Shp/Npas2 axis in regulating the oscillation of liver lipid metabolism.
In mammals, circadian rhythms are essential for coordinating the timing of various metabolic processes. The Clock gene regulates diurnal plasma triglyceride fluctuation through nuclear receptor small heterodimer partner (Shp, Nr0b2). Given that SHP is a critical regulator of metabolism in the liver, it is unknown whether SHP is necessary to coordinate metabolism and circadian rhythms. Methods: Shp(+/+) and Shp(-/-) mice on a C57BL/6 background (n=3-5/group) were fed a standard chow diet and water ad libitum. Serum and livers were collected at zeitgeber time (ZT) 2, 6, 10, 14, 18 and 22. In vivo and in vitro assays include: RNA-sequencing (RNA-seq), qPCR, VLDL production, adenovirus overexpression and siRNA knockdown, serum parameters, circadian locomotor activity, oil-red O staining, transient transfection, luciferase reporter assay, ChIP assay, gel-shift assay, Co-IP, Western blots. Results: Shp-deficiency had a robust global impact on major liver metabolic genes. Several components of the liver clock including Pgc-1?, Npas2 and Ror?/? were sharply induced in Shp(-/-) liver. At the molecular level, SHP inhibited Npas2 gene transcription and promoter activity through interaction with Ror? to repress Ror? transactivation and by interacting with Rev-erb? to enhance its inhibition of Ror? activity. Conversely, Npas2 controlled the circadian rhythm of Shp expression by binding rhythmically to the Shp promoter, which was enhanced by NADH, but not NADPH. Phenotypically, Npas2-deficiency induced severe steatosis in Shp(-/-) mice, which was attributed to the dysregulation of lipoprotein metabolism. Conclusion: Shp and Npas2 crosstalk is essential to maintain hepatic lipid homeostasis. (Hepatology 2014).