Inhibition of binding of thyroxin to transthyretin by outdoor and indoor airborne particulate matter and effects on thyroid hormone and vitamin A metabolism in rats.

Abstract

Previous studies have indicated that chemicals, e.g., hydroxylated polychlorinated biphenyls (PCBs) decrease plasma thyroxin levels through interaction with transthyretin. As a consequence, plasma vitamin A and thyroxin are depleted. Both thyroxin and vitamin A are important regulators of normal epithelial differentiation and proliferation and might act in the promotion phase of carcinogenesis. In the present report, extracts of indoor and outdoor airborne particulate matter (APM), originating from different pollution sources, were tested for a possible interaction in vitro and in vivo with thyroxin and vitamin A. Results indicated that all APM extracts significantly interfered with thyroxin binding to transthyretin. In all cases, the more polluted samples inhibited binding more than the corresponding control samples. Exposure of rats to a cigarette smoke extract induced clear changes in plasma thyroxin and triiodothyronin hormone levels. Triiodothyronin levels were reduced most. Effects on plasma vitamin A levels were also found, but these were less clear. In addition a decrease in lung vitamin A levels was found, but this decrease was not statistically significant. Exposure of rats to an outdoor APM extract induced significant changes in plasma triiodothyronin levels. Effects on thyroxin levels were not significant, although the same trend was seen as for the cigarette smoke APM extract. No changes in plasma or liver vitamin A concentrations were induced by the outdoor APM extract. Lung vitamin A levels were decreased, but again this decrease was not statistically significant. Taken together, the present report shows that APM extracts have the potency to interfere with thyroid hormone metabolism both in vitro and in vivo and probably with vitamin A metabolism as well.(ABSTRACT TRUNCATED AT 250 WORDS)