All-trans retinoic acid (ATRA)-induced TFEB expression is required for myeloid differentiation in acute promyelocytic leukemia (APL)

© 2019 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Objective: In acute promyelocytic leukemia (APL), normal retinoid signaling is disrupted by an abnormal PML-RARα fusion oncoprotein, leading to a block in cell differentiation. Therapeutic concentrations of all-trans-retinoic acid (ATRA) can restore retinoid-induced transcription and promote degradation of the PML-RARα protein. Autophagy is a catabolic pathway that utilizes lysosomal machinery to degrade intracellular material and facilitate cellular re-modeling. Recent studies have identified autophagy as an integral component of ATRA-induced myeloid differentiation. Methods: As the molecular communication between retinoid signaling and the autophagy pathway is not defined, we performed RNA sequencing of NB4 APL cells treated with ATRA and examined autophagy-related transcripts. Results: ATRA altered the expression of >80 known autophagy-related transcripts, including the key transcriptional regulator of autophagy and lysosomal biogenesis, TFEB (11.5-fold increase). Induction of TFEB and its transcriptional target, sequestosome 1 (SQSTM1, p62), is reduced in ATRA-resistant NB4R cells compared to NB4 cells. TFEB knockdown in NB4 cells alters the expression of transcriptional targets of TFEB and reduces CD11b transcript levels in response to ATRA. Conclusions: We show for the first time that TFEB plays an important role in ATRA-induced autophagy during myeloid differentiation and that autophagy induction potentiates leukemic cell differentiation (Note: this study includes data obtained from NCT00195156, https://clinicaltrials.gov/show/NCT00195156)

All-Trans-Retinoic Acid Combined With Valproic Acid Can Promote Differentiation in Myeloid Leukemia Cells by an Autophagy Dependent Mechanism

Acute myeloid leukemia (AML) is an aggressive blood cancer with an overall survival of 30%. One form of AML, acute promyelocytic leukemia (APL) has become more than 90% curable with differentiation therapy, consisting of all-trans-retinoic acid (ATRA) and arsenic trioxide (ATO). Application of differentiation therapy to other AML subtypes would be a major treatment advance. Recent studies have indicated that autophagy plays a key role in the differentiation of ATRA-responsive APL cells. In this study, we have investigated whether differentiation could be enhanced in ATRA resistant cells by promoting autophagy induction with valproic acid (VPA). ATRA sensitive (NB4) and resistant leukemia cells (NB4R and THP-1) were co-treated with ATRA and valproic acid, followed by assessment of autophagy and differentiation. The combination of VPA and ATRA induced autophagic flux and promoted differentiation in ATRA-sensitive and -resistant cell lines. shRNA knockdown of ATG7 and TFEB autophagy regulators impaired both autophagy and differentiation, demonstrating the importance of autophagy in the combination treatment. These data suggest that ATRA combined with valproic acid can promote differentiation in myeloid leukemia cells by mechanism involving autophagy

Induction of autophagy is a key component of all-trans-retinoic acid-induced differentiation in leukemia cells and a potential target for pharmacological modulation

Acute myeloid leukemia (AML) is characterized by the accumulation of immature blood cell precursors in the bone marrow. Pharmacologically overcoming the differentiation block in this condition is an attractive therapeutic avenue, which has achieved success only in a subtype of AML, acute promyelocytic leukemia (APL). Attempts to emulate this success in other AML subtypes have thus far been unsuccessful. Autophagy is a conserved protein degradation pathway with important roles in mammalian cell differentiation, particularly within the hematopoietic system. In the study described here, we investigated the functional importance of autophagy in APL cell differentiation. We found that autophagy is increased during all-trans-retinoic acid (ATRA)-induced granulocytic differentiation of the APL cell line NB4 and that this is associated with increased expression of LC3II and GATE-16 proteins involved in autophagosome formation. Autophagy inhibition, using either drugs (chloroquine/3-methyladenine) or short-hairpin RNA targeting the essential autophagy gene ATG7, attenuates myeloid differentiation. Importantly, we found that enhancing autophagy promotes ATRA-induced granulocytic differentiation of an ATRA-resistant derivative of the non-APL AML HL60 cell line (HL60-Diff-R). These data support the development of strategies to stimulate autophagy as a novel approach to promote differentiation in AML

Evaluation of the role of autophagy in acute myeloid leukaemic cell differentiation

Acute myeloid leukemia (AML) is a malignancy characterized by the accumulation of white blood cell precursors in the bone marrow and circulation. Treatment regimens for this disease are notoriously toxic and survival is poor. Pharmacological overcoming the differentiation block seen in AML is an attractive avenue of therapy, termed ‘differentiation therapy’. The best example of this is the use of all-trans-retinoic acid (ATRA) in the treatment of acute promyelocytic leukaemia (APL). The differentiation of APL cells treated with ATRA in vitro is a model that allows the examination of differentiation pathways in myeloid leukaemic cells. Using this principle, my thesis examines the activity during differentiation, of two pathways involved in post-translational protein handling - autophagy and ISGylation. Autophagy is an intracellular recycling pathway responsible for degrading aged or redundant proteins. I demonstrate that ATRA induces autophagy and this has a functional role in differentiation. Autophagy inhibition reduces myeloid differentiation while pharmacologic induction may promote differentiation in ATRA-resistant cells. Using next-generation sequencing, I explore the molecular communication between retinoids and autophagy and study the expression changes in autophagy-related genes occurring in ATRAtreated APL cells. I also demonstrate that ISGylation is activated during differentiation, a process regulating the addition of a ubiquitin-like modifier, interferonstimulated gene 15 (ISG15) to target proteins. Targeted inhibition of ISGylation impedes differentiation. I discuss the benefits and practical implications of pharmacologically targeting both autophagy and ISGylation. A thorough molecular understanding of these pathways might broaden the application of differentiation therapy to other AML subtypes

Inhibition of UBE2L6 attenuates ISGylation and impedes ATRA-induced differentiation of leukemic cells.

Ubiquitin-conjugating enzyme E2L6 (UBE2L6) is a critical enzyme in ISGylation, a post-translational protein modification that conjugates the ubiquitin-like modifier, interferon-stimulated gene 15 (ISG15), to target substrates. Previous gene expression studies in acute promyelocytic leukemia (APL) cells showed that all-trans-retinoic acid (ATRA) altered the expression of many genes, including UBE2L6 (200-fold) and other members of the ISGylation pathway. Through gene expression analyses in a cohort of 98 acute myeloid leukemia (AML) patient samples and in primary neutrophils from healthy donors, we found that UBE2L6 gene expression is reduced in primary AML cells compared to normal mature granulocytes. To assess if UBE2L6 expression is important for leukemic cell differentiation - two cell line models were employed. The human APL cell line NB4 and its ATRA-resistant NB4R counterpart, as well as the ATRA sensitive human AML HL60 cells along with their ATRA-resistant subclone - HL60R. ATRA strongly induced UBE2L6 in NB4 APL cells and in ATRA-sensitive HL60 AML cells, but not in the ATRA-resistant NB4R and HL60R cells. Furthermore, short hairpin (sh)RNA-mediated UBE2L6 depletion in NB4 cells impeded ATRA-mediated differentiation, suggesting a functional role for UBE2L6 in leukemic cell differentiation. In addition, ATRA induced ISG15 gene expression in NB4 APL cells, leading to increased levels of both free ISG15 protein and ISG15 conjugates. UBE2L6 depletion attenuated ATRA-induced ISG15 conjugation. Knockdown of ISG15 in NB4 APL cells inhibited ISGylation and also attenuated ATRA-induced differentiation. In summary, we demonstrate the functional importance of UBE2L6 in ATRA induced neutrophil differentiation of APL cells and propose that this may be mediated by its catalytic role in ISGylation

Venetoclax-based low intensity therapy in molecular failure of NPM1 mutated AML

Molecular failure in NPM1 mutated AML inevitably progresses to frank relapse if untreated. Recently published small case series show that venetoclax combined with low dose cytarabine or azacitidine can reduce or eliminate measurable residual disease (MRD). Here we report an international multicentre cohort of 79 patients treated for molecular failure with venetoclax combinations and report an overall molecular response (≥1-log reduction in MRD) in 66/79 (84%) and MRD negativity in 56/79 (71%). 18/79 (23%) patients required hospitalisation and no deaths were reported during treatment. 41 patients were bridged to allogeneic transplant with no further therapy and 25/41 were MRD negative assessed by RT-qPCR before transplant. Overall survival (OS) for the whole cohort at 2 years was 67%, event-free survival (EFS) was 45% and in responding patients there was no difference in survival in those who received a transplant using time-dependent analysis. Presence of FLT3-ITD mutation was associated with a lower response rate (64 vs. 91%, p<0.01), worse OS (HR 2.50, 95% CI 1.06-5.86, p=0.036) and EFS (HR 1.87, 95% CI 1.06-3.28, p=0.03). 18/35 non-transplanted patients became MRD negative and stopped treatment after a median of 10 months, with 2-year molecular relapse free survival of 62% from the end of treatment. Venetoclax based low intensive chemotherapy is a potentially effective treatment for molecular relapse in NPM1 mutated AML, either as a bridge to transplant or as definitive therapy