-allelic inactivation defines a homogenous clinical and molecular HCC subtype.

LAY SUMMARY

Heme (the precursor to hemoglobin, which plays a key role in oxygen transport around the body) synthesis occurs in the liver and involves several enzymes including hydroxymethylbilane synthase (HMBS). HMBS mutations cause acute intermittent porphyria, a disease caused by the accumulation of toxic porphyrin precursors. Herein, we show that HMBS inactivation is also involved in the development of liver cancers with distinct clinical and molecular characteristics.

An Intracellular Self-Assembly-Driven Uninterrupted ROS Generator Augments 5-Aminolevulinic-Acid-Based Tumor Therapy

Free radical therapy based on 5-aminolevulinic acid (ALA, a precursor of the photosensitizer protoporphyrin IX (PpIX)) has been approved by the US Food and Drug Administration for clinical tumor treatment. However, PpIX can be quickly converted into photoinactive heme, leading to unexpectedly paused production of free radicals and severely hindering its therapeutic benefits. Here, inspired by the natural biotransformation of ALA (ALA-PpIX-heme), an uninterrupted reactive oxygen species generator (URG) that converts useless heme to peroxidase mimics via intracellular self-assembly is developed. The URG is prepared by enwrapping ALA-loaded polyamide-amine dendrimers in red blood cell membrane vesicles with a further surface modification of G-quadruplex-structured AS1411. The URGs realize "¹ O₂ -•OH" uninterrupted generation through "recycling waste" in two steps: i) PpIX generates ¹ O₂ under laser irradiation; and ii) the photoinactive metabolite heme self-assembled with AS1411 to catalyze H₂ O₂ conversion into •OH. Interestingly, the specific generation of ¹ O₂ in mitochondria and •OH in nuclei further augments the free-radical-induced damage. It is demonstrated that URG can continuously produce free radicals for 6 h postirradiation, and shows 3.3-times more than that of the nonassembly group, achieving nearly 80% regression of tumors in vivo.

Role of phlebotomy in the treatment of liver damage related to erythropoietic porphyria

Liver damage affects the prognosis of patients with erythropoietic protoporphyria (EPP). However, there is no radical cure for EPP patients with severe liver damage. This study aims to investigate the effectiveness of phlebotomy in patients with severe liver damage. We examined seven patients diagnosed with EPP and liver damage between 2010 and 2020. Of the 7 cases, phlebotomy was performed in 3 cases with severe hepatic disorder, and the improvement effect of hepatic disorder was observed in all cases. In addition, as an additional study, we also investigated the mechanism by which liver damage becomes more severe. Liver biopsy samples were stained with hematoxylin and eosin and immunohistochemistry was used to examine the expression of adenosine triphosphate-binding transporter G2 (ABCG2). Liver biopsies were performed in 3 of 7 patients with EPP. Of these three patients, ABCG2 expression was low in two patients, especially in the protoporphyrin (PP) deposition area. Two patients with reduced ABCG2 expression subsequently developed severe liver damage. However, the causal relationship between the decreased expression of ABCG2 and the exacerbation of liver damage has not been directly proved, and further investigation is required in the future. This study demonstrated the effectiveness of phlebotomy in EPP patients with severe liver damage.

Liver involvement in patients with erythropoietic protoporphyria

BACKGROUND

In erythropoietic protoporphyria (EPP), which presents with severe painful phototoxicity, progressive deposition of protoporphyrins in hepatocytes and bile canaliculi may result in liver disease. Clinically EPP related liver disease ranges from mildly elevated liver enzymes to cirrhosis and acute cholestatic hepatic failure. The prevalence of liver disease in EPP, and factors predicting the risk of developing liver disease, have not been defined in a large series of unselected EPP patients.

AIM

To determine the prevalence of liver disease in EPP-patients.

METHODS

A single-center prospective unselected cohort study of 114 adult EPP patients, who underwent routine laboratory testing, abdominal ultrasonography and transient elastography to assess the presence of steatosis (controlled attenuation parameter,dB/m) and liver stiffness (kPa).

RESULTS

114 adult EPP patients were included. Elevated liver enzymes were found in 6.2% of the patients. Liver steatosis was detected in 29.0%, and significant fibrosis as assessed with liver stiffness measurements was present in 9.6% of patients. BMI positively predicted CAP-values (p = 0.026); and protoporphyrin IX levels (p = 0.043) positively predicted liver stiffness.

CONCLUSIONS

This study demonstrates a prevalence of hepatic steatosis and fibrosis in adult EPP-patients comparable to that found in the general population. Protoporphyrin IX levels correlate with increased liver stiffness in EPP.

Recognized and Emerging Features of Erythropoietic and X-Linked Protoporphyria

Erythropoietic protoporphyria (EPP) and X-linked protoporphyria (XLP) are inherited disorders resulting from defects in two different enzymes of the heme biosynthetic pathway, i.e., ferrochelatase (FECH) and delta-aminolevulinic acid synthase-2 (ALAS2), respectively. The ubiquitous FECH catalyzes the insertion of iron into the protoporphyrin ring to generate the final product, heme. After hemoglobinization, FECH can utilize other metals like zinc to bind the remainder of the protoporphyrin molecules, leading to the formation of zinc protoporphyrin. Therefore, FECH deficiency in EPP limits the formation of both heme and zinc protoporphyrin molecules. The erythroid-specific ALAS2 catalyses the synthesis of delta-aminolevulinic acid (ALA), from the union of glycine and succinyl-coenzyme A, in the first step of the pathway in the erythron. In XLP, ALAS2 activity increases, resulting in the amplified formation of ALA, and iron becomes the rate-limiting factor for heme synthesis in the erythroid tissue. Both EPP and XLP lead to the systemic accumulation of protoporphyrin IX (PPIX) in blood, erythrocytes, and tissues causing the major symptom of cutaneous photosensitivity and several other less recognized signs that need to be considered. Although significant advances have been made in our understanding of EPP and XLP in recent years, a complete understanding of the factors governing the variability in clinical expression and the severity (progression) of the disease remains elusive. The present review provides an overview of both well-established facts and the latest findings regarding these rare diseases.

A new porphyrin as selective substrate-based inhibitor of breast cancer resistance protein (BCRP/ABCG2)

The ABCG2 transporter plays a pivotal role in multidrug resistance, however, no clinical trial using specific ABCG2 inhibitors have been successful. Although ABC transporters actively extrude a wide variety of substrates, photodynamic therapeutic agents with porphyrinic scaffolds are exclusively transported by ABCG2. In this work, we describe for the first time a porphyrin derivative (4B) inhibitor of ABCG2 and capable to overcome multidrug resistance in vitro. The inhibition was time-dependent and 4B was not itself transported by ABCG2. Independently of the substrate, the porphyrin 4B showed an IC₅₀ value of 1.6 μM and a mixed type of inhibition. This compound inhibited the ATPase activity and increased the binding of the conformational-sensitive antibody 5D3. A thermostability assay confirmed allosteric protein changes triggered by the porphyrin. Long-timescale molecular dynamics simulations revealed a different behavior between the ABCG2 porphyrinic substrate pheophorbide a and the porphyrin 4B. Pheophorbide a was able to bind in three different protein sites but 4B showed one binding conformation with a strong ionic interaction with GLU446. The inhibition was selective toward ABCG2, since no inhibition was observed for P-glycoprotein and MRP1. Finally, this compound successfully chemosensitized cells that overexpress ABCG2. These findings reinforce that substrates may be a privileged source of chemical scaffolds for identification of new inhibitors of multidrug resistance-linked ABC transporters.

Ultraviolet A phototest positivity is associated with higher free erythrocyte protoporphyrin IX concentration and lower transferrin saturation values in erythropoietic protoporphyria

BACKGROUND

Erythropoietic protoporphyria (EPP) is a rare disorder of heme biosynthesis hallmarked by early-onset photosensitivity and mainly due to defective ferrochelatase activity leading to increased erythrocyte protoporphyrin IX (PPIX) levels. Evidence regarding the relationship between erythrocyte PPIX concentration and photosensitivity is limited.

METHODS

To investigate the relationship between free erythrocyte PPIX (FEP) concentration; routine laboratory tests, particularly iron metabolism biomarkers; and ultraviolet (UV) A/visible light phototesting findings, 20 genetically confirmed EPP and one XLPP treatment-naive patients were included in our study. They underwent UVA and visible light phototesting. On the same day, blood samples were collected for measurement of FEP, serum iron, transferrin, transferrin saturation, and ferritin, 25-hydroxyvitamin D, and liver enzyme levels.

RESULTS

Median FEP concentration at the time of phototesting was 57.50 (IQR: 34.58-102.70) μg/g of Hb. UVA and visible light phototesting were positive in 9 (42.9%) and 8 (38.1%) patients, respectively. Median FEP concentration was significantly higher in UVA phototest-positive patients than in those negative (64.37 [IQR: 57.45-121.82] vs 45.35 [IQR: 24.53-74.61] μg/g of Hb, respectively; P = .04486). Similarly, UVA photosensitive individuals had significantly lower median serum iron levels (61.5 [IQR: 33.5-84] μg/dL vs 109 [IQR: 63.25-154] μg/dL, respectively; P = .01862) and transferrin saturation values (15.005 [IQR: 7.0775-18.41] % vs 29.645 [IQR: 17.8225-34.3575] %; P = .0109) than those negative.

CONCLUSIONS

Our study demonstrates that UVA phototest positivity is associated with higher FEP concentration and lower transferrin saturation and serum iron concentration in EPP.

Biliary excretion of excess iron in mice requires hepatocyte iron import by Slc39a14

Iron is essential for erythropoiesis and other biological processes, but is toxic in excess. Dietary absorption of iron is a highly regulated process and is a major determinant of body iron levels. Iron excretion, however, is considered a passive, unregulated process, and the underlying pathways are unknown. Here we investigated the role of metal transporters SLC39A14 and SLC30A10 in biliary iron excretion. While SLC39A14 imports manganese into the liver and other organs under physiological conditions, it imports iron under conditions of iron excess. SLC30A10 exports manganese from hepatocytes into the bile. We hypothesized that biliary excretion of excess iron would be impaired by SLC39A14 and SLC30A10 deficiency. We therefore analyzed biliary iron excretion in Slc39a14-and Slc30a10-deficient mice raised on iron-sufficient and -rich diets. Bile was collected surgically from the mice, then analyzed with nonheme iron assays, mass spectrometry, ELISAs, and an electrophoretic assay for iron-loaded ferritin. Our results support a model in which biliary excretion of excess iron requires iron import into hepatocytes by SLC39A14, followed by iron export into the bile predominantly as ferritin, with iron export occurring independently of SLC30A10. To our knowledge, this is the first report of a molecular determinant of mammalian iron excretion and can serve as basis for future investigations into mechanisms of iron excretion and relevance to iron homeostasis.

Protective Effect of Resveratrol against Hepatotoxicity of Cadmium in Male Rats: Antioxidant and Histopathological Approaches

Cadmium (Cd) is widely used in some industries and emitted from fossil fuels. It is a heavy metal with a number of side effects, including hepatotoxicity. Resveratrol (Rs) is considered an important polyphenol, which is a secondary plant metabolite and has the ability to scavenge free radicals. The study was designed to evaluate the effects of resveratrol on Cd, which induced hepatotoxicity, by the assessment of some histopathological and biochemical alterations. Forty male albino rats were divided into four groups: the 1st group was the control group, the 2nd group was treated with Cd (5 mg/kg), the 3rd group was given Rs (20 mg/kg), and the 4th group was treated with Cd in combination with Rs intraperitoneally for 30 successive days. The results indicate that Cd increased liver enzymes alanine aminotransferase and aspartate aminotransferase (AST and ALT), alkaline phosphatase ALP and gamma-glutamyl transferase (γ-GT) while reducing the total protein level; Cd increased the malondialdhyde (MDA) level while decreasing the levels of other antioxidant enzymes super oxide dismutase, catalase and glutathione peroxidase (SOD, CAT and GPx). Serious congestion and hemorrhage related to the hepatic tissues were noticed in the Cd group, and Rs plays a major role in alleviating histopathological injuries and hepatic oxidative damage. It is clear that Rs has the ability to minimize the hepatotoxicity induced by Cd in male rats.

Resveratrol promotes liver regeneration in drug-induced liver disease in mice

Studies suggest that the bioactive polyphenolic compound resveratrol (RESV, trans-isomer), found naturally in certain foods such as red grapes and peanuts, may be able to ameliorate liver damage. However, the effects and efficacy of long-term treatment with RESV remain unclear. Here, we used an acetaminophen (APAP; 400 mg/kg/d for 15 days) overdose model to induce liver damage in C56BL/6 mice. Three days after the intoxication was stopped, we observed biochemical, histological and ultrastructural alterations in the livers of these mice. The APAP-treated animals were then given RESV (10 mg/kg/d) for 60 days. Blood and tissue were analyzed at days 7, 30 and 60. Our data show that long-term RESV treatment (60 days) ameliorates the liver injury caused by APAP intoxication, restoring histological features, ultrastructural organization and serum biochemical parameters (albumin, alanine aminotransferase). Ck18- and F4/80-positive cells (indicators of hepatocyte recovery) were reestablished and the number of α-SMA positive cells was normalized after long-term RESV treatment. Additionally, downregulation of the drug transporter BCRP was observed. Electron microscopy revealed that treatment with RESV was effective in restoring the shape and size of hepatic microvilli and normalizing both the number and viability of mitochondria. Taken together, these results indicate that long-term treatment with RESV is effective in alleviating liver injury caused by APAP administration.

Medically Important Alterations in Transport Function and Trafficking of ABCG2

Several polymorphisms and mutations in the human ABCG2 multidrug transporter result in reduced plasma membrane expression and/or diminished transport function. Since ABCG2 plays a pivotal role in uric acid clearance, its malfunction may lead to hyperuricemia and gout. On the other hand, ABCG2 residing in various barrier tissues is involved in the innate defense mechanisms of the body; thus, genetic alterations in ABCG2 may modify the absorption, distribution, excretion of potentially toxic endo- and exogenous substances. In turn, this can lead either to altered therapy responses or to drug-related toxic reactions. This paper reviews the various types of mutations and polymorphisms in ABCG2, as well as the ways how altered cellular processing, trafficking, and transport activity of the protein can contribute to phenotypic manifestations. In addition, the various methods used for the identification of the impairments in ABCG2 variants and the different approaches to correct these defects are overviewed.

Mammalian ABCG-transporters, sterols and lipids: To bind perchance to transport?

Members of the ATP binding cassette (ABC) transporter family perform a critical function in maintaining lipid homeostasis in cells as well as the transport of drugs. In this review, we provide an update on the ABCG-transporter subfamily member proteins, which include the homodimers ABCG1, ABCG2 and ABCG4 as well as the heterodimeric complex formed between ABCG5 and ABCG8. This review focusses on progress made in this field of research with respect to their function in health and disease and the recognised transporter substrates. We also provide an update on post-translational regulation, including by transporter substrates, and well as the involvement of microRNA as regulators of transporter expression and activity. In addition, we describe progress made in identifying structural elements that have been recognised as important for transport activity. We furthermore discuss the role of lipids such as cholesterol on the transport function of ABCG2, traditionally thought of as a drug transporter, and provide a model of potential cholesterol binding sites for ABCG2.

Picky ABCG5/G8 and promiscuous ABCG2 - a tale of fatty diets and drug toxicity

Structural data on ABCG5/G8 and ABCG2 reveal a unique molecular architecture for subfamily G ATP‐binding cassette (ABCG) transporters and disclose putative substrate‐binding sites. ABCG5/G8 and ABCG2 appear to use several unique structural motifs to execute transport, including the triple helical bundles, the membrane‐embedded polar relay, the re‐entry helices, and a hydrophobic valve. Interestingly, ABCG2 shows extreme substrate promiscuity, whereas ABCG5/G8 transports only sterol molecules. ABCG2 structures suggest a large internal cavity, serving as a binding region for substrates and inhibitors, while mutational and pharmacological analyses support the notion of multiple binding sites. By contrast, ABCG5/G8 shows a collapsed cavity of insufficient size to hold substrates. Indeed, mutational analyses indicate a sterol‐binding site at the hydrophobic interface between the transporter and the lipid bilayer. In this review, we highlight key differences and similarities between ABCG2 and ABCG5/G8 structures. We further discuss the relevance of distinct and shared structural features in the context of their physiological functions. Finally, we elaborate on how ABCG2 and ABCG5/G8 could pave the way for studies on other ABCG transporters.

ABCG2 Deficiency Does Not Alter Dolutegravir Metabolism and Pharmacokinetics

Dolutegravir (DTG) is a potent integrase inhibitor of human immunodeficiency virus. Because DTG is a substrate of the efflux transporter ABCG2 and ABCG2 is highly polymorphic, we asked whether dose adjustment of DTG is needed for ABCG2-deficient individuals. Using Abcg2-null mice, the current work investigated the impact of ABCG2 deficiency on DTG metabolism and pharmacokinetics. Compared with wild-type mice, no statistically significant difference was found in the systemic and tissue-specific (liver, kidney, and brain) pharmacokinetics of DTG in Abcg2-null mice. In addition, ABCG2 deficiency had no statistically significant impact on the production and excretion of DTG metabolites. In summary, this study demonstrated that deficiency of ABCG2 does not alter DTG metabolism and pharmacokinetics, suggesting that dose adjustment of DTG is not needed for individuals with ABCG2 deficiency. SIGNIFICANCE STATEMENT: The current work demonstrated that deficiency of ATP-binding cassette subfamily G member 2 (ABCG2) does not alter Dolutegravir (DTG) metabolism and pharmacokinetics, suggesting that dose adjustment of DTG is not needed for individuals with ABCG2 deficiency.