Acute myeloid leukemia (AML) is characterized by uncontrolled proliferation of myeloid progenitor cells and a block in differentiation. Differentiation therapy using all-trans retinoic acid (ATRA) is highly effective in acute promyelocytic leukemia (APL) but largely ineffective in non-APL AML. One proposed resistance mechanism is the pan-retinoic acid receptor (RAR) activity of ATRA, which activates both RARA and RARG. While RARA promotes myeloid differentiation, RARG is associated with maintenance of primitive hematopoietic cells. Therefore, the RARA-selective agonist Am80 was evaluated as an alternative to ATRA. However, in vitro analyses revealed comparable responses to ATRA and Am80 in AML cell lines and primary samples, irrespective of RARA or RARG expression levels. Primary AML samples showed marked resistance to retinoids alone, which was overcome in 72% of cases by combination with LSD1 and GCN5 inhibitors. Mechanistically, inhibition of LSD1 disrupted the CoREST complex, releasing the transcriptional repressor GFI1 and enabling activation of differentiation-associated genes. GCN5 inhibition enhanced binding of the myeloid transcription factor CEBPA at key regulatory regions. In THP-1 cells, combined LSD1 and GCN5 inhibition reduced proliferation and colony formation while inducing robust myeloid differentiation, which was dependent on functional CEBPA. In contrast, HL-60 cells required additional retinoid signaling to achieve functional differentiation, highlighting stage-specific differences in epigenetic dependency. These findings demonstrate that overcoming the differentiation block in AML requires targeting epigenetic regulators in a context-dependent manner and support subtype-specific combination strategies for differentiation therapy.
Faezeh Ghazvini Zadegan (Thu,) studied this question.