Nuclear and cytoplasmic alpha 1 (I) collagen mRNA-binding proteins

PCBP2 siRNA reverses the alcohol-induced pro-fibrogenic effects in hepatic stellate cells

PURPOSE

Type I collagen accumulates during liver fibrosis primarily because α-complex protein-2 (αCP(2)), encoded by the poly(rC) binding protein 2 (PCBP2) gene, binds to the 3' end of the collagen mRNA and increases its half-life. This study aimed to reverse the pro-fibrogenic effect of alcohol on hepatic stellate cells (HSCs) by silencing the PCBP2 gene with siRNA.

METHODS

The silencing effects of a series of predesigned PCBP2 siRNAs were evaluated in the rat hepatic stellate cell line, HSC-T6. The pro-fibrogenic effects of alcohol on the expression levels of PCBP2 and type-I collagen were examined by several methods. The effect of PCBP2 siRNA on the stability of type I collagen α1(I) mRNA was investigated by an in vitro mRNA decay assay.

RESULTS

We identified one potent PCBP2 siRNA that reversed the alcohol-induced expression of PCBP2 in HSCs. The decay rate of the collagen α1(I) mRNA increased significantly in HSCs treated with the PCBP2 siRNA.

CONCLUSION

This study provides the first evidence that alcohol up-regulates the expression of PCBP2, which subsequently increases the half-life of collagen α1(I) mRNA. Silencing of PCBP2 using siRNA may provide a promising strategy to reverse the alcohol-induced pro-fibrogenic effects in HSCs.

Lysophosphatidic acid inhibits TGF-β-mediated stimulation of type I collagen mRNA stability via an ERK-dependent pathway in dermal fibroblasts

Lysophosphatidic acid (LPA) is a serum-derived pleiotropic mediator with a potential role in wound repair. Since extracellular matrix (ECM) deposition is a critical part of wound healing, this study was designed to examine whether LPA is involved in ECM regulation. Using human dermal fibroblasts, we demonstrate that LPA counteracts transforming growth factor-beta (TGF-beta) stimulation of type I collagen mRNA and protein. This factor elicits its inhibitory effects at the posttranscriptional level via destabilization of type I collagen mRNA. Furthermore, using the mitogen-activated protein kinase kinase (MEK) inhibitor PD98059, we show that the extracellular signal-regulated kinase (ERK) pathway is a negative regulator of the TGF-beta-induced stabilization of type I collagen mRNA, and that the activation of the ERK pathway by LPA mediates their inhibitory effects on collagen production. In conclusion, this study describes a novel function for LPA as an antagonist of TGF-beta induced ECM deposition. These findings may be relevant to physiologic wound repair and may be useful in designing therapeutic agents to prevent excessive scarring.

Leptin induces increased alpha2(I) collagen gene expression in cultured rat hepatic stellate cells

Leptin is a 16-kDa hormone with an array of biologic actions. We, and others, have demonstrated that leptin is critical to the development of liver fibrogenesis both in vitro and in the lean littermates of ob/ob mice exposed to carbon tetrachloride (CCl(4)). Controversy exists as to whether leptin can act as a direct cytokine in the development of increased collagen expression, and whether ob/ob mice are resistant to potential injury from CCl(4). Here, we provide evidence that strongly suggests that leptin acts to increase nascent production of mRNA for the alpha2(I) collagen gene based upon ribonuclease protection analysis (RPA). Actinomycin D, but not cyclohexamide, or the pan-neutralizing antibody to transforming growth factor beta one (TGFbeta1), significantly diminished the effect of leptin on total alpha2(I) collagen mRNA levels. Further evidence that leptin acts directly on HSCs to alter gene expression in liver wounding is demonstrated by enhanced binding of phosphorylated signal transduction and activator of transcription factor 3 (pStat3) to a cis-inducible element (SIE) oligonucleotide by electrophoretic mobility shift assay (EMSA). This consensus sequence is responsible for production of a critical collagen transcription factor, AP-1. Finally, we have demonstrated from the ob/ob mouse model that these animals are at least as sensitive to CCl(4) as their respective lean animals as assessed by serum alanine aminotransferase (ALT) measurements. Taken together, the current data provide a continued framework that leptin is a profibrogenic cytokine and plays a key role in liver fibrosis.

Regulation of mRNA stability in mammalian cells

The regulation of mRNA decay is a major control point in gene expression. The stability of a particular mRNA is controlled by specific interactions between its structural elements and RNA-binding proteins that can be general or mRNA-specific. Regulated mRNA stability is achieved through fluctuations in half-lives in response to developmental or environmental stimuli like nutrient levels, cytokines, hormones and temperature shifts as well as environmental stresses like hypoxia, hypocalcemia, viral infection, and tissue injury. Furthermore, in specific disorders like some forms of neoplasia, thalassemia and Alzheimer's disease, deregulated mRNA stability can lead to the aberrant accumulation of mRNAs and the proteins they encode. This review presents a discussion of some recently identified examples of regulated and deregulated mRNA stability in order to illustrate the diversity of genes regulated by alterations in the degradation rates of their mRNAs.

Regulation of type I collagen mRNA in lung fibroblasts by cystine availability

The steady-state level of alpha1(I) collagen mRNA is regulated by amino acid availability in human lung fibroblasts. Depletion of amino acids decreases alpha1(I) collagen mRNA levels and repletion of amino acids induces rapid re-expression of alpha1(I) mRNA. In these studies, we examined the requirements for individual amino acids on the regulation of alpha1(I) collagen mRNA. We found that re-expression of alpha1(I) collagen mRNA was critically dependent on cystine but not on other amino acids. However, the addition of cystine alone did not result in re-expression of alpha1(I) collagen mRNA. Following amino acid depletion, the addition of cystine with selective amino acids increased alpha1(I) collagen mRNA levels. The combination of glutamine and cystine increased alpha1(I) collagen mRNA levels 6.3-fold. Methionine or a branch-chain amino acid (leucine, isoleucine or valine) also acted in combination with cystine to increase alpha1(I) collagen mRNA expression, whereas other amino acids were not effective. The prolonged absence of cystine lowered steady-state levels of alpha1(I) collagen mRNA through a mechanism involving decreases in both the rate of gene transcription as assessed by nuclear run-on experiments and mRNA stability as assessed by half-life determination in the presence of actinomycin D. The effect of cystine was not mediated via alterations in the level of glutathione, the major redox buffer in cells, as determined by the addition of buthionine sulphoximine, an inhibitor of gamma-glutamylcysteine synthetase. These data suggest that cystine directly affects the regulation of alpha1(I) collagen mRNA.

Posttranscriptional regulation of collagen alpha1(I) mRNA in hepatic stellate cells

The hepatic stellate cell (HSC) is the primary cell responsible for the dramatic increase in the synthesis of type I collagen in the cirrhotic liver. Quiescent HSCs contain a low level of collagen alpha1(I) mRNA, while activated HSCs contain about 60- to 70-fold more of this mRNA. The transcription rate of the collagen alpha1(I) gene is only two fold higher in activated HSCs than in quiescent HSCs. In assays using actinomycin D or 5,6-dichlorobenzimidazole riboside collagen alpha1(I) mRNA has estimated half-lives of 1.5 h in quiescent HSCs and 24 h in activated HSCs. Thus, this 16-fold change in mRNA stability is primarily responsible for the increase in collagen alpha1(I) mRNA steady-state level in activated HSCs. We have identified a novel RNA-protein interaction targeted to the C-rich sequence in the collagen alpha1(I) mRNA 3' untranslated region (UTR). This sequence is localized 24 nucleotides 3' to the stop codon. In transient transfection experiments, mutation of this sequence diminished accumulation of an mRNA transcribed from a collagen alpha1(I) minigene and in stable transfections decreased the half-life of collagen alpha1(I) minigene mRNA. Binding to the collagen alpha1(I) 3' UTR is present in cytoplasmic extracts of activated but not quiescent HSCs. It contains as a subunit alphaCP, which is also found in the complex involved in stabilization of alpha-globin mRNA. The auxiliary factors necessary to promote binding of alphaCP to the collagen 3' UTR are distinct from the factors necessary for binding to the alpha-globin sequence. Since alphaCP is expressed in both quiescent and activated HSCs, these auxiliary factors are responsible for the differentially expressed RNA-protein interaction at the collagen alpha1(I) mRNA 3' UTR.

Cell-specific expression of the alpha 1 (I) collagen promoter-CAT transgene in skin and lung: a response to TGF-beta subcutaneous injection and bleomycin endotracheal instillation

Transgenic mice containing a rat collagen alpha 1 (I) promoter (3.6 kilobases) fused to the reporter gene chloramphenicol acetyl transferase (CAT) express the reporter gene parallel to endogenous gene in most connective tissues other than vascular tissue [Pavlin et al. (1992): J Cell Biol 116:227-236; Bedalov et al. (1994): J Biol Chem 269:4903-4909]. We have challenged transgenic mice with subcutaneous injections of transforming growth factor-beta (TGF-beta) or intratracheal instillation of bleomycin. In situ hybridization studies of skin revealed increased CAT expression in the papillary dermis of TGF-beta treated animals. In contrast, alpha 1 (I) collagen mRNA was expressed throughout the dermis including granulation tissue and reticular dermis. Therefore, the transgenic promoter responds to TGF-beta in a subset of dermal fibroblasts. Endotracheal instillation of bleomycin induces lung fibrosis which is thought to be mediated in part by TGF-beta. CAT gene expression in lungs was increased 6-8-fold at 2 weeks post bleomycin treatment. In situ hybridization studies revealed focal areas of cells expressing both CAT and collagen genes in the interstitium. However, most regions, especially around airways, contained a subset of cells expressing the endogenous gene with little or no CAT expression as judged by in situ hybridization. These cells could be myofibroblasts that require additional cis-acting elements to activate alpha 1 (I) collagen gene expression similar to smooth muscle cells.

Differential regulation of transcription and transcript stability of pro-alpha 1(I) collagen and fibronectin in activated fibroblasts derived from patients with systemic scleroderma

Activated fibroblasts were derived from the skin of patients with systemic scleroderma (SSc), used as a model for fibrosis. Such cells are characterized by increased production of collagens and other matrix constituents. Increased collagen and fibronectin production has been correlated with similarly elevated mRNA steady-state levels. In the present study we analysed the contribution of transcriptional activity and post-transcriptional transcript stability to the increases in pro-alpha 1(I) collagen and fibronectin mRNA steady-state levels in activated (scleroderma) fibroblasts. Fibroblasts, when cultured in close contact with a three-dimensional collagenous matrix, down-regulate collagen synthesis. Culture of skin fibroblasts from two patients with SSc in three-dimensional collagen lattices, however, showed 4-fold elevated pro-alpha 1(I) collagen mRNA levels over fibroblasts from healthy donors. Transcription of the COL1A1 gene in SSc fibroblasts was induced 2-3-fold over that in controls in both monolayer and lattice cultures, accounting in part for the elevated steady-state level. A 50% decrease in transcription rate in lattice compared with monolayer culture occurred, as in control cells. In contrast, whereas control cells in lattices responded with decreased (50%) pro-alpha 1(I) collagen mRNA stability, in SSc cells these transcripts were found to be more stable (half-life of 5 h compared with 2 h in control cells). Fibronectin steady-state mRNA levels, in contrast, were not significantly regulated by the three-dimensional environment. In SSc fibroblasts, fibronectin mRNA levels were induced 1.5-4.9-fold over controls. In part, this increase appears to be due to elevated transcription, and an increase in fibronectin transcript stability was also detected. We therefore conclude that activated fibroblasts such as those derived from scleroderma patients utilize transcriptional and posttranscriptional mechanisms to maintain increased collagen and fibronectin production, which contribute to the pathogenesis of the disease.

Regulation of lysyl oxidase by basic fibroblast growth factor in osteoblastic MC3T3-E1 cells

Lysyl oxidase catalyzes the final known enzymatic step required for collagen and elastin cross-linking. A cross-linked collagenous extracellular matrix is required for bone formation. This study investigated whether lysyl oxidase, like its type I collagen substrate, is down-regulated by basic fibroblast growth factor (bFGF) in osteoblastic MC3T3-E1 cells and determined the degree of post-transcriptional control. Steady-state lysyl oxidase mRNA levels decreased to 30% of control after 24 h of treatment with 1 and 10 nm bFGF. This regulation was time-dependent. COL1A1 mRNA levels declined to less than 10% of control after 24 h of bFGF treatment. Media lysyl oxidase activity decreased consistent with steady-state mRNA changes in cultures that were refed after 24 h of growth factor treatment. Interestingly, treatment of MC3T3-E1 cells with 0.01-0.1 nm bFGF for 24 h and treatment with 1 nm bFGF for up to 12 h resulted in a modest stimulation of lysyl oxidase gene expression and enzyme activity. At least 50% of the down-regulation of lysyl oxidase was shown to be posttranscriptional. New protein synthesis was not required for the down-regulation by bFGF, but cycloheximide did increase constitutive lysyl oxidase mRNA levels 2.5-fold. We conclude that lysyl oxidase and COL1A1 are regulated similarly by bFGF in these osteoblastic cells, consistent with the in vivo effects of this growth factor on bone collagen metabolism.