Epigenetic Priming of AML Blasts for All-trans Retinoic Acid-Induced Differentiation by the HDAC Class-I Selective Inhibitor Entinostat

Abstract

All-trans retinoic acid (ATRA) has only limited single agent activity in AML without the PML-RARα fusion (non-M3 AML). In search of a sensitizing strategy to overcome this relative ATRA resistance, we investigated the potency of the HDAC class-I selective inhibitor entinostat in AML cell lines Kasumi-1 and HL-60 and primary AML blasts. Entinostat alone induced robust differentiation of both cell lines, which was enhanced by the combination with ATRA. This “priming” effect on ATRA-induced differentiation was at least equivalent to that achieved with the DNA hypomethylating agent decitabine, and could overall be recapitulated in primary AML blasts treated ex vivo. Moreover, entinostat treatment established the activating chromatin marks acH3, acH3K9, acH4 and H3K4me3 at the promoter of the RARβ2 gene, an essential mediator of retinoic acid (RA) signaling in different solid tumor models. Similarly, RARβ2 promoter hypermethylation (which in primary blasts from 90 AML/MDS patients was surprisingly infrequent) could be partially reversed by decitabine in the two cell lines. Re-induction of the epigenetically silenced RARβ2 gene was achieved only when entinostat or decitabine were given prior to ATRA treatment. Thus in this model, reactivation of RARβ2 was not necessarily required for the differentiation effect, and pharmacological RARβ2 promoter demethylation may be a bystander phenomenon rather than an essential prerequisite for the cellular effects of decitabine when combined with ATRA. In conclusion, as a “priming” agent for non-M3 AML blasts to the differentiation-inducing effects of ATRA, entinostat is at least as active as decitabine, and both act in part independently from RARβ2. Further investigation of this treatment combination in non-M3 AML patients is therefore warranted, independently of RARβ2 gene silencing by DNA methylation.

HDAC1 Is a Required Cofactor of CBFβ-SMMHC and a Potential Therapeutic Target in Inversion 16 Acute Myeloid Leukemia

“…Entinostat has also been shown to cause differentiation and apoptosis in leukemia cells expressing the fusion gene MLL-AF9. Although this fusion protein is not known to interact with HDAC1, it does require RUNX1 expression for its leukemogenic activity (48)(49)(50). These results may imply a common role for HDAC1 in RUNX1dependent leukemia.…”

MDI 301 suppresses myeloid leukemia cell growth in vitro and in vivo without the toxicity associated with all-trans retinoic acid therapy

“…Several approaches to improving retinoid efficacy have been examined in experimental studies. These include combining ATRA with inhibitors of growth factor signaling pathways [45] as well as proteasome pathway inhibitors [46] and histone deacetylase inhibitors [47,48]. At least one novel retinoid (Tamibarotene) has been tested [49] and is currently under investigation in PML [50].…”

Reprogramming acute myeloid leukemia into sensitivity for retinoic-acid-driven differentiation

“…The HDAC inhibitor VPA has been shown to restore the ability to respond to ATRA in an AML cell line and in six primary AML cases [26]. ATRA-based combination therapy with VPA or the HDAC inhibitor Entinostat enhanced histone acetylation levels on RA target sites and increased the expression of genes involved in differentiation [25,27,28]. Despite these results, several clinical trials with AML patients treated with a combination of ATRA and HDAC inhibitors, including VPA, showed limited effect on the overall survival of the patients [27,[114][115][116].…”