Expression of different conformations of p53 in the blast cells of acute myeloblastic leukaemia is related to in vitro growth characteristics

Pharmacological reactivation of p53 in the era of precision anticancer medicine

p53, which is encoded by the most frequently mutated gene in cancer, TP53, is an attractive target for novel cancer therapies. Despite major challenges associated with this approach, several compounds that either augment the activity of wild-type p53 or restore all, or some, of the wild-type functions to p53 mutants are currently being explored. In wild-type TP53 cancer cells, p53 function is often abrogated by overexpression of the negative regulator MDM2, and agents that disrupt p53-MDM2 binding can trigger a robust p53 response, albeit potentially with induction of p53 activity in non-malignant cells. In TP53-mutant cancer cells, compounds that promote the refolding of missense mutant p53 or the translational readthrough of nonsense mutant TP53 might elicit potent cell death. Some of these compounds have been, or are being, tested in clinical trials involving patients with various types of cancer. Nonetheless, no p53-targeting drug has so far been approved for clinical use. Advances in our understanding of p53 biology provide some clues as to the underlying reasons for the variable clinical activity of p53-restoring therapies seen thus far. In this Review, we discuss the intricate interactions between p53 and its cellular and microenvironmental contexts and factors that can influence p53's activity. We also propose several strategies for improving the clinical efficacy of these agents through the complex perspective of p53 functionality.

Common activities and predictive gene signature identified for genetic hypomorphs of TP53

Missense mutations that inactivate p53 occur commonly in cancer, and germline mutations in TP53 cause Li Fraumeni syndrome, which is associated with early-onset cancer. In addition, there are over two hundred germline missense variants of p53 that remain uncharacterized. In some cases, these germline variants have been shown to encode lesser-functioning, or hypomorphic, p53 protein, and these alleles are associated with increased cancer risk in humans and mouse models. However, most hypomorphic p53 variants remain un- or mis-classified in clinical genetics databases. There thus exists a significant need to better understand the behavior of p53 hypomorphs and to develop a functional assay that can distinguish hypomorphs from wild-type p53 or benign variants. We report the surprising finding that two different African-centric genetic hypomorphs of p53 that occur in distinct functional domains of the protein share common activities. Specifically, the Pro47Ser variant, located in the transactivation domain, and the Tyr107His variant, located in the DNA binding domain, both share increased propensity to misfold into a conformation specific for mutant, misfolded p53. Additionally, cells and tissues containing these hypomorphic variants show increased NF-κB activity. We identify a common gene expression signature from unstressed lymphocyte cell lines that is shared between multiple germline hypomorphic variants of TP53, and which successfully distinguishes wild-type p53 and a benign variant from lesser-functioning hypomorphic p53 variants. Our findings will allow us to better understand the contribution of p53 hypomorphs to disease risk and should help better inform cancer risk in the carriers of p53 variants.

Relation of In Vitro Growth Characteristics to Cytogenetics and Treatment Outcome in Acute Myeloid Leukemia: Prognostic Significance in Patients with a Normal Karyotype

We analyzed in vitro growth characteristics of bone marrow mononuclear cells (BMMCs) from 322 patients with acute myeloid leukemia (AML) in relation to cytogenetic abnormalities. Median colony growth was low in each of the cytogenetic changes associated with a favorable outcome. Most karyotypic abnormalities in the intermediate prognosis group were associated with low growth potential, but 11 q23 abnormalities exhibited 8 times higher in vitro growth. Cytogenetic changes that included abn(3q) seemed to display the highest colony growth in the unfavorable prognosis group, whereas isolated -7 may have been associated with limited growth potential. In vitro growth behavior was predictive of neither rate of complete remission (CR) nor survival of AML patients within the 3 cytogenetic risk groups. In contrast, colony growth differed significantly in the subgroup of patients with a normal karyotype who achieved remission with induction treatment and those who had no remission (10 versus 81.5/10(5) BMMCs; P = .015). Significantly more patients with normal cytogenetics and colony growth below the 50th percentile went into CR than did patients with colony growth above the 50th percentile (82.8% versus 71.2%). Only 4 (6.8%) of the patients in the low growth group had no remission, compared with 12 (23.1%) of the patients with higher in vitro growth (P = .031, chi-square test). In conclusion, colony growth may prove useful as a prognostic factor for early treatment failure in AML patients with a normal karyotype.

Semi-solid colony growth in acute myeloid leukemia and its relation to cytogenetic risk groups

The prognostic significance of in vitro growth characteristics of leukemic blast cells in acute myeloid leukemia (AML) is well established in the literature. This review focuses on semi-solid colony growth from bone marrow mononuclear cells (BMMCs) of 322 newly diagnosed patients with AML in relation to the three established cytogenetic risk groups. The median numbers of colonies derived from cytokine-stimulated BMMCs significantly differed between patients with favorable karyotypes (2/10(6)), intermediate cytogenetics (12/10(6)) and unfavorable abnormalities (51.5/10(6)). Colony growth in the absence of stimulatory cytokines was assessed in 113 patients. Individuals with unfavorable karyotypes exhibited significantly more often partial or full autonomous growth in vitro when compared to the intermediate and favorable prognostic group. Median stimulated colony growth of patients who entered complete remission (CR) was 9/10(6) BMMCs compared with 63 in patients with no response. Both, cytokine-stimulated as well as autonomous colony formation, significantly discriminated between patients regarding survival. However, multiple regression analysis revealed cytogenetic risk groups as the most important predictor for achieving CR, with colony growth adding no additional prognostic information. In conclusion, semi-solid colony growth predicts response to treatment and survival in AML but is clearly related to karyotypic abnormalities.

Superior effect of 9-cis retinoic acid (RA) compared with all-trans RA and 13-cis RA on the inhibition of clonogenic cell growth and the induction of apoptosis in OCI/AML-2 subclones: is the p53 pathway involved?

In the present study, the effects of 9-cis retinoic acid (RA) and 13-cis RA on acute myeloblastic leukaemia (AML) cell growth and the induction of apoptosis as well as its relationship with bcl-2 and p53 were compared with those of all-trans RA (ATRA). The study was performed with the subclones of the retinoid-sensitive OCI/AML-2 cell line. The most prominent inhibitory effect on clonogenic cell growth and morphological apoptosis was shown by 9-cis RA. In addition, Western blotting revealed the most obvious translocation of p53 from cytosol to nucleus in the case of 9-cis RA, which was the only retinoid able to change the conformation of p53 from mutational to wild type, as demonstrated by flow cytometry. There was no difference between the retinoids in the downregulation of bcl-2 as analysed by Western blotting and flow cytometry. The RA receptor (RAR)-alpha antagonist had no effect on apoptosis in any of the three retinoids studied using the annexin V method. In conclusion, this study shows that 9-cis RA was a more potent agent than ATRA or 13-cis RA in inducing growth arrest and apoptosis in the OCI/AML-2 subclones. The effect was associated with the downregulation of bcl-2 and was hardly mediated through the RAR-alpha receptor, but might be related to the activation of p53.

p53 and redox state in etoposide-induced acute myeloblastic leukemia cell death

We investigated whether p53, being a redox-sensitive protein, has a role in the responsiveness of AML cells to etoposide. Two subclones of the OCI/AML-2 cell line, the etoposide-sensitive (ES) and the etoposide-resistant (ER), were used as models. Sensitivity to etoposide was measured by trypan blue and annexin V assays. Etoposide-induced peroxide formation was associated with the induction of cell death. Evident expression of mutated p53 was observed in both subclones in basal growth conditions as analysed by Western blotting and flow cytometry. After etoposide exposure for up to 24 hours, some nuclear accumulation of p53 was observed in the ER subclone, as analysed by Western blotting. The conformation of p53, however, was not changed from mutated toward wild-type during exposure in either of the subclones as analysed by flow cytometry. In conclusion, etoposide-induced change in cellular redox state was associated with apoptosis, but was not a sufficient stimulus for p53 to make its conformation active. Thus, mutated p53 seems to have no role in etoposide-induced apoptosis.

Cell cycle regulatory protein expression in fresh acute myeloid leukemia cells and after drug exposure

Characteristics of treatment-induced cell cycle arrest are important for in vitro and in vivo sensitivity of acute myeloid leukemia (AML) cells to cytotoxic drugs. We analyzed the expression of the major G1 cell cycle regulators (p21Cip1, p27Kip1, cyclins D, cyclin E and pRb) in 41 fresh AML cell samples. The level of p27 expression was the only factor correlated with the response to chemotherapy, a high level of p27 expression being predictive of complete remission. There was a close relation between expression of pRb, cyclin D2 and FAB subtype, illustrated by the absence of both proteins in most samples having a monocytic component (M4, M5). We also assessed the expressions of pRb, cyclin E, p21 and p27 and the activity of cdk2, the major regulator of S-phase entry, after exposure to cytosine-arabinoside (AraC) and daunorubicin (DNR), and found these proteins could characterize time- and dose-dependent cellular response to each drug. We observed hyperphosphorylated pRb, increased levels of cyclin E and a high cdk2 activity, but no p21 induction, in AML cells exposed to 10(-6) M AraC. After exposure to 10(-5) M AraC, corresponding to the serum concentration reached in high-dose AraC regimens (HDAraC), a strong p21 induction was observed, associated with similarly overexpressed cyclin E and even higher cdk2 activity than after 10(-6) M AraC, while apoptosis was significantly increased. These data suggest that cdk2 activity is likely to play a role in AraC-induced apoptosis in AML cells. This mechanism may account for high efficacy of HDAraC in cells showing little sensitivity to conventional AraC doses.

Cytogenetic risk groups in acute myeloblastic leukaemia differ greatly in their semi-solid colony growth

We have analysed the results of semi-solid bone marrow cultures in 296 patients with de novo acute myeloblastic leukaemia (AML) and correlated them with the leukaemic karyotype. A favourable prognostic karyotype was found in 52 patients (group A, 18.3%), an intermediate karyotype in 163 patients (group B, 57.4%), and unfavourable cytogenetics were observed in 69 patients (group C, 24.3%). Median colony growth according to the three risk groups was 2 (range 0--344) in group A, 14.5 (range 0--5000) in group B and 50.0 (0--3000) in group C (A vs. B, P < 0.001; A vs. C, P < 0.001; B vs. C, P < 0.01). Among the patients treated with chemotherapy (n = 257), median colony growth was 10 (range 0-5000) in those who achieved complete remission (CR) compared with 56.5 (range 0-1000) in patients without remission (NR) (P = 0.002). The median colony growth of all patients [13/10(5) bone marrow mononuclear cells (BMMCs); range 0--5000] significantly discriminated between patients regarding survival (OS 11 vs. 7 months, P = 0.044). However, multiple Cox regression analysis revealed cytogenetic risk groups as the most important predictor for achieving CR, disease-free and overall survival, with colony growth adding no additional prognostic information. In 64 patients, colony growth was also investigated without the addition of exogenous cytokines. Interestingly, none of the patients with a favourable karyotype exhibited autonomous growth, whereas 50% with an intermediate and 73% of patients with an unfavourable karyotype displayed either partial or full autonomous growth in vitro (P = 0.0004). Our data suggest that the growth potential of the leukaemic clone seems to be critically influenced by the molecular changes emerging from chromosomal abnormalities.

Clinical relevance of immunohistochemical expression of p53-targeted gene products mdm-2, p21 and bcl-2 in breast carcinoma

The present study was designed to investigate the clinical/prognostic relevance of immunohistochemical expression of p53-targeted genes mdm-2, p21WAF1 and bcl-2 alone and in combination with p53 for the indirect assessment of p53 gene status in breast cancer. 141 archival breast carcinomas were immunostained, and the putative mutational status of the p53 gene was defined in 21 of them, as a control for immunohistochemistry, using the polymerase chain reaction single-strand conformational polymorphism (PCR-SSCP) analysis. Genetic changes of p53 correlated significantly with p53 protein overexpression (p = 0.01) but did not do so with any of the related molecules. Immunohistochemical p53 status was directly correlated with mdm-2 (p = 0.0001), p21 (p = 0.0004) and inversely with bcl-2 (p = 0.005) expression. bcl-2 proved to be an independent marker of prognosis, p53 only in the group of node-positive carcinomas, whereas bcl-2-/p53+ tumours revealed the worst prognosis. Mdm-2 and p21 expression was of prognostic significance neither alone nor in combination. We conclude that the detection of down-stream regulators of p53 does not increase the efficacy of immunohistochemistry in assessing the functional status of p53 in breast cancer; however, their combined analysis may help to select subgroups of patients at the extremes of risk for recurrence, or those with greater chances for survival.

p53 status of newly established acute myeloid leukaemia cell lines

We analysed the status of the p53 gene and protein in eight newly established acute myeloid leukaemia (AML) cell lines representing blast cells of either de novo leukaemia patients in first remission or patients with relapsed and chemotherapy-resistant disease causing their death. There were no mutations in the p53 gene in any of the cell lines as analysed by single-strand conformation polymorphism of amplified exons 5-8. However, the p53 protein was clearly and consistently expressed in all of these cell lines, as shown by immunohistochemistry, Western blotting and flow cytometry. The consistently expressed p53 protein was located in both the nucleus and the cytoplasm of all the cell lines and, as shown by flow cytometry, it was mostly in a conformation typical of the mutated protein. These AML cell lines offer a tool for studying the production and function of the p53 protein and its possible role in cell cycle regulation and chemoresistance as well as in the regulation of apoptosis in AML.

Intraclonal heterogeneity in the in vitro daunorubicin-induced apoptosis in acute myeloid leukemia

Leukemic cells from ten patients with acute myeloid leukemia (AML) were sorted on the basis of in vitro daunorubicin (DNR) uptake. The obtained subpopulations with high and low DNR accumulation were compared with regard to induction of apoptosis, expression of bcl-2 and p53. Heterogeneous induction of apoptosis, confined to subpopulations with high DNR uptake, was observed. The size of the DNR-induced apoptotic fraction (4% to 16%) within a given AML blast population was determined by intracellular drug accumulation and was not related to the level of bcl-2 expression. All tested leukemic samples displayed expression of p53 in a growth promoter orientation, i.e. PAb1620-/PAb240+. In two samples, however, subpopulations expressing a growth suppressor orientation of p53, i.e. PAb 1620+/PAb240-, were also present. These subpopulations were confined to high-DNR-uptake fractions and associated with the induction of apoptosis. We conclude that intraclonal heterogeneity in the intracellular drug accumulation and subsequently in DNR-induced apoptosis might allow the selection of inherently drug-resistant AML clones thus contributing to relapse of leukemia.

p53 expression in human carcinomas: could flow cytometry be an alternative to immunohistochemistry?

Several studies have shown that p53 expression has important clinical implications as an indicator of prognosis and response to chemotherapy or radiotherapy in different human tumor types. Determination of p53 expression by immunohistochemistry (IHC) has been incorporated into routine practice and its reliability has been consolidated. However, flow cytometric (FCM) analysis might represent an important objective and rapid approach. In the present study we determined p53 expression by IHC and FCM on a series of 118 human solid tumors. IHC determination was performed on histological sections and FCM analysis on cell suspensions. Low correlation coefficients (rs from 0.22 to 0.57) were observed between IHC and FCM data from individual tumors. By considering the IHC approach as the gold standard, high sensitivity and low specificity were found for FCM in detecting p53 expression. The FCM analysis of p53 expression and DNA content showed p53-positive cells in all cell cycle phases. Moreover, in most breast, lung, and colon aneuploid tumors (77%), p53-positive cells were detected only in the subpopulations with abnormal DNA content. In conclusion, FCM-p53 expression cannot be used alternatively to IHC determination, and its clinical relevance remains to be validated. Nevertheless, FCM may provide important information about p53 protein expression in the different subpopulations and cell cycle phases. (J Histochem Cytochem 46:41-47, 1998)

Thrombopoietin Upregulates the Promoter Conformation of p53 in a Proliferation-Independent Manner Coincident With a Decreased Expression of Bax: Potential Mechanisms for Survival Enhancing Effects

Thrombopoietin (Tpo) has proliferative and maturational effects on immature and more committed cells, respectively. We previously reported a role for Tpo as a survival factor in the factor-dependent human cell line M07e by demonstrating that Tpo suppresses apoptosis in the absence of induced proliferation. Wild-type p53 is a tumor suppressor gene that can play a vital role in mediating growth factor withdrawal-induced apoptosis in factor-dependent hematopoietic cells. Wild-type p53 can switch from a suppressor conformation, with an antiproliferative, pro-apoptotic phenotype, to a promoter conformation that has a diminished ability to mediate cell cycle arrest and apoptosis. In an effort to elucidate the mechanisms through which Tpo suppresses apoptosis, we investigated the effects of Tpo treatment on p53-mediated apoptosis in M07e cells. Tpo upregulated the expression of the promoter conformation of p53 in M07e cells coincident with a downregulation of Bax and Mdm2 protein levels. Protein levels of Bcl-2 and Bcl-xL did not significantly vary as a function of growth-factor stimulation. Conversely, the levels of suppressor conformation p53 were maximal when M07e was in a growth arrested state and decreased during factor stimulation. Furthermore, Tpo treatment induced an extranuclear buildup and greatly weakened the DNA binding capacity of p53. p53-specific antisense oligonucleotide treatment recapitulated the effects of Tpo treatment on the levels of Bax, Mdm-2, and Bcl-2. These results suggest that Tpo is suppressing growth factor withdrawal induced-apoptosis, at least in part, by downregulating the expression of pro-apoptotic Bax protein levels, through modulating the conformation of p53, which results in a functional inactivation of its pro-apoptotic abilities.

Thrombopoietin Upregulates the Promoter Conformation of p53 in a Proliferation-Independent Manner Coincident With a Decreased Expression of Bax: Potential Mechanisms for Survival Enhancing Effects

Thrombopoietin (Tpo) has proliferative and maturational effects on immature and more committed cells, respectively. We previously reported a role for Tpo as a survival factor in the factor-dependent human cell line M07e by demonstrating that Tpo suppresses apoptosis in the absence of induced proliferation. Wild-type p53 is a tumor suppressor gene that can play a vital role in mediating growth factor withdrawal-induced apoptosis in factor-dependent hematopoietic cells. Wild-type p53 can switch from a suppressor conformation, with an antiproliferative, pro-apoptotic phenotype, to a promoter conformation that has a diminished ability to mediate cell cycle arrest and apoptosis. In an effort to elucidate the mechanisms through which Tpo suppresses apoptosis, we investigated the effects of Tpo treatment on p53-mediated apoptosis in M07e cells. Tpo upregulated the expression of the promoter conformation of p53 in M07e cells coincident with a downregulation of Bax and Mdm2 protein levels. Protein levels of Bcl-2 and Bcl-xL did not significantly vary as a function of growth-factor stimulation. Conversely, the levels of suppressor conformation p53 were maximal when M07e was in a growth arrested state and decreased during factor stimulation. Furthermore, Tpo treatment induced an extranuclear buildup and greatly weakened the DNA binding capacity of p53. p53-specific antisense oligonucleotide treatment recapitulated the effects of Tpo treatment on the levels of Bax, Mdm-2, and Bcl-2. These results suggest that Tpo is suppressing growth factor withdrawal induced-apoptosis, at least in part, by downregulating the expression of pro-apoptotic Bax protein levels, through modulating the conformation of p53, which results in a functional inactivation of its pro-apoptotic abilities.

p53 loss and point mutations are associated with suppression of apoptosis and progression of CML into myeloid blastic crisis

A longitudinal investigation using fluorescence in situ hybridization (FISH) analysis, PCR-SSCP, and in situ detection of apoptosis by the terminal deoxynucleotidyl Transferase (TdT) method was carried out on 13 chronic myelogenous leukemia (CML) patients to study the p53 gene behavior and the apoptotic process during the course of the disease. At diagnosis, FISH showed no loss of the p53 gene on interphase nuclei, and no point mutation was detected by polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) and sequencing. During the disease course, FISH analysis showed a significative loss of allele (LOA) rate for the p53 gene in eight patients that in seven cases was associated with a suppression of apoptotic process and the progressive expansion of the p53+/p53- clone. DNA sequencing showed in two of these eight patients a point mutation on the other allele, consisting in the formation of a stop codon in one case, and in a frameshift mutation in the other. Six patients had a myeloid blastic crisis (BC), five a lymphoid BC, and the other two an erythroid and an undifferentiated BC, respectively. All patients with myeloid BC and the one with undifferentiated BC disclosed a progressive expansion of the clone with p53 loss that was associated with a significant reduction in apoptosis. On the contrary in the 5 patients with lymphoid BC no significant p53 LOA rate was observed during the course of the disease. In these patients apoptotic process also persisted in the acute phase although in a lower rate as compared to CP.

Cytokines, molecular biological abnormalities, and acute myelogenous leukemia

Leukemia cell proliferation is dependent upon cytokines produced by the leukemia cells or by the microenvironment under stimulation by leukemia cells. Abnormal expression of the p53, rb, and ras genes is associated with cytokine production, suggesting that abnormal expression can affect leukemia cells by affecting intracellular growth controls and by stimulating cytokine production. While these observations suggest that cytokines can be used to stimulate leukemia proliferation during cytotoxic therapy increasing the sensitivity to treatment, they also suggest that inhibition of cytokine affects could increase clinical responses by reducing leukemia cell regrowth between courses of therapy.

Translational regulation of human p53 gene expression

In blast cells obtained from patients with acute myelogenous leukemia, p53 mRNA was present in all the samples examined while the expression of p53 protein was variable from patient to patient. Mutations in the p53 gene are infrequent in this disease and, hence, variable protein expression in the majority of the samples cannot be accounted for by mutation. In this study, we examined the regulation of p53 gene expression in human leukemic blasts and characterized the p53 transcripts in these cells. We found control both at the level of RNA abundance and at the level of translation. Four experiments point towards translational control of human p53 gene expression. First, there is no correlation between the level of p53 mRNA and the level of p53 protein expression in blast cells. Second, in two cell lines with similar levels of p53 protein expression but with different levels of p53 mRNA, we find that there is preferential association of p53 mRNA with large polysomes in the cells with less p53 RNA. Third, translation of synthetic human p53 transcripts in cell-free extracts is inhibited by the p53 3'UTR. Fourth, the p53 3'UTR, when present in cis, can repress translation of a heterologous transcript. These observations raise the possibility that human p53 mRNA translation may be regulated in vivo by RNA binding factors acting on the p53 3'UTR.

Expression of p53 protein in oral squamous cell carcinomas and adjacent non-tumorous mucosa of the floor of the mouth: an archival immunohistochemical study using wet autoclave pretreatment for antigen retrieval

Routinely formalin-fixed and paraffin-embedded material of 22 squamous cell carcinomas of the floor of the mouth (T2NoMo, Ro), together with adjacent dysplastic or normal mucosa, were immunohistochemically investigated using a panel of four anti-p53 antibodies (CM1, PAb1801, DO7, PAb240) subsequent to wet autoclave pretreatment for antigen retrieval. p53 immunoreactivity was detected in 9/22 (40%) carcinomas with PAb1801 and DO7 antibodies, and in 8/22 cases using CM1 and PAb 240. p53-positive tumour cells accumulated predominantly at the invasive front of the carcinomas. A focal or scattered p53 immunoreactivity was observed in the adjacent normal and/or dysplastic mucosa in 17/22 (77%) cases using both CM1 and PAb1801 antibodies, in 10/22 with DO7, and in 8/22 with PAb240. This study has demonstrated examples of different p53 immunophenotypes in the non-tumorous and neoplastic oral mucosa of the same patient without significant correlation to the clinicopathological parameters studied.

Role of p53 in leukemogenesis of chronic myeloid leukemia

This review attempts to provide current information on the role played by the p53 gene in normal and leukemic hematopoiesis with particular emphasis on chronic myeloid leukemia. On the basis of the currently available data we can argue that p53 acts as a negative regulator of proliferation of myeloid mature cells and CD34+ progenitors, and its action is mediated through changes in cell cycle kinetics, mainly before the S phase. The p53-dependent pathway is also regulated by several proteins, including p16, p21, p27 (cyclin-dependent kinase [CDK] inhibitors), and a few oncogenes (bcl-2, bax, MDM-2). Although there is some information about the changes in the p53 gene seen in various types of leukemia, the functions and biological importance of these changes in the pathogenesis of leukemia are still largely elusive. During the past several years, accumulated evidence suggests that changes in the p53 gene are commonly associated with blast crisis of chronic myeloid leukemia (CML) but rarely with chronic phase, and they are represented by rearrangements, deletions and point mutations. As for most of the tumors, the majority of point mutations occur between exons 4 and 8 (hot regions). In patients with CML in blastic crisis the most frequent mechanism of p53 inactivation is complete deletion of one allele in association with a point mutation in the remaining allele.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of cell kinetics and cell cycle status by treating CD34+ chronic myeloid leukaemia cells with p53 antisense phosphorothioate oligonucleotides

Mutations of the p53 tumour suppressor gene occur in 20% of chronic myeloid leukaemia (CML) patients in blastic crisis, but it is still uncertain whether this inactivation plays a role in the pathogenesis of blastic transformation or in maintaining the leukaemic proliferation in CML, as it does in several solid tumours. We have previously shown that more than 50% of both normal and CML CD34+ cells express the p53 protein. However, haemopoietic cells at different phases of the cell cycle express p53 with different conformations, suggesting that the function of p53 may be closely regulated during the cell cycle. In order to elucidate the mechanism by which p53 suppresses cell proliferation, we evaluated the effects of inhibiting p53 expression on cell cycle and cell kinetics of chronic phase CML (n = 12) and normal (n = 7) bone marrow light-density cells and purified CD34+ progenitors by using an 18-mer modified antisense oligonucleotide which targets the region covering the six base pairs immediately before the first codon and the first four coding codons of p53. We found that the number of cells positive for the cell cycle-specific nuclear antigen Ki67 and for the BrdU monoclonal antibody (McAb) was significantly increased after p53 antisense olignucleotide treatment. At the same time, p53 protein expression was completely abrogated in both light-density and CD34+ cells. In addition, DNA analysis by flow cytometry demonstrated that the number of cells in quiescent phases of the cell cycle (G0-G1) was significantly decreased after exposure of light-density cells to p53 antisense oligomers, whereas the number of cells in S or G2-M phases was increased. Furthermore, the longer the incubation time the higher the increase in cell proliferation. Treatment of CML, cells with p53 antisense oligomers also resulted in significantly increased numbers of CFU-GM colonies. Our data suggest that p53 is a negative regulator of cell proliferation and its action is mediated through changes in cell cycle kinetics, mainly before the S phase. We can further speculate that the loss of p53 function, at the time of blastic crisis of CML, may play a role, in combination with other genetic changes (p210 BCR/ABL, Rb gene abnormality, others to be defined), in inducing disturbances in cell proliferation, differentiation, and apoptosis.

Biological features of leukaemic cells associated with autonomous growth and reduced survival in acute myeloblastic leukaemia

The blast cells from up to 70% of patients with acute myeloblastic leukaemia exhibit a variable degree of autonomous growth in vitro, which is related to the production of autocrine growth factors. It has recently been established that patients with autonomous blast cell growth have both a lower remission rate and a higher relapse rate, compared to otherwise comparable patients whose blasts exhibit non-autonomous in vitro growth. In a group of 50 patients the actuarial disease-free survival for the autonomous growth group was 11% at 5 years compared to greater than 50% for the non-autonomous growth group. This data suggests that AML blasts with autocrine growth characteristics may be resistant to cytotoxic drug therapy. Here we present further data demonstrating that AML blasts with autonomous growth are relatively resistant to the induction of programmed cell death (apoptosis) and that this is related to the autocrine production of GM-CSF. Also AML blasts with autonomous growths have aberrant expression of genes associated with resistance to apoptosis induced by cytotoxic drugs. These include high expression of the bcl-2 oncoprotein and abnormalities of expression of the p53 tumour suppressor gene. Furthermore bcl-2 expression was found to be unregulated by both exogenous and autocrine GM-CSF suggesting that the documented negative prognostic effect of autonomous growth on treatment outcome in AML, is in part due to the regulatory effect of autocrine GM-CSF on bcl-2 expression, thus protecting cells from apoptosis induced by cytotoxic drug therapy.

Decreased retinoblastoma protein expression in acute myeloblastic leukaemia is associated with the autonomous proliferation of clonogenic blasts

We have previously shown that blasts from some patients with acute myeloblastic leukaemia (AML) grow autonomously in vitro and that this growth pattern is related to the production of autocrine growth factors including granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukin-1 beta. However, another potential mechanism of autonomous growth involves the deletion of growth inhibitory molecules. One such inhibitory protein is the product of the retinoblastoma (Rb) gene which is expressed in normal haemopoietic cells and functions in cell cycle control and as a transcriptional repressor. Deletion of the Rb gene has been reported in various types of malignancy. We have examined the expression of the Rb gene product in blasts from 39 patients with AML by Western analysis. Using an antibody, Rb (Ab-2) which recognizes the C-terminal region, 11/39 samples (28%) failed to express Rb protein. The results of the Western blot analysis were confirmed by flow cytometry using a second antibody, Rb (Ab-1), against residues 300-380 of the Rb protein. Samples from seven of the 11 Rb-negative cases were studied for the expression of Rb mRNA; Rb mRNA was absent in four and three cases were positive. Analysis of the growth characteristics of these cells showed that blasts from 24/39 cases (62%) had partially or totally autonomous growth in a blast cell colony assay. Of these 24 samples with autonomous growth, 10 (41%) were Rb protein negative; in contrast, of the 15 cases with non-autocrine growth only one was Rb protein negative (7%, P < 0.05). Also, in Rb positive blasts, suppression of Rb protein production using an antisense oligonucleotide significantly increased proliferation of clonogenic AML blasts, confirming that the Rb protein acts as a negative regulator of growth in AML blasts. Our data suggest that deletion of Rb protein expression is frequently found in AML and is associated with the acquisition of autocrine growth characteristics, possibly as the consequence of derepression of genes involved in growth control and cytokine production.

Wild-type p53 is required for apoptosis induced by growth factor deprivation in factor-dependent leukaemic cells

The p53 gene is a growth control gene, abnormalities of which have been implicated in a variety of cancers. Recently wild-type p53 has been shown to exist in two interchangeable conformational variants, which can be distinguished by specific p53 monoclonal antibodies. One conformation acts as a suppressor (PAb240-/PAb1620+) and one acts as a promoter (PAb240+/PAb1620-) of cell proliferation; the latter conformation is also that of mutant p53. We have previously shown that acute myeloblastic leukaemia (AML) blasts which proliferate autonomously in vitro express only p53 in the promoter conformation. In contrast, expression of PAb1620 was found only in blasts with non-autocrine growth in vitro and was diminished following stimulation by exogenous growth factors when there was a switch to p53 in the promoter (PAb240+) conformation. As AML blasts with non-autocrine growth undergo apoptosis when deprived of exogenous growth factors, we studied whether this was mediated by wild-type p53. Antisense oligonucleotides to p53 were used to suppress p53 protein expression in blasts with non-autocrine growth and also the factor-dependent human erythroleukaemia cell line TF-1. Following growth factor deprivation for 48 h, 20.6-53.6% of control blasts were apoptotic and demonstrated a typical 'ladder' on DNA electrophoresis characteristic of internucleosomal degradation of DNA. In the presence of p53 antisense, apoptosis was suppressed despite the absence of growth factor, however cell proliferation was not stimulated. We conclude that apoptosis occurring in factor-dependent AML blasts following growth factor deprivation is mediated by wild-type p53 (PAb1620+), and that conformational change of p53 to the PAb240+ conformation occurring either by mutation or by the action of autocrine growth factors would permit leukaemic cell survival by suppressing apoptosis.