The Role of HNFs in CYP Monooxygenase Expression

The foetal liver, the major site of haematopoiesis during embryonic development, acquires additional detoxification functions near birth. The response to xenobiotic exposure with expression of several cytochromes P450 (CYP) monooxygenases and drug efflux transporters is a vital hepatic function. Expression of the genes for these proteins is regulated by nuclear receptors such as the pregnane X receptor (PXR). The expression of several xenobiotic response genes as well as of HNF-4alpha is increased in foetal hepatocytes stimulated by the hepatic maturation factors oncostatin M (OSM) and matrigel. To determine the contribution of HNF-4alpha to xenobiotic responses in the foetal liver, foetal hepatocytes containing floxed HNF-4alpha alleles were cultured, and the HNF-4alpha gene was inactivated by infection with an adenovirus containing the Cre gene. As a consequence, expression of CYP3A11 and PXR was suppressed by inac-tivation of HNF-4alpha. An HNF-4alpha binding site was characterized in the PXR promoter and found to be required for activation of the PXR promoter in foetal hepatocytes. It may be assumed that HNF-4alpha is a key transcription factor regulating responses to xenobiotics through activation of the PXR gene during foetal liver development (Kamiya et al., 2003). Several putative HNF-3 binding sites have been identified in human CYP2C 5'-flanking regions. Gene reporter experiments with proximal promoters revealed that HNF-3gamma trans-activated CYP2C8, CYP2C9, and CYP2C19 (25-, 4-, and 4-fold, respectively), but it did not trans-activate CYP2C18. However, overexpression of HNF-3gamma in hepatoma cells by means of a recombinant adenovirus induced CYP2C9, CYP2C18, and CYP2C19 mRNAs (4.5-, 20-, and 50-fold, respectively) but did not activate endogenous CYP2C8. The lack of effect of HNF-3gamma on endogenous CYP2C8 could be reversed by treating cells with the deacetylase inhibitor trichostatin A, suggesting the existence of chromatin condensation around functional HNF-3 elements in this gene (Bort et al., 2004). Within the rat CYP2E1 promoter HNF-1alpha binding sites (Ueno and Gonzalez, 1990) and within the chicken CYP2H1 promoter HNF-3alpha, HNF-3beta, and HNF-1alpha binding sites (Dogra and May, 1997) were identified. HNF-6alpha and HNF-6beta binding sites were identified in the promoters of human CYP2C12 and rat CYP2C13 (Lannoy et al., 1998; Samadani and Costa, 1996). Table 14.1 provides examples of experimental findings that demonstrate direct involvement of C/EBPs and HNFs in transcriptional control of hepatic CYP genes through interaction with their promoters.

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