Fuzzy modeling reveals a dynamic self-sustaining network of the GLI transcription factors controlling important metabolic regulators in adult mouse hepatocytes

Abstract

The GLI transcription factors, GLI1, GLI2, and GLI3, transduce Hedgehog and non-hedgehog signals and are involved in regulating development and tumorgenesis. Surprisingly, they were recently found to modulate important functions of mature liver. However, less is known about their mutual interactions and possible target genes in mature hepatocytes. To get a deeper insight into these interactions cultured mouse hepatocytes were transfected with siRNAs against each GLI factor. RNA was extracted at different times and the expression levels of the genes of interest were determined by quantitative real-time PCR. The time-dependent data were analysed by a fuzzy logic-based modelling approach. The results indicated that the GLI factors constitute an interconnected network. GLI2 inhibited GLI1 expression and was coupled with GLI3 by a positive feedback loop. The regulatory activity between GLI1 and GLI3 was more complex switching between a positive and a negative feedback loop depending on whether the level of GLI2 is low or high, respectively. Generally, this network structure enables a dynamic behaviour. When GLI2 is low, it may keep GLI1 and GLI3 activity balanced favouring the appropriate modulation of transcription factors like the Ppars and Srebp1. When GLI2 is high, it may prevent an uncontrolled amplification that may lead to cancer. In conclusion, the three GLI factors in mature hepatocytes form an interactive transcriptional network that is involved in the control of target genes associated with metabolic zonation as well as with lipid and drug metabolism. Its structure in mature cells seems different from embryonic cells.

Argininosuccinate synthase 1 and periportal gene expression in sonic hedgehog hepatocellular adenomas

“…) . This data confirmed in human tumors the role of sonic hedgehog pathway in zonation previously observed in mouse models . On the other hand, β‐catenin exon 3 mutated HCA harbored a perivenous program because Wnt/ β‐catenin was a key signaling pathway known to be involved in liver zonation and in the maintenance of a perivenous network of genes .…”

“…(18) This data confirmed in human tumors the role of sonic hedgehog pathway in zonation previously observed in mouse models. (30)(31)(32)(33) On the other hand, b-catenin exon 3 mutated HCA harbored a perivenous program because Wnt/ b-catenin was a key signaling pathway known to be involved in liver zonation and in the maintenance of a perivenous network of genes. (21,22) Moreover, as ASL and ASS1 belong to the arginine synthesis pathway, these results suggested that arginine metabolism and urea cycle were mainly activated in periportal area and potentially controlled by sonic hedgehog pathway whereas glutamine synthesis were activated in perivenous area mainly by Wnt/b-catenin pathway (Fig.…”

Hedgehog signaling is a potent regulator of liver lipid metabolism and reveals a GLI-code associated with steatosis

“…Regarding liver metabolism, we recently showed that the regulation of the IGF-axis (insulin-like growth factor) in mature hepatocytes is controlled by Hh signaling ( Matz-Soja et al, 2014 ). Moreover, we were also able to show that the TFs GLI1, GLI2 and GLI3 form a unique self-stabilizing network in hepatocytes and regulate several metabolic genes, including lipid associated factors ( Schmidt-Heck et al, 2015 ). Therefore, in this study, we have focused on the alterations in lipid metabolism as a consequence of hepatic Hh signaling modulations in vivo and in vitro.…”

“…To address this aim, a mouse model with a conditional hepatocyte-specific deletion of Smo in adult mice was established, in order to avoid interference with developmental effects of Hh signaling. For the in vitro investigations, we used RNA interference (RNAi) technology to knock down several genes of the Hh pathway in cultured hepatocytes ( Böttger et al, 2015 ; Schmidt-Heck et al, 2015 ). The results of our investigations clearly show that the Hh pathway is a strong regulator of lipid metabolism in the adult liver.…”

Tick-tock hedgehog-mutual crosstalk with liver circadian clock promotes liver steatosis

“…Further support is provided by the stimulatory effect of Ptch1 knockdown on Clock and Nr1d2 expression because the recently discovered dynamic selfsustaining network of GLI transcription factors led to the prior upregulation of Gli1 and Gli3. 14,35 Moreover, the FANTOM4 database 36 revealed specific binding sites for GLI factors within the respective clock genes, as illustrated in a transcriptomic output graph for the 3 Gli factors (Fig. 7A).…”