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

The association of blood lead levels and renal effects may be modified by genetic combinations of Metallothionein 1A 2A polymorphisms

Metallothionein (MT) is a protein with function of heavy metal detoxification. However, studies about how single nucleotide polymorphisms (SNPs) of MT genes influence lead nephropathy are relatively scarce. Therefore, our aim is to investigate the association between blood lead levels and renal biomarkers and to study whether this association is influenced by the combination of MT1A and MT2A SNPs. Blood lead, urinary uric acid (UA), and urinary N-acetyl-beta-d-glucosaminidase (NAG) levels were analyzed from 485 participants. Genotyping were performed on MT1A SNPs (rs11640851 and rs8052394) and MT2A SNPs (rs10636 and rs28366003). The combined MT1A 2A SNPs were divided into 16 groups. Among renal biomarkers, urinary UA was negatively significant associated with the time-weighted index of cumulative blood lead (TWICL), while urinary NAG was positively significant with TWICL. Furthermore, the association between urinary UA and TWICL was significantly modified by group 6 of combined SNPs (MT1A 2 A SNPs combination were AAAGGGAA, ACAGGGAA, and ACGGGGAA). In conclusion, the negative association of urinary UA and TWICL is modified by group 6, which means participants of group 6 are more susceptible to lead nephrotoxicity.

The stress response of human proximal tubule cells to cadmium involves up-regulation of haemoxygenase 1 and metallothionein but not cytochrome P450 enzymes

Enzymes of the cytochrome P450 (CYP) super-family are implicated in cadmium (Cd) -induced nephrotoxicity, however, direct evidence is lacking. This study investigated the endogenous expression of various CYP proteins together with the stress-response proteins, heme oxygenase-1 (HO-1) and metallothionein (MT) in human kidney sections and in cadmium-exposed primary cultures of human proximal tubular epithelial cells (PTC). By immunohistochemistry, the CYP members 2B6, 4A11 and 4F2 were prominently expressed in the cortical proximal tubular cells and to a lesser extent in distal tubular cells. Low levels of CYPs 2E1 and 3A4 were also detected. In PTC, in the absence of Cd, CYP2E1, CYP3A4, CYP4F2 and MT were expressed, but HO-1, CYP2B6 and CYP4A11 were not detected. A range of cadmium concentrations (0-100μM) were utilized to induce stress conditions. MT protein was further induced by as little as 0.5μM cadmium, reaching a 6-fold induction at 20μM, whereas for HO-1, a 5μM cadmium concentration was required for initial induction and at 20μM cadmium reached a 15-fold induction. The expression of CYP2E1, CYP3A4, and CYP4F2 were not altered by any cadmium concentrations tested at 48h. Cadmium caused a reduction in cell viability at concentrations above 10μM. In conclusion although cultured PTC, do express CYP proteins, (CYP2E1, CYP3A4, and CYP4F2), Cd-induced cell stress as indicted by induction of HO-1 and MT does not alter expression of these CYP proteins at 48h.

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.

Metallothionein I Isoform mRNA Expression in Peripheral Lymphocytes as a Biomarker for Occupational Cadmium Exposure

It is reported that metallothionein (MT) mRNA expression in human peripheral blood lymphocytes (HPBLs) could be used as exposure biomarkers in occupationally cadmium-exposed workers. Several MT isoforms have been identified in humans. The relationship between MT isoforms and cadmium toxicity has not been fully elucidated in occupational settings. In this study, the MT-IA, IE, IF, IX mRNA expressions in HPBLs were tested by RT-PCR technique, and the relationship between MT isoforms mRNA expression and cadmium-induced renal dysfunction was evaluated in cadmium-exposed workers. The MT-IE, IF, IX mRNA levels were found to increase with increasing blood cadmium (BCd) levels and MT-IA mRNA levels increased with increased urinary cadmium (UCd) levels. The MT-IE, IF, IX mRNA levels were significantly correlated with the BCd levels ( P < 0.05), and MT-IA mRNA levels were significantly correlated with the UCd levels. This confirmed that MT-I isoforms mRNA expression in HPBLs is a biomarker of cadmium exposure and internal dose. The MT-IA mRNA levels were significantly correlated with renal dysfunction biomarkers, such as urinary β2-microglobulin (Uβ2-MG) ( r = 0.294, P < 0.01) and urinary albumin (UALB) ( r = 0.305, P < 0.01). The latter finding indicates that MT-IA mRNA expression in HPBLs may be used as a biomarker for renal dysfunction in occupational cadmium exposure.

Potential for early involvement of CYP isoforms in aspects of human cadmium toxicity

This paper investigates the possible link between non-workplace cadmium (Cd) exposure, cytochrome P450 expression and hypertension. We present results of our investigation into the relationships between liver and kidney Cd burdens and the abundance of the CYP isoform 4A11. Our data show associations between non-workplace Cd exposure and changes in the abundance of hepatic and renal cortical CYP4A11. In liver the levels of immunochemically detectable CYP4A11 were positively correlated with tissue Cd content while in contrast CYP4A11 abundance was inversely correlated with kidney Cd burden. These differences are most likely related to the different Cd burden of the tissues. These observations suggest the potential for involvement of Cd as a mediator of CYP4A11 expression in kidney cortex and indicate that elevations in kidney Cd content may be involved in hypertension via alteration of the expression of this particular isoform. Potential mechanisms by which Cd may alter CYP4A11 expression are discussed briefly.

Induction, regulation, degradation, and biological significance of mammalian metallothioneins

MTs are small cysteine-rich metal-binding proteins found in many species and, although there are differences between them, it is of note that they have a great deal of sequence and structural homology. Mammalian MTs are 61 or 62 amino acid polypeptides containing 20 conserved cysteine residues that underpin the binding of metals. The existence of MT across species is indicative of its biological demand, while the conservation of cysteines indicates that these are undoubtedly central to the function of this protein. Four MT isoforms have been found so far, MT-1, MT-2, MT-3, and MT-4, but these also have subtypes with 17 MT genes identified in man, of which 10 are known to be functional. Different cells express different MT isoforms with varying levels of expression perhaps as a result of the different function of each isoform. Even different metals induce and bind to MTs to different extents. Over 40 years of research into MT have yielded much information on this protein, but have failed to assign to it a definitive biological role. The fact that multiple MT isoforms exist, and the great variety of substances and agents that act as inducers, further complicates the search for the biological role of MTs. This article reviews the current knowledge on the biochemistry, induction, regulation, and degradation of this protein in mammals, with a particular emphasis on human MTs. It also considers the possible biological roles of this protein, which include participation in cell proliferation and apoptosis, homeostasis of essential metals, cellular free radical scavenging, and metal detoxification.

Exposure of cultured human proximal tubular cells to cadmium, mercury, zinc and bismuth: toxicity and metallothionein induction

The kidney, in particular the proximal convoluted tubule, is a major target site for the toxic effects of various metals. However, little is known about the early effects of these metals after acute exposure in man. In the present study we have evaluated the toxicity of several inorganic metal compounds (CdCl2, HgCl2, ZnCl2, and Bi(NO3)3) and the induction of metallothionein by these compounds in cultured human proximal tubular (HPT) cells for up to 4 days. The results showed that bismuth was not toxic even at the highest dose (100 microM) used, while zinc, cadmium and mercury exhibited varying degrees of toxicity, zinc being the least toxic and mercury the most potent. A significant degree of interindividual variation between the different isolates used in these experiments was also observed. All metals used in the present study induced MT, as revealed by immunocytochemistry. All metals showed maximal induction between 1 and 3 days after treatment. Although a certain amount of constitutive MT was present in the cultures, the intensity of the staining varied with time in culture and between the different isolates studied. No correlation could be made between the intensity of the staining in control cultures (indicating total amount of constitutive MT) and the susceptibility of a given isolate to metal toxicity. Furthermore, no correlation could be made between metal-induced MT and the susceptibility of a given isolate to that particular metal.

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.