David W. Brewster

TCDD (2,3,7,8-tetrachlorodibenzo-P-dioxin) causes increases in protein kinases particularly protein kinase C in the hepatic plasma membrane of the rat and the guinea pig

To study the cause of TCDD-evoked changes in the functions of plasma membrane constituents TCDDs effects on protein kinase activities in the liver of rats and guinea pigs were investigated. TCDD was found to cause a sharp increase in both c-AMP independent and dependent protein kinase activities in plasma membrane preparations from rat liver within 48 hours from the time of administration. Such effects reached maxima around day 20, and were quite noticeable even 40 days after a single administration of TCDD. As a result of SDS-polyacrylamide gel-electrophoresis (SDS-PAGE) analysis several substrate proteins for these increased protein kinases were observed. Among them are 170 K - 150 K bands, representing EGF receptor protein. TCDD was found to particularly stimulate protein kinase C which is known to influence many enzyme and receptor functions through protein phosphorylation. The possible significance of such an action of TCDD is discussed.

Alteration of rat hepatic plasma membrane functions by 2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD)

In vivo administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to rats produced significant alterations in Na-K, Ca, Mg ATPase and protein kinase activity, and receptor binding activity to insulin, concanavalin A, glucagon and epidermal growth factor in the canaliculi-rich plasma membrane fraction of the hepatocytes. Some of the changes are dose-related, and generally follow the time course of the sign of TCDD poisoning in the rat (e.g., the loss in body weight). Such TCDD-induced changes in membrane proteins and subsequent alteration in membrane functions appear to be extensive.

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) reduces lipoprotein lipase activity in the adipose tissue of the guinea pig

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) administered to young male guinea pigs at a dose of 1 microgram/kg (single intraperitoneal injection) caused a large reduction in adipose tissue lipoprotein lipase (LPL) activity. This effect occurred rapidly; a 70% decrease was noticed after 24 hour and 80% of LPL activity was lost by 48 hours when the serum triglyceride levels increased to 175% of control levels. LPL is known to play an important role in controlling the amount of free fatty acids supplied to adipose tissues. Administration of a large dose of glucose to fasted guinea pigs, which have shown a similar weight loss, but less LPL loss than TCDD-treated animals, had the effect of elevating their adipose LPL levels back to a near normal level, whereas the same treatment caused no significant increase in the LPL levels of TCDD-treated animals. Evidence indicates that the TCDD-caused decline in LPL activity is irreversible. As a consequence, the affected guinea pigs are incapable of responding to changes in nutritional status.

Rabbit serum Hypertriglyceridemia after administration of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD)

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) caused a dose-dependent decrease of adipose tissue lipoprotein lipase (LPL) activity and caused a concomitant increase in serum triglyceride concentration in the rabbit 10 d after single ip administration of either 1 or 50 micrograms/kg. Hepatic low-density lipoprotein (LDL) binding was markedly depressed and serum cholesterol concentrations were modestly increased relative to pair-fed control animals. Serum glucose concentrations were significantly lower in the rabbit administered TCDD compared to ad libitum or pair-fed control animals, although little change was observed in serum insulin concentration. Electron microscopic examination of aortic arches 20 d after a single ip administration of 50 micrograms TCDD/kg revealed ruffling, denudation, and sloughing off of the cell surface and the appearance of macrophage-like structures in the intima and media of the endothelial cells. These alterations resemble preatherosclerotic lesions typical in animals with hyperlipidemia. It is proposed that TCDD causes hyperlipidemia in the rabbit through suppression of LPL activity and LDL receptor binding.

Effects of 2,3,7,8‐tetrachlorodibenzo‐P‐dioxin on serum insulin and glucose levels in the rabbit

Serum insulin and glucose were measured in young male rabbits after a single intraperitoneal dose of 1 or 50 micrograms/kg of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Serum insulin levels in the high dosed rabbits were significantly decreased between 15 min and 8 h post treatment, equivalent after 24 h, significantly elevated 48 h post treatment, and they were not different at 10 days post-treatment when compared to weight matched and pair-fed controls. At the low dose, rabbits showed no differences in serum insulin from controls. In the high dose group, serum glucose levels were generally not different between treated and control animals, though there was a transient hyperglycemia 1 h after treatment, and both treated groups became hypoglycemic after ten days. The results indicate that TCDD altered serum insulin levels which were not coupled to changes in serum glucose.

Reduction of adipose tissue lipoprotein lipase activity as a result of in vivo administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin to the guinea pig

Within 1 hr of intraperitoneal administration of 1 microgram 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/kg, lipoprotein lipase (LPL) activity was reduced 38% from initial levels in the adipose tissue of the guinea pig. Maximal depression was observed after 2 days and persisted throughout the 10-day observation period. Oral administration of glucose restored LPL activity in TCDD-treated animals after 1 day but only partially after 2 and 5 days, and had no effect after 10 days of exposure. Although initial (2-day) serum insulin levels were depressed, the inability of glucose to restore LPL activity after prolonged exposure was not due to malabsorption of glucose nor to changes in serum thyroxine or insulin concentration. TCDD also inhibited the lipolytic pathway in the adipocyte, but had no effect on hormone sensitive lipase (HSL). Since HSL and LPL are reciprocally regulated, it was concluded that TCDD acts on the adipocyte to uncouple HSL-LPL reciprocity as well as to reduce LPL production.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on guinea pig heart muscle☆

Cardiac effects of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) were examined using atrial muscle isolated from young or mature guinea pigs fed ad libitum, from young animals 10 or 20 days after a single ip injection of 1 microgram TCDD/kg body wt, and from control guinea pigs pair-fed to the TCDD-treated animals. Basal contraction force of left atrial muscle obtained from TCDD-treated and pair-fed control guinea pigs 10 days after treatment was significantly increased compared to atrial muscle obtained from ad libitum control animals. Atrial muscle from TCDD-treated animals 20 days after treatment had a significantly lower basal force of contraction while that from pair-fed animals was significantly higher relative to atrial muscle from ad libitum control animals. These results suggest that the TCDD-induced change in basal atrial contraction force is in part due to undernutrition and that prolonged exposure to TCDD inhibits this response observed in pair-fed animals. Furthermore, prolonged TCDD treatment shifted the dose-response curve for the positive inotropic effect of isoproterenol to the right with no statistically significant change in the maximal inotropic effect obtained with high concentrations of isoproterenol. The TCDD-induced changes in force of contraction and the dose-response curve for the positive inotropic effect of isoproterenol were similar in young TCDD-treated and mature control guinea pigs. TCDD did not alter the basal contraction frequency or the dose-response curve for the positive chronotropic effect of isoproterenol. There was no effect on cardiac lipoprotein lipase activity at 1 microgram/kg, but in animals administered 4 micrograms TCDD/kg activity was reduced. It was concluded that TCDD adversely affects atrial muscle of guinea pig heart.

Influence of 2,3,7,8-TCDD on the protein composition of the plasma membrane of hepatic cells from the rat☆

Abstract To understand the mechanism of toxic action of TCDD, a serious environmental pollutant, rats were injected with 25 μg/kg and their livers removed 10 days post-treatment. A canaliculi-rich, plasma membrane fraction was prepared and its protein composition was examined using SDS-polyacrylamide gel electrophoresis. As a result of close comparison between untreated and treated preparations it was concluded that many protein levels were reduced in the membrane from the TCDD treated rats. This phenomenon was confirmed by the finding that there is an overall reduction in binding of 3 H-concanavalin A to the TCDD-treated plasma membrane.

Differential effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on adipose tissue lipoprotein lipase activity in the guinea pig, rat, hamster, rabbit, and mink

1. Adipose tissue lipoprotein lipase (LPL) activity of several different animal species was determined after i.p. administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). 2. TCDD caused a significant reduction in LPL activity and an increase in serum triglyceride concentration in guinea pigs, rabbits, and hamsters but not rats. 3. TCDD increased adipose tissue LPL activity of mink and lowered their serum triglyceride concentration. 4. Results of this study indicate that profound differences occur in lipid metabolism between various species in response to TCDD and these changes do not appear to be related to generalized toxicity such as wasting.