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INTRODUCTION: Bromodomain and extra-terminal motif-protein inhibitors (BETi) have shown an antileukemic effect in several subtypes of acute myeloid leukemia (AML), both in vitro and in vivo (Daniel Gerlach et al. 2018 Oncogene). The effect of BETi is mainly mediated by suppression of bromodomain-containing protein 4 (BRD4) activity, an epigenetic regulator that sustains MYC and c-KIT expression, with consequences on a number of cellular processes, including proliferation and apoptosis (Kazuki Homma et al. 2017 Blood). Moreover recent findings suggested that MYC regulates through a feedback loop, a long-non coding RNA, named PVT1, which is located in the same genomic region (8q24) and is often associated with its pro-tumorigenic role (Alberto L’Abbate et al. 2018 Leukemia). PVT1 also encodes for a circular isoform (circPVT1), with MYC-dependent and independent functions. The aim of the present study is to investigate the role of BETi in the regulation of the circular and linear forms of PVT1 in AML and the functional consequences of their downregulation. METHODS: OCI-AML3 and KASUMI-1 AML cell lines were treated for 16 hours with BETi under normoxic and hypoxic conditions mimicking the bone marrow microenvironment. circPVT1 and linear PVT1 (PVT1) were quantified by qRT-PCR, using total RNA and di-polisomal fraction. The qRT-PCR results were also confirmed by RNA in situ hybridization (RNA-ISH) in combination with the S6 ribosomal marker. In addition, circPVT1 was silenced by small interfering (siRNA) in order to investigate the effects on apoptosis, proliferation, and its downstream targets in OCI-AML3 cells. RESULTS: circPVT1 and PVT1 total RNA significantly decreased in both cell lines after treatment with BETi, under normoxia and hypoxia. Notably, KASUMI-1 expressed higher PVT1 levels under hypoxia compared to normoxia. circPVT1 and PVT1 were also detected in the polysomal fraction. Interestingly, PVT1 showed mostly a nuclear localization, while circPVT1 colocalized with ribosomes. Our data on circPVT1 silencing in OCI-AML3 cells showed reduced cell growth, a slight increase in apoptosis and no effects on MYC transcript. Moreover, the interferon (IFN)-inducible serine/threonine protein kinase PKR, which has been previously reported to have a role in the innate immunity and to be inhibited by circRNAs, was induced by circPVT1-silencing in leukemic cells, as observed in T lymphocytes from systemic lupus erythematosus (Chu-Xiao Liu et al. 2019 Cell). CONCLUSIONS: Our findings show that circPVT1 and PVT1 are regulated by BETi, along with MYC, in AML cell lines under normoxia and hypoxia and that silencing of circPVT1 dampens cell proliferation. Therefore PVT1 and circPVT1 may contribute to AML pathogenesis and progression, and our results suggest that targeting circPVT1 may have therapeutic potentials in AML.