The Raf-1/MEK/ERK pathway regulates the expression of the p21(Cip1/Waf1) gene in chondrocytes.
The gene encoding the cyclin-dependent kinase inhibitor p21(Cip1/Waf1) is up-regulated in many differentiating cells, including maturing chondrocytes. Since strict control of chondrocyte proliferation is essential for proper bone formation and since p21 is likely involved in this control, we initiated analyses of the mechanisms regulating expression of p21 in chondrocytes. p21 expression and promoter activity was strongly increased during the differentiation of chondrogenic MCT cells. We have identified a 68-base pair fragment conferring transcriptional up-regulation of the p21 gene in chondrocytes. The activity of this fragment required active Raf-1 in MCT cells as well as in primary mouse chondrocytes. Inhibition of downstream factors of Raf-1 (MEK1/2, ERK1/2, and Ets2) also repressed the activity of the 68-base pair fragment in MCT cells. The chemical MEK1/2 inhibitor PD98059 reduced protein levels of p21 in MCTs and primary mouse chondrocytes. These data suggest that signaling through the Raf-1 pathway is necessary for the optimal expression of p21 in chondrocytes and may play an important role in the control of bone formation.
Beier
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F
Department of Biochemistry, University of Calgary, Calgary, Alberta T2N 4N1, Canada.
Taylor
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AC
LuValle
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P
D000818:Animals; D001483:Base Sequence; D002454:Cell Differentiation; D002460:Cell Line; D019902:Chondrocytes / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000235:genetics*; D004247:DNA; D004268:DNA-Binding Proteins; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology*; D048728:MAP Kinase Kinase Kinase 1; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism*; D020935:MAP Kinase Signaling System / Q000502:physiology*; D051379:Mice; D011401:Promoter Regions, Genetic; D017346:Protein Serine-Threonine Kinases; D050787:Proto-Oncogene Protein c-ets-2; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D012097:Repressor Proteins; D015534:Trans-Activators / Q000378:metabolism; D014157:Transcription Factors
C493957:Cdkn1a protein, mouse; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004268:DNA-Binding Proteins; C107465:ERF protein, human; C494070:Ets2 protein, mouse; D050787:Proto-Oncogene Protein c-ets-2; D011518:Proto-Oncogene Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D004247:DNA; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D048728:MAP Kinase Kinase Kinase 1; D020930:MAP Kinase Kinase Kinases; C482578:Map3k1 protein, mouse