Two established human
hepatoma cell lines, Hep3B and HepG2, were examined for aryl
hydrocarbon (
benzo[a]pyrene) hydroxylase (AHH) induction and for the presence of the murine-equivalent
aromatic hydrocarbon (
Ah) receptor. Both cell lines demonstrated
polycyclic aromatic hydrocarbon (PAH)-induced AHH activity; however, assay conditions for induction were different than those established for the control mouse
hepatoma cell line, Hepa c1-9. When cytosols from either cell line were exposed to tritiated
2,3,7,8-tetrachlorodibenzo-p-dioxin [( 3H]
TCDD) and analyzed on
sucrose gradients with or without prior
charcoal treatment, two peaks were observed at positions corresponding to 4-5 S and 8-9 S. The 8-9 S peak was identified as the probable human
Ah receptor equivalent since, like the mouse
Ah receptor, this peak: (a) was eliminated only by
cytochrome P1-450 inducers; (b) was sensitive to
protease digestion; and (c) was thermolabile. Levels of
TCDD specifically bound in the 8-9 S peak for HepG2 and Hep3B were 27 and 34 fmol/mg cytosolic
protein respectively. The level of
TCDD specifically bound was not affected by
charcoal treatment or by the addition of
sodium molybdate, which is known to stabilize
ligand binding to
steroid receptors. Incubation of Hep3B or HepG2 cells with [H]
TCDD at 37 degrees for 1 hr effected a redistribution of binding from the cytosol 8-9 S peak to a nuclear 6 S peak. The nuclear peaks from both human cell lines demonstrated similar sedimentation properties, temperature-dependence and inducer-specificity, as for the mouse nuclear
Ah receptor. Appearance of nuclear 6 S binding is consistent with a temperature-dependent translocation process, supporting the observation that these human
hepatoma cell lines contain a binding component which is similar to the mouse
Ah receptor in structure and function during AHH induction.