Background  &  Aims:  The  mammalian  circadian  clock  controls  various  aspects  of  liver

metabolism and integrates nutritional signals. Recently, we described Hedgehog (Hh) signaling

as a novel regulator of liver lipid metabolism. Here, we investigated crosstalk between hepatic

Hh signaling and circadian rhythm. 

Methods: Diurnal rhythms of Hh signaling were investigated in liver and hepatocytes from mice

with  ablation  of  Smoothened  (SAC-KO)  and  crossbreeds  with  PER2::LUC  reporter  mice.  By

using genome-wide screening, qPCR, immunostaining, ELISA and RNAi experiments in vitro we

identified relevant transcriptional regulatory steps. Shotgun lipidomics and metabolic cages were

used for analysis of metabolic alterations and behavior.

Results:  Hh  signaling  showed  diurnal  oscillations  in  liver  and  hepatocytes  in  vitro.

Correspondingly, the level of Indian Hh, oscillated in serum. Depletion of the clock gene Bmal1

in  hepatocytes  resulted  in  significant  alterations  in  the  expression  of  Hh  genes.  Conversely,

SAC-KO mice showed altered expression of clock genes, confirmed by RNAi against Gli1 and

Gli3.  Genome-wide  screening  revealed  that  SAC-KO  hepatocytes  showed  time-dependent

alterations  in  various  genes,  particularly  those  associated  with  lipid  metabolism.  The

clock/hedgehog module further plays a role in rhythmicity of steatosis, and in the response of the

liver to a high fat diet or to differently timed starvation.

Conclusions:  For  the  first  time,  Hh  signaling  in  hepatocytes  was  found  to  be  time-of-day

dependent and to feed back on the circadian clock. Our findings suggest an integrative role of

Hh  signaling,  mediated  mainly  by  GLI  factors,  in  maintaining  hepatic  lipid  metabolism

homeostasis by balancing the circadian clock.