Diverse Toxic Chemicals Disrupt Cell Function through a Common Path

0001 When technological advances in the 1930s provided the means to synthesize chemicals from petroleum and natural gas, the petrochemical industry ramped up production of diverse species of novel compounds without testing their safety. By 1940, a billion pounds of synthetic petrochemicals were produced each year. Of the more than 38 million chemicals reported in the scientifi c literature, 80,000 to 150,000 are used in commercial production. A report from the Harvard School of Public Health, published in The Lancet last November, warned that “a substantial number” of these chemicals—which show up in everything from cosmetics and textiles to rubber ducks and pacifi ers—may be capable of damaging the human brain, particularly during development. Any hope of developing large-scale toxicological screening methods depends on determining whether diverse toxic chemicals act through common pathways. Though few such mechanistic pathways have been identifi ed so far, diverse toxicants can make cells more oxidized. Oxidative stress results when harmful free radicals or other “pro-oxidants” overwhelm the cell’s anti-oxidant machinery, disrupting normal function. But changes in oxidative state also control multiple normal cellular functions, raising the possibility that toxicants might also affect these normal processes by making cells more oxidized. The signifi cance of these oxidative effects—particularly, whether they might constitute a unifying principle of toxicity—has been unclear, however, in part because different toxicants increase oxidation through different mechanisms, and because the relationship between their harmful effects and their effects on oxidative status are not well understood. In a new study, Zaibo Li and colleagues provide evidence for a novel general principle of toxicology by showing that toxicants with different chemical properties converge on activation of the same regulatory pathway with similar results. The authors monitored the response of progenitor cells isolated from the developing central nervous system to two metal toxicants, methylmercury and lead, and an organochlorine herbicide, paraquat. They found that each toxicant disrupted normal cell function by making cells more oxidized, and setting off a chain reaction that ultimately inhibited signaling pathways required for cell division. Signifi cantly, these effects occurred at environmentally relevant, low-level exposures for lead and methylmercury. The authors studied the chemicals’ effects in cells called oligodendrocyte precursor cells (OPCs), which give rise to the myelin-forming oligodendrocytes of the central nervous system. OPCs are particularly suited to toxicant screening, the authors explain, because of their sensitivity to small changes in oxidative (or redox) state, which determines whether the cells divide or differentiate. (When a cell or molecule undergoes an increase in oxidation state, it is oxidized; when it undergoes a decrease, it is reduced.) Previous studies by these authors have shown that redox changes in the range of 15%–20% can greatly alter responsiveness to extracellular signaling molecules, and that such changes may help regulate the normal development of these cells. For example, OPCs that are slightly more reduced undergo extensive cell division when grown in the presence of the cell-division stimulator platelet-derived growth factor (PDGF). In contrast, OPCs that are more oxidized, within normal physiological redox ranges, undergo differentiation. Exposing OPCs to low levels of methylmercury, similar to those found in the environment, made the cells 20% more oxidized—and inhibited cell division (as indicated by the number of cells in the synthesis phase of the cell cycle). For example, methylmercury levels as low as 20 nM (equivalent to four parts per billion) caused a 25% drop in the percentage of cells that underwent cell division in response to PDGF stimulation. Each year, methylmercury is detected in the cord blood of 600,000 infants at levels equal to or above those producing these effects. Reduced cell division, the authors show, resulted from disrupted PDGF signaling: proteins normally activated by PDGF signaling (for example, Erk1/2 and Akt) were inhibited by exposure to methylmercury (analyzed at the still sublethal exposure level of 30 nM), which also reduced the abundance of PDGF’s receptor, PDGFRá. To fi nd out which part of the PDGF pathway methylmercury targeted, the authors exposed OPCs to neurotrophin-3 (NT-3), a signaling molecule that also activates Erk1/2, a downstream component of multiple signaling pathways. Since neither Erk1/2 nor TrkC, the NT-3 receptor, were affected by the toxicant, they concluded that it must act upstream of this component. These results suggested that methylmercury might activate an enzyme—the ubiquitin ligase cCbl—that targets PDGFRá for degradation. This possibility seemed especially attractive since c-Cbl doesn’t target TrkC and is activated by Fyn, an enzyme previously shown to be activated by oxidative stress. And that’s what the authors found: methylmercury activates Fyn, which then activates c-Cbl, which in turn reduces PDGFRα levels by targeting the receptor for degradation. Methylmercury’s effects could be blocked by suppressing Fyn or c-Cbl with appropriate molecular constructs, by overexpressing PDGFRα, or by inhibiting Fyn activation with the pharmacological blocker PP1. The pro-oxidants lead and paraquat (which differ chemically both from each other and from methylmercury) produced the same effects as methylmercury on OPCs and on the PDGF pathway (about 20% increased oxidation, Fyn and c-Cbl activation, Diverse Toxic Chemicals Disrupt Cell Function through a Common Path