Inhibitory effect of dicationic diphenylfurans on production of type I collagen by human fibroblasts and activated hepatic stellate cells

A novel role of RNA helicase A in regulation of translation of type I collagen mRNAs

Type I collagen is composed of two α1(I) polypeptides and one α2(I) polypeptide and is the most abundant protein in the human body. Expression of type I collagen is primarily controlled at the level of mRNA stability and translation. Coordinated translation of α(I) and α2(I) mRNAs is necessary for efficient folding of the corresponding peptides into the collagen heterotrimer. In the 5' untranslated region (5' UTR), collagen mRNAs have a unique 5' stem-loop structure (5' SL). La ribonucleoprotein domain family member 6 (LARP6) is the protein that binds 5' SL with high affinity and specificity and coordinates their translation. Here we show that RNA helicase A (RHA) is tethered to the 5' SL of collagen mRNAs by interaction with the C-terminal domain of LARP6. In vivo, collagen mRNAs immunoprecipitate with RHA in an LARP6-dependent manner. Knockdown of RHA prevents formation of polysomes on collagen mRNAs and dramatically reduces synthesis of collagen protein, without affecting the level of the mRNAs. A reporter mRNA with collagen 5' SL is translated three times more efficiently in the presence of RHA than the same reporter without the 5' SL, indicating that the 5' SL is the cis-acting element conferring the regulation. During activation of quiescent cells into collagen-producing cells, expression of RHA is highly up-regulated. We postulate that RHA is recruited to the 5' UTR of collagen mRNAs by LARP6 to facilitate their translation. Thus, RHA has been discovered as a critical factor for synthesis of the most abundant protein in the human body.

Binding of LARP6 to the conserved 5' stem-loop regulates translation of mRNAs encoding type I collagen

Type I collagen is the most abundant protein in the human body, produced by folding of two alpha1(I) polypeptides and one alpha2(I) polypeptide into the triple helix. A conserved stem-loop structure is found in the 5' untranslated region of collagen mRNAs, encompassing the translation start codon. We cloned La ribonucleoprotein domain family member 6 (LARP6) as the protein that binds the collagen 5' stem-loop in a sequence-specific manner. LARP6 has a distinctive bipartite RNA binding domain not found in other members of the La superfamily. LARP6 interacts with the two single-stranded regions of the 5' stem-loop. The K(d) for binding of LARP6 to the 5' stem-loop is 1.4 nM. LARP6 binds the 5' stem-loop in both the nucleus and the cytoplasm. In the cytoplasm, LARP6 does not associate with polysomes; however, overexpression of LARP6 blocks ribosomal loading on collagen mRNAs. Knocking down LARP6 by small interfering RNA also decreased polysomal loading of collagen mRNAs, suggesting that it regulates translation. Collagen protein is synthesized at discrete regions of the endoplasmic reticulum. Using collagen-GFP (green fluorescent protein) reporter protein, we could reproduce this focal pattern of synthesis, but only when the reporter was encoded by mRNA with the 5' stem-loop and in the presence of LARP6. When the reporter was encoded by mRNA without the 5' stem-loop, or in the absence of LARP6, it accumulated diffusely throughout the endoplasmic reticulum. This indicates that LARP6 activity is needed for focal synthesis of collagen polypeptides. We postulate that the LARP6-dependent mechanism increases local concentration of collagen polypeptides for more efficient folding of the collagen heterotrimer.

Advances in antifibrotic therapy

Sustained progress in defining the molecular pathophysiology of hepatic fibrosis has led to a comprehensive framework for developing antifibrotic therapies. Indeed, the single greatest limitation in bringing new drugs to the clinical setting is a lack of clarity regarding clinical trial and treatment end points, not a lack of promising agents. A range of treatments, including those developed for other indications, as well as those specifically developed for hepatic fibrosis, are nearing or in clinical trials. Most are focused on attacking features of either hepatic injury and/or activated stellate cells and myofibroblasts, which are the primary sources of extracellular matrix (scar) proteins. Thus, features of injury and stellate cell activation provide a useful template for classifying these emerging agents and point to a new class of therapies for patients with fibrosing liver disease.

Hepatic fibrosis -- overview

The study of hepatic fibrosis, or scarring in response to chronic liver injury, has witnessed tremendous progress in the past two decades. Clarification of the cellular sources of scar, and emergence of hepatic stellate cells not only as a fibrogenic cell type, but also as a critical immunomodulatory and homeostatic regulator are among the most salient advances. Activation of hepatic stellate cells remains a central event in fibrosis, complemented by evidence of additional sources of matrix-producing cells including bone marrow, portal fibroblasts, and epithelial-mesenchymal transition from both hepatocytes and cholangiocytes. A growing range of cytokines and their receptors and inflammatory cell subsets have further expanded our knowledge about this dynamic process. Collectively, these findings have laid the foundation for continued elucidation of underlying mechanisms, and more importantly for the implementation of rationally based approaches to limit fibrosis, accelerate repair and enhance liver regeneration in patients with chronic liver disease.

Gene activation by bioactive glasses

Bioactive glasses have been shown to regulate gene expression in both hard and soft tissue repair. New resorbable bioactive glass constructs are now being developed that can influence gene expression in the local environment by manipulating material properties such as the surface chemistry, topography and the release of dissolution ions. The success of these scaffolds, however, may depend upon a greater understanding of the bioactive glass stimulated gene expression pathways. This will allow the construction of tissue specific scaffolds with tailored surface chemistry, topography and ion release rates. This paper summarises the advances made in understanding gene expression in response to bioactive glasses and discusses the future steps required for further insights into these molecular mechanisms.

Arg-gly-asp-mannose-6-phosphate inhibits activation and proliferation of hepatic stellate cells in vitro

AIM: To investigate the effect of arg-gly-asp-mannose-6 phosphate (RGD-M6P) on the activation and proliferation of primary hepatic stellate cells in vitro.

METHODS: Hepatic stellate cells (HSCs) were isolated from rats by in situ collagenase perfusion of liver and 18% Nycodenz gradient centrifugation and cultured on uncoated plastic plates for 24 h with DMEM containing 10% fetal bovine serum (FBS/DMEM) before the culture medium was substituted with 2% FBS/DMEM for another 24 h. Then, HSCs were cultured in 2% FBS/DMEM with transforming growth factor β1, M6P, RGD, or RGD-M6P, respectively. Cell morphology was observed under inverted microscope, smooth muscle α-actin (α-SMA) was detected by immunocytochemistry, type III procollagen (PCIII) in supernatant was determined by radioimmunoassay, and the proliferation rate of HSCs was assessed by flow cytometry.

RESULTS: RGD-M6P significantly inhibited the morphological transformation and the α-SMA and PCIII expressions of HSCs in vitro and also dramatically prevented the proliferation of HSCs in vitro. Such effects were remarkably different from those of RGD or M6P.

CONCLUSION: The new compound, RGD-M6P, which has a dramatic effect on primary cultured HSCs in vitro, can inhibit the transformation of HSCs in culture caused by TGFβ1, suppresses the expression of PCIII and decreases proliferation rate of HSC. RGD-M6P can be applied as a selective drug carrier targeting at HSCs, which may be a new approach to the prevention and treatment of liver fibrosis.