Egress of CD19(+)CD5(+) cells into peripheral blood following treatment with the Bruton tyrosine kinase inhibitor ibrutinib in mantle cell lymphoma patients.

Ibrutinib (PCI-32765) is a highly potent oral Bruton tyrosine kinase (BTK) inhibitor in clinical development for treating B-cell lymphoproliferative diseases. Patients with chronic lymphocytic leukemia (CLL) often show marked, transient increases of circulating CLL cells following ibrutinib treatments, as seen with other inhibitors of the B-cell receptor (BCR) pathway. In a phase 1 study of ibrutinib, we noted similar effects in patients with mantle cell lymphoma (MCL). Here, we characterize the patterns and phenotypes of cells mobilized among patients with MCL and further investigate the mechanism of this effect. Peripheral blood CD19(+)CD5(+) cells from MCL patients were found to have significant reduction in the expression of CXCR4, CD38, and Ki67 after 7 days of treatment. In addition, plasma chemokines such as CCL22, CCL4, and CXCL13 were reduced 40% to 60% after treatment. Mechanistically, ibrutinib inhibited BCR- and chemokine-mediated adhesion and chemotaxis of MCL cell lines and dose-dependently inhibited BCR, stromal cell, and CXCL12/CXCL13 stimulations of pBTK, pPLCγ2, pERK, or pAKT. Importantly, ibrutinib inhibited migration of MCL cells beneath stromal cells in coculture. We propose that BTK is essential for the homing of MCL cells into lymphoid tissues, and its inhibition results in an egress of malignant cells into peripheral blood. This trial was registered at www.clinicaltrials.gov as #NCT00114738.

The clinically active BTK inhibitor PCI-32765 targets B-cell receptor- and chemokine-controlled adhesion and migration in chronic lymphocytic leukemia.

Small-molecule drugs that target the B-cell antigen receptor (BCR) signalosome show clinical efficacy in the treatment of B-cell non-Hodgkin lymphoma. These agents, including the Bruton tyrosine kinase (BTK) inhibitor PCI-32765, display an unexpected response in patients with chronic lymphocytic leukemia (CLL): a rapid and sustained reduction of lymphadenopathy accompanied by transient lymphocytosis, which is reversible upon temporary drug deprivation. We hypothesized that this clinical response reflects impaired integrin-mediated adhesion and/or migration. Here, we show that PCI-32765 strongly inhibits BCR-controlled signaling and integrin α(4)ß(1)-mediated adhesion to fibronectin and VCAM-1 of lymphoma cell lines and primary CLL cells. Furthermore, PCI-32765 also inhibits CXCL12-, CXCL13-, and CCL19-induced signaling, adhesion, and migration of primary CLL cells. Our data indicate that inhibition of BTK by PCI-32765 overcomes BCR- and chemokine-controlled integrin-mediated retention and homing of malignant B cells in their growth- and survival-supporting lymph node and bone marrow microenvironment, which results in clinically evident CLL regression.

The dual HCK/BTK inhibitor KIN-8194 impairs growth and integrin-mediated adhesion of BTKi-resistant mantle cell lymphoma.

Although Bruton's tyrosine kinase (BTK) inhibitors (BTKi) have significantly improved patient prognosis, mantle cell lymphoma (MCL) is still considered incurable due to primary and acquired resistance. We have recently shown that aberrant expression of the Src-family tyrosine kinase hematopoietic cell kinase (HCK) in MCL correlates with poor prognosis, and that genetic HCK perturbation impairs growth and integrin-mediated adhesion of MCL cells. Here, we show that KIN-8194, a dual inhibitor of BTK and HCK with in vivo activity against Myd88-L265P-driven diffuse large B-cell lymphoma and Waldenström Macroglobulinemia, has a potent growth inhibitory effect in MCL cell lines and primary MCL cells, irrespective of their sensitivity to BTKi (ibrutinib and acalabrutinib). In BTKi-resistant cells this is mediated by inhibition of HCK, which results in repression of AKT-S6 signaling. In addition, KIN-8194 inhibits integrin-mediated adhesion of BTKi-sensitive and insensitive MCL cells to fibronectin and stromal cells in an HCK-dependent manner. Finally, we show that MCL cells with acquired BTKi resistance retain their sensitivity to KIN-8194. Taken together, our data demonstrate that KIN-8194 inhibits growth and integrin-mediated adhesion of BTKi-sensitive MCL cells, as well as MCL cells with primary or acquired BTKi resistance. This renders KIN-8194 a promising novel treatment for MCL patients.

Disruption of heparan sulfate proteoglycan conformation perturbs B-cell maturation and APRIL-mediated plasma cell survival.

The development and antigen-dependent differentiation of B lymphocytes are orchestrated by an array of growth factors, cytokines, and chemokines that require tight spatiotemporal regulation. Heparan sulfate proteoglycans specifically bind and regulate the bioavailability of soluble protein ligands, but their role in the immune system has remained largely unexplored. Modification of heparan sulfate by glucuronyl C5-epimerase (Glce) controls heparan sulfate-chain flexibility and thereby affects ligand binding. Here we show that Glce deficiency impairs B-cell maturation, resulting in decreased plasma cell numbers and immunoglobulin levels. We demonstrate that C5-epimerase modification of heparan sulfate is critical for binding of a proliferation inducing ligand (APRIL) and that Glce-deficient plasma cells fail to respond to APRIL-mediated survival signals. Our results identify heparan sulfate proteoglycans as novel players in B-cell maturation and differentiation and suggest that heparan sulfate conformation is crucial for recruitment of factors that control plasma cell survival.

Targeting EXT1 reveals a crucial role for heparan sulfate in the growth of multiple myeloma.

Expression of the heparan sulfate proteoglycan syndecan-1 is a hallmark of both normal and multiple myeloma (MM) plasma cells. Syndecan-1 could affect plasma cell fate by strengthening integrin-mediated adhesion via its core protein and/or by accommodating and presenting soluble factors via its HS side chains. Here, we show that inducible RNAi-mediated knockdown of syndecan-1 in human MM cells leads to reduced growth rates and a strong increase of apoptosis. Importantly, knockdown of EXT1, a copolymerase critical for HS chain biosynthesis, had similar effects. Using an innovative myeloma xenotransplantation model in Rag-2(-/-)gamma(c)(-/-) mice, we demonstrate that induction of EXT1 knockdown in vivo dramatically suppresses the growth of bone marrow localized myeloma. Our findings provide direct evidence that the HS chains of syndecan-1 are crucial for the growth and survival of MM cells within the bone marrow environment, and indicate the HS biosynthesis machinery as a potential treatment target in MM.

Impaired lymphoid organ development in mice lacking the heparan sulfate modifying enzyme glucuronyl C5-epimerase.

The development of lymphoid organs depends on cross talk between hematopoietic cells and mesenchymal stromal cells and on vascularization of the lymphoid primordia. These processes are orchestrated by cytokines, chemokines, and angiogenic factors that require tight spatiotemporal regulation. Heparan sulfate (HS) proteoglycans are molecules designed to specifically bind and regulate the bioactivity of soluble protein ligands. Their binding capacity and specificity are controlled by modification of the HS side chain by HS-modifying enzymes. Although HS proteoglycans have been implicated in the morphogenesis of several organ systems, their role in controlling lymphoid organ development has thus far remained unexplored. In this study, we report that modification of HS by the HS-modifying enzyme glucuronyl C5-epimerase (Glce), which controls HS chain flexibility, is required for proper lymphoid organ development. Glce(-/-) mice show a strongly reduced size of the fetal spleen as well as a spectrum of defects in thymus and lymph node development, ranging from dislocation to complete absence of the organ anlage. Once established, however, the Glce(-/-) primordia recruited lymphocytes and developed normal architectural features. Furthermore, Glce(-/-) lymph node anlagen transplanted into wild-type recipient mice allowed undisturbed lymphocyte maturation. Our results indicate that modification of HS by Glce is required for controlling the activity of molecules that are instructive for early lymphoid tissue morphogenesis but may be dispensable at later developmental stages and for lymphocyte maturation and differentiation.

Identification of the SRC-family tyrosine kinase HCK as a therapeutic target in mantle cell lymphoma.

Mantle cell lymphoma (MCL) is an aggressive non-Hodgkin lymphoma subtype arising from naïve B cells. Although novel therapeutics have improved patient prognosis, drug resistance remains a key problem. Here, we show that the SRC-family tyrosine kinase hematopoietic cell kinase (HCK), which is primarily expressed in the hematopoietic lineage but not in mature B cells, is aberrantly expressed in MCL, and that high expression of HCK is associated with inferior prognosis of MCL patients. HCK expression is controlled by the toll-like receptor (TLR) adaptor protein MYD88 and can be enhanced by TLR agonists in MCL cell lines and primary MCL. In line with this, primary MCL with high HCK expression are enriched for a TLR-signaling pathway gene set. Silencing of HCK expression results in cell cycle arrest and apoptosis. Furthermore, HCK controls integrin-mediated adhesion of MCL cells to extracellular matrix and stromal cells. Taken together, our data indicate that TLR/MYD88-controlled aberrant expression of HCK plays a critical role in MCL proliferation and survival as well as in retention of the malignant cells in the growth- and survival-supporting lymphoid organ microenvironment, thereby contributing to lymphomagenesis. These novel insights provide a strong rationale for therapeutic targeting of HCK in MCL.

The CXCL12gamma chemokine immobilized by heparan sulfate on stromal niche cells controls adhesion and mediates drug resistance in multiple myeloma.

BACKGROUND: The survival and proliferation of multiple myeloma (MM) cells in the bone marrow (BM) critically depend on interaction with stromal cells expressing the chemokine CXCL12. CXCL12 regulates the homing to the BM niche by mediating the transendothelial migration and adhesion/retention of the MM cells. The gamma isoform of CXCL12 (CXCL12γ) has been reported to be highly expressed in mouse BM and to show enhanced biological activity compared to the 'common' CXCL12α isoform, mediated by its unique extended C-terminal domain, which binds heparan sulfate proteoglycans (HSPGs) with an extraordinary high affinity. Here, we investigated the expression of CXCL12γ in human BM and studied its functional role in the interaction of MM cells with BM stromal cells (BMSCs). METHODS: We assessed CXCL12γ mRNA and protein expression by human BMSCs using qPCR, flow cytometry, and immunohistochemistry. CRISPR-Cas9 was employed to delete CXCL12γ and the heparan sulfate (HS) co-polymerase EXT1 in BMSCs. To study the functional roles of BMSC-derived CXCL12γ and HSPGs in the interaction of MM cells with BMSCs cells, MM cell lines and primary MM cells were co-cultured with BMSCs. RESULTS: We observed that CXCL12γ is expressed in situ by reticular stromal cells in both normal and MM BM, as well as by primary BMSC isolates and BMSC lines. Importantly, upon secretion, CXCL12γ, unlike the CXCL12α isoform, was retained on the surface of BMSCs. This membrane retention of CXCL12γ is HSPG mediated, since it was completely annulated by CRISPR-Cas9-mediated deletion of the HS co-polymerase EXT1. CXCL12γ expressed by BMSCs and membrane-retained by HSPGs supported robust adhesion of MM cells to the BMSCs. Specific genetic deletion of either CXCL12γ or EXT1 significantly attenuated the ability of BMSCs to support MM cell adhesion and, in addition, impaired their capacity to protect MM cells from bortezomib-induced cell death. CONCLUSIONS: We show that CXCL12γ is expressed by human BMSCs and upon secretion is retained on their cell surface by HSPGs. The membrane-bound CXCL12γ controls adhesion of MM cells to the stromal niche and mediates drug resistance. These findings designate CXCL12γ and associated HSPGs as partners in mediating MM-niche interaction and as potential therapeutic targets in MM.

Cell lines generated from a chronic lymphocytic leukemia mouse model exhibit constitutive Btk and Akt signaling.

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of mature CD5+ B cells in blood. Spontaneous apoptosis of CLL cells in vitro has hampered in-depth investigation of CLL pathogenesis. Here we describe the generation of three monoclonal mouse cell lines, EMC2, EMC4 and EMC6, from the IgH.TEµ CLL mouse model based on sporadic expression of SV40 large T antigen. The cell lines exhibit a stable CD5+CD43+IgM+CD19+ CLL phenotype in culture and can be adoptively transferred into Rag1-/- mice. RNA-seq analysis revealed only minor differences between the cell lines and their primary tumors and suggested that NF-κB and mTOR signaling pathways were involved in cell line outgrowth. In vitro survival and proliferation was dependent on constitutive phosphorylation of Bruton's tyrosine kinase (Btk) at Y551/Y223, and Akt(S473). Treatment of the cell lines with small molecule inhibitors specific for Btk (ibrutinib) or PI3K (idelalisib), which is upstream of Akt, resulted in reduced viability, proliferation and fibronectin-dependent cell adhesion. Treatment of cell line-engrafted Rag1-/- mice with ibrutinib was associated with transient lymphocytosis, reduced splenomegaly and increased overall survival. Thus, by generating stable cell lines we established a novel platform for in vitro and in vivo investigation of CLL signal transduction and treatment modalities.

In vivo RNA interference-mediated ablation of MDR1 P-glycoprotein.

Multidrug resistance (MDR) remains a major obstacle to successful chemotherapeutic treatment of cancer and can be caused by overexpression of P-glycoprotein, the MDR1 gene product. To further validate a knockdown approach for circumventing MDR, we developed a P-glycoprotein inhibition strategy using short hairpin RNA interference (shRNAi) and now show efficacy and target specificity in vivo. Two of eight tested shRNAi constructs targeted against human MDR1 mRNA inhibited expression of P-glycoprotein by >90%, whereas control shRNAi had no effect. Ablation of P-glycoprotein in cells stably transduced with retroviral-mediated shRNAi was documented by Western blot and functionally confirmed by increased sensitivity of MDR1-transfected cells toward the cytotoxic drugs vincristine, paclitaxel, and doxorubicin as well as by transport of (99m)Tc-Sestamibi. shRNAi-mediated down-regulation of P-glycoprotein transport activity both in cultured cells and in tumor implants in living animals could be followed by direct noninvasive bioluminescence imaging using the Renilla luciferase fluorophore, coelenterazine, a known P-glycoprotein transport substrate. Furthermore, after somatic gene transfer by hydrodynamic infusion of a MDR1-Firefly luciferase (MDR1-FLuc) fusion construct into mouse liver, the effect of shRNAi delivered in vivo on P-glycoprotein-FLuc protein levels was documented with bioluminescence imaging using d-luciferin. ShRNAi against MDR1 reduced bioluminescence output of the P-glycoprotein-FLuc reporter 4-fold in vivo compared with mice treated with control or scrambled shRNAi. Targeted down-regulation of a somatically transferred P-glycoprotein-eGFP fusion reporter also was observed using fluorescence microscopy. Our results show that shRNAi effectively inhibited MDR1 expression and function in cultured cells, tumor implants and mammalian liver, documenting the feasibility of a knockdown approach to reversing MDR in vivo.

The small GTPase Ral mediates SDF-1-induced migration of B cells and multiple myeloma cells.

Chemokine-controlled migration plays a critical role in B-cell development, differentiation, and function, as well as in the pathogenesis of B-cell malignancies, including the plasma cell neoplasm multiple myeloma (MM). Here, we demonstrate that stimulation of B cells and MM cells with the chemokine stromal cell-derived factor-1 (SDF-1) induces strong migration and activation of the Ras-like GTPase Ral. Inhibition of Ral, by expression of the dominant negative RalN28 mutant or of RalBPDeltaGAP, a Ral effector mutant that sequesters active Ral, results in impaired SDF-1-induced migration of B cells and MM cells. Of the 2 Ral isoforms, RalA and RalB, RalB was found to mediate SDF-1-induced migration. We have recently shown that Btk, PLCgamma2, and Lyn/Syk mediate SDF-1-controlled B-cell migration; however, SDF-1-induced Ral activation is not affected in B cells deficient in these proteins. In addition, treatment with pharmacological inhibitors against PI3K and PLC or expression of dominant-negative Ras did not impair SDF-1-induced Ral activation. Taken together, these results reveal a novel function for Ral, that is, regulation of SDF-1-induced migration of B cells and MM cells, thereby providing new insights into the control of B-cell homeostasis, trafficking, and function, as well as into the pathogenesis of MM.

Multidrug resistance proteins 2 and 3 provide alternative routes for hepatic excretion of morphine-glucuronides.

Glucuronidation is a major hepatic detoxification pathway for endogenous and exogenous compounds, resulting in the intracellular formation of polar metabolites that require specialized transporters for elimination. Multidrug resistance proteins (MRPs) are expressed in the liver and can transport glucuronosyl-conjugates. Using morphine as a model aglycone, we demonstrate that morphine-3-glucuronide (M3G), the predominant metabolite, is transported in vitro by human MRP2 (ABCC2), a protein present in the apical membrane of hepatocytes. Loss of biliary M3G secretion in Mrp2(-/-) mice results in its increased sinusoidal transport that can be attributed to Mrp3. Combined loss of Mrp2 and Mrp3 leads to a substantial accumulation of M3G in the liver, from which it is transported across the sinusoidal membrane at a low rate, resulting in the prolonged presence of M3G in plasma. Our results show that murine Mrp2 and Mrp3 provide alternative routes for the excretion of a glucuronidated substrate from the liver in vivo.

The effect of low pH on breast cancer resistance protein (ABCG2)-mediated transport of methotrexate, 7-hydroxymethotrexate, methotrexate diglutamate, folic acid, mitoxantrone, topotecan, and resveratrol in in vitro drug transport models.

Some cellular uptake systems for (anti)folates function optimally at acidic pH. We have tested whether this also applies to efflux from cells by breast cancer resistance protein (BCRP; ABCG2), which has been reported to transport folic acid, methotrexate, and methotrexate di- and triglutamate at physiological pH. Using Spodoptera frugiperda-BCRP membrane vesicles, we showed that the ATP-dependent vesicular transport of 1 muM methotrexate by BCRP is 5-fold higher at pH 5.5 than at physiological pH. The transport of methotrexate was saturable at pH 5.5, with apparent Km and Vmax values of 1.3 +/- 0.2 mM and 44 +/- 2.5 nmol/mg of protein/min, respectively, but was linear with drug concentration at pH 7.3 up to 6 mM methotrexate. In contrast to recent reports, we did not detect transport of methotrexate diglutamate at physiological pH, but we did find transport at pH 5.5. We also found that 7-hydroxy-methotrexate, the major metabolite of methotrexate, is transported by BCRP both at physiological pH and (more efficiently) at low pH. The pH effect was also observed in intact BCRP-overexpressing cells: we found a 3-fold higher level of resistance to both methotrexate and the prototypical BCRP substrate mitoxantrone at pH 6.5 as at physiological pH. Furthermore, with MDCKII-BCRP monolayers, we found that resveratrol, which is a neutral compound at pH < or = 7.4, is efficiently transported by BCRP at pH 6.0, whereas we did not detect active transport at pH 7.4. We conclude that BCRP transports substrate drugs more efficiently at low pH, independent of the dissociation status of the substrate.

Mice lacking Mrp3 (Abcc3) have normal bile salt transport, but altered hepatic transport of endogenous glucuronides.

BACKGROUND/AIM: Multidrug Resistance Protein 3 (MRP3) transports bile salts and glucuronide conjugates in vitro and is postulated to protect the liver in cholestasis. Whether the absence of Mrp3 affects these processes in vivo is tested. METHODS: Mrp3-deficient mice were generated and the contribution of Mrp3 to bile salt and glucuronide conjugate transport was tested in (1): an Ussing-chamber set-up with ileal explants (2), the liver during bile-duct ligation (3), liver perfusion experiments, and (4) in vitro vesicular uptake experiments. RESULTS: The Mrp3((-/-)) mice show no overt phenotype. No differences between WT and Mrp3-deficient mice were found in the trans-ileal transport of taurocholate. After bile-duct ligation, there were no differences in histological liver damage and serum bile salt levels between Mrp3((-/-)) and WT mice, but Mrp3-deficient mice had lower serum bilirubin glucuronide concentrations. Glucuronide conjugates of hyocholate and hyodeoxycholate are substrates of MRP3 in vitro and in livers that lack Mrp3, there is reduced sinusoidal secretion of hyodeoxycholate-glucuronide after perfusion with hyodeoxycholate. CONCLUSIONS: Mrp3 does not have a major role in bile salt physiology, but is involved in the transport of glucuronidated compounds, which could include glucuronidated bile salts in humans.

Mice lacking multidrug resistance protein 3 show altered morphine pharmacokinetics and morphine-6-glucuronide antinociception.

Glucuronidation is a major detoxification pathway for endogenous and exogenous compounds in mammals that results in the intracellular formation of polar metabolites, requiring specialized transporters to cross biological membranes. By using morphine as a model aglycone, we demonstrate that multidrug resistance protein 3 (MRP3/ABCC3), a protein present in the basolateral membrane of polarized cells, transports morphine-3-glucuronide (M3G) and morphine-6-glucuronide in vitro. Mrp3(-/-) mice are unable to excrete M3G from the liver into the bloodstream, the major hepatic elimination route for this drug. This results in increased levels of M3G in liver and bile, a 50-fold reduction in the plasma levels of M3G, and in a major shift in the main disposition route for morphine and M3G, predominantly via the urine in WT mice but via the feces in Mrp3(-/-) mice. The pharamacokinetics of injected morphine-glucuronides are altered as well in the absence of Mrp3, and this results in a decreased antinociceptive potency of injected morphine-6-glucuronide.

Transport of bile acids in multidrug-resistance-protein 3-overexpressing cells co-transfected with the ileal Na+-dependent bile-acid transporter.

Many of the transporters involved in the transport of bile acids in the enterohepatic circulation have been characterized. The basolateral bile-acid transporter of ileocytes and cholangiocytes remains an exception. It has been suggested that rat multidrug resistance protein 3 (Mrp3) fulfills this function. Here we analyse bile-salt transport by human MRP3. Membrane vesicles from insect ( Spodoptera frugiperda ) cells expressing MRP3 show time-dependent uptake of glycocholate and taurocholate. Furthermore, sulphated bile salts were high-affinity competitive inhibitors of etoposide glucuronide transport by MRP3 (IC50 approximately 10 microM). Taurochenodeoxycholate, taurocholate and glycocholate inhibited transport at higher concentrations (IC50 approximately 100, 250 and 500 microM respectively). We used mouse fibroblast-like cell lines derived from mice with disrupted Mdr1a, Mdr1b and Mrp1 genes to generate transfectants that express the murine apical Na+-dependent bile-salt transporter (Asbt) and MRP3. Uptake of glycocholate by these cells is Na+-dependent, with a K(m) and V(max) of 29+/-7 microM and 660 +/- 63 pmol/min per mg of protein respectively and is inhibited by several organic-aniontransport inhibitors. Expression of MRP3 in these cells limits the accumulation of glycocholate and increases the efflux from cells preloaded with taurocholate or glycocholate. In conclusion, we find that MRP3 transports both taurocholate and glycocholate, albeit with low affinity, in contrast with the high-affinity transport by rat Mrp3. Our results suggest that MRP3 is unlikely to be the principal basolateral bile-acid transporter of ileocytes and cholangiocytes, but that it may have a role in the removal of bile acids from the liver in cholestasis.

Steroid and bile acid conjugates are substrates of human multidrug-resistance protein (MRP) 4 (ATP-binding cassette C4).

Human multidrug-resistance protein (MRP) 4 transports cyclic nucleotides and when overproduced in mammalian cells mediates resistance to some nucleoside analogues. Recently, it has been shown that Mrp4 is induced in the livers of Fxr ((-/-)) mice, which have increased levels of serum bile acids. Since MRP4, like MRP1-3, also mediates transport of a model steroid conjugate substrate, oestradiol 17-beta-D-glucuronide (E(2)17betaG), we tested whether MRP4 may be involved in the transport of steroid and bile acid conjugates. Bile salts, especially sulphated derivatives, and cholestatic oestrogens inhibited the MRP4-mediated transport of E(2)17betaG. Inhibition by oestradiol 3,17-disulphate and taurolithocholate 3-sulphate was competitive, suggesting that these compounds are MRP4 substrates. Furthermore, we found that MRP4 transports dehydroepiandrosterone 3-sulphate (DHEAS), the most abundant circulating steroid in humans, which is made in the adrenal gland. The ATP-dependent transport of DHEAS by MRP4 showed saturable kinetics with K (m) and V (max) values of 2 microM and 45 pmol/mg per min, respectively (at 27 degrees C). We further studied the possible involvement of other members of the MRP family of transporters in the transport of DHEAS. We found that MRP1 transports DHEAS in a glutathione-dependent manner and exhibits K (m) and V (max) values of 5 microM and 73 pmol/mg per min, respectively (at 27 degrees C). No transport of DHEAS was observed in membrane vesicles containing MRP2 or MRP3. Our findings suggest a physiological role for MRP1 and MRP4 in DHEAS transport and an involvement of MRP4 in transport of conjugated steroids and bile acids.