Proteasomes Are Critical for Maintenance of CD133+CD24+ Kidney Progenitor Cells

Kidney progenitor cells, although rare and dispersed, play a key role in the repair of renal tubules after acute kidney damage. However, understanding these cells has been challenging due to the limited access to primary renal tissues and the absence of immortalized cells to model kidney progenitors. Previously, our laboratory utilized the renal proximal tubular epithelial cell line, RPTEC/TERT1, and the flow cytometry technique to sort and establish a kidney progenitor cell model called Human Renal Tubular Precursor TERT (HRTPT) which expresses CD133 and CD24 and exhibits the characteristics of kidney progenitors, such as self-renewal capacity and multi-potential differentiation. In addition, a separate cell line was established, named Human Renal Epithelial Cell 24 TERT (HREC24T), which lacks CD133 expression and shows no progenitor features. To further characterize HRTPT CD133+CD24+ progenitor cells, we performed proteomic profiling which showed high proteasomal expression in HRTPT kidney progenitor cells. RT-qPCR, Western blot, and flow cytometry analysis showed that HRTPT cells possess higher proteasomal expression and activity compared to HREC24T non-progenitor cells. Importantly, inhibition of the proteasomes with bortezomib reduced the expression of progenitor markers and obliterated the potential for self-renewal and differentiation of HRTPT progenitor cells. In conclusion, proteasomes are critical in preserving progenitor markers expression and self-renewal capacity in HRTPT kidney progenitors.

Pevonedistat Inhibits SOX2 Expression and Sphere Formation but Also Drives the Induction of Terminal Differentiation Markers and Apoptosis within Arsenite-Transformed Urothelial Cells

Urothelial cancer (UC) is a common malignancy and its development is associated with arsenic exposure. Around 25% of diagnosed UC cases are muscle invasive (MIUC) and are frequently associated with squamous differentiation. These patients commonly develop cisplatin (CIS) resistance and have poor prognosis. SOX2 expression is correlated to reduced overall and disease-free survival in UC. SOX2 drives malignant stemness and proliferation in UC cells and is associated with development of CIS resistance. Using quantitative proteomics, we identified that SOX2 was overexpressed in three arsenite (As3+)-transformed UROtsa cell lines. We hypothesized that inhibition of SOX2 would reduce stemness and increase sensitivity to CIS in the As3+-transformed cells. Pevonedistat (PVD) is a neddylation inhibitor and is a potent inhibitor of SOX2. We treated non-transformed parent and As3+-transformed cells with PVD, CIS, or in combination and monitored cell growth, sphere forming abilities, apoptosis, and gene/protein expression. PVD treatment alone caused morphological changes, reduced cell growth, attenuated sphere formation, induced apoptosis, and elevated the expression of terminal differentiation markers. However, the combined treatment of PVD with CIS significantly elevated the expression of terminal differentiation markers and eventually led to more cell death than either solo treatment. Aside from a reduced proliferation rate, these effects were not seen in the parent. Further research is needed to explore the potential use of PVD with CIS as a differentiation therapy or alternative treatment for MIUC tumors that may have become resistant to CIS.

Primary and Immortalized Cultures of Human Proximal Tubule Cells Possess Both Progenitor and Non-Progenitor Cells That Can Impact Experimental Results

Studies have reported the presence of renal proximal tubule specific progenitor cells which co-express PROM1 and CD24 markers on the cell surface. The RPTEC/TERT cell line is a telomerase-immortalized proximal tubule cell line that expresses two populations of cells, one co-expressing PROM1 and CD24 and another expressing only CD24, identical to primary cultures of human proximal tubule cells (HPT). The RPTEC/TERT cell line was used by the authors to generate two new cell lines, HRTPT co-expressing PROM1 and CD24 and HREC24T expressing only CD24. The HRTPT cell line has been shown to express properties expected of renal progenitor cells while HREC24T expresses none of these properties. The HPT cells were used in a previous study to determine the effects of elevated glucose concentrations on global gene expression. This study showed the alteration of expression of lysosomal and mTOR associated genes. In the present study, this gene set was used to determine if pure populations of cells expressing both PROM1 and CD24 had different patterns of expression than those expressing only CD24 when exposed to elevated glucose concentrations. In addition, experiments were performed to determine whether cross-talk might occur between the two cell lines based on their expression of PROM1 and CD24. It was shown that the expression of the mTOR and lysosomal genes was altered in expression between the HRTPT and HREC24T cell lines based on their PROM1 and CD24 expression. Using metallothionein (MT) expression as a marker demonstrated that both cell lines produced condition media that could alter the expression of the MT genes. It was also determined that PROM1 and CD24 co-expression was limited in renal cell carcinoma (RCC) cell lines.

Arsenite Exposure to Human RPCs (HRTPT) Produces a Reversible Epithelial Mesenchymal Transition (EMT): In-Vitro and In-Silico Study

The human kidney is known to possess renal progenitor cells (RPCs) that can assist in the repair of acute tubular injury. The RPCs are sparsely located as single cells throughout the kidney. We recently generated an immortalized human renal progenitor cell line (HRTPT) that co-expresses PROM1/CD24 and expresses features expected on RPCs. This included the ability to form nephrospheres, differentiate on the surface of Matrigel, and undergo adipogenic, neurogenic, and osteogenic differentiation. These cells were used in the present study to determine how the cells would respond when exposed to nephrotoxin. Inorganic arsenite (iAs) was chosen as the nephrotoxin since the kidney is susceptible to this toxin and there is evidence of its involvement in renal disease. Gene expression profiles when the cells were exposed to iAs for 3, 8, and 10 passages (subcultured at 1:3 ratio) identified a shift from the control unexposed cells. The cells exposed to iAs for eight passages were then referred with growth media containing no iAs and within two passages the cells returned to an epithelial morphology with strong agreement in differential gene expression between control and cells recovered from iAs exposure. Results show within three serial passages of the cells exposed to iAs there was a shift in morphology from an epithelial to a mesenchymal phenotype. EMT was suggested based on an increase in known mesenchymal markers. We found RPCs can undergo EMT when exposed to a nephrotoxin and undergo MET when the agent is removed from the growth media.

Contamination Assessment and Potential Human Health Risks of Heavy Metals in Urban Soils from Grand Forks, North Dakota, USA

Heavy metal (HM) pollution of soil is an increasingly serious problem worldwide. The current study assessed the metal levels and ecological and human health risk associated with HMs in Grand Forks urban soils. A total 40 composite surface soil samples were investigated for Mn, Fe, Co, Ni, Cu, Zn, As, Pb, Hg, Cr, Cd and Tl using microwave-assisted HNO3-HCl acid digestion and inductively coupled plasma mass spectrometry (ICP-MS) analysis. The enrichment factor (EF), contamination factor (CF), geoaccumulation index (Igeo), ecological risk and potential ecological risk index were used for ecological risk assessment. The park soils revealed the following decreasing trend for metal levels: Fe > Mn > Zn > Cr > Ni > Cu > Pb > As > Co > Cd > Tl > Hg. Based on mean levels, all the studied HMs except As and Cr were lower than guideline limits set by international agencies. Principal component analysis (PCA) indicated that Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Pb, Cr and Tl may originate from natural sources, while Hg, Pb, As and Cd may come from anthropogenic/mixed sources. The Igeo results showed that the soil was moderately polluted by As and Cd and, based on EF results, As and Cd exhibited significant enrichment. The contamination factor analysis revealed that Zn and Pb showed moderate contamination, Hg exhibited low to moderate contamination and As and Cd showed high contamination in the soil. Comparatively higher risk was noted for children over adults and, overall, As was the major contributor (>50%), followed by Cr (>13%), in the non-carcinogenic risk assessment. Carcinogenic risk assessment revealed that As and Cr pose significant risks to the populations associated with this urban soil. Lastly, this study showed that the soil was moderately contaminated by As, Cd, Pb and Hg and should be regularly monitored for metal contamination.

Protein interactions with metallothionein-3 promote vectorial active transport in human proximal tubular cells

Metallothionein 3 (MT-3) is a small, cysteine-rich protein that binds to essential metals required for homeostasis, as well as to heavy metals that have the potential to exert toxic effects on cells. MT-3 is expressed by epithelial cells of the human kidney, including the cells of the proximal tubule. Our laboratory has previously shown that mortal cultures of human proximal tubular (HPT) cells express MT-3 and form domes in the cell monolayer, a morphological feature indicative of vectorial active transport, an essential function of the proximal tubule. However, an immortalized proximal tubular cell line HK-2 lacks the expression of MT-3 and fails to form domes in the monolayer. Transfection of HK-2 cells with the MT-3 gene restores dome formation in these cells suggesting that MT-3 is required for vectorial active transport. In order to determine how MT-3 imparts this essential feature to the proximal tubule, we sought to identify proteins that interact either directly or indirectly with MT-3. Using a combination of pulldowns, co-immunoprecipitations, and mass spectrometry analysis, putative protein interactants were identified and subsequently confirmed by Western analysis and confocal microscopy, following which proteins with direct physical interactions were investigated through molecular docking. Our data shows that MT-3 interacts with myosin-9, aldolase A, enolase 1, β-actin, and tropomyosin 3 and that these interactions are maximized at the periphery of the apical membrane of doming proximal tubule cells. Together these observations reveal that MT-3 interacts with proteins involved in cytoskeletal organization and energy metabolism, and these interactions at the apical membrane support vectorial active transport and cell differentiation in proximal tubule cultures.

Role of HRTPT in kidney proximal epithelial cell regeneration: Integrative differential expression and pathway analyses using microarray and scRNA-seq

Damage to proximal tubules due to exposure to toxicants can lead to conditions such as acute kidney injury (AKI), chronic kidney disease (CKD) and ultimately end‐stage renal failure (ESRF). Studies have shown that kidney proximal epithelial cells can regenerate particularly after acute injury. In the previous study, we utilized an immortalized in vitro model of human renal proximal tubule epithelial cells, RPTEC/TERT1, to isolate HRTPT cell line that co‐expresses stem cell markers CD133 and CD24, and HREC24T cell line that expresses only CD24. HRTPT cells showed most of the key characteristics of stem/progenitor cells; however, HREC24T cells did not show any of these characteristics. The goal of this study was to further characterize and understand the global gene expression differences, upregulated pathways and gene interaction using scRNA‐seq in HRTPT cells. Affymetrix microarray analysis identified common gene sets and pathways specific to HRTPT and HREC24T cells analysed using DAVID, Reactome and Ingenuity software. Gene sets of HRTPT cells, in comparison with publicly available data set for CD133+ infant kidney, urine‐derived renal progenitor cells and human kidney‐derived epithelial proximal tubule cells showed substantial similarity in organization and interactions of the apical membrane. Single‐cell analysis of HRTPT cells identified unique gene clusters associated with CD133 and the 92 common gene sets from three data sets. In conclusion, the gene expression analysis identified a unique gene set for HRTPT cells and narrowed the co‐expressed gene set compared with other human renal–derived cell lines expressing CD133, which may provide deeper understanding in their role as progenitor/stem cells that participate in renal repair.

Postbaccalaureate terminal degree and career choices of students who performed undergraduate research

This study analyzed terminal degree and career choices of students who performed undergraduate research. In one analysis, the study compared terminal degree and career choices between a course-based undergraduate research experience (CURE) and traditional non-course-based undergraduate research experiences at one primarily undergraduate institution (PUI). Students who pursued postbaccalaureate programs chose terminal degrees at levels exceeding 75%, with no significant difference between a CURE experience and a traditional research experience. Analysis of terminal degree and career choices at four PUIs providing traditional research experiences showed a marked difference in the number of students pursuing terminal degrees. Two PUIs showed rates > 75%, whereas students at the other two PUIs pursued terminal degrees <50% of the time. The majority of students not pursuing terminal degrees chose M.S. degrees in education and healthcare. An analysis was also performed among students participating in traditional summer undergraduate research on a research-intensive university (RIU) campus with a medical school. Students were accepted from two programs, an NIH IDeA Network of Biomedical Research Excellence (INBRE) program recruiting students from the RIU and an NSF Research Experiences for Undergraduates (REU) program recruiting undergraduates from rural PUIs and minority-serving institutions, particularly tribal colleges. Analysis showed that >70% of the students who pursued postbaccalaureate programs chose terminal degrees. INBRE undergraduates displayed a marked preference for the M.D. degree (73.9% vs. 17.4%), whereas the REU students chose the Ph.D. degree (75.0% vs. 22.9%). American Indian students were also analyzed separately for career choice and showed an equal preference for the M.D. and Ph.D. degrees when pursuing postbaccalaureate education. Overall, the results provide evidence that undergraduate student research stimulates student careers in areas needed by the nation's citizen stakeholders.

Zinc, Zinc Transporters, and Cadmium Cytotoxicity in a Cell Culture Model of Human Urothelium

We explored the potential role of zinc (Zn) and zinc transporters in protection against cytotoxicity of cadmium (Cd) in a cell culture model of human urothelium, named UROtsa. We used real-time qRT-PCR to quantify transcript levels of 19 Zn transporters of the Zrt-/Irt-like protein (ZIP) and ZnT gene families that were expressed in UROtsa cells and were altered by Cd exposure. Cd as low as 0.1 µM induced expression of ZnT1, known to mediate efflux of Zn and Cd. Loss of cell viability by 57% was seen 24 h after exposure to 2.5 µM Cd. Exposure to 2.5 µM Cd together with 10–50 µM Zn prevented loss of cell viability by 66%. Pretreatment of the UROtsa cells with an inhibitor of glutathione biosynthesis (buthionine sulfoximine) diminished ZnT1 induction by Cd with a resultant increase in sensitivity to Cd cytotoxicity. Conversely, pretreatment of UROtsa cells with an inhibitor of DNA methylation, 5-aza-2’-deoxycytidine (aza-dC) did not change the extent of ZnT1 induction by Cd. The induced expression of ZnT1 that remained impervious in cells treated with aza-dC coincided with resistance to Cd cytotoxicity. Therefore, expression of ZnT1 efflux transporter and Cd toxicity in UROtsa cells could be modulated, in part, by DNA methylation and glutathione biosynthesis. Induced expression of ZnT1 may be a viable mechanistic approach to mitigating cytotoxicity of Cd.

Elevated glucose represses lysosomal and mTOR-related genes in renal epithelial cells composed of progenitor CD133+ cells

Hyperglycemia is one of the major health concern in many parts of the world. One of the serious complications of high glucose levels is diabetic nephropathy. The preliminary microarray study performed on primary human renal tubular epithelial (hRTE) cells exposed to high glucose levels showed a significant downregulation of mTOR as well as its associated genes as well as lysosomal genes. Based on this preliminary data, the expression of various lysosomal genes as well as mTOR and its associated genes were analyzed in hRTE cells exposed to 5.5, 7.5, 11 and 16 mM glucose. The results validated the microarray analysis, which showed a significant decrease in the mRNA as well as protein expression of the selected genes as the concentration of glucose increased. Co-localization of lysosomal marker, LAMP1 with mTOR showed lower expression of mTOR as the glucose concentration increased, suggesting decrease in mTOR activity. Although the mechanism by which glucose affects the regulation of lysosomal genes is not well known, our results suggest that high levels of glucose may lead to decrease in mTOR expression causing the cells to enter an anabolic state with subsequent downregulation of lysosomal genes.

Meta-analysis of gene expression profiling reveals novel basal gene signatures in MCF-10A cells transformed with cadmium

Cadmium (Cd²⁺) is an environmental toxicant and a human carcinogen. Several studies show an association of Cd²⁺ exposure to the development of breast cancer. Previously, we have transformed the immortalized non-tumorigenic cell line MCF-10A with Cd²⁺ and have demonstrated that the transformed cells have anchorage independent growth. In a separate study, we showed that transformation of the immortalized urothelial cells with the environmental carcinogen arsenite (As³⁺) results in an increase in expression of genes associated with the basal subtype of bladder cancer. In this study, we determined if transformation of the MCF-10A cells with Cd²⁺ would have a similar effect on the expression of basal genes. The results of our study indicate that there is a decrease in expression of genes associated with keratinization and cornification and this gene signature includes the genes associated with the basal subtype of breast cancer. An analysis of human breast cancer databases indicates an increased expression of this gene signature is associated with a positive correlation to patient survival whereas a reduced expression/absence of this gene signature is associated with poor patient survival. Thus, our study suggests that transformation of the MCF-10A cells with Cd²⁺ produces a decreased basal gene expression profile that correlates to patient outcome.

Activation of PPARγ and inhibition of cell proliferation reduces key proteins associated with the basal subtype of bladder cancer in As3+-transformed UROtsa cells

Environmental exposure to arsenite (As³⁺) has a strong association with the development of human urothelial cancer (UC) and is the 5^th most common cancer in men and the 12^th most common cancer in women. Muscle invasive urothelial cancer (MIUC) are grouped into basal or luminal molecular subtypes based on their gene expression profile. The basal subtype is more aggressive and can be associated with squamous differentiation, characterized by high expression of keratins (KRT1, 5, 6, 14, and 16) and epidermal growth factor receptor (EGFR) within the tumors. The luminal subtype is less aggressive and is predominately characterized by elevated gene expression of peroxisome proliferator-activated receptor- gamma (PPARγ) and forkhead box protein A1 (FOXA1). We have previously shown that As³⁺-transformed urothelial cells (As-T) exhibit a basal subtype of UC expressing genes associated with squamous differentiation. We hypothesized that the molecular subtype of the As-T cells could be altered by inducing the expression of PPARγ and/or inhibiting the proliferation of the cells. Non-transformed and As-T cells were treated with Troglitazone (TG, PPARG agonist, 10 μM), PD153035 (PD, an EGFR inhibitor, 1 μM) or a combination of TG and PD for 3 days. The results obtained demonstrate that treatment of the As-T cells with TG upregulated the expression of PPARγ and FOXA1 whereas treatment with PD decreased the expression of some of the basal keratins. However, a combined treatment of TG and PD resulted in a consistent decrease of several proteins associated with the basal subtype of bladder cancers (KRT1, KRT14, KRT16, P63, and TFAP2A). Our data suggests that activation of PPARγ while inhibiting cell proliferation facilitates the regulation of genes involved in maintaining the luminal subtype of UC. In vivo animal studies are needed to address the efficacy of using PPARγ agonists and/or proliferation inhibitors to reduce tumor grade/stage of MIUC.

Characterization and determination of cadmium resistance of CD133+/CD24+ and CD133-/CD24+ cells isolated from the immortalized human proximal tubule cell line, RPTEC/TERT1

Stem/progenitor cells are involved in the regeneration of the renal tubules after damage due to a toxic insult. However, the mechanism involved in the regeneration of the tubules by the stem cells is not well understood due to the lack of immortal cell lines that represent the stem/progenitor cells of the kidney. A previous study from our laboratory has shown that the immortalized cell line RPTEC/TERT1 contains two populations of cells, one co-expressing CD24 and CD133, the other expressing CD24 only. The goal of the present study was to determine if both these populations could be sorted into separate independent cultures and if so, determine their characteristic features and response to the nephrotoxicant cadmium. The results of our study show that both the populations of cells could grow as independent cultures and maintain their phenotype after extended sub-culture. The CD133⁺/CD24⁺ co-expressing cells formed multicellular spheroids (nephrospheres), a characteristic feature of stem/progenitor cells, and formed branched tubule-like structures when grown on the surface of matrigel, whereas the CD133^-/CD24⁺ cells were unable to form these structures. The CD133⁺/CD24⁺ cells were able to grow and undergo neurogenic, adipogenic, osteogenic, and tubulogenic differentiation, whereas the CD133^-/CD24⁺ cells expressed some of the differentiation markers but were unable to grow in some of the specialized growth media. The CD133⁺/ CD24⁺ co-expressing cells had a shorter doubling time compared to the cells that expressed only CD24, and were more resistant to the toxic effects of the heavy metal, cadmium. In conclusion, the isolation and characterization of these two cell populations form the RPTEC/TERT1 cell line will facilitate the development of studies that determine the mechanisms involved in tubular damage and regeneration particularly after a toxic insult.

Enrichment of genes associated with squamous differentiation in cancer initiating cells isolated from urothelial cells transformed by the environmental toxicant arsenite

Arsenic is an environmental toxicant with long-term exposure associated with the development of urothelial carcinomas. Our lab has developed an in-vitro model of urothelial carcinoma by exposing the immortal, but non-tumorigenic bladder cell line, the UROtsa, to arsenite (As³⁺). These transformed cells form tumors in immune-compromised mice, which resemble urothelial carcinomas with components of the tumor exhibiting squamous differentiation. The goal of the present study was to determine the differences in global gene expression patterns between the As³⁺-transformed UROtsa cells and the urospheres (spheroids containing putative cancer initiating cells) isolated from these cell lines and to determine if the genes involved in the development of squamous differentiation were enriched in the urospheres. The results obtained in this study show an enrichment of genes such as KRT1, KRT5, KRT6A, KRT6B, KRT6C, KRT14 and KRT16 associated with squamous differentiation, a characteristic feature seen in aggressive basal subtypes of urothelial cell carcinoma (UCC) in the urospheres isolated from As³⁺-transformed UROtsa cells. In addition, there is increased expression of several of the small proline-rich proteins (SPRR) in the urospheres and overexpression of these genes occur in UCC's displaying squamous differentiation. In conclusion, the cancer initiating cells present in the urospheres are enriched with genes associated with squamous differentiation.

The urothelial cell line UROtsa transformed by arsenite and cadmium display basal characteristics associated with muscle invasive urothelial cancers

Muscle invasive urothelial carcinomas are divided into various molecular subtypes with basal and luminal subtypes being the prominent ones. The basal muscle-invasive urothelial carcinomas are generally more aggressive at presentation and significantly enriched with squamous features. Our laboratory has developed an in-vitro model of urothelial cancer by transforming the immortalized cell line UROtsa with arsenite (As³⁺) and cadmium (Cd²⁺). In this study, we characterized the tumors formed by these transformed cell lines as more basal-like based on their gene expression patterns with increased expression of KRT1, KRT5, KRT6, KRT14, KRT16, KRT17 and CD44. In addition, histological examination of these tumor transplants showed squamous features enriched in basal muscle invasive urothelial carcinomas. The expression of these genes increased in the transformed cell lines as well as in the urospheres, which are putative cancer initiating cells/stem cells derived from the cell lines. There was also increased expression of these genes in the urospheres derived from the parent UROtsa cell line. Thus, our data shows that the As³⁺ and Cd²⁺-transformed cell lines and their derived tumor transplants have gene expression profiles similar to the basal subtype of muscle invasive bladder carcinomas with tumors having enriched squamous features. The increased expression of basal markers in the urospheres suggests that stem cells may be involved in the development of squamous differentiation seen in some of the muscle invasive bladder carcinomas.

STEERing an IDeA in Undergraduate Research at a Rural Research Intensive University

This study documents outcomes, including student career choices, of the North Dakota Institutional Development Award Networks of Biomedical Research Excellence program that provides 10-week, summer undergraduate research experiences at the University of North Dakota School of Medicine and Health Sciences. Program evaluation initiated in 2008 and, to date, 335 students have completed the program. Of the 335, 214 students have successfully completed their bachelor’s degree, 102 are still undergraduates, and 19 either did not complete a bachelor’s degree or were lost to follow-up. The program was able to track 200 of the 214 students for education and career choices following graduation. Of these 200, 76% continued in postgraduate health-related education; 34.0% and 20.5% are enrolled in or have completed MD or PhD programs, respectively. Other postbaccalaureate pursuits included careers in pharmacy, optometry, dentistry, public health, physical therapy, nurse practitioner, and physician’s assistant, accounting for an additional 21.5%. Most students electing to stop formal education at the bachelor’s degree also entered fields related to health care or science, technology, engineering, and mathematics (19.5%), with only a small number of the 200 students tracked going into service or industries which lacked an association with the health-care workforce (4.5%). These student outcomes support the concept that participation in summer undergraduate research boosts efforts to populate the pipeline of future researchers and health professionals. It is also an indication that future researchers and health professionals will be able to communicate the value of research in their professional and social associations. The report also discusses best practices and issues in summer undergraduate research for students originating from rural environments.

Human renal tubular cells contain CD24/CD133 progenitor cell populations: Implications for tubular regeneration after toxicant induced damage using cadmium as a model

The proximal tubules of the kidney are target sites of injury by various toxicants. Cadmium (Cd⁺²), an environmental nephrotoxicant can cause adverse effects and overt renal damage. To decipher the mechanisms involved in nephrotoxicity, an in vitro model system is required. Mortal cultures of human proximal tubule (HPT) cells have served, as models but are difficult to acquire and do not lend themselves to stable transfection. The immortalized human proximal tubule cell line HK-2, has served as a model but it lacks vectorial active transport and shows signs of lost epithelial features. Recently a new proximal tubule cell line was developed, the RPTEC/TERT1, and the goal of this study was to determine if this cell line could serve as a model to study nephrotoxicity. Global gene expression analysis of this cell line in comparison to the HK-2 and HPT cells showed that the RPTEC/TERT1 cells had gene expression patterns similar to HPT cells when compared to the HK-2 cells. The HPT and the RPTEC/TERT1 cell line had an increased population of stem/progenitor cells co-expressing CD24 and CD133 when compared to the HK-2 cells. The level of expression of cadherins, claudins and occludin molecules was also similar between the RPTEC/TERT1 and the HPT cells. Acute exposure to Cd⁺² resulted in necrosis of the RPTEC/TERT1 cells when compared to the HK-2 cells which died by apoptosis. Thus, the RPTEC/TERT1 cells are similar to HPT cells and can serve as a good model system to study mechanisms involved in toxicant induced renal damage.

The unique C- and N-terminal sequences of Metallothionein isoform 3 mediate growth inhibition and Vectorial active transport in MCF-7 cells

Background

The 3rd isoform of the metallothionein (MT3) gene family has been shown to be overexpressed in most ductal breast cancers. A previous study has shown that the stable transfection of MCF-7 cells with the MT3 gene inhibits cell growth. The goal of the present study was to determine the role of the unique C-terminal and N-terminal sequences of MT3 on phenotypic properties and gene expression profiles of MCF-7 cells.

Methods

MCF-7 cells were transfected with various metallothionein gene constructs which contain the insertion or the removal of the unique MT3 C- and N-terminal domains. Global gene expression analysis was performed on the MCF-7 cells containing the various constructs and the expression of the unique C- and N- terminal domains of MT3 was correlated to phenotypic properties of the cells.

Results

The results of the present study demonstrate that the C-terminal sequence of MT3, in the absence of the N-terminal sequence, induces dome formation in MCF-7 cells, which in cell cultures is the phenotypic manifestation of a cell’s ability to perform vectorial active transport. Global gene expression analysis demonstrated that the increased expression of the GAGE gene family correlated with dome formation. Expression of the C-terminal domain induced GAGE gene expression, whereas the N-terminal domain inhibited GAGE gene expression and that the effect of the N-terminal domain inhibition was dominant over the C-terminal domain of MT3. Transfection with the metallothionein 1E gene increased the expression of GAGE genes. In addition, both the C- and the N-terminal sequences of the MT3 gene had growth inhibitory properties, which correlated to an increased expression of the interferon alpha-inducible protein 6.

Conclusions

Our study shows that the C-terminal domain of MT3 confers dome formation in MCF-7 cells and the presence of this domain induces expression of the GAGE family of genes. The differential effects of MT3 and metallothionein 1E on the expression of GAGE genes suggests unique roles of these genes in the development and progression of breast cancer. The finding that interferon alpha-inducible protein 6 expression is associated with the ability of MT3 to inhibit growth needs further investigation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3355-9) contains supplementary material, which is available to authorized users.

Elevated connexin 43 expression in arsenite-and cadmium-transformed human bladder cancer cells, tumor transplants and selected high grade human bladder cancers

Connexin 43 has been shown to play a role in cell migration and invasion; however, its role in bladder cancer is not well defined. Previous studies from our laboratory have shown that the environmental pollutants arsenite and cadmium can cause malignant transformation of the immortalized urothelial cell line UROtsa. These transformed cells can form tumors in immune-compromised mice. The goal of the present study was to determine if connexin 43 is expressed in the normal human bladder, the arsenite and cadmiun-transformed UROtsa cells as well as human urothelial cancer. The results obtained showed that connexin 43 is not expressed in the epithelial cells of the human bladder but is expressed in immortalized cultures of human urothelial cells and the expression is variable in the arsenite and cadmium- transformed urothelial cell lines derived from these immortalized cells. Tumor heterotransplants generated from the transformed cells expressed connexin 43 and the expression was localized to areas of squamous differentiation. Immuno-histochemical analysis of human bladder cancers also showed that the expression of connexin 43 was localized to areas of the tumor that showed early features of squamous differentiation. Treatment of UROtsa cells with various concentrations of arsenite or cadmium did not significantly alter the expression level of connexin 43. In conclusion, our results show that the expression of connexin 43 is localized to the areas of the tumor that show squamous differentiation, which may be an indicator of poor prognosis. This suggests that connexin 43 has the potential to be developed as a biomarker for bladder cancer that may have the ability to invade and metastasize.

Loss of N-Cadherin Expression in Tumor Transplants Produced From As+3- and Cd+2-Transformed Human Urothelial (UROtsa) Cell Lines

BACKGROUND

Epithelial to mesenchymal transition is a process in which a cell experiences a loss of epithelial cell characteristics and acquires a more mesenchymal cell phenotype. In cancer, epithelial to mesenchymal transition has been proposed to play an important role during specific stages of tumor progression. The role epithelial to mesenchymal transition and mesenchymal to epithelial transition might play in toxicant-induced urothelial cancer is unknown.

METHODS

Real-time PCR, Western blotting, immuno-histochemistry and immuno-fluorescence were used to determine the expression of E- and N-cadherin in the UROtsa parent, the As+3- and Cd+2-transformed cell lines, the spheroids isolated from these cell lines as well as the tumor heterotransplants that were produced by the injection of the transformed cells into immune compromised mice.

RESULTS

This study showed that N-cadherin expression was increased in 6 As+3- and 7 Cd+2- transformed cell lines generated from human urothelial cells (UROtsa). The expression varied within each cell line, with 10% to 95% of the cells expressing N-cadherin. Tumors produced from these cell lines showed no expression of the N-cadherin protein. Spheroids which are made up of putative cancer initiating cells produced from these cell lines showed only background expression of N-cadherin mRNA, increased expression of aldehyde dehydrogenase 1 mRNA and produced tumors which did not express N-cadherin. There was no change in the expression of E-cadherin in the tumors, and the tumors formed by all the As+3 and Cd+2-transformed cell lines and cancer initiating cells stained intensely and uniformly for E-cadherin.

CONCLUSIONS

The finding that the cells expressing N-cadherin gave rise to tumors with no expression of N-cadherin is in agreement with the classical view of epithelial to mesenchymal transition. Epithelial to mesenchymal transition and N-cadherin are associated with dissemination and not with the ability to establish new tumor growth. Mesenchymal to epithelial transition and E-cadherin are viewed as necessary for a cell to establish a new metastatic site. The lack of N-cadherin expression in tumor transplants is consistent with E-cadherin expressing cells "seeding" a site for tumor growth. The study shows that a minority population of cultured cells can be the initiators of tumor growth.

SPARC Expression Is Selectively Suppressed in Tumor Initiating Urospheres Isolated from As+3- and Cd+2-Transformed Human Urothelial Cells (UROtsa) Stably Transfected with SPARC

BACKGROUND

This laboratory previously analyzed the expression of SPARC in the parental UROtsa cells, their arsenite (As(+3)) and cadmium (Cd(+2))-transformed cell lines, and tumor transplants generated from the transformed cells. It was demonstrated that SPARC expression was down-regulated to background levels in Cd(+2)-and As(+3)-transformed UROtsa cells and tumor transplants compared to parental cells. In the present study, the transformed cell lines were stably transfected with a SPARC expression vector to determine the effect of SPARC expression on the ability of the cells to form tumors in immune-compromised mice.

METHODS

Real time PCR, western blotting, immunohistochemistry, and immunofluorescence were used to define the expression of SPARC in the As(+3)-and Cd(+2)-transformed cell lines, and urospheres isolated from these cell lines, following their stable transfection with an expression vector containing the SPARC open reading frame (ORF). Transplantation of the cultured cells into immune-compromised mice by subcutaneous injection was used to assess the effect of SPARC expression on tumors generated from the above cell lines and urospheres.

RESULTS

It was shown that the As(+3)-and Cd(+2)-transformed UROtsa cells could undergo stable transfection with a SPARC expression vector and that the transfected cells expressed both SPARC mRNA and secreted protein. Tumors formed from these SPARC-transfected cells were shown to have no expression of SPARC. Urospheres isolated from cultures of the SPARC-transfected As(+3)-and Cd(+2)-transformed cell lines were shown to have only background expression of SPARC. Urospheres from both the non-transfected and SPARC-transfected cell lines were tumorigenic and thus fit the definition for a population of tumor initiating cells.

CONCLUSIONS

Tumor initiating cells isolated from SPARC-transfected As(+3)-and Cd(+2)-transformed cell lines have an inherent mechanism to suppress the expression of SPARC mRNA.

Cadherin expression, vectorial active transport, and metallothionein isoform 3 mediated EMT/MET responses in cultured primary and immortalized human proximal tubule cells

BACKGROUND

Cultures of human proximal tubule cells have been widely utilized to study the role of EMT in renal disease. The goal of this study was to define the role of growth media composition on classic EMT responses, define the expression of E- and N-cadherin, and define the functional epitope of MT-3 that mediates MET in HK-2 cells.

METHODS

Immunohistochemistry, microdissection, real-time PCR, western blotting, and ELISA were used to define the expression of E- and N-cadherin mRNA and protein in HK-2 and HPT cell cultures. Site-directed mutagenesis, stable transfection, measurement of transepithelial resistance and dome formation were used to define the unique amino acid sequence of MT-3 associated with MET in HK-2 cells.

RESULTS

It was shown that both E- and N-cadherin mRNA and protein are expressed in the human renal proximal tubule. It was shown, based on the pattern of cadherin expression, connexin expression, vectorial active transport, and transepithelial resistance, that the HK-2 cell line has already undergone many of the early features associated with EMT. It was shown that the unique, six amino acid, C-terminal sequence of MT-3 is required for MT-3 to induce MET in HK-2 cells.

CONCLUSIONS

The results show that the HK-2 cell line can be an effective model to study later stages in the conversion of the renal epithelial cell to a mesenchymal cell. The HK-2 cell line, transfected with MT-3, may be an effective model to study the process of MET. The study implicates the unique C-terminal sequence of MT-3 in the conversion of HK-2 cells to display an enhanced epithelial phenotype.

Metallothionein isoform 3 expression in human skin, related cancers and human skin derived cell cultures

Human skin is a well known target site of inorganic arsenic with effects ranging from hyperkeratosis to dermal malignancies. The current study characterizes the expression of a protein known to bind inorganic, As(3+), metallothionein 3 (MT-3). Expression of this protein was assessed immunohistochemically with a specific MT-3 antibody on human formalin-fixed, paraffin-embedded biopsy specimens in normal skin, squamous cell carcinoma (SCC), basal cell carcinoma (BCC) and melanoma. Assessment in normal skin using nine normal specimens showed moderate to intense MT-3 staining in epidermal karatinocytes with staining extending into the basal cells and moderate to intense staining in melanocytes of nevi. MT-3 immunoexpression was shown to be moderate to intense in 12 of 13 of SCC, low to moderate in 8 of 10 BCC, and moderate to intense in 12 melanoma samples. MT-3 expression in cell culture models (normal human epidermal keratinocytes, normal human melanocytes, and HaCaT cells) showed only trace expression of MT-3, while exposures to the histone deacytalase inhibitor, MS-275, partially restored expression levels. These results indicate that the epidermis of human skin and resulting malignancies express high level of MT-3 and potentially impact on the known association of arsenic exposure and the development of skin disorders and related cancers.

Quantitation of human metallothionein isoforms: a family of small, highly conserved, cysteine-rich proteins

Human metallothioneins (MTs) are important regulators of metal homeostasis and protectors against oxidative damage. Their altered mRNA expression has been correlated with metal toxicity and a variety of cancers. Current immunodetection methods lack the specificity to distinguish all 12 human isoforms. Each, however, can be distinguished by the mass of its acetylated, cysteine-rich, hydrophilic N-terminal tryptic peptides. These properties were exploited to develop a bottom-up MALDI-TOF/TOF-MS-based method for their simultaneous quantitation. Key features included enrichment of N-terminal acetylated peptides by strong cation exchange chromatography, optimization of C18 reversed-phase chromatography, and control of methionine oxidation. Combinations of nine isoforms were identified in seven cell lines and two tissues. Relative quantitation was accomplished by comparing peak intensities of peptides generated from pooled cytosolic proteins alkylated with ¹⁴N- or ¹⁵N-iodoacetamide. Absolute quantitation was achieved using ¹⁵N-iodoacetamide-labeled synthetic peptides as internal standards. The method was applied to the cadmium induction of MTs in human kidney HK-2 epithelial cells expressing recombinant MT-3. Seven isoforms were detected with abundances spanning almost 2 orders of magnitude and inductions up to 12-fold. The protein-to-mRNA ratio for MT-1E was one-tenth that of other MTs, suggesting isoform-specific differences in protein expression efficiency. Differential expression of MT-1G1 and MT-1G2 suggested tissue- and cell-specific alternative splicing for the MT-1G isoform. Protein expression of MT isoforms was also evaluated in human breast epithelial cancer cell lines. Estrogen-receptor-positive cell lines expressed only MT-2 and MT-1X, whereas estrogen-receptor-negative cell lines additionally expressed MT-1E. The combined expression of MT isoforms was 38-fold greater in estrogen-receptor-negative cell lines than in estrogen-receptor-positive cells. These findings demonstrate that individual human MT isoforms can be accurately quantified in cells and tissues at the protein level, complementing and expanding mRNA measurement as a means for evaluating MTs as potential biomarkers for cancers or heavy metal toxicity.

Prediction of the number of activated genes in multiple independent Cd(+2)- and As(+3)-induced malignant transformations of human urothelial cells (UROtsa)

BACKGROUND

Many toxic environmental agents such as cadmium and arsenic are found to be epidemiologically linked to increasing rates of cancers. In vitro studies show that these toxic agents induced malignant transformation in human cells. It is not clear whether such malignant transformation induced by a single toxic agent is driven by a common set of genes. Although the advancement of high-throughput technology has facilitated the profiling of global gene expression, it remains a question whether the sample size is sufficient to identify this common driver gene set.

RESULTS

We have developed a statistical method, SOFLR, to predict the number of common activated genes using a limited number of microarray samples. We conducted two case studies, cadmium and arsenic transformed human urothelial cells. Our method is able to precisely predict the number of stably induced and repressed genes and the number of samples to identify the common activated genes. The number of independent transformed isolates required for identifying the common activated genes is also estimated. The simulation study further validated our method and identified the important parameters in this analysis.

CONCLUSIONS

Our method predicts the degree of similarity and diversity during cell malignant transformation by a single toxic agent. The results of our case studies imply the existence of common driver and passenger gene sets in toxin-induced transformation. Using a pilot study with small sample size, this method also helps microarray experimental design by determining the number of samples required to identify the common activated gene set.

Short and long term gene expression variation and networking in human proximal tubule cells when exposed to cadmium

Cadmium (Cd²⁺) is a known nephrotoxin causing tubular necrosis during acute exposure and potentially contributing to renal failure in chronic long-term exposure. To investigate changes in global gene expression elicited by cadmium, an in-vitro exposure system was developed from cultures of human renal epithelial cells derived from cortical tissue obtained from nephrectomies. These cultures exhibit many of the qualities of proximal tubule cells. Using these cells, a study was performed to determine the cadmium-induced global gene expression changes after short-term (1 day, 9, 27, and 45 μM) and long-term cadmium exposure (13 days, 4.5, 9, and 27 μM). These studies revealed fundamental differences in the types of genes expressed during each of these time points. The obtained data was further analyzed using regression to identify cadmium toxicity responsive genes. Regression analysis showed 403 genes were induced and 522 genes were repressed by Cd²⁺ within 1 day, and 366 and 517 genes were induced and repressed, respectively, after 13 days. We developed a gene set enrichment analysis method to identify the cadmium induced pathways that are unique in comparison to traditional approaches. The perturbation of global gene expression by various Cd²⁺ concentrations and multiple time points enabled us to study the transcriptional dynamics and gene interaction using a mutual information-based network model. The most prominent network module consisted of INHBA, KIF20A, DNAJA4, AKAP12, ZFAND2A, AKR1B10, SCL7A11, and AKR1C1.

ZIP8 expression in human proximal tubule cells, human urothelial cells transformed by Cd+2 and As+3 and in specimens of normal human urothelium and urothelial cancer

Background

ZIP8 functions endogenously as a Zn⁺²/HCO₃^- symporter that can also bring cadmium (Cd⁺²) into the cell. It has also been proposed that ZIP8 participates in Cd-induced testicular necrosis and renal disease. In this study real-time PCR, western analysis, immunostaining and fluorescent localization were used to define the expression of ZIP8 in human kidney, cultured human proximal tubule (HPT) cells, normal and malignant human urothelium and Cd⁺² and arsenite (As⁺³) transformed urothelial cells.

Results

It was shown that in the renal system both the non-glycosylated and glycosylated form of ZIP8 was expressed in the proximal tubule cells with localization of ZIP8 to the cytoplasm and cell membrane; findings in line with previous studies on ZIP8. The studies in the bladder were the first to show that ZIP8 was expressed in normal urothelium and that ZIP8 could be localized to the paranuclear region. Studies in the UROtsa cell line confirmed a paranuclear localization of ZIP8, however addition of growth medium to the cells increased the expression of the protein in the UROtsa cells. In archival human samples of the normal urothelium, the expression of ZIP8 was variable in intensity whereas in urothelial cancers ZIP8 was expressed in 13 of 14 samples, with one high grade invasive urothelial cancer showing no expression. The expression of ZIP8 was similar in the Cd⁺² and As⁺³ transformed UROtsa cell lines and their tumor transplants.

Conclusion

This is the first study which shows that ZIP8 is expressed in the normal urothelium and in bladder cancer. In addition the normal UROtsa cell line and its transformed counterparts show similar expression of ZIP8 compared to the normal urothelium and the urothelial cancers suggesting that the UROtsa cell line could serve as a model system to study the expression of ZIP8 in bladder disease.

Increased neuron specific enolase expression by urothelial cells exposed to or malignantly transformed by exposure to Cd²⁺ or As³⁺

Neuron specific enolase (ENO2, γ-enolase) is a biomarker used to help identify neuroendocrine differentiation in tumors. This laboratory has shown that ENO2 might be a biomarker for exposure to cadmium and arsenite. In this study these observations are extended to the urothelial cell, where environmental exposures are strongly linked to urothelial cancer. The UROtsa urothelial cell line and its Cd²⁺- and As³⁺-transformed counterparts were used as the model. Acute exposure of the UROtsa cells to both As³⁺- and Cd²⁺-caused significant increases in ENO2 expression. Treatment with the histone deacetlyase inhibitor was also shown to significantly increase the expression of ENO2 mRNA. The expression of ENO2 was significantly elevated in the Cd²⁺- and As³⁺-transformed UROtsa cells and tumor transplants. In contrast, ENO1, was unaffected by exposure to As³⁺ or Cd²⁺. Immunofluorescence showed ENO2 associated with both the nucleus and cytoplasm and cytoplasmic ENO2 co-localized with ENO1. The findings extend the evidence suggesting a link between As³⁺ and Cd²⁺ exposure and neuroendocrine differentiation in tumors. The results suggest that ENO2 might be a biomarker of human exposure to Cd²⁺ and As³⁺ that operates through histone modification.

Arsenic, cadmium and neuron specific enolase (ENO2, γ-enolase) expression in breast cancer

Background

Neuron specific enolase (ENO2, γ-enolase) has been used as a biomarker to help identify neuroendocrine differentiation in breast cancer. The goal of the present study was to determine if ENO2 expression in the breast epithelial cell is influenced by the environmental pollutants, arsenite and cadmium. Acute and chronic exposure of MCF-10A cells to As⁺³ and Cd⁺² sufficient to allow colony formation in soft agar, was used to determine if ENO2 expression was altered by these pollutants.

Results

It was shown that both As⁺³ and Cd⁺² exposure caused significant increases in ENO2 expression under conditions of both acute and chronic exposure. In contrast, ENO1, the major glycolytic enolase in non-muscle and neuronal cells, was largely unaffected by exposure to either As⁺³ or Cd⁺². Localization studies showed that ENO2 in the MCF-10A cells transformed by As⁺³ or Cd⁺² had both a cytoplasmic and nuclear localization. In contrast, ENO1 was localized to the cytoplasm. ENO2 localized to the cytoplasm was found to co-localized with ENO1.

Conclusion

The results are the first to show that ENO2 expression in breast epithelial cells is induced by acute and chronic exposure to As⁺³ or Cd⁺². The findings also suggest a possible link between As⁺³ and Cd⁺² exposure and neuroendocrine differentiation in tumors. Overall, the results suggest that ENO2 might be developed as a biomarker indicating acute and/or chronic environmental exposure of the breast epithelial cell to As⁺³ and Cd⁺².

Comparison of expression patterns of keratin 6, 7, 16, 17, and 19 within multiple independent isolates of As(+3)- and Cd (+2)-induced bladder cancer : keratin 6, 7, 16, 17, and 19 in bladder cancer

This laboratory has generated a series of seven cadmium (Cd(+2))- and six arsenite (As(+3))-transformed urothelial cancer cell lines by exposure of parental UROtsa cells to each agent under similar conditions of exposure. In this study, the seven Cd(+2)-transformed cell lines were characterized for the expression of keratin 6, 16, and 17 while the six As(+3) cell lines were assessed for the expression of keratin 7 and 19. The results showed that the series of Cd(+2)-transformed cell lines and their respective transplants all had expression of keratin 6, 16, and 17 mRNA and protein. The expression of keratin 6, 16, and 17 was also correlated with areas of the urothelial tumor cells that had undergone squamous differentiation. The results also showed that four of the six As(+3)-transformed cell lines had expression of keratin 7 and 19 mRNA and protein and produced subcutaneous tumors with intense focal staining for keratin 7 and 19. The other two As(+3)-transformed cell lines had very low expression of keratin 7 mRNA and protein and produced subcutaneous tumors having no immunoreactivity for keratin 7; although keratin 19 expression was still present. The peritoneal tumors produced by one of these two cell lines regained expression of keratin 7 protein. The present results, coupled with previous studies, indicate that malignant transformation of UROtsa cells by Cd(+2) or As(+3) produce similar patterns of keratin 6, 7, 16, 17, and 19 in the resulting series of cell lines and their respective tumors.

Differences in the epigenetic regulation of MT-3 gene expression between parental and Cd+2 or As+3 transformed human urothelial cells

BACKGROUND

Studies have shown that metallothionein 3 (MT-3) is not expressed in normal urothelium or in the UROtsa cell line, but is expressed in urothelial cancer and in tumors generated from the UROtsa cells that have been transformed by cadmium (Cd+2) or arsenite (As+3).The present study had two major goals. One, to determine if epigenetic modifications control urothelial MT-3 gene expression and if regulation is altered by malignant transformation by Cd+2 or As+3. Two, to determine if MT-3 expression might translate clinically as a biomarker for malignant urothelial cells released into the urine.

RESULTS

The histone deacetylase inhibitor MS-275 induced MT-3 mRNA expression in both parental UROtsa cells and their transformed counterparts. The demethylating agent, 5-Aza-2'-deoxycytidine (5-AZC) had no effect on MT-3 mRNA expression. ChIP analysis showed that metal-responsive transformation factor-1 (MTF-1) binding to metal response elements (MRE) elements of the MT-3 promoter was restricted in parental UROtsa cells, but MTF-1 binding to the MREs was unrestricted in the transformed cell lines. Histone modifications at acetyl H4, trimethyl H3K4, trimethyl H3K27, and trimethyl H3K9 were compared between the parental and transformed cell lines in the presence and absence of MS-275. The pattern of histone modifications suggested that the MT-3 promoter in the Cd+2 and As+3 transformed cells has gained bivalent chromatin structure, having elements of being "transcriptionally repressed" and "transcription ready", when compared to parental cells. An analysis of MT-3 staining in urinary cytologies showed that a subset of both active and non-active patients with urothelial cancer shed positive cells in their urine, but that control patients only rarely shed MT-3 positive cells.

CONCLUSION

The MT-3 gene is silenced in non-transformed urothelial cells by a mechanism involving histone modification of the MT-3 promoter. In contrast, transformation of the urothelial cells with either Cd+2 or As+3 modified the chromatin of the MT-3 promoter to a bivalent state of promoter readiness. Urinary cytology for MT-3 positive cells would not improve the diagnosis of urothelial cancer, but might have potential as a biomarker for tumor progression.

Microarray analysis of gene expression patterns in human proximal tubule cells over a short and long time course of cadmium exposure

Numerous studies showed that renal proximal tubules cells are the cell type critically affected by chronic exposure to cadmium (Cd(2+)). The aim of the present study was to apply global gene expression technology and a human renal epithelial cell culture model (HPT) to determine whether time of exposure to Cd(2+) exerts a major influence on the resulting pattern of global gene expression. HPT cells were exposed to Cd(2+) for a short, 1-d, period of exposure (9, 27, and 45 μM) versus a longer, 13-d, period (4.5, 9, and 27 μM), with the hypothesis being that the stress response of the cells would be more active during the short time of exposure. The results showed that the differential expression of genes was very extensive for HPT cells exposed to Cd(2+) for 1 d, with more than 1848 genes displaying alterations compared to control and with the major categories of genes being involved in stress responses; cell death; checkpoint arrest, DNA repair, and the cell cycle; inflammatory responses; and cell adhesion, motion and differentiation. In contrast, HPT cells exposed to Cd(2+) for 13 d showed 923 genes to be differentially expressed, with a marked reduction in the number of differentially expressed stress response genes and a significant increase in the number of genes involved in development and differentiation. There were 387 differentially expressed genes common to both times of exposure. Data suggest that unless one is actively seeking to study the acute stress response, global gene expression technology should not be applied within an early time course of toxicant exposure.

Keratin 6 expression correlates to areas of squamous differentiation in multiple independent isolates of As(+3)-induced bladder cancer

This laboratory has shown that arsenite (As(+3)) exposure can cause the malignant transformation of the UROtsa human urothelial cell line. This single isolate formed subcutaneous tumors with a histology similar to human urothelial cell carcinoma. The tumors also displayed areas of squamous differentiation of the urothelial cells, an infrequent but known component of human bladder cancer. In the present study, five additional independent isolates of As(+3)-transformed urothelial cells were isolated and each was shown to produce subcutaneous urothelial cell tumors with a characteristic histology very similar to those described in the initial report. That there were underlying phenotypic differences in the six independent isolates was demonstrated when they were assessed for their ability to form tumors within the peritoneal cavity. It was shown that two isolates could form hundreds of small peritoneal tumor nodules, one isolate a moderate number of tumor nodules, and three isolates no or only one tumor nodule. The peritoneal tumors were also characterized for their degree of squamous differentiation of the urothelial cells and, while areas of squamous differentiation could be found, such differentiation was substantially reduced compared to subcutaneous tumors. Immunostaining for keratin 6 was tested as a potential marker for malignant urothelial cells that had undergone squamous differentiation. Keratin 6 was shown to consistently stain only cells having some evidence of squamous differentiation. Keratin 16 was shown to follow the staining pattern of keratin 6. The isolates and tumor heterotransplants were all examined for keratin 6, 16 and 17 mRNA and protein expression.

Absence of Metallothionein 3 Expression in Breast Cancer is a Rare, But Favorable Marker of Outcome that is Under Epigenetic Control

Cadmium (Cd(+2)), a known carcinogen mimics the effects of estrogen in the uterus and mammary gland suggesting its possible involvement in the development and progression of breast cancer. This lab showed through analysis of a small set of archival human diagnostic specimens that the third isoform of the classic Cd(+2) binding protein metallothionein (MT-3), is not expressed in normal breast tissue, but is expressed in some breast cancers and that expression tends to correlate with a poor disease outcome. The goals of the present study were to verify that overexpression of MT-3 in a large set of archival human diagnostic specimens tends to correlate with poor disease outcome and define the mechanism of MT-3 gene regulation in the normal breast epithelial cell. The results showed that MT-3 was expressed in approximately 90% of all breast cancers and was absent in normal breast epithelium. The lack of MT-3 staining in some cancers correlated with a favorable patient outcome. High frequency of MT-3 staining was also found for in situ breast cancer suggesting that MT-3 might be an early biomarker for breast cancer. The study also demonstrated that the MCF-10A cell line, an immortalized, non-tumorigenic model of human breast epithelial cells, displayed no basal expression of MT-3, nor was it induced by Cd(+2). Treatment of the MCF-10A cells with the demethylation agent, 5-Aza-2'-deoxycytidine, or the histone deacetylase inhibitor, MS-275, restored MT-3 mRNA expression. It was also shown that the MT-3 metal regulatory elements are potentially active binders of protein factors following treatment with these inhibitors suggesting that MT-3 expression may be subject to epigenetic regulation.

SPARC gene expression is repressed in human urothelial cells (UROtsa) exposed to or malignantly transformed by cadmium or arsenite

SPARC belongs to a class of extracellular matrix-associated proteins that have counteradhesive properties. The ability of SPARC to modulate cell-cell and cell-matrix interactions provides a strong rationale for studies designed to determine its expression in cancer. The objective of this study was to determine if SPARC expression was altered in cadmium (Cd(2+)) and arsenite (As(3+)) induced bladder cancer and if these alterations were present in archival specimens of human bladder cancer. The expression of SPARC was determined in human parental UROtsa cells, their Cd(2+) and As(3+) transformed counterparts and derived tumors, and in archival specimens of human bladder cancer using a combination of real time reverse transcriptase polymerase chain reaction, Western blotting, immunofluorescence localization and immunohistochemical staining. It was demonstrated that SPARC expression was down-regulated in Cd(2+) and As(3+) transformed UROtsa cells. In addition, the malignant epithelial component of tumors derived from these cell lines were also down-regulated for SPARC expression, but the stromal cells recruited to these tumors was highly reactive for SPARC. This finding was shown to translate to specimens of human bladder cancer where tumor cells were SPARC negative, but stromal cells were positive. Acute exposure of UROtsa cells to both cadmium and arsenite reduced the expression of SPARC through a mechanism that did not involve changes in DNA methylation or histone acetylation. These studies suggest that environmental exposure to As(3+) or Cd(2+) can alter cell-cell and cell-matrix interactions in normal urothelial cells through a reduction in the expression of SPARC. The SPARC associated loss of cell-cell and cell-matrix contacts may participate in the multi-step process of bladder carcinogenesis.

Variation of keratin 7 expression and other phenotypic characteristics of independent isolates of cadmium transformed human urothelial cells (UROtsa)

This laboratory has shown that a human urothelial cell line (UROtsa) transformed by cadmium (Cd(2+)) produced subcutaneous tumor heterotransplants that resemble human transitional cell carcinoma (TCC). In the present study, additional Cd(2+) transformed cell lines were isolated to determine if independent exposures of the cell line to Cd(2+) would result in malignantly transformed cell lines possessing similar phenotypic properties. Seven independent isolates were isolated and assessed for their doubling times, morphology, ability to heterotransplant subcutaneously and in the peritoneal cavity of nude mice, and for the expression of keratin 7. The 7 cell lines all displayed an epithelial morphology with no evidence of squamous differentiation. Doubling times were variable among the isolates, being significantly reduced or similar to those of the parental cells. All 7 isolates were able to form subcutaneous tumor heterotransplants with a TCC morphology, and all heterotransplants displayed areas of squamous differentiation of the transitional cells. The degree of squamous differentiation varied among the isolates. In contrast to subcutaneous tumor formation, only 1 isolate of the Cd(2+) transformed cells (UTCd#1) was able to effectively colonize multiple sites within the peritoneal cavity. An analysis of keratin 7 expression showed no correlation with squamous differentiation for the subcutaneous heterotransplants generated from the 7 cell lines. Keratin 7 was expressed in 6 of the 7 cell lines and their subcutaneous tumor heterotransplants. Keratin 7 was not expressed in the cell line that was able to form tumors within the peritoneal cavity. These results show that individual isolates of Cd(2+) transformed cells have both similarities and differences in their phenotype.

Basal and metal-induced expression of metallothionein isoform 1 and 2 genes in the RWPE-1 human prostate epithelial cell line

The human prostate gland has low basal expression of the metallothionein-1 and -2 proteins. In prostate cancer, MT-1/2 protein expression is variable and correlates directly with the increasing Gleason score of the tumor. The goal of the present study was to determine if the RWPE-1 cell line is a good model to elucidate the mechanisms underlying the alterations in MT-1/2 expression that occur during the development of prostate cancer. It was shown that the RWPE-1 cell line and in situ prostate tissue have identical expression profiles of MT-1 and MT-2 isoform-specific mRNAs (MT-1E, MT-1X and MT-2A) and similar levels of MT-1/2 protein. It was also shown that the RWPE-1 cells respond to Zn(+2) and Cd(+2) exposure by induction of the basally expressed MT mRNAs and the accumulation of high levels MT-1/2 protein (in excess of 10% of total protein). It was also shown that additional MT-1 mRNAs were expressed when the cells were exposed to either metal; MT-1A, MT-1F, MT-G and MT-1H for Cd(+2)-exposed cells; and, MT-1F, MT-G and MT-1H for Zn(+2)-exposed cells. The results suggest that RWPE-1 cells may be a valuable system to define the interplay between Zn(+2) concentration, Cd(+2) exposure and MT in normal and malignant prostate epithelial cells.

Transformation of human urothelial cells (UROtsa) by as and cd induces the expression of keratin 6a

BACKGROUND

Cadmium and arsenite can directly and malignantly transform the UROtsa cell line. The tumor heterotransplants produced from these transformed cells have histologic features consistent with human bladder cancer. Previous microarray analysis of total RNA from the parental and transformed cells suggested that keratin 6a was overexpressed as a result of cell transformation.

OBJECTIVES

Our goals were to verify overexpression of keratin 6a in Cd(2+)- and As(3+)-transformed UROtsa cells, the corresponding tumor heterotransplants, and human bladder cancer biopsy specimens and to assess what factors may be involved in keratin 6a overexpression.

METHODS

Expression was assessed with real-time polymerase chain reaction, Western blot analysis, and immunohistochemistry. We used the effect of addition and deletion of potential growth factors in the cell culture growth medium to assess possible pathways used in keratin 6a overexpression.

RESULTS

Cd(2+)- and As(3+)-transformed cells grown in serum-containing growth medium, as well as the derived tumor heterotransplants, overexpressed keratin 6a mRNA and protein compared with UROtsa cells grown in serum-containing growth medium. Immunostaining of keratin 6a in tumor heterotransplants showed focal staining of the tumor cells that was localized to the cytoplasm. Focal immunostaining of keratin 6a was also found in some but not all archival patient specimens of high-grade bladder cancer, confirming translation of the results to human bladder cancer. Studies on growth factor deletion and addition indicated that the level of keratin 6a expression was regulated by the presence of both insulin and epidermal growth factor (EGF). In contrast, growth factors had no effect on the elevated levels of keratin 6a expression found in transformed UROtsa cells.

CONCLUSIONS

Our present studies suggest that keratin 6a expression may be a biomarker for malignant urothelial cells that possess an activated EGF and or insulin growth factor pathway.

Cadmium, vectorial active transport, and MT-3-dependent regulation of cadherin expression in human proximal tubular cells

Previous studies from this laboratory have implicated the expression of the third isoform of metallothionein (MT-3) in the maintenance of proximal tubular vectorial active ion transport. It was shown that HK-2 cells have no expression of MT-3 and do not form domes in culture; whereas, the human proximal tubular (HPT) cells and HK-2 cells stably transfected with MT-3 [HK-2(MT-3)] form these structures. In the present study, this association was further explored by determining the effect of MT-3 expression on the expression of the E -, P -, N -, K -, and Ksp-cadherins. It was demonstrated that the HPT cells and HK-2(MT-3) cells had significant elevations in the expression of messenger RNA and protein for the E -, P -, and Ksp-cadherins compared with that of the HK-2 cells transfected with the blank vector [HK-2(blank vector)]. In contrast, the HK-2(blank vector) cells had significantly elevated expression of N- and K-cadherin compared with both the HPT and HK-2(MT-3) cell lines. These patterns of cadherin expression provide strong evidence that MT-3 might be involved in epithelial to mesenchymal transition that is postulated to occur during several disease states and in the mesenchymal to epithelial transition that occurs during normal kidney morphogenesis. A final goal of the study was to determine if Cd(+2) exposure influenced vectorial active transport of the proximal tubular cells and if this might occur through alterations in the expression of MT-3. It was shown that exposure to Cd(+2) eliminated vectorial active transport by the proximal tubular cell lines, but that Cd(+2) exposure did not reduce the expression of the MT-3 protein. The study shows that the level of MT-3 expression in HPT cells influences transepithelial resistance and cadherin expression but does not influence the Cd(+2)-induced loss of vectorial active transport.

Alterations in metal toxicity and metal-induced metallothionein gene expression elicited by growth medium calcium concentration

The calcium content of the growth medium has been shown to influence the growth and differentiation of primary epithelial cells in culture. The goal of the present study was to determine if growth medium calcium concentration could influence the susceptibility to metal toxicity and metallothionein gene expression of an immortalized human prostate-derived epithelial cell line (RWPE-1). The RWPE-1 cell line was grown in medium containing either 0.1 or 1.4 mM calcium. Confluent cells were exposed to either Zn(+2) (50, 100, or 150 microM) or Cd(+2) (3, 6, or 12 microM) for 13 days, and cell toxicity and MT gene expression were determined along the time course of exposure. It was demonstrated that the calcium content of the growth medium had a marked influence on Zn(+2) toxicity and a lesser but significant effect on Cd(+2) toxicity to the RWPE-1 cells. Calcium concentration of the growth medium was also shown to alter the accumulation of MT-1/2 protein and MT-1E, MT-1X, and MT-2A mRNAs. It was shown that MT-1/2 protein was markedly increased for metal-exposed cells grown in medium containing 0.1 mM calcium; however, the increased expression did not cause an increase in the resistance of the cells to Zn(+2) or Cd(+2) exposure. These observations show that growth medium calcium concentration can influence metal toxicity and the pattern of expression of the MT mRNAs and protein for RWPE-1 cells. The results suggest that caution should be exercised when comparing toxicological responses between cell lines that may be grown in growth formulations differing in calcium concentration.

The resistance of metallothionein to proteolytic digestion: an LC-MS/MS analysis

Metallothioneins (MTs) are a family of cysteine-rich metalloproteins which strongly bind to heavy metals, such as Cd(II), Zn(II), and Cu(I). Previous works by other group using gel electrophoresis and fluorescence showed MTs were resistant to proteolytic digestion by a variety of enzymes, raising the difficulties in proteomic identification of MTs. The present work was attempted to analyze the resistance of MTs to trypsin using LC with MS/MS (LC-MS/MS), which was able to determine the sequences of the produced peptides and thus precisely characterize the cleavages. The results showed that metal-saturated MTs were completely resistant to trypsin. This resistance problem could be overcome by the addition of EDTA to MT samples, which rendered MTs readily digested into peptides and identified by MS/MS. Interestingly, the partially metal binding MTs were digested into peptides predominantly with miss cleavages which were well dependent on the amount of heavy metals bound to MTs. An explanation for these observations was proposed. The potential applications of the MT's resistance to trypsin in isolation and identification of MTs in complex mixtures such as cultured cells was demonstrated. The preliminary data also showed the same proteomic approach of proteolytic digestion followed by MS/MS analysis may provide information on metal binding status of MTs, along with the identification of MTs in a mixture.

Simple method for identification of metallothionein isoforms in cultured human prostate cells by MALDI-TOF/TOF mass spectrometry

The present paper describes a rapid method for identification and characterization of human metallothionein (MT) isoforms in complex cell cultures using high-resolution matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF). In the proposed method, the sample preparation of MTs from cultured cells is both simple and fast. It is accomplished by trypsin cleavage of cell proteins into small peptide species, the majority of which are subsequently removed by gel filtration using beads with an exclusion limit of 4000 Da. In contrast to most cell proteins, MTs remain intact (undigested) upon being treated with trypsin, being excluded by the gel beads and thus recovered by low-speed centrifugation. To identify the protein constitutes of the MT preparation, the MT sample is divided into two parts, one for intact protein accurate mass measurement, the other for tryptic digestion followed by MS and MS/MS analyses. In the latter case, the MT proteins are denatured by the addition of EDTA which strips heavy metals from MTs and renders them susceptible to tryptic digestion. The obtained accurate mass with the unique peptide sequences of each MT isoform allows for unambiguous identification of MT isoforms in the prepared mixture. The method has been applied to RWPE-1 cells derived from normal human prostate epithelium. Four MT isoforms, 1E, 1G, 1X, and 2A, have been confidently identified, being primarily acetylated at N-termini. These results are in agreement with the expression of MT mRNAs in RWPE-1 cells determined by real-time reverse-transcription polymerase chain reaction (RT-PCR).

Urothelial Cells Malignantly Transformed by Exposure to Cadmium (Cd+2) and Arsenite (As+3) Have Increased Resistance to Cd+2 and As+3-Induced Cell Death

This laboratory has shown that both Cd(+2) and As(+3) can malignantly transform human urothelial cells. The present study examined metal resistance and the mechanism of cell death when the parental and malignantly transformed UROtsa cells were exposed to Cd(+2) and As(+3). It was shown that the malignantly transformed UROtsa cells were more resistant to the toxic effects of both metals. The assessment of the mode of cell death demonstrated that the parental UROtsa cells died by both apoptosis and necrosis when exposed to either metal. It was shown that apoptosis was the more prominent mechanism of cell death, accounting for over 50% of cell death. Apoptotic cell death was determined by the observation of fragmented nuclei using 4',6-diamidino-2-phenylindole staining, the formation of a DNA ladder, and the detection of cleaved caspase-3 and caspase-9 products in the cell lysates. Necrotic cell death was determined by measuring the release of lactate dehydrogenase into the growth medium. It was determined that the extent of apoptosis of the malignantly transformed UROtsa cells was decreased and that the extent of necrosis was increased compared to the parental UROtsa cells. These observations are consistent with in vivo studies which suggest that As(+3) can act as a tumor promoter during the regeneration of the bladder urothelium. The present in vitro studies suggest that As(+3)-induced cytotoxicity could set the stage for tissue repair due to its own inherent toxicity to normal urothelium, and then subsequently act as a tumor promoter during the regeneration process through the stimulation of the regrowth of cells that have gained increased resistance to As(+3).

Enhanced expression of metallothionein isoform 3 protein in tumor heterotransplants derived from As+3- and Cd+2-transformed human urothelial cells

This laboratory has proposed that the third isoform of the metallothionein gene family (MT-3) might be a biomarker for the development of human bladder cancer. Immunohistochemical staining of MT-3 on archival diagnostic specimens showed that only 2 of 63 (3.17%) benign bladder specimens had even weak reactivity for the MT-3 protein. In contrast, 103 of 107 (96.26%) high-grade urothelial cancers and 17 of 17 (100%) specimens of carcinoma in situ stained positive for the MT-3 protein. For low-grade bladder cancer it was shown that 30 of 48 specimens (62.5%) expressed the MT-3 protein. Using a cell culture model (UROtsa), it was demonstrated that expression of the MT-3 protein was not required for malignant transformation of urothelial cells by either Cd(+2) or As(+3). In contrast, it was shown that the cells transformed by Cd(+2) and As(+3) that did not express the MT-3 gene in cell culture, gained expression of MT-3 when grown as heterotransplants in nude mice. The gain in MT-3 expression when cells were grown as heterotransplants was also shown to occur for the MCF-7, T-47D, Hs 578t, MDA-MB-231 breast cancer, and the PC-3 prostate cancer cell lines. An analysis of MT-3 mRNA and protein expression suggested that a posttranscriptional mechanism was responsible for accumulation of the MT-3 protein. The results provide strong evidence that MT-3 could be a biomarker for the development of high-grade bladder cancer and that the expression of the MT-3 gene is not involved in the in vitro malignant transformation of UROtsa cells by Cd(+2) and As(+3).

Metallothionein-1 and -2 expression in cadmium- or arsenic-derived human malignant urothelial cells and tumor heterotransplants and as a prognostic indicator in human bladder cancer

The goal of this study was to determine if the expression of the metallothionein (MT)-1/2 proteins might serve as a biomarker for the development of bladder cancer. A retrospective analysis of MT-1/2 staining was performed on 343 tissue sections from patients referred for the diagnosis of bladder cancer. The specimens were subdivided into six categories: benign, dysplastic, low-grade cancer, high-grade cancer with no evidence of invasion, high-grade cancer with evidence of invasion, and carcinoma in situ. There was no expression of MT-1/2 in benign lesions and low-grade cancers, a low incidence of expression in dysplastic lesions and high-grade cancers with no evidence of muscle invasion, and a significantly increased incidence of MT-1/2 in high-grade cancers that had invaded the underlying matrix. The expression of MT-1/2 varied in intensity from sample to sample and was focal in its expression. It was concluded from these findings that MT-1/2 may be a prognostic marker for cancers that are progressing to invade the underlying stroma of the bladder wall. The expression of MT-1/2 was also determined in a cell culture model of human urothelium that had been malignantly transformed by Cd2+ and As3+ and shown to be capable of tumor formation in nude mice. It was demonstrated that the expression of MT-1/2 in the tumor heterotransplants was similar to the pattern found in archival specimens of high-grade bladder cancers. The MT-1/2 staining in the heterotransplants was focal in pattern, varied in intensity, and highest in the less differentiated cells of the tumor. These findings indicate that the cell culture model may serve to help define the role of MT-1/2 expression in bladder cancer invasion.

The unique N-terminal sequence of metallothionein-3 is required to regulate the choice between apoptotic or necrotic cell death of human proximal tubule cells exposed to Cd+2

This laboratory has shown that MT-3 expression determines the choice between apoptotic or necrotic cell death in Cd(+2)-exposed human proximal tubule cells. Human proximal tubule cells that express MT-3 undergo necrosis when exposed to Cd(+2), while cells that have no basal expression of MT-3 undergo apoptotic cell death. It was also shown that cells which express MT-3 were more sensitive to Cd(+2)-induced cell death than those having no basal expression. In the present study, site directed mutagenesis was used to determine if the unique N-terminal sequence of MT-3 was required for these activities regarding toxicity and cell death. The results demonstrated that HK-2 cells stably transfected with MT-3 that had been modified by converting the 2 prolines at amino acid positions 7 and 9 to threonines was no longer active in promoting necrotic cell death at lower levels of Cd(+2) exposure. This was shown in comparison to cells containing the wild type MT-3 sequence and blank vector controls as regards the % of DAPI-stained fragmented nuclei, DNA laddering, LDH release, caspase-9, and caspase-3 activation. This study demonstrates that the unique N-terminal sequence of MT-3 is required to elicit an effect on the mechanism of Cd(+2)-induced death of the proximal tubule cell. This is the identical sequence that has been shown to be responsible for the growth inhibitory activity of MT-3 in the neural system.

Expression of metallothoinein isoform 3 is restricted at the post-transcriptional level in human bladder epithelial cells

This study was designed to define the effect that overexpression of MT-3 would have on a cell culture model of bladder urothelium. Stable and inducible transfection was used to achieve overexpression of the MT-3 gene in the UROtsa cell line. When the UROtsa cells were stably transfected with the MT-3 coding sequence, there was highly elevated expression of MT-3 mRNA, but no MT-3 protein. An inducible vector showed that low basal levels of MT-3 mRNA and protein could be produced, but that induction only increased MT-3 mRNA and not protein. The clones expressing low basal levels of MT-3 protein also had reduced growth rates compared to control cells. Site directed mutagenesis was used to produce an MT-3 coding sequence where the prolines in positions 7 and 9 were converted to threonines. When this altered MT-3 was stably transfected into the UROtsa cells, the cells were able to accumulate the mutated form of the MT-3 protein. These studies show that MT-3 protein expression is inhibited by post-transcriptional control in the urothelial cell. Modifying the MT-3 protein to resemble the MT-1 isoform removes this component of post-transcriptional control and allows accumulation of the mutated MT-3 protein. The altered sequence involved in post-transcriptional control of MT-3 protein expression is the same sequence implicated in the neuronal growth inhibitory activity associated specifically with the MT-3 isoform of the MT gene family.

Post-Transcriptional Regulation of Metallothionein Isoform 1 and 2 Expression in the Human Breast and the MCF-10A Cell Line

Studies have shown, using immunohistochemical staining, that the MT-1 and MT-2 proteins (MT-1/2) are overexpressed in a substantial subset of ductal breast cancers, that overexpression occurs early in the disease process, and that this overexpression is indicative of a poor prognosis. Normal ductal breast epithelium fails to immunostain for the MT-1/2 protein, whereas the myoepithelial cells of the ducts stain intensely. There is no information regarding the expression of the mRNAs for the eight active MT-1 and MT-2 genes in normal breast duct epithelium. Microdissection of normal breast samples was used to obtain total RNA from enriched populations of ductal epithelium and myoepithelium. Analysis by reverse-transcription polymerase chain reaction (RT-PCR) demonstrated that the identity of the MT isoform-specific genes expressed (MT-2A and MT-1X) and their relative levels of expression were similar between the myoepithelial and ductal components. These findings indicate that the ductal and myoepithelial components express similar amounts of MT-2A and MT-1X mRNAs, but that they have distinctly different expression of the MT-1/2 protein. Confluent cultures of MCF-10A breast epithelial cells were exposed to Cd(+2) to test for evidence of post-transcriptional regulation of MT-1/2 protein accumulation in ductal epithelium. It was demonstrated that Cd(+2) elicited only a marginal induction of MT-1E, MT-1X, or MT-2A mRNAs, whereas, there was a marked increase in MT-1/2 protein, reaching levels of 6% of total cell protein under conditions of extended exposure. This study suggests that the mechanism underlying the finding of increased MT-1/2 protein expression in ductal breast cancer may involve, to some degree, the post-transcriptional regulation of MT-1/2 protein expression.

Expression of Metallothionein Isoform 3 (MT-3) Determines the Choice between Apoptotic or Necrotic Cell Death in Cd+2-Exposed Human Proximal Tubule Cells

This laboratory has shown that the third isoform of metallothionein (MT-3) is expressed in the human kidney in situ, including the cells of the proximal tubule. A subsequent analysis of MT-3 expression in cell cultures derived from the human proximal tubule (HPT) demonstrated that mortal HPT cells expressed MT-3, while the HPV-immortalized HK-2 cells had no expression of MT-3. In the present study, the effect of MT-3 expression on Cd(+2)-induced cytotoxicity was determined by stable transfection of the MT-3 coding sequence into the HK-2 cell line. The results demonstrated that HK-2 cells stably transfected with MT-3 were more sensitive to the cytotoxic effects of Cd(+2). Furthermore, this increase in Cd(+2)-induced cytotoxicity was correlated to an alteration in the mechanism of cell death, being changed from an apoptotic mechanism in cells not expressing the MT-3 gene to a necrotic mechanism in cells expressing the MT-3 gene. The present study provides evidence that MT-3 could play a role in controlling the choice between apoptosis and necrosis in multiple epithelial cell types of the human kidney.