The role of Polycomb group genes in all-trans retinoic acid-treated HL-60 acute promyelocytic leukemia cells

Abstract

Polycomb group (PcG) proteins are known to form multimeric complexes that suppress gene expression by epigenetic modifications1. Upon catalyzing various histone modifications, such as histone methylation and ubiquitlyation, PcG proteins are understood to regulate cell fate determination and maintenance2. PcG proteins have also emerged as common hallmarks of many cancers, which support the role of epigenetic lesions in oncogenesis3. Acute promyelocytic leukemia (APL) cells are blocked at the promyelocytic stage of myeloid differentiation, which can be resumed by all-trans retinoic acid (ATRA)4. Such differentiation therapy with ATRA is relatively safe and widely used to treat APL. Despite recent advances in therapeutic regimens, including combination therapies with anthracycline-based chemotherapy and arsenic trioxide, complete cure of the disease still remains to be achieved5. We tested the possibility of polycomb group (PcG) genes as a target for combination therapy in ATRA-treated APL cell line, HL-60. In HL-60 cells treated with 1 μM of ATRA for up to 7 days, the expression of EZH2, BMI1 and PCGF2 genes were enhanced at both mRNA and protein levels in a time-dependent manner. Cell growth was enhanced in HL-60 cells overexpressing PCGF2 gene, while it was reduced upon PCGF2 gene knockdown with lentiviral shRNA infection. The granulocytic differentiation of HL-60 by ATRA was significantly attenuated by PCGF2 gene knockdown. In addition, the expressions of HOXA7 and HOXC4 genes were enhanced, while those of HOXA9, HoxB6, HOXB7, HOXB9 and HOXC5 were reduced by ATRA in a time-dependent manner. Taken together, these results support the role of polycomb group genes, particularly PCGF2, as a differentiation regulator in HL-60 cells, which suggest PcG genes as a possible therapeutic target of acute promyelocytic leukemia.