TCDD-induced altered expression of growth factors may have a role in producing cleft palate and enhancing the incidence of clefts after coadministration of retinoic acid and TCDD

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, inducing cleft palate and hydronephrosis at doses which are not overtly maternally toxic or embryotoxic. After TCDD exposure the palatal shelves of normal size come into contact, but fail to fuse due to altered differentiation of the medial epithelial cells. These cells continue to express EGF receptors, proliferate, and differentiate into an oral-like stratified squamous epithelium. The present study examines the effect of TCDD on the expression of growth factors which are believed to regulate differentiation and proliferation in the palate. This study also examined the combined effect of TCDD and retinoic acid (RA), since in teratology studies coadministration of these agents results in an enhancement of cleft palate incidence. Embryos were exposed in vivo on Gestation Day (GD) 10 or 12 to TCDD ot TCDD + RA and the palatal shelves were dissected on GD 14-16. Growth factor expression was determined immunohistochemically using antibodies to TGF-alpha, EGF, TGF-beta 1, or TGF-beta 2. The growth factors displayed specific spatial and temporal expression in the palatal shelves. TCDD reduced the expression of TGF-alpha, EGF, and TGF-beta 1 in epithelial and mesenchymal cells. The degree of reduction was generally greater after exposure on GD 10 to TCDD alone or in combination with RA when compared to that on GD 12. The abnormal proliferation and differentiation of TCDD-exposed medial cells may be a response to reduced expression of EGF and TGF-alpha. Low levels of these factors may be related to the previously observed elevated levels of EGF receptors in medial cells. In other systems, low levels of ligand have resulted in upregulation of the EGF receptor. Continued proliferation and altered differentiation could also be attributable to decreased levels of TGF-beta 1, a factor inhibitory to epithelial proliferation. Since TGF-beta 1 stimulates mesenchymal growth and TGF-alpha and EGF stimulate epithelial proliferation, the formation of small shelves after exposure to TCDD + RA on GD 10 may be due to the severe reduction in these factors. Only a slight to moderate reduction in growth factor expression occurs after exposure to TCDD + RA on GD 12 and in this case shelves of normal size form. Since TCDD and RA appear to act in part through pathways that involve TGF-beta 1, in vitro experiments were designed to examine the involvement of TGF-beta 1 in TCDD teratogenicity.(ABSTRACT TRUNCATED AT 400 WORDS)