Revisiting the differentiation paradigm in acute promyelocytic leukemia.
Ablain Julien,de The Hugues
As the result of intense clinical and basic research, acute promyelocytic leukemia (APL) has progressively evolved from a deadly to a curable disease. Historically, efforts aimed at understanding the molecular bases for therapy response have repeatedly illuminated APL pathogenesis. The classic model attributes this therapeutic success to the transcriptional reactivation elicited by retinoic acid and the resulting overcoming of the differentiation block characteristic of APL blasts. However, in clinical practice, retinoic acid by itself only rarely yields prolonged remissions, even though it induces massive differentiation. In contrast, as a single agent, arsenic trioxide neither directly activates transcription nor triggers terminal differentiation ex vivo, but cures many patients. Here we review the evidence from recent ex vivo and in vivo studies that allow a reassessment of the role of differentiation in APL cure. We discuss alternative models in which PML-RARA degradation and the subsequent loss of APL cell self-renewal play central roles. Rather than therapy aimed at inducing differentiation, targeting cancer cell self-renewal may represent a more effective goal, achievable by a broader range of therapeutic agents.
Targeting of leukemia-initiating cells in acute promyelocytic leukemia.
Testa Ugo,Lo-Coco Francesco
Stem cell investigation
Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia (AML) with peculiar molecular, phenotypic and clinical features and unique therapeutic response to specific treatments. The disease is characterized by a single, pathognomonic molecular event, consisting of the translocation t(15;17) which gives rise to the PML/retinoic acid receptor α (RARα) hybrid protein. The development of this leukemia is mainly related to the fusion oncoprotein PML/RARα, acting as an altered RAR mediating abnormal signalling and repression of myeloid differentiation, with consequent accumulation of undifferentiated promyelocytes. The prognosis of APL has dramatically been improved with the introduction in therapy of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). The main effect of these two drugs is linked to the targeting of either RAR moiety of the PML/RARα molecule and induction of cell differentiation (ATRA) or of the PML moiety of the fusion protein and induction of leukemic cell apoptosis, including leukemic progenitors (mostly induced by ATO). These two drugs exhibited excellent synergism and determine a very high rate of durable remissions in low/intermediate-risk APLs, when administered in the absence of any chemotherapeutic drug. The strong synergism and the marked clinical efficacy of these two agents when administered together seem to be related to their capacity to induce PML/RARα degradation and complete eradication of leukemia stem cells.
[Effects of PML-RARalpha on cAMP-induced AML cell differentiation].
Jia Pei-Min,Dou Ai-Xia,Zhang Chang-Lin,Lou Ye-Jiang,Pan Xiao-Rong,Tong Jian-Hua
Zhongguo shi yan xue ye xue za zhi
To explore the molecular mechanisms of acute promyelocytic leukemia cell differentiation induced by cAMP combined with low-dose As2O3, the PR9 cell line, which was stably transfected by PML-RARa fusion gene, was used as in vitro model. The effects of PML-RARa on cAMP-induced AML cell differentiation were evaluated according to cell growth, cell morphology, cell surface antigen as well as luciferase reporter gene assay, in the cells before and after the treatment with cAMP and/or As2O3. The results showed that cAMP alone could slightly increase the expression of CD11b in the PR9 cells expressing the PML-RARa fusion protein, but could not induce these cells to differentiate. The cells presented the terminal differentiation morphology and significantly increased CD11b expression only under the treatment of cAMP combined with As2O3. In addition, PML-RARa had strong inhibitory activity on the transcription of the reporter gene containing cAMP response elements. In conclusions, the PML-RARa fusion protein could dramatically block the signaling pathway of cAMP during the AML cell differentiation.
The PML-RARalpha fusion protein and targeted therapy for acute promyelocytic leukemia.
Leukemia & lymphoma
Acute promyelocytic leukemia (APL) is an unique subtype of acute myeloid leukemia typically carrying a specific reciprocal chromosome translocation t(15;17) leading to the expression of a leukemia-generating fusion protein, PML-RARalpha. Nearly all de novo APL patients undergo disease remission when treated with all trans retinoic acid (ATRA) plus chemotherapy. APL patients that relapse following this type of therapy respond to As2O3 with disease remission once again. The mechanism of action of both ATRA and As2O3 appears to be by inducing granulocytic differentiation and this cellular differentiation seems to depend on PML-RARalpha proteolysis. ATRA treatment results in partial cleavage and complete degradation of PML-RARalpha protein in differentiation sensitive, but not in differentiation resistant APL cells. As2O3 treatment results in only complete degradation of PML-RARalpha protein in both ATRA-sensitive and -resistant APL cells. PML-RARalpha appears to cause leukemia by acting as a transcriptional repressor of RARalpha target genes and by inhibiting activity of transcription factor C/EBPalpha. Therefore, PML-RARalpha proteolysis induced by ATRA and As2O3 may play an important role in overcoming the repressive activity of PML-RARalpha and allowing cellular differentiation to proceed. This review will focus on the status of the PML-RARalpha fusion protein and its relationship to gene and differentiation induction as well as differentiation resistance of APL cells.
Differentiation therapy revisited.
de Thé Hugues
Nature reviews. Cancer
The concept of differentiation therapy emerged from the fact that hormones or cytokines may promote differentiation ex vivo, thereby irreversibly changing the phenotype of cancer cells. Its hallmark success has been the treatment of acute promyelocytic leukaemia (APL), a condition that is now highly curable by the combination of retinoic acid (RA) and arsenic. Recently, drugs that trigger differentiation in a variety of primary tumour cells have been identified, suggesting that they are clinically useful. This Opinion article analyses the basis for the clinical successes of RA or arsenic in APL by assessing the respective roles of terminal maturation and loss of self-renewal. By reviewing other successful examples of drug-induced tumour cell differentiation, novel approaches to transform differentiating drugs into more efficient therapies are proposed.
Retinoic acid and arsenic trioxide sensitize acute promyelocytic leukemia cells to ER stress.
Masciarelli S,Capuano E,Ottone T,Divona M,De Panfilis S,Banella C,Noguera N I,Picardi A,Fontemaggi G,Blandino G,Lo-Coco F,Fazi F
Retinoic acid (RA) in association with chemotherapy or with arsenic trioxide (ATO) results in high cure rates of acute promyelocytic leukemia (APL). We show that RA-induced differentiation of human leukemic cell lines and primary blasts dramatically increases their sensitivity to endoplasmic reticulum (ER) stress-inducing drugs at doses that are not toxic in the absence of RA. In addition, we demonstrate that the PERK pathway, triggered in response to ER stress, has a major protective role. Moreover, low amounts of pharmacologically induced ER stress are sufficient to strongly increase ATO toxicity. Indeed, in the presence of ER stress, ATO efficiently induced apoptosis in RA-sensitive and RA-resistant APL cell lines, at doses ineffective in the absence of ER stress. Our findings identify the ER stress-related pathways as potential targets in the search for novel therapeutic strategies in AML.
Understanding the molecular pathogenesis of acute promyelocytic leukemia.
Lo-Coco Francesco,Hasan Syed Khizer
Best practice & research. Clinical haematology
Acute promyelocytic leukemia (APL) is a distinct subset of acute myeloid leukemia (AML) associated with peculiar biologic and clinical features and requiring specific management. At the genetic level, APL is featured by a unique chromosome translocation t(15;17) which results in the PML-RARα gene fusion and chimeric protein. APL is the first example of differentiation therapy targeted to a defined genetic target i.e. PML-RARα. PML-RARα behaves as an altered retinoic acid receptor with an ability of transmitting oncogenic signaling leading to accumulation of undifferentiated promyelocytes. All-trans-retinoic acid (ATRA) induces disease remission in APL patients by triggering terminal differentiation of leukemic promyelocytes. More recently, arsenic trioxide (ATO) has been shown to contribute degradation of the PML-RARα oncoprotein through bonding the PML moiety and has shown excellent synergism with ATRA in clinical trials. Elucidating the oncogenic signaling of PML-RARα through various transcription factors and the study of APL mouse models have greatly helped to understand the molecular pathogenesis of APL. However, the precise molecular mechanism by which t(15;17) is formed and initiates leukemia remains unknown. While transforming oncogenic potential of PML-RARα has been described extensively, the mechanistic events important for the formation of t(15;17) have been taken from the model of Therapy-related APL (t-APL).