POSTTRANSCRIPTIONAL REGULATION OF BCL-2 IN ACUTE MYELOBLASTIC-LEUKEMIA - SIGNIFICANCE FOR RESPONSE TO CHEMOTHERAPY

The blast stem cells of acute myeloblastic leukemia become more sensit ive in culture to the chemotherapeutic agents cytosine arabinoside (Ar a-C) and daunorubicin (DNR) when exposed to all-trans retinoic acid (A TRA) after drug. We have proposed that down regulation of bcl-2 by ATR A is part of the mechanism of sensitization. The hypothesis is based o n reduced expression of bcl-2 mRNA, as seen in Northern blots, after A TRA. Nuclear run on experiments, however, failed to account completely for the effect at the transcriptional level, Accordingly, we looked f or post-transcriptional effects of ATRA on bcl-2, using metabolic labe lling of the protein to measure stability, We found that the half-life of bcl-2 protein is markedly shortened after treatment with ATRA. Hyd rocortisone (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 synthesiz ed bcl-2 protein is stabilized in blast cells treated with HC. Respons e to Ara-C by growth factor responsive blast cells is influenced by th e 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 c ell lines, OCl/AML-5, primarily responsive to GM-CSF, and OCl/AML-10, primarily responsive to G-CSF. Growth factor did not influence the sta bility of bcl-2 protein in either line. In contrast, Western blots sho wed that the amount of bcl-2 protein was greater in cultures with GMCS F or GM-CSF in combination with G-CSF than in cultures with G-CSF or n o added factor. This pattern was seen regardless of the mitogenic resp onse to G-CSF or GM-CSF. We interpret our findings as indicating that bcl-2 protein is transcriptionally activated; that the stability of th e protein is decreased after ATRA and increased after HC; that the amo unt of bcl-2 protein is greater in cultures with GM-CSF than in cultur es 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.