Differential expression of human metallothionein isoform I mRNA in human proximal tubule cells exposed to metals

In contrast to the single metallothionein (MT)-1 gene of the mouse, the human MT-1 gene family is composed of seven active genes and six pseudogenes. In this study, the expression of mRNA representing the seven active human MT-1 genes was determined in cultured human proximal tubule (HPT) cells under basal conditions and after exposure to the metals Cd2+, Zn2+, Cu2+, Hg2+, Ag2+, and Pb2+. Basal expression of MT-1X and MT-1E mRNA in HPT cells was similar to expression of the housekeeping gene glyceraldehyde 3-phosphate dehydrogenase. In contrast, mRNAs representing the basal expression of MT-1A and MT-1F were a minor transcript in HPT cells. Treatment of HPT cells with Cd2+, Zn2+, or Cu2+ increased the levels of MT-1E and MT-1A mRNA, but not the levels of MT-1X or MT-1F mRNA. The increase in MT-1E mRNA appeared to be influenced mainly by exposure to the various metals, whereas the increase in MT-1A mRNA was influenced more by exposure to a metal concentration eliciting a loss of cell viability. Treatment of HPT cells with the metals Hg2+, Ag2+, and Pb2+ was found to have no effect on the level of MT-1 mRNA at either sublethal or lethal concentrations. Using HPT cells as a model, these results suggest that new features of MT gene expression have been acquired in the human due to the duplication of the MT-1 gene.

Exposure of human proximal tubule cells to cd2+, zn2+, and Cu2+ induces metallothionein protein accumulation but not metallothionein isoform 2 mRNA

The organization of the human metallothionein (MT) gene family is more complex than the commonly used mouse and rat models. The human MTs are encoded by a family of genes consisting of 10 functional and 7 nonfunctional MT isoforms. One objective of this study was to determine if the accumulation of MT protein in cultures of human proximal tubule (HPT) cells exposed to metals is similar to that expected from the knowledge base obtained from rodent models. To accomplish this objective, HPT cells were exposed to both lethal and sublethal concentrations of Cd2+, Zn2+, Cu2+, Ag2+, Hg2+, and Pb2+ and MT protein levels were determined. The results were in general agreement with animal model studies, although there were some exceptions, mainly in areas where the animal model database was limited. In clear agreement with animal models, Cd2+, Zn2+, and Cu2+ were demonstrated to be potent inducers of MT protein accumulation. In contrast to the similarity in MT protein expression, we obtained evidence that the human renal MT-2 gene has a unique pattern of regulation compared to both animal models and human-derived cell cultures. In the present study, we determined that MT-2A mRNA was not induced by exposure of HPT cells to Cd2+ or the other metals, a finding in contrast to studies in both animal models and other human cell culture systems in which a high level of MT-2 mRNA induction occurs upon exposure to Cd2+ or Zn2+. While MT protein expression may be similar between humans and animal models, this finding provides initial evidence that regulation of the genes underlying MT protein expression may be divergent between species.

Differential expression of the MT-1E gene in estrogen-receptor-positive and -negative human breast cancer cell lines

The goal of this study was to determine which of the 10 functional metallothionein (MT) genes are expressed in four human breast cancer cell lines and whether expression varies among the cell lines. Using reverse transcription polymerase chain reaction (RT-PCR) technology, it was shown that there was no expression of mRNA for the MT-1A, MT-1B, MT-1F, MT-1G, MT-1H, MT-3, and MT-4 genes in any of the four cell lines. All four cell lines were shown to express mRNA for the MT-2A and MT-1X genes. The expression level of mRNA for the MT-2A gene demonstrated modest differences among the cell lines, whereas expression of the MT-1X gene was consistent. In contrast, mRNA for the MT-1E gene was expressed in only two of the four cell lines and expression correlated to the estrogen receptor status of the cell lines. The two estrogen-receptor-positive cell lines showed no mRNA expression for the MT-1E gene. In the two estrogen-receptor-negative cell lines, mRNA expression for the MT-1E gene was elevated with expression levels similar to the housekeeping gene, glyceraldehyde-3-phosphate dehydrogenase. The cellular content of MT protein was also shown to be elevated in the estrogen-receptor-negative cell lines that express MT-1E mRNA. These results suggest a possible relationship between estrogen receptor status and MT-1E gene expression in human breast cancer.

Expression of MT-3 mRNA in human kidney, proximal tubule cell cultures, and renal cell carcinoma

The human metallothionein 3 (MT-3) gene has recently been identified and characterized as a brain-specific MT having growth inhibitory activity for neuronal cells. One objective of the present study was to determine if MT-3 is brain-specific or also present in the renal system, a site for chronic toxicity due to heavy metal exposure. Using RT-PCR methodology, MT-3 mRNA was shown to be expressed in the human renal system at levels below mRNA for the beta-actin gene. MT-3 mRNA was shown to be expressed in all samples obtained from both the developing and adult renal systems, from 20 weeks of fetal age to 72 years. Cultures of human proximal tubule (HPT) cells were used to determine if MT-3 mRNA expression is influenced by metal exposure. Exposure of HPT cells to either Zn2+ or Cd2+ resulted in an early (within 24 h), but unsustained increase in MT-3 mRNA. The demonstration of MT-3 mRNA expression in the kidney indicates that MT-3 may play an important early role in the response of the cell to metal exposure. MT-3 mRNA expression was also examined in tissues and cells from three cases of renal cell carcinoma. MT-3 was found to be expressed in all three cases at levels similar to those found for normal kidney, providing evidence that MT-3 mRNA expression is not altered in this cancer.

Expression of hsc 70, but not hsp 70, in human third molar dental pulp

The constitutive (hsc 70) and inducible (hsp 70) isoforms of heat shock protein 70 are important members of the superfamily of stress related proteins that protect and promote the recovery of cells from physiological and pathologic stress. The goal of this study was to define the baseline expression of hsc 70 and hsp 70 in disease-free, minimally stressed human dental pulp of the adult 3rd molar. Immunolocalization demonstrated moderate to heavy staining intensity for hsc 70 in both the cytoplasm and nucleus of odontoblasts and fibroblasts comprising the human pulp. Endothelial and smooth muscle cells displayed weak to moderate immunoreactivity for hsc 70 in both the cytoplasm and nucleus. Schwann cells demonstrated only weak nuclear staining for hsc 70. No immunoreactivity for hsp 70 was observed in any cell type in human pulp. Western, northern, and RT-PCR analysis of pulp preparations confirmed the expression of hsc 70 mRNA and protein within components of the pulp. These results demonstrate that cells of the human pulp express, under conditions of minimal stimulation, a key component of the stress response protein superfamily. The expression of hsc 70 under conditions of minimal stress may provide pulp components an advantage in resisting cell injury when stress occurs.

Expression of heat shock protein 27 in adult human third molar dental pulp

This study is the first to define the expression of hsp 27 in the pulp of the adult human third molar. Using a monoclonal antibody against human hsp 27, immunoreactivity was demonstrated in the odontoblasts, odontoblast processes, pulp fibroblasts, and smooth muscle and endothelial cells of vessel walls. Nerves were negative. Pulp fibroblasts were characterized by cytoplasmic staining and variable nuclear staining. Odontoblasts also displayed consistent cytoplasmic staining and variable nuclear staining. Western, Northern, and RT-PCR analysis confirmed the expression of hsp 27 mRNA and protein. Hsp 27 was also shown to be present in both the unphosphorylated and phosphorylated isoforms. In general, nuclear localization and phosphorylation of hsp 27 has been correlated with cells responding to stress or other stimuli. This study demonstrates that pulp from a single human third molar provides sufficient material to support a detailed molecular analysis of gene expression.

Isoform-specific expression of metallothionein mRNA in the developing and adult human kidney

The organization of the metallothionein (MT) gene family has been demonstrated to be much more complex in humans than in the mouse, and possibly rodents in general. For humans, the MTs are encoded by a family of genes located at 16q13 representing 10 functional and 7 non-functional MT isoforms. In the present study, the 5' and 3' untranslated region sequences of the highly conserved, functional MT genes were utilized to generate primer pairs for the analysis of isoform-specific MT mRNA using reverse transcriptase-polymerase chain reaction (RT-PCR). Human kidneys from 13 weeks gestation through adulthood were examined for the expression of MT protein and mRNA. Immunohistochemical analysis demonstrated MT immunoreactivity to be confined exclusively to the proximal tubules of the adult and developing kidney. For all MT-positive cells, MT was localized in the cytoplasm and nuclear localization was variable. There was no correlation between nuclear staining and stage of development. Of the 10 MT genes examined (MT-1A, MT-1B, MT-1E, MT-1F, MT-1G, MT-1H, MT-1X, MT-2A, MT-3, and MT-4), mRNAs representing the MT-1E, MT-1F, MT-1X, and MT-2A genes were consistently expressed in all samples regardless of gestational age. There was no indication of a 'fetal form' of MT analogous to that noted to occur in human liver. Messenger RNA for the MT-1A gene was detected in 2 of 6 renal samples without correlation to gestational age. In no instance was mRNA for the MT-1B, MT-1G, MT-1H, MT-3 or MT-4 genes detected. These studies detail the initial determination of MT gene expression in the human renal system and provide the PCR primers for testing and determination of MT gene expression in other organ systems.

Expression of heat shock protein 27 in developing and adult human kidney

The expression of heat shock protein (hsp) 27 was examined in the developing and adult human kidney. Immunolocalization using a monoclonal antibody against human hsp 27 demonstrated immunoreactivity in both the developing and adult kidney. Low to moderate immunoreactivity for hsp 27 was observed in the fetal and adult proximal tubule, distal tubule, and mesangial cells of the glomeruli. Intense immunoreactivity for hsp 27 was localized to the cortical and medullary collecting ducts in both the adult and fetal kidney, with the most intense staining in the medullary regions. The loop of Henle demonstrated no immunoreactivity for hsp 27. The blastemal element of the developing kidney showed no hsp immunostaining and the ureteric bud demonstrated moderate staining. Western, northern, and reverse transcription-polymerase chain reaction (RT-PCR) analyses disclosed no significant differences in hsp 27 mRNA or protein level as a function of gestational age. An analysis of the phosphorylation state of hsp 27 showed the majority of hsp 27 to be present in the unphosphorylated isoform for both adult and fetal samples. These studies are the first to demonstrate the presence of hsp 27 in the human kidney. It is suggested that this pool of hsp 27 is constitutive as it appears in an inactivated state; localized to the cytoplasm and in an unphosphorylated state.

Enzymatic isolation and serum-free culture of human renal cells : retaining properties of proximal tubule cells

Procedures for the culture of the human renal proximal tubule (HPT) cellutilizing explanted tissue have been previously reported by this laboratory (1). Several other investigators have also reported the isolation and culture of human renal tubule cells (2, and references therein). Although explantation of tissue fragments remains an effective way to initiate cell cultures, cell outgrowth and the attainment of confluent cultures may take several weeks. The cell-culture methodology described in this chapter results in a high yield of confluent primary cultures in 7-10 d. The technique involves the digestion of minced cortical tissue with collagenase, followed by a filtering step to remove tissue fragments. The filtrate is centrifuged, and the cell pellet is resuspended in serum-free growth medium and dispensed onto prepared growth surfaces.

Exposure of human proximal tubule cells to cytotoxic levels of CdCl2 induces the additional expression of metallothionein 1A mRNA

Humans, in contrast to animals, have a complex expression of metallothionein (MT) genes which involves many MT isoforms encoded by a family of genes containing an upper limit of 12 possible functional genes. It is unknown if these human isoforms of MT have distinct functions or if they simply represent a non-essential duplication of gene function. In the present study, MT protein and mRNA for the MT-2A, MT-1A, B, E, F, and G genes was determined for 3 isolates of human proximal tubule (HPT) cells having distinct sensitivities to cadmium. For all 3 HPT isolates, the expression of MT protein and mRNA for the MT-2A, MT-1E, MT-1F and MT-1G isoforms was similar among the isolates and demonstrated no correlation to lethality. However, each isoform mRNA was expressed at different levels when compared to one another. In contrast, the expression of MT-1A mRNA differed in expression and correlated with the differing lethalities displayed by each isolate. The finding of different profiles of mRNA expression provides evidence that the MT isoforms may have unique functions and that mRNA for the MT-1A gene could be a potential marker for heavy metal exposure and/or toxicity.

Selective exposure of human proximal tubule cells to gentamicin provides evidence for a basolateral component of toxicity

The goal of the present study was to determine if cultured human proximal tubule (HPT) cells could provide evidence for a basolateral component of gentamicin toxicity. Six isolates of HPT cells were grown on Millicell filters and exposed to gentamicin either apically, basolaterally, or both apically and basolaterally. Toxicity was determined by the release of lactate dehydrogenase into the growth media. The results clearly demonstrated that basolateral exposure and combined apical and basolateral exposure to gentamicin resulted in significant levels of cell toxicity. In contrast, apical exposure to gentamicin elicited only marginal toxicity. The transepithelial flux of gentamicin was shown to be the same in either the apical to basolateral or the basolateral to apical direction. A two-step mechanism of gentamicin toxicity is proposed in order to integrate basolateral toxicity with known features of aminoglycoside nephrotoxicity.

Characterization of a monoclonal antibody recognizing selective epithelial elements of Wilms' tumors and fetal kidneys

A new antigen was detected using a monoclonal antibody generated against malignant blastema from a Wilms' tumor. This antigen showed variable expression in malignant blastemal cells but was never detected in normal blastema of fetal kidneys irrespective of gestational stage. In a series of 16 Wilms' tumors, the most intense and consistent staining was seen in tubule-associated epithelial cells. Such tubular staining is not surprising as the putative induction of malignant blastema to differentiate into malignant tubules is thought to parallel normal tubulogenesis. This antigen was also associated with epithelial cells located in a variety of fetal kidney structures. Again, the staining was most consistent in tubular epithelia. This monoclonal antibody reactive with a blastemal-epithelial-tubular (BET) antigen should be of value in studying the induction of epithelial differentiation in the normal and diseased human kidney.

Characterization of a monoclonal antibody recognizing the blastemal element of Wilms' tumors and fetal kidneys

A blastema-associated antigen (BLA-1) was detected using a monoclonal antibody against malignant blastema from a Wilms' tumor. The localization of BLA-1 was investigated in a series of nine Wilms' cases, five fetal, one childhood, and two adult kidneys. In this series, BLA-1 antibody consistently stained cell surfaces of all Wilms' tumors containing blastemal components. The same staining pattern was maintained in tumors grown as heterotransplants in nude mice. The expression of BLA-1 antigen was examined in normal blastema of fetal kidneys. BLA-1 was immunolocalized to condensed blastemal cells in the nephrogenic zone throughout gestation. In addition, kidney samples from a young child or adults contained no blastemal cells and therefore showed no blastemal cell surface staining. Glomerular mesangial cell staining was demonstrated in kidneys from 12 weeks of gestation through adulthood. This staining in developing and mature glomeruli implies that mesangial cells may be derived from condensed blastemal cells. The finding of a cell surface antigen common to Wilms' blastema, fetal blastema, and mesangial cells has not been previously demonstrated.

Expression of insulin-like growth factor binding protein 2 (IGFBP-2) in Wilms' tumors

Human Wilms' tumor (WT) expresses insulin-like growth factor (IGF) II and its cognate receptor, type 1 IGF receptor, forming a self-stimulating "autocrine loop." The biological activity of IGF-II is modulated by a class of soluble receptors called IGF binding proteins (IGFBP). To determine if IGFBP play a role in the biology of WT, extracts of nude mouse heterotransplants of three blastemal WT were examined for the ability to bind radiolabeled IGF-II by ligand blot analysis. [125I]IGF-II bound to a protein of M(r) 35 kDa. To confirm that this binding protein was being expressed by the tumor itself and not background from the host, tumor explants were prepared in cell culture. Conditioned culture media from blastemal WT cell cultures were found to contain the 35-kDa IGFBP. This secreted binding protein was identified as IGFBP-2 by screening for reactivity to characterized IGFBP antisera. Total RNA from primary WT or WT cells in culture was examined for expression of IGFBP-2 mRNA using an RNase protection assay. All three WT expressed IGFBP-2 mRNA. These data suggest a role for IGFBP-2 in the IGF-II-dependent growth of Wilms' tumor and in the developing kidney.

Serum-free culture and characterization of renal epithelial cells isolated from human fetal kidneys of varying gestational age

Cell cultures were initiated from seven human fetal kidneys that varied in gestational age from 90 days to newborn. The growth medium utilized was a 1:1 mixture of Dulbecco's modified Eagle's and Ham's F12 supplemented with selenium (5 ng/ml), insulin (5 micrograms/ml), transferrin (5 micrograms/ml), hydrocortisone (36 ng/ml), triiodothyronine (4 pg/ml), and epidermal growth factor (10 ng/ml). For all the kidney isolates, initial cell attachment occurred within 12 h through multicell spheroids, and by 24 h a rapidly growing population of cells was obtained. Confluency was reached within 3 to 6 days. A combination of light microscopy, immunohistochemistry, and ultrastructural evaluation was utilized to characterize the resulting cultures as epithelial and homogeneous within each isolate and among the isolates. That is, regardless of gestational age of the fetal kidney used as starting material, an identical or highly similar population of cells was obtained. By light microscopy, the cultures were noted to form very few domes, the number being an indication of transport activity. However, ultrastructural examination revealed that the cells were noted to form domes composed of only a few cells or "micro-domes" that would not be visible by light microscopy. Within the micro-domes as well as other areas of the monolayer an apparent absence of tight junctions was noted by routine transmission electron microscopy. However, by freeze fracture analysis cells were shown to possess sealing strands, the structural component of tight junctions. It is postulated that the tight junctions of fetal epithelial cells are structurally altered as compared to tight junctions in adult renal epithelial cell cultures.

Distinctive properties of an anaplastic Wilms' tumor and its associated epithelial cell line

Clinically the anaplastic variant of Wilms' tumor differs from the classical Wilms' tumor by its poor prognosis. To begin to understand and characterize the distinctive biology of this rare form of Wilms' tumor, a study of the histology, ultrastructure, and mRNA expression was performed on the anaplastic tumor and its associated cell line. The anaplastic tumor generated mouse heterotransplants that were readily used to establish epithelial cell cultures. The epithelial cultures, in turn, produced tumors when reinjected into nude mice. Microscopic evaluation revealed that the anaplastic epithelial cells were less differentiated than their epithelial counterpart in classical Wilms' tumors. In general the molecular profile of the anaplastic tumor was more consistent with that of an epithelial-rich classic Wilms' tumor than with the classic triphasic Wilms' tumor. Unlike the classic triphasic Wilms' tumor that contains blastema, stroma, and epithelial tubules, the anaplastic tumor expressed only marginal levels of insulin-like growth factor 2 (IGF-2) mRNA and imperceptible levels of the Wilms' tumor gene (WT-1), Pax-2, and Pax-8 mRNA. In common with the classic Wilms' tumor, the anaplastic variant retained the expression of the N-myc gene while failing to express C-myc. A comparison of cultures derived from an epithelial-rich, classic Wilms' tumor and the anaplastic Wilm's tumor indicated that both lacked IGF-2 and WT-1 mRNA expression. However, the well-differentiated epithelial cell culture derived from the classic Wilms' tumor expressed C-myc, Pax-8, and Pax-2 mRNA, none of which were expressed by the anaplastic epithelial cells. Furthermore, the well-differentiated epithelial cell component failed to express N-myc, which was expressed by both the primary triphasic Wilms' tumor and the anaplastic tumor. Overall, the findings indicate that patterns of gene expression within a single component do not correlate with the aggressive clinical behavior of the anaplastic Wilms' tumor.

Nephroblastoma (Wilms' tumor): a model system of aberrant renal development

Wilms' tumor, or nephroblastoma, is a developmental malignancy of the kidney that affects approximately 1 in 10,000 children between 1 and 6 years of age. Typically, the histology of nephroblastoma reveals a disorganized renal developmental process showing blastema and epithelia randomly interspersed in varying amounts of stroma. This developmental disruption is associated with the loss of function of the tumor suppressor gene WT-1. This gene, located on chromosome 11 at band p13, codes for a zinc finger protein that may act as a transcriptional repressor. Familial cases of Wilms' tumor fit Knudson's "two hit" model, according to which a germ line mutation of one WT-1 allele predisposes to the tumor while an additional somatic mutation of the other allele causes malignant transformation. Originally proposed for retinoblastoma, this model defines the nature of the tumor suppressor gene as a gene that is tumorigenic when inactivated. However, not all Wilms' tumor cases fit this model because the majority of Wilms' tumors do not show a mutation of WT-1. For Wilms' tumor, the loss of tumor suppression appears to be more complex than for retinoblastoma. Some of the mechanisms recognized to date involve dominant negative WT-1 mutations, interaction of the WT-1 gene product with other mutated transcription factors such as p53, loss of imprinting, and mutations of other tumor suppressor genes at 11p15 or other loci. Although classic Wilms' tumor is associated with good prognosis (85% survival), its anaplastic form is often fatal. Despite the plethora of knowledge gained in recent years, Wilms' tumor remains the center of attention for further investigation because it offers opportunities for studying normal kidney development, for understanding the molecular basis for clinically important anaplastic forms, as well as for elucidating the molecular mechanisms of tumor suppressor genes. To facilitate this task, Wilms' tumor heterotransplants have been established in nude mice. This provides an indefinite source of tumor tissue and a means to test their growth properties in response to drug treatments or molecular genetic manipulations. Furthermore, the establishment of stable Wilms' tumor cell lines is essential to investigating further the molecular basis of tumorigenesis using recombinant DNA technology.

Induction of metallothionein mRNA and protein following exposure of cultured human proximal tubule cells to cadmium

Humans have a complex expression of metallothionein (MT) genes which involves many MT isoforms encoded by a family of genes containing an upper limit of 12 possible functional genes, in contrast to most animals which have one or two functional MT genes. In the present study, human proximal tubule (HPT) cells were exposed to cadmium (Cd) to determine if these cultures might serve as a model system to study MT gene expression in the renal proximal tubule. Three independent isolates of HPT cells were shown to repeatably induce MT protein when exposed continually to a non-toxic dose of 1 microgram/ml of Cd administered as CdCl2. Accumulation of MT protein was noted within 3 h and persisted over the 16-day time course. The expression of mRNA for the MT-IIA, MT-IA, B, E, F and G genes was also assessed through 16 days of exposure to 1 microgram/ml of Cd versus control media. Of these, the mRNA for the MT-IIA, MT-IE, MT-IF and MT-IG genes were detected in the cells exposed to 1 microgram/ml of Cd. Overall, the results were supportive that the HPT cells can provide a valuable model system to study the regulation of MT gene expression as it applies to the human renal proximal tubule.

Heterogeneity in the amount of ionic cadmium necessary to elicit cell death in independent cultures of human proximal tubule cells

Eleven separate isolates of human renal proximal tubule cells (HPT) were analyzed for toxic response to ionic cadmium exposure over a 16-day period in vitro. This study demonstrates a heterogeneous response to Cd2+ exposure in isolates from different individuals with some individuals nearly 3-times more sensitive to ionic cadmium exposure than others. There was no apparent correlation to the race, sex or age of the individuals in the response to cadmium. In addition, readily identifiable culture artifacts, i.e., culture age, passage number, had no influence on the response to Cd2+ exposure and the different isolates had homogeneous baseline HPT properties and morphology. This difference in response to Cd2+ may reflect a heterogeneous response within the human population to cadmium exposure.

Aminoglycoside antibiotics alter the paracellular transport properties of cultured human proximal tubule cells

Cell cultures retaining properties of the human proximal tubule were utilized to determine whether or not the aminoglycoside antibiotics alter paracellular transport. The transepithelial resistance (RT) of the human proximal tubule (HPT) cell monolayers was determined by Ussing chamber analysis of cells grown on permeable supports. This analysis revealed that RT was reduced as a result of aminoglycoside exposure and that the reduction corresponded to the known clinical nephrotoxicities of the aminoglycosides. Variation in the aminoglycoside concentration necessary to elicit this response was documented using 14 individual cell isolates. Ultrastructural analysis provided evidence indicating that the alterations in RT were not associated with general damage to the HPT cells. An examination of the structure of the tight junctions by freeze-fracture analysis demonstrated only minimal alteration of the sealing strands as a result of aminoglycoside exposure. Consequently, the reductions in RT were not directly associated with discernible tight junction structural alterations. Alteration in the paracellular route of transport, as indicated by altered RT values, was clearly documented as a result of aminoglycoside exposure. In addition, this alteration was accompanied by an increased density of intramembrane particles within the apical cell membrane.

WT1-mediated growth suppression of Wilms tumor cells expressing a WT1 splicing variant

A human Wilms tumor cell line (RM1) was developed to test the tumor suppressor activity of WT1, a zinc finger transcription factor that is expressed in the developing human kidney and is mutationally inactivated in a subset of Wilms tumors. Transfection of each of four wild-type WT1 isoforms suppressed the growth of RM1 cells. The endogenous WT1 transcript in these cells was devoid of exon 2 sequences, a splicing alteration that was also detected in varying amounts in all Wilms tumors tested but not in normal kidney. Production of this abnormal transcript, which encodes a functionally altered protein, may represent a distinct mechanism for inactivating WT1 in Wilms tumors.

Inactivation of WT1 in nephrogenic rests, genetic precursors to Wilms' tumour

Nephrogenic rests consist of foci of primitive renal cells, typically microscopic, that are found within the normal kidney tissue of children with Wilms' tumour. To study the relationship between nephrogenic rests and the associated tumours, we screened these lesions for mutations in the 11p13 Wilms' tumour suppressor gene, WT1. In two cases in which the Wilms' tumour contained a somatic WT1 mutation, the nephrogenic rest had the identical mutation. Nephrogenic rests and Wilms' tumours are therefore topographically distinct lesions that are clonally derived from an early renal stem cell. Inactivation of WT1 appears to be an early genetic event which can lead to the formation of nephrogenic rests, enhancing the probability that additional genetic hits will lead to Wilms' tumour.

Electrical and freeze-fracture analysis of the effects of ionic cadmium on cell membranes of human proximal tubule cells

We previously reported that cell cultures of human proximal tubule (HPT) cells respond to ionic cadmium in a manner consistent with well-defined Cd(2+)-elicited responses reported for in vivo systems. However, one unique finding was that the transepithelial electrical resistance and tight junction sealing strands were altered as a result of Cd2+ exposure at micromolar concentrations. These alterations are reexamined in detail in the present report to determine whether the Cd(2+)-induced alterations are specific alterations in the tight junction structure or reflect a general alteration in the cell membrane. Exhaustive analysis of tight junction sealing strands demonstrated no significant alterations due to Cd2+ exposure, even at the concentration that elicited a significant reduction in transepithelial resistance. Further analysis of intramembrane particle distribution demonstrated a significant increase in apical intramembrane particles, indicating that Cd2+ exposure altered the characteristics of the apical cell membrane. Overall, the results were consistent with evidence of Cd(2+)-induced alteration in the apical cell membrane of the HPT cell.

Pre-exposure to glucosamine induces insulin resistance of glucose transport and glycogen synthesis in isolated rat skeletal muscles. Study of mechanisms in muscle and in rat-1 fibroblasts overexpressing the human insulin receptor

Increased routing of glucose through the hexosamine-biosynthetic pathway has been implicated in the development of glucose-induced insulin resistance of glucose transport in cultured adipocytes. Because both glucosamine and glucose enter this pathway as glucosamine-6-phosphate, we examined the effects of preincubation with glucosamine in isolated rat diaphragms and in fibroblasts overexpressing the human insulin receptor (HIR-cells). In muscles, pre-exposure to glucosamine inhibited subsequent basal and, to a greater extent, insulin-stimulated glucose transport in a time- and dose-dependent manner and abolished the stimulation by insulin of glycogen synthesis. Insulin receptor number, activation of the insulin receptor tyrosine kinase in situ and after solubilization, and the total pool of glucose transporters (GLUT4) were unaffected, and glycogen synthase was activated by glucosamine pretreatment. In HIR-cells, which express GLUT1 and not GLUT4, basal and insulin-stimulated glucose transport were unaffected by glucosamine, but glycogen synthesis was markedly inhibited. Insulin-stimulated activation of protein kinases (MAP and S6) was unaffected, and the fractional velocity and apparent total activity of glycogen synthase was increased in glucosamine-treated HIR-cells. In pulse-labeling studies, addition of glucosamine during the chase prolonged processing of insulin proreceptors to receptors and altered the electrophoretic mobility of proreceptors and processed alpha-subunits, consistent with altered glycosylation. Glucosamine-induced insulin resistance of glucose transport appears to be restricted to GLUT4-expressing cells, i.e., skeletal muscle and adipocytes; it may reflect impaired translocation of GLUT4 to the plasmalemma. The glucosamine-induced imbalance in UDP sugars, i.e., increased UDP-N-acetylhexosamines and decreased UDP-glucose, may alter glycosylation of critical proteins and limit the flux of glucose into glycogen.