title: Post-transcriptional regulation of bcl-2 in acute myeloblastic leukemia: significance for response to chemotherapy.
abstract: The blast stem cells of acute myeloblastic leukemia become more sensitive in culture to the chemotherapeutic agents cytosine arabinoside (Ara-C) and daunorubicin (DNR) when exposed to all-trans retinoic acid (ATRA) after drug. We have proposed that down regulation of bcl-2 by ATRA is part of the mechanism of sensitization. The hypothesis is based on reduced expression of bcl-2 mRNA, as seen in Northern blots, after ATRA. Nuclear run on experiments, however, failed to account completely for the effect at the transcriptional level. Accordingly, we looked for post-transcriptional effects of ATRA on bcl-2, using metabolic labelling of the protein to measure stability. We found that the half-life of bcl-2 protein is markedly shortened after treatment with ATRA. Hydrocortisone (HC) protects cells against the toxic effects of Ara-C or DNR when given before drug. HC does not alter bcl-2 expression at the level of mRNA; however, metabolic labelling shows that newly synthesized bcl-2 protein is stabilized in blast cells treated with HC. Response to Ara-C by growth factor responsive blast cells is influenced by the factor in the cultures; cells are more sensitive in cultures with G-CSF and less sensitive when GM-CSF is present. We compared two blast cell lines, OCI/AML-5, primarily responsive to GM-CSF, and OCI/AML-10, primarily responsive to G-CSF. Growth factor did not influence the stability of bcl-2 protein in either line. In contrast, Western blots showed that the amount of bcl-2 protein was greater in cultures with GM-CSF or GM-CSF in combination with G-CSF than in cultures with G-CSF or no added factor. This pattern was seen regardless of the mitogenic response to G-CSF or GM-CSF. We interpret our findings as indicating that bcl-2 protein is transcriptionally activated; that the stability of the protein is decreased after ATRA and increased after HC; that the amount of bcl-2 protein is greater in cultures with GM-CSF than in cultures with G-CSF, regardless of which factor gives the greater mitogenic response. We propose that these post-transcriptional modifications of transcriptionally activated bcl-2 account, in part, for the regulation of drug sensitivity by ATRA, HC and growth factors.
authors.affiliation: The Ontario Cancer Institute, Toronto, Canada.
authors.forename: Z B
authors.initials: ZB
authors.lastname: Hu
authors.forename: M D
authors.initials: MD
authors.lastname: Minden
authors.forename: E A
authors.initials: EA
authors.lastname: McCulloch
mesh_terms: D000893:Anti-Inflammatory Agents / Q000494:pharmacology; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D015152:Blotting, Northern; D015153:Blotting, Western; D003561:Cytarabine / Q000494:pharmacology; D003630:Daunorubicin / Q000494:pharmacology; D015536:Down-Regulation / Q000187:drug effects; D015973:Gene Expression Regulation, Leukemic / Q000187:drug effects; D016179:Granulocyte Colony-Stimulating Factor / Q000494:pharmacology; D016178:Granulocyte-Macrophage Colony-Stimulating Factor / Q000494:pharmacology; D006801:Humans; D006854:Hydrocortisone / Q000494:pharmacology; D015470:Leukemia, Myeloid, Acute / Q000188:drug therapy / Q000235:genetics* / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics* / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger / Q000378:metabolism; D012074:Remission Induction; D014158:Transcription, Genetic / Q000187:drug effects*; D015533:Transcriptional Activation / Q000187:drug effects; D014212:Tretinoin / Q000494:pharmacology; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism
chemical_list: D000893:Anti-Inflammatory Agents; D000903:Antibiotics, Antineoplastic; D000964:Antimetabolites, Antineoplastic; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D003561:Cytarabine; D016179:Granulocyte Colony-Stimulating Factor; D014212:Tretinoin; D016178:Granulocyte-Macrophage Colony-Stimulating Factor; D006854:Hydrocortisone; D003630:Daunorubicin