Expression of Menkes disease gene in mammary carcinoma cells

Identification of immune characteristic biomarkers and therapeutic targets in cuproptosis for rheumatoid arthritis by integrated bioinformatics analysis and single-cell RNA sequencing analysis

Introduction

Rheumatoid arthritis (RA) is a chronic autoimmune disorder intricately liked with inflammation. Cuproptosis, an emerging type of cell death, has been implicated in the initiation and development of RA. However, the exact alterations in the expression and biological function of cuproptosis-related genes (CRGs) in RA remain poorly understood. Therefore, our study aims to elucidate the potential association between CRGs and RA, with the goal of identifying novel biomarkers for the treatment and prognosis of RA.

Methods

In this study, we identified ten differentially expressed cuproptosis-related genes (DE-CRGs) between patients with RA and controls. Through comprehensive functional enrichment and protein-protein interaction (PPI) network analysis, we explored the functional roles of the DE-CRGs. Additionally, we investigated the correlation between DE-CRGs and immune infiltration, immune factors, diagnostic efficacy, and potential therapeutic drugs.

Results

Leveraging single-cell RNA sequencing data, we conducted a detailed analysis to elucidate alterations in various cell clusters associated with RA. Our study unveiled a significant association between DE-CRGs and diverse biological functions, as well as potential drug candidates.

Discussion

These findings provide crucial insights into the involvement of DE-CRGs in the pathogenesis of RA and shed light on potential therapeutic strategies.

Roles of Copper Transport Systems Members in Breast Cancer

BACKGROUND

The occurrence and progression of breast cancer are closely linked to copper ion homeostasis. Both copper deficiency and excess can inhibit breast cancer growth, while copper transport systems may contribute to its progression by regulating copper ion transport and the activity of associated proteins. However, a comprehensive review of the roles and applications of copper transport systems in breast cancer remains limited. In this study, we summarize the workflow of copper transport systems and the dual role of copper in cancer, highlighting the contributions of specific members of the copper transport system to breast cancer.

METHODS

A comprehensive search of the PubMed database was conducted to identify articles published over the past 30 years that focus on the relationship between copper transport system members and breast cancer. The findings were synthesized to elucidate the roles and mechanisms of these transporters in the onset and progression of breast cancer.

RESULTS

We identified 13 members of the copper transport system associated with the occurrence, progression, and mortality of breast cancer, including SLC31A1, DMT1, ATP7A, ATP7B, MTs, GSH, ATOX1, CCS, COX17, SCO1, SCO2, and COX11. Our findings revealed that, apart from STEAP, the remaining 12 members were overexpressed in breast cancer. These members influence the onset, progression, and cell death of breast cancer by modulating biological pathways such as intracellular copper ion levels and ROS. Notably, we observed for the first time that depletion of the copper storage protein GSH leads to increased copper ion accumulation, resulting in cuproptosis in breast cancer cells.

CONCLUSION

By integrating the members of the copper transport system in breast cancer, we offer novel insights for the treatment of breast cancer and copper-related therapies.

What can flies tell us about copper homeostasis?

Copper (Cu) is an essential redox active metal that is potentially toxic in excess. Multicellular organisms acquire Cu from the diet and must regulate uptake, storage, distribution and export of Cu at both the cellular and organismal levels. Systemic Cu deficiency can be fatal, as seen in Menkes disease patients. Conversely Cu toxicity occurs in patients with Wilson disease. Cu dyshomeostasis has also been implicated in neurodegenerative disorders such as Alzheimer's disease. Over the last decade, the fly Drosophila melanogaster has become an important model organism for the elucidation of eukaryotic Cu regulatory mechanisms. Gene discovery approaches with Drosophila have identified novel genes with conserved protein functions relevant to Cu homeostasis in humans. This review focuses on our current understanding of Cu uptake, distribution and export in Drosophila and the implications for mammals.

Copper and lactational hormones influence the CTR1 copper transporter in PMC42-LA mammary epithelial cell culture models

Adequate amounts of copper in milk are critical for normal neonatal development, however the mechanisms regulating copper supply to milk have not been clearly defined. PMC42-LA cell cultures representative of resting, lactating and suckled mammary epithelia were used to investigate the regulation of the copper uptake protein, CTR1. Both the degree of mammary epithelial differentiation (functionality) and extracellular copper concentration greatly impacted upon CTR1 expression and its plasma membrane association. In all three models (resting, lactating and suckling) there was an inverse correlation between extracellular copper concentration and the level of CTR1. Cell surface biotinylation studies demonstrated that as extracellular copper concentration increased membrane associated CTR1 was reduced. There was a significant increase in CTR1 expression (total and membrane associated) in the suckled gland model in comparison to the resting gland model, across all copper concentrations investigated (0-50 μM). Regulation of CTR1 expression was entirely post-translational, as quantitative real-time PCR analyses showed no change to CTR1 mRNA between all models and culture conditions. X-ray fluorescence microscopy on the differentiated PMC42-LA models revealed that organoid structures distinctively accumulated copper. Furthermore, as PMC42-LA cell cultures became progressively more specialised, successively more copper accumulated in organoids (resting<lactating<suckling), indicating a link between function and copper requirement. Based on previous data showing a function for CTR1 in copper uptake, we have concluded that under the influence of hormones and increased extracellular copper levels, CTR1 participates in uptake of copper by mammary epithelial cells, as a prerequisite for secretion of copper into milk.

Role of the P-Type ATPases, ATP7A and ATP7B in brain copper homeostasis

Over the past two decades there have been significant advances in our understanding of copper homeostasis and the pathological consequences of copper dysregulation. Cumulative evidence is revealing a complex regulatory network of proteins and pathways that maintain copper homeostasis. The recognition of copper dysregulation as a key pathological feature in prominent neurodegenerative disorders such as Alzheimer's, Parkinson's, and prion diseases has led to increased research focus on the mechanisms controlling copper homeostasis in the brain. The copper-transporting P-type ATPases (copper-ATPases), ATP7A and ATP7B, are critical components of the copper regulatory network. Our understanding of the biochemistry and cell biology of these complex proteins has grown significantly since their discovery in 1993. They are large polytopic transmembrane proteins with six copper-binding motifs within the cytoplasmic N-terminal domain, eight transmembrane domains, and highly conserved catalytic domains. These proteins catalyze ATP-dependent copper transport across cell membranes for the metallation of many essential cuproenzymes, as well as for the removal of excess cellular copper to prevent copper toxicity. A key functional aspect of these copper transporters is their copper-responsive trafficking between the trans-Golgi network and the cell periphery. ATP7A- and ATP7B-deficiency, due to genetic mutation, underlie the inherited copper transport disorders, Menkes and Wilson diseases, respectively. Their importance in maintaining brain copper homeostasis is underscored by the severe neuropathological deficits in these disorders. Herein we will review and update our current knowledge of these copper transporters in the brain and the central nervous system, their distribution and regulation, their role in normal brain copper homeostasis, and how their absence or dysfunction contributes to disturbances in copper homeostasis and neurodegeneration.

Multiple Menkes copper ATPase (Atp7a) transcript and protein variants are induced by iron deficiency in rat duodenal enterocytes

The Menkes copper ATPase (Atp7a) pumps copper into the trans-Golgi for cuproenzyme synthesis, and translocates to the basolateral membrane of enterocytes for copper export. Recent studies demonstrated that three 5' end splice variants of the Atp7a transcript exist in rat duodenum, all of which are strongly induced during iron deprivation. To explore a possible role for Atp7a (and copper) in intestinal iron absorption, the current studies were undertaken to test the hypothesis that multiple Atp7a transcript and protein variants exist in intestinal epithelial cells. Northern blot analyses using probes generated from the full-length Atp7a cDNA revealed several specific hybridization bands, all of which were more intense in RNA samples extracted from duodenal enterocytes isolated from iron-deficient rats. A PCR-based approach, using forward primers specific for the alternative 5' end splice variants and a reverse primer in exon 23, demonstrated that 3 full-length transcripts exist in rat IEC-6 cells. To identify possible Atp7a protein variants, three distinct polyclonal antisera were utilized. The specificity of the antisera was first established by western blotting and immunoprecipitation studies using samples derived from isolated rat enterocytes and Atp7a knockdown IEC-6 cells. Several specific immunoreactive bands were documented, and a unique Atp7a protein distribution in cytosolic vesicle-like structures was noted. In conclusion, multiple Atp7a transcript and protein variants exist in rodent intestinal epithelial cells and are induced by dietary iron deprivation. Further studies will be designed to determine the subcellular distribution of Atp7a protein variants and possible unique functions of each.

The multi-layered regulation of copper translocating P-type ATPases

The copper-translocating Menkes (ATP7A, MNK protein) and Wilson (ATP7B, WND protein) P-type ATPases are pivotal for copper (Cu) homeostasis, functioning in the biosynthetic incorporation of Cu into copper-dependent enzymes of the secretory pathway, Cu detoxification via Cu efflux, and specialized roles such as systemic Cu absorption (MNK) and Cu excretion (WND). Essential to these functions is their Cu and hormone-responsive distribution between the trans-Golgi network (TGN) and exocytic vesicles located at or proximal to the apical (WND) or basolateral (MNK) cell surface. Intriguingly, MNK and WND Cu-ATPases expressed in the same tissues perform distinct yet complementary roles. While intramolecular differences may specify their distinct roles, cellular signaling components are predicted to be critical for both differences and synergy between these enzymes. This review focuses on these mechanisms, including the cell signaling pathways that influence trafficking and bi-functionality of Cu-ATPases. Phosphorylation events are hypothesized to play a central role in Cu homeostasis, promoting multi-layered regulation and cross-talk between cuproenzymes and Cu-independent mechanisms.

Wilson protein expression, copper excretion and sweat production in sweat glands of Wilson disease patients and controls

Background

In Wilson disease, copper is not sufficiently excreted into bile due to the absence or malfunction of the Wilson protein copper ATPase in the excretory pathway of hepatocytes. Copper is found in sweat. It is unknown if the Wilson protein plays a role in copper excretion into sweat. It is the aim of this study to investigate Wilson protein expression in sweat glands and analysing its effects on copper excretion into sweat in controls and patients with Wilson disease.

Methods

Immunofluorescent analysis of the Wilson protein in skin samples from normal rat, LEC rat and human skin biopsies were performed. Pilocarpin-induced sweat gland stimulation by iontophoretic transfer adapted from the methods used for cystic fibrosis sweat test was used for sweat induction. Sweat volume, sweat copper concentration, serum ceruloplasmin and serum copper were analysed in 28 Wilson patients and 21 controls.

Results

The Wilson protein is expressed in human and rat sweat gland epithelia. Copper concentration in sweat is not significantly different between controls and Wilson patients. Wilson patients produce significantly smaller volumes of sweat compared to controls. Sweat production is partially reversible in Wilson patients under medical treatment for Wilson disease or after liver transplantation

Conclusion

Wilson patients show a reduced sweat production with unaltered sweat copper concentration. The Wilson protein might play an important role in physiological sweat production.

Function and regulation of human copper-transporting ATPases

Copper-transporting ATPases (Cu-ATPases) ATP7A and ATP7B are evolutionarily conserved polytopic membrane proteins with essential roles in human physiology. The Cu-ATPases are expressed in most tissues, and their transport activity is crucial for central nervous system development, liver function, connective tissue formation, and many other physiological processes. The loss of ATP7A or ATP7B function is associated with severe metabolic disorders, Menkes disease, and Wilson disease. In cells, the Cu-ATPases maintain intracellular copper concentration by transporting copper from the cytosol across cellular membranes. They also contribute to protein biosynthesis by delivering copper into the lumen of the secretory pathway where metal ion is incorporated into copper-dependent enzymes. The biosynthetic and homeostatic functions of Cu-ATPases are performed in different cell compartments; targeting to these compartments and the functional activity of Cu-ATPase are both regulated by copper. In recent years, significant progress has been made in understanding the structure, function, and regulation of these essential transporters. These studies raised many new questions related to specific physiological roles of Cu-ATPases in various tissues and complex mechanisms that control the Cu-ATPase function. This review summarizes current data on the structural organization and functional properties of ATP7A and ATP7B as well as their localization and functions in various tissues, and discusses the current models of regulated trafficking of human Cu-ATPases.

Trafficking of the copper-ATPases, ATP7A and ATP7B: Role in copper homeostasis

Copper is essential for human health and copper imbalance is a key factor in the aetiology and pathology of several neurodegenerative diseases. The copper-transporting P-type ATPases, ATP7A and ATP7B are key molecules required for the regulation and maintenance of mammalian copper homeostasis. Their absence or malfunction leads to the genetically inherited disorders, Menkes and Wilson diseases, respectively. These proteins have a dual role in cells, namely to provide copper to essential cuproenzymes and to mediate the excretion of excess intracellular copper. A unique feature of ATP7A and ATP7B that is integral to these functions is their ability to sense and respond to intracellular copper levels, the latter manifested through their copper-regulated trafficking from the transGolgi network to the appropriate cellular membrane domain (basolateral or apical, respectively) to eliminate excess copper from the cell. Research over the last decade has yielded significant insight into the enzymatic properties and cell biology of the copper-ATPases. With recent advances in elucidating their localization and trafficking in human and animal tissues in response to physiological stimuli, we are progressing rapidly towards an integrated understanding of their physiological significance at the level of the whole animal. This knowledge in turn is helping to clarify the biochemical and cellular basis not only for the phenotypes conferred by individual Menkes and Wilson disease patient mutations, but also for the clinical variability of phenotypes associated with each of these diseases. Importantly, this information is also providing a rational basis for the applicability and appropriateness of certain diagnostic markers and therapeutic regimes. This overview will provide an update on the current state of our understanding of the localization and trafficking properties of the copper-ATPases in cells and tissues, the molecular signals and posttranslational interactions that govern their trafficking activities, and the cellular basis for the clinical phenotypes associated with disease-causing mutations.

Mammary gland copper transport is stimulated by prolactin through alterations in Ctr1 and Atp7A localization

Milk copper (Cu) concentration declines and directly reflects the stage of lactation. Three Cu-specific transporters (Ctr1, Atp7A, Atp7B) have been identified in the mammary gland; however, the integrated role they play in milk Cu secretion is not understood. Whereas the regulation of milk composition by the lactogenic hormone prolactin (PRL) has been documented, the specific contribution of PRL to this process is largely unknown. Using the lactating rat as a model, we determined that the normal decline in milk Cu concentration parallels declining Cu availability to the mammary gland and is associated with decreased Atp7B protein levels. Mammary gland Cu transport was highest during early lactation and was stimulated by suckling and hyperprolactinemia, which was associated with Ctr1 and Atp7A localization at the plasma membrane. Using cultured mammary epithelial cells (HC11), we demonstrated that Ctr1 stains in association with intracellular vesicles that partially colocalize with transferrin receptor (recycling endosome marker). Atp7A was primarily colocalized with mannose 6-phosphate receptor (M6PR; late endosome marker), whereas Atp7B was partially colocalized with protein disulfide isomerase (endoplasmic reticulum marker), TGN38 ( trans-Golgi network marker) and M6PR. Prolactin stimulated Cu transport as a result of increased Ctr1 and Atp7A abundance at the plasma membrane. Although the molecular mechanisms responsible for these posttranslational changes are not understood, transient changes in prolactin signaling play a role in the regulation of mammary gland Cu secretion during lactation.

Expression, Localisation and Hormone Regulation of the Human Copper Transporter hCTR1 in Placenta and Choriocarcinoma Jeg-3 Cells

Copper is an essential trace element necessary for normal growth and development. During pregnancy, copper is transported from the maternal circulation to the fetus by mechanisms which have not been clearly elucidated. The copper uptake protein, hCTR1 is predicted to play a role in copper transport in human placental cells. This study has examined the expression and localisation of hCTR1 in human placental tissue and Jeg-3 cells. In term placental tissue the hCTR1 protein was detected as a 105 kDa protein, consistent with the size of a trimer which may represent the functional protein. A 95 kDa band, possibly representing the glycosylated protein, was also detected. hCTR1 was localised within the syncytiotrophoblast layer and the fetal vascular endothelial cells in the placental villi and interestingly was found to be localised toward the basal plasma membrane. It did not co-localise with either the Menkes or the Wilson copper transporting ATPases. Using the placental cell line Jeg-3, it was shown that the 35 kDa monomer was absent in the extracts of cells exposed to insulin, estrogen or progesterone and in cells treated with estrogen an additional 65 kDa band was detected which may correspond to a dimeric form of the protein. The 95 kDa band was not detected in the cultured cells. These results provide novel insights indicating that hormones have a role in the formation of the active hCTR1 protein. Furthermore, insulin altered the intracellular localisation of hCTR1, suggesting a previously undescribed role of this hormone in regulating copper uptake through the endocytic pathway.

ATP7B mediates vesicular sequestration of copper: insight into biliary copper excretion

BACKGROUND & AIMS

The Wilson protein (ATP7B) regulates levels of systemic copper by excreting excess copper into bile. It is not clear whether ATP7B translocates excess intrahepatic copper directly across the canalicular membrane or sequesters this copper into exocytic vesicles, which subsequently fuse with canalicular membrane to expel their contents into bile. The aim of this study was to clarify the mechanism underlying ATP7B-mediated copper detoxification by investigating endogenous ATP7B localization in the HepG2 hepatoma cell line and its ability to mediate vesicular sequestration of excess intracellular copper.

METHODS

Immunofluorescence microscopy was used to investigate the effect of copper concentration on the localization of endogenous ATP7B in HepG2 cells. Copper accumulation studies to determine whether ATP7B can mediate vesicular sequestration of excess intracellular copper were performed using Chinese hamster ovary cells that exogenously expressed wild-type and mutant ATP7B proteins.

RESULTS

In HepG2 cells, elevated copper levels stimulated trafficking of ATP7B to pericanalicular vesicles and not to the canalicular membrane as previously reported. Mutation of an endocytic retrieval signal in ATP7B caused the protein to constitutively localize to vesicles and not to the plasma membrane, suggesting that a vesicular compartment(s) is the final trafficking destination for ATP7B. Expression of wild-type and mutant ATP7B caused Chinese hamster ovary cells to accumulate copper in vesicles, which subsequently undergo exocytosis, releasing copper across the plasma membrane.

CONCLUSIONS

This report provides compelling evidence that the primary mechanism of biliary copper excretion involves ATP7B-mediated vesicular sequestration of copper rather than direct copper translocation across the canalicular membrane.

Expression and Localization of Menkes and Wilson Copper Transporting ATPases in Human Placenta

Copper is an essential trace element necessary for normal growth and development. During pregnancy, copper is transported from the maternal circulation to the fetus by mechanisms which have not been clearly elucidated. Two copper transporting ATPases, Menkes (ATP7A; MNK) and Wilson (ATP7B; WND) are known to be expressed in the placenta and are thought to have a role in copper transport to the fetus. In this study, the expression and localization of the MNK and WND proteins in the human placenta were investigated in detail using immunoperoxidase and double-label immunohistochemistry. MNK and WND are differentially localized within the placenta. MNK is present in the syncytiotrophoblast, the cytotrophoblast and the fetal vascular endothelial cells whereas WND is only in the syncytiotrophoblast. Placental levels of both proteins, measured by Western blot analysis, did not change across pregnancy. These data offer some insights into possible roles for MNK and WND within the placenta.

Marginal maternal Zn intake in rats alters mammary gland Cu transporter levels and milk Cu concentration and affects neonatal Cu metabolism

Marginal zinc intake is common and leaves women particularly vulnerable to Zn deficiency due to increased demand for Zn as a consequence of reproduction. Zn deficiency during pregnancy and lactation has been associated with secondary affects on copper metabolism in the offspring; however, the underlying mechanisms are unknown. The effects of marginal maternal Zn intake on maternal and neonatal Cu metabolism were determined in rats. Plasma, milk and tissue Cu and Zn concentrations and plasma and milk ceruloplasmin (Cp) activity were measured in dams fed a control (CON, 25 mg Zn/kg diet) or a marginal Zn diet (ZD, 10 mg Zn/kg diet) and their suckling pups. There was no effect on maternal tissue Cu or Zn or milk Zn concentration; however, plasma Cp activity was higher in dams fed ZD, suggesting that Cp activity may be a useful marker for identifying marginal Zn status. Rats fed ZD had high mammary gland Ctr1, Atp7A and Atp7B levels, milk Cp activity and Cu concentration. Immunostaining and differential centrifugation indicated that ZD also altered Ctr1 and Atp7A localization in the mammary gland. Pups from dams fed ZD had higher small intestine Cu and lower plasma Cu than CON pups. These results suggest that marginal maternal Zn intake during pregnancy and lactation increase mammary gland Cu transporter levels and alter their localization, resulting in high milk Cu levels, possibly in response to transiently elevated plasma Cu levels. The combination of high milk Cu concentration and immature neonatal Cu transport exposes the suckling neonate to excess Cu; however, whether this occurs in humans is not yet known.

Molecular and cellular aspects of copper transport in developing mammals

Copper is an essential trace element that requires tightly regulated homeostatic mechanisms to ensure adequate supplies without any toxic effects because of the ability of the metal ion to catalyze the formation of free radicals. The Cu-ATPases, ATP7A and ATP7B, play an important role in the physiological regulation of copper. Adequate supplies of copper are particularly important in developing animals, and in humans this is illustrated by mutations of ATP7A that cause the copper deficiency condition Menkes disease, which is fatal in early childhood. In contrast, mutations in ATP7B result in the genetic toxicosis, Wilson disease. We propose that the physiological regulation of copper is accomplished mainly by the intracellular copper-regulated trafficking of the Cu-ATPases. This process allows the overall copper status in the body to be maintained when levels of copper in the diet alter. A study of the defects in mouse models of Menkes and Wilson diseases has demonstrated that both ATPases play an important role in supplying copper to the developing fetus and neonate.

Copper transport to mammary gland and milk during lactation in rats

The delivery of copper to mammary gland and milk and the effects of lactation were examined in rats. Traces of (67)Cu/(64)Cu(II) were injected intraperitoneally or intravenously into virgin rats or lactating rats (2-5 days postpartum), and incorporation into blood, milk, and tissues was monitored. In virgin rats, most of the isotope first entered the liver and kidney. In lactating rats, almost 60% went directly to the mammary gland. Uptake rates and copper contents of the mammary gland were 20-fold higher in lactation. (67)Cu/(64)Cu appeared in milk and milk ceruloplasmin as rapidly as in mammary tissue and when there was no (67)Cu/(64)Cu-ceruloplasmin in the maternal plasma. Plasma (125)I-labeled albumin entered milk much more slowly. Milk ceruloplasmin (10 mg/l) had 25% of the (67)Cu/(64)Cu. Milk copper was 3.3 mg/l. Thus lactation markedly enhances the avidity of the mammary gland for copper, diverting most of it from liver and kidney to that tissue. Also, the primary source of milk ceruloplasmin is the mammary gland and not the maternal plasma.

The reproductive importance of P-type ATPases

P-type ATPases are integral membrane proteins that use the free energy of ATP hydrolysis to generate transmembrane electrochemical ion gradients to support a variety of cellular processes. They have eight signature motifs, eight or ten transmembrane domains, highly conserved phosphorylation and ATP-binding sites, and similar hydropathic profiles. This review summarizes recent insights in the relationship of P-type ATPases to successful reproduction, and the hormone dependence of some family members. Because protein topology is central to understanding the pump action of this family of enzymes, this review also describes the dramatic change in the primary structure of one family member that may mediate transcription in the uterus.

Copper in disorders with neurological symptoms: Alzheimer's, Menkes, and Wilson diseases

Copper is an essential element for the activity of a number of physiologically important enzymes. Enzyme-related malfunctions may contribute to severe neurological symptoms and neurological diseases: copper is a component of cytochrome c oxidase, which catalyzes the reduction of oxygen to water, the essential step in cellular respiration. Copper is a cofactor of Cu/Zn-superoxide-dismutase which plays a key role in the cellular response to oxidative stress by scavenging reactive oxygen species. Furthermore, copper is a constituent of dopamine-beta-hydroxylase, a critical enzyme in the catecholamine biosynthetic pathway. A detailed exploration of the biological importance and functional properties of proteins associated with neurological symptoms will have an important impact on understanding disease mechanisms and may accelerate development and testing of new therapeutic approaches. Copper binding proteins play important roles in the establishment and maintenance of metal-ion homeostasis, in deficiency disorders with neurological symptoms (Menkes disease, Wilson disease) and in neurodegenerative diseases (Alzheimer's disease). The Menkes and Wilson proteins have been characterized as copper transporters and the amyloid precursor protein (APP) of Alzheimer's disease has been proposed to work as a Cu(II) and/or Zn(II) transporter. Experimental, clinical and epidemiological observations in neurodegenerative disorders like Alzheimer's disease and in the genetically inherited copper-dependent disorders Menkes and Wilson disease are summarized. This could provide a rationale for a link between severely dysregulated metal-ion homeostasis and the selective neuronal pathology.

Uteroglobin gene transcription: what's the RUSH?

Prolactin enhances progesterone-dependent transcription of the rabbit uteroglobin gene. RUSH transcription factors are implicated in the signal transduction pathway. The RUSH acronym identifies key features of these nuclear phosphoproteins, that is, RING-finger motif, binds the uteroglobin promoter, structurally related to the SWI/SNF family of transcription factors, and helicase-like. Cloned by recognition site screening, RUSH proteins bind to an 85-bp region (-170/-85) of the uteroglobin promoter that was subsequently identified as a novel prolactin-responsive region by promoter deletion analysis. Gel shift and linker-scanning assays further reduced the RUSH target site to -160/-110. A hexameric core of MCWTDK was identified as the RUSH-specific DNA-binding site (-126/-121) by CASTing. This site overlaps authentic HNF3 beta and OCT-1 binding sites. A unique Type IV P-type ATPase that is embedded in the inner nuclear membrane binds the RING domain of RUSH. The conformationally flexible loop portion of this RING-finger binding protein (RFBP) extends into the nucleoplasm to contact euchromatin. The physical association of RFBP with transcriptionally active chromatin supports the speculation that RFBP targets RUSH transcription factors to the active uteroglobin promoter.

Cloning and characterization of an atypical Type IV P-type ATPase that binds to the RING motif of RUSH transcription factors

RUSH proteins are SWI/SNF-related transcription factors with RING finger signatures near their COOH termini. Long suspected of mediating protein-protein interactions, the RING motif was used to clone a binding partner. The RING finger binding protein (RFBP) is a Type IV P-type ATPase, a putative phospholipid pump, with conserved sequences for two loop segments, an ATP-binding site, a phosphorylation domain, and transmembrane passes potentially involved in substrate binding and translocation. However, RFBP differs from all other Type IV P-type ATPases in three ways. It has only three of four highly conserved NH(2)-terminal transmembrane passes, it is located in the inner nuclear membrane, and it binds the RING domain. Topographically the orientation of the adjacent hydrophilic domains and the determinants of transport specificity are altered. As a result, the small, hydrophilic loop extends into the perinuclear space that is contiguous with the lumen of the endoplasmic reticulum. The large, conformationally flexible loop extends into the nucleoplasm to contact euchromatin. Competitive reverse transcriptase-polymerase chain reaction and high performance liquid chromatography analysis revealed that endometrial RFBP mRNA expression is hormonally regulated. The physical association of a hormone-dependent RING finger-binding protein with transcriptionally active chromatin supports the speculation that RFBP plays a role in the subnuclear trafficking of transcription factors with RING motifs.

Defective localization of the Wilson disease protein (ATP7B) in the mammary gland of the toxic milk mouse and the effects of copper supplementation

Toxic milk (tx) is a copper disorder of mice that causes a hepatic accumulation of copper similar to that seen in patients with Wilson disease. Both disorders are caused by a defect in the ATP7B copper-transporting ATPase. A feature of the tx phenotype is the production of copper-deficient milk by lactating dams homozygous for the tx mutation; the milk is lethal to the pups. It has not been determined whether the production of copper-deficient milk is a direct consequence of impaired expression of ATP7B protein in the mammary gland. With the use of immunohistochemistry, our study demonstrated that the ATP7B protein was mislocalized in the lactating tx mouse mammary gland, which would explain the inability of the tx mouse to secrete normal amounts of copper in milk. Confocal microscopy analysis showed that, in the lactating tx mammary gland, ATP7B was predominantly perinuclear in comparison with the diffuse, cytoplasmic localization of ATP7B in the lactating normal mammary gland. Lactating tx mice showed impaired delivery of copper from the mammary gland to the milk and this was not ameliorated by dietary copper supplementation. In contrast, the normal mouse mammary gland responded to increased dietary copper by increasing the amount of copper in milk. A change in the distribution of the ATP7B protein from perinuclear in the non-lactating gland to a diffuse, cytoplasmic localization in the lactating gland of the normal (DL) mouse suggests that the relocalization of APT7B is a physiological process that accompanies lactation. We conclude that the impaired copper transport from the mammary gland into milk in lactating tx mice is related to the mislocalization of ATP7B.

Metallothionein isoform expression by breast cancer cells

Expression of metallothionein (MT) isoforms by a human breast cancer cell line, PMC42, which retains many characteristics of normal breast epithelial cells and expresses functional estrogen receptors, was examined because it has been proposed that human breast cancer cells which are estrogen receptor positive can be differentiated from those which are estrogen receptor negative, by failure to express MT-1E [J.A. Friedline, S.H. Garrett, S. Somji, J.H. Todd, D. A. Sens, Differential expression of the MT-1E gene in estrogen-receptor positive and -negative breast cancer cell lines, Am. J. Pathol. 152 (1998) 23-27]. Using RT-PCR, PMC42 cells were found to transcribe genes for the MT isoforms IE, IX and 2A but not 1A or 1H. In order to examine which of the expressed isoforms might protect against metal toxicity, the cells were challenged with high concentrations of zinc and copper. Using competitive RT-PCR, cells resistant to 500 microM zinc showed 7+/-2 fold (SD, n=3) increases in expression of MT-1X and 6+/-3 fold increases in expression of MT-2A compared to control cells in normal media. For cells resistant to 250 microM copper the corresponding increases were 37+/-13 and 60+/-20 fold, whilst for control cells treated with 250 microM copper for only 6 h, increases were 10+/-3 and 6+/-3 fold. There was only a low level of expression of MT-1E in untreated cells and but a >120 fold increase in copper- resistant cells. Thus estrogen receptor positive cells cannot, in general, be differentiated from estrogen receptor negative cells by failure to express MT-1E, as suggested by Friedline et al. (1998). Increased expression of MT-1E, as well as MT-1X and MT-2A, protects against metal toxicity in PMC42 breast cancer cells.

Expression of menkes copper-transporting ATPase, MNK, in the lactating human breast: possible role in copper transport into milk

The Menkes copper ATPase (MNK) is a copper efflux ATPase that is involved in copper homeostasis. Little is known about the intracellular localization and cell-specific function of the MNK in human tissues. To investigate a possible role for this protein in lactation, we measured its expression in sections of tissue from nonlactating and lactating human breast. Western blot analysis showed that MNK expression was greater in lactating tissue than in nonlactating tissue. By confocal immunofluorescence, the MNK was detected in luminal epithelial cells of the alveoli and ducts but not in myoepithelial cells. In the nonlactating breast epithelial cells, the MNK had a predominantly perinuclear distribution. In lactating breast tissue, the distribution of the MNK was markedly altered. Lactating epithelial cells showed a granular, diffuse pattern, which extended beyond the perinuclear region of the cell. This pattern was similar to that observed in a previous study in which cultured CHO cells were exposed to high copper concentrations. Our results suggest that relocalization of the MNK is a physiological process, which may be mediated by copper levels in the breast or by hormones and other events taking place during lactation. A vesicular pathway for copper from the Golgi into milk, similar to that of calcium, is proposed.(J Histochem Cytochem 47:1553-1561, 1999)

Null mutation of the murine ATP7B (Wilson disease) gene results in intracellular copper accumulation and late-onset hepatic nodular transformation

The Atp7b protein is a copper-transporting ATPase expressed predominantly in the liver and to a lesser extent in most other tissues. Mutations in the ATP7B gene lead to Wilson disease, a copper toxicity disorder characterized by dramatic build-up of intracellular hepatic copper with subsequent hepatic and neuro-logical abnormalities. Using homologous recombination to disrupt the normal translation of ATP7B, we have generated a strain of mice that are homozygous mutants (null) for the Wilson disease gene. The ATP7B null mice display a gradual accumulation of hepatic copper that increases to a level 60-fold greater than normal by 5 months of age. An increase in copper concentration was also observed in the kidney, brain, placenta and lactating mammary glands of homo-zygous mutants, although milk from the mutant glands was copper deficient. Morphological abnormalities resembling cirrhosis developed in the majority of the livers from homozygous mutants older than 7 months of age. Progeny of the homozygous mutant females demonstrated neurological abnormalities and growth retardation characteristic of copper deficiency. Copper concentration in the livers of the newborn homozygous null mutants was decreased dramatically. In summary, inactivation of the murine ATP7B gene produces a form of cirrhotic liver disease that resembles Wilson disease in humans and the 'toxic milk' phenotype in the mouse.

Defective copper-induced trafficking and localization of the Menkes protein in patients with mild and copper-treated classical Menkes disease

Menkes disease is an X-linked disorder of copper metabolism. An overall copper deficiency reduces the activity of copper-dependent enzymes accounting for the clinical presentation of affected individuals. The Menkes gene product (MNK) is a P-type ATPase and is considered to be the main copper efflux protein in most cells. The protein is located primarily at the trans -Golgi network (TGN), but relocalizes to the plasma membrane in elevated copper conditions to expel the excess copper from the cell. Here we report the first missense mutation which causes mild Menkes disease, a mutation in a successfully copper-treated classical Menkes patient and the effect of each mutation on the localization of MNK within the cell. Using western blot analysis, MNK was detectable in cells from both patients, but appeared to be mislocalized in the treated case. In the mild Menkes patient, the protein appeared to be located in the TGN but failed to redistribute towards the cell periphery in response to copper. This is the first description of a mutation in a Menkes patient which affects the trafficking of MNK, and the loss of this process is consistent with the clinical phenotype.

Identification of a di-leucine motif within the C terminus domain of the Menkes disease protein that mediates endocytosis from the plasma membrane

The protein encoded by the Menkes disease gene (MNK) is localised to the Golgi apparatus and cycles between the trans-Golgi network and the plasma membrane in cultured cells on addition and removal of copper to the growth medium. This suggests that MNK protein contains active signals that are involved in the retention of the protein to the trans-Golgi network and retrieval of the protein from the plasma membrane. Previous studies have identified a signal involved in Golgi retention within transmembrane domain 3 of MNK. To identify a motif sufficient for retrieval of MNK from the plasma membrane, we analysed the cytoplasmic domain, downstream of transmembrane domain 7 and 8. Chimeric constructs containing this cytoplasmic domain fused to the reporter molecule CD8 localised the retrieval signal(s) to 62 amino acids at the C terminus. Further studies were performed on putative internalisation motifs, using site-directed mutagenesis, protein expression, chemical treatment and immunofluorescence. We observed that a di-leucine motif (L1487L1488) was essential for rapid internalisation of chimeric CD8 proteins and the full-length Menkes cDNA from the plasma membrane. We suggest that this motif mediates the retrieval of MNK from the plasma membrane into the endocytic pathway, via the recycling endosomes, but is not sufficient on its own to return the protein to the Golgi apparatus. These studies provide a basis with which to identify other motifs important in the sorting and delivery of MNK from the plasma membrane to the Golgi apparatus.