The biology of acute promyelocytic leukemia

Physical and functional interaction between PML and TBX2 in the establishment of cellular senescence

Cellular senescence acts as a potent barrier for tumour initiation and progression. Previous studies showed that the PML tumour suppressor promotes senescence, although the precise mechanisms remain to be elucidated. Combining gene expression profiling with chromatin-binding analyses and promoter reporter studies, we identify TBX2, a T-box transcription factor frequently overexpressed in cancer, as a novel and direct PML-repressible E2F-target gene in senescence but not quiescence. Recruitment of PML to the TBX2 promoter is dependent on a functional p130/E2F4 repressor complex ultimately implementing a transcriptionally inactive chromatin environment at the TBX2 promoter. TBX2 repression actively contributes to senescence induction as cells depleted for TBX2 trigger PML pro-senescence function(s) and enter senescence. Reciprocally, elevated TBX2 levels antagonize PML pro-senescence function through direct protein-protein interaction. Collectively, our findings indicate that PML and TBX2 act in an autoregulatory loop to control the effective execution of the senescence program.

RNAi screen identifies UBE2D3 as a mediator of all-trans retinoic acid-induced cell growth arrest in human acute promyelocytic NB4 cells

All-trans retinoic acid (ATRA) has been widely used in differentiation therapy for acute promyelocytic leukemia (APL). ATRA binds to retinoic acid receptor (RAR) and triggers the formation of the transcription coactivator complex, which leads to changes in gene expression, APL cell-cycle arrest and differentiation, and clinical remission. The mechanisms responsible for ATRA's beneficial effects are still ill-defined. Here, we conducted a large-scale, unbiased short hairpin RNA (shRNA) screen aiming to identify mediators of ATRA-induced differentiation and growth arrest of APL cells. Twenty-six proteins were identified. They cover a wide range of cellular functions, including gene expression, intracellular signaling, cell death control, stress responses, and metabolic regulation, indicating the complexity of ATRA-induced cell growth control and differentiation in APL. One of these proteins, the ubiquitin-conjugating enzyme UBE2D3, is up-regulated in ATRA-treated acute promyelocytic NB4 cells. UBE2D3 is physically associated with cyclin D1 and mediates ATRA-induced cyclin D1 degradation. Knocking down UBE2D3 by RNA interference (RNAi) leads to blockage of ATRA-induced cyclin D1 degradation and cell-cycle arrest. Thus, our results highlight the involvement of the ubiquitin-mediated proteolysis pathway in ATRA-induced cell-cycle arrest and provide a novel strategy for modulating ATRA-elicited cellular effects.

Role of GSTP1-1 in mediating the effect of As2O3 in the Acute Promyelocytic Leukemia cell line NB4

Arsenic trioxide (As2O3) is a highly effective agent in the treatment of acute promyelocytic leukemia (APL), whereas other hematopoietic tumors are less responsive to this agent and mechanisms underlying As2O3,-resistance are poorly understood. To better understand the complex network of GSH-related pathways in As2O3 sensitivity, we investigated the role of GSH and GSH-relevant enzymes in an APL cell line sensitive to As2O3 (NB4) and in a resistant subclone (AsR). Cell proliferation, viability, and apoptosis were investigated in NB4 cells before and after treatment with 1 muM As2O3 and in AsR cells. In these experimental cell models, GSTP1-1, JNK1 and JNK2 proteins were analyzed by immunoblotting, and a kinase assay for JNK1 was performed. GSH levels as well as the activities of the enzymes glutathione peroxidase, glutathione transferase, gamma-Glutamylcysteynilsinthetase and superoxide dismutase were measured. NB4 cells treated with As2O3 showed a high level of oxidative stress and an increase of GSH levels. GSTP1-1 polymerization and JNK1 activation were detectable after 24 h and were followed by an increase of the apoptotic rate starting at 72 h. Neither GSTP1-1 polymerization nor JNK activation was found in AsR cells that showed a very low apoptotic rate. Our results suggest that APL sensitivity to As2O3 might be, at least in part, mediated by the balance between association and dissociation of JNK from GSTP1-1, depending on the redox status of the cell. Further investigation is warranted to find a way to interfere with this balance, whenever it might represent a mechanism of drug resistance.

Enhanced retinoid-induced apoptosis of MDA-MB-231 breast cancer cells by PKC inhibitors involves activation of ERK

Retinoids are vitamin A derivatives, which cause growth inhibition, differentiation and/or apoptosis in various cell types, including some breast cancer cells. In general, estrogen receptor (ER)-positive cells are retinoic acid (RA) sensitive, whereas ER-negative cells are resistant. In this report, we show that ER-negative MDA-MB-231 cells are strongly growth inhibited by retinoids in combination with a PKC inhibitor. While neither RA nor GF109203X (GF) has a significant growth inhibitory effect in these cells, RA+GF potently suppress proliferation. We found that RA+GF induce apoptosis, as shown by an increase in fragmented DNA, Annexin-V-positive cells and caspase-3 activation. Apoptosis was also induced by GF in combination with two synthetic retinoids. Expression of phosphorylated as well as total PKC was decreased by GF and this was potentiated by RA. In addition, treatment with GF caused a strong and sustained activation of ERK1/2 and p38-MAPK, as well as a weaker activation of JNK. Importantly, inhibition of ERK but not p38 or JNK suppressed apoptosis induced by RA+GF, indicating that activation of ERK is specifically required. In support of this novel finding, the ability of other PKC inhibitors to cause apoptosis in combination with RA correlates with ability to cause sustained activation of ERK.

Bexarotene: a clinical review

Bexarotene (Targretin, Ligand Pharmaceuticals Inc.) is a synthetic retinoid analog with specific affinity for the retinoid X receptor and belongs to a group of compounds called rexinoids. Early clinical trials of this drug demonstrated activity in cutaneous T-cell lymphoma. Subsequent Phase II/III trials have demonstrated a greater than 50% response rate in patients with all stages of cutaneous T-cell lymphoma who were refractory or intolerant to the previous therapy. The principal toxicities of bexarotene include central hypothyroidism, xeroderma and elevation of cholesterol and triglycerides. These toxicities can be managed with dose attenuation or addition of atorvastatin (Lipitor, Pfizer) or fenofibrate (TriCor, Abbott Laboratories). Since bexarotene has little bone marrow toxicity, it is an excellent candidate for combination therapy with other modalities useful in the treatment of cutaneous T-cell lymphoma. These include ultraviolet B irradiation, psoralen and ultraviolet A photochemotherapy, interferons, denileukin diftitox (Ontak, Ligand Pharmaceuticals Inc.) and cytotoxic chemotherapy. Bexarotene has also been investigated in the treatment of breast cancer and non-small cell carcinoma of the lung with promising early results.

JNK activation is a mediator of arsenic trioxide-induced apoptosis in acute promyelocytic leukemia cells

Arsenic trioxide induces c-jun N-terminal kinase (JNK) activation and apoptosis in acute promyelocytic leukemia (APL), where it has major clinical activity, but whether JNK is necessary to induce apoptosis is unknown. To clarify this necessity, we established 2 arsenic trioxide (As(2)O(3))-resistant subclones of the APL cell line, NB4. Both resistant lines showed little activation of JNK1 following treatment with As(2)O(3), even at doses sufficient to elicit robust activation in NB4 cells. One mechanism of resistance in these cells is up-regulated glutathione (GSH) content, and GSH depletion by l-buthionine-[S,R]-sulfoximine (BSO) restores JNK activation and As(2)O(3) sensitivity. This correlation between JNK activation and apoptosis led us to test whether inhibition of JNK would protect cells from As(2)O(3)-induced apoptosis. SEK1(-/-) mouse embryo fibroblasts (MEFs) showed diminished JNK activation following As(2)O(3) treatment and were protected from As(2)O(3)-induced but not doxorubicin-induced apoptosis. Furthermore, treatment of arsenic trioxide-sensitive APL cells with the JNK inhibitor, dicumarol, significantly increased growth and survival in response to As(2)O(3) but did not protect cells from doxorubicin. Together, these data support an essential role for JNK signaling in the induction of growth inhibition and apoptosis by As(2)O(3) and suggest that activating JNK may provide a therapeutic advantage in the treatment of cancers that do not respond to arsenic alone.

Diagnostic value of detecting fusion proteins derived from chromosome translocations in acute leukaemia

Clonal chromosomal abnormalities such as balanced translocations are characteristic features of several human leukaemias and have long been detected by conventional cytogenetics on banded metaphases. The advent of molecular biology techniques, advanced karyotyping and immunohistochemistry methods has not only allowed identification of gene involvement at altered chromosome sites and better knowledge of leukaemia pathogenesis, but also contributed important improvements in diagnosis of these heterogeneous diseases. Such novel diagnostic strategies are nowadays being increasingly used to improve leukaemia classification, and in several instances, they help to establish the most appropriate therapeutic strategy in individual patients. Moreover, at least two leukaemia-associated fusion proteins derived from chromosome translocation are specifically targeted by therapeutic approaches which result in significantly increased anti-leukaemia efficacy and reduced toxicity. In this chapter, we highlight the importance of identifying these genetic lesions at diagnosis in acute leukaemia. Further, we discuss briefly the clinical utility of detecting these alterations for prognostic assessment and evaluation of response to treatment.

Expression of bone morphogenetic proteins in acute promyelocytic leukemia before and after combined all trans-retinoic acid and cytotoxic treatment

We investigated the dynamics of bone morphogenetic protein (BMP) and their receptor mRNA expression in relation to combined treatment with all trans-retinoic acid (ATRA) and chemotherapy in four patients with acute promyelocytic leukemia (APL). Reverse transcription-polymerase chain reaction (RT-PCR) analysis of the bone marrow cells at diagnosis showed strong expression of BMP-2, -4, and -7, and their receptors RIA, RIB, and RII, parallel to the expression of promyelocytic leukemia/retinoic acid receptor alpha (PML/RARalpha) fusion gene transcripts. Therapeutic clearance of the tumor molecular marker corresponded to the absence of BMP expression, suggesting the possible role of BMPs as markers of the minimal residual disease (MRD) in APL.

The promise of retinoids to fight against cancer

Retinoids have a reputation for being both detrimental and beneficial: they are teratogens, but they also have tumour-suppressive capacity. Cell biology and genetics have significantly improved our understanding of the mechanisms that underlie the anti-proliferative action of retinoids. Recent elucidation of the pathways that are activated by retinoids will help us to exploit the beneficial aspects of this powerful class of compounds for cancer therapy and prevention.