Tyrosine kinases but not cAMP-dependent protein kinase mediate the induction of leukocyte alkaline phosphatase by granulocyte-colony-stimulating factor and retinoic acid in acute promyelocytic leukemia cells

Cytodifferentiation by retinoids, a novel therapeutic option in oncology: rational combinations with other therapeutic agents

Retinoic acid (RA) and derivatives are promising antineoplastic agents endowed with both therapeutic and chemopreventive potential. Although the treatment of acute promyelocytic leukemia with all-trans retinoic acid is an outstanding example, the full potential of retinoids in oncology has not yet been explored and a more generalized use of these compounds is not yet a reality. One way to enhance the therapeutic and chemopreventive activity of RA and derivatives is to identify rational combinations between these compounds and other pharmacological agents. This is now possible given the information available on the biochemical and molecular mechanisms underlying the biological activity of retinoids. At the cellular level, the antileukemia and anticancer activity of retinoids is the result of three main actions, cytodifferentiation, growth inhibition, and apoptosis. Cytodifferentiation is a particularly attractive modality of treatment and differentiating agents promise to be less toxic and more specific than conventional chemotherapy. This is the result of the fact that cytotoxicity is not the primary aim of differentiation therapy. At the molecular level, retinoids act through the activation of nuclear retinoic acid receptor-dependent and -independent pathways. The cellular pathways and molecular networks relevant for retinoid activity are modulated by a panoply of other intracellular and extracellular pathways that may be targeted by known drugs and other experimental therapeutics. This chapter aims to summarize and critically discuss the available knowledge in the field.

Regulation of the human tissue-nonspecific alkaline phosphatase gene expression by all-trans-retinoic acid in SaOS-2 osteosarcoma cell line

While tissue-nonspecific alkaline phosphatase (TNSALP) is a well-known indicator of bone formation and all-trans-retinoic acid a key regulator of that process, the relationship between TNSALP and retinoic acid has not yet been clearly described. The aim of the present study was therefore to clarify the mechanism by which retinoic acid modulates expression of TNSALP. After culturing SaOS-2 human osteoblastic osteosarcoma cells in the presence or absence of 10(-6) M all-trans-retinoic acid, real-time RT-PCR confirmed that retinoic acid up-regulates expression of TNSALP mRNA. Notably, this time-dependent induction of TNSALP expression was accompanied by a corresponding increase in detected catalytic activity of the enzyme. When we then isolated the 5'-upstream region of the human TNSALP gene and carried out luciferase assays with a set of deletion mutants, we found that the promoter region contains a retinoic acid response element-like motif. Moreover, electrophoretic mobility shift assays showed that the nuclear extract bound to the motif. It thus appears that retinoic acid regulates the expression of human TNSALP via a retinoic acid response element in the genes promoter region.

Induction of differentiation of retinoic acid-resistant acute promyelocytic leukemia cells by the combination of all-trans retinoic acid and granulocyte colony-stimulating factor

An acute promyelocytic leukemia (APL) cell line with natural resistance to all-trans retinoic acid (ATRA), UF-1, was induced to differentiate into mature granulocyte when treated with the combination of ATRA and granulocyte colony-stimulating factor (G-CSF), while neither of them alone was capable of inducing the differentiation effectively. Continuous presence of both agents was required for the maximal differentiation-inductive effect. Neither proliferation arrest nor induction of apoptosis preceded the differentiation. Differentiated phenotype was accompanied by growth arrest, however, not by increased apoptosis. It was assumed that cellular events at the downstream of the signaling pathways of ATRA and G-CSF cooperatively played pivotal roles in the differentiation-induction.

The induction and activation of STAT1 by all-trans-retinoic acid are mediated by RAR beta signaling pathways in breast cancer cells

Retinoic acid receptor-beta (RAR beta) and signal transducer and activator of transcription 1 (STAT1) are important mediators of the antiproliferative and apoptotic actions of retinoids and cytokines/growth factors, respectively. Expression of both RAR beta and STAT1 is lost in most breast cancer cell lines but it can be induced by retinoids in estrogen receptor-positive cells. We investigated a possible functional connection between these two mediators and present evidence supporting RAR beta as a tumor suppressor. First, by using different receptor-selective retinoids, we demonstrated that RAR beta induction in MCF-7 cells by all-trans-retinoic acid (atRA) was associated with the activation of STAT1 gene transcription. The direct involvement of RAR beta in atRA-induced STAT1 gene activation was further demonstrated by showing that transfection with an anti-sense RAR beta construct blocked atRA-induced STAT1 expression in MCF-7 cells whereas introduction of a sense-RAR beta construct resulted in STAT1 induction by atRA in MDA-MB 231 cells. In addition, we showed that STAT1 was phosphorylated/activated under atRA treatment of MCF-7 cells; this process required the involvement of RAR beta and protein synthesis. STAT1 phosphorylation/activation was accompanied by increased tyrosine kinase activity that was not due to the activation of JAK1, JAK2 or Tyk 2, suggesting the possible involvement of an unidentified tyrosine kinase.

Alkaline phosphatase, defensin gene expression and effect of myeloid cell growth factors in normal and leukemic cells

In neutrophils, four different granules are defined, i.e. azurophil, specific, gelatinase and secretary vesicles. In these granules many neutrophil-specific constituents are identified. Some of these constituents have already been cloned and their gene expressions studied. In such constituents, alkaline phosphatase (ALP) and defensin are well known, although their functions are not yet fully clarified. ALP is present in secretary vesicles and has important roles in the diagnosis of some myeloid disorders. On the other hand, defensin is the most abundant functional peptide of neutrophils and is present in azurophillic granules. Which are subdivided into defensin-rich and defensin-poor granules. This review describes the expression of ALP and the defensin gene in normal and leukemic cells and the effect on these genes of myeloid growth factors, such as granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor and interleukin 3.

Cross-talk between retinoic acid and interferons: molecular mechanisms of interaction in acute promyelocytic leukemia cells

All-trans retinoic acid (ATRA) and interferons (IFNs) are active anticancer agents. ATRA is capable of inducing complete remission in acute promyelocytic leukemia (APL) patients, whereas IFNalpha is successfully used in the treatment of the stable phase of chronic myeloid leukemia. ATRA and IFNs have shown synergistic interactions in various experimental conditions and represent a potentially useful therapeutic combination in the treatment of various types of leukemias and solid tumors. The molecular basis of these interactions are poorly understood and need to be elucidated. In this review, we summarize a series of recent observations concerning the molecular mechanisms underlying the cross-talk between the intracellular pathways activated by ATRA and IFNs in APL cells. In APL blasts, IFNs regulate the expression of retinoic acid receptors, and ATRA, in turn, modulates the levels and the state of activation of members of the Jak-STAT second messenger pathway. This demonstrates a two-way interaction between ATRA and IFNs, which leads to cross-modulation of genes normally under the control of the retinoid and the cytokine. These data may be relevant in the context of a rational use of the combination between ATRA and IFNs in the clinical management of myeloid leukemias.

Stat1 Is Induced and Activated by All-Trans Retinoic Acid in Acute Promyelocytic Leukemia Cells

Treatment of freshly isolated acute promyelocytic leukemia (APL) cells and the myelogenous leukemia cell lines, NB4, HL-60, and U937, with all-trans retinoic acid (ATRA) results in a remarkable elevation in the amounts of Stat1α and Stat2 proteins. Stat1α protein levels are augmented by ATRA as a consequence of elevated amounts of the corresponding transcripts. The retinoid increases the levels of nuclear complexes that are capable of binding to interferon (IFN)-regulated consensus sequences and contain Stat1 and/or Stat2 proteins, and causes a rapid and long-lasting elevation in Stat1α tyrosine phosphorylation. Transient transfection experiments show that ATRA enhances the transactivating properties of Stat1α observed on an appropriate reporter gene, in the presence of the RARα retinoic acid receptor, but not in the presence of the PML-RAR protein. Treatment of NB4 cells with ATRA is associated with a remarkable upregulation of the two IFN-responsive genes IFN-responsive factor 1 and 2′-5′ oligoadenylate synthetase, as well as with an augmentation in the levels of IFNα secretion. Our data show that ATRA is capable of modulating the amounts and the state of activation of some of the components of the IFN intracellular signaling pathways. They also suggest that the retinoid can bypass IFN/IFN-receptor interactions and induce the expression of IFN-regulated genes.

Leukocyte alkaline phosphatase a specific marker for the post-mitotic neutrophilic granulocyte: regulation in acute promyelocytic leukemia

Leukocyte alkaline phosphatase (LAP) is the product of the gene coding for the liver/bone/kidney-type alkaline phosphatase. In the normal hematopoietic system, the only cell type expressing LAP in basal conditions is the post-mitotic neutrophilic granulocyte. Thus LAP represents a specific and restrictive marker for the terminal maturation of the neutrophilic granulocyte. The study of the factors and the molecular mechanisms responsible for the expression of LAP in cells undergoing granulocytic maturation may shed light on this complex biological process. Acute promyelocytic leukemia (APL) represents a unique biological model in which it is possible to investigate neutrophilic differentiation. APL blasts undergo rapid and irreversible maturation towards cells morphologically and biochemically resembling normal mature granulocytes upon in vivo and in vitro challenge with all-trans retinoic acid (ATRA). In this cellular context, we studied the endogenous factors involved in the expression of LAP. The phosphatase is not synthesized in undifferentiated APL blasts and it is expressed only upon treatment with combinations between ATRA and a second cyto-differentiating signal. The second signal may be given by G-CSF, cAMP analogs, IL-6 and to a lesser extent by IL-1 beta. The molecular mechanisms underlying the induction of LAP by combinations of ATRA and G-CSF or cAMP analogs were studied in detail and are the object of this review.

Interferons induce normal and aberrant retinoic-acid receptors type alpha in acute promyelocytic leukemia cells: potentiation of the induction of retinoid-dependent differentiation markers

Treatment of the acute promyelocytic (APL) cell line NB4 with interferon alpha (IFN(alpha)), as well as IFN(beta) and gamma, results in an increased expression of the transcripts coding for retinoic-acid receptor type alpha (RAR(alpha)) and the leukemia-specific retinoic acid receptor PML-RAR. Transcriptional induction of the RAR(alpha) and PML-RAR mRNAs is rapid and it is parallelled by an increase in the corresponding proteins. Up-regulation of RAR(alpha) and PML-RAR gene expression by IFN(alpha) is accompanied by a strong potentiation in the induction of 2 retinoid-dependent granulocytic markers, i.e., granulocyte-colony-stimulating factor receptor mRNA and leukocyte alkaline phosphatase. However, IFN(alpha) does not have any effects on the retinoid-dependent regulation of the myeloid surface markers CD11b and CD33. The IFN-dependent increase in RAR(alpha) levels and the enhancing effect of the cytokine on retinoid-dependent granulocytic markers expression may be a characteristic of PML-RAR positive cells, since the phenomena are not observed in HL-60 promyelocytes. Interferons as well as retinoids inhibit the growth of NB4 cells, although the 2 classes of compounds do not significantly interact in terms of anti-proliferative activity. These results suggest the possible use of combinations between IFNs and retinoic acid in the cyto-differentiating treatment of APL patients.