Mechanism of benzo(a)pyrene induction of alpha-human chorionic gonadotropin gene expression in human lung tumor cells

Abstract

Human lung cells (ChaGo) derived from a bronchogenic carcinoma synthesize and secrete in the culture medium the alpha subunit of the glycoprotein hormone, human chorionic gonadotropin (alpha-hCG). The synthesis of alpha-hCG by ChaGo cells could be further stimulated by treatment with sublethal concentrations of the polycyclic aromatic hydrocarbons (PAHs), benzo(a)pyrene (gaP), or dimethylbenzanthracene. The production of alpha-hCG could be correlated to the levels of alpha-hCG-specific mRNA sequences in control and PAH-treated cells. Further analysis of the RNA species (Northern blot) revealed that the level of the mature (~1.0 kb) and the high molecular weight alpha-hCG specific nuclear RNA sequences (~2.2 and 5 kb) were all greater in PAH-treated cells.Addition of [3H]BaP (0.25 #g/ml) in the culture medium of ChaGo cells led to immediate uptake of the radioactive compound apparently by simple diffusion. SDS PAGE and subsequent fluorography revealed that the radioactive compound interacted and formed covalent complexes with cytoplasmic and nuclear proteins. This covalent interaction of the [3H]BaP molecule with cellular proteins could be significantly inhibited by either inhibiting the activity of the enzyme aryl hydrocarbon hydroxylase with 7,8-benzoflavone or by reducing the cellular concentration of the enzyme by simultaneous incubation with cycloheximide. These results suggested that in ChaGo cells, the observed covalent complexes were formed by the interaction of the BaP metabolites with cellular proteins. The concentrations at which 7,8-benzoflavone or cycloheximide inhibited (a) formation of metabolites from [3H]BaP and (b) their covalent interaction with cell protein did not affect the BaP-induced stimulation of alphahCG gene expression. However, the cytotoxic effects of BaP in ChaGo cells seemed to be exerted by the metabolism of the compounds. Results presented in this report suggest that gap metabolism and the interaction of the metabolites with cell proteins were not essential for the BaP-induced modulation of alpha-hCG gene expression.Polycyclic aromatic hydrocarbons (PAHs) ~ are currently recognized as one of the major classes of environmental carcinogenic pollutants (1). Of the PAHs, benzo(a)pyrene (BaP) is Abbreviations used in this paper: AHH, aryl hydrocarbon hydroxylase; BaP benzo(a)pyrene; BF, 7,3-benzoflavone; ChaGo cells, human lung bronchogenic carcinoma cells; DMBA, dimethylbenzanthracene; alpha-hCG, the alpha subunit of human chorionic gonadotropin; alpha-hCG mRNA, alpha-hCG-specific mRNA sequences; PAHs, polycyclic aromatic hydrocarbons; SSC, 3 M NaC! and 0.3 M sodium acetate. accepted as the prototype (2, 3). Ectopic hormone and inappropriate gene expression are commonly found to be associated with neoplastic transformation of cells. BaP and several other members of the PAHs have been found to induce aryl hydrocarbon hydroxylase (AHH) in different mammalian cell systems (4). These inducers of AHH also enhance the expression of alpha-fetoprotein gene (5); activities of phospho...