Retinoic acid-mediated growth arrest of EBV-immortalized B lymphocytes is associated with multiple changes in G1 regulatory proteins: p27Kip1 up-regulation is a relevant early event.
EBV-immortalized lymphoblastoid B cell lines (LCLs) are a suitable in vitro model for the study of EBV-related lymphoproliferative disorders of immunosuppressed patients. We have previously shown that 9-cis-, 13-cis- and all-trans-retinoic acid (RA) powerfully inhibit LCL proliferation at concentrations corresponding to therapeutically achievable plasma levels (10(-6) M). Herein we show that RA-induced LCL accumulation in the G0/G1 phases correlated with the loss of the catalytic activity of all three G1-associated CDKs (CDK2, CDK4 and CDK6) and with increased levels of underphosphorylated pRb and, in some LCLs, p130. LCLs arrested in G0/G1 by RA also showed a significant decrease in the protein levels of cyclins D2, D3 and A, together with a reduction in the amount of cyclin D associated with CDK4 and CDK6, probably accounting for the inhibition of the relative kinase activity. In addition, RA-treated LCLs showed a marked up-regulation of the CDK inhibitor (CKI) p27Kip-1 at the protein but not mRNA level, which correlated with a progressive increase of p27Kip-1 in CDK2 complexes (more than 2.5-fold) and with a reduction in the active phosphorylated form of CDK2. p27Kip-1 may also contribute to the inhibition of CDK4 kinase activity, as the amount of CDK4-associated p27Kip-1 was increased by 50% after RA exposure. p27Kip-1 up-regulation stably persisted for more than one week after RA withdrawal concomitantly with the maintenance of the proliferative block. Moreover, neutralization of TGFbeta did not affect the growth inhibitory activity of RA, suggesting that LCL growth arrest induced by these retinoids is probably not mediated by a pathway directly involving TGFbeta. Overall, these results demonstrate that RA treatment of EBV-immortalized B lymphocytes is associated with multiple effects on G1 regulatory proteins, including p27Kip1 up-regulation, decreased levels of cyclins D2, D3 and A, and inhibition of CDK2, CDK4 and CDK6 activity, which ultimately result in reduced pRb phosphorylation and G0/G1 growth arrest.
Division of Experimental Oncology 1, Centro di Riferimento Oncologico, Aviano (PN), Italy.
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Dolcetti
D001402:B-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle; D018797:Cell Cycle Proteins; D002455:Cell Division; D002461:Cell Line, Transformed; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D050759:Cyclin-Dependent Kinase Inhibitor p21; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D016213:Cyclins / Q000096:biosynthesis; D016193:G1 Phase; D004854:Herpesvirus 4, Human; D006801:Humans; D015474:Isotretinoin / Q000494:pharmacology*; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism; D011233:Precipitin Tests; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011506:Proteins; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein / Q000378:metabolism; D050717:Retinoblastoma-Like Protein p130; D016212:Transforming Growth Factor beta / Q000494:pharmacology; D025521:Tumor Suppressor Proteins; D015854:Up-Regulation
C493956:CDKN1A protein, human; D018797:Cell Cycle Proteins; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D008869:Microtubule-Associated Proteins; D010750:Phosphoproteins; D011506:Proteins; D011518:Proto-Oncogene Proteins; C493837:RBL2 protein, human; D016160:Retinoblastoma Protein; D050717:Retinoblastoma-Like Protein p130; D016212:Transforming Growth Factor beta; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; C495900:CDK4 protein, human; C495911:CDK6 protein, human; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases; D015474:Isotretinoin