0216 Is There a Daily Rhythm in Alcohol Craving and Does It Vary by Circadian Timing?

Introduction

People with later circadian timing tend to consume more alcohol, potentially due to altered rhythms in when and how much they crave alcohol throughout the day. However, whether circadian factors play a role in alcohol craving has received scant attention. Here, we investigated if the daily rhythm of alcohol craving varied by circadian timing in two independent studies of late adolescent and young adult drinkers.

Methods

In Study 1, 32 participants (18–22 years of age; 61% female; 69% White) completed momentary reports of alcohol craving five times a day for 14 days. Participants wore wrist actigraphs and completed two in-lab assessments of dim light melatonin onset (DLMO). Average actigraphically-assessed midpoint of sleep on weekends and average DLMO were used as indicators of circadian timing. In Study 2, 231 participants (21–35 years of age; 28% female; 71% White) completed momentary reports of alcohol craving six times a day for 10 days. Average midpoint of self-reported time-in-bed on weekends was used to estimate circadian timing.

Results

Multilevel cosinor analysis revealed a 24-hour daily rhythm in alcohol craving which was moderated by circadian timing in both studies (p’s<0.05). In both Study 1 and 2, people with later circadian timing had a later timed peak of craving. In Study 1, but not Study 2, later circadian timing predicted a blunted amplitude in craving.

Conclusion

Findings support a daily rhythm in craving that varies by individual differences in circadian timing. Because craving is an important predictor of future alcohol use, the findings implicate circadian factors as a useful area to advance alcohol research and potentially improve interventions.

Support

R21AA023209; R01DA044143; K01AA021135; ABMRF/The Foundation for Alcohol Research.

Association of the Epstein-Barr virus latent membrane protein 1 with lipid rafts is mediated through its N-terminal region

The latent membrane protein 1 (LMP1) encoded by the Epstein-Barr virus acts like a constitutively activated receptor of the tumor necrosis factor receptor (TNFR) family and is enriched in lipid rafts. We showed that LMP1 is targeted to lipid rafts in transfected HEK 293 cells, and that the endogenous TNFR-associated factor 3 binds LMP1 and is recruited to lipid rafts upon LMP1 expression. An LMP1 mutant lacking the C-terminal 55 amino acids (Cdelta55) behaves like the wild-type (WT) LMP1 with respect to membrane localization. In contrast, a mutant with a deletion of the 25 N-terminal residues (Ndelta25) does not concentrate in lipid rafts but still binds TRAF3, demonstrating that cell localization of LMP1 was not crucial for TRAF3 localization. Moreover, Ndelta25 inhibited WT LMP1-mediated induction of the transcription factors NF-kappaB and AP-1. Morphological data indicate that Ndelta25 hampers WT LMP1 plasma membrane localization, thus blocking LMP1 function.

The role of Epstein-Barr virus in neoplastic transformation

In this review, we focus on new data from basic, translational and clinical research relating to the Epstein-Barr virus (EBV). Beside its well-known tropism for B lymphocytes and epithelial cells, EBV also infects T lymphocytes, monocytes and granulocytes. After primary infection, EBV persists throughout the life span in resting memory B cells, from where it is reactivated upon breakdown of cellular immunity. In the process of neoplastic transformation, the EBV-encoded latent membrane protein 1 (LMP1) oncogene represents the major driving force. LMP1 acts like a constitutively activated receptor of the tumor necrosis factor receptor family and allows the amplification or bypassing of physiological regulatory signals through direct and indirect interactions with proteins of the tumor necrosis factor receptor-associated factor (TRAF) family. TRAF2-mediated NF-kappaB activation, AP-1 induction and JAK3/STAT activation may result in sustained proliferation leading to lymphoma. The ability of LMP1 to suppress germinal center formation and its capacity to mediate its own transcriptional activation shed new light on the pathogenesis of EBV-associated latency type II lymphoproliferations like Hodgkin's disease and angioimmunoblastic lymphadenopathy. The carboxy terminus of LMP1 is also a reliable marker for individual EBV strain identification and thus offers new possibilities in tracing the molecular events leading to posttransplant lymphoproliferative disorders (PTLDs). Cytotoxic T lymphocytes directed against well-characterized epitopes of EBV latency genes represent an already successful and promising therapeutic approach to EBV-associated lymphomas, in particular PTLDs.

Do automated red cell exchanges relieve priapism in patients with sickle cell anemia?

Priapism is a dramatic, painful complication for some men afflicted by sickle cell anemia. Although the natural history remains unclear, many believe replacing the patient's abnormal red blood cells (RBCs) with normal RBCs by apheresis is effective. However, no controlled trials have demonstrated its effectiveness. We exchanged 7 men after medical management failed. All procedures reduced sickle hemoglobin levels to < 30%. Two patients underwent emergency automated red cell exchanges without any detumescence or reduction of pain. The remaining 5 patients were exchanged non-emergently; 4 experienced no detumescence or relief of pain. One adult experienced resolution 8 h postexchange. However, he had a history of "stuttering" priapism. All required decompression procedures. Automated RBC exchanges were not effective in achieving detumescence or reducing pain.

Latent membrane protein 1 associated signaling pathways are important in tumor cells of Epstein-Barr virus negative Hodgkin's disease

The latent membrane protein 1 (LMP1) oncogene of Epstein-Barr virus (EBV) is selectively expressed in the Reed-Sternberg (RS) cells of EBV-associated Hodgkin's disease (HD). However, no differences in clinical presentation and course are found between EBV positive and EBV negative forms of HD suggesting a common pathogenetic mechanism. We have studied the LMP1 associated signaling pathways and their dominant negative inhibition in the myelomonocytic HD-MyZ and the B-lymphoid L-428 HD cell lines. In both EBV negative cell lines expression of LMP1 is associated with the formation of multinuclear RS cells. Dominant negative inhibition of NF-kappa B mediated signaling at the step of I kappa B-alpha phosphorylation results in increased cell death with only a few typical RS cells resistant to overexpression of the dominant negative inhibitor I kappa B-alpha-N delta 54. However, dominant negative inhibition of NF-kappa B mediated signaling at the early step of TRAF2 interaction results in the formation of multinuclear cells in both cell lines and, in addition, in clusters of small mononuclear cells in the HD-MyZ cell line. In HD-MyZ cells overexpression of the powerful JBD-inhibitor of the JNK signal transduction pathway is restricted to small cells and never observed in RS cells. These small cells undergo apoptosis as shown by the TUNEL technique. Apoptosis of small cells is still observed after co-transfection of JBD and LMP1 but in addition a few apoptotic HD-MyZ cells with large fused nuclear masses are identified suggesting that specific inhibition of JNK leads also to apoptosis of LMP1 induced RS cells. Thus, activation of the JNK signaling pathway is also important in the formation of Reed-Sternberg cells. Our findings are consistent with a model where all three LMP1 associated functions, i.e. NF-kappa B mediated transcription, TRAF2 dependent signaling, and c-Jun activation act as a common pathogenetic denominator of both EBV negative and EBV positive HD.

Sequence polymorphisms between latent membrane proteins LMP1 and LMP2A do not correlate in EBV-associated reactive and malignant lympho-proliferations

The latent membrane proteins LMP1 and LMP2A are co-expressed in most malignancies associated with Epstein-Barr virus (EBV). In contrast with the transforming LMP1 oncoprotein, LMP2A is expressed in lymphocytes of healthy EBV carriers and considered to maintain viral latency. Critical for these LMP2A functions are a transmembranous epitope recognized by specific cytotoxic T lymphocytes (CTLs) and the N-terminal immunoreceptor tyrosine-based activation motif (ITAM), blocking B-cell receptor signaling. To characterize ITAM and CTL motifs of LMP2A and to correlate them with C-terminal variants of LMP1 including the 30-bp deletion variant (LMP1delta), comparative sequence analysis was performed on 76 samples from patients with reactive and malignant lympho-proliferation (infectious mononucleosis, n=21; tonsillar hyperplasia, n=16, chronic lympho-proliferation, n = 9; Hodgkin's disease, n = 8; Non-Hodgkin's lymphoma, n = 5; AIDS-related large-cell lymphoma, n=17). The CTL motif was conserved in all but 2 cases (C426-->S). The ITAM motif was characterized by strictly conserved YXXL sequences in all cases, with a sequence polymorphism in between. The B95.8 prototype was found in 17% (13/76) of cases, while in 72% a variant with 3 point mutations (166796 C-->A, 166805 C-->A, 166810 C-->T) was detected; 11% had 1 or 2 of these mutations in addition to G-->A at 166793. In the C terminus of LMP1, a hypervariable region including LMP1delta was described in 61% of cases. There was no significant association of a particular LMP2A variant with either malignant phenotype or LMP1delta, demonstrating that the functional domains of LMP2A are conserved and that the sequence polymorphisms in LMP1 and LMP2A are independent.

1997 Judson C. Hickey Scientific Writing Awards. An innovative investment method for the fabrication of a closed hollow obturator prosthesis

An investment method is described for fabrication of a closed hollow obturator based on the use of three sections of a denture processing flask. The thickness of the obturator portion is minimized and all surfaces exposed to the oral cavity are processed with heat-polymerized acrylic resin. The technique eliminates additional steps noted in the literature, allowing for the prosthesis to be processed to completion from the wax trial denture. This technique provides a lightweight and seamless obturator and can be used for complete or partially edentulous cases. Clinical and laboratory time are minimized while fabricating a durable, virtually water-tight prosthesis that can be used alone or in conjunction with an extraoral prosthesis.

Molecular and functional analysis of the Epstein-Barr virus LMP1 oncogene promoter in lymphoproliferative diseases

The expression of the Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) oncogene is tightly regulated by viral and cellular factors. LMP1 is present in the majority of nasopharyngeal carcinoma tumor cells and in Reed-Sternberg cells from Hodgkin's lymphoma, in which the only EBV nuclear antigen detected is EBNA1. The aim of this study was to test whether mutations affecting LMP1 gene expression were present in lymphoproliferative disorders, and, if so, whether their presence correlated with the clinical course of the disease. For this purpose we characterized the LMP1 promoter region from seven cases including two patients with aggressive Hodgkin's disease and two with atypical lymphoproliferative syndromes. Our results show that the sequences -298 to +29 relative to the transcription start site diverged up to 9.3% when compared with the prototype EBV strain B95-8. The cAMP responsive-like element (CRE), located at positions -37 to -44, was found to be mutated in 3 of the 7 cases. Functional analysis of transfected human embryonic kidney 293 cells using the firefly luciferase reporter gene revealed that mutations within the CRE site led to a 70% mean decrease in reporter activity. Our analysis indicates that in lymphoproliferative disorders, naturally occurring LMP1 variants that exhibit weak promoter activity are still associated with clinically progressive disease.

Natural 30 base pair and 69 base pair deletion variants of the LMP1 oncogene do stimulate NF-kappaB-mediated transcription

An increasing number of reports shows a link between the Epstein-Barr virus (EBV) and lymphoid neoplasia. The latent membrane protein 1 (LMP1) is likely to play a determinant role in this process since this EBV encoded protein has oncogenic properties and is usually expressed in EBV-associated lymphoproliferative diseases (LPD), except Burkitt's lymphoma. We previously identified in LPD patients mutational hot spots and a 30 bp or 69 bp deletion in the LMP1 gene region coding for the C-terminal domain. These deletions are located in an area shown to be important for the activation of the transcription factor NF-kappaB. These findings lead us to test whether these natural deletion variants may have a functional effect. We measured the stimulation of their activity using a luciferase reporter plasmid containing NF-kappaB responsive elements. We tested the NF-kappaB inducing activity of four naturally occurring LMP1 deletion variants. Our results show that these deletion variants activate NF-kappaB to the same level as the wild-type form, indicating that the crucial residues for NF-kappaB activation are conserved among the variants isolated and lie within the last 32 amino acids of the C-terminal domain of the LMP1 oncogene.

Expression of the LMP1 oncoprotein in the EBV negative Hodgkin's disease cell line L-428 is associated with Reed-Sternberg cell morphology

The latent membrane protein 1 (LMP1) oncogene is one of the major proteins synthesized by the Epstein-Barr virus (EBV) and is expressed in Reed-Sternberg (RS) cells of EBV-associated Hodgkin's disease (HD). We have studied the effect of this oncoprotein on the formation of RS cells in the EBV-negative HD cell lines L-428 and KM-H2 as well as on the formation of multinucleated cells in the mononuclear human embryonic kidney cell line 293. LMP1 prototype (B95-8) and its naturally occurring carboxy terminal 30 base pair deletion variant LMP1-del were transfected into the cell lines and cytocentrifuge preparations were analysed after 24, 48, 72, 144, 216, and 240 h. While no oncoprotein expression was seen in the KM-H2 cell lines, expression of LMP1 and LMP1-del was observed in the L-428 and 293 cell lines. In the HD cell line L-428 oncoprotein expression was infrequent but when observed was very strong and preferentially associated with multinucleated RS cell morphology (71% of LMP1 positive cells). This is in contrast with the untransfected or transfected but not expressing cells where intermediate mononuclear elements predominated over multinucleated RS cells (< 3%). Frequent oncoprotein expression was observed in the 293 cells and again was associated with multinuclearity. These LMP1 expressing 293 giant cells showed strong expression of ICAM-1(CD54), not detectable in the untransfected cells. In the LMP1-del transfectants giant cells with more than four nuclei were frequently observed. However, giant cells were much less frequent in 293 cells transfected with the amino terminal deletion variant of LMP1 or the lytic form of LMP1, known to induce low NF-kappa B activation compared to the LMP1 prototype. Therefore, LMP1 mediated NF-kappa B activation appears to be involved in polycaria formation. The strong association of LMP1 expression with multinuclearity in a genetically unstable condition -the L-428 and 293 cells show multiple chromosomal abnormalities-suggests that this oncoprotein including its naturally occurring carboxy terminal deletion variant promote the formation of multinuclear cells, in particular of RS cells in EBV-associated HD.

Pumping iron in the '90s

The role o f iron in cell division, cell death and human disease has recently gained increased attention. The best studied process for iron uptake into mammalian cells involves traps ferrin and its receptor. This review discusses evidence supporting the existence of other routes by which iron can enter mammalian cells. Specifically, iron uptake by the cell-surface GPI-linked traps ferrin homologue, melanotransferrin or p97, is described and possible functions of this traps ferrin-independent pathway are proposed.

Coincident expression and distribution of melanotransferrin and transferrin receptor in human brain capillary endothelium

One method of iron transport across the blood brain barrier (BBB) involves the transferrin receptor (TR), which is localized to the specialized brain capillary endothelium. The melanotransferrin (MTf) molecule, also called p97, has been widely described as a melanoma specific molecule, however, its expression in brain tissues has not been addressed. MTf has a high level of sequence homology to transferrin (Tf) and lactoferrin, but is unusual because it predominantly occurs as a membrane bound, glycosylphosphatidylinositol (GPI) anchored molecule, but can also occur as a soluble form. We have recently demonstrated that GPI-anchored MTf provides a novel route for cellular iron uptake which is independent of Tf and its receptor. Here we consider whether MTf may have a role in the transport of iron across the BBB. The distributions of MTf, Tf and the TR were studied immunohistochemically in human brain tissues. The distributions of MTf and TR were remarkably similar, and quite different from that of Tf. In all brain tissues examined, MTf and the TR were highly localized to capillary endothelium, while Tf itself was mainly localized to glial cells. These data suggest that MTf may play a role in iron transport within the human brain.

Reactive microglia specifically associated with amyloid plaques in Alzheimer's disease brain tissue express melanotransferrin

Several investigations have implicated the involvement of metals in neuropathologies. In particular, the disruption of iron metabolism and iron transport molecules have been demonstrated in Alzheimer's disease (AD). We have identified a novel pathway of iron uptake into mammalian cells involving melanotransferrin, or p97, which is independent of the transferrin receptor. Here we investigated whether there is a possible link between this molecule and the pathology of AD. The distributions of melanotransferrin, transferrin and the transferrin receptor were studied immunohistochemically in brain tissues from AD cases. In brain tissues from AD, melanotransferrin and the transferrin receptor were highly localized to capillary endothelium, while transferrin itself was mainly localized to glial cells. In brain tissue derived from AD patients, melanotransferrin was additionally detected in a subset of reactive microglia associated with senile plaques. Our demonstration that melanotransferrin mediates iron uptake through a pathway independent of the transferrin receptor indicates that this mechanism may have a role in AD.

Deletion variants within the NF-kappa B activation domain of the LMP1 oncogene prevail in acquired immunodeficiency syndrome-related large cell lymphomas and human immunodeficiency virus-negative atypical lymphoproliferations

This sequencing study of 17 acquired immunodeficiency syndrome-related lymphomas (9 primary brain, 8 systemic) and 8 human immunodeficiency virus-negative atypical lymphoproliferations expressing large amounts of the latent membrane protein 1 (LMP1) of Epstein-Barr virus was performed to characterize the carboxy terminal NF-kappa B activation domain of LMP1 at the molecular level in an immunocompromised host. In-frame deletions within the NF-kappa B activation domain were identified in all but 2 primary brain lymphomas, 4 systemic lymphomas, and 4 atypical lymphoproliferations, ie, in 60% of cases. In addition, non silent point mutations (range 1 to 5, mean 3.3) were detected in all cases. Although all changes occurred within the first 100 nucleotides of the carboxy terminal NF-kappa B activation domain--a critical sequence for the protein half-life--not a single point mutation was found in the remaining 62 nucleotides, necessary for malignant transformation. Such a clustering of nonrandom sequence variations, associated with a high oncoprotein expression in immunocompromised hosts, suggests that this part of the LMP1 oncogene behaves as a hypervariable region with natural selection of growth-promoting variants through prolongation of the protein half-life.

EBV-associated lymphoproliferative syndrome with a distinct 69 base-pair deletion in the LMP-1 oncogene

We describe an immunocompetent 12-year-old boy with chronic EBV infection and lymphoid interstitial pneumonitis. Lymph node biopsies showed effacement of the architecture with polymorphic cellular infiltrates, consisting predominantly of T cells and natural killer cells. No clonal rearrangement of TCR or immunoglobulin genes was seen. DNA was extracted from hilar lymph nodes; sequencing of the carboxy terminal region of the latent membrane protein 1 (LMP-1) oncogene revealed a 69 base-pair deletion and four point mutations. Immunosuppressive treatment with prednisone and cyclosporine reversed the lymphadenopathy.

Detection of restricted isoform expression and tyrosine phosphatase activity of CD45 in murine dendritic cells

CD45 is a cell surface transmembrane tyrosine phosphatase. It is expressed as distinct protein isoforms via alternative splicing of exons 4, 5 and 6. In T and B lymphocytes, CD45 is thought to play a critical role in antigen-dependent signaling through their respective antigen receptor complexes. However, the isoform expression and enzymatic activity of CD45 in other leukocytes remains largely unknown. Here, we examine the isoform expression and phosphatase activity of CD45 in murine dendritic cells (DC). Flow cytometric double-labeling analysis and biochemical analysis of purified splenic DC CD45 demonstrate that DC express both the CD45RB and CD45R0 isoforms. Flow cytometric analyses of freshly isolated splenic DC and thymic DC also indicate the expression of CD45RB and CD45R0 on these DC populations. In addition, we find that purified splenic DC CD45 possesses a high level of intrinsic tyrosine phosphatase activity. These data therefore establish the restricted isoform expression pattern of CD45 in murine DC and demonstrate that cells lacking specific antigen receptor complexes have active tyrosine phosphatase activity associated with CD45.

A deletion mutant of the LMP1 oncogene of Epstein-Barr virus is associated with evolution of angioimmunoblastic lymphadenopathy into B immunoblastic lymphoma

The latent membrane protein 1 (LMP1) oncogene is one of the major proteins synthesized by the Epstein-Barr virus (EBV). It is expressed in Reed-Sternberg cells of Hodgkin's disease (HD), tumor cells of nasopharyngeal carcinoma (NPC), and immunoblasts of angioimmunoblastic lymphadenopathy (AILD). A particular LMP1 deletion mutant was recently identified in NPC and clinically and histologically aggressive HD. We studied two patients with AILD that subsequently progressed into immunoblastic lymphoma (IBL) in order to investigate whether the LMP1 deletion mutant was implicated in progression of AILD into IBL. Immunohistology and in situ hybridization were performed on diagnostic biopsies. DNA extracted from fresh frozen material was used for rearrangement studies and polymerase chain reaction (PCR) based amplification and sequencing of portions of the LMP1 gene. Immunohistochemistry revealed B cell origin of both cases of IBL. In the first patient clonal rearrangement of the immunoglobulin heavy-chain gene was present in IBL but not in AILD. In this patient, scattered immunoblasts of AILD and numerous tumor cells of B-IBL were shown to contain EBV transcripts (EBER1) and to express LMP1. Sequence analysis of the LMP1 gene from AILD and IBL in the first, and from IBL in the second patient, revealed identical deletions and point mutations. This LMP1 deletion mutant is identical to those which have been reported in HD and NPC. Its association with evolution of AILD into B-IBL, aggressive HD and NPC, suggests that this particular mutant is more widespread than originally thought and is clinically relevant.

Mutational hot spots within the carboxy terminal region of the LMP1 oncogene of Epstein-Barr virus are frequent in lymphoproliferative disorders

We have recently identified in Epstein-Barr virus (EBV) positive Hodgkin's disease (HD) a variant of the latent membrane protein 1 (LMP1) oncogene characterized by four point mutations and a 30 base pair deletion. These findings led us to test whether such mutants were also present in other lymphoproliferative disorders (LPD). We analysed 98 EBV DNA positive cases (67 LPD, 15 benign conditions, 16 lymphoblastoid cell lines) by PCR for deletions within the LMP1 gene. DNA sequencing of the region coding for the carboxy terminal protein domain was performed on 24 cases. In 13 cases the same combination of 4 point mutations at positions 168,320, 168,308, 168,295 and 168,225 was identified. Of these cases, 12 had an additional point mutation at position 168,357 and eight at position 168,355, and nine had a 30 base pair deletion including nucleotides 168,285 to 168,256. These deletion mutants were identified in HD, angioimmunoblastic lymphadenopathy, B-immunoblastic lymphoma, peripheral T-cell lymphoma, and two lymphoblastoid cell lines. Our findings reveal a high frequency of non-random point mutations at preferential sites within the 3' (carboxy terminal) region of the LMP1 oncogene. The association of these mutational hot spots with LPD suggests that they are involved in EBV related lymphomagenesis and that they define a clinically relevant EBV strain.

Persistence of the same viral strain in early and late relapses of Epstein-Barr virus-associated Hodgkin's disease

Twelve cases of relapsing Hodgkin's disease were investigated for the presence of Epstein-Barr virus (EBV). Of these, 7 cases contained EBV gene products (LMP1, EBER RNA) in the diagnostic Reed-Sternberg cells and variants at first presentation and at relapse(s), whereas 5 cases were negative at both first diagnosis and relapse. Among the 7 EBV-positive cases, material for DNA extraction was available in 2 cases at both diagnosis and relapse(s). Ig and T-cell receptor gene rearrangements displayed a germline configuration in the 2 cases. However, Southern blot analysis of the terminal repeats (TR) of EBV genome showed that, in 1 of the 2 cases, the fragment was of the same size at diagnosis and in the subsequent two relapses (1 early and 1 late). The second case contained monoclonal EBV genome at diagnosis, but the Southern analysis of the TR was negative at relapse. The latent membrane protein (LMP1) sequence analysis confirmed the persistence of a distinctive viral strain in each of the 2 cases with individual abnormalities within the carboxy terminal region (5 point mutations and a 30-bp deletion for the first case and 6 point mutations for the second case). The persistence of a given strain in early and late relapses is evidence towards the view that in Hodgkin's disease such relapses are related to a single residual tumor cell clone.

Transport and expression in human melanomas of a transferrin-like glycosylphosphatidylinositol-anchored protein

Melanotransferrin, also called p97, is a cell surface glycoprotein which was first described as a marker antigen for human melanoma cells. Although p97 has a striking structural similarity to human serum transferrin and lactoferrin, its function has not yet been determined. One feature that distinguishes p97 from the other members of the transferrin family is the presence of a stretch of 24 hydrophobic amino acids at the C terminus, previously assumed to form a proteinacious transmembrane domain. In this study, sensitivity to bacterial phosphatidylinositol-specific phospholipase C, biosynthetic labeling with [3H]ethanolamine, and partitioning in Triton X-114 are used to establish that p97 is expressed at the cell surface as a glycosylphosphatidylinositol-anchored protein. In addition, to gain insight into the intracellular transport of p97, biosynthetic transport studies were performed on a melanoma cell line. These studies resulted in the identification of an additional form of p97 which is found in the medium and which likely does not originate from an alternatively spliced form of the p97 mRNA. These findings, together with our recent observation of the co-localization of p97 and the transferrin receptor in brain capillary endothelium (W. A. Jefferies, M. R. Food, R. Gabathuler, S. Rothenberger, T. Yamada, and P. L. McGeer, manuscript submitted) raise important questions about the function of the two forms of p97 detected and the possible involvement of this protein in a cellular iron uptake mechanism that is independent from the transferrin/transferrin receptor system.

The internalization signal and the phosphorylation site of transferrin receptor are distinct from the main basolateral sorting information

Wild-type human transferrin receptor (hTfR), like endogenous canine receptor, is expressed almost exclusively (97%) at the basolateral membrane of transfected Madin-Darbey canine kidney (MDCK) cells. We investigated the role of two distinct features of the hTfR cytoplasmic domain, namely the endocytic signal and the unique phosphorylation site, in polarized cell surface delivery. Basolateral location was not altered by point mutation of Ser24-->Ala24, indicating that phosphorylation is not involved in vectorial sorting of hTfR. The steady state distribution of hTfR was partially affected by a deletion of 36 cytoplasmic residues encompassing the internalization sequence. However, 80% of the receptors were still basolateral. As assessed by pulse-chase experiments in combination with biotinylation, newly synthesized wild-type and deletion mutant receptors were directly sorted to the domain of their steady state residency. Although both receptors could bind human transferrin, endocytosis of the deletion mutant was strongly impaired at either surface. These data indicate that the predominant basolateral targeting signal of hTfR is independent of the internalization sequence.

In vivo and in vitro modulation of the mRNA-binding activity of iron-regulatory factor. Tissue distribution and effects of cell proliferation, iron levels and redox state

The mRNA-binding protein, iron-regulatory factor (IRF) has a central role in iron metabolism. It coordinately increases transferrin-receptor mRNA stability and inhibits translation of ferritin and erythroid delta-aminolevulinate synthase mRNA by binding to specific mRNA structures, the iron-responsive elements (IRE). In gel-retardation assays, IRF had a broad tissue distribution, showing activity in cytosolic extracts from 12 mouse organs tested. In all these extracts, IRF could be further activated in vitro by 2-mercaptoethanol. In cultured mouse 3T6 fibroblasts, growth stimulation after low serum arrest increased IRF activity 10-fold, mainly through activation of existing inactive IRF. No change was observed during progression of 3T6 cells through the cell cycle. IRF activation by iron chelators has been postulated to result in the reduction of an intramolecular sulfhydryl group. In a search for redox conditions that regulate IRE binding of IRF, we studied several compounds in vitro or in vivo. Hemin, known to inactivate IRF in vivo, showed a similar, reversible effect in vitro, presumably by oxidizing IRF. However, this did not appear to be relevant for the mode of IRF regulation in vivo. Addition of protoporphyrin IX to intact cells induced IRF activity almost to the same extent as desferrioxamine. This effect was inhibited by iron salts, indicating that IRF is activated in vivo through depletion of a chelatable iron pool. In vitro activation by reductants other than 2-mercaptoethanol suggested some selectivity in their access to relevant sulfhydryl groups, but did not reveal which natural redox-sensitive compound might regulate IRF in vivo. However, in cultured cells, inactivation of free IRF by the sulfhydryl-specific oxidizing agent diamide was much more rapidly reversed than inactivation by iron salts. This indicates the direct involvement of a cellular reductant in setting IRF activity and suggests a rate-limiting IRF conformation that is reached only in the presence of iron, but not after diamide oxidation.

The mRNA-binding protein which controls ferritin and transferrin receptor expression is conserved during evolution

A post-transcriptional regulatory protein, termed iron regulatory factor (IRF), that binds specifically to the iron-responsive elements of ferritin and transferrin receptor mRNA, has recently been identified in the cytoplasm of human and mouse cells. Activation of this factor by low intracellular iron levels leads to inhibition of ferritin translation and an increase of TR mRNA stability. To investigate whether these feedback regulatory mechanisms are conserved during evolution, we analysed cytoplasmic extracts from 12 different species for a specific IRE-binding activity. We found mRNA-binding proteins in chicken, frog, fish and fly, which are equivalent to human and mouse IRF in gel-retardation assays with radiolabeled RNA transcripts. Competition experiments, molecular weight determinations, and modulation of the mRNA-binding activity in response to intracellular iron levels or reduction by beta-mercaptoethanol indicate that IRF has similar structural and functional properties in these different species.

A role for the cytoplasmic domain in transferrin receptor sorting and coated pit formation during endocytosis

The cytoplasmic domain of transferrin receptor (TR) is essential for endocytosis of this transmembrane protein. We have investigated by electron microscopy the association of wild-type and cytoplasmic deletion mutant human TR with coated pits at the surface of transfected L cell lines. Approximately 15% of wild-type TR was concentrated in coated pits, regardless of the level of TR expression. In contrast, only 2% of deletion mutant TR was present in these structures. We also correlated the frequency of coated pits with the level of TR expression in different transfected L cell lines. Expression of more than 3 x 10(6) wild-type TR per cell was accompanied by up to a 4-fold increase in coated pits compared with nontransfected Ltk- cells. No such increase was observed in a cell line expressing a similarly high level of cytoplasmic deletion mutant TR. These results indicate that the cytoplasmic domain plays an active role in sorting and endocytosis of TR by providing an assembly site for coated pit formation.

Endocytosis of the transferrin receptor requires the cytoplasmic domain but not its phosphorylation site

The transferrin receptor (TR) mediates cellular iron uptake by bringing about the endocytosis of transferrin. We investigated whether the cytoplasmic domain of 65 N-terminal amino acids or phosphorylated sites within this domain constitute a structure that is required for TR endocytosis. To test this hypothesis, we modified the cytoplasmic serine residues or introduced a deletion of 36 amino acids by in vitro mutagenesis of a cDNA expression vector for human TR. Upon expression in transfected mouse Ltk- cells, both the wild-type and phosphorylation site mutant receptors mediated transferrin internalization, whereas the truncated receptor did not. These results provide evidence that the cytoplasmic domain, or part of it, is essential for internalization of the TR, but argue against a role for receptor phosphorylation in endocytosis.