Phenobarbital induction of cytochrome P4501A1 is regulated by cAMP-dependent protein kinase-mediated signaling pathways in rainbow trout hepatocytes.

Phenobarbital (PB) induces CYP1A1 at the transcriptional level and causes nuclear translocation of the aromatic hydrocarbon (Ah) receptor in primary cultures of rainbow trout hepatocytes (1). The results from this study suggest that PB induction of CYP1A1 in rainbow trout hepatocytes is regulated by cAMP-dependent pathways (PKA), whereas TCDD induction is not dependent upon PKA. Epinephrine, which increases cAMP levels and activates PKA-dependent pathways, was a potent inhibitor of PB induction, while having no effect on TCDD induction of CYP1A1 gene expression. When PKA-dependent pathways were inhibited, PB induction of CYP1A1 gene expression was greatly potentiated, whereas TCDD induction was affected to a lesser extent. Inhibitors of calcium-phospholipid-dependent protein kinase (PKC) had modest or no effect on PB and TCDD induction of CYP1A1, respectively. Whether the relatively weak-to-no inhibition of CYP1A1 in response to PKC inhibitors in fish is due to differences in the types and levels of PKC isoenzymes, cell permeability, protocol, or the role of PKC in the mechanism of CYP1A1 induction in fish remains to be established. PB induced persistent and transient increases in the intracellular calcium concentration. This may be an important factor regulating PKC which may have a role in PB-mediated induction of CYP1A1 gene transcription.