Differentiation therapy of leukemia: 3 decades of development

Abstract

A characteristic feature of leukemia cells is a blockade of differentiation at a distinct stage in cellular maturation. In the 1970s and 1980s, studies demonstrating the capabilities of certain chemicals to induce differentiation of hematopoietic cell lines fostered the concept of treating leukemia by forcing malignant cells to undergo terminal differentiation instead of killing them through cytotoxicity. The first promising reports on this notion prompted a review article on this subject by us 25 years ago. In this review, we revisit this interesting field of study and report the progress achieved in the course of nearly 3 decades. The best proof of principle for differentiation therapy has been the treatment of acute promyelocytic leukemia with all-trans retinoic acid. Attempts to emulate this success with other nuclear hormone ligands such as vitamin D compounds and PPARgamma agonists or different classes of substances such as hematopoietic cytokines or compounds affecting the epigenetic landscape have not been successful on a broad scale. However, a multitude of studies demonstrating partial progress and improvements and, finally, the new powerful possibilities of forward and reverse engineering of differentiation pathways by manipulation of transcription factors support the continued enthusiasm for differentiation therapy of leukemia in the future.

Figure 1

Ligands of nuclear hormone receptors. Depicts schematically the molecular mechanisms of differentiation induction of nuclear hormone receptor agonists. (A) All-trans retinoic acid (ATRA) for acute promyelocytic leukemia (APL). The characteristic chromosomal translocation t(15;17)(q22;q21) in APL leads to the production of a fusion protein between the PML protein and the retinoic acid receptor alpha (RARα). This fusion product is able to form homodimers and disrupt normal RARα signaling. It binds to retinoic acid response elements (RAREs) of target genes and recruits corepressors (Co-Rs) such as histone deacetylases (HDACs) and DNA methyltransferases (DNMTs), and sequesters retinoic X receptor (RXR) and the wild-type PML protein (PML), which finally leads to repression of genes necessary for granulocytic differentiation. Treatment with pharmacological concentrations of ATRA causes a conformational change of the PML-RARα fusion product leading to the release of the corepressors, recruitment of histone acetyl transferases (HATs), and relief of transcriptional repression. This causes the treated APL cells to undergo terminal granulocytic differentiation and finally apoptosis. (B) Biologically active vitamin D [1,25(OH)₂D₃] binds to the nuclear vitamin D receptor (VDR), which heterodimerizes with retinoic X receptor (RXR). This activated complex binds to vitamin D response elements (VDREs) in the promoter regions of genes inducing cell-cycle arrest, apoptosis, and differentiation in cancer cells. Furthermore, it leads to an up-regulation of the antimicrobial peptide cathelicidin (CAMP) in myeloid cells. (C) Thiazolidinediones (TZDs) bind to peroxisome proliferator activated receptor gamma (PPARγ). This activated complex acts as a transcription factor by heterodimerizing with (RXR) and binding to PPARγ-responsive elements in the promoter regions of target genes involved in cell-cycle arrest, apoptosis, growth inhibition, and differentiation of cancer cells.

Figure 2

Block of differentiation by disruption of hematopoietic transcription factors in myeloid and lymphoid leukemia. Depicts the molecular mechanisms leading to disruption of the transcription factor CCAAT/enhancer binding protein alpha (C/EBPα) in myeloid leukemia and the paired box gene 5 (Pax5) in lymphocytic leukemia. Disruption of these transcription factors blocks hematopoietic cells in their early stages of differentiation. Treatment with substances altering the epigenetic settings such as histone deacetylase inhibitors or demethylating agents may partly overcome the block in differentiation. Forced expression of the normal counterpart of the disrupted transcription factor can often re-establish differentiation. In the future, synthesis of small molecules specifically targeting transcription factors or nuclear reprogramming and gene therapy may provide useful tools for correcting defective differentiation in hematologic malignancies.

Figure 3

Timeline of milestones in differentiation therapy of leukemia. NHR indicates nuclear hormone receptor; TZD, thiazolidinediones; ATRA, all-trans retinoic acid; ATO, arsenic trioxide; and HDAC, histone deacetylase.