Reduced levels of alpha 1 (I) collagen mRNA in cells immortalized by mutant p53 or transformed by ras

Endo180 binds to the C-terminal region of type I collagen

Type I collagen is a fibril-forming heterotrimer composed of two alpha1 and one alpha2 chains and plays a crucial role in cell-matrix adhesion and cell differentiation. Through a comprehensive differential display screening of oncogenic ras target genes, we have shown that the alpha1 chain of type I collagen (col1a1) is markedly down-regulated by the ras oncogene through the mitogen-activated protein kinase pathway. Although ras-transformed cells are no longer able to produce and secrete endogenous collagen, they can still adhere to exogenous collagen, suggesting that the cells express a collagen binding factor(s) on the cell surface. When the region of col1a1 encompassing the C-terminal glycine repeat and C-prodomain (amino acids 1000-1453) was affinity-labeled with human placental alkaline phosphatase, the secreted trimeric fusion protein could bind to the surface of Ras-transformed cells. Using biochemical purification followed by matrix-assisted laser desorption/ionization mass spectrometry analysis, we identified this collagen binding factor as Endo180 (uPARAP, CD280), a member of the mannose receptor family. Ectopic expression of Endo180 in CosE5 cells followed by in situ staining and quantitative binding assays confirmed that Endo180 indeed recognizes and binds to placental alkaline phosphatase. The interaction between Endo180 and the C-terminal region of type I collagen appears to play an important role in cell-matrix adhesion.

Transcriptional repression of the alpha 1(I) collagen gene by ras is mediated in part by an intronic AP1 site

We have previously shown that transformation of fibroblasts by ras results in transcriptional inhibition of the alpha 1(I) gene. An alpha 1(I)-hGH chimeric plasmid containing 3.7 kb of 5' flanking and 4.4 kb of alpha 1(I) transcribed sequence was regulated appropriately by ras in a transient transfection assay. In contrast, a similar plasmid containing alpha 1(I) DNA from -220 to +500 was virtually unresponsive to ras. The regions from -3700 to -220 and +500 to +4400 contributed equally to the ras-mediated inhibition of the parental plasmid. Deletion analysis indicated that a short fragment, between +500 and +890 in the first intron of the alpha 1(I) gene, was recognized differently in ras-transformed and wild-type cells. A previously described AP1 site in this fragment stimulated alpha 1(I) transcription in Rat1 fibroblasts but was inactive in ras-transformed cells. Mobility shift assays using nuclear extracts from the two cell types demonstrated differences in binding to the alpha 1(I) AP1 site. We conclude that ras transformation suppresses the function of a cell-specific enhancer in the first intron of the alpha 1(I) collagen gene.

Regulation of collagen I gene expression by ras

Although transformation of rodent fibroblasts can lead to dramatic changes in expression of extracellular matrix genes, the molecular basis and physiological significance of these changes remain poorly understood. In this study, we have investigated the mechanism(s) by which ras affects expression of the genes encoding type I collagen. Levels of both alpha 1(I) and alpha 2(I) collagen mRNAs were markedly reduced in Rat 1 fibroblasts overexpressing either the N-rasLys-61 or the Ha-rasVal-12 oncogene. In fibroblasts conditionally transformed with N-rasLys-61, alpha 1(I) transcript levels began to decline within 8 h of ras induction and reached 1 to 5% of control levels after 96 h. In contrast, overexpression of normal ras p21 had no effect on alpha 1(I) or alpha 2(I) mRNA levels. Nuclear run-on experiments demonstrated that the transcription rates of both the alpha 1(I) and alpha 2(I) genes were significantly reduced in ras-transformed cells compared with those in parental cells. In addition, the alpha 1(I) transcript was less stable in transformed cells. Chimeric plasmids containing up to 3.6 kb of alpha 1(I) 5'-flanking DNA and up to 2.3 kb of the 3'-flanking region were expressed at equivalent levels in both normal and ras-transformed fibroblasts. However, a cosmid clone containing the entire mouse alpha 1(I) gene, including 3.7 kb of 5'- and 4 kb of 3'-flanking DNA, was expressed at reduced levels in fibroblasts overexpressing oncogenic ras. We conclude that oncogenic ras regulates the type I collagen genes at both transcriptional and posttranscriptional levels and that this effect, at least for the alpha 1(I) gene, may be mediated by sequences located either within the body of the gene itself or in the distal 3'-flanking region.

Regulation of collagen I gene expression by ras

Although transformation of rodent fibroblasts can lead to dramatic changes in expression of extracellular matrix genes, the molecular basis and physiological significance of these changes remain poorly understood. In this study, we have investigated the mechanism(s) by which ras affects expression of the genes encoding type I collagen. Levels of both alpha 1(I) and alpha 2(I) collagen mRNAs were markedly reduced in Rat 1 fibroblasts overexpressing either the N-rasLys-61 or the Ha-rasVal-12 oncogene. In fibroblasts conditionally transformed with N-rasLys-61, alpha 1(I) transcript levels began to decline within 8 h of ras induction and reached 1 to 5% of control levels after 96 h. In contrast, overexpression of normal ras p21 had no effect on alpha 1(I) or alpha 2(I) mRNA levels. Nuclear run-on experiments demonstrated that the transcription rates of both the alpha 1(I) and alpha 2(I) genes were significantly reduced in ras-transformed cells compared with those in parental cells. In addition, the alpha 1(I) transcript was less stable in transformed cells. Chimeric plasmids containing up to 3.6 kb of alpha 1(I) 5'-flanking DNA and up to 2.3 kb of the 3'-flanking region were expressed at equivalent levels in both normal and ras-transformed fibroblasts. However, a cosmid clone containing the entire mouse alpha 1(I) gene, including 3.7 kb of 5'- and 4 kb of 3'-flanking DNA, was expressed at reduced levels in fibroblasts overexpressing oncogenic ras. We conclude that oncogenic ras regulates the type I collagen genes at both transcriptional and posttranscriptional levels and that this effect, at least for the alpha 1(I) gene, may be mediated by sequences located either within the body of the gene itself or in the distal 3'-flanking region.