Piperine impairs cytochrome P4501A1 activity by direct interaction with the enzyme and not by down regulation of CYP1A1 gene expression in the rat hepatoma 5L cell line.

Modulation of benzo(a)pyrene (BP) metabolism and regulation of CYP1A1 gene expression by piperine in 5L cells in culture was studied. Treatment of cultures with 60 microM piperine for different time periods inhibited metabolism of BP by 50% within 4-8 h. Piperine uptake in 5L cells attained saturation plateau at 8 h and this related with the maximal impairment of BP metabolism. Exposure of cell cultures to piperine for 24 h indicated activation of CYP1A1 gene transcription. There was a 10-fold increase in CYP1A1mRNA and an approximately 7-fold increase in arylhydrocarbon hydroxylase (AHH) activity, while treatment with benzanthacene (BA) increased CYP1A1mRNA by 86- and AHH by 56-fold. Combined treatment with BA plus piperine further increased CYP1A1mRNA contents by about 25%. The BA-inducible AHH activity, however, registered a decrease of about 45%. Piperine neither destroyed CYP1A1-protein nor affected the total cellular protein contents. Exposure of cultures to 0.01 to 3.0 microM trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene for 24 h reduced maximum cellular growth by about 50%. Piperine at 60 microM offered full protection against the diol cytotoxicity. The results, suggest that piperine mediated inhibition of the AHH activity and consequent suppression of the procarcinogen activation is the result of direct interaction of piperine with CYP1A1-protein and not because of down regulation of the CYP1A1 gene expression.