Involvement of histone deacetylases (HDAC) in the molecular regulatory pathway of cholesterol 7α-hydroxylase: an in vitro approach with selective HDAC inhibitors

Cholesterol 7α-hydroxylase (CYP7A1) is the rate-limiting enzyme in the conversion of cholesterol to bile acids (BA) which is the major metabolic pathway of cholesterol disposal in mammals. CYP7A1 gene expression is repressed by BA returning to the liver via enterohepatic circulation. We previously showed that BA stimulate the sequential recruitment of HDAC 7, 3, and 1, and of the corepressors Silencing Mediator of Retinoid and Thyroid receptors-α (SMRT-α) and N-CoR1 on CYP7A1 promoter that finally repress CYP7A1 gene transcription. Moreover, we showed that valproic acid (VPA) and trichostatin A (TSA), two non-selective HDAC inhibitors (HDAC-i), stimulate CYP7A1 expression in vitro and in vivo by preventing the negative feed-back exerted by BA and consequently decreasing serum cholesterol in mice. Based on these studies, the aim of this work is to define the importance of specific HDACs on cholesterol catabolism to bile acids. By using siRNA oligonucleotides we were able to show that HDAC7 plays an important role in CYP7A1 regulation. To understand the relative importance of the activity of different HDACs in BA biosynthetic pathway, we tested different HDAC-I on human hepatoma cell lines, the pan-inhibitor SAHA, the class 1 selective HDAC-I MS275, and the class 2 selective HDAC-I MC1568. Our results show that the class 1 selective HDAC-I MS275 prevents the repressive effect of BA on CYP7A1 expression, whereas the pan inhibitor SAHA and the class 2 HDAC selective inhibitor MC1568 allow only a partial recovery of CYP7A1 expression after BA treatment. Our results highlight the role of class 1 HDACs in the formation of the repressive complex acting on CYP7A1 promoter and suggest that the inhibition of class 2 HDAC activity may not be sufficient to prevent the inhibitory effect of bile acids on CYP7A1 promoter. Acknowledgment: supported by grants from the FP6 LSHM-CT 2006-037498 and Cariplo Foundation 2008.2511.