Increased cdc2 and cdk2 kinase activity by retinoid X receptor gamma-mediated transcriptional down-regulation of the cyclin-dependent kinase inhibitor p21Cip1/WAF1 correlates with terminal differentiation of squamous cell carcinoma lines.
The chemotherapeutic agent and vitamin A metabolite retinoic acid (RA) has been used to treat many tumor types. The effects of RA are mediated by a family of ligand-dependent transcription factors, the RA receptors and the retinoid X receptors (RXR). Alterations in retinoid receptor expression have been implicated in tumorigenesis. Previous studies have shown lack of RXR-gamma expression in squamous cell carcinoma (SCC) lines. To begin to elucidate the role of RXR-gamma in the malignant transformation of SCCs, we expressed RXR-gamma in SCC lines by stable transfection. SCC lines expressing RXR-gamma produced large numbers of flat cells with abundant cytoplasm, which died and detached from the culture dish. These cells morphologically resembled the differentiated cells of normal stratified squamous epithelium in culture. These cells did not exhibit the characteristic DNA fragmentation pattern of apoptotic cells, nor did they label in a fluorescent apoptosis assay. RNase protection and Western blot analysis revealed induction of RA-responsive involucrin and keratin 10 expression, early markers of terminal differentiation. RXR-gamma expression produced significant reduction in levels of RA-responsive genes including the cyclin-dependent kinase inhibitors p21Cip1/WAF1 and p27Kip1, resulting in increased cdc2 and cdk2 kinase activity and RB phosphorylation. We concluded that RXR-gamma induced terminal differentiation in SCC lines, suggesting a potential tumor suppressor function for this transcription factor.
Crowe
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DL
Center for Craniofacial Molecular Biology, University of Southern California, Los Angeles 90033, USA.
Shuler
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CF
D015153:Blotting, Western; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D042846:CDC2-CDC28 Kinases; D002294:Carcinoma, Squamous Cell / Q000378:metabolism / Q000473:pathology*; D018797:Cell Cycle Proteins; D016923:Cell Death; D002454:Cell Differentiation; D048429:Cell Size; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000235:genetics* / Q000378:metabolism; D015536:Down-Regulation; D004789:Enzyme Activation; D016147:Genes, Tumor Suppressor; D006801:Humans; D053550:Keratin-10; D007633:Keratins / Q000235:genetics; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D010766:Phosphorylation; D011233:Precipitin Tests; D011498:Protein Precursors / Q000235:genetics; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D012333:RNA, Messenger / Q000032:analysis; D018168:Receptors, Retinoic Acid / Q000235:genetics / Q000502:physiology*; D016160:Retinoblastoma Protein / Q000378:metabolism; D047488:Retinoid X Receptors; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D014162:Transfection; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins
C493956:CDKN1A protein, human; D018797:Cell Cycle Proteins; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; C507493:KRT10 protein, human; D008869:Microtubule-Associated Proteins; D011498:Protein Precursors; D012333:RNA, Messenger; D018168:Receptors, Retinoic Acid; D016160:Retinoblastoma Protein; D047488:Retinoid X Receptors; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D053550:Keratin-10; C032253:involucrin; D007633:Keratins; D017346:Protein Serine-Threonine Kinases; D016203:CDC2 Protein Kinase; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases