Mechanisms of Pb,Cd, As InteractionsEPA Grant Number: R827161
Title: Mechanisms of Pb,Cd, As Interactions
Investigators: Fowler, Bruce A.
Institution: University of Maryland - Baltimore
Current Institution: University of Maryland
EPA Project Officer: Louie, Nica
Project Period: November 15, 1998 through November 14, 2001 (Extended to November 14, 2003)
Project Amount: $832,000
RFA: Chemical Mixtures in Environmental Health (1998) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Health Effects , Land and Waste Management , Health , Safer Chemicals
Early detection of renal disease from occupational or environmental exposure to nephrotoxic chemicals is currently limited by the lack of sensitive and/or chemical-specific tests. Previous studies from this laboratory and others have demonstrated increased excretion of specific porphyrin combinations in the urines of rats exposed to Pb, Cd, or As alone or as the possible mixture combinations for 10 weeks. The primary objective of the present studies is to further these findings specifically identifying the mechanisms of interactive effects between these common metal/metalloid combinations by studying mechanisms by which these elements alter sensitive subcellular systems such as the heme and porphyrin pathway and chemical/mixture specific proteinuria patterns in a major target organ system, the kidney. These studies will be conducted in relation to an examination of the chemical species of these elements in the kidney and the protective stress protein response and markers of cell injury and cell death. The interpretative capacity of these studies for evaluating Pb, Cd, or As either alone or in mixtures, should provide a mechanistic basis for improved risk assessments based upon quantitative measurement of biomarkers which are directly linked to mechanisms of renal cell injury from specific combinations of Pb, Cd, and As at the lowest observed effect levels (LOELS) for each combination of these elements.
The nephrotoxicity of Pb, Cd, or As either alone or as mixtures will be evaluated both in vitro and in vivo using a factorial design approach following determination of lowest observable effect levels (LOELS) for each element using highly sensitive biomarkers of toxicity such as alterations in the renal heme and porphyrin pathway, stress protein induction patterns, chemical-specific proteinuria patterns in relation to both molecular and moprhological markers of cell injury and cell death. For in vitro studies, primary cultures of both male and female rat and human proximal tubule epithelial cells will be studied. In vivo studies will utilize the previously developed male rat model with exposures via drinking water. Data from the above analyses will be correlated with intracellular speciation studies for these elements with particular emphasis on the roles of metallothionein and renal lead-binding proteins in mediating the bioavailability of cadmium and lead to sensitive biochemical systems respectively. Displacement of lead from the lead-binding proteins by non-metallothionein bound cadmium will be studied in terms of increasing the bioavailability of lead to the sensitive biochemical systems noted above. The roles of the arsenical methylation pathway in mediating the bioavailability of biologically active arsenical species to these sensitive systems will be evaluated. Statistical evaluations and interpretation of the data from the 2x2x2 factorial design mixture studies will be conducted using analysis of variance (ANOVA) procedures.
It is expected that these studies will confirm the previously observed additive Pb, Cd, As interactions in renal proximal tubule cells at individual LOEL exposure levels for each element using state of the art biomarker endpoints for both in vitro and in vivo exposures. The marked interaction between Pb and Cd noted in prior studies should be explained on the basis of molecular competition between the renal metallothionein pool for cadmium and the ability of the non-metallothionein fraction to displace lead from the renal lead-binding protein pool which results in disruption of lead intranuclear inclusion body formation and the increased bioavailability of lead to other sensitive biochemical systems such as the heme biosynthetic pathway with resultant additive mixture-specific increases in porphyrin excretion. It is also expected that there will be additive mixture B specific alterations in renal tubule cell gene expression patterns and increased protein excretion under multi-element exposure conditions.
Improvements in Risk Assessment or Risk Management: Pb, Cd and As are the most common toxic inorganic agents in Superfund sites and frequently occur as mixtures. Risk assessments for these mixtures have been hampered by the availability of actual mechanism-based data which utilize sensitive biomarkers as toxic endpoints. The direct comparison of rat and human renal epithelial cells from in vitro studies should strengthen confidence in data derived from in vivo exposure studies with regard to general conclusions derived animal to human extrapolations for risk assessment purposes in relation to low level exposures to mixtures of these inorganic agents.