Defining the ligand-dependent proximatome of the sigma 1 receptor

Sigma 1 Receptor (S1R) is a therapeutic target for a wide spectrum of pathological conditions ranging from neurodegenerative diseases to cancer and COVID-19. S1R is ubiquitously expressed throughout the visceral organs, nervous, immune and cardiovascular systems. It is proposed to function as a ligand-dependent molecular chaperone that modulates multiple intracellular signaling pathways. The purpose of this study was to define the S1R proximatome under native conditions and upon binding to well-characterized ligands. This was accomplished by fusing the biotin ligase, Apex2, to the C terminus of S1R. Cells stably expressing S1R-Apex or a GFP-Apex control were used to map proximal proteins. Biotinylated proteins were labeled under native conditions and in a ligand dependent manner, then purified and identified using quantitative mass spectrometry. Under native conditions, S1R biotinylates over 200 novel proteins, many of which localize within the endomembrane system (endoplasmic reticulum, Golgi, secretory vesicles) and function within the secretory pathway. Under conditions of cellular exposure to either S1R agonist or antagonist, results show enrichment of proteins integral to secretion, extracellular matrix formation, and cholesterol biosynthesis. Notably, Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) displays increased binding to S1R under conditions of treatment with Haloperidol, a well-known S1R antagonist; whereas Low density lipoprotein receptor (LDLR) binds more efficiently to S1R upon treatment with (+)-Pentazocine ((+)-PTZ), a classical S1R agonist. Furthermore, we demonstrate that the ligand bound state of S1R correlates with specific changes to the cellular secretome. Our results are consistent with the postulated role of S1R as an intracellular chaperone and further suggest important and novel functionalities related to secretion and cholesterol metabolism.

dTtc1, a conserved tetratricopeptide repeat protein, is required for maturation of Drosophila egg chambers via its role in stabilizing electron transport chain components

We recently identified the Drosophila ortholog of TTC1 (dTtc1) as an interacting partner of Egalitarian, an RNA adaptor of the Dynein motor. In order to better understand the function of this relatively uncharacterized protein, we depleted dTtc1 in the Drosophila female germline. Depletion of dTtc1 resulted in defective oogenesis and no mature eggs were produced. A closer examination revealed that mRNA cargoes normally transported by Dynein were relatively unaffected. However, mitochondria in dTtc1 depleted egg chambers displayed an extremely swollen phenotype. Ultrastructural analysis revealed a lack of cristae. These phenotypes were not observed upon disruption of Dynein. Thus, this function of dTtc1 is likely to be Dynein independent. Consistent with a role for dTtc1 in mitochondrial biology, a published proteomics screen revealed that dTtc1 interacts with numerous components of electron transport chain (ETC) complexes. Our results indicate that the expression level of several of these ETC components was significantly reduced upon depletion of dTtc1. Importantly, this phenotype was completely rescued upon expression of wild-type GFP-dTtc1 in the depleted background. Lastly, we demonstrate that the mitochondrial phenotype caused by a lack of dTtc1 is not restricted to the germline but is also observed in somatic tissues. Our model suggests that dTtc1, likely in combination with cytoplasmic chaperones, is required for stabilizing ETC components.

In vivo proximity biotin ligation identifies the interactome of Egalitarian, a Dynein cargo adaptor

Numerous motors of the Kinesin family contribute to plus-end-directed microtubule transport. However, almost all transport towards the minus-end of microtubules involves Dynein. Understanding the mechanism by which Dynein transports this vast diversity of cargo is the focus of intense research. In selected cases, adaptors that link a particular cargo with Dynein have been identified. However, the sheer diversity of cargo suggests that additional adaptors must exist. We used the Drosophila egg chamber as a model to address this issue. Within egg chambers, Egalitarian is required for linking mRNA with Dynein. However, in the absence of Egalitarian, Dynein transport into the oocyte is severely compromised. This suggests that additional cargoes might be linked to Dynein in an Egalitarian-dependent manner. We therefore used proximity biotin ligation to define the interactome of Egalitarian. This approach yielded several novel interacting partners, including P body components and proteins that associate with Dynein in mammalian cells. We also devised and validated a nanobody-based proximity biotinylation strategy that can be used to define the interactome of any GFP-tagged protein.

Dynein light chain-dependent dimerization of Egalitarian is essential for maintaining oocyte fate in Drosophila

Egalitarian (Egl) is an RNA adaptor for the Dynein motor and is thought to link numerous, perhaps hundreds, of mRNAs with Dynein. Dynein, in turn, is responsible for the transport and localization of these mRNAs. Studies have shown that efficient mRNA binding by Egl requires the protein to dimerize. We recently demonstrated that Dynein light chain (Dlc) is responsible for facilitating the dimerization of Egl. Mutations in Egl that fail to interact with Dlc do not dimerize, and as such, are defective for mRNA binding. Consequently, this mutant does not efficiently associate with BicaudalD (BicD), the factor responsible for linking the Egl/mRNA complex with Dynein. In this report, we tested whether artificially dimerizing this Dlc-binding mutant using a leucine zipper would restore mRNA binding and rescue mutant phenotypes in vivo. Interestingly, we found that although artificial dimerization of Egl restored BicD binding, it only partially restored mRNA binding. As a result, Egl-dependent phenotypes, such as oocyte specification and mRNA localization, were only partially rescued. We hypothesize that Dlc-mediated dimerization of Egl results in a three-dimensional conformation of the Egl dimer that is best suited for mRNA binding. Although the leucine zipper restores Egl dimerization, it likely does not enable Egl to assemble into the conformation required for maximal mRNA binding activity.

Optimal RNA binding by Egalitarian, a Dynein cargo adaptor, is critical for maintaining oocyte fate in Drosophila

The Dynein motor is responsible for the localization of numerous mRNAs within Drosophila oocytes and embryos. The RNA binding protein, Egalitarian (Egl), is thought to link these various RNA cargoes with Dynein. Although numerous studies have shown that Egl is able to specifically associate with these RNAs, the nature of these interactions has remained elusive. Egl contains a central RNA binding domain that shares limited homology with an exonuclease, yet Egl binds to RNA without degrading it. Mutations have been identified within Egl that disrupt its association with its protein interaction partners, BicaudalD (BicD) and Dynein light chain (Dlc), but no mutants have been described that are specifically defective for RNA binding. In this report, we identified a series of positively charged residues within Egl that are required for RNA binding. Using corresponding RNA binding mutants, we demonstrate that specific RNA cargoes are more reliant on maximal Egl RNA biding activity for their correct localization in comparison to others. We also demonstrate that specification and maintenance of oocyte fate requires maximal Egl RNA binding activity. Even a subtle reduction in Egl's RNA binding activity completely disrupts this process. Our results show that efficient RNA localization at the earliest stages of oogenesis is required for specification of the oocyte and restriction of meiosis to a single cell.

Deficiency in the endocytic adaptor proteins PHETA1/2 impairs renal and craniofacial development

A critical barrier in the treatment of endosomal and lysosomal diseases is the lack of understanding of the in vivo functions of the putative causative genes. We addressed this by investigating a key pair of endocytic adaptor proteins, PH domain-containing endocytic trafficking adaptor 1 and 2 (PHETA1/2; also known as FAM109A/B, Ses1/2, IPIP27A/B), which interact with the protein product of OCRL, the causative gene for Lowe syndrome. Here, we conducted the first study of PHETA1/2 in vivo, utilizing the zebrafish system. We found that impairment of both zebrafish orthologs, pheta1 and pheta2, disrupted endocytosis and ciliogenesis in renal tissues. In addition, pheta1/2 mutant animals exhibited reduced jaw size and delayed chondrocyte differentiation, indicating a role in craniofacial development. Deficiency of pheta1/2 resulted in dysregulation of cathepsin K, which led to an increased abundance of type II collagen in craniofacial cartilages, a marker of immature cartilage extracellular matrix. Cathepsin K inhibition rescued the craniofacial phenotypes in the pheta1/2 double mutants. The abnormal renal and craniofacial phenotypes in the pheta1/2 mutant animals were consistent with the clinical presentation of a patient with a de novo arginine (R) to cysteine (C) variant (R6C) of PHETA1. Expressing the patient-specific variant in zebrafish exacerbated craniofacial deficits, suggesting that the R6C allele acts in a dominant-negative manner. Together, these results provide insights into the in vivo roles of PHETA1/2 and suggest that the R6C variant is contributory to the pathogenesis of disease in the patient.This article has an associated First Person interview with the first author of the paper.

The Egalitarian binding partners Dynein light chain and Bicaudal-D act sequentially to link mRNA to the Dynein motor

A conserved mechanism of polarity establishment is the localization of mRNA to specific cellular regions. Although it is clear that many mRNAs are transported along microtubules, much less is known about the mechanism by which these mRNAs are linked to microtubule motors. The RNA binding protein Egalitarian (Egl) is necessary for localization of several mRNAs in Drosophila oocytes and embryos. Egl also interacts with Dynein light chain (Dlc) and Bicaudal-D (BicD). The role of Dlc and BicD in mRNA localization has remained elusive. Both proteins are required for oocyte specification, as is Egl. Null alleles in these genes result in an oogenesis block. In this report, we used an shRNA-depletion strategy to overcome the oogenesis block. Our findings reveal that the primary function of Dlc is to promote Egl dimerization. Loss of dimerization compromises the ability of Egl to bind RNA. Consequently, Egl is not bound to cargo, and is not able to efficiently associate with BicD and the Dynein motor. Our results therefore identify the key molecular steps required for assembling a localization-competent mRNP.

Renal iron accelerates the progression of diabetic nephropathy in the HFE gene knockout mouse model of iron overload

Diabetic nephropathy (DN) is the most common cause of end-stage renal disease associated with high mortality worldwide. Increases in iron levels have been reported in diabetic rat kidneys as well as in human urine of patients with diabetes. In addition, a low-iron diet or iron chelators delay the progression of DN in patients with diabetes and in animal models of diabetes. Possible maladaptive mechanisms of organ damage by tissue iron accumulation have not been well studied. We recently reported that iron induced the retinal renin-angiotensin system (RAS) and accelerated the progression of diabetic retinopathy. However, whether iron regulates the systemic RAS is unknown. To explore if iron alters the expression of intrarenal RAS and its role in the progression of DN, we used the high Fe iron (HFE) knockout mouse, a genetic model of systemic iron overload. We found that diabetes upregulated the expression of iron regulatory proteins and augmented tissue iron accumulation in the kidneys of both type 1 and type 2 diabetic mouse models. Iron accumulation in the kidneys of HFE knockout mice was associated with increase in serum and intrarenal renin expression. Induction of diabetes in HFE knockout mice using streptozotocin caused a much higher accumulation of renal iron and accelerated the progression of nephropathy compared with diabetic wild-type mice. Treatment of diabetic mice with the iron chelator deferiprone reversed the renin upregulation and reduced kidney injury. Thus, our results establish a new link between renal iron and RAS activity. Exploring the mechanisms of iron-induced RAS activation further may have a significant therapeutic impact on hypertension and DN.

Iron Overload Accelerates the Progression of Diabetic Retinopathy in Association with Increased Retinal Renin Expression

Diabetic retinopathy (DR) is a leading cause of blindness among working-age adults. Increased iron accumulation is associated with several degenerative diseases. However, there are no reports on the status of retinal iron or its implications in the pathogenesis of DR. In the present study, we found that retinas of type-1 and type-2 mouse models of diabetes have increased iron accumulation compared to non-diabetic retinas. We found similar iron accumulation in postmortem retinal samples from human diabetic patients. Further, we induced diabetes in HFE knockout (KO) mice model of genetic iron overload to understand the role of iron in the pathogenesis of DR. We found increased neuronal cell death, vascular alterations and loss of retinal barrier integrity in diabetic HFE KO mice compared to diabetic wildtype mice. Diabetic HFE KO mouse retinas also exhibited increased expression of inflammation and oxidative stress markers. Severity in the pathogenesis of DR in HFE KO mice was accompanied by increase in retinal renin expression mediated by G-protein-coupled succinate receptor GPR91. In light of previous reports implicating retinal renin-angiotensin system in DR pathogenesis, our results reveal a novel relationship between diabetes, iron and renin-angiotensin system, thereby unraveling new therapeutic targets for the treatment of DR.

A new isoform of Drosophila non-muscle Tropomyosin 1 interacts with Kinesin-1 and functions in oskar mRNA localization

Recent studies have revealed that diverse cell types use mRNA localization as a means to establish polarity. Despite the prevalence of this phenomenon, much less is known regarding the mechanism by which mRNAs are localized. The Drosophila melanogaster oocyte provides a useful model for examining the process of mRNA localization. oskar (osk) mRNA is localized at the posterior of the oocyte, thus restricting the expression of Oskar protein to this site. The localization of osk mRNA is microtubule dependent and requires the plus-end-directed motor Kinesin-1. Unlike most Kinesin-1 cargoes, localization of osk mRNA requires the Kinesin heavy chain (Khc) motor subunit, but not the Kinesin light chain (Klc) adaptor. In this report, we demonstrate that a newly discovered isoform of Tropomyosin 1, referred to as Tm1C, directly interacts with Khc and functions in concert with this microtubule motor to localize osk mRNA. Apart from osk mRNA localization, several additional Khc-dependent processes in the oocyte are unaffected upon loss of Tm1C. Our results therefore suggest that the Tm1C-Khc interaction is specific for the osk localization pathway.

Amino acid metabolism inhibits antibody-driven kidney injury by inducing autophagy

Inflammatory kidney disease is a major clinical problem that can result in end-stage renal failure. In this article, we show that Ab-mediated inflammatory kidney injury and renal disease in a mouse nephrotoxic serum nephritis model was inhibited by amino acid metabolism and a protective autophagic response. The metabolic signal was driven by IFN-γ-mediated induction of indoleamine 2,3-dioxygenase 1 (IDO1) enzyme activity with subsequent activation of a stress response dependent on the eIF2α kinase general control nonderepressible 2 (GCN2). Activation of GCN2 suppressed proinflammatory cytokine production in glomeruli and reduced macrophage recruitment to the kidney during the incipient stage of Ab-induced glomerular inflammation. Further, inhibition of autophagy or genetic ablation of Ido1 or Gcn2 converted Ab-induced, self-limiting nephritis to fatal end-stage renal disease. Conversely, increasing kidney IDO1 activity or treating mice with a GCN2 agonist induced autophagy and protected mice from nephritic kidney damage. Finally, kidney tissue from patients with Ab-driven nephropathy showed increased IDO1 abundance and stress gene expression. Thus, these findings support the hypothesis that the IDO-GCN2 pathway in glomerular stromal cells is a critical negative feedback mechanism that limits inflammatory renal pathologic changes by inducing autophagy.

DNMT1 is essential for mammary and cancer stem cell maintenance and tumorigenesis

Mammary stem/progenitor cells (MaSCs) maintain self-renewal of the mammary epithelium during puberty and pregnancy. DNA methylation provides a potential epigenetic mechanism for maintaining cellular memory during self-renewal. Although DNA methyltransferases (DNMTs) are dispensable for embryonic stem cell maintenance, their role in maintaining MaSCs and cancer stem cells (CSCs) in constantly replenishing mammary epithelium is unclear. Here we show that DNMT1 is indispensable for MaSC maintenance. Furthermore, we find that DNMT1 expression is elevated in mammary tumors, and mammary gland-specific DNMT1 deletion protects mice from mammary tumorigenesis by limiting the CSC pool. Through genome-scale methylation studies, we identify ISL1 as a direct DNMT1 target, hypermethylated and downregulated in mammary tumors and CSCs. DNMT inhibition or ISL1 expression in breast cancer cells limits CSC population. Altogether, our studies uncover an essential role for DNMT1 in MaSC and CSC maintenance and identify DNMT1-ISL1 axis as a potential therapeutic target for breast cancer treatment.

Regulation of the cholesterol efflux transporters ABCA1 and ABCG1 in retina in hemochromatosis and by the endogenous siderophore 2,5-dihydroxybenzoic acid

Hypercholesterolemia and polymorphisms in the cholesterol exporter ABCA1 are linked to age-related macular degeneration (AMD). Excessive iron in retina also has a link to AMD pathogenesis. Whether these findings mean a biological/molecular connection between iron and cholesterol is not known. Here we examined the relationship between retinal iron and cholesterol using a mouse model (Hfe(-/-)) of hemochromatosis, a genetic disorder of iron overload. We compared the expression of the cholesterol efflux transporters ABCA1 and ABCG1 and cholesterol content in wild type and Hfe(-/-) mouse retinas. We also investigated the expression of Bdh2, the rate-limiting enzyme in the synthesis of the endogenous siderophore 2,5-dihydroxybenzoic acid (2,5-DHBA) in wild type and Hfe(-/-) mouse retinas, and the influence of this siderophore on ABCA1/ABCG1 expression in retinal pigment epithelium. We found that ABCA1 and ABCG1 were expressed in all retinal cell types, and that their expression was decreased in Hfe(-/-) retina. This was accompanied with an increase in retinal cholesterol content. Bdh2 was also expressed in all retinal cell types, and its expression was decreased in hemochromatosis. In ARPE-19 cells, 2,5-DHBA increased ABCA1/ABCG1 expression and decreased cholesterol content. This was not due to depletion of free iron because 2,5-DHBA (a siderophore) and deferiprone (an iron chelator) had opposite effects on transferrin receptor expression and ferritin levels. We conclude that iron is a regulator of cholesterol homeostasis in retina and that removal of cholesterol from retinal cells is impaired in hemochromatosis. Since excessive cholesterol is pro-inflammatory, hemochromatosis might promote retinal inflammation via cholesterol in AMD.

L-2-oxothiazolidine-4-carboxylic acid attenuates oxidative stress and inflammation in retinal pigment epithelium

PURPOSE

Oxidant- and inflammation-induced damage to retinal pigment epithelial (RPE) cells is central to the pathogenesis of age-related macular degeneration (AMD). Thus, developing novel strategies to protect these cells is important. We reported previously on the robust antioxidant and therefore cell-protective effects of the cysteine pro-drug L-2-oxothiazolidine-4-carboxylic acid (OTC) in cultured human RPE cells. New reports citing a novel anti-inflammatory role for OTC in addition to the known glutathione-stimulating and antioxidant properties emerged recently; however, this role has not been evaluated in RPE cells or in intact retina. Given the crucial causative roles of oxidative stress and inflammation in AMD pathogenesis, knowing whether OTC might exhibit a similar benefit in this cell and tissue type has high clinical relevance; thus, we evaluated OTC in the present study.

METHODS

ARPE-19 and primary RPE cells isolated from wild-type, Gpr109a(-/-) , or Slc5a8(-/-) mouse eyes were exposed to TNF-α in the presence or absence of OTC, followed by analysis of IL-6 and Ccl2 expression with real-time quantitative polymerase chain reaction or enzyme-linked immunosorbent assay. Cellular and molecular markers of inflammation and oxidative stress (i.e., IL-1β, TGF-β, ABCG1, ABCA1, reduced glutathione, and dihydroethidium) were evaluated in Ccl2(-/-)/Cx3cr1(-/-) double knockout mice on rd8 background (DKO rd8) treated with OTC (10 mg/ml) in drinking water for a period of 5 months.

RESULTS

OTC treatment significantly inhibited the expression and secretion of IL-6 and Ccl2 in TNF-α-stimulated ARPE-19 cells. Studies conducted using DKO rd8 animals treated with OTC in drinking water confirmed these findings. Cellular and molecular markers of inflammation were significantly suppressed in the retinas of the OTC-treated DKO rd8 animals. Subsequent in vitro and in vivo studies of the possible mechanism(s) to explain these actions revealed that although OTC is an agonist of the anti-inflammatory G-protein coupled receptor GPR109A and a transportable substrate of the sodium-coupled monocarboxylate transporter SMCT1 (SLC5A8), these properties may play a role but do not explain entirely the anti-inflammatory effects this compound elicits in cultured RPE cells and the intact mouse retina.

CONCLUSIONS

This study represents, to our knowledge, the first report of the suppressive effects of OTC on inflammation in cultured RPE cells and on inflammation and oxidative stress in the retina in vivo.

Induction of the cystine/glutamate exchanger SLC7A11 in retinal pigment epithelial cells by the antipsoriatic drug monomethylfumarate

PURPOSE

Oxidative stress is a common pathological factor in degenerative retinal diseases; therefore, identifying novel strategies for its limitation is critically important and highly relevant clinically. Along these lines, our present goal was to evaluate the effect(s) of the fumarate ester and antipsoriatic agent monomethylfumarate (MMF) on the expression and functional activity of the cystine/glutamate exchanger SLC7A11 (system xc(-)), a transport system critical to potentiation of antioxidant signaling in retina.

METHODS

ARPE-19 and primary mouse RPE cells were cultured in the presence or absence of varying concentrations of MMF (0-5000 μM) for 0 to 24 hours. MMF (10 mM) was also delivered intravitreally to mouse eyes. RT-PCR, radiolabeled uptake, Western blotting, and glutathione (GSH) assays were then used to evaluate the effects of MMF on endogenous antioxidant machinery.

RESULTS

MMF induced system xc(-), Nrf2, and hypoxia-inducible factor 1α (Hif-1α) in cultured RPE cells. Additionally, the compound was recognized as a transportable substrate by the Na(+)-coupled monocarboxylate transporter SLC5A8 (SMCT1). In vivo these factors were evidenced by a significant increase in retinal levels of GSH.

CONCLUSIONS

MMF stimulates multiple pathways in retinal cells that potentiate cellular events leading to the upregulation of genes/mechanisms that function to protect retina against various forms of insult; upregulation of system xc(-) is one such consequence. To our knowledge, this is the first report that fumarate esters, compounds already employed clinically for other indications, are effective in retina via xc(-) induction. This novel, hitherto unknown mechanism helps to explain the antioxidant feature of these compounds and highlights their therapeutic potential in retina.

Loss of Hfe leads to progression of tumor phenotype in primary retinal pigment epithelial cells

PURPOSE

Hemochromatosis is a disorder of iron overload arising mostly from mutations in HFE. HFE is expressed in retinal pigment epithelium (RPE), and Hfe(-/-) mice develop age-related iron accumulation and retinal degeneration associated with RPE hyperproliferation. Here, the mechanism underlying the hyperproliferative phenotype in RPE was investigated.

METHODS

Cellular senescence was monitored by β-galactosidase activity. Gene expression was monitored by real-time PCR. Survivin was analyzed by Western blot and immunofluorescence. Migration and invasion were monitored using appropriate kits. Glucose transporters (GLUTs) were monitored by 3-O-methyl-D-glucose uptake. Histone deacetylases (HDACs) were studied by monitoring catalytic activity and acetylation status of histones H3/H4.

RESULTS

Hfe(-/-) RPE cells exhibited slower senescence rate and higher survivin expression than wild type cells. Hfe(-/-) cells migrated faster and showed greater glucose uptake and increased expression of GLUTs. The expression of HDACs and DNA methyltransferase (DNMTs) also was increased. Similarly, RPE cells from hemojuvelin (Hjv)-knockout mice, another model of hemochromatosis, also had increased expression of GLUTs, HDACs, and DNMTs. The expression of Slc5a8 was decreased in Hfe(-/-) RPE cells, but treatment with a DNA methylation inhibitor restored the transporter expression, indicating involvement of DNA methylation in the silencing of Slc5a8 in Hfe(-/-) cells.

CONCLUSIONS

RPE cells from iron-overloaded mice exhibit several features of tumor cells: decreased senescence, enhanced migration, increased glucose uptake, and elevated levels of HDACs and DNMTs. These features are seen in Hfe(-/-) RPE cells as well as in Hjv(-/-) RPE cells, providing a molecular basis for the hyperproliferative phenotype of Hfe(-/-) and Hjv(-/-) RPE cells.

GPR109A as an anti-inflammatory receptor in retinal pigment epithelial cells and its relevance to diabetic retinopathy

PURPOSE

Retinal pigment epithelium (RPE) expresses GPR109A, a receptor for the vitamin niacin and the ketone body β-hydroxybutyrate (β-HB). Because diabetes results in elevated levels of β-HB, here we studied expression of the receptor in diabetic retina. We also investigated its functional relevance in RPE.

METHODS

Retinal expression of GPR109A in diabetic mice and postmortem human eyes was evaluated by quantitative PCR (qPCR). ARPE-19 cells and primary wild-type and Gpr109a(-/-) mouse RPE cells were exposed to TNF-α in the presence or absence of niacin or β-HB, followed by analysis of IL-6 and Ccl2 expression via real-time qPCR and ELISA.

RESULTS

GPR109A expression was increased in diabetic mouse and human retina. TNF-α increased the expression and secretion of IL-6 and Ccl2 in ARPE-19 cells. Niacin and β-HB suppressed these effects, implicating GPR109A as the target responsible for mediation of the observed effects. Primary RPE cells from wild-type mice behaved similarly. In contrast, GPR109A ligands failed to suppress TNF-α-induced expression and secretion of IL-6 and Ccl2 in primary RPE cells from Gpr109a(-/-) mice, confirming that the observed anti-inflammatory effects were mediated specifically by Gpr109a.

CONCLUSIONS

GPR109A plays an anti-inflammatory role in RPE and its expression is upregulated in diabetes. Inflammation is a key causative factor in the pathogenesis of diabetic retinopathy. We speculate that the increased expression of GPR109A and elevation of its ligand β-HB in diabetes are mechanisms by which the tissue attempts to fight inflammation in this disease. Pharmacological activation of GPR109A may therefore have therapeutic potential in clinical management of diabetic retinopathy.

Abstract 3599: Expression and function of HFE in mammary gland iron homeostasis and its relevance to breast cancer

Emerging evidences on the role of iron-regulatory proteins in tumor growth provide new insights into the role of iron in cancer. Iron, though essential for normal cellular function, is toxic if present in excess. Hemochromatosis is an autosomal recessive disease of iron overload linked to mutations in HFE gene [Histocompatability leukocyte antigen class I-like protein involved in iron (FE) homeostasis]. HFE interacts with transferrin receptors, TfR1 and TfR2, and inhibits cellular iron uptake. The prevalence of the disease is very high with homozygosity incidence of ∼1 in 300 and heterozygosity incidence of ∼1 in 10. The disease manifests with liver cirrhosis/cancer, cardiomyopathy, diabetes, and arthritis. Several studies have shown increased prevalence of HFE mutations in breast cancer patients across all races. There is also evidence of increased levels of iron in breast cancer tissue. Although excessive iron and oxidative stress have been postulated to play a role in breast cancer, the expression/function of HFE in mammary gland iron homeostasis and how mutations in HFE affect breast cancer progression have not been studied directly. Here we demonstrate the expression of HFE mRNA in mouse mammary gland and primary mammary epithelial cells. The mRNA was absent in HFE-/- mice. Immunohistochemistry showed localization of HFE protein in luminal and myoepithelial cells. Similar staining was also seen in primary mammary epithelial cells from wild type mice but not from HFE-/- mice. We then examined the physiological role of HFE in mammary gland. Ferritin is a cytoplasmic protein involved in iron storage, and increase in intracellular iron levels results in increased level of ferritin. Recently, ferritin has also been shown to promote angiogenesis. Western blot and immunohistochemical analysis revealed that mammary tissue from HFE-/- mice had increased ferritin levels in an age-dependent manner. Thus, for the first time we show that HFE-/- mice have increased iron accumulation in the mammary gland. Immunostaining with antibody against 4-hydroxynonenal, one of the byproducts of lipid peroxidation, showed that HFE-/- mammary tissue have oxidative stress mediated by iron accumulation. MTT assay and thymidine incorporation showed that primary mammary epithelial cells from HFE-/- mice proliferate faster than the cells from HFE+/+ mice. Increase in histone deacetylase (HDAC) activity has been implicated in many cancers. We found that mammary tissue from HFE-/- mice has increased HDAC activity compared to wild type tissue. This shows that functional loss of HFE in mammary tissue can also promote cell cycle progression via epigenetic mechanisms. Based on these data, we hypothesize that tissue iron is an important modifier of breast cancer progression and that mutations in HFE (heterozygosity as well as homozygosity) may enhance the incidence and/or progression of breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3599. doi:1538-7445.AM2012-3599

Polarized distribution of heme transporters in retinal pigment epithelium and their regulation in the iron-overload disease hemochromatosis

PURPOSE

FLVCR, BCRP, and PCFT/HCP-1 represent the three heme transporters identified thus far in mammalian cells, but there is very little known about their expression and regulation in the retina. In this study, the expression of these transporters in mouse retina and retinal pigment epithelium (RPE) and their regulation in the iron-overload disease hemochromatosis were examined.

METHODS

The expression of FLVCR, BCRP, and PCFT in mouse retina and primary mouse RPE cells was studied by RT-PCR and immunofluorescence. Polarized localization of the transporters in RPE was studied by co-localization using a specific marker of the RPE apical membrane. Uptake of heme in primary RPE cells was determined using zinc-mesoporphyrin, a fluorescent heme analogue. The regulation of heme transporters by iron overload was studied in two genetic models of hemochromatosis (HFE-null mouse and HJV-null mouse) and in two nongenetic models of iron overload (cytomegalovirus infection and treatment with ferric ammonium citrate).

RESULTS

All three heme transporters were expressed in the retina and RPE. In the RPE, the expression of FLVCR was restricted to the apical membrane, and the expression of BCRP and PCFT was restricted to the basolateral membrane. In all cases of iron overload, the expression of FLVCR and PCFT was upregulated and that of BCRP was downregulated.

CONCLUSIONS

Hemochromatosis is associated not only with excessive accumulation of free iron in the retina and RPE but also with excessive accumulation of heme. Since heme is toxic at high levels, as is free iron, heme-induced oxidative damage may also play a role in hemochromatosis-associated retinal pathology.

Transport via SLC5A8 (SMCT1) is obligatory for 2-oxothiazolidine-4-carboxylate to enhance glutathione production in retinal pigment epithelial cells

PURPOSE

To evaluate the role of SLC5A8 in the transport of 2-oxothiazolidine-4-carboxylate (OTC) and to determine whether OTC augments glutathione production in RPE cells, thereby providing protection against oxidative stress.

METHODS

SLC5A8-mediated transport of OTC was monitored in Xenopus laevis oocytes by electrophysiological means. Saturation kinetics, Na(+)-activation kinetics, and inhibition by ibuprofen were analyzed by monitoring OTC-induced currents as a measure of transport activity. Oxidative stress was induced in ARPE-19 cells and primary RPE cells isolated from wild type and Slc5a8(-/-) mouse retinas using H(2)O(2), and the effects of OTC on cell death and intracellular glutathione concentration were examined.

RESULTS

Heterologous expression of human SLC5A8 in X. laevis oocytes induced Na(+)-dependent inward currents in the presence of OTC under voltage-clamp conditions. The transport of OTC via SLC5A8 was saturable, with a K(t) of 104 ± 3 μM. The Na(+)-activation kinetics was sigmoidal with a Hill coefficient of 1.9 ± 0.1, suggesting involvement of two Na(+) in the activation process. Ibuprofen, a blocker of SLC5A8, inhibited SLC5A8-mediated OTC transport; the concentration necessary for half-maximal inhibition was 17 ± 1 μM. OTC increased glutathione levels and protected ARPE-19 and primary RPE cells isolated from wild type mouse retinas from H(2)O(2)-induced cell death. These effects were abolished in primary RPE isolated from Slc5a8(-/-) mouse retinas.

CONCLUSIONS

OTC is a transportable substrate for SLC5A8. OTC augments glutathione production in RPE cells, thereby protecting them from oxidative damage. Transport via SLC5A8 is obligatory for this process.

Cystamine prevents haloperidol-induced decrease of BDNF/TrkB signaling in mouse frontal cortex

The role of brain-derived neurotrophic factor (BDNF) has been implicated in the pathophysiology as well as treatment outcome of schizophrenia. Rodent studies indicate that several antipsychotic drugs have time-dependent (and differential) effects on BDNF levels in the brain. Earlier studies from our laboratory have indicated that long-term treatment with haloperidol (HAL) decreases BDNF, reduced GSH and anti-apoptotic marker, Bcl-xl protein levels and increases the expression of pro-apoptotic proteins in rat frontal cortex. Furthermore, findings from human as well as rodent studies suggest that treatment of schizophrenia must involve the neuroprotective strategies to improve the neuropathology and thereby clinical outcome. In the present study, we investigated the potential of cystamine (CYS), an anti-oxidant and anti-apoptotic compound, to prevent HAL-induced reduction in BDNF, GSH, and Bcl-xl protein levels in mice and the signaling mechanism(s) involved in the beneficial effects of CYS. The results indicated that CYS as well as cysteamine (the FDA-approved precursor of CYS) increased BDNF protein levels in mouse frontal cortex 7 days after treatment. CYS co-treatment prevented chronic HAL treatment-induced reduction in BDNF, GSH, and Bcl-xl protein levels. CYS treatment enhanced TrkB-tyrosine phosphorylation and activated Akt and extracellular signal-regulated kinase (ERK)1/2, downstream molecules of TrkB signaling. In addition, in vitro experiments with mouse cortical neurons showed that CYS prevented the HAL-induced reduction in neuronal cell viability and BDNF protein levels, and increase in apoptosis. BDNF-neutralizing antibody as well as K252a, a selective inhibitor of neurotrophin signaling blocked the CYS-mediated neuroprotection. Moreover, CYS-mediated neuroprotection is also blocked by LY294002, a phosphatidylinositol 3-kinase inhibitor or PD98059, a mitogen-activated protein kinase kinase (MEK) inhibitor. Thus, CYS protects cortical neurons through a mechanism involving TrkB receptor activation, and a signaling pathway involving phosphatidylinositol 3-kinase and MAPK. The findings from the present study may be helpful for the development of novel neuroprotective strategies to improve the treatment outcome of schizophrenia.