ATP-Binding Cassette Transporter C4 is a Prostaglandin D2 Exporter in HMC-1 cells

Structural basis of prostaglandin efflux by MRP4

Multidrug resistance protein 4 (MRP4) is a broadly expressed ATP-binding cassette transporter that is unique among the MRP subfamily for transporting prostanoids, a group of signaling molecules derived from unsaturated fatty acids. To better understand the basis of the substrate selectivity of MRP4, we used cryogenic-electron microscopy to determine six structures of nanodisc-reconstituted MRP4 at various stages throughout its transport cycle. Substrate-bound structures of MRP4 in complex with PGE1, PGE2 and the sulfonated-sterol DHEA-S reveal a common binding site that accommodates a diverse set of organic anions and suggest an allosteric mechanism for substrate-induced enhancement of MRP4 ATPase activity. Our structure of a catalytically compromised MRP4 mutant bound to ATP-Mg2+ is outward-occluded, a conformation previously unobserved in the MRP subfamily and consistent with an alternating-access transport mechanism. Our study provides insights into the endogenous function of this versatile efflux transporter and establishes a basis for MRP4-targeted drug design.

Inflammation-Induced Attenuation of Prostaglandin D2 Elimination across Rat Blood-Brain Barrier: Involvement of the Downregulation of Organic Anion Transporter 3 and Multidrug Resistance-Associated Protein 4

Prostaglandin (PG) D₂ is a lipid mediator, and in the brain, overproduction of PGD₂ is reportedly involved in the progression and exacerbation of neuroinflammation. The objective of this study was to elucidate PGD₂ efflux transport, under normal and inflammatory conditions, across the blood-brain barrier (BBB), which is formed by brain capillaries. Elimination of [³H]PGD₂ across the BBB of normal and lipopolysaccharide (LPS)-induced inflammatory rats was examined by the intracerebral microinjection technique. After intracerebral injection, the percentage of [³H]PGD₂ remaining in the ipsilateral cerebrum decreased with time, with a half-life of 13 min. This [³H]PGD₂ elimination across the BBB was significantly inhibited by the co-administration of unlabeled PGD₂, which suggests carrier-mediated PGD₂ efflux transport at the BBB. In isolated rat brain capillaries, mRNA expression of organic anion transporter (Oat) 3, organic anion-transporting polypeptide (Oatp) 1a4, and multidrug resistance-associated protein (Mrp) 4 was observed. In addition, co-administration of substrates/inhibitors for Oat3, Oatp1a4, and/or Mrp4, such as benzylpenicillin and cefmetazole, reduced [³H]PGD₂ elimination across the BBB. Data suggest that Oat3 and Mrp4, but not Oatp1a4 are involved in PGD₂ elimination across the BBB, as Oatp1a4-expressing Xenopus (X.) oocytes did not show the significant [³H]PGD₂ uptake compared with water-injected X. oocytes. In LPS-treated rats, [³H]PGD₂ elimination across the BBB and mRNA expression levels of Oat3 and Mrp4 were significantly decreased. Our data suggest that Oat3- and Mrp4-mediated PGD₂ elimination across the BBB is attenuated under inflammatory conditions.

Recent advances in studies of SLCO2A1 as a key regulator of the delivery of prostaglandins to their sites of action

Solute carrier organic anion transporter family member 2A1 (SLCO2A1, also known as PGT, OATP2A1, PHOAR2, or SLC21A2) is a plasma membrane transporter consisting of 12 transmembrane domains. It is ubiquitously expressed in tissues, and mediates the membrane transport of prostaglandins (PGs, mainly PGE₂, PGF_2α, PGD₂) and thromboxanes (e.g., TxB₂). SLCO2A1-mediated transport is electrogenic and is facilitated by an outwardly directed gradient of lactate. PGs imported by SLCO2A1 are rapidly oxidized by cytoplasmic 15-hydroxyprostaglandin dehydrogenase (15-PGDH, encoded by HPGD). Accumulated evidence suggests that SLCO2A1 plays critical roles in many physiological processes in mammals, and it is considered a potential pharmacological target for diabetic foot ulcer treatment, antipyresis, and non-hormonal contraception. Furthermore, whole-exome analyses suggest that recessive inheritance of SLCO2A1 mutations is associated with two refractory diseases, primary hypertrophic osteoarthropathy (PHO) and chronic enteropathy associated with SLCO2A1 (CEAS). Intriguingly, SLCO2A1 is also a key component of the Maxi-Cl channel, which regulates fluxes of inorganic and organic anions, including ATP. Further study of the bimodal function of SLCO2A1 as a transporter and ion channel is expected to throw new light on the complex pathology of human diseases. Here, we review and summarize recent information on the molecular functions of SLCO2A1, and we discuss its pathophysiological significance.