Sensitive nonradioactive dot blot/ribonuclease protection assay for quantitative determination of mRNA

Toxicity, dioxin-like activities, and endocrine effects of DDT metabolites--DDA, DDMU, DDMS, and DDCN

BACKGROUND, AIM, AND SCOPE

2,2-bis(chlorophenyl)-1,1,1-trichloroethane (DDT) metabolites, other than those routinely measured [i.e., 2,2-bis(chlorophenyl)-1,1-dichloroethylene (DDE) and 2,2-bis(chlorophenyl)-1,1-dichloroethane (DDD)], have recently been detected in elevated concentrations not only in the surface water of Teltow Canal, Berlin, but also in sediment samples from Elbe tributaries (e.g., Mulde and Havel/Spree). This was paralleled by recent reports that multiple other metabolites could emerge from the degradation of parent DDT by naturally occurring organisms or by interaction with some heavy metals. Nevertheless, only very few data on the biological activities of these metabolites are available to date. The objective of this communication is to evaluate, for the first time, the cytotoxicity, dioxin-like activity, and estrogenicity of the least-studied DDT metabolites.

METHODS

Four DDT metabolites, p,p'-2,2-bis(chlorophenyl)-1-chloroethylene (DDMU), p,p'-2,2-bis(chlorophenyl)-1-chloroethane (DDMS), p,p'-2,2-bis(4-ch1oropheny1)acetonitrile (DDCN), and p,p'-2,2-bis(chlorophenyl)acetic acid (DDA), were selected based on their presence in environmental samples in Germany such as in sediments from the Mulde River and Teltow Canal. O,p'-DDT was used as reference in all assays. Cytotoxicity was measured by neutral red retention with the permanent cell line RTG-2 of rainbow trout (Oncorhynchus mykiss). Dioxin-like activity was determined using the 7-ethoxyresorufin-O-deetylase assay. The estrogenic potential was tested in a dot blot/RNAse protection-assay with primary hepatocytes from male rainbow trout (O. mykiss) and in a yeast estrogen screen (YES) assay.

RESULTS

All DDT metabolites tested revealed a clear dose-response relationship for cytotoxicity in RTG-2 cells, but no dioxin-like activities with RTL-W1 cells. The dot blot/RNAse protection-assay demonstrated that the highest non-toxic concentrations of these DDT metabolites (50 μM) had vitellogenin-induction potentials comparable to the positive control (1 nM 17β-estradiol). The estrogenic activities could be ranked as o,p'-DDT > p,p'-DDMS > p,p'-DDMU > p,p'-DDCN. In contrast, p,p'-DDA showed a moderate anti-estrogenic effect. In the YES assay, besides the reference o,p'-DDT, p,p'-DDMS and p,p'-DDMU displayed dose-dependent estrogenic potentials, whereas p,p'-DDCN and p,p'-DDA did not show any estrogenic potential.

DISCUSSION

The reference toxicant o,p'-DDT displayed a similar spectrum of estrogenic activities similar to 17β-estradiol, however, with a lower potency. Both p,p'-DDMS and p,p'-DDMU were also shown to have dose-dependent estrogenic potentials, which were much lower than the reference o,p'-DDT, in both the vitellogenin and YES bioassays. Interestingly, p,p'-DDA did not show estrogenic activity but rather displayed a tendency towards anti-estrogenic activity by inhibiting the estrogenic effect of 17β-estradiol. The results also showed that the p,p'-metabolites DDMU, DDMS, DDCN, and DDA do not show any dioxin-like activities in RTL-W1 cells, thus resembling the major DDT metabolites DDD and DDE.

CONCLUSIONS

All the DDT metabolites tested did not exhibit dioxin-like activities in RTL-W1 cells, but show cytotoxic and estrogenic activities. Based on the results of the in vitro assays used in our study and on the reported concentrations of DDT metabolites in contaminated sediments, such substances could, in the future, pose interference with the normal reproductive and endocrine functions in various organisms exposed to these chemicals. Consequently, there is an urgent need to examine more comprehensively the risk of environmental concentrations of the investigated DDT metabolites using in vivo studies. However, this should be paralleled also by periodic evaluation and monitoring of the current levels of the DDT metabolites in environmental matrices.

RECOMMENDATIONS AND PERSPECTIVES

Our results clearly point out the need to integrate the potential ecotoxicological risks associated with the "neglected" p,p'-DDT metabolites. For instance, these DDT metabolites should be integrated into sediment risk assessment initiatives in contaminated areas. One major challenge would be the identification of baseline data for such risk assessment. Further studies are also warranted to determine possible additive, synergistic, or antagonistic effects that may interfere with the fundamental cytotoxicity and endocrine activities of these metabolites. For a more conclusive assessment of the spectrum of DDT metabolites, additional bioassays are needed to identify potential anti-estrogenic, androgenic, and/or anti-androgenic effects.

The secretory granule protein syncollin localizes to HL-60 cells and neutrophils

The secretory granule protein syncollin was first identified in the exocrine pancreas where a population of the protein is associated with the luminal surface of the zymogen granule membrane. In this study we provide first morphological and biochemical evidence that, in addition to its pancreatic localization, syncollin is also present in neutrophilic granulocytes of rat and human origin. By immunohistological studies, syncollin was detected in neutrophilic granulocytes of the spleen. Furthermore, syncollin is expressed by the promyelocytic HL-60 cells, where it is stored in azurophilic granules and in a vesicular compartment. These findings were confirmed by fractionation experiments and immunoelectron microscopy. Treatment with a phorbol ester triggered the release of syncollin indicating that in HL-60 cells it is a secretory protein that can be mobilized upon stimulation. A putative role for syncollin in host defense is discussed.

Regulation of bcl-2 gene expression in human breast cancer cells by prolactin and its antagonist, hPRL-G129R

To gain insight into the molecular basis of human prolactin (hPRL) antagonist induced apoptosis, we compared the differential gene expression profile of four human breast cancer cell lines following treatment with hPRL and its antagonist (hPRL-G129R). Among the genes identified, the bcl-2 gene was of particular interest. We found that bcl-2 mRNA was up regulated in three of the four cell lines that were treated with hPRL. To further confirm these results, real time RT-PCR and ELISA analyses were used to detect bcl-2 mRNA and Bcl-2 protein, respectively, in 11 different breast cancer cell lines after hPRL or hPRL-G129R treatment. Our data suggests that Bcl-2 is up-regulated in response to hPRL stimulation and is competitively inhibited by hPRL-G129R in the majority of the cell lines tested. Thus, we propose that the anti-apoptotic role of hPRL in breast cancer is mediated, at least in part, through regulation of Bcl-2.

Genomic organization, chromosomal localization and tissue specific expression of the murine Pxmp2 gene encoding the 22 kDa peroxisomal membrane protein (Pmp22)

Peroxisomes are subcellular organelles with important functions in lipid metabolism that are found in virtually all eucaryotic cells. The peroxisomal membrane contains a number of integral and peripheral membrane proteins involved in the import of peroxisomal matrix proteins and the transport of metabolites across the membrane. The most abundant peroxisomal membrane protein (Pmp) in rat peroxisomes is Pmp22, a 22 kDa protein of unknown function that is encoded by the Pxmp2 gene. To investigate the function of the Pxmp2 gene, we have initiated mouse knockout studies. The expression level of the Pxmp2 mRNA in mice was investigated by Northern blot analysis. Pxmp2 RNA was shown to be differentially expressed with highest expression levels in liver, kidney and in heart tissue. Comparison with other peroxisomal marker genes revealed that the expression of Pxmp2, Pmp70 (Pxmp1) and catalase was regulated independently. Using 5' and 3' RACE we have cloned the full-length cDNA of murine Pxmp2 which comprises 863 nucleotides and have isolated a genomic clone containing the entire murine Pxmp2. We have analyzed the complete intron/exon structure of the Pxmp2 gene which contains five exons spanning about 11 kb on the genomic clone. All intron/exon splice junctions conform to the GT/AG rule. Sequence analysis of the Pxmp2 5' flanking region revealed that it was devoid of a TATA box, but characteristic promoter elements were identified within 250 base pairs upstream of the transcriptional start site. Using a mouse/hamster radiation hybrid panel, Pxmp2 was localized on mouse chromosome 5 at 59 cM.

Temperature-dependent vitellogenin-mRNA expression in primary cultures of rainbow trout (Oncorhynchus mykiss) hepatocytes at 14 and 18 degrees C

In order to study the influence of temperature on vitellogenin gene and estrogen receptor gene expression in primary hepatocytes from rainbow trout (Oncorhynchus mykiss), cells were exposed to 17beta-estradiol, bisphenol-A and nonylphenol for 48 and 96 hr. Induction of vitellogenin-mRNA expression was detected in a non-radioactive dot blot/RNAse protection assay and by RT-PCR. In the dot blot/RNAse protection assay, the estrogenic potentials of bisphenol-A and nonylphenol were about 10(4)- to 10(5)-fold and 10(5)-fold lower than that of 17beta-estradiol, respectively. The relative estrogenic potential did not show any difference between 14 and 18 degrees C. In contrast, at 18 degrees C, RT-PCR analysis revealed increased amounts of vitellogenin- and estrogen receptor-mRNA after 12 and 24 hr of exposure to 17beta-estradiol, if compared to 14 degrees C. Owing to increased vitellogenin gene expression at 18 degrees C, the sensitivity of primary hepatocytes to 17beta-estradiol and bisphenol-A could be increased.

Sibling species in the Llutzomyia longipalpis complex differ in levels of mRNA expression for the salivary peptide, maxadilan

Maxadilan is a small ( approximately 7 kDa) protein found in the saliva of sand fly species in the Lutzomyia longipalpis complex, vectors of the parasite causing visceral leishmaniasis, Leishmania chagasi. It is a potent vasodilator and also has immunomodulatory affects. Maxadilan recovered from different sibling species of the Lu. longipalpis complex differ in amino acid content by as much as 23%, however all variants possess equivalent vasodilatory activity. Therefore, the dramatic differences in vasodilatory activity of the saliva from different sibling species is probably due to differences in the amounts of maxadilan in their saliva. This is significant because it has been suggested that maxadilan may influence the pathogenesis of leishmanial infections. In this study we measured the amount of maxadilan messenger RNA (mRNA) per pair of salivary glands from individual sand flies by quantitative reverse transcription polymerase chain reaction (RT-PCR) using a competitive method. We report a method using the gene of interest, in this case maxadilan, amplified by the PCR from genomic DNA, as a competitor in the quantitative RT-PCR, taking advantage of differences in the size of these products due to the presence of an intron. Significant differences in amounts of maxadilan mRNA among colonies from Central and South America are described. We found a strong correlation between the amount of maxadilan mRNA detected in salivary glands of different Lu. longipalpis sibling species and previously described differences in the size of erythemas produced by the bite of these species. Therefore, variation in the amount of mRNA suggests that differences in the vasodilatory properties of saliva among the different sibling species are the result of differences in the amount of maxadilan present in the saliva and not differences in the potency of maxadilan peptide variants. The geographical distribution of species with high or low levels of maxadilan gene expression are concordant with the distribution of atypical cutaneous leishmaniasis resulting from infection with Le. chagasi, lending credence to earlier suggestions that maxadilan may be involved with visceralization of this parasite.

Regulation of neurogenesis by interactions between HEN1 and neuronal LMO proteins

Basic-helix-loop-helix transcription factors regulate neurogenesis and neuronal differentiation by as yet unknown mechanisms. We show that an embryonic neuronal-specific basic-helix-loop-helix protein, HEN1 (also known as NSCL1 or NHLH), interacts with ‘LIM only’ proteins. Examination of the expression patterns of XHEN1 and XLMO-3, the Xenopus homologues of these human genes, reveals extensive overlap during early neurogenesis: at the onset of gastrulation on the dorsal side of the blastopore lip and, subsequently, in the prospective neural plate. Binding of XLMO-3 increases the transcriptional activity of XHEN1 in vivo. Co-expression of these two genes in Xenopus embryos induces a cascade of expression of neuronal-specific basic-helix-loop-helix proteins that leads to neuronal differentiation. We propose that XHEN1, in concert with XLMO-3, is a critical regulator of neurogenesis.

Measurement of vitellogenin-mRNA expression in primary cultures of rainbow trout hepatocytes in a non-radioactive dot blot/RNAse protection-assay

The induction of vitellogenin synthesis both in vivo and in vitro has proven to be a reliable biomarker for assessing the estrogenic activity of individual substances and the more complex effluents of sewage treatment plants. However, due to the requirement of radioactively labelled nucleotides, the measurement of vitellogenin-mRNA has not been widely used in routine testing--even though this technique promises elevated sensitivity. In order to develop a practicable, reliable and cost-effective bioassay suitable for routine testing, a combined dot-blot/RNAse protection assay, utilising digoxigenin-labelled cRNA transcripts of plasmid psg5Vg1.1 was used for the quantification of vitellogenin-mRNA in isolated rainbow trout (Oncorhynchus mykiss) hepatocytes. By re-cloning the Vg1.1 insert into a pGemZf7(-)-vector, the sense-transcript of Vg1.1 was utilized as a standard for the quantification of vitellogenin-mRNA concentrations. Male rainbow trout hepatocytes were cultured as monolayers in pure M199 medium. The addition of serum supplements did not result in increased expression of vitellogenin-mRNA following 17 beta-estradiol administration. This indicates that for this assay no supplementation of the culture medium is necessary. After addition of 17 beta-estradiol, hepatocytes exhibited an exponential time-dependent expression of vitellogenin-mRNA over a period of 144 h. The dot blot system was sufficiently sensitive to detect vitellogenin-mRNA following addition of 1 microM 17 beta-estradiol after 6 h of incubation. However, the amount of vitellogenin-mRNA expressed was found to be a function of both incubation time and inducer concentration. Prolonged incubation times were therefore required to enhance the sensitivity of the system. After a 96-h incubation, detection limits for 17 beta-estradiol were between 100 pM and 1 nM. Vitellogenin-mRNA could not be detected in untreated hepatocytes. The vitellogenin-mRNA dot blot/RNAse protection assay was further used as a tool for assessing the estrogenic potential of the xenoestrogens nonylphenol and bisphenol A, which exhibited estrogenic activities approximately 2000-fold less than the natural inducer 17 beta-estradiol. The vitellogenin-mRNA response to 17 alpha-ethinylestradiol reached maximum efficacy down to the lowest tested concentration of 10(-9) M. The assay also successfully identified estrogenic activity in selected waste water samples.

Maturation of peroxisomes in differentiating human hepatoblastoma cells (HepG2): possible involvement of the peroxisome proliferator-activated receptor alpha (PPAR alpha)

We have studied the alterations of peroxisomes in the human hepatoblastoma cell line HepG2, induced to differentiate by long-term cultivation (20 days without passaging) using morphological and biochemical techniques as well as mRNA analysis. Ultrastructural studies revealed alterations in shape and size of peroxisomes, with significant increases in mean diameter and formation of small clusters exhibiting heterogeneous staining for catalase after 20 days in culture. These alterations of peroxisomes correspond to the changes described during the maturation process from prenatal to adult human hepatocytes. As revealed by Northern and Western blotting there was marked elevation of the mRNA (190%) and protein (180%) of the peroxisomal branched-chain acyl-CoA oxidase. This protein is the key regulatory enzyme for the side chain oxidation of cholesterol for bile acid synthesis, a pathway associated with mature hepatocytes. Concomitantly a marked increase of bile canaliculi was noted by light and electron microscopy. This differentiation process was confirmed also by the increase of albumin synthesis (mRNA: 160%; protein: 190%) which is generally used as a differentiation marker of hepatocytes in culture. Interestingly, the mRNA for peroxisome proliferator-activated receptor alpha (PPAR alpha) increased drastically by almost 390% and its corresponding protein by 150%, suggesting its involvement in maturation of the peroxisomal compartment in differentiating HepG2 cells. In contrast to the wellknown increases during the drug-induced peroxisome proliferation of cytochrome P450 4A, multifunctional enzyme 1, palmitoyl-CoA oxidase and the 70-kDa peroxisomal membrane protein, those proteins were either not altered or only slightly elevated during the differentiation process, suggesting that peroxisome proliferation and maturation are two distinct and differentially regulated processes.

Extremely-rapid RNA detection in dot blots with digoxigenin-labeled RNA probes

An RNA dot blot technique is described which uses digoxigenin-labeled antisense RNA probes and a highly-sensitive chemiluminescent substrate. Detection is extremely rapid: 10 s for even low abundance mRNA.

Impairment of peroxisomal structure and function in rat liver allograft rejection: prevention by cyclosporine

BACKGROUND

During allograft rejection, cytokines and lipid mediators contribute to cell injury and organ failure. Peroxisomes play a crucial role in lipid metabolism, including the degradation of lipid mediators by peroxisomal beta-oxidation. Therefore, we investigated the alterations of hepatic peroxisomes after allogeneic rat liver transplantation.

METHODS

MHC-incompatible Dark Agouti (RT1a) donor rats and Lewis (RT1(1)) recipient rats were used for allogeneic transplantation. For immunosuppression, a group of these animals received cyclosporine (CsA) intraperitoneally (1 mg/kg body weight per day). Lewis rats were used for isogeneic transplant combination. Ten days after transplantation, livers were investigated using morphometrical methods for determination of peroxisomal diameter and volume density. The activities of peroxisomal catalase (CAT) and acyl-coenzyme A oxidase (AOX) were determined, and the corresponding proteins were evaluated by quantitative immunocytochemistry and immunoblotting. The expressions of mRNAs encoding CAT and AOX were investigated by Northern blotting.

RESULTS

The volume density and diameter of peroxisomes were significantly decreased in allogeneic transplanted livers but were unchanged in CsA-treated animals. Both the activities of CAT and AOX and their protein levels were significantly reduced in liver allografts. Moreover, the corresponding mRNA levels of CAT and AOX were decreased significantly in liver allografts, whereas CsA treatment led to an increase of those mRNAs. Isogeneic transplanted livers showed only a slight reduction of the corresponding enzyme values.

CONCLUSIONS

Peroxisomes are severely affected both morphologically and functionally after allogeneic liver transplantation. These results suggest that impairment of peroxisomal lipid beta-oxidation could contribute to the pathogenesis of the rejection process by decreased catabolism of lipid mediators involved in the regulation of the inflammatory response. CsA, in addition to its immunosuppressive effects, may contribute to allograft survival by maintenance of those important peroxisomal functions.

Effect of sodium butyrate on the expression of genes transduced by retroviral vectors

We have studied the effects of sodium butyrate (NaBu) on the expression of genes transduced by retroviral vectors and stably expressed in two salivary gland-derived cell lines, A5-DAP and A5-BAG, established earlier. These cell lines were obtained by infecting A5 cells with the retroviral vectors DAP and BAG, respectively, and by selecting neomycin-resistant transduced cells. A5-DAP cells express human placental alkaline phosphatase (PLAP) and A5-BAG cells bacterial beta-galactosidase, both under the control of the viral long terminal repeat (LTR) enhancer-promoter. NaBu in the concentration of 2-8 mM inhibited the growth of A5-DAP cells, and induced the expression of heat-stable PLAP. These effects of NaBu were dose-dependent. Induction of PLAP in clones of A5-DAP cells that express different basal levels of the enzyme was not correlated with the relative inducibilty by NaBu. Exposure to 4 mM NaBu for 48 h increased the PLAP mRNA level by 31%. A5-DAP cells released, in a time-dependent manner, PLAP into the culture medium. Cells treated with NaBu released more PLAP than untreated cells in proportion to their elevated level of the enzyme. The parent A5 cells also express a low level of tissue non-specific type alkaline phosphatase, which was also induced by NaBu. NaBu inhibited the growth of A5-BAG cells also, and increased the beta-galactosidase level. These data indicate the genes transduced by retroviral vectors can be induced by NaBu, which most likely interacts with the viral LTR.

Tubular peroxisomes in HepG2 cells: selective induction by growth factors and arachidonic acid

We showed recently the plasticity of the peroxisomal compartment in the human hepatoblastoma cell line HepG2 as evidenced by the presence of elongated tubular peroxisomes measuring up to 5 microm next to much smaller spherical or rod-shaped ones (0.1-0.3 microm). Since the occurrence of tubular peroxisomes in a given cell in culture is synchronized, with neighboring cells containing either small spherical or elongated tubular peroxisomes, cell counting of immunofluorescence preparations stained for catalase was used for the quantitative assessment of the dynamics of the peroxisomal compartment and the factors regulating this process. Initial studies revealed that the formation of tubular peroxisomes is primarily influenced by the cell density as well as by lipid- and protein-factors in fetal calf serum, being independent of an intact microtubular network. Biochemical studies showed that the occurrence of tubular peroxisomes correlated with the expression of the mRNA for 70 kDa peroxisomal membrane protein (PMP70), but not with that of matrix proteins. By cultivation of cells in serum- and protein-free media specific factors were identified which influenced the formation of tubular peroxisomes. Among several growth factors tested, nerve growth factor (NGF) was the most potent one inducing tubular peroxisomes and its effect was blocked by K252b, a specific inhibitor of neurotrophin receptor pathway, suggesting the involvement of signal transduction in this process. Furthermore, from several polyunsaturated fatty acids (PUFA) which all induced tubular peroxisomes, the arachidonic acid (AA) was the most potent one. Our observations suggest that tubular peroxisomes are transient structures in the process of rapid expansion of the peroxisomal compartment which are induced either by specific growth factors or by polyunsaturated fatty acids both of which are involved in intracellular signaling.

TNF-alpha downregulates the peroxisome proliferator activated receptor-alpha and the mRNAs encoding peroxisomal proteins in rat liver

We have studied the effects of TNF-alpha on the mRNAs coding for the peroxisome proliferator activated receptor alpha (PPAR-alpha), and for catalase (Cat), acyl-CoA oxidase (AOX), multifunctional enzyme (PH), and beta-actin in rat liver. Total RNA was isolated from livers of male SD-rats 16 h after administration of a single dose of 25 microg TNF-alpha and mRNAs were analyzed by a novel dot blot RNase protection assay. The mRNAs for PPAR-alpha and for Cat, AOX and PH were significantly reduced by TNF-treatment. In addition, the level of PPAR-alpha protein was also decreased after TNF. In contrast, the mRNA for beta-actin was markedly increased implying that the effect of TNF on PPAR-alpha and the peroxisomal mRNAs is highly selective. This effect may have important implications in perturbation of the lipid metabolism induced by TNF-alpha.