title: Induction of p53-independent p21 during ceramide-induced G1 arrest in human hepatocarcinoma cells.
abstract: Ceramide is known to induce pRb (retinoblastoma gene product) dephosphorylation through the activation of ceramide-activated protein phosphatase (CAPP) during G1 arrest, but other molecular mechanisms linked to regulation of pRb dephosphorylation during ceramide-induced G1 arrest are poorly understood. In this paper, we investigated whether p21, a cdk (cyclin-dependent kinase) inhibitor, is involved in the induction of pRb dephosphorylation during ceramide-induced G1 arrest. In SK-Hep-1 cells, the addition of ceramide resulted in pRb dephosphorylation and G1 arrest. The activity of cdk2 was inhibited in response to ceramide during this process. p21 protein and mRNA were remarkably induced, while the protein level of p53, known as a transcriptional activator of p21, was not elevated at the same condition. p21 induction was also observed in the Hep3B cells lacking a functional p53 after exposure to ceramide. Although p21 is induced in ceramide-treated Hep3B cells, Hep3B cells do not induce G1 arrest, because Hep3B cells are deficient in a functional pRb protein. To confirm that pRb is a critical target for the induction of G1 arrest by inhibiting cdk2 activity through p53-independent p21, pRb-expressing vector was transfected into Hep3B cells. After treatment with ceramide, pRb-expressing cells (pRb+/+), but not pRb-/- cells, were arrested in G1 phase. In pRb+/+ cells, ceramide-mediated G1 arrest was accompanied by the accumulation of hypophosphorylated pRb and p21 associated with cdk2. Together, these results suggest that p21, induced through p53-independent pathway, participates in the induction of pRb dephosphorylation by inhibiting cdk2 activity during ceramide-mediated G1 arrest in hepatocarcinoma cells.
authors.affiliation: Department of Life Science, College of Natural Sciences, Chung-Ang University, Seoul, South Korea.
authors.forename: W H
authors.initials: WH
authors.lastname: Kim
authors.forename: K H
authors.initials: KH
authors.lastname: Kang
authors.forename: M Y
authors.initials: MY
authors.lastname: Kim
authors.forename: K H
authors.initials: KH
authors.lastname: Choi
mesh_terms: D015152:Blotting, Northern; D015153:Blotting, Western; D042846:CDC2-CDC28 Kinases; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D002469:Cell Separation; D002518:Ceramides / Q000378:metabolism* / Q000494:pharmacology; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors; D016213:Cyclins / Q000378:metabolism*; D005434:Flow Cytometry; D016193:G1 Phase / Q000187:drug effects*; D006801:Humans; D008113:Liver Neoplasms / Q000378:metabolism*; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011233:Precipitin Tests; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors; D012333:RNA, Messenger / Q000378:metabolism; D013997:Time Factors; D015533:Transcriptional Activation; D014162:Transfection; D014407:Tumor Cells, Cultured; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*
chemical_list: C493956:CDKN1A protein, human; D002518:Ceramides; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D012333:RNA, Messenger; D016159:Tumor Suppressor Protein p53; C101954:N-caproylsphingosine; D011494:Protein Kinases; C102333:histone H1 kinase; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases