ABCC4 impacts megakaryopoiesis and protects megakaryocytes against 6-mercaptopurine induced cytotoxicity

The role of ABCC4, an ATP-binding cassette transporter, in the process of platelet formation, megakaryopoiesis, is unknown. Here, we show that ABCC4 is highly expressed in megakaryocytes (MKs). Mining of public genomic data (ATAC-seq and genome wide chromatin interactions, Hi-C) revealed that key megakaryopoiesis transcription factors (TFs) interacted with ABCC4 regulatory elements and likely accounted for high ABCC4 expression in MKs. Importantly these genomic interactions for ABCC4 ranked higher than for genes with known roles in megakaryopoiesis suggesting a role for ABCC4 in megakaryopoiesis. We then demonstrate that ABCC4 is required for optimal platelet formation as in vitro differentiation of fetal liver derived MKs from Abcc4-/- mice exhibited impaired proplatelet formation and polyploidization, features required for optimal megakaryopoiesis. Likewise, a human megakaryoblastic cell line, MEG-01 showed that acute ABCC4 inhibition markedly suppressed key processes in megakaryopoiesis and that these effects were related to reduced cAMP export and enhanced dissociation of a negative regulator of megakaryopoiesis, protein kinase A (PKA) from ABCC4. PKA activity concomitantly increased after ABCC4 inhibition which was coupled with significantly reduced GATA-1 expression, a TF needed for optimal megakaryopoiesis. Further, ABCC4 protected MKs from 6-mercaptopurine (6-MP) as Abcc4-/- mice show a profound reduction in MKs after 6-MP treatment. In total, our studies show that ABCC4 not only protects the MKs but is also required for maximal platelet production from MKs, suggesting modulation of ABCC4 function might be a potential therapeutic strategy to regulate platelet production.

Human immunodeficiency virus protease inhibitors interact with ATP binding cassette transporter 4/multidrug resistance protein 4: a basis for unanticipated enhanced cytotoxicity

Human immunodeficiency virus (HIV) pharmacotherapy, by combining different drug classes such as nucleoside analogs and HIV protease inhibitors (PIs), has increased HIV-patient life expectancy. Consequently, among these patients, an increase in non-HIV-associated cancers has produced a patient cohort requiring both HIV and cancer chemotherapy. We hypothesized that multidrug resistance protein 4/ATP binding cassette transporter 4 (MRP4/ABCC4), a widely expressed transporter of nucleoside-based antiviral medications as well as cancer therapeutics might interact with PIs. Among the PIs evaluated (nelfinavir, ritonavir, amprenavir, saquinavir, and indinavir), only nelfinavir both effectively stimulated MRP4 ATPase activity and inhibited substrate-stimulated ATPase activity. Saos2 and human embryonic kidney 293 cells engineered to overexpress MRP4 were then used to assess transport and cytotoxicity. MRP4 expression reduced intracellular accumulation of nelfinavir and consequently conferred survival advantage to nelfinavir cytotoxicity. Nelfinavir blocked Mrp4-mediated export, which is consistent with its ability to increase the sensitivity of MRP4-expressing cells to methotrexate. In contrast, targeted inactivation of Abcc4/Mrp4 in mouse cells specifically enhanced nelfinavir and 9-(2-phosphonylmethoxyethyl) adenine cytotoxicity. These results suggest that nelfinavir is both an inhibitor and substrate of MRP4. Because nelfinavir is a new MRP4/ABCC4 substrate, we developed a MRP4/ABCC4 pharmacophore model, which showed that the nelfinavir binding site is shared with chemotherapeutic substrates such as adefovir and methotrexate. Our studies reveal, for the first time, that nelfinavir, a potent and cytotoxic PI, is both a substrate and inhibitor of MRP4. These findings suggest that HIV-infected cancer patients receiving nelfinavir might experience both enhanced antitumor efficacy and unexpected adverse toxicity given the role of MRP4/ABCC4 in exporting nucleoside-based antiretroviral medications and cancer chemotherapeutics.

Crucial role for phylogenetically conserved cytoplasmic loop 3 in ABCC4 protein expression

The ABC transporter ABCC4 is recognized as an ATP-dependent exporter of endogenous substances as well as an increasing variety of anionic chemotherapeutics. A loss-of-function variant of zebrafish Abcc4 was identified with a single amino acid substitution in the cytoplasmic loop T804M. Because this substituted amino acid is highly conserved among ABCC4 orthologs and is located in cytoplasmic loop 3 (CL3), we investigated the impact of this mutation on human and zebrafish Abcc4 expression. We demonstrate that zebrafish Abcc4 T804M or human ABCC4 T796M exhibit substantially reduced expression, coupled with impaired plasma membrane localization. To understand the molecular basis for the localization defect, we developed a homology model of zebrafish Abcc4. The homology model suggested that the bulky methionine substitution disrupted side-chain contacts. Molecular dynamic simulations of a fragment of human or zebrafish CL3 containing a methionine substitution indicated altered helicity coupled with reduced thermal stability. Trifluoroethanol challenge coupled with circular dichroism revealed that the methionine substitution disrupted the ability of this fragment of CL3 to readily form an α-helix. Furthermore, expression and plasma membrane localization of these mutant ABCC4/Abcc4 proteins are mostly rescued by growing cells at subphysiological temperatures. Because the cystic fibrosis transmembrane conductance regulator (ABCC7) is closely related to ABCC4, we extended this by engineering certain pathogenic CFTR-CL3 mutations, and we showed they destabilized human and zebrafish ABCC4. Altogether, our studies provide the first evidence for a conserved domain in CL3 of ABCC4 that is crucial in ensuring its proper plasma membrane localization.

Multi-drug resistance protein 4 (MRP4)-mediated regulation of fibroblast cell migration reflects a dichotomous role of intracellular cyclic nucleotides

It has long been known that cyclic nucleotides and cyclic nucleotide-dependent signaling molecules control cell migration. However, the concept that it is not just the absence or presence of cyclic nucleotides, but a highly coordinated balance between these molecules that regulates cell migration, is new and revolutionary. In this study, we used multidrug resistance protein 4 (MRP4)-expressing cell lines and MRP4 knock-out mice as model systems and wound healing assays as the experimental system to explore this unique and emerging concept. MRP4, a member of a large family of ATP binding cassette transporter proteins, localizes to the plasma membrane and functions as a nucleotide efflux transporter and thus plays a role in the regulation of intracellular cyclic nucleotide levels. Here, we demonstrate that mouse embryonic fibroblasts (MEFs) isolated from Mrp4(-/-) mice have higher intracellular cyclic nucleotide levels and migrate faster compared with MEFs from Mrp4(+/+) mice. Using FRET-based cAMP and cGMP sensors, we show that inhibition of MRP4 with MK571 increases both cAMP and cGMP levels, which results in increased migration. In contrast to these moderate increases in cAMP and cGMP levels seen in the absence of MRP4, a robust increase in cAMP levels was observed following treatment with forskolin and isobutylmethylxanthine, which decreases fibroblast migration. In response to externally added cell-permeant cyclic nucleotides (cpt-cAMP and cpt-cGMP), MEF migration appears to be biphasic. Altogether, our studies provide the first experimental evidence supporting the novel concept that balance between cyclic nucleotides is critical for cell migration.

Deregulated hepatic metabolism exacerbates impaired testosterone production in Mrp4-deficient mice

The physiological role of multidrug resistance protein 4 (Mrp4, Abcc4) in the testes is unknown. We found that Mrp4 is expressed primarily in mouse and human Leydig cells; however, there is no current evidence that Mrp4 regulates testosterone production. We investigated its role in Leydig cells, where testosterone production is regulated by cAMP, an intracellular messenger formed when the luteinizing hormone (LH) receptor is activated. Because Mrp4 regulates cAMP, we compared testosterone levels in Mrp4(-/-) and Mrp4(+/+) mice. Young Mrp4(-/-) mice had significantly impaired gametogenesis, reduced testicular testosterone, and disruption of Leydig cell cAMP homeostasis. Both young and adult mice had impaired testosterone production. In Mrp4(-/-) primary Leydig cells treated with LH, intracellular cAMP production was impaired and cAMP-response element-binding protein (CREB) phosphorylation was strongly attenuated. Notably, expression of CREB target genes that regulate testosterone biosynthesis was reduced in Mrp4(-/-) Leydig cells in vivo. Therefore, Mrp4 is required for normal Leydig cell testosterone production. However, adult Mrp4(-/-) mice are fertile, with a normal circulating testosterone concentration. The difference is that in 3-week-old Mrp4(-/-) mice, disruption of gonadal testosterone production up-regulates hepatic Cyp2b10, a known testosterone-metabolizing enzyme. Therefore, defective testicular testosterone production de-regulates hepatic Cyp-mediated testosterone metabolism to disrupt gametogenesis. These findings have important implications for understanding the side effects of therapeutics that disrupt Mrp4 function and are reported to alter androgen production.

All-trans retinoic acid suppresses Stat3 signaling during skin carcinogenesis

Squamous cell carcinoma (SCC) of the skin is the most clinically aggressive form of nonmelanoma skin cancer. We have determined the effects of all-trans retinoic acid (ATRA), a naturally occurring chemopreventive retinoid, on signal transducer and activator of transcription 3 (Stat3) signaling during the development of skin SCC. Stat3 is a transcription factor that plays a critical role in cell proliferation and survival, and it is constitutively active in several malignant cell types. We have previously shown that Stat3 is required for the initiation, promotion, and progression of skin SCC. ATRA is a highly efficient suppressor of tumor formation in the two-stage mouse skin carcinogenesis model and we have shown that this effect correlates with the suppression of the B-Raf/Mek/Erk signaling pathway. In this study, we have determined the pattern of Stat3 phosphorylation throughout the course of the two-stage protocol, both in the presence and absence of ATRA. We have used both SENCAR mice and K5.Stat3C transgenic mice, which express the Stat3C protein, a constitutively active form of Stat3, in the skin. Using Western blotting and immunohistochemical staining with phosphospecific antibodies, we show that coadministration of ATRA suppressed the 12-O-tetradecanoylphorbol-13-acetate-induced phosphorylation of Stat3 in both models, but was only able to suppress tumor formation in the SENCAR mice. Surprisingly, ATRA actually enhanced tumor formation in 12-O-tetradecanoylphorbol-13-acetate-treated K5.Stat3C mice. We hypothesize that ATRA blocks tumor formation, at least in part, by targeting events upstream of Stat3, such as the B-Raf/Mek/Erk pathway, and that in the K5.Stat3C mice, in which Stat3 activity is constitutive, it cannot suppress tumor formation.

Identification of the B-Raf/Mek/Erk MAP kinase pathway as a target for all-trans retinoic acid during skin cancer promotion

Background

Retinoids have been studied extensively for their potential as therapeutic and chemopreventive agents for a variety of cancers, including nonmelanoma skin cancer (NMSC). Despite their use for many years, the mechanism of action of retinoids in the prevention of NMSC is still unclear. In this study we have attempted to understand the chemopreventive mechanism of all-trans retinoic acid (ATRA), a primary biologically active retinoid, in order to more efficiently utilize retinoids in the clinic.

Results

We have used the 2-stage dimethylbenzanthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis model to investigate the chemopreventive effects of ATRA. We have compared the gene expression profiles of control skin to skin subjected to the 2-stage protocol, with or without ATRA, using Affymetrix 430 2.0 DNA microarrays. Approximately 49% of the genes showing altered expression with TPA treatment are conversely affected when ATRA is co-administered. The activity of these genes, which we refer to as 'counter-regulated', may contribute to chemoprevention by ATRA. The counter-regulated genes have been clustered into functional categories and bioinformatic analysis has identified the B-Raf/Mek/Erk branch of the MAP kinase pathway as one containing several genes whose upregulation by TPA is blocked by ATRA. We also show that ATRA blocks signaling through this pathway, as revealed by immunohistochemistry and Western blotting. Finally, we found that blocking the B-Raf/Mek/Erk pathway with a pharmacological inhibitor, Sorafenib (BAY43-9006), induces squamous differentiation of existing skin SCCs formed in the 2-stage model.

Conclusion

These results indicate that ATRA targets the B-Raf/Mek/Erk signaling pathway in the 2-stage mouse skin carcinogenesis model and this activity coincides with its chemopreventive action. This demonstrates the potential for targeting the B-Raf/Mek/Erk pathway for chemoprevention and therapy of skin SCC in humans. In addition our DNA microarray results provide the first expression signature for the chemopreventive effect of ATRA in a mouse skin cancer model. This is a potential source for novel targets for ATRA and other chemopreventive and therapeutic agents that can eventually be tested in the clinic.

Retinoids and skin: microarrays shed new light on chemopreventive action of all-trans retinoic acid

Despite the use of retinoids in the clinic for many years, their mode of action in the prevention of skin cancer is still unclear. Recent microarray analyses of the chemopreventive effect of all-trans retinoic acid (ATRA), one of the primary naturally occurring biologically active retinoids, in the two-stage mouse skin chemical carcinogenesis model have provided novel insight into their action. Comparison of the gene expression profiles of control skin to skin subjected to the two-stage protocol for 3 wk, with or without ATRA, has shown that approximately half of the genes regulated by 12-o-tetradecanoylphorbol-13-acetate (TPA) are oppositely regulated when ATRA is coadministered with TPA. It was further shown the Raf/Mek/Erk branch of mitogen-activated protein (MAP) kinase pathway contains a disproportionate number of oppositely regulated genes, thereby implicating it as one of the key pathways involved in tumor promotion by TPA, that is blocked by ATRA. This result has pointed the way toward the detailed study of Raf/Mek/Erk pathway signaling in skin cancer development and its potential as a target pathway for chemoprevention by ATRA and other chemopreventive drugs.