Docosahexaenoic acid (DHA) regulates the expression of cytochrome P450 2B1 (CYP 2B1) in rat primary hepatocytes in response to xenobiotics. Ceramide, a lipid signaling molecule, is involved in various physiological processes and can be generated by the hydrolysis of sphingomyelin via sphingomyelinase (SMase). DHA activates SMase and increases ceramide formation in vitro. Ceramides differentially enhance adenylyl cyclase activity in vitro depending on the chain length of their fatty acids. In addition, the cAMP-dependent PKA pathway down-regulates CYP 2B1 expression induced by phenobarbital (PB). In the present study, we determined the effect of DHA on SMase transactivation and the downstream pathway in CYP 2B1 expression induced by PB. SMase was activated by DHA 2 h after treatment, and D609 (an SMase inhibitor) attenuated the inhibition of PB-induced CYP 2B1 expression by DHA. Ceramide formation reached a maximum 3 h after DHA administration. C2-ceramide dose-dependently inhibited PB-induced CYP 2B1 expression and increased intracellular cAMP concentrations. SQ22536 (an adenylyl cyclase inhibitor) and H89 (a PKA-specific inhibitor) partially reversed the inhibition of PB-induced CYP 2B1 expression by C2-ceramide. These results suggest that stimulation of SMase, generation of ceramide and activation of the cAMP-dependent PKA pathway are involved in the inhibition exerted by DHA.
