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.

p53-mediated differentiation of the erythroleukemia cell line K562

The tumor suppressor gene p53 can mediate both apoptosis and cell cycle arrest. In addition, p53 also influences differentiation. To further characterize the differentiation inducing properties of p53, we overexpressed a temperature-inducible p53 mutant (ptsp53Val135) in the erythroleukemia cell line K562. The results show that wild-type p53 and hemin synergistically induce erythroid differentiation of K562 cells, indicating that p53 plays a role in the molecular regulation of differentiation. However, wild-type p53 did not affect phorbol 12-myristate 13-acetate-dependent appearance of the megakaryocyte-related cell surface antigens CD9 and CD61, suggesting that p53 does not generally affect phenotypic modulation. The cyclin-dependent kinase inhibitor p21, a transcriptional target of p53, halts the cell cycle in G1 and has also been implicated in the regulation of differentiation and apoptosis. However, transiently overexpressed p21 did neither induce differentiation nor affect the cell cycle distribution or viability of K562 cells, suggesting that targets downstream of p53 other than p21 are critical for the p53-mediated differentiation response.

Involvement of the retinoblastoma protein in monocytic and neutrophilic lineage commitment of human bone marrow progenitor cells

The retinoblastoma gene product (pRb) is involved in both cell cycle regulation and cell differentiation. pRb may have dual functions during cell differentiation: partly by promoting a cell cycle brake at G(1) and also by interacting with tissue-specific transcription factors. We recently showed that pRb mediates differentiation of leukemic cell lines involving mechanisms other than the induction of G(1) arrest. In the present study, we investigated the role of pRb in differentiation of human bone marrow progenitor cells. Human bone marrow cells were cultured in a colony-forming unit-granulocyte-macrophage (CFU-GM) assay. The addition of antisense RB oligonucleotides (alpha-RB), but not the addition of sense orientated oligonucleotides (SO) or scrambled oligonucleotides (SCR), reduced the number of colonies staining for nonspecific esterase without affecting the clonogenic growth. Monocytic differentiation of CD34(+) cells supported by FLT3-ligand and interleukin-3 (IL-3) was correlated to high levels of hypophosphorylated pRb, whereas neutrophilic differentiation, supported by granulocyte colony-stimulating factor (G-CSF) and stem cell factor (SCF), was correlated to low levels. The addition of alpha-RB to liquid cultures of CD34(+) cells, supported with FLT3-ligand and IL-3, inhibited monocytic differentiation. This was judged by morphology, the expression of CD14, and staining for esterase. Moreover, the inhibition of monocytic differentiation of CD34(+) cells mediated by alpha-RB, which is capable of reducing pRb expression, was counterbalanced by an enhanced neutrophilic differentiation response, as judged by morphology and the expression of lactoferrin. CD34(+) cells incubated with oligo buffer, alpha-RB, SO, or SCR showed similar growth rates. Taken together, these data suggest that pRb plays a critical role in the monocytic and neutrophilic lineage commitment of human bone marrow progenitors, probably by mechanisms that are not strictly related to control of cell cycle progression.

p53-dependent and -independent differentiation of leukemic U-937 cells: relationship to cell cycle control

Observations based on overexpression of the suppressor gene p53 or interference with endogenous p53 support a role for p53 in mediating not only growth inhibition and apoptosis but also differentiation. The aim of this study was to characterize the mechanisms of p53-dependent differentiation in the monoblastic leukemia cell line U-937. These cells were transfected with a mutant of the p53 gene expressing wild-type p53 at a permissive temperature. The results showed that wild-type p53 and interferon (IFN)-gamma were able to work synergistically to promote differentiation. This cooperative response was not associated with early G1 arrest of the cell cycle, indicating that p53 can mediate differentiation by mechanisms other than those used for mediating G1 arrest. The differentiation response to transfected p53 with or without INF-gamma was inhibited by cyclic adenosine monophosphate (cAMP)-inducing agents (dibutyryl cyclic adenosine 3':5'-monophosphate, forskolin, and 3-isobutyl-1-methylxanthine) in a dose-dependent manner. In contrast, the differentiation response of p53-negative U-937 cells to 1,25-dihydroxychole-calciferol or all-trans retinoic acid was enhanced by cAMP-inducing agents at optimal concentrations and inhibited at higher concentrations. In addition, 1,25-dihydroxycholecalciferol-mediated differentiation could be achieved in cells arrested in G1 by concomitant incubation with cAMP-inducing agents, indicating that differentiation can occur in the absence of proliferation. In conclusion, the results of this study indicate that p53-dependent and -independent differentiation can occur independently of cell cycle regulation.

The tumor suppressor gene p53 can mediate transforming growth [corrected] factor beta1-induced differentiation of leukemic cells independently of activation of the retinoblastoma protein

Although the involvement of the tumor suppressor gene p53 in normal hematopoiesis is uncertain, it can give rise to differentiation signals in leukemic cells. It is not clear, however, whether differentiation merely is a consequence of the ability of p53 to arrest cell proliferation or whether hitherto unknown molecular mechanisms are responsible for the p53-mediated differentiation. To further explore the role of p53 in leukemic cell differentiation, we investigated whether transforming growth factor beta1 (TGF-beta1), a cytokine involved in cell cycle control at several levels, can cooperate with wild-type p53 to induce differentiation of monoblastic U-937 and erythroleukemic K562 cells. Indeed, wild-type p53-expressing cells were found to be more sensitive to TGF-beta1-induced differentiation than control cells, lending support to the idea that p53 is of importance for differentiation induction of leukemic cells. In addition, it is shown that TGF-beta1 can suppress p53-mediated cell death, thus reinforcing the differentiation response. The cyclin-dependent kinase inhibitor p21 and the retinoblastoma protein (pRb) are downstream effectors of p53-mediated growth arrest. Therefore, the roles for these molecules in p53-mediated differentiation were examined. The p53-dependent signals of differentiation were associated with induction of p21 in both cell lines investigated. However, activation of pRb by induced hypophosphorylation and concomitant decreased growth rate on p53-mediated differentiation was observed only in U-937 cells expressing an inducible, temperature-sensitive form of p53 but not in K562 cells constitutively expressing p53. Thus, our data suggest a role for p53 in the regulation of differentiation in leukemic cells that can be independent of its ability to activate pRb and arrest cell proliferation.

Altered expression of the retinoblastoma tumor-suppressor gene in leukemic cell lines inhibits induction of differentiation but not G1-accumulation

The retinoblastoma tumor-suppressor gene, RB, has been implicated in tumor suppression, in regulation of the cell cycle, and in mediating cell differentiation. RB is necessary for hematopoiesis in mice, and aberrant RB-expression is associated with the progress and prognosis of leukemia. We have used antisense oligonucleotides, established clones stably expressing an antisense RB construct, and also established clones over expressing the retinoblastoma protein (pRb) to study the role of RB expression in monocytic differentiation induced by all-trans retinoic acid (ATRA) or 1-alpha-25-dihyroxycholecalciferol (Vit D3) in the monoblastic cell line U-937 and erythroid differentiation induced by transforming growth factor beta1 (TGFbeta1) and hemin in the erythroleukemic cell line K562. A reduction in pRb production in antisense RB-transfected U-937 clones was shown. Antisense oligonucleotides as well as expression of the antisense RB construct suppressed differentiation responses to ATRA or Vit D3, as judged by the capability to reduce nitro blue tetrazolium, by the appearance of monocyte-related cell surface antigens and by morphologic criteria. K562 cells showed decreased differentiation response to TGFbeta1, but not to hemin, when incubated with antisense oligonucleotides. U-937 antisense RB-transfected cells were also suppressed in their ability to upregulate levels of hypophosphorylated pRb when induced to differentiate. Although U-937 cells incubated with antisense oligonucleotides and clones expressing the antisense RB construct were hampered in their ability to differentiate on incubation with ATRA or Vit D3, the induced G0/G1-accumulation was similar to differentiating control cells treated with ATRA or Vit D3. Intriguingly, U-937 clones overexpressing RB were also inhibited in their differentiation response to ATRA or Vit D3 but not inhibited in their ability to respond with G0/G1 accumulation when induced with these substances. The results indicate that pRb plays a role in induced differentiation of U-937 cells as well as K562 cells involving mechanisms that, at least partially, are distinct from those inducing G1 accumulation.

Cell differentiation in acute myeloid leukemia

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.

Plasma exudation in the skin measured by external detection of conversion electrons

A novel technique for measurement of plasma exudation in the skin is described. Transferrin labelled in vivo with indium-113m is used as a plasma tracer. The conversion electrons from 113mIn are detected with a polystyrene crystal mounted on a photomultiplier tube. Owing to the short range of the electrons in tissue, background radiation from tracer circulating in underlying tissue will be very small, allowing plasma exudation in the skin to be detected with a high signal to noise ratio. The characteristics of the detector system are described in model experiments using sheets of mylar to simulate soft tissue. The acute inflammatory response to histamine provocation was studied in guinea pig skin. A dose-related increase in count rate representing vasodilatation and plasma exudation was detected over the skin after histamine provocation. The electron radiation system appears suitable for detection of low levels of superficial radioactivity and for pathophysiological studies of the skin.

Expression of the p53 tumor suppressor gene induces differentiation and promotes induction of differentiation by 1,25-dihydroxycholecalciferol in leukemic U-937 cells

Leukemic U-937 cells, which lack normal p53, were stably transfected with a temperature-sensitive mutant of p53 to investigate the consequences for growth and differentiation. On induction of wild-type p53 activity at the permissive temperature, some of these cells underwent maturation as judged by the capacity for oxidative burst and the appearance of monocyte related cell surface molecules. Moreover, wild-type p53-expressing cells were more sensitive than p53-negative control cells to induction of differentiation by 1,25-dihydroxycholecalciferol; a twofold to fourfold increase of the fraction of cells showing signs of terminal maturation was observed when wild-type p53-expressing cells were incubated with 1,25-dihydroxycholecalciferol at concentrations that only slightly affected control cells. Whereas wild-type p53 activity per se induced maturation of certain cells, other underwent cell death judging from the reduced capability to exclude trypan blue and the appearance of fragmented DNA in flow cytometric analysis. The p53-induced cell death could be inhibited by incubation with 1,25-dihydroxy-cholecalciferol, but not all-trans retinoic acid. Thus, 1,25-dihydroxycholecalciferol, seemed to increase the survival of wild-type p53-expressing cells and to cooperate with wild-type p53 to induce differentiation. The data imply that p53-mediated maturation in U-937 cells depends on optimal regulation of signals for differentiation, survival and proliferation, and suggest a role for p53 in the differentiation induction of leukemic cells.

Skin plasma exudation and vasodilatation monitored by external detection of conversion electrons

We have examined the plasma exudation response of inflammation in guinea pig skin by a noninvasive method and have evaluated the influence of vasodilatation. Indium radionuclides have been used to label plasma and blood and conversion electrons have been detected by an external detector. Transferrin (79,600 Da) was labeled by 111In or 113mIn in vivo and red blood cells were labeled by 111In in vitro. These tracers were given to separate groups of anesthetized guinea pigs and baseline activities were recorded from shaved skin surface areas. Skin prick tests with histamine and saline were performed and time-activity curves were recorded. The measurements with 111In-transferrin and 111In-labeled red blood cells demonstrated that histamine produced dose-dependent accumulation of plasma (up to a 6.5-fold increase) and blood (up to a 2.0-fold increase) in the skin. Hence, about one-third of accumulation of plasma induced by histamine may be explained by vasodilatation. With 113mIn-transferrin as plasma tracer greater effects of histamine were recorded, probably reflecting that the measurements also included deeper sections of the skin. We conclude that the intensity of accumulation of plasma in skin inflammation can be monitored by external detection of conversion electrons from 111In- and 113mIn-transferrin, and that the influence of vasodilatation can be estimated by detection of 111In-labeled red blood cells.

[Hematopoiesis. Biological mass production closely regulated by cytokines]

Haematopoiesis is regulated by unrelated, pleiotropic, and diverse regulatory molecules, cytokines, whose membrane receptors are related and restricted to a few families manifesting sequence homology. Most members of the cytokine receptor family which lack tyrosine kinase activity are composed of multiple chains. An accessory signal transducer can be shared by members of the receptor family. Cytokine receptor oligomerisation is required for signal transduction, which includes phosphorylation of receptors and cytoplasmic proteins. Upon ligand binding, the receptors for erythropoietin and G-CSF form homodimers, whereas other members of the receptor family form hetero-oligomers in order to generate high-affinity receptor and signal transduction. In their cytoplasmic part, cytokine receptors contain distinct functional domains, proximal and distal to the membrane, that generate separate signals. Cytokines can be used to minimise chemotherapy-induced neutropenia and treat chronic neutropenia, and to shorten the period of aplasia following bone marrow transplantation.