The aryl hydrocarbon receptor pathway: a model for biological switching

Abstract

The shape of the dose-response curve may vary depending on whether one examines response at a population or a single cell level. Populations of cells may exhibit a graded response whereas single cell responses may have threshold or switch-like behavior. Studies in vivo and in vitro using primary hepatocyte cultures have shown that induction of CYP1A1 in the liver exhibits switch-like behavior in response to PCB 126 (3,3',4,4',5-pentachlorobiphenyl) (Chubb LS 2002; French et al. 2004). The goal of the present study was to determine if two liver cell lines (H4IIE rat hepatoma and Hepa lclc7 mouse hepatoma) also show switch-like behavior and develop experimental models for studying mechanisms of these switch-like responses. Both cell lines were analyzed via concentration-response and time-course studies using quantitative real-time PCR, revealing a sigmoid concentration-response curve for CYP1A1 mRNA induction at the population level. To study CYP1A1 protein induction on a single cell level, flow cytometry was employed. In both cell lines the distribution of fluorescence increased with increasing concentrations of PCB 126. The switch behavior was more pronounced in the H4IIE cells than in the Hepa lclc7 cells, exhibiting a well-defined shift of induction from the "off" to the "on" state. The concentration-response curve at the single cell level appeared more switch-like with two populations of cells — basal levels and maximally induced. Our data support the hypothesis that PCB 126 induces CYP1A1 in a switch-like fashion in H4IIE rat hepatoma cells. These cells can now be used to study the mechanism of the biological switch. The AhR pathway is modulated by multiple co-activators and by phosphorylation. Our research reveals a likely role for PKC in this switch response as evidenced by dramatically reduced CYP1A1 induction after inhibition of PKC. We have also demonstrated the regulation of several proteins/phosphoproteins by PCB 126 and that the MAPK pathways ERK and JNK are not activated by PCB 126 in H4IIE cells. This may indicate that the switch-like induction of CYP1A1 by PCB 126 involves the PKC signaling pathway and does not involve the MAPK pathways in this system.