Ellen K. Silbergeld

Animal antibiotic use has an early but important impact on the emergence of antibiotic resistance in human commensal bacteria

Antibiotic use is known to promote the development of antibiotic resistance, but substantial controversy exists about the impact of agricultural antibiotic use (AAU) on the subsequent emergence of antibiotic-resistant bacteria among humans. AAU for animal growth promotion or for treatment or control of animal diseases generates reservoirs of antibiotic-resistant (AR) bacteria that contaminate animal food products. Mathematical models are an important tool for understanding the potential medical consequences of this increased exposure. We have developed a mathematical model to evaluate factors affecting the prevalence of human commensal AR bacteria that cause opportunistic infections (e.g., enterococci). Our analysis suggests that AAU hastens the appearance of AR bacteria in humans. Our model indicates that the greatest impact occurs very early in the emergence of resistance, when AR bacteria are rare, possibly below the detection limits of current surveillance methods.

Lead as a carcinogen: Experimental evidence and mechanisms of action†

Recent epidemiological and experimental work confirms that inorganic lead compounds are associated with increased risks of tumorigenesis. In animals, these risks can be induced at doses that are not associated with organ toxicity and in mice that do not produce alpha-2 urinary globulin in the kidney. Thus the mechanisms of lead carcinogenicity are unlikely to be fully explained as toxicity-related sequelae of high dose exposure or as a rat-specific response involving overexpression of a renal protein. Plausible mechanisms of lead carcinogenicity include direct DNA damage, clastogenicity, or inhibition of DNA synthesis or repair. Lead may also generate reactive oxygen species and cause oxidative damage to DNA. Recent data indicate that lead can substitute for zinc in several proteins that function as transcriptional regulators, including protamines. Lead further reduces the binding of these proteins to recognition elements in genomic DNA, which suggests an epigenetic involvement of lead in altered gene expression. These events may be of particular relevance in transplacental exposures and later cancer.

Mechanisms of lead neurotoxicity, or looking beyond the lamppost.

Despite several decades of research on the neurotoxicology of lead and its continued prominence as a major environmental and occupational health hazard, the mechanisms of its toxic action in the nervous system are still unknown. The differential effects of lead exposure in young children and adults, as well as inconsistencies between in vivo and in vitro studies, suggest that lead toxicity may have multiple mechanisms in the central nervous system (CNS). Two are: neurodevelopmental toxicity, possibly involving interference with cell adhesion molecules, resulting in miswiring of the CNS during early development and possibly permanent dysfunction; and neuropharma‐cological toxicity, which might involve interactions between lead and calcium and lead and zinc, resulting in interference with neurotransmission at the synapse. This may be reversible.— Silbergeld, E. K. Mechanisms of lead neurotoxicity, or looking beyond the lamppost. FASEB J. 6: 3201‐3206; 1992.

Variability in human metabolism of arsenic.

Estimating the nature and extent of human cancer risks due to arsenic (As) in drinking water is currently of great concern, since millions of persons worldwide are exposed to arsenic, primarily through natural enrichment of drinking water drawn from deep wells. Humans metabolize and eliminate As through oxidative methylation and subsequent urinary excretion. While there is debate as to the role of methylation in activation/detoxification, variations in arsenic metabolism may affect individual risks of toxicity and carcinogenesis. Using data from three populations, from Mexico, China, and Chile, we have analyzed the distribution in urine of total arsenic and arsenic species (inorganic arsenic (InAs), monomethyl arsenic (MMA), and dimethyl arsenic (DMA). Data were analyzed in terms of the concentration of each species and by evaluating MMA:DMA and (MMA+DMA):InAs ratios. In all persons most urinary As was present as DMA. Male:female differences were discernible in both high- and low-exposure groups from all three populations, but the gender differences varied by populations. The data also indicated bimodal distributions in the ratios of DMA to InAs and to MMA. While the gene or genes responsible for arsenic methylation are still unknown, the results of our studies among the ethnic groups in this study are consistent with the presence of functional genetic polymorphisms in arsenic methylation leading to measurable differences in toxicity. This analysis highlights the need for continuing research on the health effects of As in humans using molecular epidemiologic methods.

Lead effects on protamine–DNA binding

BACKGROUND Lead impairs male fertility and may affect offspring of exposed males, but the mechanisms for this impairment are not completely clear. Protamine P1 and P2 families pack and protect mammalian sperm DNA. Human HP2 is a zinc-protein and may have an important role in fertility. As lead has affinity for zinc-containing proteins, we evaluated its ability in vitro to bind to HP2 and its effects on HP2-DNA binding. Methods and Results UV/VIS spectroscopic data indicated that HP2 binds both Pb(2+) and Zn(2+)(as chloride salts). They also provided evidence that thiol groups mainly participate for Zn(2+)-binding; however, HP2 has additional binding sites for Pb(2+). The mobility shift assay showed that lead interaction with HP2 caused a dose-dependent decrease on HP2 binding to DNA, suggesting that lead may alter chromatin stability. CONCLUSIONS These in vitro results demonstrate that lead can interact with HP2 altering the DNA-protamine binding. This chemical interaction of lead with protamines may result in chromatin alterations, which in turn may lead to male fertility problems and eventually to DNA damage.

Lead Inhibits Secretion of Osteonectin/SPARC without Significantly Altering Collagen or Hsp47 Production in Osteoblast-like ROS 17/2.8 Cells

In an effort to better understand the consequences of lead (Pb2+) on skeletal growth, the effects of Pb2+ were investigated using ROS 17/2.8 bone-like cells in vitro. These studies revealed that Pb2+ (4.5 x 10(-6) M -4.5 x 10(-7) M) has little or no effect on cell shape except when added immediately following seeding of the cells. However, proliferation of ROS cells was inhibited, in the absence of serum, at concentrations of 4.5 x 10(-6) M Pb2+. Protein production was generally increased, however, the major structural protein of bone, type I collagen, production was only slightly altered. Following treatment of ROS cells with Pb2+, intracellular levels of the calcium-binding protein osteonectin/SPARC were increased. Osteonectin/SPARC secretion into the media was delayed or inhibited. Coincident with retention of osteonectin/SPARC there was a decrease in the levels of osteonectin/SPARC mRNA as determined by Northern analysis. These studies suggest that processes associated with osteonectin/SPARC translation and secretion are sensitive to Pb2+.

Health Risks Associated with Prenatal Metal Exposure

A symposium entitled Health Risks Associated with Prenatal Metal Exposure was held at the 33rd Annual Meeting of the Society of Toxicology (SOT) in Dallas, Texas. The symposium was cosponsored by the Metals and Reproductive and Developmental Specialty Sections of SOT and was designed to elaborate the health risks associated with in utero exposure to metals commonly found in the workplace and/or ambient environment on the mother and developing offspring. Epidemiological and toxicological evidence that demonstrates the health effects and underlying mechanisms associated with exposure to arsenic (As), lead (Pb), and methyl mercury (MeHg) were discussed, as well as the legal ramifications and personal implications associated with prenatal metal exposure. The following is a summary of each of the individual presentations.

Neurotoxic effects of endocrine disruptors.

Endocrine disrupting chemicals are a newly defined category of environmental contaminants that may affect animal and human populations by interfering with normal hormone action. There is substantial concern that these agents could have a range of subtle and long-lasting effects. Because of the sensitivity of the developing central nervous system to low levels of endogenous gonadal hormones during development, the central nervous system may be a target for the action of endocrine disrupting chemicals.

Mercury — are we studying the right endpoints and mechanisms

Mercury and methylmercury are major environmental contaminants. New concerns have arisen based upon reports of increasing levels in biota and ecosystems, as well as concerns for low level toxicity. Despite the substantial knowledge about mercury toxicity, there are gaps in our knowledge that limit the development of science based risk assessments. First, mechanistic research on mercury/methylmercury toxicity is not clearly related to sublethal and noncytotoxic effects. Second, relatively little attention has been paid to nonneurotoxic effects of mercury, specifically, immunototoxic effects. Because of this, caution is needed in evaluating the health risk of mercury, particularly if important sources of ongoing releases are not identified or adequately controlled.

Male-Mediated Reproductive Toxicity: Effects on the Nervous System of Offspring

Male reproductive toxicity deals primarily with infertility measures. The possibility of toxic effects subtly manifested in the offspring of exposed males is rarely considered. This is possibly due to the lack of research and testing dealing with this type of toxicity. Paternally-mediated effects on the development of offspring are not commonly assessed in toxicity studies. This oversight is partly due to the standard reproductive toxicity testing parameters of EPA and other regulatory agencies. Also, it is due to the lack of attention in epidemiological studies to exposure or co-exposure of fathers to reproductive and developmental toxicants. In both cases, developmental toxicity studies concentrate on maternal exposures both prior to pregnancy and during gestation. Developmental toxicity due to paternal exposure is rarely considered, even in the cases of recognized reproductive toxicants.