Dioxin is an extremely potent carcinogen. In highly exposed people, the most commonly observed toxicity is chloracne, a pathological response of the skin. Most of the effects of dioxin are attributed to its activation of the aryl hydrocarbon receptor (AHR), a transcription factor that binds to the Ah receptor nuclear translocator (ARNT) to regulate the transcription of numerous genes, including CYP1A1 and CYP1B1. In cultures of normal human epidermal keratinocytes dioxin accelerates cell differentiation, as measured by the formation of cornified envelopes. We show that this acceleration is mediated by the AHR; also, that dioxin increases the expression of several genes known to be regulated by ARNT, which have critical roles in the cornification and epidermal barrier function of the skin. Importantly, we demonstrate that all of these responses are opposed by ligand-activation of the EGF receptor (R), an important regulator of keratinocyte cell fate. In the CYP1A1 enhancer, EGFR activation prevents recruitment of the p300 coactivator, although not affecting the binding of the AHR or ARNT. The total cellular level of p300 protein does not decrease, and overexpression of p300 relieves EGFR-mediated repression of transcription, indicating that p300 is a critical target for the repression of the AHR complex by EGFR signaling. These results provide a mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin is able to disrupt epidermal homeostasis and identify EGFR signaling as a regulator of the AHR. This signaling may modulate the incidence and severity of chloracne and be of therapeutic relevance to human poisonings by dioxin.

https://www.pnas.org/content/pnas/106/11/4266.full.pdf

EGF receptor signaling blocks aryl hydrocarbon receptor-mediated transcription and cell differentiation in human epidermal keratinocytes

Trophic importance of Spartina alterniflora production and decomposition to the marsh-estuarine ecosystem

Dredge and fill activities in estuaries have many environmental effects, most, although not all, of them deleterious. These effects include reduced light penetration by increased turbidity; altered tidal exchange, mixing, and circulation; reduced nutrient outflow from marshes and swamps; increased saltwater intrusion; and creation of an environment highly susceptible to recurrent low dissolved oxygen levels. Coral, oysters, and barnacles are particularly vulnerable to the effects of siltation. Both estuarine flora and fauna may be harmed by contaminants released into the water column by dredging operations. Ways to mitigate the effects of dredge and fill operations include careful pre- and post-construction environmental studies; use of bridging to create roadbeds where coastal wetlands cannot be avoided; use of a turbidity diaper and other means to control turbidity; dredging during periods of low benthic populations or during tides that would carry coarser sediments away from productive areas such as oyster reefs; and thoughtful disposal of spoil, such as locating spoil sites on the uplands with proper diking.

Estuarine dredge and fill activities: A review of impacts

The effects of dredging to remove sediments contaminated with polychlorinated biphenyls (PCBs) were studied in the South Branch of the Shiawassee River in south-central Michigan. Caged fingernail clams, Sphaerium striatinum (Lamarck), and fathead minnows, Pimephales promelas Rafinesque, were used to monitor the bioavailability of PCBs, and these data were compared with changes in water column concentrations before dredging, during dredging and up to six months after dredging was completed. Monitoring of water, clams and fish during dredging confirmed that significant amounts of PCBs were released from the sediments. At all locations downstream and in the area of the dredging, there were increases in the availability of PCBs for at least six months. PCB concentrations in clams and fish in the dredged zone increased from 64.5 to 87.95 μg/g dry weight and from 13.82 to 18.30 μg/g dry weight, respectively, after dredging. Downstream (11 km), post-dredge uptake was also higher; however, clams showed less uptake than fish. This implied different uptake routes for the two organisms. There was no noticeable change in total PCB concentration in the water after dredging.

In situ experiments were run to determine uptake curves and time to uptake equilibrium for both clams and fish. Results of tests for equilibrium uptake by fatheads agreed with literature values. Uptake maxima occurred within 20 to 30 d. For clams, maximum uptake for Aroclor 1242 occurred after 9 d. Log bioconcentration factors for clams ranged from 2.6 to 4.5 for Aroclor 1242 and from 3.0 to 4.4 for Aroclor 1254; for fish, they ranged from 3.0 to 4.4 for Aroclor 1242 and from 4.5 to 5.5 for Aroclor 1254.

PCB availability assessment of river dredging using caged clams and fish

Great blue heron (Ardea herodias) eggs and prey items were collected from six colonies in Oregon and Washington, USA, during 1994 to 1995. Contaminant concentrations, reproductive success, and biomagnification factors were determined and effects of residue levels were measured by H4IIE rat hepatoma bioassays. Mean residue concentrations in heron eggs and prey items were generally low. However, elevated concentrations of polychlorinated biphenyls (PCBs) were detected in eggs and prey from Ross Island on the Willamette River. Biomagnification factors varied among sites. Sites were not significantly different in H4IIE tetrachlorodibenzo-p-dioxin equivalents (TCDD-EQs), although the TCDD-EQ for Karlson Island was 9 to 20 times greater than that of any other site. Large differences existed between toxic equivalents calculated from egg residue concentrations and TCDD-EQs, which indicated nonadditive interactions among the compounds. Tetrachlorodibenzo-p-dioxin equivalents and nest failure were positively correlated with TCDD concentration. Fledging and reproductive rates were similar to those determined for healthy heron populations, however, indicating that any adverse effects were occurring at the individual level and not at the colony level. Our results support the use of great blue herons as a biomonitor for contamination in aquatic ecosystems. Their relatively low sensitivity to organochlorine contaminants and high trophic position allows contaminant accumulation and biomagnification without immediate adverse effects that are often seen in other, more sensitive species.

A.J. Hafferty

Papers

Release of polychlorinated biphenyls (PCB) in a salt-wedge estuary as induced by dredging of contaminated sediments