Akt Inhibition Differently Controls PRC2 Components and Synergizes with Dual EZH2/1 Inhibitor in the Treatment of Multiple Myeloma

PI3K/Akt pathway is constitutively activated in multiple myeloma (MM). A plethora of studies extensively investigated Akt inhibitors, alone or in combination; however, the outcomes in hematological malignancies were largely unsatisfactory, emphasizing the need for critical preclinical evaluations. Polycomb repressive complex 2 (PRC2) components, EZH2 and its related homolog EZH1, induce H3K27me3 to silence the transcription of target genes. Recent studies ensured that EZH2 inhibition alone is not sufficient to completely disrupt the oncogenic functions of PRC2. With the importance of PRC2 as a therapeutic target in MM, we aimed to investigate the mechanisms by which Akt inhibition may impact PRC2 function, and test whether targeting both EZH2 and EZH1 together with Akt inhibition is a promising treatment strategy for MM.

We herein evaluated the cytotoxic effect of TAS-117, a potent and selective non-competitive Akt inhibitor, against different MM cell lines and found that responsive cell lines tended to have significant levels of activated Akt, coupled with low/deleted PTEN. Then, we examined signaling-epigenetic crosstalk on EZH2 level. TAS-117 significantly down-regulated EZH2 mRNA and protein in dose- and time-dependent manners, while H3K27me3 levels were rather maintained or elevated, suggesting compensation by EZH1. As EZH2 is a direct target for E2F1, we focused on Rb-E2F pathway as a regulatory mechanism for EZH2. TAS-117 induced marked down-regulation of E2F1 and E2F2. Moreover, TAS-117 induced the up-regulation of CDKN1B, in addition to the inactivation of cyclins and cyclin dependent kinases, hence, hypo-phosphorylated Rb, thereby stabilizing Rb-E2F1 complex and diminishing free E2F1 available for binding to its target genes, including EZH2 promoter. This prompted us to examine the impact of TAS-117 combination with either dual EZH2/1 inhibitor, UNC1999, or selective EZH2 inhibitor, GSK126. In agreement, UNC1999, but not GSK126, synergistically enhanced TAS-117-induced cytotoxicity, confirmed by combination index calculation, and provoked MM cell apoptosis.

As we observed an increase in H3K27me3 levels after TAS-117 treatment, we hypothesized that EZH1 function was augmented. Consistently, we found that EZH1 was markedly up-regulated after TAS-117 treatment in dose- and time-dependent manners. Importantly, EZH1 knockdown significantly enhanced the sensitivity of myeloma cells to TAS-117-induced cytotoxicity. To clarify the molecular mechanisms underlying EZH1 up-regulation, we performed RNA-seq followed by KEGG pathway analysis for up-regulated genes in TAS-117-treated group. We focused on FOXO pathway enrichment as it is a crucial target in MM treatment using Akt inhibitors. We then focused on FOXO3 as it was the main FOXO family gene expressed in MM cells according to our RNA-seq data. We examined the nuclear localization of FOXO3 following TAS-117 treatment. We found that TAS-117 significantly enhanced the nuclear accumulation of FOXO3, as depicted by both the immunostaining images and the digital calculations of the nuclear subset of FOXO3. Murine Ezh1 promoter was shown to be bound by Foxo transcription factors (TFs) in neuronal progenitors, T-regulatory cells, CD8⁺ cells, and pre-B cells. More than 80% of FOXO3-binding sites share the common binding motif, GTAAACAA, which was found both in human EZH1 (+48 from the TSS) and mouse Ezh1 (+77 from the TSS) promoter regions. So, we hypothesized that FOXO3 may be a regulatory partner for human EZH1 gene in myeloma cells in response to TAS-117 treatment. To this end, we performed ChIP-qPCR analysis for TAS-117-treated and -untreated cells. TAS-117 promoted the binding of FOXO3 to EZH1 promoter, in addition to one of the canonical FOXO3 targets, BIM promoter. To further confirm our results, we expressed shRNA against FOXO3 (shFOXO3) in MM cells which, interestingly, induced the down-regulation of EZH1 mRNA.

In conclusion, the present results defined novel signaling-epigenetic crosstalk between PI3K/Akt pathway and PRC2 components, EZH2 and EZH1, and demonstrated that Akt inhibition can differently modulates EZH2 and EZH1 levels via Akt downstream effectors, E2F1 and FOXO3, respectively. Therefore, targeting both EZH2 and EZH1 in addition to Akt inhibition may be a promising rationale to eradicate MM, leading to significant advances in treatment.