Leukemia (2004) 18, 219–226 & 2004 Nature Publishing Group All rights reserved 0887-6924/04 $25.00 www.nature.com/leu REVIEW Interleukin-3 receptor in acute leukemia U Testa1, R Riccioni1, D Diverio2, A Rossini1, F Lo Coco3 and C Peschle1 1Department of Hematology and Oncology, Istituto Superiore di Sanita`, Viale Regina Elena 299, Rome, Italy; 2Dipartimento di Biotecnologie Cellulari, Universita` ‘‘La Sapienza’’, Rome, Italy; and 3Cattedra e Divisione di Ematologia, Universita` Tor Vergata, Ospedale S Eugenio, Rome, Italy Recent studies indicate that abnormalities of the interleukin-3 Several lines of evidence have indicated that AML1/ETO is receptor (IL-3R) are frequently observed in acute myeloid insufficient by itself to induce leukemia, but instead alters the leukemias (AMLs) and may contribute to the proliferative advantage of leukemic blasts. This review analyzes the self-renewal capacities of hematopoietic stem cells, resulting in evidences indicating that the IL-3R represents one of the target the formation of a preleukemic population that lacks a great 5 molecules involved in the stimulation of proliferation of AMLs, growth advantage. For the development of an overt leukemic and the overexpression of the IL-3Ra chain may represent one condition, AML1/ETO-induced effects must be associated with of the mechanisms contributing to the development of a highly the effects of secondary mutations that cooperate with the malignant leukemic phenotype. Furthermore, there is evidence primary mutation and ultimately result in a growth advantage that the IL-3Ra is a marker of leukemic stem cells, at variance 6,7 with normal stem cells that are IL-3RaÀ. Finally, the IL-3R may and in a block of cell differentiation. In line with these represent an important target for the development of new findings, a high frequency of NRAS and c-kit mutations have þ antileukemic drugs. been observed in AML patients with AML1/ETO leukemias. Leukemia (2004) 18, 219–226. doi:10.1038/sj.leu.2403224 Therefore, it was proposed that fusion proteins encoding Published online 4 December 2003 hybrid transcription factors require genetic complementation Keywords: leukemia; growth factors; interleukins; proliferation; with mutant or activated kinases. Activated kinases could differentiation interfere with the cell cycle and complement the differentia- tion-blocking activity of chimeric transcription factors. This model is supported also by additional important observations: (a) fusion proteins, such as BCR/ABL or TEL/AML1 or AML1/ The current model on the pathogenesis of acute myeloid ETO, are found in normal subjects at a frequency about 100 leukemias (AMLs) 8,9 times the corresponding leukemia rates, thus indicating that their presence in few hemopoietic cells is per se not sufficient Several lines of experimental evidence suggest that multiple for the development of a leukemic condition; (b) the analysis of mechanisms are involved in the genesis of human tumors and, the clonal relationship of concordant leukemia in identical particularly, of AMLs.1,2 Among these different pathogenetic mozygotic twins strongly argues in favor of a parental in utero mechanisms, the occurrence of chromosomic translocations 10 origin of pediatric leukemias. with consequent generation of fusion genes and corresponding In the recent years several genes have been identified, fusion proteins play a major role in the genesis of these diseases. mutated or abnormally expressed in AML, that confer a Stem cells are the main target for these key genetic events in proliferative survival advantage to leukemic blasts. These genes acute leukemias. The major contribution of fusion proteins, such are characterized by their involvement in the control of cell as PML/RARa, AML1/ETO, AML1/Evi1 or CBFb/SMMHC, con- proliferation. Thus, the hemopoietic receptor tyrosine kinase sists in the capacity to block the differentiation process at a Flt3 is constitutively activated in 30–35% of cases of AML, as a specific stage (reviewed in Tenen).3 Studies in transgenic mice consequence of internal tandem-repeat mutations in the harboring the fusion gene in their genome clearly indicate that juxtamembrane domain, or activating loop mutations (reviewed the simple expression of this gene in hemopoietic cells greatly 11 in Gilliland and Griffin). Similarly, Flt3 is frequently over- contributes to the differentiation block and, in some cases, also 12 expressed in secondary, therapy-related AMLs and in lympho- in part to the proliferative stimulation, but per se is not sufficient 13 4 blastic leukemia with MLL rearrangements and with for the development of a leukemic condition. This conclusion 14 hyperdiplody. Finally 5–10% of patients with myelodysplasia is directly supported by several experimental models, such as have mutations of the Flt3 gene. The occurrence of a significant transgenic animals expressing the PML/RARa fusion gene: these association between the presence of Flt3 mutations in AML and animals, in spite a high expression of the transgene, need 1–2 the number of blast cells strongly indicates that the mutated years before the development of leukemia. This time is required receptor induces a proliferative biologic effect in leukemic for the acquisition in hemopoietic cells of additional mutations, blasts. Activating loop mutations have been identified in the mainly occurring at the level of genes involved in the control of hematopoietic receptor tyrosine kinase c-kit in 5% of AML cell proliferation, whose alterations are required for tumor 15 cases. Furthermore, activating loop mutations in RAS family development. Interestingly, similar observations have been also members, mainly NRAS and KRAS, have been identified in 15– made in M2 leukemias associated with the t(8;21) translocation 16 25% of cases. It was estimated that mutations of some and the consequent formation of the AML1/ETO fusion protein. components of the receptor tyrosine kinase pathway (including Flt3, c-kit, VEGFRs, c-fms and RAS) occur in 450% of AML Correspondence: Dr U Testa, Department of Hematology and patients. Oncology, Istituto Superiore di Sanita`, Viale Regina Elena 299, 00161 Rome, Italy; Fax: þ 39 064 938 7087; E-mail: [email protected] Recent studies indicate that abnormalities of the interleukin-3 Received 22 July 2003; accepted 15 October 2003; Published online receptor (IL-3R) are frequently observed in AMLs and may 4 December 2003 contribute to the proliferative advantage of leukemic blasts.17 IL-3R in acute leukemia U Testa et al 220 This review will analyze the evidences indicating that the IL-3R GM-CSFR acquired biologic activity on cell differentiation and may represent one of the target molecules involved in the losted its activity on cell proliferation of FDPC-mix cells.23 stimulation of proliferation in AMLs and particularly, that its According to these observations, it was suggested that IL-3 overexpression may represent one of the mechanisms involved exerts its biologic activities mostly at the level of the control of in the pathogenesis of a highly malignant leukemic phenotype. proliferation of hemopoietic progenitors, while the predominant This review will not cover the basic physiology of the IL-3R, as effect of GM-CSF is on the differentiation of these cells. well as the signal transduction pathways induced by the The analysis of the signal transduction pathways triggered by activation of this receptor since this topic was covered by an IL-3R activation is out the scope of the present review, and the excellent review, recently published on this journal.18 reader is referred to an excellent review recently published on this journal.18 Here, only a brief summary of this process is briefly oultined. After ligand binding, the IL-3R/GM-CSFR bc Basic background becomes phosphorylated, and through the recruitment of adaptor proteins such as Shc, Grb2, and Sos activates the Ras Normal hematopoiesis is a multistep process in which lineage signaling pathway. The activated Ras pathway in turn stimulates development occurs as a consequence of the initial stochastic Raf followed by the downstream induction of the MAPK differentiation of pluripotent stem cells with the generation of pathway related to ERK1 and ERK2, linked to increased multipotent hemopoietic progenitors and the subsequent differ- expression of the transcription factors c-fos and c-jun. The entiation along the different hemopoietic lineages of these cells activation of the MAPK pathway is not restricted only to ERK1 under the ordered effects of a number of growth factors. These and ERK2, but involves also p38, and JNK/SAPK. Another very growth factors may be classified according to the level of their important transduction pathway induced during the activation biological action as early-acting hemopoietic growth factors of the IL-3R is represented by the phosphatidylinositol 3-kinase (HGFs), multilineage HGFs and late-acting HGFs. The prototype (PI-3K)/AKT pathway: the p85 subunit of PI3 K associates with of multilineage HGF is represented by IL-3. the bc of the activated IL-3R at the level of phosphorylated Ser- The main biologic activity of IL-3 is exerted at the level of the 585,24 recruits PKB/AKT by phosphorylation of its serine progenitor compartment, where this cytokine stimulates the residues and transmits several survival signals. Ultimately, these survival and proliferation of multipotent cells; particularly, IL-3 various transcription pathways influence in a specific way gene stimulates the development of multilineage colonies from transcription activating