ABSTRACT
Acute promyelocytic leukemia (APL) is uniquely sensitive to treatment with all-trans retinoic acid (ATRA), which results in the expression of genes that induce the terminal granulocytic differentiation of the leukemic blasts. Here we report the identification of two ATRA responsive genes in APL cells, ID1 and ID2. These proteins act as antagonists of basic helix-loop-helix (bHLH) transcription factors. ATRA induced a rapid increase in ID1 and ID2, both in the APL cell line NB4 as well as in primary patient cells. In addition, a strong downregulation of E2A was observed. E2A acts as a general heterodimerization partner for many bHLH proteins that are involved in differentiation control in various tissues. The simultaneous upregulation of ID1 and ID2, and the downregulation of E2A suggest a role for bHLH proteins in the induction of differentiation of APL cells following ATRA treatment. To test the relevance of this upregulation, ID1 and ID2 were overexpressed in NB4 cells. Overexpression inhibited proliferation and induced a G0/G1 accumulation. These results indicate that ID1 and ID2 are important retinoic acid responsive genes in APL, and suggest that the inhibition of specific bHLH transcription factor complexes may play a role in the therapeutic effect of ATRA in APL.
Subject(s)
DNA-Binding Proteins , G1 Phase/drug effects , Leukemia, Promyelocytic, Acute/genetics , Repressor Proteins , Resting Phase, Cell Cycle/drug effects , Transcription Factors , Tretinoin/pharmacology , Basic Helix-Loop-Helix Transcription Factors , Cell Differentiation/drug effects , Cell Differentiation/genetics , Cell Line, Tumor , Cell Proliferation/drug effects , Cells, Cultured , Clone Cells/drug effects , Colony-Forming Units Assay , DNA-Binding Proteins/drug effects , DNA-Binding Proteins/genetics , DNA-Binding Proteins/metabolism , DNA-Binding Proteins/pharmacology , Dose-Response Relationship, Drug , Gene Expression Regulation, Neoplastic , Humans , Inhibitor of Differentiation Protein 1 , Inhibitor of Differentiation Protein 2 , Leukemia, Promyelocytic, Acute/metabolism , Leukemia, Promyelocytic, Acute/pathology , Repressor Proteins/drug effects , Repressor Proteins/genetics , Repressor Proteins/pharmacology , Transcription Factors/drug effects , Transcription Factors/genetics , Transcription Factors/metabolism , Transcription Factors/pharmacology , Translocation, GeneticABSTRACT
Acute promyelocytic leukemia (APL) is characterized by a block in differentiation of hematopoietic cells at the promyelocytic stage of development. This disease is uniquely sensitive to treatment with pharmacological doses of all-trans retinoic acid (ATRA), and the combination of ATRA with chemotherapy has improved the durable disease-free survival in these patients to up to 80%. APL is characterized by chromosomal translocations that lead to the fusion of the retinoic acid receptor-alpha (RARalpha) to various partner genes. RARalpha functions as a ligand-inducible transcription factor and the aberrant RARalpha fusion proteins contribute to leukemic transformation by dominant inhibition of the expression of target genes that are important for cellular differentiation. This may at least in part be explained by an abnormally strong interaction with corepressor proteins leading to deacetylation of DNA and silencing of target genes. Most RARalpha fusion proteins can still be induced to transactivate genes, but only at very high doses of ligand, explaining why pharmacological doses of ATRA are necessary to obtain a therapeutic effect in these patients.
