Molecular cloning and chromatin structure analysis of the murine alpha1(I) collagen gene domain

DNase I-hypersensitive sites enhance alpha1(I) collagen gene expression in hepatic stellate cells

Liver fibrosis is characterized by a dramatic increase in the expression of type I collagen. Several deoxyribonuclease (DNase) I-hypersensitive sites (HS) have been located in the distal 5'-flanking region of the alpha1(I) collagen gene that are specific to collagen-producing cells. To assess the role of the DNase I-HS in regulating alpha1(I) collagen gene expression in hepatic stellate cells (HSCs), 3 transgenic mouse lines expressing collagen-alpha1(I) reporter genes were used (Krempen et al. Gene Expr 1999;8:151-163). The pCol9GFP transgene contains the collagen gene promoter (-3122 to +111) linked to the green fluorescent protein (GFP) reporter gene. The pCol9GFP-HS4,5 transgene contains HS4,5 and pColGFP-HS8,9 contains HS8,9 positioned upstream of the collagen promoter in pCol9GFP. HSCs isolated from transgenic mice containing pCol9GFPHS4,5 and pColGFP-HS8,9 showed earlier and higher GFP expression patterns than HSCs isolated from pCol9GFP mice. HSCs from pCol9GFP-HS4,5 showed the highest levels of GFP expression and culture-induced expression correlated with induction of the endogenous alpha1(I) collagen gene. After CCl(4) administration, pCol9GFP-HS4,5 mice showed increased GFP expression compared with pCol9GFP mice in both whole liver extracts and isolated HSCs. Several sites for DNA-protein interactions in both HS4 and HS5 were identified that included a binding site for activator protein 1. In conclusion, DNase I-HS4,5 enhance expression of the alpha1(I) collagen gene promoter in HSCs both in vitro and in vivo after a fibrogenic stimulus. The collagen-GFP transgenic mice provide a convenient and reliable model system to investigate the molecular mechanisms controlling increased collagen expression during fibrosis.

delta Ef1 binds to a far upstream sequence of the mouse pro-alpha 1(I) collagen gene and represses its expression in osteoblasts

The transcription of type I collagen genes is tightly regulated, but few cis-acting elements have been identified that can modulate the levels of expression of these genes. Generation of transgenic mice harboring various segments of the mouse pro-alpha1(I) collagen promoter led us to suspect that a repressor element was located between -10.5 and -17 kilobase pairs. Stable and transient transfection experiments in ROS17/2.8 osteoblastic cells confirmed the existence of such a repressor element at about -14 kilobase pairs and showed that it consisted in an almost perfect three-time repeat of a 41-base pair sequence. This element, which we named COIN-1, contains three E2-boxes, and a point mutation in at least two of them completely abolished its repressor effect. In gel shift assays, COIN-1 bound a DNA-binding protein named delta EF1/ZEB-1, and mutations that abolished the repressor effect of COIN-1 also suppressed the binding of delta EF1. We also showed that the repressor effect of COIN-1 was not mediated by chromatin compaction. Furthermore, overexpression of delta EF1 in ROS17/2.8 osteoblastic cells enhanced the inhibitory effect of COIN-1 in a dose-dependent manner and repressed the expression of the pro-alpha 1(I) collagen gene. Thus, delta EF1 appears to repress the expression of the mouse pro-alpha 1(I) collagen gene, through its binding to COIN-1.

Activation of a fibroblast-specific enhancer of the proalpha2(I) collagen gene in tight-skin mice

OBJECTIVE

Reporter transgenes were introduced into the type 1 tight-skin (Tsk1/+) mouse model of scleroderma to test the hypothesis that fibroblast-specific genetic programs are activated in fibrosis.

METHODS

Transgenes harboring upstream fragments of the 5' flanking region of the mouse proalpha2(I) collagen gene (Col1a2), linked to a 400-bp minimal Col1a2 promoter driving an Escherichia coli beta-galactosidase (LacZ) reporter gene, were introduced into Tsk1/+ mice by breeding. Expression of these transgenes, which function as lineage-specific markers of fibroblast differentiation, was compared between the Tsk-LacZ mice and non-Tsk littermates. Responsiveness of these constructs to the profibrotic cytokine, transforming growth factor beta1 (TGFbeta1), was investigated by transient transfection of reporter constructs in tissue-culture cells.

RESULTS

There was significant activation of reporter genes harboring the upstream enhancer in Tsk1/+ mice starting from 1 week of age. This was maximal at 6 weeks old (mean +/- SD 237 +/- 24% of non-Tsk controls; P= 0.001). Recombinant TGFbeta1 significantly activated reporter genes regulated by the upstream enhancer in transient transfection, and Tsk-LacZ fibroblasts showed elevated LacZ expression in tissue culture.

CONCLUSION

These data suggest that activating signals in Tsk1/+ mice may act via fibroblast-specific regulatory elements within the murine Col1a2 gene. Although TGFbeta has been implicated in the pathogenesis of fibrosis, and reporter genes regulated by the upstream enhancer appear to be TGFbeta responsive in vitro, our results suggest that fibroblast-specific pathways may also be involved.

Cell type-specific transcription of the alpha1(VI) collagen gene. Role of the AP1 binding site and of the core promoter

Analysis of the chromatin of different cell types has identified four DNase I-hypersensitive sites in the 5'-flanking region of the alpha1(VI) collagen gene, mapping at -4.6, -4.4, -2.5, and -0.1 kilobase (kb) from the RNA start site. The site at -2.5 kb was independent from, whereas the other three sites could be related to, alpha1(VI) mRNA expression. The site at -0.1 kb was present in cells expressing (NIH3T3 and C2C12) but absent in cells not expressing (EL4) the mRNA; the remaining two sites were apparently related with high levels of mRNA. DNase I footprinting and gel-shift assays with NIH3T3 and C2C12 nuclear extracts have located a binding site for transcription factor AP1 (activator protein 1) between nucleotides -104 and -73. When nuclear extracts from EL4 lymphocytes were used, the AP1 site-containing sequence was bound by proteins not related to AP1. The existence of the hypersensitive site at -0.1 kb may be related to the binding of AP1 and of additional factors to the core promoter (Piccolo, S., Bonaldo, P., Vitale, P., Volpin, D., and Bressan, G. M. (1995) J. Biol. Chem. 270, 19583-19590). The function of the AP1 binding site and of the core promoter in the transcriptional regulation of the Col6a1 gene was investigated by expressing several promoter-reporter gene constructs in transgenic mice and in cell cultures. The results indicate that regulation of transcription of the Col6a1 gene by different cis-acting elements (core promoter, AP1 binding site and enhancers) is not completely modular, but the final output depends on the specific interactions among the three elements in a defined cell type.