Haemopoietic growth factors: their role in acute myeloblastic leukaemia

Cytokines in the differentiation therapy of leukemia: from laboratory investigations to clinical applications

Differentiation therapy of leukemia is the treatment of leukemia cells with biological or chemical agents that induce the terminal differentiation of the cancer cells. It is regarded as a novel and targeted approach to leukemia treatment, based on our better understanding of the hematopoietic process and the mechanisms of its deregulation during leukemogenesis. Clinically, differentiation therapy has been most successful in acute promyelocytic leukemia using all-trans-retinoic acid as the inducer, either alone or in combination with chemotherapy. This review presents evidence that a number of hematopoietic cytokines play important roles in both normal and aberrant hematopoietic processes. In vitro laboratory investigations in the past two decades using well-characterized myeloid leukemic cell lines and primary blast cells from leukemia patients have revealed that many hematopoietic cytokines can trigger lineage-specific differentiation of leukemia cells, which may have important implications in the clinical setting. Moreover, our current understanding of cytokine interactions and the molecular mechanisms of cytokine-induced leukemic cell differentiation will be discussed in the light of recent findings. Finally, ways in which laboratory research on cytokines in the differentiation therapy of leukemia can lead to the improved design of protocols for future clinical applications to leukemia therapy will also be addressed.

Leukemic cells and the cytokine patchwork

BACKGROUND

In leukemia, the clonal population is characterized by a hierarchical organization. Although the majority of the leukemic population is generated after post-determinic divisions, a subset of cells retain undifferentiated "blast" morphology. In addition, leukemic cells often have numerical or structural chromosomal abnormalities, aberrant gene expression patterns, and abnormal cell surface marker profiles. Despite these differences when compared to normal bone marrow and blood cells, leukemic cell survival and proliferation, just like that of normal progenitor cells, is influenced by hematopoietic growth factors. A major issue is whether differential regulation of normal and leukemic hematopoietic cells by cytokines can be exploited in antileukemic treatment or, in contrast, whether in vivo cytokine therapy may even be harmful to the patients.

PROCEDURE

Here we review the results of recent experimental and clinical observations that investigated the influence of cytokines on leukemic cell growth and differentiation in vitro and in vivo.

RESULTS

The majority of studies indicate that hematopoietic growth factors are involved in the regulation of proliferation and terminal differentiation of leukemic blast cells. Genetic aberrations involving cytokines or their receptors may contribute to leukemogenesis. Abundant interactions, cross-lineage stimulation, and aberrant response patterns seem to transform the complex cytokine network regulation of normal hematopoiesis into an even more interlaced "patchwork" that controls leukemic hematopoiesis.

CONCLUSIONS

Since hematopoietic growth factors are present in high serum concentrations in patients with acute leukemia and myelodysplastic syndromes, consequences of possible interactions should be kept in mind even when well-defined human recombinant factors in single application are to be involved in antileukemic protocols.

Binding of monoclonal anti-interleukin 2 antibody to nucleoli in acute leukemia blast cells

Neutralizing anti-IL-2, anti-IL-3, and anti-IL-6 monoclonal antibodies (MAbs) were used for the immunoenzymatic detection of the respective cytokines in blast cells of 38 patients with acute myeloid (AML) and lymphoid (ALL) leukemias by the APAAP-technique. In 20/24 AML-cases (83%) blast cells showed intranuclear staining with MAb anti-IL-2 (DMS-1). In 17 cases reaction was restricted to the nucleoli, in 4 cases additional cytoplasmic staining was observed. Only 2/13 (15%) of the ALL cases showed anti-IL-2 staining. In contrast to IL-2, neither IL-3, IL-6 nor IL-2R alpha-chains were detected in any of the acute leukemias tested. The anti-IL-2 staining of nucleoli in AML cells is distinct from the cytoplasmic staining which is observed in PHA-activated normal lymphocytes and in a minority of AML cells.

Oncogenes, malignant transformation, and modern medicine

During the past decade there have been remarkable strides in the understanding of the basic mechanism of cancer. It is now clear that there is a set of genes, known as oncogenes, that can cause cells to become malignant if their expression is altered, either by mutation or overexpression. The products of these genes include growth factors, growth factor receptors, signal tranduction proteins, and DNA binding proteins. The normal cellular counterparts of these genes play very important roles in the regulation of growth and proliferation by normal cells. Another set of genes, anti-oncogenes, also play an important role in preventing abnormal cell proliferation. The remarkable explosion of understanding of the pathophysiology of malignancy has led to a common unifying concept of malignant transformation that applies to all tumors. It is likely that these new insights will lead to improved and more specific treatments for malignant disease in the next decade.

The role of growth factors in haemopoietic development: clinical and biological implications

Mature blood cells of all lineages are derived from a single class of cell, the haemopoietic stem cell. Stem cells are pluripotent and capable of almost limitless self-renewal. In the bone marrow they form part of a hierarchy that includes progenitor cells, which are more restricted in the lineages their progeny can adopt, and precursor cells, which are committed to differentiation. The mechanisms that regulate progression through this hierarchy are not fully understood, but evidence suggests that both bone marrow stromal cells and soluble growth factors have a role in controlling haemopoiesis. Four growth factors act on progenitor cells to promote their survival, proliferation, differentiation, and maturation: interleukin-3 (IL-3), granulocyte/macrophage-colony stimulating factor (GM-CSF), granulocyte-CSF (G-CSF), and macrophage-CSF (M-CSF). They can also activate the function of mature cells. Considerable overlap is found in the target cells for these four growth factors. We have found that growth factors acting in synergy can recruit more primitive cells than had previously been appreciated. These factors can also determine the lineage that the progeny of multipotential progenitors will adopt. Thus, colony-stimulating factors (CSFs) have the potential to regulate the development of primitive haemopoietic cells in vivo. The properties of CSFs have made them useful in treating malignant disease: G-CSF, in particular, has been used to reduce the period of neutropaenia that follows cytotoxic therapy for various malignancies. The success of these early trials gives ground for cautious optimism about the clinical use of these compounds.