Gösta Bergh

Cell differentiation in acute myeloid leukemia

Abstract: Acute myeloid leukemia (AML) is characterized by a differentiation block leading to accumulation of immature cells. Chromosomal translocations in AML affect transcription factors that are involved in regulation of myeloid differentiation. Aberrant expression of these factors interferes with differentiation events and has a role in the pathogenesis of AML through superactivation or (dominant negative) repression of genes regulating proliferation and differentiation or by interference with assembly of the transcription complex for these genes. The maturation arrest can be reversed by certain agents as judged by results from investigations of myeloid leukemic cell lines and from treatment of acute promyelocytic leukemia (APL) patients with all‐trans retinoic acid. Inactivation of the p53 and retinoblastoma (Rb) tumor suppressor genes is also associated with the pathogenesis of leukemia through effects on the cell cycle, and manipulation of these genes can affect differentiation of AML cells. With differentiation therapy, when successful as in APL, the leukemic cell mass is reduced to allow restoration of normal hematopoiesis and clinical remission, but the disease is not cured. However, initial reduction of the cell mass by maturation can increase the probability for cure with chemotherapy. Overexpression of suppressor genes may increase the probability for differentiation. Most probably, particular molecular defects of subgroups of AML have to be explored to find optimal strategies for treatment including both blocking the cell cycle, promoting terminal differentiation, and inducing apoptosis as well as strengthening the immune response.

Forced expression of the cyclin-dependent kinase inhibitor p16(INK4A) in leukemic U-937 cells reveals dissociation between cell cycle and differentiation

OBJECTIVE The aim of this study was to investigate how the tumor suppressor protein p16(INK4A) interferes with growth and differentiation of leukemic U-937 cells. MATERIALS AND METHODS U-937 clones constantly overexpressing the cyclin-dependent kinase inhibitor p16(INK4A) were established. Clones transfected with empty vector were used as controls. The effects of high-level expression of p16(INK4A) on proliferation and cell cycle progression were investigated (cell cycle distribution, proliferation rate, analyses of different cell cycle regulatory proteins). The effect of introduction of p16(INK4A) on capacity for induced differentiation, assayed by capacity to reduce nitroblue tetrazolium, was determined. RESULTS Overexpressed p16(INK4A) protein was active as judged by its ability to bind to CDK-4 in a coimmunoprecipitation assay. Clones overexpressing p16(INK4A) grew slower than controls, without any apparent effects on the phosphorylation status of the retinoblastoma protein (pRb). Instead, p16(INK4A) overexpression affected the phosphorylation status of pRb-related pocket protein p130, which was detected in its growth-restraining hypophosphorylated form. Despite an enhanced tendency to accumulate in G(0)/G(1), p16(INK4A)-overexpressing cells were less sensitive to induction of differentiation with vitamin D(3) or ATRA than control cells. CONCLUSIONS Constitutive expression of p16(INK4A) in U-937 cells resulted in decreased proliferation as a result of activated p130 rather than pRb. Also, we showed that introduction of p16(INK4A) into U-937 cells impaired their capacity to differentiate. Moreover, the results support the notion that cell differentiation and cell cycle progression are dissociated and independently regulated processes.

Receptors for hematopoietic regulatory cytokines: Overview of structure and function

The production of blood cells is regulated by the action of external factors, cytokines, that can be released by many cell types. In the first place, a population of multipotent stem cells, mostly in the resting Go phase of the cell cycle, but with self-renewal capacity, gives rise to progenitor cells that are predetermined for differentiation into all kinds of blood cells. Expression of genes for cytokine receptors leads to external regulation of hematopoiesis by cytokines which bind to the receptors, resulting in modifications of proliferation and differentiation, as cytokines are not only growth factors, but are also maturation factors capable of directing hematopoiesis towards functionally competent cells. What is more, they are survival factors capable of suppressing programmed cell death (apoptosis). This is of particular importance for the viability of stem cells which must be preserved. Thus cytokines can act at all positions of the hematopoietic family tree, and the response can differ from proliferation and differentiation of progenitor cells to functional activation of mature cells. Under physiological conditions, during constitutive hematopoiesis, the regulatory cytokines are produced locally, for instance by stromal ceils of the microenvironment, and act locally in a paracrine manner [2].