Cellular & Molecular Biology Letters最新文献

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.

Inter-organelle communication via membrane contact sites (MCSs) is essential for the efficient functioning of eukaryotic cells, facilitating coordination among approximately 20 distinct organelles, each with unique metabolic profiles. Among these interactions, mitochondria-endoplasmic reticulum (ER) contacts (MERCs) are particularly significant, encompassing about 5% of the mitochondrial surface. Key proteins involved in MERCs include inositol 1,4,5-trisphosphate receptor (IP3R), voltage-dependent anion channel (VDAC), glucose-regulated protein 75 (GRP75), Sigma1 receptor (Sig-1R), vesicle-associated membrane protein (VAMP)-associated protein B (VAPB), protein deglycase DJ-1, and protein tyrosine phosphatase interacting protein 51 (PTPIP51), with new proteins continually being identified for their roles in these structures. At these contact sites, metabolic exchanges involve calcium (Ca²⁺), lipids, reactive oxygen species (ROS), and proteins. MERCs enable efficient molecular exchanges through temporary bridges mainly formed by the ER, the organelle with the largest surface area. These contacts are crucial for maintaining mitochondrial dynamics, which is essential for cellular homeostasis, and they are notably impacted in pathological states such as metabolic dysfunction-associated steatotic liver disease (MASLD), alcohol-related liver diseases (ALD), and viral hepatitis. Dysfunctional MERCs can lead to mitochondrial fragmentation, increased ROS production, impaired autophagy, and disrupted protein trafficking, thereby exacerbating senescence and cellular aging. Senescence is a cell fate initiated by stress, characterized by stable cell-cycle arrest and a hypersecretory state, and is an underlying cause of aging and many chronic conditions, including liver diseases. The hallmarks of senescence-such as macromolecular damage, cell cycle withdrawal, deregulated metabolism, and a secretory phenotype-are well established. However, recent studies have demonstrated that senescence is a heterogeneous process, with molecular markers varying according to the stressors that induce it. This review focuses on the functional aspects of MERCs in hepatic senescence and their impact on liver diseases, and explores the potential of targeting MERCs to address hepatocytic senescence.

Nuclear receptor-binding SET domain (NSD) proteins have been initially described as methyltransferases specific to lysine-36 in histone H3 and associated with active chromatin. However, their role in the regulation of transcription and in overall cellular physiology is much more complex, especially in mammals. The emerging diversity of their targets and, accordingly, the processes in which NSD proteins are involved, shows the importance of their noncanonical functions. A wide functionality apparently requires a complicated control system ensuring proper spatial and temporal activation of NSD methyltransferases. In this review, we discuss the role of NSD proteins in transcription, genome topology, mitosis, oncogenesis, immunity, DSB repair, and known mechanisms regulating their activity.