TGFβ pathway represses hepatic ribosome biogenesis and protein synthesis by regulating p70S6K-S6RP proteins.

[BACKGROUND] Transforming growth factor-beta (TGFβ)-superfamily signaling has been implicated in the regulation of hepatocyte growth and regeneration after acute or chronic liver injury. However, the precise mechanisms underlying TGFβ signaling in the distinct hepatic cell types during the progression of liver fibrosis remain largely unknown. We aim to identify the downstream molecular mechanisms of TGFβ-signaling modulation on hepatocytes.

[METHODS] To modulate TGFβ-superfamily signaling in vivo, Smad3 or Smad7 were adenovirally overexpressed in mouse liver. Parallelly, hepatosphere cultures were treated with recombinant TGFβ1 and subjected to transcriptomic analysis. These data were compared with transcriptomes from Smad7-overexpressing livers. To broaden the analysis, publicly available RNA-seq datasets from TGFβ-treated hepatic stellate cells and hepatocellular carcinoma lines were meta-analyzed. Finally, human liver tissues from cirrhotic and healthy individuals were examined for fibrosis and ribosome biogenesis markers to validate murine findings.

[RESULTS] Acute hepatic overexpression of Smad3 induced a transient fibrotic phenotype in the mouse liver. In hepatosphere cultures, TGFβ1 treatment suppressed key components of ribosomal assembly, whereas Smad7 overexpression exerted the opposite effect in the mouse liver, thus highlighting ribosome biogenesis as a major cellular process negatively regulated by the TGFβ superfamily. Inhibition of TGFβ signaling via Smad7 increased hepatic protein content (a critical parameter for restoring hepatic homeostasis upon liver damage), activated the nucleolus, and prompted the production of ribosomal pre-mRNAs without affecting p53 levels. Mechanistically, SMAD7-mediated inactivation of TGFβ signaling triggered selectively the p70S6K-S6RP regulatory axis, independently of cellular myelocytomatosis oncogene (c-MYC), mechanistic target of rapamycin (mTOR), and mitogen-activated protein kinase (MAPK) pathways. Importantly, analysis of hepatic tissue from cirrhotic patients and controls unveiled a negative association between TGFβ signaling and ribosome biogenesis in fibrotic livers. Complementary meta-analysis of RNA-seq data demonstrated that TGFβ regulates ribosome biogenesis in a cell type-specific manner, suppressing it in hepatocytes while enhancing it in hepatic stellate cells, consistent with their distinct functional states and transcriptional landscapes.

[CONCLUSIONS] Collectively, our data reveal a SMAD-dependent regulatory role of TGFβ-superfamily signaling on hepatocytes that is tightly connected with hepatic growth to ensure proper energy homeostasis and metabolism. This is a critical regeneration parameter, which is closely related to the restoration of hepatic mass, especially following liver injury and fibrosis.