Constitutive expression of the promyelocytic leukemia-associated oncogene PML-RARα in TFI cells: Isoform-specific and retinoic acid-dependent effects on growth, bcl-2 expression, and apoptosis

Two major isoforms of PML-RARα are associated with (15;17)-positive acute promyelocytic leukemia (APL); however, functional differences between these isoforms have been difficult to define, and the molecular mechanism by which each isoform contributes to the pathogenesis of APL is not fully understood. To address these issues, the 'short' (S) and 'long' (L) isoforms of PML-RARα were constitutively expressed in the factor-dependent human erythroleukemia cell line, TF1. Expression of the L, but not the S, isoform inhibited growth of these cells in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). In the absence of GM-CSF, the S isoform partially protected against apoptosis, while the L isoform accelerated cell death. Treatment with all-trans retinoic acid (ATRA) inhibited cell growth and caused apoptosis only in PML-RARα-expressing cells, and these effects of ATRA were more marked in cells expressing the L isoform. ATRA treatment also led to down-regulation of bcl-2 and endogenous RARα in PMI.-RARα-expressing cells, but had little effect on the level of exogenously expressed PML- RARα. We conclude that (1) subtle differences exist in the biologic activities of the L and S isoforms of PML-RARα, and (2) both isoforms are capable of transducing an ATRA-mediated signal that leads to down-regulation of bcl-2 and induction of programmed cell death.