Antineoplastic Agents/pharmacology; Arsenicals/pharmacology; Fusion Proteins, bcr-abl/metabolism; Gene Expression Regulation, Leukemic/drug effects; Genes, ras; Leukemia, Promyelocytic, Acute/metabolism; Proto-Oncogene Proteins c-abl/metabolism; Apoptosis/drug effects; Arsenicals/therapeutic use; Benzamides; Enzyme Inhibitors/pharmacology; Fibroblasts/drug effects; Fusion Proteins, bcr-abl/genetics; Humans; Imatinib Mesylate; Leukemia, Promyelocytic, Acute/drug therapy; Leukemia, Promyelocytic, Acute/genetics; Monocytes/drug effects; Oxides/pharmacology; Oxides/therapeutic use; Philadelphia Chromosome; Piperazines/pharmacology; Precursor Cell Lymphoblastic Leukemia-Lymphoma/pathology; Protein Isoforms/genetics; Protein Isoforms/metabolism; Proto-Oncogene Proteins c-abl/genetics; Pyrimidines/pharmacology; Receptors, Retinoic Acid/metabolism; Retinoic Acid Receptor alpha; Transgenes; Translocation, Genetic; Tumor Cells, Cultured; U937 Cells; Up-Regulation