Hepatic Hedgehog signaling contributes to the regulation of IGF1 and IGFBP1 serum levels

Abstract

BackgroundHedgehog signaling plays an important role in embryonic development, organogenesis and cancer. In the adult liver, Hedgehog signaling in non-parenchymal cells has been found to play a role in certain disease states such as fibrosis and cirrhosis. However, whether the Hedgehog pathway is active in mature healthy hepatocytes and is of significance to liver function are controversial.FindingsTwo types of mice with distinct conditional hepatic deletion of the Smoothened gene, an essential co-receptor protein of the Hedgehog pathway, were generated for investigating the role of Hedgehog signaling in mature hepatocytes. The knockout animals (KO) were inconspicuous and healthy with no changes in serum transaminases, but showed a slower weight gain. The liver was smaller, but presented a normal architecture and cellular composition. By quantitative RT-PCR the downregulation of the expression of Indian hedgehog (Ihh) and the Gli3 transcription factor could be demonstrated in healthy mature hepatocytes from these mice, whereas Patched1 was upregulated. Strong alterations in gene expression were also observed for the IGF axis. While expression of Igf1 was downregulated, that of Igfbp1 was upregulated in the livers of both genders. Corresponding changes in the serum levels of both proteins could be detected by ELISA. By activating and inhibiting the transcriptional output of Hedgehog signaling in cultured hepatocytes through siRNAs against Ptch1 and Gli3, respectively, in combination with a ChIP assay evidence was collected indicating that Igf1 expression is directly dependent on the activator function of Gli3. In contrast, the mRNA level of Igfbp1 appears to be controlled through the repressor function of Gli3, while that of Igfbp2 and Igfbp3 did not change. Interestingly, body weight of the transgenic mice correlated well with IGF-I levels in both genders and also with IGFBP-1 levels in females, whereas it did not correlate with serum growth hormone levels.ConclusionsOur results demonstrate for the first time that Hedgehog signaling is active in healthy mature mouse hepatocytes and that it has considerable importance for IGF-I homeostasis in the circulation. These findings may have various implications for mouse physiology including the regulation of body weight and size, glucose homeostasis and reproductive capacity.

Canonical and non-canonical Hedgehog signalling and the control of metabolism

“…Interestingly, the HCV replication-secretion cycle involves the intracellular pool of PI4P and takes advantage of the VLDL secretory machinery [164–166]. Besides damage-induced response, Hedgehog signalling is also active in healthy hepatocytes and contributes to Igf-1 homeostasis [167]. …”

Hepatic JAK2 protects against atherosclerosis through circulating IGF-1

“…Besides JAK2, other upstream regulators of hepatic IGF-1 may mediate its effects, such as FGF21, PPARγ, and components of the hedgehog signaling pathway including Smoothened (66)(67)(68). Furthermore, recent clinical studies have shown ω-3 supplementation and baclofen, a β-p-chlorophenyl derivative of γ-aminobutyric acid (GABA), in increasing plasma IGF-1 concentrations to varying extent (69)(70)(71).…”

The many faces of Hedgehog signalling in the liver: Recent progress reveals striking cellular diversity and the importance of microenvironments

“…The latter cells, at least their majority, do not express cilia [1], in contrast to activated HSC. The existence of this unusual type of Hh signalling (cyclopamine responsive) is most evident for healthy hepatocytes [6]. In addition, SMO-independent, truly non-canonical signalling, has been observed at least in part of the cell types (Fig.…”

“…Although the authors refer to the IHH-positive hepatocytes as ''injured cells'' [1], proof of the actual injury is lacking. Within this context, it should be emphasised that Ihh in normal liver has been recognised as a target gene of Wnt/ß-catenin signalling [10], and, correspondingly, expression could be demonstrated in healthy hepatocytes around the central veins [6,10] where Wnt signalling is highest (cf. [2]).…”

“…1. Besides canonical (cilia-and SMO-dependent) signalling in endothelial cells, cholangiocytes [3], activated hepatic stellate cells (HSC) [4] and progenitor cells [1,5], there is obviously cilia-independent but SMO-dependent signalling in mature healthy hepatocytes [6], Kupffer cells and, most probably, in quiescent HSC. The latter cells, at least their majority, do not express cilia [1], in contrast to activated HSC.…”