HEPATIC NUCLEAR RECEPTOR EXPRESSION AND REGULATION OF BILE ACID SYNTHESIS IN HUMANS

Bile acid synthesis plays a crucial role in cholesterol homeostasis. Recent evidence has highlighted the role of nuclear receptors in the regulation of the expression and activity of cholesterol 7alpha-hydroxylase (CYP7A1), the limiting enzyme, in cellular and animal models. Understanding the regulatory role of nuclear receptors in humans might help to define molecular targets for pharmacological intervention aimed to enhance hepatic cholesterol degradation. AIM of the present study was to analyze the expression of CYP7A1 and a number of related nuclear receptors in human liver. METHODS. Surgical liver biopsies were obtained in 40 patients; 30 of them were untreated, presenting gallbladder stones (12), non-metastatic abdominal cancer (10) or obstructive cholestasis (8); 10 subjects were receiving standard dose of CDCA (3), UDCA (5) or cholestyramine (2). mRNA levels of CYP7A1 and of the main nuclear receptors involved in its regulation (FXR, SHP, LRH-CPF-1, HNF-4, PGC-1) were assayed by real-time RT-PCR, using custom-designed primers and with sybr-green as an intercalator of double-stranded DNA. RESULTS. CYP7A1 mRNA showed a high degree of variability. No difference was detected between untreated gallstone and gallstone-free subjects regarding the expression of CYP7A1 and other genes, with the exception of PGC-1 which was significantly (p < 0.05 on a log scale) less expressed in gallstone subjects. In untreated, non-cholestatic subjects no correlation could be detected between CYP7A1 or other genes and age. Stepwise regression analysis of data from all non-cholestatic subjects, with CYP7A1 mRNA levels as the dependent variable, showed the strongest correlation with HNF-4 as the independent variable (r = 0.471 on a log scale, p < 0.05), all other genes (including SHP) bringing non-significant further contribution to the correlation. A very strong direct correlation (r = 0.880 on a log scale, p< 0.05) was detected between HNF-4 and LRH-CPF-1 expression. Finally, no difference was observed between cholestatic and non-cholestatic patients.CONCLUSIONS. Our data suggest that HNF-4 might play a relevant role in the regulation of CYP7A1 transcription in humans; on the other hand no evidence for a suppressive role of SHP, which was well documented in cellular models, was observed. As a whole, the interrelationships between the different nuclear receptors, and their physiological role, have still to be defined. Data on CYP7A1 expression support the view that post-transcriptional, and possibly post-translational levels of regulation may also play a critical role in the control of bile acid synthesis.