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The discovery of effective therapeutic treatments for cancer via cell differentiation instead of antiproliferation remains a great challenge. Cyclin-dependent kinase 2 (CDK2) inactivation, which overcomes the differentiation arrest of acute myeloid leukemia (AML) cells, may be a promising method for AML treatment. However, there is no available selective CDK2 inhibitor. More importantly, the inhibition of only the enzymatic function of CDK2 would be insufficient to promote notable AML differentiation. To further validate the role and druggability of CDK2 involved in AML differentiation, a suitable chemical tool is needed. Therefore, we developed first-in-class CDK2-targeted proteolysis-targeting chimeras (PROTACs), which promoted rapid and potent CDK2 degradation in different cell lines without comparable degradation of other targets, and induced remarkable differentiation of AML cell lines and primary patient cells. These data clearly demonstrated the practicality and importance of PROTACs as alternative tools for verifying CDK2 protein functions.

Retinoic acid (RA) is a well-known differentiation inducer that exerts its effects by binding to nuclear RA receptors. Retinoic acid receptor α (RARα), as an important nuclear RA receptor, is activated upon RA binding and facilitates the transcription of target genes related to differentiation, which ultimately initiates cell differentiation. Previous studies have found that the transcriptional activity of RARα is regulated by various post-translational modifications, which influence its DNA binding efficiency, transactivation ability and even lead to degradation. Post-translational modifications of RARα, as a consequence, play an important role in the RA-induced differentiation process. Therefore, in this review, we focus on recent advances in the understanding of how these modifications affect the activity of RARα as well as strategies to increase the differentiation effect of RA treatment in cancer cells based on RARα modifications, which may promote the development of novel effective differentiation therapies for cancer treatment., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Folate, an essential micronutrient, is a critical cofactor in one-carbon metabolism for many cellular pathways including DNA synthesis, metabolism and maintenance. Folate deficiency has been associated with an increased risk of neurological disease, cancer and cognitive dysfunction. Dihydrofolate reductase (DHFR) is a key enzyme to regulate folate metabolism, however folate/DHFR activity in oligodendrocyte development has not been fully understood. Here we show that folate enhances oligodendrocyte maturation both in vitro and in vivo, which is accompanied with upregulation of oligodendrocyte-specific DHFR expression. On the other hand, pharmacological inhibition of DHFR by methotrexate (MTX) causes severe defects in oligodendrocyte survival and differentiation, which could be reversed by folate intake. We further demonstrate that folate activates a metabolic regulator AMPKα to promote oligodendrocyte survival and differentiation. Moreover, activation of AMPKα partially rescues oligodendrocyte defects caused by DHFR-inhibition both in vitro and in vivo. Taken together, these findings identify a previously uncharacterized role of folate/DHFR/AMPKα axis in regulating oligodendrocyte survival and myelination during CNS development.

A series of novel indene-derived retinoic acid receptor α (RARα) agonists have been designed and synthesized. The use of receptor binding, cell proliferation and cell differentiation assays demonstrated that most of these compounds exhibited moderate RARα binding activity and potent antiproliferative activity. In particular, 4-((3-isopropoxy-2,3-dihydro-1 H -inden-5-yl)-carbamoyl)benzoic acid ( 36d ), which showed a moderate binding affinity, exhibited a great potential to induce the differentiation of NB4 cells (68.88% at 5 μM). Importantly, our work established indene as a promising skeleton for the development of novel RARα agonists.

The success of all-trans retinoic acid (ATRA) in differentiation therapy for patients with acute promyelocytic leukemia (APL) highly encourages researches to apply this therapy to other types of acute myeloid leukemia (AML). However, AML, with the exception of APL, fails to respond to differentiation therapy. Therefore, research strategies to further sensitize cells to retinoids and to extend the range of AMLs that respond to retinoids beyond APLs are urgently needed. In this study, we showed that TAK165, a HER2 inhibitor, exhibited a strong synergy with ATRA to promote AML cell differentiation. We observed that TAK165 sensitized the AML cells to ATRA-induced cell growth inhibition, G0/G1 phase arrest, CD11b expression, mature morphologic changes, NBT reduction and myeloid regulator expression. Unexpectedly, HER2 pathway might not be essential for TAK165-enhanced differentiation when combined with ATRA, while the enhanced differentiation was dependent on the activation of the RARα/STAT1 axis. Furthermore, the MEK/ERK cascade regulated the activation of STAT1. Taken together, our study is the first to evaluate the synergy of TAK165 and ATRA in AML cell differentiation and to assess new opportunities for the combination of TAK165 and ATRA as a promising approach for future differentiation therapy.

Small ubiquitin-related modifier-1 (SUMO-1) modification has been implicated in many important cellular processes, including cell cycle progression, apoptosis, cellular proliferation, and development, but its role in all-trans-retinoic acid (ATRA)-induced differentiation processes of cancer cells remains unclear. Here, we report for the first time that ATRA-induced differentiation of leukemia and osteosarcoma is accompanied by a decrease in the level of SUMO-1 protein. Our results also demonstrated that depletion or inhibition of SUMO-1 blocks ATRA-induced differentiation, suggesting that SUMO-1 is critical for the differentiation effect of ATRA. Further studies indicated that SUMO-1-promoted ATRA-induced differentiation might be associated with the stabilization of retinoic acid receptor α (RARα), protecting it from degradation. Moreover, our results suggested that Lys399 is a major site for SUMO-1 conjugation of RARα. We also found that RARα enhanced the transcription of its target genes, which might also contribute to the enhanced differentiating effects of ATRA; however, mutation of Lys399 of RARα inhibits the extents of both SUMO-1 modification and ATRA-induced differentiation. Together, these results indicate that SUMO-1 modification of RARα is a potent mechanism for balancing proliferation and differentiation by controlling the stability of RARα in cancer cells. SUMO-1 modification may thus serve an important role in controlling ATRA-induced cell differentiation in cancers., (© 2014 FEBS.)

A breakthrough in clinical oncology was achieved as All-trans-retinoic acid (ATRA) sparked intensive differentiation therapy research. However, differentiation therapy is limited because ATRA is the sole efficient agent. Dasatinib is reported to induce myeloid differentiation of acute myeloid leukemia (AML) cells in vitro, but its mechanism remains unclear. Furthermore, the ability of dasatinib to cause differentiation of AML cells has not yet been proven. We assessed the contribution of autophagy to dasatinib-induced differentiation of AML cells. We found that dasatinib induces myeloid differentiation of AML cells accompanied with autophagy induction. Pharmacological inhibition of autophagy by 3-MA, Wortmannin, LY294002 and chloroquine block dasatinib-induced AML cell differentiation, whereas the induction of autophagy by rapamycin enhances AML cell differentiation. Our results suggest that retinoic acid receptors alpha (RARα) may not be involved in dasatinib-induced differentiation. In addition, we further illustrated that even low concentration of dasatinib can enhance ATRA-induced differentiation capability through initiation of autophagy. Taken together, we conclude that autophagy enhances the dasatinib-induced differentiation, which may provide theoretical support for developing dasatinib as a promising strategy for future differentiation therapy in AML patients., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Dasatinib (BMS-354825) is a FDA-approved multitargeted kinase inhibitor of BCR/ABL and Src kinases. It is now used in the treatment of chronic myelogenous leukemia (CML) with resistance or intolerance to prior therapies, including imatinib. Here we report a novel effect of dasatinib on inducing the differentiation of acute myeloid leukemia (AML) cells through MEK/ERK-dependent activation of signal transducer and activator of transcription 1 (STAT1). We found that dasatinib could induce the differentiation of AML cells as demonstrated by the expression of differentiation marker CD11b, G0/G1 phase arrest and decreased ratio of nucleus to cytoplasm. Of note, dasatinib induced robust phosphorylation of STAT1 both at Tyr701 and Ser727 as well as the redistribution of STAT1 from the cytoplasm to the nucleus, thus leading to the transcription of STAT1-targeted genes. Knocking down STAT1 expression by shRNA significantly attenuated dasatinib-induced differentiation, indicating an important role of STAT1 in myeloid maturation. We further found that dasatinib-induced activation of STAT1 was regulated by the MEK/ERK kinases. The phosporylation of MEK and ERK occurred rapidly upon dasatinib treatment and increased progressively as differentiation was induced. MEK inhibitors PD98059 and U0216 not only inhibited the phosphorylation of STAT1, but also abrogated dasatinib-induced myeloid differentiation, suggesting that MEK/ERK dependent phosphorylation of STAT1 might be indispensable for the differentiating effect of dasatinib in AML cells. Taken together, our study suggests that STAT1 is an important mediator in dasatinib-induced differentiation of AML cells, whose activation requires the activation of MEK/ERK cascades.

In this paper we report a new myeloid differentiation effect of bortezomib (BTZ) in acute myeloid leukemia (AML) cell lines and primary patient-derived AML cells; this effect was assayed by investigating growth-inhibition, cell morphology, differentiation markers, and nitro-blue tetrazolium reduction. We show that BTZ induces the phosphorylation of several mitogen-activated protein (MAP) kinases, including MEK/ERK, c-Jun N-terminal kinase (JNK), and p38 MAPK. BTZ-induced cell differentiation is almost completely reversed by PD98059, an inhibitor of MEK, which also attenuates the increase in phospho-JNK p46. However, p38 activation does not appear to be required for the differentiation induced by BTZ. Furthermore, the differentiation effect of BTZ is associated with increased protein level of signal transducer and activator of transcription-1 (STAT1), a molecular determinant of myeloid differentiation, due to effects on both its synthesis and degradation. In short, this study reveals that BTZ activates the MEK/ERK cascade, which further up-regulates the expression and activity of the key myeloid transcription factor STAT1, thus promoting myeloid differentiation. These findings contribute to an unexpected potential mechanism for the antitumor activity of BTZ in AML., (Copyright © 2011 Wiley Periodicals, Inc.)

Unlike its cytotoxicity in p53-functional cell lines, Nutlin-1, the small-molecule inhibitor of murine double minute (MDM2), significantly enhanced the differentiation-inducing activity of all-trans retinoic acid (ATRA) in HL60 and NB4 cells (p53-nonfunctional) but not in U937 cells (p53 wild-type). Moreover, we demonstrated that the synergistic differentiation-inducing activity of Nutlin-1 combined with ATRA appeared in a p53-independent manner. In the present study, we found that ATRA could selectively induce expression of p-glycoprotein (p-gp) in HL60 and NB4 cells but not in U937 cells. Investigation of p-gp-ATPase activity showed that Nutlin-1 and ATRA were likely to act as p-gp transport substrates. Furthermore, Nutlin-1 enhanced the ability of ATRA to induce expression of the myeloid differentiation-related transcription factor C/EBPβ and to reduce expression of c-myc. Additionally, the expression of retinoic acid receptor α (RARα) was further reduced in cells treated with ATRA in combination with Nutlin-1. Taken together, the mechanisms of synergistic differentiation-inducing activity of Nutlin-1 combined with ATRA could be attributed to Nutlin-1 competitive binding to p-gp, leading to ATRA efflux inhibition, and then the differentiation pathways involved were therefore further activated. Nutlin-1 might be a useful adjuvant with ATRA for patients with retinoid-resistant leukemia induced by overexpression of p-gp.

All-trans retinoic acid (ATRA) is nowadays considered to be the sole efficient agent for differentiation-based therapy in leukemia; however, the mechanisms of ATRA's biological effects remain largely unknown. Here we first reported that ATRA-induced myeloid leukemia differentiation was accompanied with the increased level of ubiquitin-protein conjugates and the upregulation of proteasome activity. To explore the functional role of the activated proteasome in retinoic acid (RA) signaling, the effects of proteasome inhibitors on RA-induced cell differentiation were determined. Our results demonstrated that inhibition of ATRA-elevated proteasome activity obviously promoted the myeloid maturation program triggered by ATRA, suggesting that the overactivated proteasome is not beneficial for ATRA's effects. Further studies demonstrated that the synergistic differentiating effects of ATRA and proteasome inhibitors might be associated with the protection of retinoic acid receptor alpha (RARα) from degradation by the ubiquitin-proteasome pathway (UPP). Moreover, the accumulated RARα was able to enhance the transcription of its target gene, which might also contribute to the enhanced differentiation of leukemia cells. Together, by linking the UPP to ATRA-dependent signaling, our data provide a novel insight into studying the mechanisms of ATRA-elicited cellular effects and imply the possibility of combination of ATRA and proteasome inhibitors in leukemia therapy., (© 2010 Wiley-Liss, Inc.)

Neuroblastoma (NB) is the most common extracranial solid tumor in childhood. Differentiated human NBs are associated with better outcome and lower stage; induction of differentiation is considered to be therapeutically advantageous. All-trans retinoic acid (ATRA) has been shown to induce the differentiation of neuroblastoma (NB) cell lines. The proteasome inhibitor bortezomib inhibits cell growth and angiogenesis in NBs. Here, we investigated the synergistic effect between bortezomib and ATRA in inducing NB cell differentiation in different NB cell lines. Bortezomib combined with ATRA had a significantly enhanced antiproliferative effect. This inhibition was characterized by a synergistic increase in neuronal differentiation. At the same time, the combination therapy showed little neuronal toxicity which was assessed in primary cultures of rat cerebellar granule cells by the MTT assay, PI staining. The combination of bortezomib and ATRA triggered increased differentiation through the activation of proteins, including RARα, RARβ, RARγ, p-JNK and p21, compared with ATRA treatment alone. Using JNK inhibitor SP600125 to block JNK-dependent activity, the combination therapy-induced neuronal differentiation was partially attenuated. In addition, p21 shRNA had no effect on the combination therapy-induced neuronal differentiation. The in vivo antitumor activities were examined in human NB cell xenografts and GFP-labeled human NB cell xenografts. Treatment of human NB cell CHP126-bearing nude mice with ATRA plus bortezomib resulted in more significant tumor growth inhibition than mice treated with either drug alone. These findings provide the rationale for the development of a new therapeutic strategy for NB based on the pharmacological combination of ATRA and bortezomib.

While neuroblastoma accounts for 15% of childhood tumor-related deaths, treatments against neuroblastoma remain scarce and mainly consist of cytotoxic chemotherapeutic drugs. Currently, maintenance therapy of differentiation induction is the standard of care for neuroblastoma patients in clinical, especially high-risk patients. However, differentiation therapy is not used as a first-line treatment for neuroblastoma due to low efficacy, unclear mechanism, and few drug options. Through compound library screening, we accidently found the potential differentiation-inducing effect of AKT inhibitor Hu7691. The protein kinase B (AKT) pathway is an important signaling pathway for regulating tumorigenesis and neural differentiation, yet the relation between the AKT pathway and neuroblastoma differentiation remains unclear. Here, we reveal the anti-proliferation and neurogenesis effect of Hu7691 on multiple neuroblastoma cell lines. Further evidence including neurites outgrowth, cell cycle arrest, and differentiation mRNA marker clarified the differentiation-inducing effect of Hu7691. Meanwhile, with the introduction of other AKT inhibitors, it is now clear that multiple AKT inhibitors can induce neuroblastoma differentiation. Furthermore, silencing AKT was found to have the effect of inducing neuroblastoma differentiation. Finally, confirmation of the therapeutic effects of Hu7691 is dependent on inducing differentiation in vivo , suggesting that Hu7691 is a potential molecule against neuroblastoma. Through this study, we not only define the key role of AKT in the progression of neuroblastoma differentiation but also provide potential drugs and key targets for the application of differentiation therapies for neuroblastoma clinically. This study not only defines the key role of AKT in the progression of neuroblastoma differentiation but also provides potential drugs and key targets for the application of differentiation therapies for neuroblastoma clinically. [Display omitted] [ABSTRACT FROM AUTHOR]

Background: Chemotherapy is the only therapy option for the majority of AML patients, however, there are several limitations for this treatment. Our aim was to find a new chemotherapy strategy that is more effective and less toxic. Methods: MTT assays and a xenograft mouse model were employed to evaluate the synergistic activity of all-trans retinoic acid (ATRA) combined with topotecan (TPT). Drug-induced DNA damage and apoptosis were determined by flow cytometry analysis with PI and DAPI staining, the comet assay and Western blots. Short hairpin RNA (shRNA) and a RARα plasmid were used to determine whether RARα expression influenced DNA damage and apoptosis. Results: We found that ATRA exhibited synergistic activity in combination with Topotecan in AML cells, and the enhanced apoptosis induced by Topotecan plus ATRA resulted from caspase pathway activation. Mechanistically, ATRA dramatically down regulated RARα protein levels and led to more DNA damage and ultimately resulted in the synergism of these two agents. In addition, the increased antitumor efficacy of Topotecan combined with ATRA was further validated in the HL60 xenograft mouse model. Conclusions: Our data demonstrated, for the first time, that the combination of TPT and ATRA showed potential benefits in AML, providing a novel insight into clinical treatment strategies. [ABSTRACT FROM AUTHOR]

• Osteosarcoma is a disease resulting from osteogenic differentiation defects. • Differentiation therapy for osteosarcoma has received extensive attention. • Various agents have been shown to induce terminal differentiation in osteosarcoma. • The exploration of new inducers and biomarkers is the way forward. Differentiation therapy involves the use of agents that can induce differentiation in cancer cells, with the irreversible loss of tumour phenotype. The application of differentiation therapy in osteosarcoma has made progress because of a better understanding of the potential links between differentiation defects and tumorigenesis. Here, we review recent studies on differentiation therapy for osteosarcoma, describing a variety of differentiation inducers. By highlighting these examples of drug-induced osteosarcoma cell differentiation, we can acquire unique insights into not only osteosarcoma treatment, but also novel approaches to transform differentiating drugs into more effective therapies for other solid tumours. [ABSTRACT FROM AUTHOR]

Aim: Chemotherapy-induced reactive oxygen species (ROS) not only contribute to apoptosis, but also trigger autophagy. Since autophagy is reported to protect cancer cells from apoptosis, this weakens the therapeutic effect of chemotherapy. This study aimed at identifying the key molecules that determine the cellular response to ROS and, therefore, provide better strategies to increase chemotherapeutic efficiency. Results: Increasing concentrations of N-(4-hydroxyphenyl) retinamide (4-HPR)-treatment pushed autophagy down to apoptosis in a dose-dependent manner, and 4-HPR-induced ROS contribute to this process. Since we found that ASK1-regulated JNK1 and p38 are responsible for 4-HPR-induced autophagy and apoptosis, respectively, we further utilized co-immunoprecipitation followed by liquid chromatography-tandem mass spectrometry analysis to identify proteins that specifically bind to ASK1 under different oxidative states. Of note, DJ-1, a crucial antioxidant protein, was identified. Interestingly, DJ-1 functions as a redox sensor that senses ROS levels and determines the cellular response to 4-HPR: Under mild oxidative stress, moderate oxidation of DJ-1 is recruited to inhibit the activity of ASK1 and maintain cell viability by activating autophagy; under a lethal level of oxidative stress, excessive oxidized DJ-1 dissociates from ASK1 and activates it, thereby initiating p38 activation and enabling the cells to commit to apoptosis. Moreover, the depletion of DJ-1 increases the sensitivity of tumor cells to 4-HPR both in vitro and in vivo. Innovation: Our results reveal that the different oxidation states of DJ-1 function as a cellular redox sensor of ROS caused by 4-HPR and determine the cell fate of autophagy or apoptosis. Moreover, the results suggest that DJ-1 might be a potent therapeutic target for cancer treatment. Conclusion: ROS-mediated changes in the oxidation state of DJ-1 are involved in 4-HPR's effect on pushing autophagy down to apoptosis. Consequently, this change mediates ASK1 activation by regulating DJ-1-ASK1 complex formation and determines the cell fate of autophagy or apoptosis. Antioxid. Redox Signal. 21, 1443-1459. [ABSTRACT FROM AUTHOR]

A characteristic feature of leukemia cells is a blockade of differentiation in cellular maturation. All-trans-retinoic acid (ATRA) has been successfully applied for the treatment of M3-type AML (APL, 10 %), but it fails to demonstrate a significant efficacy on the remaining patients with non-APL AML (90 %). Therefore, the research for strategies to extend the efficacy of ATRA-based therapy to non-APL AML is a key avenue of investigation. Here, we evaluate the synergetic effect of CDK2 inhibition and ATRA in AML both in vitro and in vivo. We have determined that both the CDK2 depletion and pharmacological inhibitor of CDK2 significantly sensitize three subtypes of AML cells (including two non-APL cells) to ATRA-induced cell differentiation. RNA-sequence results indicate that transcription activation of differentiation and maturation pathways plays an important role in this synergetic effect. Furthermore, the down-regulation of CDK2 sensitized AML cells to ATRA-induced engraftment prevention of leukemia cells in NOD-SCID mice and promotes the primary AML blasts differentiation when combined with ATRA. Thus, our work not only provides relevant experimental evidence for further validating CDK2 as a target for differentiation therapy, but also uncovers the future clinical application of CDK2 inhibitors in ATRA-based differentiation therapeutics for AML. [ABSTRACT FROM AUTHOR]